i hy ih Ht " (yas ania Ceidett Bewole ad favainionoee enn Wh fas) ‘ ea eeshe ail § f a vient ; \ ao a) Bf) ; ag i fiye ee i f sa iigaaiat ae enh iv ieee pseriapeea ical avaniealgiadetatayaite seaediee etttey pce) poh nabes beh ot bee Heat licence ae ity i sl naa ifs ae es ati - ide i eehedidetgl oe a F en oe ats #0 ne . ne = ai Sine tate ag Sia nape Vel eeu ah ag itt ae sine ‘ pie hi it a0 i: W ia Sado te . ae rN : ie i Ss A . te « a be ae a Ae Asa ¢' ‘ Hit: et fe nia ae ae be ae d Bit a Py ile pide i ju ie bat ssi it) saint ine ne a Wn “ ini ia dada vi Seas * ay y ne AO) ae mega Hs ce a eeieneae an: nha Hh ' ituety ts tee whee a ni aitaiart nasa ak vce a i He i nS ‘ 8 an ify Hi us y¢ a Hh i i at . 7 ela ibyhetelcsite aes a ; i les ee Fae $y Het Hit fim ui : . lt aunts a oe : . Bie ra i id its) : : ae fe f — eudtel i it fh it Be : Hania ae ae a oe ss si Fehu And Tot: Bat f al ie Qe AS Re Pa yf n y ayy bY vy 19 A A aN it eins , ry eat AY anes; Wen ; bey j ( Geet CG VOURNAL OF GEOLOGY THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS THE CAMBRIDGE UNIVERSITY PRESS LONDON THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI THE MISSION BOOK COMPANY SHANGHAI pa a EE Fat ae THE MOOK NAL OF GEOLOGY A Semi-Quarterly Magazine of Geology and Related Sciences Q. KX EDITED BY THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY With the Active Collaboration of STUART WELLER, ALBERT JOHANNSEN, Invertebrate Paleontology Petrology EDSON S. BASTIN ROLLIN T. CHAMBERLIN, Economic Geology Dynamic Geology ASSOCIATE EDITORS SIR ARCHIBALD GEIKIE, Great Britain JOHN C. BRANNER, Leland Stanford Junior Uni- CHARLES BARROIS, France versity ALBRECHT PENCK, Germany RICHARD A. F. PENROSE, Jr., Philadelphia, Pa, HANS REUSCH, Norway WILLIAM H. HOBBS, University of Michigan GERARD DEGEER, Sweden FRANK D. ADAMS, McGill University SIR T. W. EDGEWORTH DAVID, Australia CHARLES K. LEITH, University of Wisconsin BAILEY WILLIS, Leland Stanford Junior WALLACE W. ATWOOD, Clark University University WILLIAM H. EMMONS, University of Minne- CHARLES D. WALCOTT, Smithsonian sota Institution ARTHUR L. DAY, Carnegie Institution VOLUME XxXIx JANUARY-DECEMBER, 1921 THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS Published February, March, May, June, August, September, November, December, 1921 Composed and Printed By The University of Chicago Press Chicago, Illinois, U.S.A. Contents or Votume X XIX NUMBER I THE MECHANICAL INTERPRETATION OF JOINTS. II. Walter H. Bucher FEATURES OF A Bopy OF ANORTHOSITE-GABBRO IN NORTHERN NEW York. William J. Miller A New Foro oF Diplocaulus. M. G. Mehl A GLACIAL GRAVEL SEAM IN LIMESTONE AT RIPON, Weconen F. T. Thwaites STRAND MARKINGS IN THE Peering Senmmenonna OF Canes County, OKLAHOMA. Sidney Powers SUMMARIES OF PRE-CAMBRIAN LITERATURE OF ora et Edward Steidtmann EpiIToRIAL NOTE REVIEWS .. NUMBER II VOLCANIC EARTHQUAKES. Charles Davison THE STRATIGRAPHIC AND FAUNAL RELATIONSHIPS OF THE Weenies Group, MippLE EOCENE OF CALIFORNIA. Bruce L. Clark VULCANISM AND MountarIn-MAxkinc: A SUPPLEMENTARY NOTE. Rollin T. Chamberlin 5 SUMMARIES OF PRE-CAMBRIAN Eeeearene OF NOE Awirnan. Edward Steidtmann REVIEWS NUMBER III THE MINERALOGRAPHY OF THE FELDSPARS. Part I. Harold L. Alling INTRODUCTION . FELDSPAR COMPONENTS . Two-CoMPONENT SYSTEMS ‘THREE-COMPONENT SYSTEMS EXAMINATION OF CHEMICAL Anmees OF ipenpenes Microscopic EXAMINATION OF NATURAL FELDSPARS APPLICATION OF THE MINERALOGRAPHY OF THE FELDSPARS TO GEOLOGICAL PROBLEMS . APPENDIX . eal 166 173 188 194 205 213 242 254 258 275 279 Vi CONTENTS OF VOLUME XXIX NUMBER IV DIFFUSION IN SILICATE Metts. N. L. Bowen . THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION AND EEN Core DIFFERENTIATION OF IGNEOUS Rocks. J. H. L. Vogt , RuSSELL ForK Fautt oF SOUTHWEST VIRGINIA. Chester K. Went- worth : : : : Y : STUDIES OF THE Cucmm 4 OF Gunnar, William Herbert Hobbs REVIEWS NUMBER V DIASTROPHISM AND THE FORMATIVE Processes. XIV. GROUND- WorK FOR THE STUDY OF MEGADIASTROPHISM Part I. SumMMARY STATEMENT OF THE GROUNDWORK ALREADY Law. Thomas C. Chamberlin Part Il. THe INTIMATIONS OF SHELL DanerRNAnN. ‘Retin T. Chamberlin THE PHYSICAL CHEMISTRY OF THE Causmennmenaan AND enenne DIFFERENTIATION OF IGNEOUS Rocks. II. J.H.L. Vogt Types oF Rocky MouNTAIN STRUCTURE IN SOUTHEASTERN IDAHO. George Rogers Mansfield 5 Discussion oF ‘‘SUMMARIES OF PRE-CAMBRIAN Teraarenanann OF INGE AMERICA” BY EDWARD STEIDTMANN. ‘Terence T. Quirke THE NATURE OF A SPECIES IN PALEONTOLOGY AND A NEW KIND OF TyprE SPECIMEN. Edward L. TROXELL REVIEWS NUMBER VI THEORETICAL CONSIDERATIONS OF THE GENESIS OF ORE DEPOSITS. R. H. Rastall Nore ON A PossIBLE FACTOR IN ‘Cansens OF ‘Ghonescen Chane. Harlow Shapley THE PLEISTOCENE SUCCESSION eee “Agena, Tana, AND THE aoe AGE OF THE MAMMALIAN Fossit Fauna. Morris M. Leighton THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION AND MAGMATIC DIFFERENTIATION OF IGNEOUS Rocks. III. J. H. L. Vogt CYCLES OF EROSION IN THE PIEDMONT PROVINCE OF PENNSYLVANIA. F. Bascom THE HoRIzONTAL Mormons OF ‘(Gansencnmes AND THE RUNES NEAR THEIR SURFACE. H. A. Brouwer REVIEWS RECENT Pam eeTtoIes PAGE 295 318 351 370 387 391 416 426 444 469 475 480 487 502 505 515 540 560 578 580 CONTENTS OF VOLUME XXIX NUMBER VII THE MARINE TERTIARY OF THE WEST COAST OF THE UNITED STATES: Its SEQUENCE, PALEOGEOGRAPHY, AND THE PROBLEMS OF CORRE- LATION. Bruce L. Clark OUTLINE OF PLEISTOCENE HISTORY OF Wtissracteens Wonmine Beane Leverett : THe PHYSICAL CHEMISTRY OF THE Chusneenazeonon AND VENER DIFFERENTIATION OF IGNEOUS Rocks. IV. J. H. L. Vogt . SUGGESTIONS AS TO THE DESCRIPTION AND NAMING OF SEDIMENTARY Rocks. A. J. Tieje REVIEWS RECENT PaaCAMONS NUMBER VIII DIASTROPHISM AND THE FORMATIVE PROCESSES. XV. THE SELF- COMPRESSION OF THE EARTH AS A PROBLEM OF GEOLOGY. T. C. Chamberlin EXAMPLES OF SQUEEZING DiS FROM Noman Were Steinar Foslie : ; f : : GEOLOGIC RECONNAISSANCE IN Bawa erect N.H. Darton . PAGE 583 615 627 650 667 677 679 701 720 ERRATA Page 569, lines 14, 15, and 16, should read: “seanticline do not move at the same rate, and the upper parts which were originally above the downward-moving secondary geosyncline may in a later stage of evolution be above the” vill NUMBER ; uses GEOLOGY. ee ae SEM -QuaRreRiy eee a rages Nee EDITED BY ‘THOMAS C. CHAMBERLIN’ AND ROLLIN D. SADBERW, With the Active ollznoration of A ALBERT IO HANN: SEN, Reviolony, : ROLLIN T. CHAMBERLIN, ‘Dynamic Geology ‘ * ASSOCIATE EDITORS JOHN C. BRANNER, Leland Stanford Junior University ~~. RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. WILLIAM H. HOBBS, University of Michigan’ FRANK D. ADAMS, McGill University : CHARLES K. LEITH, University oi Wisconsin eat ‘EDGEWORTH DAVID, Australia , WALLACE W. ATWOOD, Clark Universi‘v BAILEY WILLIS, Leland Stanford Junior University WILLIAM H. EMMONS, University of Minnesota LES D. WALCOTT, Smithsonian Institution : Seis L, DAY, Carnegie Institution. JANUARY- FEBRUARY 1921 a THE MECHANICAL INTERPRETATION OF JOINTS. il. - is WaLTER H. BucHER I OF A BODY oe ANORTHOSITE- GABBRO IN NORTHERN NEW YORK. 4 Wittiam J. MILLER 29 NEW FORM OF DIPLOCAULUS stata ORSON TSS aN . : M. G. Mem 48 THE UNIVERSITY, OVE (© PCG.) PRESS | CHICAGO, ‘ILLINOIS, WSs Ay gi ‘THE CAMBRIDGE UNIVERSITY PRESS, Lonpow She eee THE SER UeEN KABUSHIKI-KAISHA, Toxvo, Osaka, Kyoto, FUKUOKA, ae ee a THE MISSION BOOK COMPANY, Saancuar eae ketmiar 1921 EDITED BY THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY ‘ f With the Active Walhortian of STUART WELLER _ ALBERT JOHANN SEN" Invertebrate Paleontology , Petrology ; EDSON S. BASTIN | ROLLIN ‘TL. CHAMBERLIN. 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Acceptance for mailing at special rate of postage provided for in Section 1103, Act of October 3, 1917, authorized on July 15, 1918. Among articles to appear in early numbers of the Journal of Geology are the following: Diastrophism and the Formative Processes. XIV. Megadiastro- phism. By T. C. CHAMBERLIN and R. T. CHAMBERLIN. _ The Physical Chemistry of the Crystallization and Magmatic Differ- entiation of Igneous Rocks. By J. H. L. Vocr. The. Mineralography of the Feldspars. Bye, aA Line: Volcanic Earthquakes. By Cartes Davison. The Stratigraphic.and Faunal Relationships of the Meganos Group, Middle Eocene of California. By Bruce L. CLARK. Types of Rocky Mountain Structure in Southeastern Idaho. By GEORGE RoGERS MANSFIELD. Summaries of Pre-Cambrian Literature of North America. Papers VI and VII. By Epwarp STEIDTMANN. Russell Fork Fault: of Southwest Virginia. By CHESTER K. _ WENTWORTH. Cycles of Erosion in the Piedmont Province of Pennsylvania. By F. Bascom. Through the generosity of an associate editor of the Journal, Dr. R. A. J. Penrose, Jr., the announced reduction to six issues will- not be necessary for Volume XXIX.— Subscribers will receive eight issues without change in price. (See Editorial Note in this number.) Outlines of Geologic History with Contributions from Walker Museum ; : Especial Reference to North America Vol I. No: 5 The Flora of the Eagle Creek Formation By RALPH W. CHANEY Symposium Organized by BAILEY WILLIS Compilation Edited by ROLLIN D. SALISBURY (5 tiers w and all readers of geologic literature will welcome the publication, in book form, of an important series of essays and discussions on the subject of geologic correlation under the title, Outlines of Geologic History with Especial Reference to North America. They embody the present state of knowledge and opinion concerning many of the funda- mental problems of North American geology, and form an admirable supplement to earlier treatises and manuals. The value of the book is greatly enhanced by the fifteen paleogeographic maps by Bailey Willis which accompany the papers. § The Eagle Creek formation is exposed along the bottom of the Columbia River gorge on the Ore- gon side. It is the oldest forma- tion in the region. § The monograph contains a num- ber of drawings, tables, and nearly a hundred halftones. Paper, $1.00, postpaid $1.10 THE UNIVERSITY ue CHICAGO PRESS CHICAGO - - ILLINOIS 316 pages, Svo, cloth; price $1.50, postpaid $1.65 The University of Chicago Press || Chicago - - Illinois The Geography of the Ozark Highland of Missouri By CARL ORTWIN SAUER, the University of Michigan (Publications of the Geographic Society of Chicago) The Ozark Highland of Missouri was selected for investigation because of its unusual wealth of geographic responses and because little is known concerning its con- ditions and possibilities. 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A valuable feature of the volume is the 44 figures in the text and 26 plates. xvitit-246 pages, Svo, cloth; $3.00, postpaid, $3.20 The University of Chicago Press Chicago, Illinois Goode’s Base Maps and Graphs A series of outline maps for all classes of work in applied sciences and the various fields ofresearch. Prepared by J. Paul Goode, Professor of Geography, University of Chicago. These maps are adapted for use in every grade from the university to the common school: in geography, including commercial or economic geography, in physiography, geology, botany, zodlogy, anthropology and ethnology, sociology, economics, politics, and history. The maps have been prepared by being first drawn on a large scale, to insure accuracy of detail, and then greatly reduced in the engraving. In the quality of the drawing they are superior to most maps used in books and magazines. =a) All maps are perforated for insertion in a standard notebook cover with page 8 X 104 inches. The larger sizes fold once or twice into this cover. A new classification has been arranged, but it should be noted that the maps themselves have not been altered. New maps and graphs are shown in blackface type. Write for prices. SERIES I SERIES II SERIES Ill SERIES IV Maps I IOI ee sate The World: on Mercator’s projection. rorHc abhi 301Hc The World (continents): Homalographic projection. 201HcE ay The World (continents, Eastern half): Homalo- graphic projection. 201HcW..... The World (continents, Western half): Homalo- graphic projection. to1Ho Sine ets The World (oceans): Homalographic projection. 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THE UNIVERSITY OF CHICAGO PRESS, CHICAGO, ILLINOIS Walker Prizes in Natural History B* THE provisions of the will of the late Dr. William Johnson Walker two prizes are annually offered by the Boston Society or Naturat History for the best memoirs written in the English language, on subjects proposed by a Committee appointed by the Council. For the best memoir presented a prize of sixty dollars may be awarded; if, however, the memoir be one of marked merit, the amount may be increased to one hundred dollars, at the dis- cretion of the Committee. For the next best memoir a prize not exceeding fifty dollars may be awarded. Prizes will not be awarded unless the memoirs presented are of adequate merit. The competition for these prizes is not restricted, but is open to all. It is nevertheless the tradi- tion of the Society that the founder of these prizes intended them more in the nature of encourage- ment to younger naturalists than as rewards for the work of mature investigators. 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The Society assumes no responsibility for publication of manuscripts submitted, and pub- lication should not be made before the Annual Meeting of the Society in May. SUBJECT FOR 1921 AND 1922: Any subject in the field of Natural History BOSTON SOCIETY OF NATURAL HISTORY GLOVER M. ALLEN, Secretary BOSTON, MASS., U.S.A. The Geology of Vancouver and Vicinity By EDWARD M. J. BURWASH 14 half-tones, 10 line drawings, and 2 colored maps A thorough and authoritative survey of the region, made more interesting by the remarkable photographs, which include a profile of the Grouse Mountain spur, the Lions from Mount Brunswick, a view of Red Mountain from Black Tusk Mountain, Mount Garibaldi from the south, a glacial section from a cutting on Keith Road, and others equally inter- esting. One colored map. shows the topography of Vancouver range and the other is a geological map of Vancouver and vicinity. 112 pages, paper covers; $1.50, postpaid $1.60 THE UNIVERSITY OF CHICAGO PRESS CHICAGO : - - : - ILLINOIS VOLUME XXIx NUMBER 1 THE BOwWKNAL OF GEOLOGY JANUARY FEBRUARY ro27 THE MECHANICAL INTERPRETATION OF JOINTS WALTER H. BUCHER University of Cincinnati Je IE JEL OUTLINE Mour’s THEORY Mour’s THEORY APPLIED TO EXPERIMENTAL DATA THE ELLIPSOID OF STRAIN PLANES OF SHEARING PRODUCED BY IRROTATIONAL AND ROTATIONAL STRAINS PLANES OF SHEARING IN SHALES HorizoNTAL COMPRESSIVE STRAINS IN Camas Low-ANGLE FAULTING I. MOHR’S THEORY OF RUPTURE! BY LOUIS BRAND Let P denote a point of a body at which the state of stress is to be investigated. For this purpose imagine a sphere of infinitesi- mal radius described about P as center (Fig. 10); then the totality of the stresses at all the surface elements of the sphere constitute the state of stress at the point P. A surface element of the sphere may be specified by means of the vector radius to the element. The surface v thus means an infinitely small plane surface tangent to the sphere at the end of ry. In the following, the stresses shown t This account of Mohr’s theory of rupture was written at the writer’s request by Professor Louis Brand, of the University of Cincinnati, to whom he wishes herewith to express his sincere thanks. ; 2 WALTER H. BUCHER acting upon the sphere are those due to outside matter. Since we assume that the stress distribution within the body is continuous, the stresses that correspond to the two surface elements at the end of a diameter of the sphere are therefore numerically equal - and opposite in sign, except for negligibly small quantities. FIG. 10 We shall now obtain the relation between the unit stresses px, p2 at the two surfaces r;, 7. Form a parallelepiped with three pairs of planes tangent to the sphere, one pair normal to 7,, another to 72, while a third pair is normal to a radius 7; which is perpendicular to both r, and 7,. Figure to shows the projection of this parallele- piped upon a plane through P normal to 7;. Both faces normal to 7; project into a rhombus. The other four faces are rectangles of equal area, dA, and project into lines. Now resolve the stresses px: into three components: parallel to 7,, parallel to r,, and normal THE MECHANICAL INTERPRETATION OF JOINTS 3 to 72,73. Similarly resolve the stresses p, into three components: parallel to r,, parallel to 7;, and normal to 7,, 7;. Of these com- ponents, only the first named of each set, namely, Px COS (oz) P2 COS (p.7:), have moments about 7;. Hence for equilibrium as regards rotation about 7;, we have 2dA +p; COS (pr 2) -PC,=2dA * p2 COS (p27:)° PC, ; or, since PC, = PC., Px COS (172) = pz COS (p21). (x) The angles indicated are those between the stresses and outwardly directed radii. The fundamental equation (1) may be stated as follows: Tf px, p2 are unit stresses at the surface elements r;, fo, the projection of p; upon r, is equal to the projection of p. wpon 1;. We proceed to put equation (1) in a more usable form. Let us regard angles in the plane of 7;, r. as positive when clockwise; and let s,, s, denote radii in this plane 90° ahead of r,, 7, respectively. Now resolve p,; into components as follows: (1) o:=p: Cos (p:/s), parallel to 7:; (2) t=: cos (p:5:), parallel to s,; (3) parallel to 73. Also resolve p2 into the components: (1) ¢.=p. cos (p2r2), parallel tO 72; (2) T2=p2 COS (p52), parallel to s,; (3) parallel to 7; The normal stresses o;, 02 (i.e., normal to their surface elements) are positive when directed outward from P, or when they are fensions. The components 7;, 72 of the shearing stresses are positive when they produce clockwise moments about 7;. We now transform the cosines in (1) by means of the formula for the angle between two directions in space: cos (p:2) =Cos (prs) COS (721) + cos (px52) COS (7251) COS (pr) + COS (7275) =cos (pyr) Cos (7172) 0s (px5:) sin (r12), cos (p,7:) =cos (p.72) Cos (712) + COS (p252) COS (7:52) Cos (p273) COS (7773) =cos (p,r2) cos (7:72) — COs (p22) sin (7272). 4. WALTER H. BUCHER Substituting the values in (1) and noting the above values for oy, G2, Tx, T2, We have ox COS (772) +7: Sin (772) =o, COS (7x72) —Tz Sin (7,72), or (o.—o;) COS (7472) =(7,+72) sin (7472). (2) As a first application of this equation we have Tz+T2=0 when (%72)=00. (3) In this case the shearing stresses are numerically equal but opposite in sign. Next, let 7, be any fixed radius in the plane 7,, r.; and write b= (rors), b+ AG=(For2)} C2=01+Ac, T2=71+Ar. Then (2) assumes the form Ao cos Ad = (27,;+Ar) sin Ad; dividing through by A¢ and letting A@ approach zero, we obtain ant (a the subscript being no longer needed. From this equation it appears that o increases with ¢@ when 7 has the direction of increas- ing . If ¢ is not constant over the sphere it must reach a minimum o, and a maximum go, at certain diameters which we denote by x and zg respectively. At these points = =o for al] diametral planes; hence from (4) the fofal shear vanishes at x and g. Also, from (2), cos (xz) =o, so that x and z are perpendicular. Again, if y is a third diameter, perpendicular to both x and zg, the components of shear in both yx and yz planes vanish by virtue of (3), and hence the total shear for y is also zero. The three mutually perpendicular diameters x, y, z, are called the principal axes, the planes xy, yz, 2x, the principal planes, and the stresses oy, Fy, o;, the principal stresses THE MECHANICAL INTERPRETATION OF JOINTS 5 at the point P. When the principal stresses are unequal the nota- tion has been chosen so that ¢,<0, = 0,; values of 6 corresponding to different values of x in this case are given in the following table: O02 Ke aaillo ak 5) 2 3 4 SLO ZO Or SOs 7 Ge Oo” Be AO” BS AS” The principal circle for the state in which o,=—c,, that is, a circle about O as center and tangent to the envelope tangents, determines the ultimate torsional strength: p,=O7. The ulti- mate shearing strength is given by 7,=OS; for this ordinate represents the maximum shear for zero normal stress. From the geometry of the figure, o10> ey So T,=3V o.O2. P3 Gi ee ae 102 When a; and a, are nearly equal we have p,=7,=40, a result in concordance with many old and modern experiments and not satisfactorily explained by the earlier theories of rupture. II. MOHR’S THEORY APPLIED TO EXPERIMENTAL DATA Opi Os =, connects the angle of shearing O20; with two important physical constants, the ultimate tensile strength and the crushing strength. Within certain limits, that is to say, inso far as the curve T,, =f(c) between the points G, and G, approaches a straight line, it confirms Mohr’s formula, cos @= IO WALTER H. BUCHER Hartmann’s observation that the angle of shearing is independent of the nature and intensity of the stresses involved. It also brings out clearly, in a qualitative way, the very different attitude of the planes of shearing in brittle and ductile substances. This becomes apparent when we introduce into it the average values of ultimate strengths contained in the following table: TABLE I* ki Tons, |) Se Granite (Gv) 50o> cae ee eee ToS Ou 25 Glassi(vaniousysorts) hens amin cone oe eno 4.3-8.7 | 2.5-6.0 | I5-— Mila oles (avs): vevce te arcs cpnisy oka cer eles emieietees chek enone 4.0 Ona5 II HBIVEeSton ess rye cathe i CR nes aha a 6.8 0.7 9-7 AGES tOMe{(Givia) ees. eseoks eke UCI nee Bo On 5 7 Glassi(Comimontereen) saree eeceeeennee Bio) 1.5 6.7 OMe (CASTS cele ee ral cre ae Ns Rete ie era Re 40 Fo 5 Ga ZAM CRCAS tea 4 discrete: coat ccrecaye Muosine AREA SIS moon (10) 2.5 (4+) Georgia Yellow Pine (across grain)............... On7] 0.3 Doe Coppersicastaytin sn acce oka o eri em nea (20) - 12 a9 LEST SCS) Ga ta ety tar eaten Sen A ee ans toa. Ole eet Ee aR 35 I Coppers sOlis eee A. LA ies tuahn poole iter myeaiie ae 15 15 I SIUC CR rae sR Gera, Magee Mm gene Mec ecc a 5 5 I Winhitei@aks(Gicrossiorain) seen eeee einer I I i Wiarter@alker (yatiny esa) peepee er ane ee 3.5 5 0.7 Iron, wrought (av. good bars)................... 16-20 25 0.6-0.8 Georgia Yellow Pine (with grain)................ D5 6 0.4 * All figures in this table, excepting those in the third row, from H. H. Suplee, The Mechanical Engineer’s Reference Book (4th ed., 1913). They represent at best only average values and are used here only in a qualitative sense. Values of K2 given in parenthesis indicate loads producing 10 per cent reduction in original lengths. All figures are given in the original in lbs. per sq. in. The values for various sorts of glass are taken from Winkelmann and Schott, quoted in O. D. Chwolson, Lehrbuch der Physik, Vol. I (Braunschweig, 1902), pp. 709 and 712. } ‘Fine-grained, compact, very strong and durable” graywacke (of Hamilton age). G. P. Merrill, Stones for Building and Decoration (2d ed., 1897), p. 322. According to the formula, the angle 6 will differ the more from go the greater the ratio of the values of crushing and tensile strength. Correspondingly, we find that substances like glass shear at very acute angles,t while for mild steel the angle varies between 80° and 100°.? 20°-30° in the case of glass of thick microscopic slides, according to tests made by the writer. 2 See results of experiments by W. Mason and G. H. Gulliver, as given in W. Mason, “The Liiders’ Lines on Mild Steel,” Proc. Phys. Soc. of London, Vol. XXIII (1911), p. 306. THE MECHANICAL INTERPRETATION OF JOINTS II In the case of those substances in which the tensile strength is greater than the crushing strength, the formula would indicate that the shearing angle is obtuse. That this is indeed the case, for instance in the striking case of wood cut with the grain, may be seen from Figure 3, page 17, of Leith’s Structural Geology. This leads us to the important generalization that the angle of shearing of a material is the more acute the more brittle the sub- stance is, and vice versa. In fact, it seems possible, if not probable, that in the hands of the physicist the angle of shearing will be made the chief criterion of brittleness. The formula also brings out clearly the fact that the angle of shearing is independent of the hardness of the material. In the table given above wrought iron ranks with oak and pine wood. Small cubes of a “‘brittle’’ rubber, sold under the trade-name ‘soap rubber,”’ produce shearing planes in the form of pyramids exactly like those seen in cubes of sandstone in ordinary crushing tests, with apical angles of 50° or even less. This shows that the shearing angle is also independent of the absolute amount of defor- mation of which the substance is capable below the elastic limit. Strongly ductilet bodies, on the other hand, like soap, shear at very obtuse angles. t The writer knows that in this paper he is using the word “ductile” in a sense which is sure to be severely criticized. He would be delighted to see such criticism lead to a fruitful open discussion of the fundamental conceptions involved in the deformation of solids. At this place the restricted sense in which the word “‘ ductility” is used here may be defined best by giving it as one of several purely empirical charac- teristics of solids under deformation. Substances differ in 1. The force required to produce the same absolute amount of deformation, (Small: rubber, wet clay; large: steel.) 2. The absolute amount of deformation required to reach the elastic limit. (Small: steel, wet clay; large: rubber.) 3. The percentage of any given deformation which remains permanent when the stress is removed. (o per cent=perfect elasticity; 1oo per cent=perfect plasticity.) 4. The additional force required to produce an additional amount of permanent deformation (negative, zero, positive). 5. The time required to produce the same absolute amount of permanent deformation without rupture. 6. The position of the point of rupture with reference to the yield point. (Point of rupture yield point.) In this paper a substance is called “brittle” when its point of rupture lies near its yield point. It is called “the more ductile” the farther beyond the yield point its point of rupture lies. In a “perfectly ductile” substance it lies an infinite distance beyond. Ina “perfectly brittle” substance it is reached before the yield point. 12 WALTER H. BUCHER When rocks are subjected to deformation in nature, the question whether they will break or bend depends largely on the degree of brittleness they possess under the given conditions and not on their hardness. ‘This explains why geologists have been justified in the attempt to reproduce in the laboratory the various structures exhibited by the hard materials of the earth’s crust by the use of soft clay, mixtures of clay and plaster of paris, and even wet sand. The use of the angle of shearing as an index of brittleness opens up new possibilities for standardizing the materials used in such experiments to accurately reproduce definite actual conditions. Mohr’s formula could be quantitatively correct only, if the curve Tm=f(o) were practically a straight line between the circles of ultimate tensile and compressive stress. This, however, is not true. The form of the curve, therefore, must first be determined experimentally for each substance. From it the variable 6 can be computed, from point to point by analogous formulas. This was carried out for the first time, as far as the writer knows, in a series of excellent experiments by Karman.? He used an apparatus in which small cylinders of rock could be subjected simultaneously to hoop and longitudinal pressures in such a way that either pressure could be controlled without changing the other. The results of his experiments are embodied in stress- strain diagrams which in the most striking way show the fact that the materials used—marble and sandstone—change step by step with increasing hoop pressure from a state of perfect brittleness to one of perfect ductility. In this respect Karman’s experiments supplement beautifully the brilliant investigations of Adams. With low hoop pressures shearing occurred in the rock cylinders resulting in the formation of Liiders’ lines on the polished surfaces and, with lowest hoop pressures, leading to rupture. In the following table? the observed values of the angles of shearing at various hoop pressures are placed side by side with the values computed according to Mohr’s graphic construction. t The angle of shearing can readily be determined for many substances by means of an ordinary vise, if small cubes (x cm3) are used. Th. von Karman, “Festigkeitsversuche unter allseitigem Druck,” Zeitschr. des Vereins deutscher Ingenieure, Vol. LV (10911), pp. 1749-57. 3 Tables 1 to 4 of K4rman’s paper. THE MECHANICAL INTERPRETATION OF JOINTS 13 The agreement is sufficiently close to strongly support Mohr’s theory. In addition, however, the figures reveal the striking fact that as the material changes, under the action of all-sided pressure, from a brittle to a ductile substance, the angle of shearing grows progressively less and less acute. Karman’s experiments have, therefore, completely verified in the case of one and the same sub- stance the inference that the less brittle a substance is the larger is its angle of shearing. TABLE II Effective Hoop Pressure | Longitudinal | 6 Observed | 6 Observed | - o2=03 Pressure without with Computed in Atmospheres o1—02 Reduction* | Reduction in Atmospheres | 0 gio || oe sa 53, 235 2100 59 5 5 Ile re OS ee eee 500 2650 72° 65° 63° 685 2880 83° 70° Be ° 690 38° 38° 40° SHIMCNUOMOs.o 0005600068 280 2040 70° 60° 63° 555 2580 82° 73% 70° * To find the true angle at which rupture actually took place, it is necessary to reduce the observed angle to the value it had when the rock cylinder was deformed under its load. We can, however, go even one step farther. If growing circum- ferential pressure at right angles to the direction of maximum (compressive) stress increases the ductility of a substance, and with it the angle of shearing, circumferential tension must decrease it, that is, render the substance more brittle. This is completely born out by the experiments published in 1911 by W. Mason.t. He subjected tubes of mild steel to longitudinal compressive stress simultaneously with the application of interior hydrostatic pressure, and in one series of experiments, made the tubes undergo longi- tudinal tension while applying water pressure externally. The angle facing the direction of maximum compressive stress, in the absence of circumferential pressure, measured? approximately 100°. With growing tension normal to the direction of compression, tW. Mason, ‘‘The Liiders’ Lines on Mild Steel,’ Proc. Phys. Soc. of London, Wolexexalin(Gor1),) pp. 305-33. 2 Ibid., Table B, values at bottom of column. I4 WALTER H. BUCHER the angle changed from 100° to values as low as 84° and event 70°. These values remained the same, whether the compressive stress acted longitudinally or as hoop stress. Here, then, we have a substance which normally shears under compression at an angle of 100° changed to one shearing at 80° through the action of tension in all directions normal to that of the axial compression, or, using the word in the sense defined above, we may say, the substance has been made more brittle. III.’ THE ELLIPSOID OF STRAIN The attempt to correlate joint planes with stress-strain relations, to which the first part of this paper was devoted exclusively, is by no means new. Steidtman’s splendid paper on ‘‘The Secondary Structures of the Eastern Part of the Baraboo Quartzite Range, Wisconsin’ is well known, and Leith, in his lectures and in his Structural Geology’ has impressed on the younger generation of geologists the importance of shearing planes in the mechanics of rock fractures by the use of a wire-netting model. Unfortunately, however, the model as well as the earlier dis- cussions by other writers, give expression only to that case in which the planes of shearing form an obtuse angle in the direction of compressive stress. The outstanding characteristic of the strain ellipsoid illustrated by Leith’s wire-netting model, is the fact that the elongation in the direction of one principal stress equals the shortening in the direction of the other principal stress, or, that the area of the strained surface remains unchanged. A simple mathematical consideration shows that when a circle is changed into an ellipse of equal area, the angle of the lines of no distortion facing the direc- tion of shortening, must always exceed 90° (Fig. 14). To reduce this angle to the smaller value characteristic of all brittle materials, we must assume the longitudinal shortening to be smaller than t 'W. Mason, “‘The Liiders’ Lines on Mild Steel,” Proc. Phys. Soc. of London, Vol. XXIII (1911), Table D, bottom of column (complementary angle). 2 Jour. Geol., Vol. XVIII (1910), pp. 259-70. 3 New York: Henry Holt & Co., 1913, pp. 18-20. THE MECHANICAL INTERPRETATION OF JOINTS 15 the transverse elongation (Fig. 15), that is, increase the area of the Figure under deformation.* This, however, leads to the conclusion that under simple non- rotational stress a brittle body suffers an increase of volume. Since the angle of shearing is the more acute the more brittle a substance is, we must expect the increase of volume under stress to be the greater the more brittle a material is. Fic. 14.—Diagram showing the position of the lines of no distortion in an ellipse of strain derived from a circle of equal volume. The angle of shearing is obtuse. This seems indeed to be the case. Chwolson? gives the following formula connecting the modulus of volume increase (under tension), n, with the modulus of longitudinal strain (Young’s modulus), a, lateral strain Aioe. JPOUSS CIS WYO) || a || a B longitudinal strain n=a(t—20). According to this formula, o=0.5, when 7=0, that is, when the volume remains unchanged during deformation. The change of t For the mathematical proof of this statement the writer is again under obligation to Dr. Brand. 20. D. Chwolson, Lehrbuch der Physik, Vol. I, p. 713. 16 WALTER H. BUCHER volume, therefore, must be the greater the more a differs from o. 5, that is, the smaller it is. The following values of o for different substances completely confirm the inference drawn from the graphic representation of the strain ellipsoid. Paraffin ©.50 Caoutchouc 0.50 Copper °.348 Mild Steel 0.304 Tron 0.243 to 0.310 Zinc 0.205 Glass 0.197 to 0.319 That brittle bodies suffer an increase of volume when deformed under tension, is well known. ‘That such an Fic. 15.—Diagram showing the position of the jncrease of volume also lines of no distortion in an ellipse derived from a : actually accompanies circle of smaller volume (increase of volume accom- : panies deformation). The angle of shearing is acute. deformation under one- sided compression, as demanded by the graphic construction of the strain ellipsoid, seems to be proved by the experiments made by Kahlbaum and Seidler? and more recently by Lea and Thomas. It is essential, therefore, before we use the strain ellipsoid for the interpretation of shearing planes in nature, that we decide which form of the ellipsoid corresponds to the conditions of the specific case. IV. PLANES OF SHEARING PRODUCED BY IRROTATIONAL AND ROTATIONAL STRAINS We may now return to the interpretation of planes of shearing observed in nature. We have learned that Hartmann’s law applies to brittle substances only, that is, that only in brittle materials the QO. D. Chwolson, Lehrbuch der Physik, Vol. I, p. 714. *R. Kahlbaum and Seidler, Zeitschr. Anorg. Chem. (1902), pp. 29-30, 254-94. 3 F. C. Lea and W. N. Thomas, ‘‘Change in Density of Mild Steel Strained by Compression beyond the Yield-Point,” Engineering, Vol. C (1915), pp. 1-3. THE MECHANICAL INTERPRETATION OF JOINTS 17 acute angle of shearing planes faces the direction of the com- pressive stress. We may now extend the law by adding, that in ductile substances it is the obtuse angle that faces the direction of the compressive stress. Before attempting to apply the law to any specific case, there- fore, we must decide whether the material under the given con- ditions had the properties of a brittle or those of a ductile substance. On the other hand, when the direction of the greatest principal stress is known, the position of the joint planes produced by it may be used to determine the degree of ductility which the material possessed at the time of shearing. All the cases so far discussed involve irrotational strains only. The arrangement of shearing planes due to rotational strains, as illustrated by Leith’s wire-netting model and discussed in his book on Structural Geology, is, of course, only possible in ductile sub- stances, as a glance at the angle of the shearing planes will show. This model has, however, been applied successfully to some striking cases of jointing in quartzites. We may approach the problem involved in these interesting cases by turning to ‘an illustration in Van Hise’s “Principles of North American Pre-Cambrian Geology,” page 652.1 Figure 131 shows layers of quartzite alternating with thin beds of more slaty character. The harder beds are traversed by two systems of intersecting joints, both forming angles of 50°—70° with the bedding planes, that is, forming acute angles of approximately 60° facing the bedding planes. In the intercalated slaty beds, however, only one of these two joint systems is developed. It consists of more numerous joints inclined but 20° or less to the bedding planes. If the comple- mentary symmetrical set were developed, the angle formed by the two systems facing the bedding planes would be 130° in these less brittle slaty beds, instead of 60° as in the brittle purer beds. From this relation of the shearing angles in the two types of rock it is evident that the joints in the more brittle beds are due to the normal component of the stress acting on the beds. They t Sixteenth Ann. Rept. U.S. Geol. Survey, Part I (1896), p. 652. See also C. K. Leith, “Rock Cleavage,” U.S. Geol. Survey, Bull. 239 (1905), Pp. 123, Fig. 37. 18 WALTER H. BUCHER must, therefore, have been developed essentially through irrotational strain. | The differential movement between adjoining beds required by the structural relations indicated in the text, was obviously largely limited to slippage parallel to the bedding within the thin slaty layers. It is important to note, however, that the direction of movement here, as in several cases referred to in the first part of this paper, has influenced the number and nature of the two opposed diagonal joint systems in the brittle beds. Those inclined in the direction of drag produced by the differential movement are more numerous, closed, and slickensided, while the opposite set is represented by few and gaping joints. The closely spaced joints in the slaty beds, on the other hand, may partly be due to true rotational strain. Essentially the same considerations apply to the case discussed in detail by Steidtmann' and by Leith in his Structural Geology. Here the joint system opposed to the ‘‘drag”’ action of the differ- ential movement between the beds is represented by but a few “‘open gashes or tension joints.” But their presence is sufficient to indicate that in the center of the quartzite beds the effect of rotational strain has been entirely subordinate to that of the irrotational strain produced by the component normal to the bedding planes. The writer has the suspicion that this will be found to be true most generally in bedded rocks, in which differential movement between the beds is largely accomplished by slippage along bedding planes chiefly within layers of soft rock acting as lubricants. V. PLANES OF SHEARING IN SHALES The absolute and relative values of the crushing and tensile strengths of a rock play a fundamental roll in the deformation of rocks along the face of natural and artificial excavations.? When the depth of an excavation has reached the point at which the vertical component of the stress set up by the removal of t Jour. Geol., Vol. XVIII (1910), p. 261, Fig. r. 2 See the excellent analysis of the factors involved in D. F. MacDonald, ‘‘Excava- tion Deformations,” Congrés géologique international, Compte rendu de la XIIe session (Canada, 1913), pp. 779-92. 2) THE MECHANICAL INTERPRETATION OF JOINTS oryQ “WeUUTDUID ‘x91 YAO JSoM ‘Sopeys Uspy Ul zNeyZ ysn1y} pue pjoq—1 ‘org = ———— Ss = Wy a => = \i\ Se P= Die 2S SS pasanod a= Dig= = =) es M Ta f= 2S Se Li fe ee = ——- = PE En ae —— th LLB jisodary wip) | pear HU 20 WALTER H. BUCHER material at the toe of the new steep slope exceeds the elastic limit, rupture along planes of shearing will take place sooner or later in brittle materials. Ductile substances, on the other hand, such as a OMtS.) 10) [See Zones: Fic. 18.—Diagrammatic contour map of the surface of a bed of shale buckled and cut by an inclined joint plane. Superimposed on it is the outline of a new stream bed cut into this structure after rejuvenation. clays, will flow, causing the lower part of the steep slope to bulge out and the bottom of the excavation to buckle up.t Van Horn has given a detailed description of this buckling at the base of a rock slide.2 Small and entirely local anticlines which =D. F. MacDonald, “Excavation Deformations,’ Congrés géologique interna- tional, Compte rendu de la XIIé session (Canada, 1913) p. 791, Fig. 3. 2 Frank R. Van Horn, “Landslide Accompanied by Buckling, and Its Relation to Local Anticlinal Folds,” Bull. Geol. Soc. Amer., Vol. XX (1910), pp. 625-32. THE MECHANICAL INTERPRETATION OF JOINTS ait quite obviously owe their origin to this process, are of common occurrence in the bottoms of ravines cut into clays or shales, and are often directly asso- ciated with landslip terraces. Identical surficial anticlinal buckles? which have been observed under a cover of glacial | drift without any connection with steep slopes or landslides, probably owe their origin to similar stress relations resulting from the cracking or other deformation of the Pleistocene ice cover. In the vicinity of Cincinnati, wherever the most recent rejuvenation has cut into the bottom of ravines within the Eden shales, similar bucklings are quite common. Frequently, however, these anticlines are not only overturned, but faulted, generally in the form of a minia- ture overthrust, such as shown in the Figures 16 and 17. In the light of the preceding discus- sion, it appears highly probable that these miniature ‘“‘reversed faults’? have nothing to do with horizontal compres- sive stresses. The shales are distinctly ductile, as is implied by the very exist- ence of the anticlines due to flowage. They are, however, ductile only to a limited degree. After the strain has reached a certain limit, they rupture FIG. 19.—Sec. 1 shows the structure exposed on the south side of the stream channel shown in Fig. 18. Secs. 2 and 3 are drawn parallel to and 5 and to feet, respectively, south of, sec. 1. Sec. A shows the result of squeezing out the shale layers from underneath the joint plane in such a way that the lower portion of sec. 2 is pushed out so as to rest under the upper portion of sec.t. In sec. B this process is carried farther, the lower portion of 3 pushed out so as to rest under- neath the upper portion of 1. along planes of shearing. Here the obtuse angle formed by two tE.g., T. C. Hopkins and W. M. Smallwood, “Discussion of the Origin of Some Anticlinal Folds near Meadville, Pennsylvania,” IV, No. 1, 18 (quoted from Van Horn, Joc. cit.). Bull. Syracuse University, Ser. 2 For instance, G. K. Gilbert, ‘Dislocation at Thirtymile Point, New York,’ Bull. Geol. Soc. Amer., Vol. X (1899), pp. 131-34; F. R. Van Horn, “Local Anticlines in the Chagrin Shales at Cleveland, Ohio,” ibid., Vol. XXI (1910), pp. 771-73. 3E. L. Braun, “The Cincinnatian Series and Its Brachiopods in the Vicinity of Cincinnati,” Jour. Cinc. Soc. Nat. Hist., Vol. XXII (1916), No. 1. 22 WALTER H. BUCHER symmetrical joint planes points to the action of compressive stress in a perpendicular direction, or especially to the horizontal tensile Fic. 20.—Block diagram representing the pitching end of an anticline. Sec. a-a’=sec. 1 below. Sec. b-b’=sec. 2 below. These sections show, in addition, the position of two inclined joint planes striking parallel to the axis of the fold. Sec. A shows the result of squeezing out the shale layers from underneath the joint plane in such a way that the lower portion of sec. 2 is pushed out so as to rest under the upper portion of sec. 1. stress which results when the upper layers are forced up by the pressure of the flowing layers of shale underneath. Whenever a stream removes a portion of such an anticline, as indicated on the map sketch in Figure 18, the tendency exists to THE MECHANICAL INTERPRETATION OF JOINTS 23 squeeze the dipping beds up from underneath the joint plane, out into the channel. Sections A and B (Fig. 19) show the result which is brought about entirely by a horizontal flowage of the beds beneath the joint plane in a direction perpendicular to the plane of the paper, and not to any horizontal compressive stresses acting within the plane of the paper. The joint nature of these ‘“‘fault” planes is brought out beauti- fully by the exposure shown in Figure 17, which for years has been well exposed in Westfork Creek at Cincinnati, just above the schoolhouse. There can be little doubt that essentially the same interpretation applies to other similar occurrences.* VI. HORIZONTAL COMPRESSIVE STRAINS IN GRANITE Compressive strains of considerable magnitude, essentially in a horizontal direction, exist at widely separated localities in all states of New England, if not throughout the whole region. In the quarries, the strains find expression in various ways. Vertical drill holes are flattened to an elliptical cross-section,? the cores between contiguous borings are crushed, and cracks open up diagonally from the channels. In the process of quarrying new fissures open up with a dull explosive noise,4 or new sheetlike partings form’ or old sheets buckle up.° With a detailed knowledge of most of the excellent exposures of sheeted granite in New England at his command, Dale has come to the conclusion that shrinkage in cooling, or changes of temper- ature, or other forms of weathering have played, at best, only a secondary réle in the production of the sheet structure; that the t A. H. Purdue, “Illustrated Note on a Miniature Overthrust Fault and Anticline,”’ Jour. Geol., Vol. TX (1901), pp. 341-42; C. E. Decker, unpublished manuscript, see Fig. 17, p. 39, Jour. Geol., Vol. XXVI (1918). 2T. N. Dale, “The Granites of Vermont,” U.S. Geol. Survey, Bull. 404, p. 18, Fig. 2; Bull. 354, p. 34. 3 [bid., Bull. 354, pp. 96 and 126; Bull. 313, pp. 12 and 142. 4Tbid., Bull. 313, pp. 34 and 142. 5 [bid., Bull. 404, pp. 97 and 107. 6 Ibid., Bull. 313, Pl. VII, A. 24 WALTER H. BUCHER main factor has been horizontal compressive stress such as now finds expression in the region." This is not the place nor the time to enter into-a discussion of this complex problem. ‘The final word has not yet been spoken. If we assume, for the time being, a diastrophic origin of the observed strains, we can point to three observations favoring such a view. t. For at least one locality, the famous Quincy quarries south of Boston, Dale records the observation that “‘this strain in some quarries appears to increase with their depth.’” 2. At Fletcher Quarry, on Robeson Mountain, in Washington County, Vermont, Dale observed what he called ‘‘double-sheet” structure. Here, instead of the usual single set of sheets, two such sets, intersecting at an angle of about 42° are exposed. If we analyze their position according to the method described in the first part of this paper we find that the bisectrix of the acute angle, that is, the direction of the greatest principal (compressive) stress, trends N. 60 W.-S. 60 E. and that it differs but slightly from the horizontal, being slightly directed downward toward the southeast. The least (tensile) principal stress, on the other hand, is practically directed upward, in the direction of easiest relief from the horizontal pressure. Fortunately, there is, at the same locality, ““a marked north- east-southwest compressive strain in the upper part of the quarry, raising the sheets and even forming new sheet partings.” The direction of this strain is essentially that which would result from the compressive stress, acting from the northwest, inferred from the ‘“double-sheets.” While this may, of course, be a mere coin- cidence, it certainly is suggestive of a causal connection. None of the other quite numerous cases in which Dale records the direction of compressive strain, together with data concerning the position of the sheet structure, can be used to test this matter «T. N. Dale, ‘The Granites of Connecticut,” U.S. Geol. Survey, Bull. 484 (1911), Pp. 29-36. 2 U.S. Geol. Survey, Bull. 354, p. 96. 3 If we had reason to believe that the granite had been in a ductile state when these planes of parting were formed, the stresses would have to be reversed. Butall observations seem to speak against this possibility. THE MECHANICAL INTERPRETATION OF JOINTS 25 further. For with only one of the two intersecting planes of shearing given, we cannot even guess at the position of the principal stresses.* 3. The New England region is that part of the United States of which we know most definitely and quantitatively that it has under- gone considerable deformation in post-glacial time. DIRORLAL | NOTE In Number 6, Volume XXVIII (1920), of the Journal of Geology, it was announced that, because of the extremely high cost of print- ing, it would be necessary to limit Volume XXIX (1921) to six numbers. This announcement was made with great regret. It is therefore with correspondingly great pleasure that the Journal now is able to make the announcement that, through the gener- _ osity of one of its associate editors, Dr. R. A. F. Penrose, Jr., for- merly connected with the Department of Geology in the University of Chicago, the announced reduction in the volume will not be necessary. During the year 1921, the Journal will continue to be published semi-quarterly, as heretofore. The editors of the Journal and the University appreciate deeply the generous support of Dr. Penrose. Their feeling will be shared, we believe, by all who are interested in the science of geology. Certain changes in the editorial policy of the Journal have been adopted. ‘These are printed near the bottom of the inside page of the first cover of this number, and the attention of contributors is called to them. 87 REVIEWS Fifteenth Biennial Report, Colorado Bureau of Mines, for 1917 and 1918. Denver: The State Printers, 1919. The mining is considered by counties and by products. The his- tory, recent development, production, and markets for the various ores are discussed. Non-metallic products are included; also a short note on oil shale possibilities. In general the report shows that the mining industry of the state is _ declining. Since 1915 the production of gold in the Cripple Creek dis- trict, the chief gold center of the state since 1893, has decreased from $13,683,494 to $8,300,000 (estimated) in 1918. The production of silver in Lake County (the leading silver-producing county) has fallen from 4,154,913 ounces in 1907 to 2,353,530 ounces (estimated) in 1918, although 1914 was a relatively good year. The production of lead has decreased less than that of the precious metals, but the decrease in both copper and zinc has been considerable in recent years. Lake County produces more silver, lead, copper, and zinc than any other. In 1916 the state produced nearly $5,000,000 worth of tungsten, but the esti- mate for 1918 is less than half this figure, due to decreased demand and possibly to the irregularity of the veins. Colorado leads the world in the production of molybdenum, the main deposit (said to be the largest known) being in the western part of Summit County. In 1918 the state had an estimated production of 94,000 pounds of uranium, the largest except in 1914. Two million pounds of vanadium (largest production to date) is the estimate for 1918. The total mineral.production of the state to 1917 is as follows: Gola eh ihc Mente oan nurs $623;047,100)) |. 54 tues 6 ee Silver wactle LAr nee en 466,463,217 593,790,442 fine ounces Lead sy Dah Ms aie Trea ae 173,909,020 3,962,140,896 pounds Copper v2.2) ene ae een 355755,139 237,422,282 pounds VAX CMMI ARENA aM CREO US ete 106,310,030 1,484,929,849 pounds $1,405,484,565 | Dane 88 REVIEWS 89 Coals and Structure of Magoffin County, Kentucky. By Itry B. BROWNING and Puitip G. RUssELL. Frankfort: Kentucky Geological Survey, 4th Series, Vol. V, Pt. II, with geologic ection and maps, 1919. Pp. x+552. This is a detailed report on the subject named in the title. The columnar section accompanying the report shows twenty-three beds of coal, not all workable, most of which are in the Pottsville Series. It is stated that only three horizons in the 1,200 foot section are sufficiently persistent and well defined to be serviceable as horizon markers. It is stated that all the strata exposed are of marine origin. RD ES: Oil and Gas Resources of Kansas. By RAaymonp C. Moore and Winturop P. Haynes. Lawrence: State Geological Survey of Kansas, Bulletin 3. 391 pages, 4o plates. The volume contains a historical sketch of the oil and gas industry of the state, and brief discussions of a general nature on (x) the origin of oil and gas, (2) their migration and accumulation, and (3) methods of production, refining, etc. These discussions are followed by a sum- mary of the stratigraphy of Kansas (pp. 78-173), including the fullest account to date of the sub-surface crystalline rocks of the state. These rocks (granite) are said to constitute a buried ridge nearly 175 miles long and 10 to 25 miles wide, trending in a northeast-southwest direction (really north-northeast, south-southwest) from the Nebraska line near Bern, to northern Butler County. Its highest elevation is at the north, where its top is about 600 feet below the surface, and its maximum height above the surrounding crystalline rock floor probably is 2,500 feet or more. The age of the granite is conjectured to be pre-Cambrian, and to have been uplifted in the late Mississippian or early in the Pennsylvanian. These preparatory chapters precede the main topic of the bulletin, the production of oil in Kansas (pp. 194-397). Most of the oil of the state is from the Pennsylvanian system, but the Permian, and perhaps the Mississippian, have yielded some. The production of oil in 1916, the last year for which data are given, was about 8,750,000 barrels, more than twice that of any preceding year. In 1916 more than 3,600 new wells were completed, about ro per cent of them dry. Asmall but clear geological map of the state accompanies the volume, also a map showing the distribution of oil and gas. oo REVIEWS The volume bears no date on title-page, or elsewhere where a date is naturally looked for, though the date 1917 appears under the state printer’s name. Its publication appears to have been delayed, as so many other volumes have been in recent years. R.. Dass Petroleum and Natural Gas in Indiana. By W. M. Loeay, State Geologist. Fort Wayne: The Department of Conservation, Division of Geology, 1920. Pp. 279. Like the preceding, this volume appropriately discusses the general fundamental questions concerning the origin and accumulation of oil and gas, and methods of finding it (pp. 10-48). A summary of the stratigraphy of the state (pp. 50-62) is followed by reports on the several counties. A map showing the oil and gas areas of the state accompanies the report. R. Ds: The Sand and Gravel Resources of Missouri. By C. L. DAKE. Rolla: Missouri Bureau of Geology and Mines. Vol. XV, ad ser. (1918). Pp. xii+274, 17 plates. A useful volume, dealing not only with the geological phases of the subject, but with the industrial phases as well. It is not restricted to surface sands and gravels, but includes available materials of these types in formations from the Cambrian up. Incidentally the volume presents a brief, up-to-date summary of the stratigraphic succession of the state, which is welcome and useful. The volume should be of value to those engaged in most sorts of construction work, both now and in the future, as well as to geologists. Re Des: The Physical Features of Anne Arundel County. By Homer P. LirTLeE and OTHERS. Baltimore: Maryland Geological Sur- WN, UONGs Jeo), Baa, OS, ©. This county report covers the physiography, geology, mineral resources, soils, climate, magnetism, and forests. The county lies in the coastal plain, and formations older than the Cretaceous therefore are wanting. One of the striking features of the geology of the region is the large number of unconformities in the Coastal Plain series. There REVIEWS QI are, for example, seven Cretaceous formations, each bounded above and below by an unconformity. Much the same may be said of the later formations. The Cretaceous strata of the region have a total thickness of 720 feet, the Eocene, 160 feet, the Miocene, 100 feet, the Pliocene (?), 40 feet, and the Pleistocene about 100 feet. R. D.S. Onaping Map-Area. By W. H. Coxiins. Ottawa: Canadian Geological Survey, Memoir 95, 1917. Pp. viiit157, pls. 11, figs. 8, map. A very concise report on the geology of an area of approximately 3,500 square miles the center of which is 50 miles north of Sudbury. The area lies within the southern part of the pre-cambrian shield and its topography is that of a hummocky plateau 875 to 1,450 feet above the sea. The most important physiographic features antedate glacia- tion. ‘The two intersecting series of parallel lake basins, in the south- west quarter of the area, probably follow faults. The solid rocks, all pre-Cambrian, are separable by a great uncon- formity into a pre-Huronian group and a Huronian group. The pre- Huronian consists of a schist-complex and intrusive granite-gneisses. The schist-complex consists of volcanics and subordinately of water- deposited tuffs, iron-formation, and other sediments. ‘The structure of this schist-complex, wherever determinable, is that of low anticlinoria and synclinoria. Dynamic metamorphism has converted the original volcanics and sediments into chlorite and sericite or paragonite schists. Near the granite-gneiss batholiths the effects of contact metamorphism are very marked. This schist-complex represents a period of extensive vulcanism and the formation of shallow-water or land deposits. The intrusive granite-gneiss series is dominantly granodiorites with which are associated a great variety of amphibolites, diorites, aplites, pegma- tites, and other types. ‘The diversity of types is explained by primary differences in the intruding magma, magmatic differentiation, and large- scale magmatic assimilation of older rocks. Crenulated interlocking contacts of larger mineral individuals with irregular shape and orienta- tion are textural features very characteristic of these assimilated prod- ucts. Good photomicrographs are shown to illustrate these features. In the future these criteria may prove of great assistance in determining this obscure type of metamorphic rocks. The Huronian rocks constitute the Cobalt series, divisible into two parts. The lower part (Gowganda formation, o-3,000 feet thick) 92 REVIEWS is composed of conglomerates, greywackes characterized by incomplete weathering and imperfect sorting, and a few beds of limestone. By many geologists familiar with this general region this formation in part at least is thought to be of Glacial origin. The upper part of the series con- sists of quartzites (chiefly Lorrain quartzite). As compared with the pre- Huronian, the Cobalt series is little metamorphosed or folded. In most places the Gowganda formation grades up into the Lorrain quartzite, but at some localities there is evidence of an erosion unconformity between the two. This local unconformity may be the result of overlap and probably does not represent a great time-gap. Both the pre-Huronian and Huronian are intruded by dikes and sills, probably of Keweenawan age. Many different rock-types ranging from norites to aplites are represented and here again the field evidence and relationships make it clear that this diverse petrological variety is due in some cases to original differences in the composition of the magma, in others to assimilation of country rocks, or to magmatic differentiation. Calcite and the association of quartz, chalcopyrite, and silver-cobalt- nickel minerals, which constitute the silver-cobalt veins of the area, are believed to be among the subsidiary differentiates. Primary calcite is found sparingly in the diabase dikes and abundantly in the aplite dikes. In two cases the aplite dikes merge into calcite veins. These sills and dikes and all older rocks of the region are cut by porphyritic olivine diabase dikes. The numerous gold-quartz veins near West Shiningtree Lake are irregularly mineralized and the gold content'is low. In this general region the post-Cobalt diabases have gold-bearing quartz veins associated with them. ‘The gravels along the Vermillion river have been worked for placer gold, but they are rather lean. Small silver-cobalt veins occur at Gowganda. The future commercial importance of the several iron ranges of the area is very doubtful. JS We Contributions to the Mineralogy of Black Lake Area, Quebec. By EUGENE PoItTEvVIN and R.P.D. GRAHAM. Canadian Geological Survey, Mus. Bull) No, 27, 1915. Pp. 103, pls. c2;aneseaas A detailed study of the minerals of the chromite and asbestos pits in the Black Lake area, Megantic County, Quebec. This is a very productive area, in the serpentine belt of the eastern townships. The country rocks consist of a complex of igneous rocks, ranging from the most REVIEWS 93 basic to the most acidic in composition, and from late Cambrian to pre-Devonian in age. These igneous rocks probably take the form of thick laccoliths, and the different rock varieties are arranged in the order of decreasing basicity. In many cases erosion has removed the acidic members of the series. Serpentine itself is the least abundant rock of the area, but the most important economically. Thirty-four mineral species are described from the area. In many cases their origin is given, especially the alumino-silicates rich in lime such as diopside, vesuvianite, and grossularite, which occur as dikes in the peridotite and are not the products of contact metamorphism. The CaO content for these minerals is thought to have been extracted by magmatic waters from the already consolidated portions of the igneous mass. Microscopic diamond crystals were found in the chro- mite, which is further evidence of the primary origin of chromite. Eleven new forms of diopside are recorded, with a number of illustrative drawings. Colerainite, H;Mg,A1SiOs, is a new mineral species found in Coleraine Township, and its physical properties are described in detail with a number of chemical analyses. The mode of origin of the various varieties of serpentine is described with chemical analyses. Good views of the pits and microphotographs are given. J. F. W. Report on Braxton and Clay Counties. By Ray V. HENNER. West Virginia Geological Survey, 1917. Pp. 883, pls. 29, figs. 16. A report on the mineral resources of the area with a discussion of its general geology. Aside from soils the principal wealth of the two counties is in the oil and gas pools, building-stone, and clay and shale for brick. The report is accompanied by topographic and geo- logic maps. Part I considers briefly the physiography and history of the develop- ment of the region. The counties are in the central part of the state, on the eastern flank of the Appalachian geosyncline. Their present topography is that of a deeply dissected plateau. Part II is an account of the general geology. The structure is simple, consisting of a gentle dip to the northwest, interrupted by gentle folds. The stratigraphic range is from the upper Devonian through the Paleozoic. Some Pleistocene river terrace deposits are present. A detailed description accompanied by sections is given for each formation present. 04 REVIEWS Part III discusses the mineral resources, the chief of which are oil and gas. Their development is of recent date. But few wells have been driven into the Chemung, and none below it, the present, known producing horizons being limited to the Pennsylvanian and Mississip- pian. Coal-mining operations, while on a large scale, are insignificant when compared to those of other counties of the state. The author esti- mates that the total available tonnage that may eventually be recovered is about 4,440,000,000. While there is not a single brick or pottery plant utilizing clays within the counties, there is an almost inexhaustible supply of raw materials as well as cheap fuel. Sandstone for road maca- dam and building purposes is abundant. In Clay County about half of the land is unfit for agricultural purposes, and it is suggested that this land be reforested. Part IV consists of several paleontological contributions. W. A. Pierce presents some notes on the fossils of the Winefrede limestone and Uffington shale in which he notes the absence of a marine fauna. Pro- fessor E. C. Case describes the leg bone of a pareiasaurus-like reptile found in the Conemaugh series. I. C. White gives a few notes on the Conemaugh and Permian of the region, and comes to the conclusion that “not only the reptilian life, but also the plant and insect life of the Conemaugh series supports the conclusion that the beginning of red sediments in the Conemaugh marks the dawn of Permian time while there is nothing in the marine life of the epoch to contradict the same when properly interpreted. Attached to the report is an appendix giving the elevations above mean tide for the area. A. C. McF. The Mackenzie River Basin. By CHARLES CAMSELL and WYATT Matcoitm. Canadian Geological Survey, Memoir 108, 1919. Pp: 154, pls) 24, fer and map: This is a compilation of what is known concerning the geology of the Mackenzie River basin, which is about 1,350 miles long and too miles wide at the mouth of the river and 900 miles wide near the center, with a total area of about 682,000 square miles. Parts of three chief physiographic provinces are included in this area and each one runs almost the whole length of the basin. They are the Laurentian Plateau on the east, the Great Central Plain of North America in the center, and the Cordilleran region on the west. REVIEWS 95 The outstanding characteristics of each of these three provinces is given along with a description of the Mackenzie River, including its lakes and larger branches. Early pre-Cambrian rocks outcrop in the eastern part of the basin and consist of various schists, slates, limestones, and quartzites intruded by granites and gneisses. These are overlaid unconformably by sand- stones, limestones, and basic flows and intrusives of late pre-Cambrian age. The Paleozoic is represented by a series of limestones, shales, and sandstones, not subdivided and of unknown age. A series of limestones and shales is classed as the Devonian, but the basal part is Upper Silurian in age, according to fossil evidence. Beds of gypsum are interbedded with these basal limestones, and the strata above the gypsum beds are fractured and folded, which is thought to be the result of expansion due to the alteration of beds of anhydrite to gypsum. The Mesezoic is represented by the Cretaceous sandstones and shales which occupy nearly the whole of the valleys of Athabasca and Peace rivers. Traced northward from Peace River these formations show three changes: a decrease in thickness, replacement of sandstone by shale, and a substitution of subaerial for marine conditions of deposition. In the sandstones in the basin of Athabasca River there are a number of workable seams of coal as well as extensive deposits of bituminous sands. A few small areas of Tertiary sands and clays overlie the Cre- taceous with slight unconformity. lLignite seams occur in these beds. Only the highest parts of the Rocky and Mackenzie mountains escaped Pleistocene glaciation. The ice from the Keewatin Glacier, which moved north, west, and south, entered this area, as well as ice from the mountains to the west. Glacial and lacustrine deposits are very extensive. Descriptions are given of the bituminuous sands, with a discussion of their possible utilization, also notes on the various coal horizons, gypsum beds, salt springs, and clays of the area. Cobalt, gold, hematite, lead, zinc, and nickel are known to exist but very little is known as to their extent. A gas-bearing horizon in the Cretaceous has been known for twenty years, also petroleum from borings on the Peace River. During the last two years, active prospecting for gas and oil has been carried on with favorable results. This is a valuable compilation and contribution to the geology of this little-known region. In the future it will serve as the starting- point for geologists working there. A good bibliography is given. To 285 Ae 96 REVIEWS Report on Berkeley, Morgan and Jefferson Counties. By G. P. GRIMSLEY. West Virginia Geological Survey, 1916. Pp. 644, pls. 37, figs. 20. This report covers an area of 768 square miles, comprising what is known as the “‘Eastern Panhandle,” and is accompanied by topographic and geologic maps. Part I is concerned with the physiography, climate and industrial development of the area. Part II deals with the general geology. The structure is that typically developed in the Appalachians, consisting of parallel folds and faults. The deformation is not intense. Stratigraphically the rocks range from the Algonkian through the Carboniferous. Detailed and generalized sections, with local faunal lists, are given. Questions of correlation are discussed, and attention is given to the origin of the Catskill formation which is well developed in the region. Part III is a discussion of the mineral resources, the more important of which include glass-sand, limestone clays, and road materials. There is included with each a discussion of the preparation and uses of the raw material. As a result of the vast deposits of Cambro-Ordovician limestones, the limestone and lime industries are well developed. ‘The Stones River formation contains limestone of great purity, and furnishes a high-grade fluxing material and a high-grade lime. An abundant supply of clay and shale for brick is available, the Martinsburg shale being of impor- tance. Sandstone, quartzite, and limestone for road metal are found in abundance. Iron ore is of negligible importance but the region is admirably situated with respect to transportation, coal production and limestone fluxes, for the steel industry. Attached to the report are two appendixes: (a) levels above the tide in the Eastern Panhandle region; (6) location of true meridian lines in the Eastern Panhandle region. A. C. McF. . qualifications briefly. PAST AND PRESENT By SAMUEL WENDELL WILLISTON ona Professor of Paleontology in the Umirersity of Chicago ( ak and gaetodtiae geographical the best-known feature of the Illi- has been prepared for the benefit ho are interested in knowing more : Tegion | ce can be Elo By 1 in Peds Williston, who is widely known: as a student, of extinct reptiles and as'the author of | American Permian Vertebrates, which has now beeome a standard work, presents in this new volumea summary, divested as far as possible of as aie aid its ce funneeade has f eS ou as as beautiful knowledge concerning the reptiles of the seas, lakes, and rivers of past and present times. ~ _ The numerous illustrations, in large part from the pen or brush. of the author himself, include not only living types and twenty-four restorations * of extinct forms, but also many figures elucidat- ing the structures and habits of the animals. art and the UE settle- nd development of the region; Part the geology of the park; and Part Til, botany. 130 pages, cloth; $2. 00, age $2.15 | viti--ase pages, royal 8v0, cloth; $3.00, postpaid $3.20 The University of Chicago Press ae - rt - Illinois oe Ciniversity Press ‘POSITIONS OF ALL KINDS i) Never was the demand so great for qualified teachers and 1a “specialists. For ten years we have given our time and |} | energy to this work. Write for our free literature. State Co-operative Instructors’ Association ii Marion - - Indiana “ossil Plants. A Text-Book for Students of Botany and Geology. By A. C. ie SEWARD, M.A.,F.R.S. In4gvols. Demy 8vo. Vol. I. With 112 illustrations. $3.60. Vol. II. With 266 Btcreitions: $5.60. Vol. III. With 253 illustrations. $6.50. Vol. IV. With 390 illustrations. $6.50. >alaeontology—Invertebrate. By Henry Woops, M.A., F.G.S. Crown 8vo. Fifth ie edition. _ Revised and enlarged, with 173 illustrations. $3.75. Petrology for Students: an Introduction to the Study of Rocks under the ~ oe Microscope. By Atrrep Harker, M. A., F.R.S., F.G.S. Fifth edition, revised. Crown 3 8vo. $2.75. Treatise on ee eae By W. J. Lewis, M.A. Demy 8vo. $4.50. : ee: 8vo. 2 .00. cto to e eey By J. E. Marr, ScD, F.RS., F.G.S. Crown 8vo. $r.30. "The MacMillan ee Meonc: in the United States 64 and 66 Fifth Avenue, New York City | WATER REPTILES OF THE | unnecessary scientific details, of our present \ INE INKS anv ADHESIVES | Wha For those who KNOW a j ven of EO PEERSE onde Sami eae or baju Deas suptemag =. us ae is true that the se Ww Drawiaeinte | the very latest and best i _ Eternal Writing Ink eric a writing must go to the R o : ) Taurine. Mucilage —}}, It is true that the world $ record 1 Hl ag St ns Deeien tea haces actual gross speed in typing was ma vashey Prt ight j Liquid Paste. =: - on a Remington. Office Paste .... : Vegetable Glue,. Etc. But underlying all of the oc tee Finest snd Best Inks and Adhesives Remington facts is the uni Emancipate yourself from the use of corrosive ‘and —the surpassing strength whicl ill-smelling inks and adhesives. and adopt the “Higgins Inks and Adhesives. They will bé a every Remington to . reliable ‘revelation ‘to you, they are so sweet, ye well “put up, and withal so efficient. f ice At Dealers Generally y are 0 constant use. Mee | a Remington—it laste aN CHAS. . M. HIGGINS & CO., Mfrs. Branches: Chicago, London (Incorporate a). 68 (271 Ninth Street ee een i N. Vi 374 Becadway, ‘THE ORIGIN OF THE EARTH By THOMAS C. CHAMBERLIN Head of the penere ent of Geology, The University on ieee). ee of “The Univer of Chicago Barenve Series” ; xii +272 pages, fue; cloth; $1. 75 ae postpaid $1 90 ws SOS FROM. THE PREFACE E saa ‘“Tn telling the story of this search for the mode by which the Barri came - into being, we “have let the incidents that led the i inquiry on from one stage to another fall in with the steps. of the inquiry. itself.. It is in keeping with the purposes of this series of booklets that the motives which set researches going — should have their place with the quests that arose from them. . ... . The final — story of the birth of the earth will come only after a time, when the vestiges of — creation have been more keenly discerned and more faithfully rendered than i 19/4 i peeable now.’ Py THE UNIVERSITY OF ‘CHICAGO PRESS” CHICAGO - ! es “NUMBER 2 With the Active Collaboration of ALBERT JOHANNSEN, Petrology ROLLIN T. CHAMBERLIN, Dynamic Geology _ ASSOCIATE EDITORS Tarr JOHN C. BRANNER, Leland Stanford Junior University RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. WILLIAM H. HOBBS, University of Michigan FRANK D. ADAMS, McGill University CHARLES K. LEITH, University oi Wisconsin WALLACE W. ATWOOD, Clark Univeisiiv ~ WILLIAM H. EMMONS, University of Minnesota ARTHUR L. DAY, Carnegie Institution” ea ic gr le aoe SCE - Cartes DAVISON 07 ‘AND FAUNAL RELATIONSHIPS | OF THE MEGANOS GROUP, OF ee aa a ia - - - = s - Bruce L. CLARK 125 ~~ 166 | 173 188 UNIVERSITY OF CHICAGO PRESS _ CHICAGO, eine U.S. A. g THE CAMBRIDGE UNIVERSITY PRESS, Lonpon 7EN-KABUSHIKL-KAISHA, Tokyo, OSAKA, Kvoro, FUKUOKA, Sn THE — BOOK COMPANY, SHANGHAI : Paks 2 ‘ Belge ts + vt Me ages h q 7 rah: eo mh f Retna: Mar 1921. f EDITED BY THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY With the Active Collaboration of STUART WELLER : ALBERT JOHANNSEN ~ 5 Invertebrate Paleonteloey : Petrology EDSON S. BASTIN ROLLIN T. CHAMBERLIN % =f Economic Geology : Dynamic Geology ea The Journal of Geology is : published hy the win dieity of Chicago at the University of chaser P: 5750 Ellis Avenue, Chicago, Illinois, semi-quarterly, on or about the following dates: February 1, Mar May 1, June 15, August 1, September 15, November 1, December 15. {| The subscription price is per year; the price of ae copies is 65 cents. 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While the cost of printing remains as now, the Journal cannot supply reprints without charge, but reprints will be furnished authors at cost, if ordered in pee Ol publication. a ‘3 Entered as second-class matter, March 20, 1893, at the Post-office at Chicago, Ill., under the Act of March 3, 1879. r ‘ ay Acceptance for mailing at special rate of postage provided for in Section 1103, Act of October ‘ae x917, authorized © July 5, 1918. ; By Sa % Among articles to appear in early numbers of the Journal of Geology are the following: The Physical Chemistry of the Crystallization and Magmatic Differ- entiation of Igneous Rocks. By J. H. L. Voer. Types of Rocky Mountain Structure in Southeastern Idaho. By GEORGE ROGERS MANSFIELD. Cycles of Erosion in the Piedmont Province of Pennsylvania. By F. Bascom. Russell Fork Fault of Southwest Virginia. By CHESTER K. . WENTWORTH. Discussion of ‘‘Summaries of Pre-Cambrian Literature of North America,’ by Edward Steidtmann. By T. T. QUIRKE. 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Complete list with prices will be sent upon request The University of Chicago Press 5750 Ellis Avenue Chicago, Illinois VOLUME XxXIx NUMBER 2 THE WOURNAL OF GEOLOGY FEBRUARY-MARCH 1921 VOLCANIC EARTHQUAKES CHARLES DAVISON Birmingham, England Volcanic earthquakes, according to the late Professor Mercalli, are those which have their centers of maximum intensity under or close to the cones of active or semi-extinct volcanoes.* Pro- fessor Omori somewhat enlarges this definition. “A volcanic earthquake,” he says, ““may be defined as a seismic disturbance, which is due to the direct action of the volcanic force, or one whose origin lies under, or in the immediate vicinity of, a volcano, whether active, dormant or extinct.”? Of the two definitions, the latter is the wider in its scope, for it includes the earthquakes which visit extinct volcanoes, such as the Alban Hills near Rome—earthquakes which differ in no important particular from those of an active volcano like Etna or of a dormant volcano such as M. Epomeo in the island of Ischia. Mercalli follows up his definition of volcanic earthquakes by describing their important properties. The earthquakes, he says, (rt) are felt many times in an area which is very restricted, although the shocks are violent; (2) they precede slightly, but sometimes accompany or follow, the eruptions of the neighboring volcano; and (3) they are repeatedly felt in the same area with similar characters *G. Mercalli, Vulcani e Fenomeni Vulcanici in Italia (1883), p. 355. 2F. Omori, Bull. Imp. Earthquake Inv. Com., Vol. VI (1912), p. 8. 97 98 © CHARLES DAVISON so long as the volcano maintains its activity without notable change. Tectonic earthquakes differ in the following respects from volcanic earthquakes: (1) for the same intensity at the epicenter, they are felt over a much wider area; (2) though they sometimes occur in the immediate neighborhood of active volcanoes, they are as a rule quite independent of volcanic eruptions; and (3) great tectonic earthquakes seldom revisit the same district except at wide intervals of time. It will be seen later that these are not the only important differences between volcanic and tectonic earthquakes. For the present, however, it may be inferred that volcanic earthquakes as a rule originate in shallow foci which are confined to a limited region, while tectonic earthquakes spread from deeply seated foci that are subject to continual change. Professor Omori notices that, under his definition, tectonic earthquakes may occasionally be included—earthquakes which disturb large areas’ and are recorded by seismographs at very distant stations. For instance, about thirty hours before the | eruption of the Usu-san (north Japan) on July 25, 1910, an earth- quake occurred that was felt to a distance of 87 miles from the volcano and was recorded at a distance of 475 miles.2 The begin- ning of the great eruption of the Sakura-jima (south Japan) on January 12, 1914, was followed after a few hours by an earth- quake that damaged houses in Kagoshima and the surrounding country and was recorded in European observatories and probably in all parts of the world. Moreover, one month later, on February 13, another strong earthquake took place during an eruption of the Iwo-jima, a volcano belonging to the same chain as the Sakura- jima. To these examples may be added the destructive Hawaiian earthquake of April 2, 1868, which originated in or near the southern part of Hawaii at nearly the same time as great eruptions of The occurrence of such earthquakes has long been recognized. For instance, G. P. Scrope, in his Considerations on Volcanos (1825), states that ‘those shocks .. . . which are felt to a considerable distance, are probably caused by new rents produced in the solid subjacent strata supporting or surrounding the mountain, and enter into that class of earthquakes which were discussed in a former chapter,” that is, of tectonic earthquakes (p. 155). 2F. Omori, Bull. Imp. Earthquake Inv. Com., Vol. V (1911), p. 15; Vol. VI (1912), p. 9. 3 Ibid., Vol. VIII (1914), p. 23; see also Nature, Vol. XCII (1914), pp. 716-17. VOLCANIC EARTHQUAKES 99 Kilauea and Mauna Loa, and was certainly felt in the island of Kauai, about 350 miles from the epicenter, or probably over an area of 375,000 square miles.’ e/ /® Kirishima / if / Be Sakura-jima Scale of Miles y Suwanose Fic. 1.—Map of volcanic chain in south Japan * It is of some consequence to decide whether such earthquakes should be regarded, as they would be under Professor Omori’s definition, as volcanic earthquakes. The course of the volcanic « See an admirable paper by H. O. Wood, ‘On the Earthquakes of 1868 in Hawaii,” Bull. Seis. Soc. of America, Vol. IV (1914), pp. 169-203. 100 CHARLES DAVISON chain of southern Japan referred to in the last paragraph is shown in the accompanying sketch map (Fig. 1), and it is interesting to notice the progressive awakening from north to south of the volcanic foci along its course. On November 18, 1913, the Kirishima-yama broke out in strong eruption, which lasted into the following year. The Sakura-jima followed on January 12, 1914; and, about a month later, the Iwo-jima. When three volcanoes, situated as these are and all of infrequent activity, break into eruption so nearly together, and when two of the eruptions are accompanied by strong and deeply seated earth- quakes, it would seem natural to ascribe both phenomena to a common cause—the earthquakes directly, and the eruptions indirectly—to the stress accumulation along the whole volcanic chain." It seems to me, then, that tectonic earthquakes which originate in the immediate neighborhood of volcanoes and even concur- rently with eruptions of the same, are not directly of volcanic origin, and should not, by reason of such proximity, be regarded as volcanic earthquakes. I venture, therefore, to suggest that the definitions of such earthquakes given by the two distinguished seismologists referred to should be somewhat modified and should be replaced by the following: A volcanic earthquake is an earth- quake directly due to the operations which result or tend to result in a volcanic eruption or is due to relative movements, by whatever cause they may be produced, along fractures of the volcanic mass, whether the volcano itself is active, dormant, or extinct. Adopting this as the definition of a volcanic earthquake, I - propose in the first section of this paper to describe the earth- quakes connected with a few typical volcanoes, namely, certain Japanese volcanoes (the Usu-san, the Asama-yama, and the Sakura-jima) and Etna, as examples of active volcanoes, and M. Epomeo in Ischia and the Alban Hills near Rome as examples of dormant and extinct volcanoes respectively. In the second section is given a summary of the characteristic phenomena of volcanic earthquakes; and in the third and last section the modes of origin of volcanic earthquakes will be considered. «F, Omori, Bull. Imp. Earthquake Inv. Com., Vol. VII (1914), pp. 23-24. VOLCANIC EARTHQUAKES IOI I. DESCRIPTION OF SOME VOLCANIC EARTHQUAKES I. JAPANESE VOLCANOES Usu-san and Sakurayima.—The Usu-san is situated near the southwest end of Hokkaido, the northern island of Japan, and the Sakura-jima in the Bay of Kagoshima on the south coast of Kyushu, the southern island. The last eruption of the Usu-san began at Io P.M. on July 25, 1910, and that of the Sakura-jima at 10 A.M. on January 12, 1914. Both eruptions were preceded by a large number of earth- quakes. Those of the Usu-san outburst began on July 21. At Nishi-Monbets, about 5 miles from the center of the volcano, 25 shocks were felt on July 22, 1100n July 23, 351 on July 24, and 165 on July 25. Once the eruption had begun, the seismic fre- quency decreased. At Sapporo, about 44 miles from the volcano, the earthquakes were registered by a horizontal pendulum seis- mograph, the numbers being 1 (at 4:18 P.M.) on July 21, 3 on July 22, 23 on July 23, 76 on July 24, 84 on July 25; after the eruption there was a rapid decline in frequency, the numbers being 26 on July 26, 15 on July 27, 5 on July 28, 6 on July 29, and 1 on July 30. The eruption continued until the end of the year." At Kagoshima, which is about 6 miles from the center of the Sakura-jima, the first earthquake was felt at 3:41 A.M. on January II, 1914. At the Kagoshima Observatory the earthquakes were recorded by a Gray-Milne seismograph, the average hourly fre- quency being 4.1 from 3 to 11 A.M. on January 11, 12.4 from 11 AM. to 8 P.M., and 19.5 from 8 P.M. on January 11 to Io A.M. on January 12. The greatest hourly numbers were 28 at 8-9 P.M. on January 11 and 27 at 3-4 A.M. on January12. The total number of shocks registered from 3 A.M. on January 11 to 10 A.M. on January 12 was 418. After the first eruption at the last-mentioned hour, there was a marked decline in earthquake frequency, the mummers bempy: 1TO-1r A.M, £7; If noon, 11; NOOn-1-P.M., 6; I=2 DIM, Be QR WAL Se BoM IN, AR eS eM, IE Oey Zp At 6:30 P.M., the seismograph was injured by the strong tectonic earthquake referred to above.’ t Tbid., Vol. V (1911), pp. 8-17. 2 [bid., Vol. VIII (1914), pp. 9-14, 22-27. 102 CHARLES DAVISON Thus, both eruptions were preceded by a marked increase in seismic frequency, followed by a marked decrease, the maxima occurring 24 hours before the first outburst of the Usu-san and about 13 and 6 hours before that of the Sakura-jima.* Observations were made with a portable horizontal tromometer erected by Professor Omori at Nishi-Monbets (5 miles from the Usu-san) from July 30 to August 6, and at the West-Kohan School (Sobets) at the foot of the East Maru-yama from August 6 to Io. At this place, which is close to the nearest craterlet, series of well-defined, small, quick, unfelt vibrations, called micro-tremors by Professor Omori, were registered, which were entirely absent from the records at Nishi-Monbets. The mean range of motion was in every case less than one-tenth of a millimeter; but the principal periods of the tremors (.53, 1.08, 1.59, 2.14 seconds) were prac- tically identical with those of earthquake vibrations recorded at Nishi-Monbets (.53, 1.01, 1.58, 2.43 seconds). It would seem, then, that the micro-tremors are in reality true earthquake vibrations, but so weak that they cannot be recorded more than a few miles from the origin. Professor Omori notices that the shortest of the foregoing periods is approximately one-half, one- third, and one-quarter of the other periods. He also shows that moderate explosions from even the nearest craterlet were not as a rule accompanied by marked micro-tremors; whereas violent explosions from that and other craterlets were usually accompanied, and often preceded by several minutes, by well-pronounced micro-tremors. ‘The tremors, however, were not confined to the epochs of explosions. They sometimes occurred when the smoke ejections from the different craterlets were insignificant and even when they had completely ceased. At such times, as Professor Omori suggests, eruptions were perhaps prevented by the tem- porary stoppage of the craterlets.’ Asama-yama.—The Asama-yama, one of the greatest of Japa- nese volcanoes, rises from the plateau of the central island of 1 Professor Omori’s observations give precision to a fact which has long been known. “It is,” says G. P. Scrope in his Considerations on Volcanos (1825), “‘a remark common to the observations made on almost all volcanic eruptions, that local earthquakes always precede the emission of lava currents, and cease while the lava is flowing, to recommence when it has stopped” (p. 155). 2 Op. cit., Vol. V (1911), pp. 31-38. VOLCANIC EARTHQUAKES 103 Japan to a height of 8,140 feet above the sea. After a prolonged period of rest, it has been subject for about six years to a series of strong explosions, the first of which occurred on February 13, 1908. Observations with horizontal pendulum seismographs were instituted by Professor Omori at two stations on the southwest flank of the volcano, Yuno-taira and Ashino-taira, at heights of 6,306 and 4,422 feet above the sea. Owing to weather conditions, the observations at the former station were confined to the summer months. The seismograms obtained at these places showed that there were two distinct types of earthquakes, some being independent of any outburst of the volcano, while others were invariably the results of explosions. The two types of earthquakes are characterized by several marked differences, of which the following are the more important: a) The shocks without explosions consisted only of minute quick vibrations, while those with explosions consisted of slow movements (of as much as 2.6 and 5.3 seconds’ period), on which after a few seconds quick vibrations were superposed. b) The earthquakes without explosions were distinctly stronger than the others, probably because, as Professor Omori suggests, a great part of the energy of the explosions is expended in the projection of rock fragments and débris. Of 1,485 earthquakes without explosions, 21 per cent were sensible at Yuno-taira; of 8,847 earthquakes with explosions, only 0.3 per cent were sensible. Moreover, the strong earthquake of May 26, 1908, which did not accompany an explosion and which evidently originated in the volcano itself, was felt over an area of 2,400 square miles.’ c) The earthquakes without explosions were of shorter duration than those with explosions, the averages for the former being 16.7 seconds in 1911 and 15.0 seconds in 1912, and for the latter 33.2 seconds in 1911 and 32.7 seconds in 1912. d) The two types of earthquakes alternate in frequency. During the two years, 1911-12, the maxima of the earthquakes ‘It may be added that the Usu-san explosions of 1910 were preceded by many sensible earthquakes, a few of them strong; but, as a rule, they were unaccompanied by sensible shocks. 104 CHARLES DAVISON without explosions occurred at 3-4 A.M., 8-9 A.M., I-2 P.M., and 8-9 P.M., while the minima of the earthquakes with explosions occurred at 3-4 A.M., 8-9 A.M., 2-3 P.M., and g-10 P.M. The minima of the former occurred at 6-7 A.M., 11-12 A.M., 6-7 P.M., and 11-12 P.M., while the maxima of the latter occurred at 5-6 A.M., NOON-I P.M., 4-5 P.M., and 1-2 A.M. In August, 1911, the earthquakes with eruptions attained their maximum frequency (205) for the year, and the earthquakes without eruptions their minimum frequency (38); in October, 1912, there was a maximum of earthquakes with eruptions (626) and a minimum of earthquakes without eruptions (10). A similar relation is shown in the varia- tion of frequency from one year to another, as will be seen in the following table:* EARTHQUAKES WITHOUT ERUPTIONS EARTHQUAKES WITH ERUPTIONS . Strong Small Sensible Unfelt Total Explosions Ouburee Total TOTES hai tse 57 321 378 ° 577 577 ST ONEA} Bul Reel 124 563 687 ° III IIII IMO a eR 6 28 34 25 7IOL 7126 TODA ects wi clecs II 37 48 I 30 ate LOWS seer aces 44 65 109 ° TOT Okeueweysieies: 64 165 229 ° 2 2 2. ETNA Etna rises from a nearly circular base, measuring 428 square miles, to a height of 10,870 feet. Earthquakes occur on all sides of the mountain, and, for many years past, they have been espe- cially frequent beneath its southeastern flank. In the present section, I propose to describe a few typical earthquakes on this flank, and to refer very briefly to the distribution of the epicenters within the area covered by the volcano. The earthquakes belong as a rule to one of two classes, according as the greater axes of their meizoseismal areas are directed nearly along or perpendicular to the radu from the central crater. They may therefore be called radial and perimetric earthquakes, respec- tively. t After May 5, 1914, there was no strong outburst of the Asama-yama. VOLCANIC EARTHQUAKES 105 As examples of radial earthquakes on the southeastern flank, I have selected three which have been studied in perhaps greater detail than others, namely, the Fondo Macchia earthquake of July 19, 1865, the Fleri earthquake of August 8, 1894, and the Linera earthquake of May 8, 1914. The Fondo Macchia earthquake of October 15, 1911, is given as an example of a perimetric ~ earthquake. Radial earthquakes.—On Baenuany 20, 1805, a great eruption took place on the east-northeast flank of Etna, which lasted nearly twelve weeks. Eighty-eight days after its close, on July 10, Fondo Macchia was com- pletely destroyed by an earthquake, and the neigh- boring villages of Baglio, Rondinello, Scaronazzi, and S. Venerina were seriously damaged, though not ruined. The meizoseismal area was a narrow band directed W.NW. andr ESE. a little over 4 miles in length and about 2 Eieieivide. Outside this | its band, the shock was disas- Fic. 2.—Map of earthquakes on southeast trous within an area 5 miles flank of Etna. long and 14 miles wide, represented by the curve A on the sketch map (Fig. 2). The intensity of the shock diminished rapidly toward the north, and more slowly toward the south, but at no place more than 12 miles from the epicenter was the shock felt. The area dis- turbed cannot therefore have exceeded 113, and was probably less than 100, square miles. A number of after-shocks followed this earthquake, 24 being felt in July (3 of them very strong at : ‘ ° Acireale Scale of Miles 106 CHARLES DAVISON Fondo Macchia), and several others, more or less slight, in the following month, the series ending on August 23." The earthquake of August 8, 1894, differed in two respects from many Etnean earthquakes. It occurred while the volcano was in a state of only moderate activity, and it disturbed an area unusually large for the district, although small for a shock of such intensity. It was preceded by a violent shock at 1:58 P.M. on August 7, strongest at Fleri, Zerbate, etc., and felt generally at Catania and Zafferana, and by a few persons at Nicolosi and Trecastagni. Three other shocks followed in the meizoseismal area before the occurrence of the principal shock of the series at 6:16 A.M. on August 8. By this shock, the villages of Fleri, Pisano, Zerbate, etc., were ruined. The meizoseismal area, about 4 miles long from northwest to southeast and 2 miles wide, is represented by the curve B in Figure 2. From this area, the intensity of the shock diminished outward rather, but not very, rapidly, the dis- turbed area including the whole base of Etna and probably on the whole as much as 800 square miles. Within or near the epicentral district, at least 15 after-shocks were felt before the end of the month.’ Few, if any, Etnean earthquakes have thrown so much light on the nature and origin of volcanic earthquakes as the remarkable series which culminated in the Linera earthquake of May 8, 1914. They have been studied by Professor G. Platania in one of the most valuable memoirs that we possess on volcanic earthquakes.’ In all respects except in the shallowness of the foci, they resemble true tectonic earthquakes. They were preceded and followed by long series of accessory shocks, and, along the axis of the meizo- seismal area, there were displacements of a pre-existing fault. Omitting instrumental shocks, the whole series contained 55 earthquakes, 21 being fore-shocks from April 28 to May 7, and 33 after-shocks from May 8 to June 4. Five of the fore-shocks and =A. Riccd, Boll. Soc. Sism. Ital., Vol. XVI (1912), pp. 27-31; M. Baratta, Boll. Soc. Geogr. Ital. (1894), pp. 12-13, and I Terremoti d’Italia, pp. 442-43. 2M. Baratta, Boll. Soc. Geogr. Ital. (1894), pp. 6-9, 23. 3 “Sul periodo sismico dei maggio 1914 nella regione orientale dell’Etna,” Pubdl. dell’Ist. di Geogr. Fis. e Vulcan. della R. Univ. di Catania, No. 5, 1915. VOLCANIC EARTHQUAKES 107 seven of the after-shocks were strong. The broken line on the map (Fig. 3) includes all the places in which houses were damaged by the different shocks of the series. The smaller curves represent the meizoseismal areas of the more important shocks. Thus, the curves 1 and 2 indicate the meizoseismal areas of the double earth- quake of May 7 at 6:35 P.M., houses being damaged at Piano d’Api (No. 1) and at Pennisi and Fiandaca (No. 2). Seay At Io P.M. on the same day, poo i S © Fondo Macchia another strong earthquake | Se caused damage at Fossalac- GaN / Dagala g ~ qua (No. 3) along a fracture which may be a continuation of that at Fiandaca (No. 2). The principal earthquake occurred at 7:2 P.M. on May 8, and was ruinous within the area bounded by the ellipse (No. 4), the zone of complete oe: ruin being much less wide. Ne © (Vn, With this earthquake also Lat oer Y, ener there was a zone of serious viagrande damage to houses at Dagala Scale of Miles (No. 5), separated from the MERE A aL OUNace i principal zone by a wide tract of little or no damage to Prop- Fic. 3.—Map of Linera earthquakes of 1914 erty. Lastly, the curve No. 6 shows the boundary of slight damage near Viagrande wrought by the after-shock of May 26. The meizoseismal area of the principal earthquake (bounded by the curve C in Fig. 2 and No. 4 in Fig. 3), within which the ruin was practically complete, is very elongated, being about 4; miles long, with a maximum width near Linera of 13 miles. In this zone, not only are the houses completely razed to the ground, but the ground itself is crushed. Along the axis of this zone, there runs a slightly sinuous fracture with, in parts, a secondary nearly parallel 108 CHARLES DAVISON fracture separated by a distance of from 2 to 50 or more yards. The most destructive effects of the earthquake are concentrated along this fracture, starting from Passopomo, through Linera, to beyond Mortara. Here the railway line was seriously damaged, the rails being displaced and contorted. The fracture can be followed as far as the seacoast, bending slightly to the south in the neighborhood of Sta. Tecla. Almost throughout the fractured zone there is a change of level, in some places of only an inch or two in others of 15 or 16 inches, and in one, near the seacoast, of more than 3 feet, the ground of the southwest side being left at a higher level than that on the northwest side.t The fracture is by no means a recent one, for it has been known since 1879, and Professor Platania states that displacements have occurred along it in previous earthquakes. Nor is it the only fracture along which movements have taken place during this series of earthquakes. There is a second at Fiandaca (No. 2), which was observed during the earthquakes of 1894 and 1907, and which probably corresponds with another at Fossalacqua (No. 3). Near this fracture occurred the greatest damage wrought by the two strong fore-shocks of May 7. During the interval covered by this series of earthquakes there was a marked increase in the activity of Etna, though the different shocks were not coincident with the volcanic explosions. Perimeiric earthquakes.—On September 10, 1911, a great eruption of Etna began, and, notwithstanding the extraordinary energy of the early phenomena, ceased after only thirteen days of activity. Three weeks later occurred the destructive Fondo Macchia earth- quake of October 15, t911. ‘This earthquake was, however, pre- ceded by at least 1o fore-shocks, the first 5 of which occurred on September 30 at Pisano (intensity 7, Mercalli scale), October 9 (2 shocks) at Piano d’Api (intensity 5), and October 14 at S. Venerina (intensity 6). The principal shock (intensity 10) occurred at 9:52 A.M. on October 15, and was followed by 4 slight after- shocks on October 24 and 1 on November 9. ‘There is also some evidence of horizontal displacement. A high wall in the upper part of the fractured zone is curved and shifted toward the southeast. VOLCANIC EARTHQUAKES 109 The isoseismal lines of intensities 10, 8, 6, 4, and 2 (Mercalli scale) are shown in Figure 4, the line of intensity 10 being also represented by the curve D in Figure 2. The meizoseismal area (bounded by the iso- seismal 8) is a slightly sinuous band, running from N.NW. to S.SE., 4 miles long, about 3 mile wide, and 1% square miles in area. Within it is a band, in- cluding Fondo Mac- chia, in which the destruction of build- ings was complete, this band being about 3 miles long and } mile wide. Notwithstand- ing the great intensity within this band, the shock was not felt at places 6 miles to the west, the area within the isoseismal 4 con- tained not more than about 70 square miles, and the whole district shaken only about 230 square miles. The mean duration of the shock was about 8 seconds. 1 I | fo} | Linguaglossa I | ! I ! Piedemonte + Riposto © Giarre Zafferana © ; Fleri 0: 6 j Trecastagni’, : i ° o a, 1 aes ° . . 7| Acireale Viagrande Scale of Miles {0} 5 10 Fic. 4.—Map of Fondo Macchia earthquake of October 15, 1911. During the eighteen years (1893-1911) preceding this earth- quake, 27 shocks more or less strong originated within the elliptical IIo CHARLES DAVISON area represented by the outer dotted line in Figure 4. Six of these earthquakes were of ruinous strength, with their epicenters at Fondo Macchi, Zerbate, S. Leonardello, and Aci Platania, all included within the area represented by the smaller dotted ellipse in Figure 4. The two ellipses have their longer axes coinciding with those of the isoseismal lines of the Fondo Macchia earth- quake of 1911. Close to these axes and nearly parallel to them, runs a fault known as the Timpa della Scala and represented by the broken line on the map. The western limb at the south end has undergone, and is still undergoing, elevation; while the eastern limb at the north end is subsiding.* Distribution of Etnean earthquakes.—A few of the Etnean earth- quakes disturb the whole area of the volcano; but the majority are strongly felt in one or a few of the villages scattered over the mountain sides. In such cases the villages affected cannot be far distant from the epicentral areas, and Dr. M. Baratta, in his great history of Italian earthquakes,? has thus found it possible to dis- tinguish the principal seismic zones of this volcanic region. These are twelve in number, the places which give their names to the different zones being shown in Figure 5, in which the dotted line marks out the base of the volcano: (1) Linguaglossa, to the NE.; (2) Randazzo, to the N.; (3) Aderné-Bronte-Maletto, to the W.; (4) Santa Maria di Licodia, to the S.W.; (5) Paterno, to the S.SW.; (6) Belpasso, to the S.; (7) Nicolosi, to the S.; (8) Trecastagni, to the S.SE.; (9) Acireale, to the SE.; (10) Zafferana-Pisano-S. Venerina, to the E.SE.; (11) Macchia Region, to the E.; and (12) Giarre-Riposto, to the E. Of these zones, the most important at present are those of Santa Maria di Licodia, Nicolosi, Trecastagni, Acireale, Zafferana- Pisano-S. Venerina, and the Macchia Region. The Fleri earth- quake of 1894, described above, is included in the Acireale zone, the Linera earthquake of 1914 in the Zafferana zone, and the Fondo Macchia earthquakes of 1865 and 1911 in the Macchia zone. The Nicolosi zone gives rise to many and violent earth- quakes, and it is remarkable that some of the greatest are quite tA, Riccd, Boll. Soc. Sis. Ital., Vol. XIX (1912), pp. 9-38. 2 IT Terremoti d’Italia, pp. 829-33. VOLCANIC EARTHQUAKES III local. For instance, the earthquake of May 11, 1901, damaged many houses in Nicolosi (intensity 8), and yet was only just per- ceptible at a distance of 3 or 4 miles." The earthquake of May 14, 1898, is typical of the S. Maria di Licodia zone; nearly all the Pry a ' | oS . \ he BOSS SSN SET RS ae TAS oa -— \ Suet \ eeO \ MO ae Randazzo \ eh EL Se > 4 ‘ ‘ Ag SO vt \ TNs. ) Sik x Ack i . Pore dik Linguaglossa ee Maletto Oem Pe ee, ~ / ryx\\ . Si= \ Phy SN ies Piedemonte © fo) f Bronte a Le f 4 ‘ : ete i Crater :~ ‘ a Riposto \5 S iM Nilo ° Giarre ! ° i Fondo Macchia ‘ ? Zafferana O 5 1 ! Aderno S.Venerina u ° : © Pisano ‘ Fleri © Ne Biancavilla . ° Ae , ‘¢ Licodia Trecastagni , 2 ° Onn mae, ; SN Nicolosi : ° ol Acireale \ 2 IES, Belpasso Viagrande { \ ! Paterno ° ‘ 0 Mascalucia Ne sy pe im ne U7 ne SS SS \ \ N . Scale of Miles SES ~ NGG mM aahaat Tk = Sel ) \\ 7 saluas ——= MS SONVOAN Y OZNAUOT ‘SATIN JO 9109S XN Z| NvVS XA1dWoo iaeie206N0 ‘aspg 4D NOraL [ULL] presses Twsva = SOUINOVA 152 BRUCE CLARK the basal conglomerate of the Tejon rests on the granite. Just back of the ranch-house, a little farther west, a remnant of the basal Meganos sandstone outcrops below the basal conglomerate. Here the unconformity is evident. Traced east of this locality, the Meganos beds are found to thicken rapidly, reaching their maximum thickness near the head of Pleito Canyon about three miles to the east, where the Meganos beds have an estimated thickness of more than one thousand feet. As shown on the map, these beds thin out rapidly farther east, and in the canyon of Salt Creek, only a little more than four miles distant, their thickness probably is not more than one hundred feet. The conglomerate of the basal _ Tejon was traced to Tecuya Creek in the next large canyon east of Salt Creek. In Grape Vine Canyon the basal conglomerate of the Tejon was found to be separated from the granite by about twenty-five feet of unfossiliferous, coarse arkosic sandstone, together with a few feet of dark shales. Thus the beveled Meganos is transgressed by the Tejon from west to east in the vicinity of Grape Vine Canyon, only a very small part of the Meganos being left, and in the next canyon to the east of Live Oak Canyon the Meganos beds fail to appear. Lithology—The Meganos outcrops are best exposed between Pleito and San Emigdeo canyons. The basal beds consist of several hundred feet of fairly indurated, coarse, reddish-gray arkosic sandstone. The upper part of the section is composed principally of sandy shales and platy shaly sandstone. The Tejon beds of this region have a thickness of a little more than two thousand feet. The thickness in the vicinity of Grape Vine Canyon was estimated to be about twenty-four hundred feet. In the vicinity of this canyon the beds consist principally of medium-fine buff-colored sandstone, with lenticular harder, calcareous fossiliferous layers. This section, as already described by Anderson and Dickerson, is very uniform in lithology. To the west these beds become finer, and in the vicinity of San Emigdeo Canyon the larger part of the section might be described as a mudstone. In places lenses of conglomerate are found with the finer sediments, and at one horizon not very far from the base is a heavy layer of conglomerate that can be traced FAUNAL RELATIONSHIPS OF THE MEGANOS GROUP 153 for a considerable distance. It would appear that the Tejon beds in this last general locality may be delta deposits rather than typical marine deposits, such as those to the east in the vicinity of Grape Vine Canyon. This is borne out by the paucity of the fauna as well as by the lithology. Fauna of the type Tejon.—The faunas obtained from different horizons in the type section of the Tejon, as found in Grape Vine Canyon, were studied by Dickerson. The invertebrate species were listed and.a number of new species described by him.t Dick- erson’s conclusion, with which the writer agrees, was that the fauna obtained from the various horizons in the type Tejon, taken as a whole, isa unit. It has already been pointed out that Dicker- son believed that these beds were somewhat younger than the Turritella andersoni beds at Coalinga or his lower Tejon from the south side of Mount Diablo, which beds of both localities are referred by the writer to the Meganos. In discussing this fauna, he says? Beds about three hundred feet above the base (University of California locality 458) yielded an excellent fauna. This fauna, however, does not differ ~ essentially from that of the beds higher in the section. The faunas from several other localities which are listed below do not differ materially from one another, but appear to represent one phase only. This faunal unity is in consonance with the sedimentary record as Anderson described it... .. The writer is in complete agreement with Anderson’s view as expressed here in relation to the type Tejon. However, beds both higher and lower than the Eocene of Canada de las Uvas occur in other parts of the state, notably in the vicinity of Mount Diablo, along Cantua Creek, Coalinga Quadrangle, and at the Marysville Buttes. As quoted in the paragraph above, Dickerson recognized that the fauna of the type Tejon was higher than that from the Lower Eocene beds on the south side of Mount Diablo, and higher than his so-called lower Tejon at Coalinga, the Turritella andersoni beds, which latter beds are here referred to the Meganos Group. He correlated the fauna of the type Tejon with that of his Rimella tR. E. Dickerson, ‘“‘Fauna of the Type Tejon; Its Relation to the Cowlitz Phase of the Tejon Group of Washington,” Proc. Cal. Acad. Sci., Vol. V (1915), No. 3, PP- 33-98. 2R. E. Dickerson, op. cit., p. 40. 154 BRUCE CLARK simplex zone. With this correlation I do not agree. As stated — in the discussion on p. ooo, the species Rimella simplex has not been found in the vicinity of Mount Diablo. The specimens from the south side of Mount Diablo, determined as such by Dickerson, belong to a new species which appears to be character- istic of the Meganos horizon. The so-called Rimella simplex beds of Mount Diablo come within the Meganos part of the section, and contain the typical species of that horizon. EOCENE OF THE CAMULOS QUADRANGLE,’ VENTURA COUNTY General.—The fourth Eocene section studied during the summer of 1918 is that of the Camulos Quadrangle of Ventura County, California. The Eocene outcrops are found on both sides of the Simi Valley, the best and most complete section being in the hills on the south side of the valley, the strike of the beds almost paralleling the valley in an east-and-west direction. The late W. A. Waring described and mapped the geology of this area. He recognized two Eocene divisions in this section, the Martinez and the Tejon, stating that apparently the Martinez (Lower Eocene) graded up into the Tejon. The fauna figured and described by him in his paper as Tejon is that of the Meganos. However, the Tejon also is represented in this section resting unconformably upon the Meganos. Lithology—This general area is being mapped and described by Dr. William S. W. Kew of the United States Geological Survey. According to him, the maximum thickness of the beds here referred to the Meganos is about three thousand five hundred feet. They consist principally of bluish-gray shales and shaly sandstones. Massive conglomerates are found near but not at the base. No sharp line of division between the Martinez and the Meganos has been found in this section. This, very. possibly, is due to the lack of sufficient detailed work. The Tejon here consists of a series of about one thousand five hundred feet of coarse sand- «The eastern half of Camulos Quadrangle comprises the Santa Susana and Calabasas quadrangles. 2W. A. Waring, ‘Stratigraphic and Faunal Relations of the Martinez to the Chico and Tejon of Southern California,” Proc. Cal. Acad. Sci., 4th ser., Vols Vil (1917), No. 4, pp. 41-124, Pls. 7-16. FAUNAL RELATIONSHIPS OF THE MEGANOS GROUP 155 stones, cross-bedded sandstones, and conglomerates. Above this is a great thickness of land-laid beds which are generally correlated with the Sespe formation. Unconformity.—The contact between the Tejon and Meganos of this section is marked by conglomerates and conglomeratic sandstones. At a number of localities true. basal conglomerate was found. The unconformity between the beds of these two horizons is also brought out by the mapping. On the south side of the Simi Valley near its east end the Meganos beds have a thickness of about one thousand five hundred feet; traced west- ward they thin out rapidly and near the west end of the valley disappear, due to overlap of the Tejon beds. This disappearance of the Meganos beds takes place in a very short distance, there being an appreciable difference in strike between the beds of the two horizons, which could only have been the result of crustal movements. | Fauna.—tThe following is a list of species obtained from the basal beds of the Tejon of this section, University of California locality 3311: Cardium brewerit Gabb Ficopsis remondit Gabb Corbicula, n. sp. ? Natica hornit Gabb Glycimeris sagitata Gabb Pseudoperissolax blakei (Conrad) Marcia? n. sp. Turritella uvasana Conrad Tellina, sp. Turris (Surculites) sinuata -Gabb Amauropsis alveata Conrad Turris (Surcula) io Gabb Crepidula pilium (Gabb) Whitneya ficus Gabb Though this fauna is a small one, the writer feels confident in his correlation of these beds with those of the typical Tejon, because: (1) of the presence of an angular unconformity between the beds containing these species and those containing a typical Meganos fauna; (2) because it is believed that a number of the. species listed above are characteristic of the Tejon. All are very common in the fauna obtained from the type section of the Tejon, and only four of the species have been found in beds of Meganos age: Amauropsis alveata Conrad, Ficopsis remondit Gabb, Natica hornit Gabb, and Pseudoperissolax blaket (Conrad). The fauna obtained from the Meganos of this general section is one of the best preserved and largest from any known section 156 BRUCE CLARK belonging to that epoch of deposition. A very large percentage of the species are common to the Meganos of the Coalinga section, as well as to that of the Mount Diablo region." Correlation.—It was from these Eocene beds in Ventura County that convincing evidence was first obtained that the Meganos belongs to the same horizon as that of the Eocene of Marysville Buttes and Table Mountain near Oroville, California, the beds of which localities contain the fauna of the Siphonalia sutterensis zone. The large number of highly ornamented species common to the Meganos of the Ventura County region and to the Eocene of these other localities seems to show conclusively that we are dealing with beds that are nearly, if not exactly, contemporaneous. One of the localities, from which the writer has obtained the best-preserved Meganos fauna in the Ventura County area, is along Aliso Canyon about four miles northeast of the east end of Simi Valley. Here were found a number of the species which have been regarded as characteristic of the Siphonalia sutterensis zone. The following quotation is taken from the published abstract of one of Dickerson’s papers in which he refers to this section: A year ago Mr. Reginald Stoner discovered a locality in the Santa Susana Mountains, on Aliso Canyon of Devil Creek, just beneath the Miocene strata. The fossils from this locality represent a lower phase of the Siphonalia sutter- ensis zone and the fauna is essentially the same as the Siphonalia sutterensis zone of the Roseburg Quadrangle, on Little River near the confluence with the Umpqua. In the Simi Hills, a few miles away from the locality discovered by Mr. Stoner, the Rimella simplex zone of the middle Tejon stage occurs; the general absence of this zone through most of the Coast Range region is probably due to extensive erosion during the interval between upper Eocene and Oligocene time. Dickerson, at the time the above-mentioned paper was written, supposed that these beds containing the fauna which he recognized t For the list of the described species from the Meganos of the area under discussion the reader is referred to the list on pages 158-59. 2 R. E. Dickerson, ‘‘Occurrence of the Siphonalia Sutterensis Zone, the Uppermost Tejon Horizon in the Outer Coast Ranges of California,” Bull. Geol. Soc. America, Vol. XXIX (1917), p. 163. FAUNAL RELATIONSHIPS OF THE MEGANOS GROUP 157 as that of his Siphonalia sutterensis zone were at the top of the Eocene in this general section, and used this as corroborative evi- dence in support of his belief that this zone belongs to the upper- most Eocene horizon known on the West Coast. At that particular locality these beds are in unconformable contact with beds of Lower Miocene age. Further stratigraphic work by Kew, however, has shown that to the west other beds come in between these Eocene beds of Aliso Canyon and the Lower Miocene, and not more than four miles from that locality nearly 3,500 feet of other strata are found between. These include beds of true Tejon age, together with a considerable thickness of land-laid beds which have generally been called the Sespe formation. This is the section already referred to, in which there is a marked unconformity between the Meganos and the Tejon. Thus mapping and faunal work in this region show conclusively that the Eocene beds of Aliso Canyon, correctly considered by Dickerson as representing his Siphonalia sutterensis zone, lie unconformably below beds which contain a typical Tejon fauna. FAUNA OF THE MEGANOS OF CALIFORNIA The following is a list of the described species from the different Eocene localities in California which are now believed to be of Meganos age. The list is as complete as can be made at the present time. The writer takes entire responsibility of the specific determinations. The fauna, as listed here, represents only a comparatively small part of the known fauna, since a very large proportion of the known Meganos species has not yet been described. When this fauna is more thoroughly worked up, the evidence for the correlation of the different sections here described will appear more conclusive. The general localities from which the species listed on pages 158-59 have been obtained are indicated in the columns on the side, as follows: M., Marysville; T.M., Table Mountain; Mt.D., Mount Diablo; Coal., Coalinga; Ca., Camulos and Calabasas Quadrangles; T.T., Type Tejon. 158 BRUCE CLARK TABLE I M. | T.M. |Mt.D.| Coal. | Ca. | or. Anthozoa: Flabellum (?) merriami Nomland...........|....0.|..---|.---- XX ||eeel eee Stephanophyllia californica Nomland........ > a eae XK |. eee ae eee Trochocyathus imperialis Nomland..........|....-|...-- < Xi AS el eee Trochocyathus perrint Dickerson............ Xoiaaee x Kleene al See Thamnasteria sinuata Nomland.....:.......].....|....- XX nece oll Be eee Turbinolia dickersoni Nomland.............|...--|---+. x Xe Turbinolia pusillanima Nomland ...........|.....|..... x Xe Echinodermata: Schizaster diabloensis Kew....... Sainte leat eas Xk cal oeee Ceeee Pelecypoda: Acila gabbiana Dickerson..... EUs doe roccane x x x PRE oa dlcic XP ARE GA BATE RONG > ogo geddouesdpaousodladsad|dscac X- Nek sare ete eee Arca. horni Gabb; n. Vari... jh fec cs ote we sllee ee cle ene | sei |e eee | ene Cardium brewerit Gabb, n. subsp........... lalla x x Perillo oc Cardium marysvillensis Dickerson.........-| X Xx x x XV |seeee Corbula diletata Waring... «02 5c) ees: o oo cist ne «lll self ee el reel ete | eee CrassarellizesilisdsWickersoneey eer eee eee |e eee XK. lace eee Cucullacaymorani Varin eae eels ere inte oe ee ele | ee | een eee xo eee Di plodonta cretacea (Gabba. 1-1): eee ae la e selene | eerie | eee | eae eee Ghiamerisiinesnoensis Wickersoniy. see eee |e Geen arises (ed. Glvcimeris marysvillensis Dickerson......... > Gan Ps esol oeoosilonG culls ols oc Gly cimentsymajor, Stanton Mena jee ee |e |e be eaten (sor olladS ac Isocardvum tejonensis Waring. .......-----:| X |... x x xX Viet ae bed aniresmocnsissDickeTsOnen eee eet eee eit | erie Xo Naceasea ete ikedascanbiConradacsr cer mocceree aoe eae oe ee ell eee x< XO vile x Moapgia (2) Gorin IDORKEROD, oasn00000080clls0000os00cllooo0 Xi) les Seeleeee Modiolus ornatus Gabb..................--- x x x x x x Nucula coopert Dickerson................. > Gn RRP eee biniatdliacodello coos Phacordes gyrata Gabby i icias feos so b6 «ae sb ioe So aes | ie eet] ee eee | | ee Spisula tejonensis Packard...........1....-- x S< x” lei x ae SS DUSULC IMCLPTO NII C Aran NM Peet rete, aren ele. Grell etetevel| etece ee pen nein sete alas iersc Tellina sutterensis Dickerson..............- » et REI Semallciciccollo cea ciao < Fellinailonga\ Gabby oe hs ok ais nels 3 bo she eeode | 631 ol| 2 sie-acell ee ole all eee eene| eeee Pellinarémondi) Gabbs 33 256 is dots so e0\s bie): || eh ee /e0 << Cll stereo eRe oes | eee RivelarweaverveDickersoneeer meee Cele see acre liaise | eel D> haliciocis Venericardia planicosta merriami Dickerson. .| X? | X?| X?].....].----]..- Gastropoda: aa Acmaea ruckmani Dickerson...............|.-+.. KV ccoave aloe eae eee Amauropsis alveata (Conrad)............... Seana x x x x Ancilla (Oliverata) californica Cooper....... x x PG ic orer Kh \ iene ae Bittium featherensis Dickerson.............|..... > Gan eRe laiolyliava-c.a%s|io'o cbc Bittium longissimum Dickerson............- yaaa nen (Orolo dooallaoccalloco0c Calliostoma arnoldi Dickerson.............. Sine Pen eco Soclige oasis sce » Calypirae excentrica (Gabb)............:... x x x x x x Cancellaria irelaniana (Cooper)............| X > ee PAPI sho ollscin cielo 6 oa c Cancellaria stantont Dickerson............. x x x x x x Caricella stormsiana Dickerson............. Pp Ga ere eer S| IAs eval la bie Gallo 0-0 0 Cerithiopsis orovillensis Dickerson...........|..... bs ee Mo mel| ames nllos60< Chrysodomus? martini (Dickerson) = Phos MOLI EDI CKerSOn Reece er x S< X |e ocala eee Clavilithes tabulata Dickerson .............. > Sima ieee letra cteisl fc. acho 6 Ke ie Cordiera gracilluma Cooper..............-- x PM EI arin.c rilcicioo ollac onc Exilia perkinsiana Cooper..............+.. OK hilttcere XX ence Sa ees FAUNAL RELATIONSHIPS OF THE MEGANOS GROUP 159 TABLE I—Continued M. | T.M. | Mt.D.| Coal. | Ca. | T.T. Gostropoda (continued): Ficopsis rémondii Gabb, n. var............. x >< x XK x< x Fusinus lineatus Dickerson................ Sell deste sect oes et cre ettal ere eae yan Fusinus merriamt Dickerson.............-- DS Glebe o|S-c preiesl aey eee hte eee ted beceaiere Galeodea sutterensis Dickerson ............. x x Dee ll-eienadg Xeon Jira, CUA PSODE WESTIN sobbcb0cdenedooced|asnodienoodloaces x SS ss ete Wietuloanarrisy Dickersomy = ese acct |loe oyelledcrdel| eee SSC Taos heer Mitra simplicissima Cooper............---- Grill ainereste acura lpeveuer ess x< x WMotopophorus sirtatus Gabb ...............||..---|-.--- DSi lveiceenal ka easel (e Sik Monodonta wattsi Dickerson...............| X De TEIN ae ac ere on | eaters Hee Ot Murex nashi Dickérson ................05. SC) Alesha east liicabbecacy | cca onenta stonenmigtemenchee iNaticavesterp Dickersonus see aan eee ae elieee x DCH Fakesierat eal ee Ate INGBED [LO TEBACEIC Obie RG 8.5 oc obis a ooo eee Bieta | oolecie] eects lototeural | S| aces nelle Niaivcasubvovesa. (COopen eansetaadee eee oleae ele see. Si atte ll erie arene Naeiica hannibal Dickerson. 5 2422-45245 2+25)\o2+ cele oes soe 5- = < Sun ia ce Waticamucionnis Gabbaar needs sone eae dale: Dein ire a Meares 2 STI, Economic Geoloey ! » ROLLIN T. CHAMBERLIN, Dyna Geology ASSOCIATE EDITORS D ore, aS Britain : i JOHN C. BRANNER, Leland Stanford Junior University ; - RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. ~ WILLIAM H. HOBBS, University of Michigan FRANK D. ADAMS; McGill University ; : CHARLES K. LEITH, University 01 Wisconsin (ORTH DAVID, Australia. ~ WALLACE W. ATWOOD, Clark Univers ty — S, Leland Stanford Junior University WILLIAM H. EMMONS, University of Mianmesota) ALI ane; Smithsonian Institution ARTHUR L. DAY, Carnegie Institution x APRIL-MAY 1921 Haroitp L. ALLING > 5 v9) 5 q re oO Fy 4 ei eS is) o i wW n Lae) > 2) ty -_ : ae aes Gn ee IS rah em) San ok Os CREE A ARE a ay cee Oa MOI TS 8 a a ee hot CA at a a a en ae gem vt SYSTEMS SURE ec cts duc cas ee ee ce or Bo hes aban = Gy Oe hen ee one Ss = = ie; = 3 ee eS 254 4 . Sh I ran SG oe a Poin S25 = 270 a EDITED BY THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY “4 With the Active Collaboration of oe Be We STUART WELLER ALBERT onan Pe Invertebrate Paleontology -Petrolc EDSON S. BASTIN ROLLIN T. CHAMBERLIN Economic Geology i r May 1, June ts, rene x Sa nieher 15, November 1, December 1 5. per year; the price of single copies is 65 cents. 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COLBY Assistant Professor of Geography, the University of Chicago | | } i The fundamental idea of the book is to make available to the busy teacher the material on this subject which is scattered widely through literature. The course which has been given for a number of years at the University of Chicago has resulted in the collection and organization of the material most suitable for the purpose. The book contains short magazine articles and materials from Canadian, Mexican, and United States government publications. The selection of the material has been governed by the concept that there is a direct relation between the environment and ___ the economic activities of the region. The author aims to survey the different sections ' of the country from the viewpoint of industrial opportunity. It is thought that this collection of materials will make possible the development of courses in this important f subject. i Svo, cloth; estimated 300 pages, $3.50, postpaid $3.70 : | THE INTIS Oe att ee PRESS CHICAGO : ILLINOIS GEOGRAPHY OF ILLINOIS By DOUGLAS C. RIDGLEY . Professor of Geography in the Illinois State Normal University The author’s aim is to provide an authoritative study of the state, presenting in compact form the wide range of physical influences which make up the geographic environment in which men live. The natural features and natural resources of the state are treated in some detail. The great occupations of mankind— agriculture, mining, manufacturing, transportation, and trade —are discussed with sufficient fulness to give an adequate idea of their development and present importance within the state. The population census of 1920 for Illinois is treated in a final chapter. The book is designed to be of interest to the busy citizen who wishes to know his state as a unit in its present-day activities; to teachers and pupils who would know Illinois well enough to interpret other regions in com- parison with the home state; to all who wish to learn the reasons for the high rank of Illinois in many lines of human endeavor. The reader will find among the references listed in the bibli- ography a wealth of material with which to continue a study of the state and its resources as fully as may be desired. xvitt+- 355 pages, r6mo, cloth; $2.50, postpaid $2.65 THE UNIVERSITY OF CHICAGO PRESS CHICAGO - - = 0 (3 82 0 = Seabees PREFACE The writer’s experience in the Adirondack Mountains of New York state has led him to realize the lack of published information concerning the perthitic feldspars. The dominant feldspar of the Algoman augite syenite of the region is perthite (microperthite). This Precambrian rock is a differential phase of an important rock unit which grades from syenites, through quartz syenites, and syenite granites to true granites. In passing from the femic to the salic phases it is readily observed that the feldspar becomes more potassic and passes into what is commonly known as orthoclase, and thence into microcline. Thus the Algoman syenite-granite rocks raise two questions: What is perthite? And what is the difference between orthoclase and microcline ? The attempt to answer these and similar questions led the writer to make the investigation of which the following paper is the result. iii VOLUME XXIxX NUMBER 3 THE pew KNAL OF GEOLOGY VA EVAN OZ Ts THE MINERALOGRAPHY OF THE FELDSPARS PARE I HAROLD L. ALLING Rochester, New York Introduction (Crystalline) Solid Solutions Isomorphism and Solid Solutions The Feldspar Components The Potassium Component Orthoclase Microcline Dimorphism of the Potassium Component The Sodium Component Albite Barbierite The Calcium Component Anorthite The Barium Component Celsian Carnegieite Two-Component Systems The Soda-Lime Feldspars—Plagioclase Series The properties of the Soda-lime Feldspars Classification of the Plagioclase Series The Potash-Soda Feldspars—The Perthite Series ‘Undercooling Specific Gravities of the Potash-Soda Feldspars The Optical Properties of the Potash-Soda Feldspars Classification of the Potash-Soda Feldspars The Potash-Lime Feldspars—The “‘Oranite”’ Series The Barium-Potash Feldspars—Hyalophane Series 193 194 HAROLD L. ALLING Three-Component Systems Ternary Diagrams The Potash-Soda-Lime Feldspars Physical Properties of the Potash-Soda-Lime Feldspars Extinction Angles Specific Gravities Classification — Examination of Chemical Analyses of Feldspars - Microscopic Examination of Natural Feldspars Examples of Plagioclase Feldspars Examples of Potash-Soda Feldspars Applications of the Mineralography of the Feldspars to Geological Problems Case One—Location of a Fault Case Two—Ortho-amphibolites versus Paramphibolites Appendix The Solubilities of the Feldspar Components Conclusions INTRODUCTION The rock-forming minerals can be studied (1) as solids capable of assuming definite geometrical forms—crystallography; (2) as optical media that affect light in characteristic ways—determina- tive petrography; (3) as chemical substances—the chemistry of silicon compounds; and (4) as the end products of crystallization of melts—geophysical chemistry, or as here proposed—mineralog- raphy. This last classification embodies the point of view which has been adopted in this paper for the study of the feldspars, treating them as chemical substances formed by the solidification of melts. When the important mineral groups are fully examined, and their various thermo-equilibrium diagrams published, then the interpre- tative petrographer will be able to unravel the life-histories of rocks with much greater accuracy and in finer detail than is possible at present. Appreciating the value of metallographic methods as applied to the elucidation of the feldspar system, it is therefore proposed that these silicates be examined with the aid of the phase rule. The phase rule has shed a flood of light upon the nature and con- stitution of alloys, and promises to be of equal value in the study of magmas and their crystallization into igneous rocks. THE MINERALOGRAPHY OF THE FELDSPARS 195 The components here considered are the K-, Na-, Ca-, and Ba-feldspars, which are assumed to be stable under the conditions of the present investigation. There is no difficulty in under- standing the term “‘component’’ employed here as an example of the nomenclature of the phase rule. It is well for the purpose of this discussion that we understand the term “phase.” ‘‘ Phases are the homogeneous states, whether of freedom, solution, or combination, and whether solid, liquid, or gaseous, into which the components present pass or group themselves .... the phases are the transitory stages, states, or conditions, physical cr chemical, through which the components pass as they are heated up and cooled down, or as their pressure rises and falls.’”! The mathematics of the phase rule applies to systems which are in a state of perfect equilibrium. It cannot be disputed that many metallic alloys and silicate complexes (silicate alloys) are in a state of imperfect or false equilibrium. So frequently does this condition occur that the phase-rule method of considering such systems has been the object of considerable criticism. These critics are entirely justified so far as the strict application of the rule is concerned, but apparently overlook the value of thermo- equilibrium diagrams. ‘The use of the diagrams and the application of the phase rule are distinct. Such diagrams help to explain the phenomena of crystallization and microscopic textures of silicate and metallic alloys. Howe’? points out that hardened steel is a metastable system, that is, it is not in equilibrium, and yet that fact does not depreciate the value of the iron-carbon diagram to the steel manufacturer. A condition of imperfect equilibrium can be, definitely indicated upon the diagram. Ostwald’ says: ‘‘In spite of my great admiration for the progress that has been made in metallography through the introduction into this field of the con- cepts of chemical equilibrium, the phase rule and van’t Hoff’s -concept of solid solution, I cannot help emphasizing the need of caution in all this, for those concepts are all based upon the truth 1H. M. Howe, The Metallography of Steel and Cast Iron, 1916, p. 240. 2 I(iilon (De Beit 3 Wolfgang Oswald, Theoretical and Applied Colloid Chemistry, translated by Martin H. Fischer (1917), p. 108. 196 HAROLD L. ALLING of certain assumptions. .... The belief that true equilibria are attained in these solid mixtures . . . . lacks support.” This quotation is quite typical of such criticisms: it gives the impression that in studying a system, such as the feldspars, by means of the phase rule the laws of physical chemistry are violated. Metallographers such as Howe, for example, are recognizing the metastability of many systems. Many feldspars are metastable solid solutions. It would appear that the most satisfactory method of attack would be a laboratory study of the melting and freezing phenomena of the feldspars. ‘The work of Allen and Day’ has shown, however, that little information can be secured through this means because of the high viscosity of the alkali feldspars when above their fusion temperatures. Even when in a molten condition it is impossible to effect crystallization. Watts? using pyrometric cones in his work upon artificial mixtures of natural potash-soda feldspars has found it very difficult if not impossible to secure reliable thermal data. Thus the method of attack here is in part a reverse process. By a study of textures and properties of various specimens differ- ing in their chemical composition, the thermo-equilibrium diagrams of the binary systems can be inferred by analogy with the diagrams of other systems and by means of suggestions found in the metal- lurgical and mineralogical literature. The writer suggests that the application of the methods of the metallographer to the study of silicate systems for the purpose of increasing our knowledge concerning them may properly be called the science of mineralog- raphy. Murdoch? has already proposed the use of this term in a slightly different sense to cover the employment of the metallurgical (metal- lographic) microscope in the study of non-transparent minerals in the same way that it has been used for years in the study of metals. tF—. T. Allen and A. L. Day, “The Isomorphism and Thermal Properties of the [Plagioclase] Feldspars,”’ Carnegie Inst. of Wash. Pub. 31, 1905. 2A.S. Watts, ‘‘The Feldspars of the New England and North Appalachian States,” U.S. Bur. Mines Bull. 92, 1916. 3 Joseph Murdoch, Microscopial Determination of the Opaque Minerals, 1916, p. iii. THE MINERALOGRAPHY OF - THE FELDSPARS 197 It seems to the writer that the analogy is not perfect. Metal- lography deals with the constitution, crystallization, and micro- scopic textures of metallic alloys and their relation to their physical properties. Such study must necessarily be guided by constant . references to the thermo-equilibrium diagram of the system under consideration. In the examination of opaque minerals, such as the sulphide ores, the diagrams are not at hand, and not until then should the term mineralography be employed. Furthermore it is questionable whether it is best to limit mineralography to the opaques as contrasted with the transparent minerals. Mineralogy and petrography become mineralographic when the phase rule is considered.and the interpretation of mineral compositions and rock textures studied in the light it sheds upon them. ‘This paper is an attempt to discuss the essential nature and relationships of the feldspars, together with an explanation of the methods used in studying them. Discussions of some phases of this subject have appeared in German, but no adequate study of the entire system has been attempted in this country. The following brief summary indicates the substance of the discussion, though not the precise order of presentation. Many of the principles that apply to metallography and metals apply also to minerals. One of these essential principles is that of solid solutions. To handle this principle the method known to physical chemists as the phase rule has been applied to metal- lography, necessitating the construction and interpretation of thermo-equilibrium diagrams to set forth the constituents and the processes which form metallic alloys. _ Now the principle of solid solutions is applicable also to minerals, and is indispensable to a thorough knowledge of their composition. The mineralogist and the geologist should therefore avail themselves of the phase-rule method, and should understand the use of its diagrams if they would thoroughly grasp the nature of the con- stituents and of the physical phenomena which have given them their rocks and minerals, the end products of such processes. This study involves: (1) the construction of thermo-equilibrium diagrams; (2) the interpretation of such diagrams; (3) the applica- tion of these diagrams to the rock-forming minerals. 198 HAROLD L. ALLING The present-day conception of the chemical nature of the silicates is the result of slow development. In 1846 Laurent sug- gested that the silicates should be considered to be salts of several acids rather than a single one, and by 1865 ortho-, meta-, and trisilicic acid had become a firmly established nomenclature. Later Vernadsky? pointed out that in some aluminum-bearing silicates the element aluminum seemed to possess the characteristics ofanacid. ‘Thus the theory that some silicates are aluminosilicates instead of simple silicates was developed and is entertained by many geochemists. While it would be instructive to follow this phase of the subject in greater detail it is outside of the main pur- pose of this paper. However certain aspects of the alumino- silicate theory need to be considered in attacking the problem of the relationships between the soda, potash, and lime feldspars. ‘““Schwantke? while reasoning over the réle of the lme-silicate which occurs mixed with the potash silicate K,AISi¢O,6 in orthoclase built up theoretical conclusions of much interest. ... . In order to make the formulas of albite, anorthite, and orthoclase more analogous to each other, we may write them Na,AL,SiSi,O., K,ALSi,Si,O,6, Ca,ALALSi,O.%, and Ba,ALALSi,O;6.’”4 Bayley® says that chemically, the feldspars may be regarded as isomorphous mixtures of the four compounds, KSiAISi,0s, NaSiAlSi,Os, CaAlAISi,Os, and BaAlAISi,Og, each of which except the fourth has been found nearly pure in nature..... The feldspars have also been regarded as salts‘of the acid H;AISi,O3 in which the | hydrogen is replaced by various radicals, thus (KSi)AISi,0s, orthoclase; (NaSi)AISi,Os, albite: (CaAl)AISi,Os, anorthite; and (BaAl)AISi,Os, celsian. There are several objections to Bayley’s conceptions. In the first place, as will be pointed out later, the t Laurent, “Sur les Silicates,” Comp. Rend., XXIII (1846), 1050-58. 2 W. Vernadsky, “Uber die Sillimanitgruppe und die Rolle des Aluminums in den Silicaten,”’ Bull. d. Russ. Ges. d. Naturf., 1891, nr. 1-100. (In Russian.) 3 A. Schwantke, ““Die Beimischung von Ca in Kalifeldspath und die Myrmekit- bildung,” Centralbl. fiir min. Geol. und Pal. (1909), pp. 311-18. 4J. J. Serderholm, ‘‘On Synantectic Minerals,” Bull. de la commo. geol. de Fin- lande No. 48, 1916, pp. 90-91. 5 William S. Bayley, Descriptive Mineralogy, Appleton (1917), p. 408. ita iae ee e THE MINERALOGRAPHY OF THE FELDSPARS 199 feldspars do not form a complete isomorphous series. Secondly, it is not customary to regard silicon as a base-forming element. (CRYSTALLINE) SOLID SOLUTIONS The whole subject of solid solutions was opened up in a paper by van’t Hoff" which appeared in 1890, bearing the title of ‘Solid Solutions.” He said: ‘“‘If we regard a solid solution as a solid, homogeneous mixture of several substances, the composition of which can be changed without destroying the homogeneity, analogous to solutions in liquids as the solvent, it should not be difficult to cite cases which belong unconditionally in this cate- gory.” This definition of solid solutions gives us a concept that isa most valuable aid to the study of substances in the solid state. Again van’t Hoff defines a solution as ‘‘a homogeneous mixture, the composition of which can undergo continuous variation within the limits of its stable existence,’’”? or in other words a solid solution is a solid homogeneous complex of several substances, whose pro- portions may vary without loss of homogeneity. A solution may be defined in terms of the phase rule as ‘‘a homogeneous mixture which undergoes a change in composition in producing a new phase.”’? Washburn says that “‘a solution may be defined as a one-phase system composed of two or more molecular species.’’4 Ordinarily the term “solution”’ has been limited to solids dis- solved in liquids. But recent investigation has shown that the conception of solutions should be extended to include gases and solids as actual solvents. The metallographer and the geologist are beginning to speak of “‘solid solutions,’ a term which has cleared the atmosphere and opened the way toward a better understanding of matter in the solid state. The terms “‘solid solution” and “crystalline solid solution” have both been used in referring to solids dissolved in solids. But the latter term is not as common, therefore the term ‘‘solid solu- tion’’ will be used in this discussion. tJ. H. van’t Hoff, “Solid Solutions,” Zeitschr. phys. Chem., V (1890), p. 322. 2 As cited by J. V. Elsden, Principles of Chemical Geology, 1910, p. 116. 3 J. L. R. Morgan, The Elements of Physical Chemistry, 1918, p. 147. 4 Edward W. Washburn, Principles of Physical Chemistry, McGraw-Hill, 1915. 200 HAROLD L. ALLING Solid solutions are frequently referred to as ‘‘mixed crystals,” a term probably derived from a too literal translation of the German ‘‘Mischkrystalle.”’ This translation is unfortunate since the term may suggest a merely mechanical mixture of crystalline substances like sand. As the term is liable to convey an entirely erroneous impression, it should be avoided in the interest of clearness. Owing to the restricted nature of molecular motion in crystals and the high viscosity of this state of aggregation, crystalline solutions in a state of equi- librium are seldom met with in practice, because the attainment of equilibrium in a reasonable length of time is so frequently prevented by the restraints upon the free movements of the molecules. This fact prevents satisfactory laboratory work on the problem. In geology we have at our disposal enormous lengths of time in which perfect equilibrium may be established, a factor, the impor- tance of which is not always appreciated and which of course is rarely attained in the laboratory. Although the writer is inclined to the view that the feldspars are aluminosilicates which in many cases are capable of forming solid solutions, nevertheless the usual nomenclature (ortho-, meta-, and trisilicates) will be employed without necessarily implying any particular theory of the chemical nature of these minerals. ISOMORPHISM AND SOLID SOLUTION Recognizing that isomorphous substances must be completely soluble in each other some mineralogists have come to the con- clusion that the terms “isomorphism” and ‘‘solid solution” are synonymous. As this matter is of some importance here it will be discussed. “The establishment of complete solid solution between the [plagioclase] feldspars raises the whole question of the use of the terms solid solution and isomorphism. Some authors use isomorphism to designate complete solid solution, others speak freely of limited isomorphism, and still others use the term in its original significance of simple crystallographic similarity.’” 1. W. Washburn, Principles of Physical Chemistry, 1915, Pp- 117. 2.N. L. Bowen, Amer. Jour. Sci. (4), XXXV (1913), 595. THE MINERALOGRAPHY OF THE FELDSPARS 201 Hlawatsch,’ in a very complete review, concludes that two minerals are isomorphous when: (1) they possess like crystal forms; (2) when their chemical composition is strictly analogous; (3) when they are capable of forming homogeneous ‘mixed crys- tals.’ The etymology of the term isomorphism strictly means ‘““same form.’”’ We know, however, that two minerals that are isomorphous do not possess identically the same crystal form but differ from one another in many details. Though they are similar they are not identical. In addition to the crystallographic mean- ing of the term a similarity in chemical composition is implied. On the other hand in viewing such substances mineralographically the term is applied to systems where complete solubility in the solid condition exists among the components. One of the difficulties in the correct use of these terms is illus- trated by their application to the plagioclase feldspars. Albite is soluble in all proportions in anorthite; both of these minerals are triclinic but their formulas as generally written indicate respectively a trisilicate and an orthosilicate. Their mutual solubility has suggested to mineralogists that their chemical structures ought to be similar, being similar salts of the same acid as suggested by Schwantke? and Bayley: But this gets us into an opposite difficulty. If the different ° salts are salts of the same acid, and have similar structures, how is it then that certain pairs are completely soluble while others are only partially so? To this question no direct answer is forth- coming, for ‘“‘very little is yet known about the physical and chemical conditions which determine the solubility of a substance in a (liquid) solvent. In fact the essential nature of the process of solution must be regarded as at present uncertain. It has been noticed that solution is more likely to occur if the [components] are alike chemically, but no general rule can be framed.’’* On the other hand when we consider solids and (crystalline) solid t Hlawatsch, Zeitschr. fiir Kryst., Vol. LI (1912), pp. 417-01. 2 A. Schwantke, “Die Beimischung von ‘Ca’ in Kalifeldspath und die Myrmekit- bildung,” Centralbl. fiir min. Geol. und Pal., 1909, pp. 311-18. 3 William S. Bayley, Descriptive Mineralogy, Appleton, 1917, 408. 4W.C. D. Whetham, Theory of Solution, Cambridge University Press (1902), p. 78. 202 HAROLD L. ALLING. solutions we find a more satisfactory answer to our query. Some fine work on this subject has been done by the Italians, especially by Ciamician and his co-workers. Ciamician* and Bruni? have shown that single-ring organic compounds can form solid solutions only with other single-ring com: pounds. Thus, benzene (not benzine), Figure 1-(1), can form solid solutions only with such compounds as: thiophene, Figure 1-(2), pyrrol, Figure 1-(3), and pyridine, Figure 1-(4). In the same way a double-ring compound can only form a solid solution with another double-ring compound. Thus naphthalene, Figure 1-(s5), only with quinoline, Figure 1-(6), etc. Under that same rule applied to triple- ring compounds anthracene, Figure 1-(7), forms solid solutions with its isomer, phenanthrene; with carbozol, Figure 1-(8), or with other triple-ring structures. The complete solubility of one solid in another is more likely to occur if the chemical structure of each is similar. The similarity of the two, however, does not necessarily have to be so close as the term “isomorphism”? would imply. For example some feldspars contain nephelite in solid solution, but this mineral cannot be regarded as isomorphous with the normal feldspar components. It does not seem unreasonable to assume that the mineralographic term “solid solution” is more comprehensive than the crystallo- graphic term ‘‘isomorphism.”’ “It is convenient to regard . . . . [an isomorphous series] as formed by the replacement of one element or radical by another isomorphous with it, rather than as a mixture of different individual molecules.’ It certainly is true that the term “‘replacement” is very commonly used, and no criticism to such use can be made provided that the conception of the solubilities of the components is not lost sight of, but there is danger of confusion in such cases. To the eco- nomic geologist the word “‘replacement”’ immediately suggests such phrases as “‘pyrite replaced by chalcocite” or ‘“‘limestone replaced ™ Ciamician, Zeitschr. fiir Physt. Chem., XIII (1894), 1; XVIII (1894), 51; XLIV (1903), 505. 2G. Bruni, Rendiconti dell’ Accademia dei Lincet., Vol. VIII (1899), p. 570. 3A. J. Moses and C. L. Parsons, Mineralogy, Crystallography, and Blowpipe Analysis, 5th ed., 1916, p. 234. [alin 2 THE MINERALOGRAPHY OF THE FELDSPARS 203 eg AAA | C C-H era! | BCC new Git Cob IM Me Flee glee Fic. 1.—Structural chemical formulas of single-, double-, and triple-ring organic compounds, illustrating structures that form solid solutions. 1. Benzene. 2. Thio- phene. 3. Pyrrol. 4. Pyridine. 5. Naphthalene. 6. Quinoline. 7. Anthracene. 8. Carbozol. H ae LTT Et H 204 HAROLD L. ALLING by ore” due to metasomatism. The term, however, is used in a very different sense by mineralogists. Is it chemically correct to speak of an element in a formula as “partially replaced” or “replaceable” by another? A replacement in a chemical sense means double decomposition taking place through chemical reaction, involving a thermal change; chemical reaction and ther- mal change being functions of each other. Thus if the potassium of orthoclase is ‘‘replaced”’ by sodium by chemical reaction then there should necessarily be a change in the thermal state of the system, the absorption or liberation of heat. While it cannot be stated positively that there is no change in the thermal state when liquid albite is added to liquid orthoclase at the same temperature, it seems more than likely that the albite will pass into solution without greatly disturbing the thermal equilibrium. The same principle probably applies to solid minerals, although it would be very difficult to prove it experimentally. Of all the mineralogical systems studied up to the present time none is better known and understood than that of the plagioclase feldspars. It can be asserted with considerable emphasis that the variations in composition in the series are not due to chemical replacement but that they constitute a series of solid solutions. Moreover the plagioclase feldspars are not unique in nature, for many similar systems undoubtedly exist to which the same prin- ciple applies. The reader should not overlook the fact that in this discussion of the feldspars we are dealing with them from the point of view which considers them solid solutions, and mixtures of solid solu- tions, and not “mixtures, admixtures, and mixed crystals” nor “molecules” which have certain portions “‘replaceable” by analo- gous units. Thus the evidence that can be brought to bear upon this difficult problem points to the conclusion that NaAISi,Os, when it exists as albite, and CaAl,Si,0s as anorthite probably possess similar struc- tures. The suggestion of Bayley that the NaAISi,O3 in orthoclase is barbierite (see later under ‘‘Feldspar Components’’) may be true but in view of the fact that other evidence points to a possible inversion of albite to barbierite (in the same way that orthoclase oe THE MINERALOGRAPHY OF THE FELDSPARS 205 probably changes to microcline) at temperatures below the freezing- point of any melt in the system, it does not seem very likely that the soda feldspar in a magma slowly cooling at great depths could take the form of barbierite. It is very likely that barbierite and microcline, when found, are usually not in their original form, but are the result of inversion, or transition. The possible inversion of the potash and soda feldspars intro- duces a new element into the subject. Microscopic evidence does not seem to verify the supposition that microcline and barbierite are high-temperature forms, but rather that they are the result of inversion or that the presence of mineralizers has lowered the temperature of freezing below the inversion range of these minerals. From the foregoing we conclude that orthoclase and albite have dissimilar structures, while barbierite and orthoclase possess similar atomic groupings. In an analogous way albite and micro- cline are probably alike. FELDSPAR COMPONENTS Modern textbooks on mineralogy list the following feldspars: (1) orthoclase, (2) microcline, (3) soda-orthoclase, (4) soda microcline, (5) anorthoclase, (6) albite, (7) oligoclase, (8) andesine, (9) labradorite, (10) bytownite, (11) anorthite, (12) plagioclase, (13) perthite, (14) celsian, (15) hyalophane, (16) carnegieite, and (17) anemousite. All the feldspars from orthoclase to and including perthite (1 to 13 inclusive) belong to a three-component system where the three units are: (1) potassium feldspar, (2) sodium feldspar, (3) calcium feldspar. Or if celsian, hyalo- phane, carnegieite, and anemousite be included then all the 17 mem- bers fit into a five-component system, of which the following are the components: Type Empirical Formula Potassium Feldspar KAISi,Og Sodium Feldspar NaAlSi,Os Calcium Feldspar CaAl,Si,08 Barium Feldspar BaAl,S1,03 Carnegieite Na,ALSi.O0s 206 HAROLD L. ALLING THE POTASSIUM COMPONENT Orthoclase.—This mineral (dp6os, straight, and kas, angle), KAISi,Os, is usually regarded as a salt of trisilicic acid, H,$1,Os. In regard to its chemical composition zm nature and its distinction from microcline we shall have more to say. Microcline.—The name (uxpds, small, and xXivew, to incline) has reference to the fact that the angle (89°30’) between the two perfect cleavages differs but little from a right angle. Although the chemical formula of microcline is given as identical with that of orthoclase the petrographer usually has little difficulty in recog- nizing and separating it from orthoclase by the multiple pericline or ‘‘gridiron”’ twinning of the former as revealed by the microscope. Orthoclase either does not twin in this manner or does so on such an extremely fine scale as to be submicroscopic. Whether there is any physical or chemical difference between the two potash feld- spars has been a matter of debate for some time. Investigators are divided between the two general theories; one group maintains that they are one and the same mineral, and differ from one another only in the magnitude of the twinning. On the contrary, another group maintains that there is a fundamental difference between the two minerals which although of identical composition possess different chemical structures. Dimorphism of the Potassium Component.—‘‘It appears highly probable that if the cross twinning and interpenetration of micro- cline become so minute as to be invisible under the microscope the crystals would be indistinguishable from those of orthoclase, and would, in fact, possess all the properties of that mineral. Many authors regard orthoclase as pseudo-symmetric; if this be so, all the feldspars may be in reality anorthic [triclinic].””* According to E. Mallard and A. Michel-Lévy it seems highly probable that orthoclase and microcline are not dimorphous, but identical, since they: proved that the optical behavior of orthoclase would be a necessary conse- quence of an intimate multiple twinning of microcline lamellae after the albite and pericline law.? «H. A. Miers, Mineralogy, p. 460. 2 Rosenbusch-Iddings, Microscopic Physiography of the, Rock-Forming Minerals, Pp. 320. ote ee Sea THE MINERALOGRAPHY OF THE FELDSPARS 207 On the other hand Vogt speaks of the “probable occurrence of an inversion point in the potassic feldspar from an alpha to a beta modification, viz., from orthoclase to microcline.’”’ Barbier? has expressed the opinion that potash feldspar exists in two distinct forms which he regards as dimorphous, and which he says is a case of polymorphism. Assuming that there are two dimorphous forms it is not known whether the difference is due to “‘polymer- ism” or “‘isomerism.”’ ‘The term “‘polymerism” is applied where two compounds possess the same chemical composition but differ in their molecular weights.* For example, butyric acid, C,HsO., and alde- hyde, C,H,O, differ by polymerism. On the other hand “isomerism”’ is used when two or more compounds possessing the same chemical composition differ in physical or chemical properties but have the same molecular weight, such as levulose and dextrose. If the difference between orthoclase and microcline is due to polymerism then the following formulae might express the relation: (KAISi,Os)1 and (KAISi,Os) (n--m) where and m are whole numbers. If the difference is due to isomerism then the kinship of the two minerals may be repre- - sented, as suggested by Clarke,‘ as follows: O=Si—O—K O=Si-O—-K O O Si—O = OF 0-2 Al 4 5 Osi La 0 t As cited by C. H. Warren, Proc. Amer. Acad. Arts. Sci., LI, No. 3 (1915), p. 144. 2 Ph. Barbier, ‘‘ Researches sur la composition chemique de feldspaths potassique,”’ Bull. Soc. frang. minéral., XX XI (1908), 152-67. 3 Since the examination of crystals by X-ray spectra has shown that our conception of molecules in solid matter has no real basis, it is well to bear in mind that such expressions as here used are subject to subsequent revision. 4F. W. Clarke, U.S. Geol. Surv. Bull. 588, p. 12. 208 HAROLD L. ALLING The exact nature of the isomeric equivalences among the feldspars is not clear; they may be due to the structure of the salts independently of the acids they represent, or to isomerism among the acids themselves. Barbier sought to find distinct chemical differences between the two and suggested that the minerals may be distinguished by the fact that orthoclase often contains traces of lithium and rubidium while microcline does not seem to carry them. Boeke? pointed out that the two salts may have a selective solubility for these rarer alkalies, or that these control the magnitude of the twinning. But such a distinction is not fundamental as is shown by Vernadsky? when he found an unquestioned specimen of microcline from the Ilmen Mountains that contained rubidium to the extent of 3.13 per cent of Rb,O. It is generally stated by those who believe that there is no physical-chemical difference between orthoclase and microcline that the specific gravities of the two are identical. It is true that they closely approximate each other, as would be expected in the case of minerals so closely allied, but the slight differences are significant when definitely charted after careful analysis as is shown by the diagram, Figure 7, on page 231. Let us defer final conclusions on this argument till we come to its more detailed examination. Another difference between the two minerals is the varying values of the indices of refraction. Weinschenk has stated it very clearly when he calls attention to the fact that orthoclase has lower indices of refraction in all crystallographic directions than anortho- clase, while those of microcline are a little lower in one direction and a trifle higher in others.4 Still another difference indicating a fundamental distinction between orthoclase and microcline is the thermal constants of these minerals as shown in Table I.° tF. W. Clarke, ‘Constitution of the Natural Silicates,” U.S. Geol. Surv. Bull. 558, P. 35. : ioe E. Boeke, Grundlagen der Physikalisch-Chemischen Petrographie (1915), ‘ : Vernadsky, Bull. Soc. Min., Vol. XXXVI (1914), p. 258. 4 Weinschenk-Clark, Petrographic Methods (1912), p. 330. 5D. M. Liddell, Metallurgists’ and Chemists’ Handbook, second ed., 1918, pp. 207-8; Joseph W. Richards, Metallurgical Calculations, 1918, p. 140. a THE MINERALOGRAPHY OF THE FELDSPARS 209 If the figures are reliable there should be some basis for the opinion that the two similar minerals may be isomeric forms of the same substance. TABLE I Mineral Melting-Point Latent Heat of Fusion Specific Heat Orthoclase.......... 1200° roo cals. 0.1877 Microcline........ ay | I17o° 83 cals. 0.107* *See W. P. White, Amer. Jour. Sci. (4), Vol. XLVII (January, 1919), p. 17, for more modern values of the specific heat of microcline. Harker says: If orthoclase and microcline are dimorphous, the latter must clearly be the lower [temperature] form. Where it occurs with the apparent characters of a primary mineral it is the latest product of crystallization and is charac- teristic of the most acid of granites and especially of pegmatites..... The conversion of orthoclase to microcline, or the setting up of microcline structure in orthoclase, has been attributed to dynamic causes." Another explanation of the microclinic texture in potash feld- spar is that it is solely due to stresses set up by dynamic forces and is not explained by the theory of dimorphism. Thus Rosenbusch’ remarks: The fact that microcline is almost wholly confined to the older eruptive rocks which have been subjected to processes of faulting and pressure, together with the observation that normal orthoclase assumes the microstructure of microcline when it has experienced strong pressure, leads to the supposition that microcline-structure is a pressure phenomenon. ‘The correctness of this assumption can be verified in many cases. The occurrence of crystals of microcline in cavities, however, proves that it has not been produced in this way in all cases. Still the absence of microcline from unaltered extrusives is notable. The writer is fully aware that microclinic texture is frequently the result of dynamic stresses, but is inclined to the view that the stresses of dynamic metamorphism permit orthoclase, which is metastable at temperatures below its inversion point, to change to microcline. His conclusion is that pressure does not produce micro- cline from orthoclase; it only initiates and accelerates the change. t Alfred Harker, Natural History of Igneous Rocks (1909), p. 259- 2 Rosenbusch-Iddings, Microscopical Physiography of Rock-Forming Minerals, P- 320. : 210 HAROLD L. ALLING Working with thin sections and crushed fragments of the same specimen it was frequently observed that while the thin section showed the feldspar frequently twinned the crushed fragments of the same material were often untwinned. This applies to plagioclase as well as to the potash-soda varieties. In fact some ‘‘adularias” and ‘‘microclines”” when examined in the form of thin plates (removed from the specimen by a knife blade) do not exhibit twinning while in thin sections of the same specimen the characteristic twinning is at once manifest. An excellent example of this phenomenon is the ‘‘microcline” from San Diego County, California. The sug- gestion is strong that sufficient pressure to develop twinning was present in! the grinding process necessary for the preparation of the thin section. If soda orthoclase is metastable at normal tempera- tures then it follows theoretically that inversion to soda microcline would be hastened by heating. ‘This proved to be the case. The San Diego County material was crushed, sieved, and sized, and then divided into four samples. One sample was not heated. No. 2 was heated one hour in a quartz glass crucible over a special Bunsen (Scimatco) burner. No. 3 was heated three hours, and No. 4, five hours. It was found that the percentage of the twinned fragments increased with the duration of heating. The significance of this simple experiment is the raising of the question whether thin sections can be relied upon uniformly as a means of proper identification of the feldspars. The experimental work of Allen and Day’ shows that the determination of the melting temperatures of the alkali feldspars and the thermal-reaction points indicative of inversions is extremely difficult. Working with natural microcline from Mitchell County, North Carolina, and employing ‘thermal apparatus .. . . suffi- ciently sensitive to detect any unsteadiness of a tenth of a degree [centigrade] with certainty, not the slightest trace of an absorption or release of heat was found.’’ All such ‘“‘phenomena appeared to be effectively veiled by some property [of the substance], presum- ably the viscosity.’’ In natural magmas the presence of mineral- izers “acting as solvents, keeps the minerals in a fluid condition until the temperature is far below that at which they would t Allen and Day, Carnegie Institute Pub. 31. THE MINERALOGRAPHY OF THE FELDSPARS 211 otherwise solidify, thereby making possible their crystalline devel- opment.’’? : While it seems as if the matter cannot be definitely settled by laboratory work unless fluxes or high pressures are employed, yet the writer is of the opinion that microcline is an isomer of orthoclase. This implies that above the transformation tempera- ture orthoclase is the stable mineral, while below it microcline is the normal form. If orthoclase passes this point on cooling with- out inverting, then the mineral exists in a metastable condition. THE SODIUM COMPONENT Albite——Albite (albus, Latin for white) is the soda feldspar, NaAlSi,Os, which crystallizes in the triclinic or anorthic system.. Structurally it can be represented by the same general formulas given above by substituting Na for K. Barbierite.—It was not until recently that a monoclinic form of the soda component was even suspected. Barbier and Prost? found a sodium feldspar that is monoclinic and isomorphous with orthoclase, which Clarke? lists as ‘“‘barbierite isomeric with albite.”’ Schaller? says that the existence of a monoclinic soda feldspar iso- morphous with orthoclase must be admitted. Thus we may have a relationship in the soda component identical with that prevailing in the potash feldspar. The thermal ranges of barbierite are unknown. THE CALCIUM COMPONENT Anorthite —Anorthite (av, not, and épos, straight) is a triclinic lime feldspar, but unlike the other components it is usually regarded as a salt of orthosilicic acid (H,SiO,) instead of the trisilicic. The formula is commonly written: CaAl,Si,O; or CaAl,(SiO,),. So far as known no isomeric form of the lime component exists, consequently the term ‘‘lime component” and ‘‘anorthite” can be used indiscriminately. The above three components, :potash, soda, and lime, are the most important feldspar constituents. Two other components however ought to be mentioned. t Weinschenk-Johannsen, Fundamental Principles of Petrology, 1916, p. 41. 2 “Sur Vexistence d’un feldspath sodique monoclinique isomorphe de 1’orthoclase,”’ Bull. Soc. Chem. (1908), II, 804. 3B, W. Clarke, U.S. Geol. Surv. Bull. 588, p. 35. 4 Bull. Soc. Min., XXXIII (1910), 320. 212 HAROLD L. ALLING THE BARIUM COMPONENT Celsian.—Celsian BaAl,Si,Og ‘‘is monoclinic and isomorphous with orthoclase.’’* No triclinic isomeric form of celsian is definitely known. CARNEGIEITE Our knowledge of carnegieite, Na,ALSiOs, the polymer of nephelite, has been derived almost entirely from the researches of the Geophysical Laboratory of the Carnegie Institution, although Thugutt? obtained crystals of it by rapidly cooling “nephelite hydrate.’ Carnegieite was first recognized as occurring in a cinder cone on the island of Linosa,? east of Tunis. It is “‘chemi- cally, as well as physically, a feldspar, differing somewhat from those belonging to the orthoclase-albite-anorthite series. It may represent a member of a new series, containing variable amounts of carnegieite.”’4 It has the same chemical composition as nephelite but it differs therefrom by dimorphism. It is triclinic and often twins polysynthetically after the albite law and less frequently after the pericline law. Carnegieite is reported as having a specific gravity of 2.571 at 25°C. The reader is referred elsewhere for more details, especially to Bowen.5 No better summary of the feldspar components can be offered than by giving a modification of a tabulation suggested by Wash- ington.°® : Fieldspar Group “Albite” Subgroup (R’AISi,Os) be tie (R’AISi,Os) triclinic (R” ALSi,O)s Monoeliniey etc) vicrseine KAISi,Os Albite NaAlSi,Os CONTalaraiSuberoup Anorthoclase (K,Na)AISi,O3 (R’AISi,Os) Monoclinic “Anorthite” Subgroup Orthoclase KAISi,O (R’AL,Si.0s) triclinic Barbierite NaAlSi,Og Anorthite CaA1,Si.Og Celsian BaAl,Si,Og Carnegieite Na-ALSi.03 tF, W. Clarke, U.S. Geol. Surv. Bull. 588, p. 35, 1914. 2S. J. Thugutt, Newes Jahrb., Beilage Band 9 (1894), p. 561. | 3H. S. Washington, Jour. Geol., XVI (1908), 10; H. S. Washington and F. E. Wright, Amer. Jour. Sci. (4), XXVI (1908), 187, and XXIX (1910), 52-70. 4J. P. Iddings, Rock Minerals (1911), p. 243. 5 N. L. Bowen, Amer. Jour. Sci. (4), XX XIII (1912), 551-73. 6H. S. Washington, ‘Suggestion for Mineral Nomenclature,” Amer. Jour. Sct. (4), XX XIII (February, 1912), 140. THE MINERALOGRA PHY OF THE FELDSPARS 213 The method of approach here is the application of the phase rule to the feldspar system. The original presentation of the phase rule was garbed in mathematical terms. For the involved equations as set forth by Gibbs the thermo-equilibrium diagram has been substituted. ‘To the metallographer the construction and inter- pretation of these diagrams present no great difficulties, but to the average geologist they are conventions that possess little or no meaning. As an understanding of the diagrams of the feldspar system is essential to what follows, a section at the end of the paper in the Appendix is introduced in which their construction and interpretation are discussed. For most purposes it is sufficient to consider that the binary systems, soda-lime and potash-barium feldspars, constitute a series of solid solutions, the thermo-equilibrium diagrams of which are classified by Roozeboom as Type I. On the other hand the binaries, orthoclase-albite and orthoclase-anorthite, are repre- sented by the eutectiferous type of diagram. ‘Thus the feldspar binaries may be classified as follows: Limited Solubility Aaa ae \ Ee eRSEnY. Type SIStelle arise per mare Soda-lime feldspars Potash-soda feldspars INIDISTS 5 sas cota eee ae Plagioclase series | Perthite series _ SYSUC RRS aoe eee Potash-barium feldspars Potash- lime feldspars INAIND SS elt Hig eee Re Cee Hyalophane series No name* * The writer proposes on a later page the term ‘‘oranite.” TWO-COMPONENT SYSTEMS THE SODA-LIME FELDSPARS—-PLAGIOCLASE SERIES The soda-lime feldspars or the plagioclase series is the best known isomorphous series in the mineralogy of the rock-forming minerals. Tschermak in 1864 propounded the theory that the plagioclase feldspars are isomorphous mixtures of albite and anor- thite as is indicated by the formula: m(NaAISi,Os) +2(CaAL,Si,Os). Vogt developed this theory, and established it from indirect evi- dence. It was experimentally demonstrated by the classical work 214 HAROLD L. ALLING THERMO-EQUILIBRIUM DIAGRAM OF THE PLAGIOCLASE FELDSPARS (Ame |): AP ames | |: aR ey See |: Sh == Ln AB OLIG AWD LAB BYT AN Fic. 2.—Thermo-equilibrium diagram of the plagioclase feldspars, based upon a molecular percentage (after Bowen) THE MINERALOGRAPHY OF THE FELDSPARS 215 of Allen and Day. Figure 2 is the equilibrium diagram? of the soda-lime series, based upon a molecular percentage instead of a weight percentage as has been done in all the other diagrams. The study of the solid-solution type of diagram shows us that the resulting crystals freezing from a mutual solution will not be homogeneous in composition, unless simultaneous or subsequent adjustment takes place, and consequently will vary in their optical properties as may be seen in zonal-grown crystals. But in deep- seated igneous rocks where the time of cooling is long, the non- homogeneous crystals gradually become uniform in composition by readjustment or exchange with each other and with the surrounding liquid. This process occurring between crystals is known as diffusion. All gradations between beautifully zoned plagioclase crystals and perfectly homogeneous ones occur in nature. The degree of homogeneity is therefore a function of the rate of chill, or, as the metallurgists would say, a measure of the rapidity of the quenching of the silicate alloy. Some zonal textures are, however, due to more complex processes such as the reabsorption of the margins of the already solid crystals and the adding of a new coat- ing or layer deposited thereon. ‘The writer believes that undue emphasis has been paid to the latter explanation of such textures and entertains the view that a large proportion of zonal crystals are not due to “‘magmatic corrosion,” as it is called, but to normal magmatic crystallization under the influence of rapid chill. Sometimes the crystal zones are sharply defined, each possessing fairly constant physical and chemical properties. ‘These can be explained by irregularities in the rate of cooling. The reversal of the order of zoning may be due to undercooling or the exposure of crystals to a liquid of a composition different from that of the one from which they crystallized. The Properties of the Soda-Lime Feldspars.—It would seem as though our present knowledge of the physical properties of the plagioclase series was as complete as could be desired. Through tA. L. Day and E. T. Allen, “The Isomorphism and Thermal Properties of the [Plagioclase] Feldspars,” Carnegie Inst. Pub. 31 (1905). ’ 2N. L. Bowen, ‘“‘ Melting Phenomena of the Plagioclase Feldspars,” Amer. Jour. Sci. (4), XXXV (1913), 583. 3.N. L. Bowen, Amer. Jour. Sci. (4), XXXV (1913), 507. Under the circumstances it is HAROLD L. ALLING others, we have come to possess comprehensive data, more especially d to the optical properties. the studies of Michel-Lévy, Lacroix, Fouqué, Iddings, and many 216 in regar ‘oinsy aq} Jo do} oy} 3@ WMOYS SI SerIIS dy} JO UOTJVOMIsse[D VYT, “WeiseIp wntAqymMba-ow1y} ay} JO 9AIMd snpmnbi] 94} YIM J9Yy}0904 ‘sredspjoy aseporsed sy} Jo serrzodoad [eotskyd oy} Jo Jo[qG—'f “91,7 av oO Ol od oe Ov OS 09 OL Og 06 ool AW gel 2 Lg] SW Yor yaUuIAX” - pales AY AED AG ~ E91 eG @unjeiseqwat - @ ajeoe xeratay wapuyz - T a\e9¢9 ee Pi Lal ra iS ee e Ea 100 ‘Suy xq ----- i Olo Suyxey, ——— eS ie ee ea Pale ie sees ee a Pale an oS oe Sq eae SINICCI ALMOadVYdVvT SNISHGNV SSV1009I10 FLIGW the relation between these optical and physical properties and their rather a surprising fact that very few authors correlate or show less-known thermal properties. THE MINERALOGRAPHY OF THE FELDSPARS 27 Groth’ says that the physical properties of a completely isomor- phous series “are continuous functions of their composition.” In fact it may be safe to regard a binary system to be isomorphous from a knowledge of these physical curves alone even though the system has not been subject to thermal investigation. Thus for the plagioclase series we can draw the following curves: (1) specific gravity, which in case of artificial feldspars is a straight line, (2) indices of refraction, alpha, beta, and gamma values, and (3) extinction angles on the (oro) and (001) crystallographic faces. Although these functions have long been known to petrographers it may be that their presentation in graphic form, as shown in Figure 3, demonstrates relationships which hitherto have not been sufficiently emphasized. Classification of the Plagioclase Series —The plagioclase series is classified into six subdivisions, albite, oligoclase, andesine, labradorite, bytownite, and anorthite. These subdivisions are not a random group of minerals with definite composition arbitrarily classified into a group but constitute a series of steps or gradation from one end member of the series, albite, to the other end member, anorthite, and consist of these end members in reasonably definite but differing proportions. There is, however, a lack of agreement among petrographers as to the limits of the variations that may occur in these subdivisions without altering the mineral names which have been assigned to them respectively. F. C. Calkins? has lessened the difficulty by suggesting a decimal standard of composition as a basis of classification. This method, adopted by the writer, gives the following proportions as applied to the members of this series. It will be noticed that each of the minerals varies within definite limits as to the percentage of the two con- stituents which it may contain without losing its identifying name. Albite, from Abyoo Any to Abgo Anio Oligoclase, from Aboo Ani to Abyo Anjo Andesine, from Abjyo Anjo to Abso Anso Labradorite, from Abs. Ango to Ab3o Anyo Bytownite, from Abjo Anjo to Abyo Ango Anorthite, from Aby Ang to Aby Anioo t Paul Groth, Chemical Crystallography (1906), p. 96. 2F. C. Calkins, Jour. Geol., XXV, 157-59. 218 HAROLD L, ALLING It will be observed that the term “‘albite”’ is used in two senses: as a component of the system, that is pure NaAlSi,O,; and as a mineral capable of a range in composition within definite limits. All mineralogists, consciously or unconsciously, use this dual nomenclature: (1) albite as a component; (2) albite as a mineral found in nature; a distinction necessary to make. No modifica- tion of the classification limits of the plagioclase series can reduce these two uses to one. The metallographer has a similar problem. His components are metals, his solid solutions and definite chemical compounds are ‘‘meterals.”* Meterals are strictly analogous to minerals. Consequently we see that in mineralogy there is no term corresponding to the metallographer’s term “metal.” There would be a distinct gain if we possessed a name for the components of a mineralogical system. The writer is using the word “‘minal”’ to convey the meaning here expressed. Las there is albite as a minal, and albite as a mineral. The theoretical percentages given above are based upon the assumption that the plagioclase series is a simple binary system. While such a conception is sufficient for most cases, a little study shows that natural specimens almost invariably. contain: some potash feldspar, the maximum being 10 to 12 per cent of the total in specimens near the albite end of the series, and decreasing as the percentage of anorthite increases. The potash component does not enter into the system in quite the same manner as the other two members for it is not a completely isomorphous component. Thus in classifying natural specimens one is compelled to consider the plagioclase series as a three-component rather than a two- component system. Now the question arises what is the particular form of the potash feldspar which is found in the plagioclase feldspars ? Miers’ says since most of the plagioclase [feldspars] contain potash, -we have to suppose either that monoclinic orthoclase can form [imperfect] isomorphous mixtures with triclinic plagioclase, or that the potassium feldspar is dimorphous, and that a modification exists belonging to the triclinic system and capable of entering into these mixtures. tH. M. Howe, Metallography of Steel and Cast Iron, p. 232; Albert Sauveur, Metallography and Heat Treatment of Iron and Steel, pp. 293-94. 2H. A. Miers, Mineralogy (Macmillan, 1902), p. 452. JouRNAL oF GEoLocy, VoL. XXIX, No. 3 Prarie Microperthite (hyperperthite) in augite-syenite-granite, Ausable Forks, Clinton County, New York. Polarized light. 50. Specimen 61. THE MINERALOGRAPHY OF THE FELDSPARS 219 The question is, is it orthoclase or microcline that occurs in plagio- clase? We have learned from our discussion of isomorphism that it is probably microcline. THE POTASH-SODA FELDSPARS—THE PERTHITE SERIES Before the advent of Vogt’s equilibrium diagram and Warren’s masterful discussion of the potash-soda series it was held that this system was similar to if not: identical with the plagioclase series; the two components being regarded as perfectly isomorphous.' But the Winchells? question this and say: ‘‘The close relation and gradation of optical properties corresponding to a gradation in chemical composition which exists in the plagioclase feldspars does not exist, or at least, has not been established, in the soda- potash feldspars.” Harker? states that “we have to do with WO! 5; ee pte’ series, Or- Ab, and Or-An, with a wide hiatus in the middle.” There would thus seem to be a hopeless difference of opinion regarding the potash-soda series. It will be shown later that these two extreme views are not as antagonistic as now appears. Zaptee? is right when he says that “‘very little has been written regarding perthite, or perthitic intergrowth.” J. H. L. Vogt’ was the first to give us an elaborate paper regarding the alkali feldspars, but his paper leaves many questions still obscure. Makinen® has studied the perthites from the peg- matites of Finland. But it has remained for Warren’ to sum up the status of the perthite series and to discuss it quantitatively. While the binary system, potash-soda feldspars, has not as yet been investigated thermally there seems but little doubt that tP. Macnair, Introduction to the Study of the Rocks and Guide to the Rock Col- lections in Kelvingrove Museum (1911), p. 28; J. P. Iddings, “Obsidian Cliff, Yellow- stone National Park,” U.S. Geol. Oe conc Ann. Rept., pp. 269-70; A. H. Phillips, Mineralogy (1912), p. 408. ?.N. H. and A. N. Winchell, Elements of Optical Mineralogy, p. 210. 3 Alfred Harker, Natural History of Igneous Rocks, p. 244. 4 Carl Zapfee, Econ. Geol., VII, 137. 5 J. H. L. Vogt, Tschermak’s mineral. und petrog. Mitt. (1905), p. 24. 6E. Makinen, ‘Die Granitpegmatite von Tammela in Finnland,”’ Bull. a’ Comm. Geol. de Finlande (1913), pp. 1-101. 7 Chas. H. Warren, Proc. Amer. Acad. Arts. and Sci., LI (1915), 125-54. 220 HAROLD L. ALLING it is of the general character shown in Figure 4 with the two com- ponents so regarded for the present only partially soluble in each other in the solid state, that it is a eutectiferous series. ‘The first attempts to represent the diagram of the potash-soda feldspars were based upon the assumption that each component maintained uniform physical and chemical properties throughout heating and cooling. This assumption of course eliminated the possibility of their possessing dimorphous modifications. Consequently Vogt’s diagram is extremely simple, and some of the lines are only approxi- mate. Warren however suggested that the solubility lines should be drawn somewhat inclined instead of vertical as shown by Vogt. Harker illustrates by a diagram a eutectiferous system of which one component is dimorphous but did not apply it directly to the potash-soda feldspars. Warren stated the probability of the dimorphism of the potash component, orthoclase to microcline, and constructed his diagram with this in mind. Marc* has a different conception, which, although it possesses considerable merit, cannot be discussed in detail here. For the present we can derive considerable light upon the nature of the potash-soda feldspars by considering the diagram as given by Warren (Fig. 4). In spite of the fact that the locations of the various points and the slopes of the curves of Warren’s diagram are only approximate at the best it is believed that they are sufficiently accurate for our purpose. Warren has emphasized the truth that the two crystal- line phases that make up perthitic intergrowths are solid solutions and not pure components. ‘That is if we define orthoclase as pure KAISi,Os and albite as pure NaAlSi,O; then the usual definition of perthite is not entirely satisfactory. Before the application of the phase rule to silicate systems the theory that the imbedded spindles had been introduced from without, subsequent to the solidification of the feldspar, could be regarded as plausible.* But today such an idea is abandoned by most workers in the field. In Figure 4 the composition of the original feldspar melt with an assumed value of 60 per cent K-feldspar and 4o per cent Na- feldspar is represented by the vertical line NOC. Now following t Robert Marc, Chemische Gleichgewichtslehre (1911), p. 102. 20. Wenglein, Ing. Diss. Kiel, 1903. THE MINERALOGRAPHY OF THE FELDSPARS 221 the method explained in the Appendix, the crystals will be found to have a composition of R through S toward A, as the tempera- ture falls from T to T,. The composition of the liquid on the other ‘ we — - b- - ee | 1 L ‘ \ ' e iy ‘ ' t \ iy t t H Fic. 4.—Thermo-equilibrium diagram of the potash-soda series, based upon a weight percentage (after Warren). hand is indicated by the line NOE. Let us examine the crystal— a solid solution, which is of variable composition provided there is no readjustment taking place between the crystal phases and 222 HAROLD L. ALLING the melt, each increment of which is in various degrees of satura- tion in respect to the sodium component. Then the phenomenon of diffusion (due to osmotic pressure) should take place tending to bring about a homogeneous mass. Let us suppose this diffusion takes place to completion while the temperature remains con- stant at J,. A homogeneous solid solution nearly but not abso- lutely saturated with the sodium component results. We observe that the line AL is inclined, a fact which informs us that as the tem- perature falls the solubility likewise decreases. A temperature will soon be reached where saturation is complete. As “‘exsolution’’? in solids is up to this point excessively slow, due to the high vis- cosity of that state, a condition of a supersaturation will occur. Assume that the feldspar is at normal temperature (HL/JKM1), then in the course of geologic time this supersaturated crystalline mass will gradually separate into the two phases and give perthitic intergrowth. The perfect orientation of the crystal units in the host mineral would be maintained and hence we would expect to find that albitic phase “in irregularly lenticular layers .... in planes parallel to (801) or (100) and that both feldspars could have (oro) in common.’? This is probably the common form of perthite, due to exsolution of a supersaturated solid solution. As this is not a eutectic mixture it is well to be cautious in applying the term to all intergrowths found in rock sections. In fact many intergrowths that have the appearance of being eutectics may be due to secondary or subsequent processes. Whitehead? has emphasized this and illustrates the striking similarity in appear- ance between a true eutectic of 70 per cent of silver and 30 per cent of copper, and certain intergrowths which are found in sulphide « There is no satisfactory word to express the phenomenon of the separation of two crystal phases due to supersaturation. “Precipitation”? cannot be used as it designates a chemical relation; there is no chemical reaction taking place; it is merely a physical change. Warren uses the term “‘unmixing.”’ This is not satisfactory for we are not dealing with original mixtures but with solutions. If we were observing a liquid solution we could say “crystallizes out of solution” but the homogeneous mass is already crystalline and such an expression would be unfortunate. It is really the opposite of ‘‘passing into solution,’ hence the term ‘“‘exsolution” is pro- posed. The German term is “entmischung.” 2J. P. Iddings, Rock Minerals (1911), p. 239. 3 W. L. Whitehead, Econ. Geol., XI, 1-13. cee eae = a THE MINERALOGRAPHY OF THE FELDSPARS 228 ores. His opinion is that the great majority of such intergrowths are due to secondary metasomatic replacement and not to the decrease in solubility of the components as they pass from the liquid to the solid phase. They are not eutectics in the sense in which the metallographer would use the term. C. H. Smyth, Jr.," says in regard to the microperthite con- tained in a ‘‘gneiss” from the Adirondack Mountains (which is, in all probability, the Adirondack augite-syenite of Algoman age): A very marked feature in a majority of the sections examined is the great abundance of the microperthite intergrowth of orthoclase and plagioclase. . .. . In most instances the microperthite has the appearance of that of a contemporaneous crystallization of the two feldspars; but enough sections contain absolute proof of its secondary nature to render it extremely probable that in this gneiss it is never an original intergrowth. Evidence of this second- ary origin is seen in the plagioclase spindles passing unbroken across cracks in the orthoclase and in the evident optical continuity of the material of the spindles and secondary feldspar filling cavities and cracks adjacent to the microperthite. Smyth gives the reader the impression that “contemporaneous crystallization of the two feldspars”’ and their ‘‘secondary origin” are incompatible. The writer would agree that the spindles could have formed subsequent to solidification and thus are secondary in this sense. Yet the material found in the two phases was present at the time the rock solidified. Smyth has apparently overlooked the question of the relative solubilities of the two _ phases with lowering temperature. The examination of a large number of perthitic feldspars in thin section reveals the fact that most of them did not have an original eutectic composition, but have assumed their present form through the agency of exsolution. However, on the margins of the grains or in the interstitial spaces, perthite feldspars are frequently found, and are interpreted as representing the eutectic mixture. An excellent example of this type of feldspar is the pegmatite (graphic granite) from Bedford, New York. Howe? makes the distinction between the eutectoid and the conglomerate of cementite (Fe,C) and ferrite (alpha iron) in steels. tC. H. Smyth, Jr., Trans. New Vork Acad. Sci., XII (1893), 204. 2H. M. Howe, Metallography of Steel and Cast Iron (1916), pp. 71, 161. 224 HAROLD L. ALLING The latter is the eutectic mixture. The eutectoid marks a change in the solid state and is called “‘pearlite,”’ while the eutectic is referred to as “‘primary pearlite”’ or ‘“‘ledeburite,” a distinction well to make. The eutectoid pearlite is somewhat analogous to perthite due to exsolution. As this exsolution is a slow process the appearance of the two phases may not occur until many thou- sands of years after the freezing of the magma. ‘There would be a gain for the sake of clearness if the literature of petrography can receive the term ‘‘perthoid” to refer to intergrowths of potash- soda feldspars due to exsolution, while “‘ perthite”’ could be reserved for the true eutectic. Undercooling.—Hitherto it has been assumed that each new phase made its appearance at the temperature at which it theo- retically formed. In the laboratory this condition is rarely secured, while in nature, pressure, gases, and time are factors which pro- foundly affect the crystallization of a system. Inertia often causes a melt to remain in the liquid condition although the temperature may be below the freezing-point. This phenomenon is termed ‘“‘undercooling.”’ The bridging of the eutectic gap by undercooling is discussed with the aid of the diagram shown in Figure 5. ‘Three variables are represented: temperature, composition, and degree of equi- librium; and consequently a three-dimension model is necessary for representation. The back plane, ABT, ET-., is: the normal diagram of the potash-soda feldspars, after Vogt and Warren. The plane nearest the reader is the metastable diagram when the two components A, and B, are completely soluble in the solid. The form assumed by the liquidus and solidus curves comes within Roozeboom’s classification of Type III. The analogous points of the two diagrams are connected. The field of the rear plane where two solid phases are in equilibrium, CDGOF, becomes more and more restricted in passing to the front plane and comes to an end at E,J. Stated in words, this indicates that the solubility of the two solid phases in each other becomes increasingly greater until complete solubility prevails. The converse of this is that if homogeneous crystals are formed upon freezing through the process of undercooling then because they are supersaturated in a | THE MINERALOGRAPHY OF THE FELDSPARS 225 ll c& 2 - » BE BRS Be LIL WP KL Bai | LB Nee A we —_\ ME A /\ fs SLU LILES ZA ATA ESSPo £\ WE 7 Ls Z Ls T— \ WAC ar / NL [——V7 he? SN Wi ay WES iI TAY. SE Fic. 7.—Extinction angles of the potash-soda series. The upper diagram shows values of the extinctions for the simple binaries, microcline, Mi; albite, Ab; and orthoclase, Or; albite, Ab, on the two faces, (oro) and (oor). The lower diagram shows the alkalic portions of the ternary diagram, K-, Na-, Ca-feldspars. The values of the extinction angles are shown by isogenic lines. Both diagrams are tentative. HAROLD L. ALLING 232 ‘“soda orthoclase”’ is ranges of composition. In general the term clear in its meaning. Rogers' points out a danger in employing the term soda orthoclase in that it is somewhat ambiguous, for ‘‘soda-orthoclase may mean an orthoclase in which a portion of the potassium is replaced? by sodium or it may mean that sodium compound cor- responding to orthoclase.” For the latter it is better “to use a distinctive name for the monoclinic feldspar in which sodium pre- dominates molecularly? over potassium.” For such a mineral the term barbierite has been proposed. The writer fully appreci- ates the necessity for such distinctions, and maintains that soda orthoclase should mean an orthoclasic feldspar with some sodium component dissolved in it (Winchell’s [K,Na]AISi,Os) while sodium orthoclase should refer to barbierite. The limits of the range here proposed for soda orthoclase are K-feldspar 90, Na-feldspar 10, to K-feldspar 70, Na-feldspar 30. There exists considerable uncertainty regarding the composi- tion of anorthoclase. Rosenbusch* gives as the range of this mineral the following ratios: Na-feldspar 67, K-feldspar 33- Na-feldspar 82, K-feldspar 18. These ratios suggest that anor- thoclase is analogous to soda orthoclase, that is, it is a feldspar consisting chiefly of the sodium component with an appreciable amount of potash feldspar dissolved in it. Yet a study of the available chemical analyses of “‘anorthoclase’’ would suggest that these limits should be extended farther toward the potash side of the diagram, embracing in many cases the range where, under equilibrium conditions, perthite occurs. It is quite reasonable therefore to consider that many anorthoclases are undercooled metastable solid solutions of the two alkali components. A clear tA. F. Rogers, ‘‘The Nomenclature of Minerals,” Proc. Amer. Phil. Soc., LII (1913), 610. 2 It is well to recall the objection to an expression of this kind. “Replaced” is unsatisfactory in a physical-chemical sense. Professor Johannsen informs me that he has used ‘‘proxied by” instead of “replaced by.” ‘There is no misunderstanding of that term. 3 The application of X-rays to crystal structure has shown that molecules do not exist as such in solids. 4 Rosenbusch-Iddings, Microscopic Physiography of the Rock-Forming Minerals, Pp. 340. THE MINERALOGRAPHY OF THE FELDSPARS 233 conception, such as this, seems necessary to supplant the present nomenclature which is very unsatisfactory due to its indefinite- ness. The question arises whether it is best to restrict ‘“‘anorthoclase”’ to the central portion of the equilibrium diagram and to under- cooled homogeneous crystals of perthitic composition, or to limit it to albitic feldspars containing the potash component up to a maximum of 20 per cent. After due consideration the former proposal is the one here adopted. ‘The average of the 47 available analyses of this mineral is higher than 20 per cent in the potash component and is therefore within the perthitic range. If the term “‘soda orthoclase” is used for that portion of the potash-soda series indicated by the range Or,,Ab,,-Or,,.Ab,. and the term “‘anorthoclase’’ for the range in the center of the equilibrium diagram, then it is necessary to supply a term for albitic feldspars containing the potash component up to 20 percent. For this range of solid solutions, K-feldspar 20, Na-feldspar 80- K-feldspar 5, Na-feldspar 95, the term “potash albite”’ is proposed. Anorthoclase is here used as the name to designate the range, K-feldspar 70, Na-feldspar 30- K-feldspar 20, Na-feldspar 80, when it is a supersaturated undercooled metastable homogeneous solid solution, potentially perthite, through the intermediate stages of cryptoperthite and microperthite. The emphasis that mineralogists place upon the distinction between monoclinic and triclinic crystals has resulted in consider- able confusion about the distinction between soda orthoclase, anorthoclase, and soda microcline. For example, the interesting classification of Klockmann’ will illustrate this point: Monoctinic TRICLINIC FORMULA Component* Isomorphous Mixture Component* Tsomorphous Mixture KAISi,0g....| Orthoclase | Microcline Anorthoclase WaAlsO:...| Unknown} {| -00? Orthoclase’ || Aihite PaANSrOn) Unknowne (ee... Anorthite | Plagioclase * Free translation of ‘‘Selbstandig.”” { For this modification of the sodium component the term barbierite has been proposed. ™Klockmann, Lehrbuch der Mineralogie (1912), p. 488. 234 HAROLD L. ALLING He furthermore puts in parentheses after “anorthoclase,” ‘soda microcline’’* and ‘‘microcline albite.”? A similar procedure is followed by Iddings* who speaks of the potash-soda feldspars rich in soda as ‘‘soda microcline (anorthoclase).”’ It seems to the writer that this confusion is due to the failure to recognize the possible dimorphism of each of the components in the series, that is, there may be two distinct binary systems in one: the so-called monoclinic series with orthoclase and barbierite as end members, and the triclinic potash-soda feldspars with micro- cline and albite as components. If this is recognized then it is logical that the two should be classified as follows: Monoclinic: Orthoclase, soda orthoclase, monoclinic anortho- clase, potash barbierite, and barbierite. Triclinic: Microcline, soda microcline, triclinic anorthoclase, potash albite, and albite. The interrelationship that exists between the monoclinic and triclinic anorthoclases has been pointed out by Dana‘ who says that the axial angle of anorthoclase varies with the temperature, ‘“becoming monoclinic in optical symmetry between 86° and 264°C. but again triclinic on cooling. This is true of those containing a little calcium” (anorthite). Although the writer is somewhat cautious in proposing new terms—for the literature of mineralogy and petrography is already burdened with many useless names, some of which are worse than useless—yet there has been no systematic attempt to subdivide the potash-soda series into definite ranges analogous to oligoclase, andesine, labra- dorite, etc. Calkins’ decimal principle as applied to the plagio- clase series is so logical that its application to the potash-soda series is worth attempting. The terms hypoperthite and hyperperthite may appear strange to the petrographer but the metallographer will recognize old friends. ‘The nomenclature proposed is an adaptation of the terms hypoeutectoid and hypereutectoid as applied to steels. The word eutectoperthite is self-explanatory. If such refinement in classify- ing the system is neither possible nor desirable, then perthite can t Natronmikroklin. 2 Mikroklinalbite. 3 Joseph P. Iddings, Rock Minerals, 1911, p. 235. 4 James D. Dana, System of Mineralogy, sixth ed., 1892, p. 324. THE MINERALOGRAPHY OF THE FELDSPARS 235 be substituted for the three names in the center of the table as is indicated at the sides. Monoclinic Ratios Triclinic Orthoclase OrrooAbo -OrgAbro | Microcline Soda orthoclase | Orgo Abro -OrzAb3. | Soda microcline : Hypoperthite Orjo Ab3o -Or,;Ab;; | Hypo- Perthite ee ) te perthite | (stable) . Eutectoperthite | Or,; Ab;; -Or,;Abe; | Eutecto- { Anorthoclase Anorthoclase : enetast An) perthite } (metastable) Hyperthite Or; Abs; -OrzAbg | Hyperthite Potash barbierite| Orz. Abs. -Or; Aby; | Potash albite Barbierite Or; Aby; -Oro Abyoo | Albite Some criticism may arise in that the above classification is more detailed than is warranted by the determinations possible with the petrographic microscope upon natural specimens. The writer feels, however, that with greater care and proper emphasis, microscopic distinctions are possible that will approach the accuracy now obtainable in classifying the soda-lime series. THE POTASH-LIME FELDSPARS—* ORANITE”’ SERIES One of the strange facts of mineralogy is that the potash-lime feldspars are considered rare in nature, and are little discussed in the literature. Harker’ says ‘‘the relation between orthoclase and anorthite are doubtless of the same general kind [orthoclase- albite], though the higher melting-point of the latter mineral will presumably throw the eutectic point somewhat nearer to ortho- clase.” Bayley? remarks that “mixtures’of the potash and calcium molecules? are extremely rare as minerals, but that they have been formed experimentally in the laboratory.” All available data lead us to conclude that the KAISi,Os-CaAL,Si,Os system is similar to the potash-soda series, except that we do not have to consider an isomeric modification of anorthite, which reduces the system to one of less complexity. The eutectic temperature is probably higher and the lines of solubilities are nearer to the sides of the diagram. Alfred Harker, Natural History of Igneous Rocks, p. 246. 2Wm. S. Bayley, Descriptive Mineralogy, 1917, p. 408. 3 “Components” is to be preferred for reasons already given. 236 HAROLD L. ALLING The reason why it is rare in nature is not far to seek; a rock high in potash and lime but low in soda is rare, and when found the two feldspathic phases occur as separate and distinct grains. This can be explained on the basis that melts of the potash-lime feldspars, especially when rich in the latter component, are much less viscous than the potash-soda ranges. Consequently their separation into distinct identities is much more common, and intergrowths are rare. That the latter do occur, however, the writer feels convinced. In the Adirondack Mountains the anorthosite and the augite syenite, both of Algoman age,’ occur in igneous contact with each other with such field relations as to indicate that certain batho- lithic masses of these two rocks invaded the country rocks (chiefly Grenville sediments) at about the same time, although the anor- thosite is probably the older of the two in all cases. Magmatic assimilation of the syenite by the anorthosite has taken place to some extent in zones where they adjoin one another. The feldspar of the syenite is microperthite (antiperthite) (hyperperthite) while that of the anorthosite is acid labradorite (Ab,;An,,Or;). In the syntectic rock, marking the zone of contact, microscopic examina- tion reveals intergrowths of potassic feldspar and labradorite or orthoclase with an appreciable amount of lime (‘‘lime-orthoclase”’) holding blebs of either labradorite or bytownite. It is obvious that such rocks are not “typical” and that the process of the development of these potash-lime feldspars is more involved than the simple freezing of a magma and consequently these cannot be pointed to as good examples of this rather neglected family. But to return to more normal rocks, we find that the feldspathic content of most granites, syenites, monzonites, etc., is not limited to one species of feldspar. Even a hasty petrographic study of slides of these rocks shows “‘orthoclase and plagioclase.” In the monzonites and granodiorites both alkali and plagioclase feldspars are present. Occasionally a basic representative of these carries in addition to the potassic or alkali feldspars, basic plagioclase, ™H. L. Alling, ‘‘Some Problems of the Adirondack Pre-Cambrian,” Amer. Jour. Sci. (4), XLVIII (July, tog), 62; ‘‘Geology of the Lake Clear Region,” N.Y. State Mus. Bull. 207-8 (1919), pp. 119-20. THE MINERALOGRAPHY OF THE FELDSPARS 22y7 labradorite, or even bytownite. In such rocks the feldspars are undoubtedly approaching, as a limit, the potash-lime binary sys- tem. The infrequency of intergrowths of these feldspars is the cause of the failure to recognize the system in nature. Referring to the triangular plot of the feldspar analyses (see Fig. 19), it will be seen that a few specimens called ‘‘labradorite” and “anorthite”? are approaching the side of the triangle occupied by the potash-lime feldspars. The lack of specimens of this binary system prevents any accurate attempts being made to outline or to plot the physical and optical properties. They may, however, be inferred. The specific-gravity curves are steeply inclined, extending from 2.58 for orthoclase to 2.765 for anorthite. The gravity line for microcline and soda-microcline probably does not reach the high lime ranges, and ends somewhere between the two limits. The indices of refraction likewise are more inclined. In the center of the diagram, _ these lines have no practical significance, as undercooled meta- stable crystals of analogous to anorthoclase are probably unknown or very rare in normal rocks. When we come to the nomenclature and classification of the binary, we enter virgin fields. What name shall be applied to the intergrowths described above as occurring in the Adirondacks ? Shall perthite be used? There is considerable objection to such a practice; it would be extending the meaning of a well-established term and the modern tendency is in the opposite direction. Rogers' has proposed a slight extension of the term perthite to include intergrowths of two alkali feldspars when the orientation of the blebs differs from the customary relation. This implies that a certain element of textural habit is associated with the composi- tional significance. All of this leads to the obvious conclusion that a new word is required. The writer proposes, for intergrowths of potassic and high lime plagioclase feldspars, either eutectics or due to exsolution, the term ‘“‘oranite.” Its derivation tells the composition: orthoclase-anorthite—ite, the mineralogical ending. The same name can include intergrowths of soda microcline and potash anorthite. tA. F. Rogers, Jour. Geol., XXI (1913), pp. 202-7. 238 HAROLD L. ALLING It will be seen that in order to limit the feldspar species to definite areas in a three-component ternary diagram the potash- lime series must be classified, in analogous fashion, to that already proposed for the soda-lime and potash-soda binaries. For feld- spars of the K-Ca series, analogous to soda orthoclase and potash albite, the terms “lime orthoclase” and “‘potash anorthite,’’ respec- tively, are suggested as meeting the present requirements. THE BARIUM-POTASH FELDSPARS—-HYALOPHANE SERIES The name hyalophane is said to have been proposed by Walters- hausen in 1855: from the Greek, tados, “glass,” and gavecba, “to appear,” alluding to its transparency. It is described as a barium-bearing feldspar found in transparent crystals similar to adularia. It is exceedingly rare in nature, the most famous locality being in the white dolomite of Binnenthal, where it was formed as the result of igneous contact action. The melting-point of celsian is not known, and furthermore very few suggestions have been found in the literature that would indicate the probable nature of the thermo-equilibrium diagram. Iddings gives: m(KAISi,Os), 1(BaAlSi,Os), suggesting isomor- phism similar to that possessed by the plagioclase series. Winchell gives a plot of the indices of refraction which incline toward higher values with increasing barium content, which is strictly analogous to the plagioclase system. Iddings supplies data for the construc- tion of the specific-gravity curve, which takes the form of a straight line. Clarke? quotes the opinion of Standmark: that “the mineral celsian . . . . is monoclinic and isomorphous with orthoclase.” Klockmann‘ likewise expresses the same view. In speaking of. hyalophane ina more restricted sense, Clarke says: ‘‘Hyalophane and other barium feldspars are mixtures of orthoclase and celsian.”’ In consistence with this view both Winchell and Iddings employ symbols and ratios which show a continuous range from Or to Cn, as follows: Or,, Cn;, Or,.Cn,, Or,Cn,, Or,Cn,, Or,Cn,,and Cn Figaae 1W.S. Waltershausen, Pogg. Ann., XCIV, 134. 2F. W. Clarke, U.S. Geol. Surv. Bull. 588 (1914), p. 35. 3 Standmark, Zeitschr. fiir Kryst. und Min., Vol. XL (1907), p. 89. 4 Klockmann, ‘‘Isomorphe mischung von K,Al1,SisOx6 mit BaAl,Si,Os,” Lehrbuch der Mineralogie (1912). THE MINERALOGRAPHY OF THE FELDSPARS 239 HYALOPHANE SERIES —-— -Exti net. Angles —— Specific Gr Indices. Refr 10 20 30 40 50 60 70 80 90 90 80 70 60 SO 40 0 20 les Or Barium Restricted Potash Cn Orthoclase Hyalopbhane Celsian Fic. 8.—Plot of the physical properties of the hyalophane series. The uniform character of the curves implies that the orthoclase celsian system is a series of solid solutions, and that no chemical compound exists between the components. Data taken from Iddings and Winchell. 240 HAROLD L. ALLING On the other hand the conception of a definite chemical com- pound within the series has been expressed by a number of min- eralogists. Miers™ says hyalophane corresponds to the formula K,BaAl,SisO., (not isomorphous [K.,Ba]) and this can be expressed as a mixture of BaAI,Si,O3 with 2 (KAISi,Os), i.e., as compounded. of two molecules of orthoclase with one molecule of barium silicate similar to anorthite. This union is exactly analogous to the mixture of albite and anorthite in the (plagioclase) group . . . . but in hyalophane the mixture appears to be only in one definite proportion, so that the mineral is to be regarded as a double salt tather than a solid solution. A similar view is taken by Moses and Parsons? who give hyalo- phane the following formula: (K.,Ba)AL(SiO,), as though the fundamental acid was metasilicic. Let us see how such an inter- pretation is possible. If we employ one unit of the barium and two of the potash components then 2X (KAISi,Os) = K.AI,SicOy6, a trisilicate Pa etcn , an orthosilicate. Now ; | Aiba Nl ekOu_KBaAl,(SiO)s And furthermore see = (K,Ba)Al,(SiO,),, which looks like a metasilicate but it may be far from being one. By taking two trisilicate units and one orthosilicate unit we get the result. The writer must take exception to such an interpretation. Furthermore Moses and Parsons indicate by the comma between the K, and Ba that the ratio between them is not constant. In other words the ratio of the number of units of the two components varies. If it varies then the so-called formula would be much more complex and depart from the form assumed by a metasilicate. tH. A. Miers, Mineralogy, p. 461. 2A. J. Moses and C. L. Parsons, Mineralogy, Ceysialieenae and Blowpipe Analysis, fifth ed., p. 493, 1916. mid we THE MINERALOGRAPHY OF THE FELDSPARS 241 Their formula, upon the basis that the end members are orthoclase and celsian, is misleading. Groth? discusses the difficulty of distinguishing between isomor- phous “mixtures”? and compounds when there appears to be a definite and fixed ratio between the end members of a given series. In referring to the “triclinic feldspars”’ [plagioclase ?] he says: The predilection toward certain definite mixture ratios in the series named is probably connected with the fact that apparently a regular distribution of the two kinds of atomic groups provides a particularly stable equilibrium of the crystal structure, since it occurs also with isomorphous substances of completely analogous chemical constitution.? The fact that many ‘isomorphous mixtures [occur] in simple stoichio- metric proportions appear in certain cases to possess greater sta- bility than do those in other proportions.”” The modern “view of Pace structure .- - shows . +. that the” formation of a crystal from two different kinds of chemical molecules,? even though these differ very slightly from each other, will give a particularly stable structure when the molecules? take part in this formation in regularly alternating manner; since such a substance has as much right to the name of ‘molecular compound’ as to that of ‘isomor- phous mixture,’ it is evident that that view does not permit of any sharp boundary between the two ideas.’4 Groth points out the inherent failure of the usual mineralogical methods of attack to distinguish compounds from isomorphous mixtures, solid solutions. Upon such problems mineralography sheds considerable light. Unless a compound is unstable at its melting temperature, the liquidus curve assumes a maximum in the form of an arch. In all cases there is an abrupt change in the direction and slope of the curves showing the physical properties of the series. This is clearly emphasized by the CaSiO,-MgSiO, (Pseudowollastonite-Clinoenstatite) diagram.s Both of the end members of the series form eutectiferous mixtures with the com- mon double salt, diopside. If the specific-gravity curve is plotted t™P. Groth, Chemical Crystallography, translation by H. Marshall, 1906, p. 98. 2P. Groth, Neues Jahrb. f. Mineral., II (1903), 93 ff. 3 Components. 4P. Groth, Chemical Crystallography (1906), p. 105. 5 Allen and White, Amer. Jour. Sci. (4), Vol. XXVII (1909); Ferguson and Mer- win, Amer. Jour. Sci. (4), Vol. XX XVIII, August, 1919. 242 HAROLD L. ALLING in conjunction with the diagram, it will be observed that it experi- ences a sharp change in direction at the point indicating diopside. Now in the hyalophane series, we possess sufficient data to show that the specific-gravity curve is straight and therefore that there is no “molecular compound”’ between orthoclase and celsian. Consequently we must reject the theory stated by Miers, Moses, and Parsons as untenable. THREE-COMPONENT SYSTEMS TERNARY DIAGRAMS In dealing with two-component systems we have to consider two variables—the temperature and the composition. This necessitates the employment of two-dimensional diagrams. But with three components we must employ three dimensions, in other words, solid figures. ‘These are bulky affairs which are difficult to represent upon paper. ‘There are two methods by means of which this may be accomplished: first, by perspective drawings (special form of ‘“‘block diagram’’); or second, by plan drawings, ignoring the vertical co-ordinate (temperature), and projecting the liquidus surface to the base. The base is an equilateral triangle. The lines forming its side are the plan views of the binary systems. The corners, then, represent the pure components. This method, developed by Roozeboom, divides the sides of the equilateral triangle into 100 parts. The percentage composition of each of the three components, forming the ternary system, is obtained from its position and the distance of the point P (see Fig. g) from the three sides of the triangle in directions parallel to the sides. The procedure of representing a three-component mixture or solution by a point within the triangle can best be shown by an illustration. Let us suppose we wish to represent a mixture com- posed of 50 per cent of L, 30 per cent of M, and 20 per cent of N. First 50 units are measured off on the side of the triangle LM (Fig. 9) from the corner M. Let this be point A. Next the constructional line AB is drawn parallel with the line MN, as shown. From L, 30 units are measured off on the side WL. This is point C. From C the line CD is drawn parallel to LN. THE MINERALOGRAPHY OF THE FELDSPARS 243 AB and CD intersect at point P. Through P the line EF is drawn parallel to the remaining side, ML. The point P represents the composition of this ternary system. By consulting the figures at the side of the triangle the method of representation is made clear. 90 _ 80 70 60 50 40 350 O ay (0 _ 29 30 40 50 60 70 60 90 Fic. 9.—Ternary diagram, illustrating the method of indicating by a point (P) the composition of a specimen consisting of 50 per cent of L, 30 per cent of M, and 20 per cent of V. ; By means of such triangular diagrams we can represent in a most direct manner the thermo-equilibrium diagram of a three- component system. If we employ a sufficient number of these figures we could discuss the complete feldspar system with the aid of the phase rule. The following are the ternary systems that would be necessary to that end: (1) potash-soda-lime, (2) potash- soda-barium, (3) potash-soda-carnegieite, (4) soda-lime-barium, 244 HAROLD L. ALLING (5) soda-lime-carnegieite, (6) soda-barium-carnegieite, (7) lime- barium-carnegieite, (8) potash-barium-carnegieite, (9) potash- barium-lime, and (10) potash-lime-carnegieite. As it is not possible, within the limits of the present paper, to discuss all of these ternary systems, we are compelled to confine our considerations to the most important series, the potash-soda- lime feldspars. THE POTASH-SODA-LIME FELDSPARS The first important paper outlining the ternary system, K-, Na-, Ca-feldspars, appeared in 1905 from the pen of J. H. L. Vogt. Tracings of his original figures are here reproduced in Figure to. The two upper diagrams illustrate the conventional method of representing the space model. The plagioclase series will be ~ recognized as occupying the back plane. It will be seen that Vogt’s conception of the solidus TandTay was that it assumed a straight line, which recent laboratory work has modified to a concave one as will be recalled by consulting Figure 2. The binary solubility lines extending from the eutectic temperatures to the base of the space model, which indicate low or normal tem- peratures, are not drawn. In the plan view the ternary solubility lines, /g and kz, at eutectic temperature, are shown projected to the base. The plan view may represent the ternary diagram at eutectic temperatures, or the diagram at normal temperatures provided the binary solubility lines are vertical. Vogt’s original conception was that these lines were vertical. However, Warren has correctly suggested that these lines should be inclined, approaching the sides of the binary diagram with lowering tem- perature. This modification is called for in order to explain the formation of perthites (“‘perthoids’’) due to exsolution. The Or-An binary is more or less hypothetical. If the plan view be considered as a transverse section of the diagram cut at eutectic temperature, the areas AnAbgh and kiOr represent solid solutions of the components at this temperature. At normal temperature these areas should be more restricted than is actually shown by Vogt, occupying less space on the diagram. Positions outside of these areas represent compositions where tJ. H. L. Vogt, Tschermak’s Mineralog. und Petrogr. Mitt. (1905), pp. 24 et seq. THE MINERALOGRAPHY OF THE FELDSPARS 245 Fic. 10.—The ternary diagram of the potash-soda-lime feldspar, after Vogt. The first diagram is the perspective method of representing the solid model. The one to the right is the projection of the same to the base. The lower diagram shows the change in composition during crystallization. The circles represent the composi- tion of the first-formed feldspars, the arrows show the direction of the change in composition taken by the subsequent forming minerals, and the arrow heads them- selves indicate the composition of the last-formed minerals. 246 HAROLD L. ALLING two-phase systems coexist in equilibrium. Because of the great viscosity of the high alkali feldspars we can expect that solid solu- tions can exist as metastable systems in the regions between the ternary solubility lines, but with increased lime content, regions near An, the ability of these feldspars to undercool and remain homogeneous at normal temperatures decreases to a point where intergrowths are rare. The line EE, joins the eutectic points of the two eutectiferous binaries; and is consequently called the eutectic line. Although Vogt in the perspective drawing shows the eutectic line EE, hori- zontal, the probability is that £ is situated at a higher temperature than E,. We do not know the actual temperatures of the binary eutectics, but there is some evidence to suspect that the main portion of the solidus surface slopes down from £ to £,. Figure 11 shows the liquidus and the solidus surfaces of the ternary system projected to the base and represented by contours of equal temperatures: isotherms. These diagrams are only approximate, but in spite of that fact it must be remembered that they constitute the rather meager basis upon which our knowledge rests. Vogt pointed out that the feldspars which first crystallize from a magma are of different composition from those that form during the later stages of freezing. The early formed crystals are of the orthoclastic or microclinic type if the ratio of the potash component to the soda member plus anorthite was greater than the eutectic ratio. If the ratio, on the other hand, was less than that of the eutectic then the feldspars formed during the later stages of crys- tallization would be plagioclase. These phenomena have been indicated in Vogt’s diagram in the lower part of Figure ro. The little circles represent the composition of the first-formed feldspars, and the arrows show the direction of the change in composition taken by the subsequent forming minerals, the arrow heads them- selves indicating the composition of the last-formed minerals. The diagram clearly shows that during the freezing of a melt of eutectic composition the feldspars separate from that ratio toward both the potash and plagioclase areas. In other words the feld- spars ‘‘split along the eutectic line.” The consequences that THE MINERALOGRAPHY OF THE FELDSPARS 247 POTASH SODA- LIME FELDSPARS -F oe LIQUIDUS E 120 1530 Na-F ISOTHERMS Ca: Fic. 11.—The liquidus and solidus surfaces of the ternary system, potash-soda- lime feldspars, projected to the base and represented by contours of equal tempera- tures or isotherms. 248 HAROLD L. ALLING follow from this phenomenon are that, in the simple freezing ot a magma, the maximum number of stable feldspathic phases is two, and these possess fairly constant compositions. The feld- spathic content of igneous rocks, therefore, is comparatively simple in contrast with its complexity generally found in sedimentary rocks. This fact constitutes a useful criterion in distinguishing orthogneisses from paragneisses. The complete thermo-equilibrium space model of these feld- spars should show the dimorphism of the potash and the soda components. The discussion of this phase of the subject has shown that we do not possess sufficient data to supply the lines, the surfaces, and the spaces within the model to make it complete. The lack of this information seriously, handicaps the interpretative petrologist. It is hoped, in spite of the obvious difficulties, that laboratory experimental work may supply the missing infor- mation. Physical Properties of the Potash-Soda-Lime Feldspars.—In deal- ing with a two-component system the change in the physical properties and their relations to the change in composition is best indicated by lines upon a plane surface. In the case of three com- ponents, as here under consideration, these variables are repre- sented by surfaces. The most convenient method of indicating to the eye the nature of these surfaces is by the use of contours. On these the liquidus and the solidus surfaces may be shown by lines of equal temperature or isotherms, the value of the extinction angles by isogonic lines, and the values of the specific gravity by lines indicating equal density, etc. All of these contoured surfaces are drawn upon triangular bases showing the interrelationships of these properties to the composition itself. To be able to show with accuracy properties of the potash-soda-lime feldspars by means of these surfaces is an ideal not yet fully realized. If the reader will bear in mind that the following diagrams are conjectural because of our total lack of definite information about the potash- lime feldspars and to be studied as the stratigrapher studies and interprets his paleogeographic maps then there will be no mis- understanding in regard to them. ‘The writer feels that there is great value in the construction of these tentative diagrams, for THE MINERALOGRAPHY OF THE FELDSPARS 2409 they demonstrate, as no other method can, the nature of these minerals and the constants by means of which they are identified. Extinction Angles: Inasmuch as the extinction angles of the plagioclase and the potash-soda series constitute the most serviceable means of identification, it is important that some attempt be made to draw the isogonic lines of the orthoclase-albite- anorthite and the microcline-albite-anorthite systems for the two faces (o10) and (oor). This involves the use of four triangular diagrams, which are reproduced as diagrams 1 to 4 in Figure 12. When the extinction angle of natural specimen has been meas- ured an inspection of the proper diagram shows that considerable compositional range is indicated by its curve. The extinction angle, therefore, does not appear to constitute a conclusive identi- fication of composition. The common procedure is to ignore the least-abundant component and thus attempt to reduce the system _ toa simple binary. The writer is convinced, however, that this sacrifices considerable accuracy. It is much more accurate to assume the presence of a small amount of the third component rather than to ignore it altogether. As a result of examining nearly 1,300 analyses of feldspars the following empirical rule is offered: Albites contain an average of 6 per cent of the potash component; oligoclases, 8 per cent; andesines, 7 per cent; labra- dorites, 6 per cent; bytownites, 4 per cent; and anorthites, 3 per cent. Even though these figures are not constant (see “potash oligoclase’’ for example) yet the writer is convinced that if these are assumed the petrologist will be nearer to the truth than if the third component is ignored. A similar set of figures might be set up for the potash-soda side of the triangle. In soda orthoclase and soda microcline the percentage of the lime component is 20r3 percent. In potash albite and anorthoclases of hyperper- thitic composition the percentage of the lime component is greater, reaching in some instances as high a value as 15 or 18 per cent. This is suggested in diagram 5 in Figure 12 by the irregular line within the triangle. It represents the approximate average com- position of 954 natural feldspars actually recast and plotted. The point of intersection of this compositional line and of the measured extinction angle line represents the true composition of 250 HAROLD L. ALLING POTASH-SODA-LIME FELDSPARS Orn" EXTINCTION ANGLES P Or Fic. 12.—The diagrams, 1 to 4, are attempts to show the extinction angles of the potash-soda-lime feldspars by means of isogenic lines. Diagram 5 shows the approximate average composition of natural feldspars. The intersection of this compositional line and the proper isogenic lines determines the composition of the specimen under examination. Diagrams 6 and 7 show the specific gravities of the orthoclase-barbierite-anorthite and the microcline-albite-anorthite systems. All of these diagrams are tentative. THE MINERALOGRAPHY OF THE FELDSPARS 251 the specimen more accurately than the intersection of the isogonic line and the side of the triangle. The latter procedure is the one commonly followed although the petrologist does not express the method in these terms. An important fact, which needs to be emphasized, is that in using the conventional extinction curves of the textbooks to determine the percentage of the soda component in a plagioclase, it is not possible to determine the percentage of the lime and potash members with anywhere near the same accuracy. The reason for this fact is that the isogonic lines are nearly parallel to the potash-soda side of the triangle. It follows then that the most satisfactory means of determining the true composition. the percentage of each of the three components, is to consider that the value of the extinction angles gives the percentage of the soda component only. Knowing this fact it can be ascertained what particular subdivision of the series is being examined. By con- sulting the compositional line or by reference to the average per- centage of the potash component in the natural plagioclase, the approximate amount of the third component is found. The amount of the line component is the remainder. This can be illus- trated: Suppose the extinction angles of a specimen were found to be 23° on the (oro) face and 10° on the (oor) face. The theoretical composition of this specimen would be found from the plot shown in Figure 3 to be Ab,,An¢., but by the method here suggested the value of the Ab alone is correct. Now it has been found that most labradorites contain an average of 6 per cent of the potash com- ponent. Thus the amount of the lime member is 60 minus 6, or 54 per cent. Therefore the composition of the specimen is K-feldspar, 6; Na-feldspar, 40; Ca-feldspar, 54. The fact that the isogonic lines are nearly parallel to the potash-soda side of the triangle is the reason why it is easier to determine the composition of the plagioclase feldspars than that of the potash-soda series; for the isogonic lines intersect that side of the diagram more frequently. To state it in another way, there is a greater change in the value of the extinction angles per unit change in composition. As most petrographic determinations of extinction angles are only approximate it follows that an error in 252 HAROLD L. ALLING the extinction angles of the plagioclase feldspars does not involve such a large error in the determination of the composition as it does in the case of the potash-soda feldspars. This is, perhaps, the reason that petrographers have thought it was impossible to determine, microscopically, the chemical composition of the latter series. Specific Gravities: We have seen that the specific-gravity curves for the potash-soda series probably are represented by two non-parallel lines; the upper lines representing the density of the monoclinic modifications and the lower those of the triclinic forms. The specific gravities of the plagioclase feldspars, Ab-An, can properly be represented by a straight line inclined upward from albite to anorthite. ‘Taking the curves for these binary systems it has been possible to construct the diagrams 6 and 7 in Figure 12 for the ternary system. ‘They are instructive, even though they lack confirmation on the potash-lime side. They also illustrate the difficulty in the determination of the potash-soda series in that only a few lines intersect the potash-soda binary side, while a greater number cut the plagioclase side. The diagrams indicate an inclined flat surface of the space model which they represent. One diagram is for the orthoclase- barbierite-anorthite system; the other, the microcline-albite- anorthite feldspars. Two others might be drawn: orthoclase- albite-anorthite and microcline-barbierite-anorthite. , CLASSIFICATION It is important that we attempt to sum up the ternary system, potash-soda-lime feldspars, by offering a reasonable classification of the same. . To accomplish this the writer has had to venture upon untrodden ground and therefore realizes his limitations. To overcome some of the objections that may be raised against it on the ground that it is too comprehensive or too complicated, the scheme is offered in two forms: one which may be called the tech- nical classification and the other the popular one. They are shown in Figure 13. In proposing these classifications absolutely new names have been avoided so far as possible. For the row of areas beginning with potash oligoclase and ending with potash bytownite the THE MINERALOGRAPHY OF THE FELDSPARS 253 2 & ke) ESS POTASH/POTASH OLIGO. /ANDESINE C«Fetp Fic. 13.—Proposed schemes for classification of the potash-soda-lime feldspars. The upper diagram is the “‘technical’’ while the lower one is the “popular” scheme. Orthoclasic feldspars are not indicated; the microclinic forms being shown instead. 254 HAROLD L. ALLING writer has merely extended the term “potash” as given to the feldspar from Tyveholmen, Norway, to other plagioclasic feldspars with a potash-component content greater than 10 per cent. The prefixes hypo-, eutecto-, and hyper- have been pointed out before as adaptations of metallographic nomenclature. The term ‘‘oranite”’ is employed for feldspars of the potash-lime ranges analogous to perthite. The derivation of the term is indicated by the first two letters of orthoclase and anorthite with the mineralogical ending -ite. Thus with these prefixes and the term ‘‘oranite”’ many areas can be supplied with suitable names. In the triangles between hypoperthite and hypo-oranite the proposed terms ‘‘para- perthite” and “‘para-oranite”’ are not very satisfactory. The Greek Tapa- possesses a wide range of meanings, one of which implies ‘‘a position alongside of” and ‘‘beside of,’ which is the meaning desired here. Criticism of the term, however, may be made on the ground that the geologist and the mineralogist use paragneiss, paraschist, paragenesis, paramorphism, paramorph, etc., without attaching this significance to the prefix. It is a term which can be temporarily used until a better one is found. EXAMINATION OF CHEMICAL ANALYSES OF FELDSPARS It is very desirable to know what is the actual composition of natural specimens of feldspars, and how these have been classified. Consequently many chemical analyses have been examined. About 1,300 analyses of feldspars from all parts of the world were collected from the literature. Of this number 954 were considered to be suitable for recasting and plotting. In dealing with such a large number of analyses it was obviously impossible to recast each by first obtaining the molecular ratio of each oxide and combining them in the usual manner. Therefore short cuts to approximately the same results were used. It was first assumed that each specimen was composed of only three components, the potash, soda, and lime feldspars, excepting in case where BaO was determined, indicating the presence of celsian. The percentage of each component was determined directly from the percentage of the characteristic base. Thus all of the K,O was assumed to be in KAISi,O3; all the Na.O in NaAISi,Os, and THE MINERALOGRAPHY OF THE FELDSPARS 255 the CaO in CaAl,Si,0s. The small amounts of extraneous bases. and water, such as Fe,0,, FeO, MgO, H.O, etc., that are very fre- quently present in natural specimens, have been ignored. The following factors were used in calculating the percentage of each of the three components: Component Formula Oxide Factor Rotashwees ane KAISi,Og K,0O 16.85 Sodium....... NaAlSi,Og Na,O 11.83 Mime sec — CaAl,Si,03 CaO 20.21 The sum of these three components should be too per cent theo- retically but as a matter of fact it rarely was. The majority of these totals was about 96 per cent, indicating if the analyses were accurately made, that the specimens were only 96 per cent pure feldspar. The inferior quality of many of the analyses or the probable presence of additional components in the system were emphasized when the sum of the feldspar components was con- siderably above or below 100. All those showing a total below 85 and above 110 have been rejected. The three feldspar components were then proportionately raised or lowered to 100 per cent. All the calculations were performed on a 20-inch slide rule; the errors resulting from its use being within the limits of the chemical analyses and within plotting range upon the triangular co-ordinate base used (Fig. 19). It is known that not all of the Na,O is necessarily in NaAl1Si,Os in every case. ‘This base may be present in the form of nephelite or carnegieite in addition to the albite. Foot and Bradley’ have pointed out such a possibility and say: “‘Albite sometimes occurs, associated with an excess of the constituents . . . . either in free condition, as corundum or silica, or in combination as nephelite.”’ When the sum of the three feldspar components, calculated from a reliable chemical analysis, exceeds 100 per cent then the number of components is probably in excess of the three assumed. This con- dition is far more common than is usually supposed. Washington t Foot and Bradley, ‘‘On Solid Solutions in Minerals, III,” ‘‘The Constant Com- position of Albite,” Amer. Jour. Sci. (4), XXXVI, 47. 256 ‘ HAROLD L. ALLING and Wright" have shown that the plagioclase from Limosa contains an appreciable amount of carnegieite in solid solution. The chemical analysis is as follows: SIO see amen ere en ete Sona) (CaO ie Sean) nee 10.66 ALE Oya a Pecienaieres rey eterno 20.450 NaO: ict A eee 5-40 Bez O a rrennia Uiyar. attest elo (65. » RaQ. Uh ea 74 COWS tvs eee cae ec mnt xcs 17 | FLO: 2 s-ctis eee 36 I [eed Oe aies ee BRS Baa By Onan ete ee .05 Total... tk cee 100.30 They recast the analyses in terms of four components: Potash=component. ces... - KAIST Os rare eee 4.48 Sodium component........-: IN@ATSIEOs reese eee 36.16 Mimeicomponcnte seem aie CaAlSiO; eee sane CAGMEPIEIt eC. yah ines nai ae Nal ALSO; ee eee 5-58 If, however, the presence of the carnegieite in the feldspar was not suspected and the analysis recast on the basis of only the three components then the result would be as follows: RotashicomipPoncnte ean ree 4.48 Sodium componente rere ee 54.20 Litas GOmANOOMGMNE, okodncsodoobenzscade 53-78 with the excessive total of 112.46 per cent.. A short method of obtaining the proper composition of a carnegieite bearing feldspar can be secured by setting up the ratio, Na-component without carnegieite: 1 = Na-component with carnegieite: .6667. Because of the probability that many of the analyses here recast and plotted, even though tested by the method above men- tioned, are inferior to those now being made in many laboratories, there are limitations to the conclusions that can be safely drawn from their study and comparison. Yet it is believed that they illustrate beyond much doubt that the term “‘orthoclase” is used in a very loose manner, quite inconsistent with present-day stand- ards. Fair maximum and minimum limits for the range assumed by natural “‘orthoclase”’ specimens among the analyses examined were: K-feldspar Na-feldspar Ca-feldspar AONE S sitters 87.60 ies (OS E35 CORE Saher: 49.20 48 .60 220 « H. S. Washington and F. E. Wright, “A Feldspar from Limosa and the Existence of a Soda-Anorthite (Carnegieite),” Amer. Jour. Sci. (4), XXIX (1910), 52-70. THE MINERALOGRAPHY OF THE FELDSPARS Del Furthermore in only a very few cases where the three oxides, Na,O, K.O, and especially CaO, had been looked for, was any one of the three components entirely wanting. Such a revelation may not be surprising to the petrographer for he knows that it is usually impossible to determine with a microscope the composition of the alkali feldspars with anything like the accuracy obtainable in case of the plagioclase series. The writer has secured some sixty specimens of so-called ortho- clase from many world-famous localities and has satisfied himself after careful petrographic examination of them in thin sections and in crushed fragments that in the majority of cases the mineral is not orthoclase at all, but that it is a microcline relatively high in soda and more frequently a microclinic perthite. It is but a reasonable assumption, therefore, that the specimens which fur- nished the material from which the analyses have been made had not been examined petrographically, for if such examinations had been made, the name orthoclase would not have been applied to them in such a careless manner. The names given in Figure 19 are the original ones published in connection with the chemical analyses which, as already stated, have been secured from many sources. The works on mineralogy by Dana, Hintze, Bayley, etc., have contributed many. Various bulletins of the state and federal geological surveys have been consulted. The volume of the Asches on The Silicates in Chemistry and Commerce has fur- nished a considerable number of analyses of the plagioclase series. The analyses themselves are not here reproduced but the references to the literature are given in the bibliography. Hintze does not distinguish the analyses of orthoclase from those of microcline, grouping them together. ‘This necessitates the symbol for ‘‘orthoclase and microcline.” The purpose of this triangle diagram did not warrant an extreme degree of accuracy and consequently the circles have been located as close to the actual recast figures as possible without any overlapping, which would cause undue confusion in recognizing the different species there represented. In many ways the diagram (Fig. 19) speaks for itself. It clearly indicates that most if not nearly all feldspar specimens are three-component systems. 258 HAROLD L. ALLING MICROSCOPIC EXAMINATION OF NATURAL FELDSPARS The present section contains the results of microscopic and chemical analyses of typical feldspars from many parts of the world. The optical properties of the feldspars were only determined to the extent necessary for an identification of the species. The extinction angles on (oro) and (oor) were determined by Michel- Levy’s “statistical”? method* on crushed fragments, which were properly sized by passing through sieves of 100 mesh, caught on screens of 120 mesh, and then placed in a suitable mounting fluid —such as clove oil. A large share of the determinations were made with monochromatic (sodium) light and the stage of the petrographic microscope rotated from each position to extinction. The recorded results are the averages of five readings. The orientation of the fragments was ascertained in the following manner. The faces were recognized by the cleavage, the inter- ference figure, and the twinning. The fragments broken into plates parallel to the base (cor) were found to be more common than those parallel to (o10); the former were often identified by traces of the albite twinning. The (oro) faces are more likely to have parallel edges due to the cleavage. | For this experimental work the writer naturally chose speci- mens of feldspars whose chemical composition was known, and whose chemical analyses were later recast into percentages of the three feldspar components. Upon this basis the interrelationship of the optical properties and the composition were ascertained which enabled the writer to draw the diagrams of the extinction angles of the system. The reader must remember that the close check between the chemical analyses and that inferred from the optical characters is the result of using the chemical analyses for the purpose of establishing the relations between chemical com- position and physical properties, and so lead to methods of iden- tification. The percentage of the feldspar phases, when more than one was present in the specimens, was determined by a method analogous * Michel-Levy, ‘“‘De Vemploi du microscope polarisant a lumiére parallele pour Vétude des plaques minces les roches eruptives,” Ann. des Mines (December, 1877), PP: 392; 471. THE MINERALOGRAPHY OF THE FELDSPARS 259 to that developed by Rosiwal. The outlines of the different phases in a given field of the thin section were traced upon paper with the aid of a camera lucida. The areas of these grains were measured by a polar planimeter. The sum of the areas occupied by the grains of the different minerals was assumed to be propor- tional to their volumes. By multiplying the volumes by the specific gravities of the minerals the proportion by weight was secured and then calculated to too per cent. Usually four different micro- _ scopic fields in each slide were analyzed and their results averaged. Care was taken to use an optical system (objectives and oculars) which would give the largest practical field. The composition of each phase was determined by the extinction angles of crushed fragments. The first portion of each table shows the percentage of each of the phases present and their composition. ‘The composi- tion of each phase is given in percentages of each of the three feld- spar components, totaling roo per cent. In the second portion of the tables the percentage of each component is calculated upon a basis of 100 per cent for the entire specimen. The sum of the different components thus obtained gives the composition of the entire specimen after the manner of a recast chemical analysis. Consequently a chemical analysis, recast, would have to be rear- ranged by distributing the components into the various phases present in order to appreciate the true nature of the specimen. This indicates that the perthitic feldspars are much more com- plicated than is generally thought. The accuracy of the proper distribution of the components into the phases is directly depend- ent upon the accuracy of the thermo-equilibrium diagram and the degree of undercooling of the feldspar system under considera- tion. Until the diagrams of these minerals can be put upon a quantitative basis our examinations will be approximate only. 260 HAROLD L. ALLING EXAMPLES OF PLAGIOCLASE FELDSPARS (Specimen 961.) Microscopic examination, thin section: Broad albite twinning. Extinction angles, crushed fragments: r. ‘“Albite,” Amelia Court House, Virginia. (o10) 18.3° (Con) iceOn Inferred composition: K-feldspar........ 1.0 Na-feldspar! 2/5.) 97.0 Ca-feldspar........ 250 CHEMICAL AND MICROSCOPIC ANALYSES SiO. Al,03 CaO MgO Na.O K.0 H.0 Fe.0; Total Te Raresee cree O7OOM ete 7/2 I.50 .03 10.01 130 era there eee 100.80 ON Ae Be pie) gee O8/544 iy LOSZ5 hos elles wena Il.67 343 ek oh eee 99.890 BR ew varies: 68.22 | 19.06 TAOn 5 Sih II.47 220 .69 -15 | 100.19 K-feldspar Na-feldspar Ca-feldspar iG Aloe orca a6 2.24 90.60 7.14 DESPA pe eta ane 2.49 O7-52..\ «| (esl ene eee eee BONG ee rad ele? 96.9 I.9 Microscopic 1.0 97.0 2.0 Classification: Albite. Foote and Bradley, Amer. Jour. Sci. (4), XXXVI (1913), 47, Chem. News, XLVI (1882), 204. Dana, System of Analyses I and 2. after Robertson and Musgrave. Mineralogy. Albite 15. Analysis 3. Allen and Day, Carnegie Inst. Pub. 31, p. 48, G. P. Merrill. 2. ‘‘Oligoclase,”’ Arendal, Norway. (Specimen 960.) Microscopic examination, thin section: Perfectly normal plagioclase with albite twinning. Extinction angles, crushed fragments: (010) 4.0° (001) 1.0° Index of refraction, beta: 1.547 Inferred composition, ignoring potash component Ab,;An,;. Inferred composition, considering potash component: Kefeldsparw. ssn er 10.0 Na-feldspar;...... 75.0 Ca-feldspar....... 15.0 JOURNAL oF GEoLocy, VoL. XXIX, No. 3 JPicom, JUL A. Albite, Amelia Court House, Virginia. Polarized light. _ X30. Specimen 961. B. Labradorite, near Nain, Labrador. Polarized light. X30. Specimen 967. THE MINERALOGRAPHY OF THE FELDSPARS 261 CHEMICAL AND MICROSCOPIC ANALYSES CaO K.0 Na.O MgO H.0O Total 2.60 GSO 1 GoOB WS oao0'50 .90 | 100.00 BoA, || Ds) || Os VUhallasnchoe IOI.37 K-feldspar SiOz AO; Ti o.6 SOR ORE 63.53 | 24.05 2 6 5.0 OC eR En 63.51 | 23.00 Pep eyitesleie cokes 8 11.8 2 oc og SO eae Tm Microscopic..... 10.0 Na-feldspar 74-5 13-7 76.0 hat 6 9/ 75.0 I5.0 Classification: Oligoclase. Analysis fr. Asch, The Silicates in Chemistry and Commerce. which are duplicated. Asch 9, Des Cloizeaux, Bull. Soc. Min. 7 (1884), 225. Analyses 9 and 34, Asch 34, Rosales, Pogg. Ann. (1842), 55, 109. Analyzed by Dirvell. Analysis 2. lyzed by Hagen. Asch, ibid., Analysis 71, Hagen, Pogg. Ann. 44 (1838), 329. Ana- See Dana, System of Mineralogy. Extinctions on: ARENDAL CaO Percentage oor Oo 2.50 0-2 IO-12.5 2.60 O-1.5 Q-I2 2.81 o-2 IO-12 4.20 .5-1I 2-4 3. “Oligoclase, Sunstone,” Tvedestrand, Norway. (Specimen 970.) Microscopic examination, thin section: Oligoclase with inclusions of hema- tite, which are in all probability due to exsolution.t Extinction angles, crushed fragments: Mallard* Schuster ft Andersent Dana§ Alling (OHO) 2 isa. 2°—5° 3°34" Bae 2°-4° 2.5" (oor) 1°-1°27" T°r0! ig 1°30' I.0 * EK. Mallard, Bull. Soc. Min. France, IV (1880), 104. t Max Schuster, Tschernaks Min. und Petrog. Mitt., III (1880), 164. { Olaf Andersen, Amer. Jour. Sci. (4), XXX (1915), 379-80. § James D. Dana, System of Mineralogy, sixth ed., p. 336. Inferred composition from the writer’s measurements, ignoring the potash component: Ab;Anjo. Inferred composition, considering potash component: K-feldspar........ 7.0 Na-feldspar....... 70.0 @a-feldspar 2.244230 ™ Olaf Andersen, Amer. Jour. Sci. (4), XXX (1915), 379. 262 HAROLD L. ALLING CHEMICAL AND MICROSCOPIC ANALYSES SiO. Al,O; Fe.0; CaO K.0 Na.O Total (Amally sister nciie eroen Ono Ae || 2a.9%7 0.36 | 4.78 T26) 8.50 | 100.00 K-feldspar Na-feldspar Ca-feldspar Chemical occas nee 7.45 69.62 22.93 Mucrascopice sence reo FAO 70.0 23.0 Classification: Oligoclase. Analysis. James D. Dana, System of Mineralogy. Oligoclase and oligoclase- albite No. 11. Asch, The Silicates in Chemistry and Commerce. No. 40. Scheerer, Poggendorff’s Annalen, 64, 1845, 153. (Specimen 967.) Microscopic examination, thin section: Normal plagioclase with albite twinning. Small inclusions of rutile, ilmenite (?), diopside, and hematite. Extinction angles, crushed fragments: 4. “Labradorite,”’ near Nain, Labrador. (010) 20° (Con))ys 25 Maximum, Zone | (010) 33° Inferred composition, ignoring potash component: Ab,,Ans¢. Inferred composition, considering potash component: Kefeldspatan seemer 250 Na-feldspar....... 44.0 Ca-feldspar....... 54.0 CHEMICAL AND MICROSCOPIC ANALYSES SiO: Al.O3 CaO Na.O K.0 Ign Fe.0; | MgO Total Deer yckaeel FORGOn 27/50 LOmLOn 5 OO PAOy | ated -70 Io 99.80 De ee reerate ten nraee FOSS || 27oRS || WO88 | S07 £6 Benes 1.38 Aa 100.75 (ae a eaiale enamine A Sin |i Onl | LOMOOM |G inne 58 .56 269) eee T00.02 K-feldspar Na-feldspar Ca-feldspar Di evvaearsuchaicintens 2.50 44.70 52.41 Di trsvatalaler gainers 2.21 45.10 52.61 CTE ICTR GRE ORCE BRS 43.80 Fenon Microscopic. . 2.0 44.0 54.0 Classification: Labradorite. Analyses. James D. Dana, System of Mineralogy, Labradorite Nos. 23, 24, 25. THE MINERALOGRAPHY OF THE FELDSPARS 263 EXAMPLES OF POTASH-SODA FELDSPARS 1. “Orthoclase,” “Adularia coated with chlorite,’ Scopi, Switzerland. (Speci- men 988.) Microscopic examination, thin section: Clear, untwinned potassic feldspar. Extinction angles, crushed fragments: (or0) Shon (cor) °.4° Inferred composition, considering potash component: Kefeldsparseen eee g2.0 INa-teldspar.. 22... 6.0 @azfeldspareeoee 250 This is one of the very few specimens examined to which the term “‘ortho- clase” can be assigned in accordance with the nomenclature here adopted. Classification: True orthoclase. 2. “Chesterlite,” Poor House Quarry, West Chester, Pa. (Specimen 997.) Microscopic examination, thin section: Broad phantom twinning, suggest- ing microcline. Extinction angles, crushed fragments: (o10) 6.0° (oor) WO) Inferred composition, considering the potash component: Keteldsparaeerc see: 83.0 INa-teldspars same ce 14.0 @a-feldspars: a0. 4-4- 3.0 CHEMICAL AND MICROSCOPIC ANALYSES SiO: ALO; K.0O Na:O Ign Fe.0 CaO MgO Total yi. 5 See ene 64.97 | 17.65 | 14.02 | 1.69 .65 .50 .50 .27 | 100.36 K-feldspar Na-feldspar Ca-feldspar Chemical... . 82.8 14.2 Lee BO Microscopic. . 83.0 14.0 BES Classification: Popular and technical: Soda microcline. Analysis. James D. Dana, System of Mineralogy. Miicrocline 5. Hintze, Handbuch der Mineralogie. ‘“‘Orthoclase and Microcline Kalifeldspath,’’ CCXCII. 264 HAROLD L. ALLING 3. “Orthoclase, var. Adularia,” Eggerhorn, Switzerland. (Specimen 989.) Microscopic examination, thin section: Very coarse phantom twinning with wavy extinction. Some areas comparatively free from microclinic texture, and appear like perfect development of orthoclasic feldspar. Extinction angles, crushed fragments: Untwinnedh ye. ee (One) eee 7.2 Untwinneds 2 ane ener (CoT)\s. oeciere ey Twannedl (yi. seers ee (Cro) eae 6.5 CE WAR ede a ta i ween aise (Cor) hae 17 0 Inferred composition: Keteldspat. -ote0ee 82.0 Na-feldspar® jenn 16.0 Ca-feldspars eevee 2.0 The probable character of the feldspar is that it is in the process of invert- ing from soda orthoclase to soda microcline. Classification: Soda orthoclase—soda mitrocline. 4. “Microcline,” Georgetown, Maine. (Specimen 995.) Microscopic examination, thin section: It is readily seen that the speci- men is a microclinic microperthite. Extinction angles, crushed fragments: Potash phase...55...-. (Cro) saa 6.0° (Glois) Stun TSS Soda phaseurnia eerie (CuO) ins be eee AvOw (Com enone ae ELOn Inferred Composition Potash Phase Soda Phase K-feldspar........... 88.0 I5.0 Na-feldspar.......... 5.0) 750 Ca-feldspatgaerrente 7.0 10.0 QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage K-feldspar Na-feldspar Ca-feldspar Soda microcline........... 83.5 88.0 | 5.0 7.0 Oligoclasehwen neta wee 16.5 I5.0 75.0 10.0 K-feldspar Na-feldspar Ca-feldspar Soda microcline........... WB 4.2 [Pe RHE ORS foo cicc.c Olizoclasé Asn eee eee Do & 12.3 1.6) oleate sree JOURNAL oF GEoLocy, VoL. XXIX, No. 3 Pais, JUL eer 1) 166 ee Nepyeommmemmt ce et mete SE £ A. Soda orthoclase inverting to soda microcline, called “Orthoclase, var. Adularia,”’ Eggerhorn, Switzerland. Polarized light. X30. Specimen 989. B. Microcline microperthite (hypoperthite), called “‘Amazon- stone,” Amelia Court House, Virginia. Polarized light. 30. Specimen 992. i & e tied © tae ue THE MINERALOGRAPHY OF THE FELDSPARS 265 CHEMICAL AND MICROSCOPIC ANALYSES SiO. Al.O; Fe.0; CaO Na.O K.0 Ign Total 5000 0.00 CE nen OS SON LOMSON eeaeeieieLesS 6&5 || 2207 llosooaell @@otert 10690 000 65.23 | 20.09 oF® lor ocooll Boe) || wir Co -36 | 99.99 K-feldspar Na-feldspar , Ca-feldspar 30.0 0 OG SSO Eran 85.8 5.3 8.9 2 9 0.0 0 ORO a nae 80.2 19.8 Al Soaecattee ievrooaset's Microscopic......... 76.0 16.5 7.5 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite. Analysis r. Collection of A. A. Robbins, now on exhibition in the New York State Museum, Albany, New York. Analysis 2. E.S. Bastin, U.S. Geol. Surv. Bull. 420, p. 24. . “Microcine Amazonstone,’ Amelia Court House, Amelia County, Virginia. (Specimen 992.) Microscopic examination, thin section: Microclinic microperthite. Some of the intergrowths of the potash and soda phases have an appear- ance as though they were primary, due to the freezing of the eutectic mixture. ; The albite-oligoclase occurs in stringers which vary in thickness and are not continuous. The “linkage” areas between these blebs are charac- terized by a much finer Scotch-plaid type of twinning in the soda micro- cline which appears to be akin to anorthoclase. These areas then may be regarded as a supersaturated crystalline solid solution changing to microperthite by exsolution. Extinction angles, crushed fragments: Potash phase........ (ONO) ese eres 6.4° (Geld) See aie aes Tania Soda phasemey sae (GIO) a lascan e 2 ROg (om) epee ends BO Inferred Composition Potash Phase Soda Phase K-feldspar........... 80.0 2.0 Na-feldspar.......... 18.0 88.0 Ca-feldspar.......... 2.0 10.0 266 HAROLD L. ALLING QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage K-feldspar Na-feldspar Sodapmicroclinesmeren seer QI 80 18 Albite-oligoclase........... 9 2 88 Sodasmicroclineanee renee eee ree TR 16.2 All bite-olteoclaseiaae renee eee eee see Sit ed] TRO tall sta a rdeclts note hoy kevenenoll Cerone rere 73.4 23.9 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite. Ca-feldspar 6. ‘‘Microcline,”’ Etta Mine, one mile south of Keystone, South Dakota. (Specimen 965.) Microscopic examination, thin section: Microcline microperthite with quartz, diopside, sericite, and carbonates. growths is oligoclase. + Extinction angles, crushed fragments: Potashyphasenys..--- (Gro) eee 6.0° (Son) isis eRe 17eon Sodalphasers qatar (oLo)ee HR 7 O° (Con) EME eee Bn Or Inferred Composition Potash Phase Soda Phase iKketeldspareenneeeeiee 80. 3 Na-feldspar.......... 17. 80. Ca-feldspar.......... Be 17. QUANTITATIVE MICROSCOPIC ANALYSIS The sodic phase of the inter- Phase Percentage K-feldspar Na-feldspar Ca-feldspar Soda microcline........... 87.5 80 17 3 Oligoclase We ean TD, B 80 17 Sodamicroclinesy aueen eee err ieee 70. 14.9 2.6 Oligoclase ae wha se vsverses weer eeereece ee redeces ae ag 10.0 2.12 PLO tall nse Le cee 8 cts te ate etcl leer ream Meyanae ks 7OnG 24.9 4.8 Classification: Popular: Microcline microperthite. Technical: Microcline hypoperthite. 7. “Microcline,” San Diego County, California. THE MINERALOGRAPHY OF THE FELDSPARS 267 (Specimen 958.) Microscopic examination, thin section: Soda microcline, showing but faint microclinic twinning, and in a few areas, entirely clear. Slivers removed from the specimen with the gentle application of the knife blade show no twinning. The suggestion is strong that in grinding pieces for thin sections sufficient pressure was present to hasten the inversion of the soda orthoclase to soda microcline. To test this theory crushed frag- ments were heated in a quartz crucible over a Scimatco burner for one hour, three hours, and five hours. The percentage of the twinned speci- mens was measured in each case after the fragments had cooled. The results are tabulated below: . . Number of Number of Time of Heating . = Percentage of No. in Hours Pernned Er gments Te ecaen Twinned Fragments Ths Gg Ghar eee eee ° 28 64 19.6 Do ccc CTE eae I 53 go 36.8 festivals ces css 3 56 48 54.0 ee EA eh icy efesieile''s & « 5 81 34 65.0 Extinction angles, crushed fragments: Twinned (or0) (oor) 17.0 6.5° ° Untwinned (010) 7.0° (oor) TS These observations point to the fact that the specimen is in the act of inverting from the one modification to the other. QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage K-feldspar Na-feldspar Ca-feldspar Silica Soda microcline.... 80 80 18 2 Wligoclase......... 17 2 85 TQM Taegan ioe ccensis QUWEICA cauee ae GH le ta ees encarta lan Me mt ence 100 SOG@asmicrocline, »..|.....2....- 64.0 14.4 Beatie ot Pa OI cee MT ROGIASE Ee oreotsetheoni|lcctols-a ies e ans 0.3 I4.5 Dy facing al eas a ea (QUERIED 3 5 ww 5 icy es orl ess Be rere yo aN enc Uday ls na POR eg a 3 Mota... ke al ee a eet 64.3 28.9 3.8 3.0 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite. 268 HAROLD L. ALLING 8. “Orthoclase,” Sanidine Porphyry, Drachenfels, Siebenebirge, Rhenish p Prussia. (Specimen 986.) Microscopic examination, thin section: The slide slows phenocrysts of sanidine, usually zonally grown, the central] portion of which is slightly more potassic than the margins. Extinction angles, crushed fragments, the average of a large number. (o10) O23) (oor) 6.0° Inferred composition: ; . Kfeldsparn Wages oes 62 Na-teldspareauscjcri tre 34 Cazteldspan em eemer 4 CHEMICAL AND MICROSCOPIC ANALYSES SiO. Al,03 CaO K.0 Na.O | MgO Ign Total 65.87 | 18.53 -95 | 10.32 | 3.02 -39 -44 | 99.92 ° * K-feldspar Na-feldspar Ca-feldspar Chemicaleeeseseeeae 61.6 33-7 4.7 Microscopic.......: 62.0 34.0 4.0 Classification: Anorthoclase, potentially hypoperthite. Analysis. James D. Dana, System of Mineralogy. Orthoclase No.6. (Rg. Min. Ch. 1003, 1860.) g. “Sunstone,” Delaware County, Pennsylvania. (Specimen F 3-974.) Microscopic examination, thin section: The slide reveals that the specimen is microclinic perthite of two periods of development. The microcline intergrown with the albitic feldspar of the first generation is holding blebs of soda-rich feldspar that are clearly the result of a later development. It is believed that the albite phase of the second generation and the small flakes of hematite are due to the decrease in solubility of these constituents of the solid phase—they are due to exsolution. Extinction angles, crushed fragments: Potash phase........ (Gio) Se ee ee SS? (Com) scr. T7107 SOG dup haschrr 3 see (GIO) een 21.2” (Gord anes 4.0° Inferred Composition Potash Phase Soda Phase K-feldspar........... 80 2 Na-feldspar.......... 17 97 Ca-feldspar.......... 3 I Oligoclase Microscopic examination. really a microcline microperthite with some accessory quartz. Extinction angles, crushed fragments: Potash phase Soda phase Inferred Composition K-feldspar Na-feldspar Ca-feldspar eee ee ee eo oe ee ee © © © Potash Phase 22 2 eo © © © ew © oo oe © ee © et ew eee coo ec eee Soda Phase 2 80 18 QUANTITATIVE MICROSCOPIC ANALYSIS Phase Soda microcline. . . Soda microcline. . . Percentage eee ee eee ee ee oe Classification: K-feldspar Popular: Microcline microperthite. - Technical: Microcline hypoperthite. Na-feldspar 25 80 THE MINERALOGRAPHY OF THE FELDSPARS 269 QUANTITATIVE MICROSCOPIC ANALYSIS Phase Bhasen pecan ii maecen di dase base K-feld. | Na-feld.| Ca-feld.| Fe.O; 80 fo3.5 | Soda micro. | 80 17 3} GENE Perthite {K-feld. | 6.5 | Albite 2 2 07 I feawl| eowsrcsiece ea \Na-feld. | 19.5 |(19.5 | Albite 1 2 07 Te) eet ees Hematite... .|........-- AS eeaes voce teva eauetcaerrenel eal Mrchaeaua lfareamvtictet| tdi dc 100 89 ((Sodapnni cromeams5Onon en 2e7) lez)? lll eseiers Perthit {Ka-feld. | Albite 2 st || Soi HUlf|steciage Bees \Na-feld. 19.5 (Albite x || 23. Pinan eee HVE MA LILES sralol-) << aisiers o ose sltsot i ott| leah pen ENGas Cecchi oeen cuca | ai a EUG aden rit 25 Roe Stare are ces cial eeeoucncnecaeacusied leet esroimaialemtes 60.3 | 36.7 2 55 55 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite. 10. ““Microcline,’’ Verona, Ontario, Canada. (Specimen 950.) The microscope shows that the specimen is Ca-feldspar 270 HAROLD L. ALLING rr. “Microcline-Amazonstone,”’ Mineral Hill, Pennsylvania. (Specimen 964.) Microscopic examination, thin section: Under the microscope it is readily seen that there are two generations of perthitic intergrowths. The potash phase, which is intergrown with a soda-rich feldspar, is in turn full of secondary blebs of a soda phase. This may be explained as due to the fact that complete separation of the two phases was accomplished at relatively high temperatures, while the subsequent separation has taken place at more moderate temperatures. The latter phenomenon is probably due to decreased solubility as determined by the slope of the solubility- saturation curve, line AZ in Figure 4. The fact that the “secondary” blebs of potash feldspar in the original member, rich in soda, are not seen, even under high magnifications (950 diameters), testifies to the nearly vertical character of the corresponding curve, BM in Figure 4. Extinction angles, crushed fragments: Potash phases ee oe (oro) \er Tink (Cons nce 16.9° Sodasphasea eis are (CRO) acct eee 19.0 (COT) tem ees 4.8° QUANTITATIVE MICROSCOPIC ANALYSIS | Phase Phase Aaa ea Phase pees Nea oe SiO: 85 -(|f9° Sodasmicro, || 80.0) si7onimes o 2 ui < K-feld. \10 Albite 2 BON OSHOm MeO Perthite : citnahe Na-feld 19.6 |(19.6 | Albite x 2.0152 0n MEO) @Quantzeenerre Quartz oF lia eee ee es Wich cllendia co" 5 0'¢ «oc 100 (K-feld. 8 j Soda micro: |.57 6) |\s129em meme Perthite..... { q |Albite 2 12) || eo 2) ie | Na-feld. 19.6 (Albite x 6 | 18.6 4) @uantzaneeer Quartz... WA Ne ba bas oe we lL BOs eS | eee 4 ATG tall suka setts coy scee opel estes cucees Vea cael ease here eee eR Bal | Yeinb |) 2.8 4 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite. 12. “Orthoclase,”’ near Unionville, Chester County, Pennsylvania. (Speci- men 990.) Microscopic examination, thin section: Typical microcline microperthite with inclusions of diopside, quartz, and untwinned feldspar thought to be anorthoclase. JOURNAL or GroLocy, VoL. XXIX, No. 3 Prate IV i ) i P) 4 he A. Microcline microperthite (hypoperthite), called “‘Amazon- stone,’ Mineral Hill, Pennsylvania. Polarized light. X3o. Specimen 964. B. Microcline microperthite (hypoperthite), called ‘‘ Amazon- stone,” near Florissant, California. Polarized light. 30. Specimen 971. THE MINERALOGRAPHY OF THE FELDSPARS Extinction angles, crushed fragments: Potash) phase;ass. (Gide) ea a ee 7 Oo (Com) Ramee ase 109) Oe Sodas phases sere (Chie) anes Raat 115} (Com) praia ee TO Inferred Composition | Potash Phase Soda Phase K-feldspar........... 79.0 2.0 Na-feldspar.......... 19.0 89.0 Ca-feldspar.......... 2.0 g.o QUANTITATIVE MICROSCOPIC ANALYSIS Phase . Percentage K-feldspar Na-feldspar Ca-feldspar Soda microcline.... 71.5 79.0 19.0 2.0 Oligoclase......... 24.1 2.0 89.0 9.0 CYWBUREZ So 6.6’ ict Bresctesne Oke ie eee RE MESsne | crc tse ei |e Pua Lea ge PE Soda microcline....}........... 56.5 13.6 it di, PO CIASE! sr cirsce =o se alee tes a5 2A. 242 Qi. o o'0\d tb Ghats Beal a ecneMnenO ei 6. LOIRE Goto CaniG| | re once coer cers Seana MING Gal Desvayerat pete | sichbs. = ae crete 57-0 35.0 3.6 Classification: Popular: Microcline microperthite Technical: Microcline hypoperthite 27% Silica 13. ‘““Microcline Amazonstone,” near Florissant, California. (Specimen 971.) Microscopic examination, thin section: Intergrowths of potash and soda rich feldspars. Microclinic twinning beautifully shown. Extinction angles, crushed fragments: Rotash)paasese sae a. (Cro) BB (Com) a ee 09/ () Sodas phase mmeieus eae (IO) ais oes T2Oy (Como ake mini ALOR Inferred Composition Potash Phase Soda Phase kételdspaipecrm nee : 80 3 INasteldsparen eee sa. 18 285 (anteldspakeeneeeinnne 2 12 272 Phase Soda microcline.... Oligoclase......... Soda microcline.... Oligoclase......... Classification: HAROLD L. ALLING QUANTITATIVE MICROSCOPIC ANALYSIS Percentage K-feldspar ‘ Na-feldspar Ca-feldspar 69.6 80 18 2.) Ol Ae 30.4 3 85 12° Pistepatehatenci ane 55-6 12.6 I.4 £ oh soe Pe tere et ae Ae) 25.9 3.6 2 bis soneleretae a uiraiie heel 56.5 38.5 5.0 100 Popular: Microcline microperthite. Technical: Microcline hypoperthite. 14. “Oligoclase,” Eganville, Ontario, Canada. Specimen 962.) Microscopic examination, thin section: The specimen has been incorrectly There is some oligoclase in the specimen but it is intergrown with soda microcline. identified; it is a microcline microperthite—not oligoclase. Besides the feldspar phases, biotite and quartz are present. Extinction angles, crushed fragments: Potashyphases eee. (10) eee ERIS). = (Conch eae 18 .0° Soda phases ise (Gro) axermenee Seon (Gon) hb e hone To0 LEGS Me cee Ver (OO) ie ween nee SOM Index of refraction, soda phase, beta, 1.546 Inferred Composition Potash Phase | Soda Phase K-feldspar........... 70 8 Na-feldspar.......... 28 75 Ca-feldspar.......... 2 07 QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage Na-feldspar K-feldspar Ca-feldspar Soda microcline.... 62 70 28 2). | | Ree Oligoclases.. 24... 38 8 75 iy! | ae Sodaymicroclines| seers 44.4 7 6 Ti2) | lint eee Oligoclase sty 5. ew alice Bef 27.4 6.5: |. kee APO tal Sie se aie Nee 47.6 44.7 7.7 100.0 Classification: Popular: Microcline microperthite. Technical: Microcline hypoperthite. JOURNAL OF GEoLocy, VoL. XXIX, No. 3 PLATE V * B A. Microcline microperthite (hypoperthite), called “Oligo- b) clase,” Eganville, Ontario, Canada. Polarized light. x 30. Specimen 962. B. Anorthoclase, Frederiksvarn, Norway. Polarized light. X30. Specimen o8t. 15. “Anorthoclase,” Frederiksvérn, Norway. THE MINERALOGRAPHY OF THE FELDSPARS 273 (Specimen 98T.) Microscopic examination, thin section: There is a faint suggestion of microclinic twinning on a very fine scale. Some areas appear to be entirely free and have slightly different optical characters. There are veinlets of oligoclase and stringers of soda microcline but they are too small for quantitative measurements. Extinction angles, crushed fragments: Untwinned......... (ne) eo Se ELLOy (Com)its teens 2.00 awantaedy,'.. < Seke 2c. ls (GIO) eee: Boke (Con) ier ee. eee Ook) It follows from examination of the chart, Figure 8, that the untwinned fragments are orthoclasic, that is monoclinic (?), while the twinned pieces are microclinic, triclinic (?). Inferred composition: I feldSmai was mya Marae ae Genes 45 INasteld/spanpe sya ttc assis desist 5 53 Carfeldspari: Wis wensadecis sae 2 CHEMICAL AND MICROSCOPIC ANALYSES SiO. Al.O3 Fe.03 CaO K.0 Na.O H:.0 Total HM eect cacvareiey ncaa a! « 65.19 | 19.99 63 .48 | 7.03 | 7.08 .38 | 100.78 Dé 0p OEE GOROSM IETS 7.70 seen: 587 || 7508 || ©oF4 Jocooae 99.44 K-feldspar Na-feldspar Ca-feldspar Es o SHO Nectar 44.09 57.65 2.26 Bence oD RD OGRAG 44.41 53.82 77 MVIIcrOSCOpic....-.... 45.0 53-0 2.0 Classification: Anorthoclase, potentially eutectoperthite.? 16. “Orthoclase,’ East DeKalb, St. Lawrence County, New York. (Specimen 995.) Microscopic examination, thin section: A typical microcline micro- perthite, not orthoclase. A few inclusions of apatite, calcite, phogopite, and hematite. The latter due to exsolution. Extinction angles, crushed fragments: Potash phase........ (Ciito))) een 8.0° (Gon) ee eras soe TeOw Sodayp ase cee ery (Oro) Meek oe 10.0° (oro tO ae hi. 5 t Analyses t and 2. Carl Hintze, Handbuch der Mineralogie. Kalifeldspath, CCXXIX and CCXXX, 1414, 1802. 274 HAROLD L. ALLING Inferred Composition Potash Phase Soda Phase Keteldsparses see. seer 72 IO Na-feldspar.......... 25 80 Ca-teldspatenneeeeeee 3 IO QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage K-feldspar Na-feldspar Ca-feldspar Soda microcline. . . Bit. & 72.0 25.0 3.0 Oligoclase........ 48.5 10.0 80.0 10.0 Sodasmicroclinesr|peaeee eee eee 360.0 13.0 2.0 Oligoclase: (Sa.5.5| S32 eee 5.0 39.0 5-0 pROtal ae eee eee es, Se 41.0 52.0 7.0 CHEMICAL AND MICROSCOPIC ANALYSES Analysis. A. A. Robbins, Collection on exhibition in the New York State Museum, Albany, N.Y. SiO. Al.O3 MgO CaO Na.0O K.0 H.0 SO; Total 68.60 19.82 14, .96 4.57 5.25 .30 tr 99.64 } K-feldspar Na-feldspar Ca-feldspar Chemical..... 41.5 51.8 6.7 Microscopic...| 41.0 52.0 7.0 Classification: Popular: Microcline microperthite. Technical: Microcline eutectoperthite. ” Lenni, Delaware County, Pennsylvania. 17. “Orthoclase, var. Delawarite, (Specimen 9gT.) Microscopic examination, thin sections: It is evident that this specimen is what a few mineralogists call antiperthite.t That is, the host is a soda- rich feldspar, while the blebs or “inclusions” are high in potash. The nomenclature here adopted calls for hyperperthite. This type is rare in nature. Extinction angles, crushed fragments: Soda phiasetan 2 isa (OLO) teeta 14.0° (GOT) ner Aner ANG (Sie els teticee Bee (GOT) eaten wey TOMSe t Ernst Weinschenk and R. W. Clark, Petrographic Methods (1912), p. 326. JOURNAL oF GrEoLocy, VoL. XXIX, No. 3 PLATE VI A. Microcline microperthite (eutectoperthite), called “‘Orthoclase,”” East DeKalb, St. Lawrence County, New York. Polarized light. X30. Specimen gg5x. B. Antiperthite (hyperperthite), called ‘‘Orthoclase,” var. Delawarite,”’ Delaware County, Pennsylvania. Polarized light. X30. Specimen got. THE MINERALOGRAPHY OF THE FELDSPARS 275 QUANTITATIVE MICROSCOPIC ANALYSIS Phase Percentage| K-feldspar | Na-feldspar | Ca-feldspar Silica morasimalbDlten.csck des aoe. bos 80 8 90 2 fo) Primary soda microcline.... . 10 87 se) B ° Primary oligoclase.......... 2 17 80 3B ° Ml HIME IEC edie oie. Sa aleve 2 09) 75 3 5 MOMMA Zeist 2 ese tie cieeeve races 3 ° ° ° I0o Secondary soda microcline.... 2 89 8 3 ° ovashvallottes. 22.2 nce. eco slee ens oes 6.4 71.0 1.6 fo) Primary soda microcline.....]........ 8.7 0 8} fo) Primary oligoclase.,.......:-||.4-.- i 34 1.6 -06 ° MMpvnmieksibe qe acc .s ete dee niieeaece «© 34 it§ - 06 sit BP Te gts Zep e ea ar Sut are sa eA cou il iach wa abe soe) lay anes cheer Shandy | axeest ar ovenay eyes 3-0 Secondary soda microcline...|....... 2 D5] .20 09 ° OGEILS ois Gams ect Citroen aecad eae 265] 76.30 2 Bot Classification: Popular: Antiperthite. Technical: Hyperperthite. APPLICATIONS OF THE MINERALOGRAPHY OF THE FELDSPARS TO GEOLOGICAL PROBLEMS CASE ONE—LOCATION OF A FAULT The Problem.—One of the graphite properties in the Adirondack Mountains visited by the writer in 1917! had been abandoned ‘because the ore was cut off by a fault. The writer had this infor- mation when he entered the field, but to his unpleasant surprise he was unable to locate with any satisfaction the faults even though slickensided surfaces were found on the walls of the old workings. As it was deemed very desirable to locate the faults with some degree of accuracy, the writer carried the problem into the labora- tory for solution. Method of Attack.—Preliminary examination of the graphite- bearing schist showed that it was chiefly composed of potassic feldspar. The suggestion of Rosenbusch? that the development of microcline structure in orthoclase is due to pressure was recalled. tH. L. Alling, New York State Museum Bull. 199, pp. 61, 68-70, 1918. 2 Rosenbusch-Iddings, Microscopic Physiography of the Rock Making Minerals, Dr 320. 276 HAROLD L. ALLING The writer has already discussed this phenomenon and has reached the conclusion that pressure does not produce microcline from orthoclase; it only starts and accelerates the change. This sug- gestion seemed a promising method of attack. Unless the graphite- bearing schist had suffered very severe regional metamorphism the potassic feldspar would still exist in the metastable condition which we know as orthoclase. But under the stress and jar of faulting the feldspar would take on the microclinic type of twinning as a consequence of the inversion of orthoclase to microcline. Micro-- scopic examination would locate the fault. | Another trip into the field resulted in a collection of a suite of specimens from numerous localities in and about the old workings. Petrographic Study.—Examination of the slides from these specimens showed that the writer’s supposition was entirely correct. Some were composed of orthoclasic feldspar while others showed microclinic types. Inter pretation.—It was concluded from quantitative microscopic analyses that specimens which showed a high orthoclasic content came from areas that were free from faulting, and that specimens showing soda microcline were situated in zones affected by faulting. Resulis.—In 1918, the following year, the writer took his map of the Rock Pond workings, giving the results to the petrographic study, back into the field and erected piles of stones where faulting had been deduced from the slides. From the position of these cairns it was possible to trace a group of faults that cut off the ore on three slides. Careful examination of the walls of the pits revealed conclusive evidence of the correctness of the interpretation. Slickensides and breccias were where the microscope had indicated that they should be. CASE TWO—ORTHO-AMPHIBOLITES VERSUS PARAMPHIBOLITES The Problem.—In many pre-Cambrian areas where ancient sedi- ments have been invaded by igneous rocks, and subjected to con- tact and regional metamorphism, the character of the original rocks becomes profoundly altered, both in regard to mineralogical and structural relationships, under these forces. Both limestones and calcareous shales become metamorphosed into paraschists, THE MINERALOGRAPHY OF THE FELDSPARS Dolel which are composed largely of hornblende. These are hornblende schists, or paramphibolites. In a similar manner old basic erup- tives, such as diabases, diorites, and gabbros, are metamorphosed to orthoschists and gneisses. They may be called ortho-amphibolites. “The origin of the amphibolites is a question of the highest impor- tance in the elucidation of the geology of the [Haliburton and Bancroft] area, as well as one of great interest from a petrographical standpoint . . . . [examination has] shown, beyond a doubt, that amphibolites, which, in many cases cannot be distinguished apart, have been produced by the action of granitic instrusions on lime- stone. There is also reason to believe that other amphibolites have been produced in still other ways [for there is evidence that some amphibolites are] of undoubted igneous origin.’ In the Adirondack Mountains ortho- and paramphibolites present a difficult problem. In areal mapping this problem is of scientific interest only, but when these are encountered upon mining properties the distinction between the two types becomes essential. The writer has encountered amphibolites? where it was impossible to classify the rock. In some doubtful cases the rocks were studied petrographically. Petrographic Study and Inter pretation.—Specimens were collected from rock masses where field relations pointed to a definite origin. Microscopic examinations revealed striking similarities and a few differences. ‘The similarities need not be touched upon; it is the latter that are important. If the rock is sedimentary in origin and derived from calcareous shales as Cushing suggests,} quartz would be expected to occur, as unmetamorphosed shales almost universally carry some quartz. Thus if any original quartz is present in an amphibolite, it gives it a sedimentary look, for basic (subsalic, femic) rocks are usally lacking in this mineral. On the other hand the absence of quartz suggests an igneous origin, tF..D. Adams and A. E. Barlow, Canada, Dept. Mines, Geol. Surv. Mem. 6, pp. 158-59, IgIo. 2See J. F. Kemp and H. L. Alling, ‘The Geology of the Ausable Quadrangle,” New York State Mus. Bull. (In preparation.) 3H. P. Cushing, New York State Mus. Bull. 169, p. 19, and Bull. ror, p. 15, IQI4. 278 HAROLD L. ALLING but this may not be a safe criterion, in that quartz may have been reorganized into meta- and trisilicates. Seeking for a more reliable distinction the pyroxene-amphibole (the pyribole of Johannsen)' content was examined. It is held by many geochemists? that pyroxene is a high-temperature mineral, while amphibole is a lower-temperature form. The change from one to the other being a paramorphic (or ‘“‘autometamorphic”’) one —a change readily brought about by the stresses of dynamic and static metamorphism—the inversion of pyroxene to amphibole furnishes some aid in the problems in hand. If a large amount of pyroxene, such as augite, is found in an amphibolite it suggests an igneous origin. But under the stress of severe metamorphism this inversion may be complete. Martin’ found this to be true of the amphibolite inclusions in the granitic rocks in St. Lawrence County. Thus the absence of augite does not prove a sedimentary parentage, but merely suggests it. This criterion, like the former, is therefore regarded as inconclusive. Hunting for additional criteria, the writer investigated the feldspars in turn. It was found that the igneous types usually contained a simple range of feldspars, such as ro per cent of soda orthoclase and 20 per cent of andesine, while the sedimentary rocks frequently exhibited a motley collection; cover- ing a much wider range. Very commonly soda orthoclase, soda microcline, microperthite, oligoclase, and labradorite were seen in a single microscopic slide.4 Adams’ states that the amphibolite occurring near Jack Lake, Ontario, to which an igneous origin must be ascribed, is “composed almost exclusively of hornblende, and plagioclase feldspar. The hornblende is rather light green in color in ordinary light... . . The plagioclase is clear and fresh in appearance, and rather basic t Albert Johannsen, Jour. Geol., XIX, p. 319, I9II. 2J. V. Elsden, Principles of Chemical Geology, p. 114, 1910; Becke, Tsch. Mineral und Petrog. Mitt., 16, pp. 327-36; F. W. Clarke, U.S. Geol. Surv. Bull. 616, p. 386; Lacroix, Mineralogie de la France, I (1893-95), pp. 668-69. 3 J. C. Martin, New York State Mus. Bull. 185 (1916), p. 157. 4H. L. Alling, Amer. Jour. Sci. (4), XLVIII (1919), pp. 61-62. 5F. D. Adams, “On the Origin of the Amphibolites of the Laurentian Area of Canada,” Jour. Geol., XVII (1909), pp. 1-18; F. D. Adams and A. E. Barlow, “ Geology of the Halburton and Bancroft Areas,” Province of Ontario, Canada, Dept. of Mines, Geol. Survey Branch, Mem. 6 (1910), pp. 160-61. SN eee ere ee THE MINERALOGRAPHY OF THE FELDSPARS 279 in character .. . . [and limited to] a single species .. . . ,a large proportion of [which] is frequently untwinned.”” The paramphibo- lites derived from the action of granitic intrusives and metamorphism upon Grenville limestone ‘are composed of quartz, microcline, orthoclase, and plagioclase.’* This selective habit of the feldspars is explained on the ground that in the freezing of a magma the feldspars “‘split along the eutectic line.” If the feldspar composi- tion, in the magma, was on the potash side of the eutectic line the resulting crystals would be dominantly the orthoclase type of feld- spar, while if it was on the other side plagioclase (plus a little potash feldspar) would result. But if the position of the molten feldspar was on or near the eutectic line the solid minerals would be divided on freezing into orthoclase (carrying a little soda feldspar in solid solution) and plagioclase. Conclusion.—The criteria may be summed up as follows: Sedimentary Origin Igneous Origin Original quartz High pyroxene content Motley collection of feldspars Evenly “split” feldspars How successfully these criteria have been applied to amphibo- lites whose origin was not forthcoming from the field relations cannot as yet be stated, but hope is entertained that some progress has been made in this difficult problem.’ APPENDIX? THE SOLUBILITIES OF THE FELDSPAR COMPONENTS! In order to understand the nature and construction of minerals from a mineralographic point of view, it is preferable to commence our consideration with reference to the state of homogeneous fusion. Although it may be that certain pairs of silicic salts cannot tF. D. Adams, ‘“‘On the Origin of the Amphibolites of the Laurentian Area of Canada,” Jour. Geol., XVII (1909), p. to. 2 Since this was in type a recent paper in this Journal on the ‘‘Feldspar Method’ of distinguishing sedimentary and igneous metamorphics has appeared. 3 This section is introduced to furnish the reader who is not familiar with the meaning and interpretation of thermo-equilibrium diagrams a simple explanation of their construction and value. 4’The manner of presenting this topic has been patterned very closely after that. of Rosenhain (Introduction to Physical Metallurgy, p. 78, 1915). It is of interest to note the similarity of mineralography to metallography. 280 HAROLD L. ALLING be made to mix in all proportions while in the molten condition, it is highly probable that, in the great majority of cases, they can be mixed with one another in the fluid state in any relative propor- tion. In this respect these fluids resemble such liquids as water and alcohol. We may, in fact, safely carry this analogy much further, and regard mixtures of two molten silicic salts as simple solutions of the two constituents in one another. Since our interest naturally centers in the solid mineral which results from the solidification of such melts the question which lies before us is what happens to a mutual solution of two silicic salts when the temperature is lowered so that the material undergoes solidification ? The answer is that there are two opposite modes of solidification adopted by such systems, and a range of intermediate modes con- necting these extremes. The one extreme is (a) the case in which on solidification the mineral crystallizes while still remaining a solution, i.e., the crystals which are formed ultimately attain the same composition as the molten liquid from which they crystallize. Such crystallized solutions are usually termed “‘solid solutions” in view of the fact that they are at the same time solids and solutions. This is the case when perfect isomorphism exists. Such solid solutions should not be regarded as compounds, and no single chemical formula can be employed to express the mineral as a whole. The other extreme of the mode of solidification (6) is that in which the state of solution which exists in the liquid condition is largely or entirely destroyed by the passage into the solid state, the two constituents separating more or less completely during the process of crystallization. This condition occurs when limited isomorphism prevails. THE EQUILIBRIUM DIAGRAM The most comprehensive and satisfactory method of presenting and describing the nature and constitution of minerals belonging to a given system consists in a diagram—the thermo-equilibrium diagram—which is based primarily upon thermal data. The con- struction of such a diagram is based upon the determination of the temperature of the specimen at various times during a heating or cooling process. The usual method consists in taking temperature 281 THE MINERALOGRAPHY OF THE FELDSPARS “WeISeIP WINIIqI{INba a4} JO WIOF [PUOTJUPAUOD dy} SI voIe podeys-IeSI9 oy} VTTYA ‘SoAIND ainyeiadurs}-oul} aie ‘S pur “y “Ee ‘2 ‘1 . ‘suonsed,, ay], ‘“UOT}IPUOD pI[OS 9Y} UT I9Yy}0 Yoo ut aqQnjos Ajo}0;duI0D o1@ YOIYM syusuodurod omy Jo WeiseIp WntIqI[Mbe-our19y} & Jo UOTJINIYSUOD oY} SuNeIisN{[I weiseiq—v1 ‘org =] m xz a) m Re) > = ¢ p.2) m 282 HAROLD L. ALLING readings at fixed intervals of time and then plotting the results with temperatures as ordinates and times as abscissas. A time- temperature curve is thus obtained which indicates the behavior of the mineral in a most direct way. So long as the substance is simply raised or lowered in temperature at a steady rate, this curve follows a smooth course; a departure from this smoothness indicates that there has been either an evolution or an absorption of heat within the specimen. Such a change in the shape of the curve indicates a change of state, either in phase, or in modification. Time-temperature curves are obtained for a binary system from specimens composed of the two constituents in varying amounts from 100 per cent of one to 100 per cent of the other. The con- struction of the equilibrium diagram from these time-temperature curves is illustrated in Figure 14. Five time-temperature curves are shown as partitions in the end of a box, numbered 1, 2, 3, 4, and 5. The critical points or places where the curves change in direction are indicated by A’, B, D, F, and H’. These mark the points where crystallization commences and A, C, G, and dH, indicate where the solidification is complete, if the specimen is allowed to cool. If, however, the specimen is reheated, it will theoretically at least pass through the identical behavior except in the reverse order. That is, the points A, C, G, and H are deter- mined by the initial melting, and the points A’B, D, F, and H’ by complete liquefaction. Now these time-temperature curves (partitions) enable us to construct the equilibrium diagram by projecting or drawing con- struction lines parallel with the base from the points already men- tioned back to the vertical plane, as is indicated in Figure 14. Removing the time-temperature curves, which are merely scaf- folding, the diagram remains as a conventional method of indi- cating the crystallization behavior of an isomorphous series, i.e., a series of solid solutions. In Figure 15 the opposite extreme of a binary system is shown. The diagram is constructed in the same manner, from the projec- tion of the critical points of a series of time-temperature curves. In this case the two components are completely insoluble in the solid state. It will be noticed that the upper line, A,B,£,, repre- THE MINERALOGRAPHY OF THE FELDSPARS 283. senting the freezing temperatures (and called the liquidus) falls off or is depressed as amounts of the second component are added, JUN LVEAdWIL Fic. rs.—Diagram illustrating the construction of the thermo-equilibrium diagram of a eutectiferous system until (near the center of the present diagram) a point is reached where the sum of the two components freezes at a temperature lower than in any other proportion. This is the ‘“‘eutectic point” HAROLD L. ALLING 284 ‘ ' Yeon 22 Siena it vac pmcen cass aeeccees ‘ ] Sor chogg saat aes were cae ae ae eect eewece Composi- tion of melt taken as 60 per cent of V and 40 per cent of M. For discussion see text. Fic. 16.—Thermo-equilibrium diagram of a series of solid solutions. Roozeboom’s Type I. THE MINERALOGRAPHY OF THE FELDSPARS 285 (eb, easy, Tnxrés, melting). The line PC,E£,F,L is the other curve of the diagram, the solidus. Now having described the manner in which an equilibrium diagram is constructed let us see what it can tell us of the process of solidification of an isomorphous series. Figure 16 is a diagram of such a system (Type I of Roozeboom’s classification), the time-temperature construction curves not being shown. The temperatures form the ordinate and the composition the abscissa. Above the liquidus line TyD,C,B,ATy the solution of the two components as a liquid is a mutual one, that is they mix in all proportions. Between the liquidus and the solidus—the lens- shaped area—is the area in which solidification is going on and is occupied by both liquid and solid phases. Below the solidus the system is solid. Now let us trace in detail what happens during the freezing of a melt composed of 60 per cent of WV and 40 per cent of M as is indicated by the vertical line XY. Above A the system is liquid, but as the temperature, falling, reaches the liquidus line, crystallization commences, precipitating crystals of the composi- tion A’. The composition of such a crystal is obtained by con- structing the horizontal dash line from A to the solidus. Thus the first crystal formed has a composition of A’. The remaining liquid has a composition of A. With continued lowering of the temperature the point represented by B is reached. The composi- tion of the crystal is B’ and that of the liquid is B,. Temperature Composition of Crystal Composition of Liquid IPs AW A T, IBY JB iP Cc’ CG ay, D All liquid frozen It will readily be seen that in the phenomenon above, where we are assuming, for the time being, that no adjustment of the crystals between themselves or between the crystals and the liquid takes place, the resulting crystals will have a wide range in composition, or a single crystal will be built up concentrically of zones of variable composition. It follows from the examina- tion of the diagram that the center of such crystals will have a 286 HAROLD L. ALLING composition richer in the component that possesses the higher freezing-point than the margins. Such zonal crystals are common among the plagioclase feldspars. Not all zonal textures, however, are to be explained in this way, as will be explained later in more detail. All available information leads to the conclusion that this type of diagram is the one to which the plagioclase feldspars (the soda-lime series) are to be assigned. In the field of mineralog- raphy our knowledge of other and similar systems is very incom- plete. In addition to the plagioclase feldspars, the garnets, the scapolites, the micas, the alums, and certain ranges of the pyroxene and amphibole families may be mentioned. But in the field of metallography, binary systems of solid solutions are better under- stood. The following systems may be noted: Ag-Au, Ag-Pd, Au-Pd, Bi-Sb, Co-Fe, Co-Ni, Cu-Pd, Cu-Pt, Fe-Mn, In-Pb.* There is no fundamental difference between isomorphous minerals and the alloys belonging to this group. Now let us examine in more detail the other extreme (0), that of a eutectiferous system, the two components of which are entirely insoluble in each other when in the solid state. In Figure 17 the composition of the melt chosen is the same as that used before, namely 60 per cent of N and 4o per cent of M. On cooling the melt the freezing commences at A; the resulting crystal having a composition of A, or pure NV, the liquid, a composi- tion of A. This can be expressed by saying that the composition of the crystal “varies” or “‘slides” down the line A,B,C,D, while that of the remaining liquid “slides” down the liquidus from A through B and C toward E where it freezes, but on passing to the solid phase the solution £ is rendered a mechanical mixture as the two components separate, theoretically at least, into pure NV and pure M. This mechanical mixture is termed the eutectic mixture. This tells us how such a system will look under the microscope, either in thin sections or as etched polished slabs. It will consist of crystals of pure N surrounded by minute crystals of pure M@ and pure N. The groundmass of the small crystals will be the eutectic mixture. It is possible to reverse the scheme and deter- mine the composition of the original melt by determining the 1C. H. Desch, Meiallography (1913), p. 401. THE MINERALOGRAPHY OF THE FELDSPARS 287 amount of the crystals of pure N and of the small crystals which constitute the eutectic mixture, and “sliding” back up the lines until A is reached. Y eS ES SS ES SS ee ee N 10 20 3040 50 60 70 80 90 N M90 80 70 60 SO 40 30 20 10 Mm PERCENTAGE COMPOSITION uJ ie =) FE < x Wl Oo. = uJ E Fic. 17.—Thermo-equilibrium diagram of a eutectiferous system 288 HAROLD L. ALLING We are not greatly interested in Figure 17 for it was introduced as a stepping-stone to Figure 18 with which we shall have much to do. Intermediate between the two diagrams already discussed is a type of diagram shown in Figure 18 (Roozeboom’s Type V) which represents a condition where limited solubility between the two components prevails in the solid condition. The lines FD, and GD, are the assumed solubility lines, showing the limits of solubility. Again, going through the same procedure as before, we take a melt composed of 60 per cent of N and 4o per cent of M and allow the temperature to fall to T, indicated by the point A on the liquidus. At this point the composition of the crystals separating into solid form is A. This is not pure WV as it would be in the previous Figure 17 but it is a solid solution, 91 per cent N and g per cent M, the percentage being found by dropping a perpendicular line from A; to the “Percentage Composition” scale at the bottom of the Figure 18. The liquid remaining has an approximate composition of A. As the temperature falls the change in the composition of the crystals is represented by the liquidus curve from A through B and C toward H. When the eutectic point £ is reached the eutectic mixture freezes, being com- posed of solid solution D,, 80 per cent NV and 20 per cent M, and solid solution D,, 20 per cent N and 80 per cent M. Thus the resulting solid mineral is composed of crystals—solid solutions— having a range in composition represented by the curve A,D, sur- rounded by the eutectic mixture which is composed of two solid. solutions, D, and D,. The discussion of these diagrams has been merely for the pur- pose of attempting to explain the meaning and the use of what are known as thermo-equilibrium diagrams. CONCLUSIONS 1. The application of the phase rule and thermo-equilibrium diagrams to the feldspar system enables the mineralogist and the petrographer to secure a much better conception of the true physical-chemical nature of these minerals. This method of investigation throws considerable light upon the character of many other mineralogical systems. THE MINERALOGRAPHY OF THE FELDSPARS 289 2. The thermo-equilibrium diagram of the iron-carbon binary system offers the mineralogist a method of studying the alloys which Liquid SOLUTION SOLID SOLUTION bed & = = q (° 4 ry a = fed) - M90 80 70 60 50 40 30 20 PERCENTAGE COMPOSITION Somes Fic. 18.—Thermo-equilibrium diagram of a eutectiferous, limited-solubility system. The general type probably represented by the potash-soda series. 290 HAROLD L. ALLING in his domain are called minerals. Familiarity with this system is of great assistance in clarifying the problems of mineralogy. 3. The feldspars belong to a five-component system, of which the end members are the potash, soda, lime, barium, and carnegieite feldspars. For most purposes it is only necessary to consider the potash, soda, and lime components. Every specimen of feldspar found in nature contains a certain amount of each of these three components. ; 4. The plagioclase and the hyalophane series constitute a series of solid solutions. The potash-soda and the potash-lime series possess only limited solubility, and constitute eutectiferous systems. 5. It is believed that both the potash and the soda feldspars are dimorphous, each existing in two isomeric forms: each component crystallizing either in monoclinic or triclinic modifications, depending upon the temperature and the viscosity of the magma; that ortho- clase and albite are high-temperature modifications and that microcline and possibly (?) barbierite are relatively low-tempera- ture forms. 6. The complete solubility of albite in anorthite, and vice versa, presupposes that their chemical structures are analogous, even though albite is usually regarded as a “‘trisilicate”’ and anorthite an “orthosilicate.” This presupposes that the feldspars are aluminosilicates. Since orthoclase and albite (components) are not completely soluble in each other (when perfect equilibrium prevails), they probably possess somewhat dissimilar chemical structures. 7. Some feldspars contain nephelite in solid solution but this mineral cannot be regarded as isomorphous with the normal feld- spars. ‘Therefore the mineralographic term ‘‘solid solution” is more comprehensive than the crystallographic term “‘isomorphism.” 8. All feldspars are solid solutions and mixtures of solid solu- tions, and therefore do not possess definite chemical compositions. No single chemical formula can be assigned to a single species. Inasmuch as labradorite is a mineral, the chemical composition of which is not fixed, and, furthermore, the mineral is often found zonally grown, the usual definition of a mineral as a homogeneous THE MINERALOGRAPHY OF THE FELDSPARS 291 natural inorganic substance of definite chemical composition needs revision. 9. That the plagioclase feldspars are not ‘“molecules,’’ portions of which are “replaceable” by analogous units. This statement applies with almost equal force to the potash-soda series as well. to. While the term ‘“‘mixed crystals” is frequently used to signify “‘solid solutions,” yet it should be avoided for the sake of clearness. t1. Perthite is an intergrowth of two (or more) solid solutions, and not an intergrowth of the simple components, microcline (or sometimes orthoclase) and acid plagioclase. The two phases are solid solutions, one rich in potash and the other rich in soda. 12. That most perthites are not the direct result of the freezing of a magma, but are the result of subsequent processes, where the decrease in solubility of one phase for the other with falling temperature is the principal factor. The inversion of orthoclase to microcline is regarded by some as also a contributing cause. Perthites are commonly the result of the process here called “‘exsolution.”” Such perthites (or ‘‘perthoids’”’) are analogous to pearlite in steels. 13. That many anorthoclase specimens are supersaturated, undercooled metastable solid solutions, potentially perthite through the intermediate stages of crypto- and microperthite. 14. That intergrowths of potash-rich and lime-rich solid solu- tions occasionally are found. To such intergrowths the term oranite (the first two letters of orthoclase and anorthite and the ending -ite) has been applied. 15. That the feldspars of many basic monzonites and the granodiorites are approaching, as the limit, the potash-lime binary system. But because such feldspars are not as viscous at their melting temperatures as the potash-soda series, they separate more completely into definite identities, and consequently the orantitc feldspars are not usually recognized as such. 16. The significance of the process of exsolution is that many so-called inclusions in mineral grains are due to secondary processes, and consequently are of late rather than of early development. 292 HAROLD L. ALLING This complicates the methods of determining the order of crystal- lization of the minerals in a rock. | 17. Primary perthite, due to the freezing of the eutectic mixture, analogous to ledeburite in steels, is probably uncommon in nature. 18. The potash feldspar of pegmatitic origin, the usual museum variety of “‘orthoclase,” is soda microcline and very rarely, if at all, ‘‘orthoclase.”’ In fact orthoclase (nearly pure potash feldspar without microclinic characteristics) is very rare in nature. tg. Some ‘‘adularias”’ and “microclines’’ show microclinic twin- ning when examined in thin sections, but in thin plates or in crushed fragments they do not exhibit twin striations. The suggestion is strong that the pressure, which the specimens experienced in the grinding process in the preparation of the thin section, has hastened the inversion of the metastable soda orthoclase to soda microcline. This raises the question whether complete reliance can be placed upon thin sections in the identification of the feldspar species. 20. That microclinic and orthoclasic feldspars, with a content . of the potash component higher than 85 per cent of the whole, are exceedingly rare in nature. It is far more accurate to speak of soda microcline and soda orthoclase than of microcline and ortho- clase. Very frequently specimens of so-called microcline or orthoclase are found upon examination to be perthitic as well. Before assigning a name to a museum specimen, it should be microscopically examined. 21. That the inversion of soda orthoclase to soda microcline is often hastened by the pressure set up by regional or static meta- morphism, but that the pressure does not produce soda microcline from soda orthoclase; it only initiates and accelerates the change. The tendency to change is inherent; the pressure merely starts the process. 22. All plagioclase specimens contain some potash component; the average is in the neighborhood of 5 per cent. It is more accu- rate to assume that the potash component is present to this extent than it is to ignore it altogether. The extinction angles of the soda-lime feldspars enable the petrographer to ascertain the amount of the soda component present but they do not determine the amount of the lime component. The percentage of the potash THE MINERALOGRAPHY OF THE FELDSPARS 203 feldspar is inferred from the average of the chemical analyses and the lime component is the remainder of the 100 per cent. 23. Many zoned plagioclase feldspars are to be explained by the process of normal crystallization under rapid chill instead of ““magmatic corrosion.”” Homogeneous crystals of plagioclase are probably due to readjustment between crystal phases, or between them and the surrounding unfrozen liquid, in a slowly cooling magma. ‘The degree of homogeneity is therefore a function of the rate of chill. Some zoned feldspars are undoubtedly the result of more complex processes in which the phenomenon of undercooling plays an important rdle. ° 24. The physical properties of a series of solid solutions are direct functions of the composition. If the properties, such as specific gravity, indices of refraction, extinction angles, etc., are plotted in conjunction with the thermo-equilibrium diagram they assume the form of smooth curves which rise and fall with the freezing (liquidus) curve. A break, a cusp, or a sharp change in direction in these curves at least suggests a discontinuity in the chemical properties of the system. Many involved formulas of minerals will probably be abandoned when they are shown to be solid solutions and mixtures of solid solutions of simple end mem-— bers. The formula for hyalophane, “(K,,Ba)Al,(SiO,),,” cannot be entertained as possessing any true value. | 25. In attempting to classify the feldspars on a basis of their true composition several new names have been proposed. See Figure 13 for schemes of classification. ‘‘ Anorthoclase”’ is defined in paragraph 13 and its range delimited as Or,Ab,o-Or,;Abes. The term “soda microcline” is confined to microclinic feldspars containing 10 to 30 per cent of the soda component. For albitic feldspars containing from 5 to 20 per cent of the potash component the term ‘‘potash albite”’ is proposed. 26. Because there are more contours of equal extinctions (isogonic lines) cutting the plagioclase side of the triangle (see Fig. 12, facing p. 251) than the number of those cutting the potash-soda side, the identification of the soda-lime feldspars can be accomplished with comparative ease. Nevertheless proper identification of the subspecies of the potash-soda series can be 204 HAROLD L. ALLING made by means of the extinction angles on the different faces, together with the indices of refraction. 27. The application of these physical-chemical principles to the feldspar system furnishes, as a result, the means of solving practical geological problems. Two illustrations of the application of these conclusions to actual field problems are given on pages 275 to 279 inclusive. BIBLIOGRAPHY OF CHEMICAL ANALYSES OF FELDSPARS Asch, D. and W., The Silicates in Chemistry and Commerce (1914), pp. 411-27. Barbier and Canara Bull. Soc. Franc., Mineral. (z910), XXXII, 81. Bastin, E. S., U.S. Geol. Surv. Bull. 420, p. 24. , U.S. Geol. Surv. Bull. 445, p. 123. Puce W.S., Descriptive Mineralogy, 1917. Clarke, F. W., U.S. Geol. Surv. Bull. 419, (1910), pp. 257-60. Dana, James D., System of Mineralogy. DeSchmid, H. S., Canada, Dept. of Mines, Mines Br. No. gor (1916), p. 9. Foot and Bradley, Amer. Jour. Sci (4), XXX, 151. , Amer. Jour. Sci (4), XXXVI, 47. Hintze, Carl, Handbuch der Mineralogie, III, 1406 ff. Iddings, J. P., U.S. Geol. Surv., 7th Ann. Rept., pp. 269-70. Jannettaz, E., Les Roches (Paris, 1910), p. 85. Robbins, A. A., Collection in New York State Museum, Albany, New York. Stose, G. W., and Lewis, J. V., Bull. Geol. Soc. Amer., XXVII, 642, 1916. Warren, C. H., Proc. Amer. Acad. Arts and Sci., 51, No. 3, 1915. Watts, A. S., Bur. Mines, Bull. 92, pp. 11-17, 1916. Williams, J. F., Arkansas Geol. Surv., II (1890), 58-60, 74, 76, 239-42, 271, 3590-61. Zapfee, Carl, Econ. Geol., VII, 157. POTAS @ ORTHOCLASE MICROCLINE 8 @ ORTHOCLASE 4»> MICROCLINE PERTHITE ANORTHOCLASE © ALBITE SODA FELDSPA Fic. 19.—Plot of 954 recast chemical analyses of natural feldspars. The chemical analyses and are not those which the writer would employ in many cases plagioclase feldspars contain considerable amounts of the potash component. The « sds Bi pment { POTASH FELDSPAR ® Mae (C) ® ORTHOCLASE a\% \ ® OLIGOCLASE ® MICROCLINE a8 e 9 © ANDESINE [gis Eo y c) 9 ORTHOCLASE +> MICROCLINE © LABRADORITE PERTHITE 17) BYTOWNITE © ANORTHOCLASE ANORTHITE © ALBITE UNCLASSIFIED 90 |imE FELDSPAR SODA FELDSPAR !0 i i tion with the i i ted are those published in connec Fic. r9.—Plot of 954 recast chemical analyses of natural feldspars. The names assigned to the Specie en eee Soe ae Oe aeaaeeeam chemical analyses and are not those which the writer would employ in many cases, Note the concentration of ee fs poate ence ee ee oe Plagioclase feldspars contain considerable amounts of the potash component. 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At the same time it has often been inferred that large quantities of matter have dif- fused through considerable distances. ‘This has been done for the purpose of explaining various associations of igneous rocks and especially the formation of basic border phases. Since Becker’s destructive criticism of this view’ many geologists, but not all, have been less willing to assign to diffusion any important rdéle in the production of such features. Becker’s objection was based on the extreme slowness of diffusion in all cases where its rate had been determined. The principal data then available referred to the diffusion of salts in aqueous solution, and it was on the basis of reasonable deduction t American Journal of Science (4) Vol. III (1897), p. 27- 205 296 : N. L. BOWEN from these data that Becker’s objection was raised. There has been as yet no discovery of a general principle connecting the rate of diffusion of matter in solution with other physical constants. Each substance has its coefficient of diffusivity (in a given medium) characteristic of the substance and determinable only by experi- ment. The coefficient is the constant factor, k, in the equation expressing Fick’s law of the rate of diffusion, viz., dc @¢ an” ag? and is evidently equal to the number of grams which diffuse past — I square centimeter of any plane in unit time when the concentra- tion gradient normal to the plane is unity. This is a very small quantity for substances investigated. For most salts in aqueous solution it is of the order 3 X107° in cm.’ per second at the ordinary temperature and increases rapidly with rise of temperature; for molten metals it is considerably larger and of the order 3 X10-°, again increasing with rise of temperature.” In the case of the quite different type of matter, molten silicates, it is perhaps not probable, but nevertheless not inconceivable, that the coefficient of diffusivity should be fairly large at the high temperatures concerned in spite of the usual high viscosity. Only actual measurements can make us at all certain that we have a firm basis of fact for the discussion of diffusion in molten silicates. Practically no measurements have been made. Endell has demon- strated the fact of the interdiffusion of lime and microcline glass.’ Schulze has measured the rate of migration of silver ion in glass. Indeed, it is perhaps principally from the problems of glass manu- facture that we gain our impressions of diffusion in molten silicates. There diffusion is often excessively slow, but it should be noted that glasses belong for the most part among the very viscous silicate mixtures. On account of the lack of experimental data it was considered advisable to undertake some measurements of the 1 Roberts-Austin found for the coefficient of diffusivity of gold into molten lead k=3.47X10-5 at 492°. Roy. Soc. London, Phil. Trans., Vol. 187A (1896), p. 383. See also Van Orstrand and Dewey, U.S. Geol. Survey, Prof. Paper 95-G. 1915. 2K. Endell, Silikat-Zeitschrift, Vol. I, p. 195. 3 Ann. phys., Vol. XL (1913), P- 335: DIFFUSION IN SILICATE MELTS 207 rate of diffusion in fused rock-forming silicates. Those here described are to be considered of a preliminary nature. They are not devised with the purpose of establishing precise values for diffusion coefficients from which general theoretical conclusions might be drawn, though this is recognized as a desirable ultimate goal. They may serve rather to aid the geologist in deciding what he may and what he may not reasonably attribute to diffusion. THE METHOD OF EXPERIMENT The method followed was that of permitting the diffusion against gravity of a heavy liquid placed in the lower part of a crucible into a lighter liquid in the upper part. Ostwald has said that to make accurate experiments on diffusion is one of the hardest problems in practical physics on account of the difficulty experi- enced in eliminating convection currents. It is to be noted that this is especially true of aqueous solutions. Water is a thin liquid with a relatively high coefficient of thermal expansion. The driving force of convection, viz., difference of density, is therefore large and the resistance to it (viscosity) small. In silicates, how- ever, these conditions are reversed, the viscosity being relatively great and the thermal expansion relatively small. While a small difference of temperature may establish convection in aqueous solutions, it is not to be expected to have a comparable effect in silicates. In the case of aqueous solutions the density gradient resulting from the composition gradient may be very small, but in the case of the silicates used it is quite large. If the relative densities of diopside and plagioclase liquids at high temperatures are comparable with those at lower temperatures, it can readily be shown that it would be necessary to have a temperature gradient of 20° per mm. in order to counteract the density gradient due to I per cent per mm. change of composition. Gradients of com- position of this magnitude exist throughout most of the period of experiment. Such temperature gradients are, however, entirely lacking; not only this, but it is easy to make the moderate tem- perature gradient that does exist of such a sign that it acts together with the composition gradient instead of counteracting it. This is accomplished, of course, by making the temperature of the upper 208 N. L. BOWEN part of the column higher than that of the lower part. With this end in view the crucible was suspended in most cases in a part of the furnace where the temperature increased upward. In one experiment recorded here this method of eliminating convection was replaced by a method involving the use of a bath of molten gold to obtain a uniform temperature. The platinum crucible containing the charge was protected from the molten gold by a tube of silica glass.t. The silica glass was rather soft at the tem- perature of the experiments and required reinforcement by a tube of Marquardt porcelain. The one result obtained by this method showed no significant difference from a result obtained by the method of suspending the bare crucible in the furnace. There seemed, therefore, to be no reason for preferring the use of the gold bath and it was not carried farther. To make assurance doubly sure in the way of minimizing possible convection, the charge was made small. This acts in three ways: to make a possible lateral difference of temperature small, to render difficult the initiation of convection currents, and to decrease the velocity of possible currents. The crucibles used were, therefore, 5 to 6 mm. in diameter and Io to 20 mm. deep. In order to simplify the conditions of diffusion the crucibles were right circular cylinders without flare or rounded bottoms. The temperature was kept constant partly by continual watch- ing and regulation and partly by using current from a storage battery. In some cases an automatic regulator was used, designed by W. P. White of this Laboratory. In each case the heavy material taken was diopside. It was first melted and then chilled to a firm cake of glass ( 7 2.8 54) in the bottom of the crucible. The lighter material put in the top was one of the plagioclases Ab.An,, Ab,,An,, or Ab,An, (c= 2.483, 2.533, 2.591 respectively). The temperature was in all cases about 1500°, that is, it was above the melting temperature of both layers, so that the experiments deal with diffusion of one liquid silicate into another. « The use of silica glass in this manner was suggested by J. C. Hostetter, formerly of this Laboratory, who also kindly worked the glass into the desired form. Se DIFFUSION IN SILICATE MELTS 299 The charge was raised quickly to the desired temperature by plunging it into a furnace already somewhat above that tem- perature. After holding it for the desired length of time the charge was rapidly cooled by removal from the furnace, and the composition of the glass at various levels in the crucible was determined by measuring its refractive index, the relation between composition and refractive index having been previously deter- mined on mixtures of known composition. It may appear that the drastic temperature differences that arise when the cold crucible is placed in the hot furnace would be bound to set up violent convection, but fortunately this is a matter that can easily be ascertained by running a blank test. For this purpose a charge was prepared in the ordinary manner and allowed to remain in the furnace only a few minutes, when it was removed and the distribution of composition determined. It was found that the distribution was as it should be after diffusion for a short period with no random variations such as would result from convection. In earlier experiments an ordinary thick-walled platinum crucible was used and the charge was always badly shattered in cooling. When not too numerous, the fragments were fitted together to reconstruct the original charge and the composition determined at various levels by removing a little powder with a file and determining its refractive index by the immersion method. The error involved in the measurement of the distance from the top or the bottom of the charge was large, and the results were only rough approximations. In later experiments the crucible was made of platinum foil 0.03 mm. thick. On contracting, the glass pulls the weak walls of the crucible with it and remains unshattered. The platinum foil was then peeled off and the cylin- der of glass was ground to a wedge whose edge was parallel to the axis of the cylinder. The faces of the wedge were polished and the refractive index of the glass at various points was determined on the goniometer by the method of minimum deviation. The exact distance of the points from the bottom of the cylinder was measured by means of the scale on the centering screws of the goniometer. 300 N. L. BOWEN Though numerous measurements were made by the early, rougher method, only the few made by the later method will be described. It may be noted, however, that the earlier results, within their rather large limits of error, agree with the later. In order to obtain a visual impression of the diffusion of material, such as is obtained when copper sulphate crystals are placed in the bottom of a vessel of water, the layer of diopside was in some cases colored by the addition of 1 per cent Fe,O, which imparts to the glass a marked green color. In all cases the change of color was sensibly coincident with the change of refractive index; in a layer where the index fell rapidly the color faded rapidly; in cases where an upper layer was found unaffected in refraction it was likewise uncolored.* In summary it may be stated that the mode of procedure was as follows: A layer of diopside was placed in the bottom of a crucible with a layer of plagioclase above it, diffusion was per- mitted at constant temperature (above the melting-temperature of both layers) for a definite period, the charge was quenched, and the composition determined at various depths by measuring the refractive index of the glass. GRAPHICAL PRESENTATION OF RESULTS The results (Table I) may be presented graphically as in TABLE I DIFFUSION RESULTS EXPERIMENT NO. 24 Depth of diopside layer 1.8 mm. Total deptho.5 mm. Time 48 hrs. Distance from bottom (amma) eects Oo8 Wok set eth chs} nN Oro§ ° 2 13 853 “ORS Vol. per cent diopside. 39.5 39.6 38. ih it 5-3) (O-3iees 7.5) Anommses ° Plagioclase of upper layer AbzAny. EXPERIMENT NO. 27 Depth of diopside layer 3.2 mm. Total depth 11.7 mm. Time 48 hrs. Distance from bottom (oan) Peer ese I.0 2.0 3.0 4:0 5.0 6.0 7:0 S:0mG.OMtonomsamem Vol. per cent diopside.52.0 51.6 50.7 49.6 46.5 33-7 9.7 2-4 0.7 ° ° Plagioclase of upper layer Ab2An,. «This result might be taken as indicating that selective diffusion was unimpor- tant, though not necessarily absolutely lacking. DIFFUSION IN SILICATE MELTS 301 EXPERIMENT NO. 21 Depth of diopside layer 3.9 mm. Total deptho.1 mm. Time 22 hrs. Distance from bottom (mm )is ess cas On IOC HON A TOMS On OFOMm HOM OnOn ms OnS Vol. per cent diopside.. 48.0 47.9 47-5 45-5 43-0 41.0 36.5 30.5 29.5 Plagioclase of upper layer Ab,An,. Immersed in bath of molten gold. EXPERIMENT NO. 37 . Depth of diopside layer 7 mm. Total depth 10.2 mm. Time 17 hrs. Distance from bottom (Gani) aoe oiee Sree Qi wen ar lee oye: -7 6.3 Vol. per cent diopside. 73.0 72.8 72.3 70.5 70 8.8 7 6 66. Plagioclase of upper layer Ab,;An,. EXPERIMENT NO. 25 Depth of diopside layer 3.2 mm. Total depth 14.5 mm. Time 22.5 hrs. Distance from bottom Grinie eee rol ahe3 ON 3) 07/43) 6-3) O23 1Os3 PL .3) L223) 13h3 14-2 Vol. per cent diop- Bide 4215 30-3) 301.320.924.420. 3 20.1 17.8172) 15-0 14.6) 0.5, ©.0 71-3 Plagioclase of upper layer Ab;Anz. Figures 1-5, in which the ordinates represent height in millimeters above the bottom and the abscissae composition in units per cent of diopside. The initial condition will then be represented by two vertical lines indicating two uniform layers, one at 100 per cent diopside and of a length corresponding to the depth of the diopside layer, the other at o per cent diopside representing the layer of plagioclase above it and of appropriate length. These are joined by a horizontal line indicating instantaneous change of com- position. The final condition will be represented by a curve on which any point represents the composition at the corresponding level. The slope at any point indicates the composition gradient at that point, the curve approaching more nearly to the horizontal the greater the composition gradient. The figures need little discussion since they present the results better than can be done in words. THEORETICAL CONSIDERATIONS As noted on an earlier page, concentrations in a mass under- going diffusion may be calculated on the basis of Fick’s law. In its application to the present case, and in the form most useful for 302 N. L. BOWEN calculation of the concentration at any point in a diffusion cylinder, the equation™ becomes l-« L—(2m—*) 2V kt 2V kt 26=Co a2. cP dg+—?- e-®d6+etc. | I We ae —l—x —l—(2m—*) 2V kt 2V kt where the term in brackets in the limits is successively x, 2m—x, 2m+x, 4m—x, 4m-+x, etc., and where c is the volume concen- tration at any point at distance x from the base of the column, / is the thickness of the bottom layer of original uniform concentration Co, m is the total length of column, ¢ is the time elapsed, and k the constant of diffusivity. For the examples in hand the series is rapidly convergent. With the aid of this equation we may, then, calculate the concentration at various points after a certain period of time and for a certain value of k& (or, more simply, for a certain value of the product kt) and draw a curve representing the theo- retical distribution of concentration. Curves of this kind were drawn and it was found that in no case could a calculated curve be obtained that would coincide with the observed curve. Of the calculated curves a certain one was chosen and was plotted on each of the figures as a dotted curve. The theoretical curve chosen in each case was that which showed approximately the same concentration at the upper surface as that actually found. The curves therefore coincide at their upper ends, but at other depths wide divergence is shown between the full curve of actual concentration and the dotted curve of theoretical concentration. In all cases this divergence is of a systematic kind, the actual con- centration showing a smaller gradient in the diopside-rich layers and a larger gradient in the diopside-poor layers than the theo- retical concentration. Thisis shown particularlyplainly in Figure 1, where the diopside-rich layers have reached practical uniformity while the upper layers show a very strong gradient. This uniform : : 6 9 q 2 «The equation is not so formidable as it appears, {| of fds being merely the probability integral whose value, for various values of g in the limits, can be looked up in the tables. DIFFUSION IN SILICATE MELTS //} 10, 9) Gym ENP hey Th ER Lnitial Condition ie Final i 48 hrs. 7A NR ee ce Calculated , k= 00/5 Fic. 1.—Diffusion experiment No. 27 | i —.—.— Initial Condition Final i YB AIS. Calculated ., K= OU fi,snl0 20 30 40.50 60 70 80 GOnURWE Fic. 2.—Diffusion experiment No. 24 Afy,l0 20 3 ¥0 50 60 70 80 LDR: 28) 304 N. L. BOWEN divergence from theory may be explained by assuming that the coefficient of diffusivity is not a constant but is itself a function of concentration and is greater for diopside-rich mixtures than for those poor in diopside. In those experiments with aqueous solu- tions, where the highest degree of correspondence with theory is obtained, the solutions are always kept dilute so that the medium into which diffusion is taking place is sensibly constant. Under these conditions theoretical concentrations calculated on the basis’ of a constant value of the coefficient of diffusivity are in marked accord with observed values. In the present case, however, there is a continual and very important change in the nature of the diffusion medium as time progresses, and no constancy is to be expected in the value of the coefficient of diffusivity. ‘There is no necessity, therefore, for regarding the results as showing divergence from Fick’s law, the results being reconcilable with theory if it is assumed, as mentioned above, that the diffusivity is a function of concentration. Einstein has developed for dilute solutions a theoretical relation between diffusivity and viscosity which makes them inversely proportional.t While experimental results do not entirely confirm his theory, they suggest its correctness if certain disturbing factors such as hydration could be evaluated.? At any rate, if we assume that the diffusivity is an inverse function of viscosity, we obtain a natural explanation of our experimental results. The viscosity of the diopside-rich mixtures is much less than that of the plagioclase- rich mixtures. The coefficient of diffusivity for the diopside- rich mixtures should be correspondingly greater, and this we have found to afford a natural explanation of the deviation from theo- retical values calculated in the ordinary way. Moreover, as one would expect, this deviation is more marked for the plagioclase liquid Ab,An, whose viscosity is very much greater than that of diopside liquid, and less marked for the plagioclase liquid Ab,An, where the viscosity contrast is not so great, while liquid Ab,Anz occupies an intermediate position in this particular (cf. Figs. 1, 3, and 5). Qualitatively, then, the experimental results suggest t Ann. d. Physik, Yol. XVII (1905), p. 549. 2. W. Oholm, Medd. K. Wettenskapsakad Nobelinstitut 2, No. 26, p. 21. S _ Sy © Ee S 2 SSeS eS pres jo) AbAn/0 20 JO 4 30 Ab, /0 20 30 40 50 60 70 80 GODUPIUE DIFFUSION IN SILICATE MELTS Fic. 3.—Diffusion experiment No. 21 Shins Initial Condition final fe Vhs Fic. 4.—Diffusion experiment No. 37 Linmersed 1 ratten gold. a LI nitial Condition Final “ ZA V4, Vp) ar anee Calculated 1 K=0/4 ! | | | | 6) 70 80 ADDIE IOS 306 N. L. BOWEN agreement with theory if the variation of the coefficient of diffusivity is taken into account. It is possible, too, that from the observed values a definite quantitative relation between the coefficient of diffusivity and the concentration could be calculated, but the writer has not been able to find any attack upon a problem of this Initial Condition Final po ZOU Catulated 1» k=02 AbAn,/0 20 30 40 50 60 70 80 90 TOPSWE Fic. 5.—Diffusion experiment No. 25 kind in the various treatments of diffusion of concentration or of temperature. All of these adhere to a constant value of the coefhi- cient of diffusivity. While in some respects it is highly desirable to check the present results more thoroughly along the lines indicated above, yet, for the purposes of the present paper, this is unneces- sary. The results afford us very definite information as to the DIFFUSION IN SILICATE MELTS BON magnitude of diffusion in silicate melts, which was the objective in mind when the work was undertaken. VALUES OF DIFFUSIVITY We cannot speak of a diffusivity “‘constant” in connection with the present results, but we may take the amount of diopside which penetrates to the surface layer as an indication of the average diffusivity of diopside in a liquid mixture of diopside and plagio- clase. In this sense we find the “average diffusivity” of diopside in Ab,An, mixture from Experiment No. 24 (Fig. 1), =0.015 in cm.” per day, in Ab,An, mixture from Experiment No. 21 (Fig. 3), k=o.14, and in Ab,An, mixture from Experiment No. 25 (Fig. 5), k=o.2. We therefore observe a progressive increase in the value of k with increase in the amount of anorthite in the diffusion mixture. Since it is well known that plagioclases rich in anorthite afford less viscous liquids than those rich in albite, we have further evidence of the increase of rate of diffusion with decrease of vis- _cosity. If we compare the results of Experiment No. 21 (Fig. 3) and 37 (Fig. 4), we find again an increase in the value of the diffu- sivity, being k=o.14, and k=o.3, respectively.t In both these examples, however, the plagioclase Ab,An, was used, the difference being that an increasing proportion of diopside was added. Asa consequence, a higher value of the diffusivity is found for the mixture which was richer in diopside and therefore of lower vis- cosity. All of the results are therefore consistent with the assump- tion that the diffusivity varies inversely with the viscosity, which is in turn dependent on composition. By way of comparison of the diffusivities here found with measured values of diffusivities for other substances, it may be noted that for common salt diffusing in water k=1 at 15°C., for gold in molten lead at 492°C., k=3, for solid gold in solid lead at 150°C., k=o.0043 in the same units as those used above. The tIn Figure 4 no calculated curve is shown for the reason that the calculated curve for k=o.3 sensibly coincides with the observed curve. Since the final result will be uniformity in all cases, whether the diffusivity varies with composition or not, a close approach of the calculated curve to the observed curve is to be expected in cases where diffusion is far advanced (Fig. 4). On the other hand, where the upper layers have not yet been affected, the greatest divergence between observed and calculated values is to be expected (Figs. 1 and 2). 308 N. L. BOWEN diffusivities obtained for the silicates are, therefore, much smaller than those of salts in solution and those of molten metals, but much greater than those of solid metals. Some of the higher values obtained for the silicates are comparable with those of certain relatively viscous organic liquids. The diffusivity constant of glycerine in propyl alcohol at 17°C. is, for example, approxi- mately o. 2. APPLICATION OF RESULTS It is not at all likely that the diffusivities of substances in mutual solution in rock magmas can be significantly greater than those determined for the plagioclase-diopside mixtures, and in many viscous magmas they would no doubt be considerably less. For the purpose of applying the results to diffusion problems in petrogenesis a value has been taken very close to the highest, viz., 0.25, which is at the same time convenient in calculations. The Soret action is one of the diffusion phenomena that has been con- sidered of possible importance in magmas. It has been found in the laboratory that if a tube containing a solution is heated at one end and cooled at the other there is usually a concentration of the solute toward the cold end which depends upon the difference of temperature, the relative concentrations being, for many cases, inversely as the absolute temperatures. In cooling magmas the margin must be regarded as having a lower temperature than the interior, and there should presumably be a tendency toward a greater concentration of some substance or substances at the cooler margin. This introduces the possibility of composition differences in different parts of an entirely liquid magma, the differences being brought about by: diffusion. If cooled entirely by conduction, the temperature of a magma brought into contact with cold country rock should at the border quickly assume a value midway between that of the magma and that of the country rock. For a long period thereafter cooling at the margin is very slow (see Fig. 6).” We may imagine that the original temperatures of the magma and ‘that of the surrounding rocks are such that during this long period of maintenance midway between them the magma is still above its temperature of beginning of crystallization. Here we would have t See also Lane, Aun. Rept. Geol. Surv. Michigan for rgo9, Fig. 18, p. 152. DIFFUSION IN SILICATE MELTS 309 the most favorable case conceivable for the establishment of a composition gradient as a result of a temperature gradient according to the Soret principle. These conditions would evidently obtain either when the temperature of the magma was very much above the crystallization temperature, or when that of the surrounding rock was not very much below it, the latter being the more likely case. The magma intruded into hot surroundings, perhaps into a cognate intrusive not yet cooled, is, therefore, the most favorable 6 Fic. 6.—Curves of cooling of an intrusive igneous sheet 20 m. thick D [nerves subject for the working of the Soret action. Yet when we realize that the diffusivity of mass is, according to our determinations, from 10,000 to 100,000 times smaller than the diffusivity of tem- perature in rocks, it is apparent that the temperature of any igneous body will fall too rapidly to allow sufficient time for the Soret phenomenon to manifest itself. This statement may perhaps be more readily appreciated if the Soret action is stated more definitely as a diffusion problem. In order to do so we may assume that the osmotic pressure is 310 N. L. BOWEN proportional to the absolute temperature and that for this reason diffusion takes place until the concentration is inversely proportional to the absolute temperature. In other words, the effective con- centration is, initially, inversely proportional to the absolute temperature, and diffusion takes place until the effective concen- tration is uniform. In applying these considerations to a cooling mass of rock, we may take for simplicity a tabular body. A solu- tion of the problem of the cooling of such a body is given by the equation l—x 2V kt Oo == e—*'dB Vo —l—x 2V kt where @ is the temperature at any point distant « from the margin, 6, the original temperature of the magma, the temperature of the wall rock being taken as zero and / is thickness of the intrusive. If we take a tabular body of thickness 20 m. (i.e., 10 m. from center to margin) we may calculate the temperature in any plane at given distance from the margin at the end of any period of time. The results of such calculations are shown graphically in Figure 6, the curves representing the distribution of temperature at the end of various periods of time if the diffusivity is taken as 0.0118 in cm.? per second. In this figure the temperature scale has no necessary absolute significance, o of the scale being merely the initial temperature of the surrounding rock and 1 of the scale being the initial temperature of the magma. It will be noted that at the end of one year the temperature at the margin is about halfway between the initial temperature of the surrounding rocks and that of the magma, while the temperature at the center is much higher. If it is supposed that the whole mass is still above its crystallization temperature, then the Soret action should be operative, that is, the effective concentration at any point should be proportional to the absolute temperature, and diffusion should take place until the effective concentration was uniform. Partly for simplicity and partly for the sake of obtaining an especially marked Soret effect we shall assume that the temperature scale of Figure 6 represents DIFFUSION IN SILICATE MELTS Qua absolute temperature, 1.e., o of the scale is o° absolute and 1 of the scale is 1000° absolute. After one year the temperatures at the margin and at the center are 490° and 760° respectively. Then i : Ohi: the effective concentration at the center should be eS times that at the margin and diffusion should take place until the effective concentration is uniform, or until the real concentration at the center is times that at the margin. The problem is to find how long a time it would require for this diffusion to take place. Infinite time would, of course, be required to allow the process to go to completion, but we may find the time needed to give any assigned approach to this condition. As a first step we may calculate the time necessary for the acquirement, from any arbitrary initial condition, of a concen- tration gradient represented by the curve showing the thermal gradient at the end of one year. ‘This may be done by assuming a condition analogous to our experiments, viz., that all the material was first concentrated in a meter layer and by diffusion had acquired the gradient referred to. With the aid of equation (I) we find this to be very nearly true when V é¢ in the limits of the integral has the value 500. If we take & as having a value close to the highest found in any of the experimental determinations, viz., 0.25, then t=10° days. But this is not the time we wish to know; it is that required to go on from this condition to practical uni- formity. Again we find from the equation that practical uni- formity (1:0.996) is obtained from the arbitrary initial condition when V kf=1000 or when f,=4X10° days. From this we get the desired time ¢,—#=3X10° days, or nearly 10,000 years. This shows that it would need about 10,000 years to obtain nearly the full theoretical Soret effect required by the curve of temperature distribution after one year in a mass of the dimensions chosen. In the meantime, as shown by the curves of Figure 6, the whole mass would have cooled to the temperature of the surrounding rocks. Even if we imagine it to be still above its crystallization temperature at the end of four years, it will be noted that most of the temperature gradient has been destroyed at that time so that 312 N. L. BOWEN there is no reason for the action continuing even for four years. It should be observed that, although the times have been computed for a body of a definite size, the solution is really of a general nature, for if the body were z times as thick, the time on the one- year curve would be changed to n? years and the time required for the Soret phenomenon would be 7? times as large. Nothing is gained, therefore, for the Soret effect by making the body larger or smaller. In assuming that the scale of Figure 6 represents absolute temperature, we have, of course, taken an impossible condition for any body of rock. This would mean that the surrounding rocks were initially at o° absolute and the magma had not yet begun to crystallize at 490° absolute=217°C. The assumption was made on account of the convenience of referring both concentration and temperature gradients to the same curve, but even if we make reasonable assumptions as to the temperature of the magma and of wall rock, our conclusion will not be affected. We may even assume that the Soret effect for silicates is many times that deduced ‘from the theoretical (absolute temperature) relation, yet the outstanding fact remains that the time required to produce a significant amount of concentration of material by diffusion is enormously greater than that required for the mass to cool off. It may be noted that in speaking of a concentration toward the cool margin no mention has been made of what is concentrated. The reason is, of course, that it is not known. Ordinarily it is stated that the solute is concentrated toward the margin, but no distinction of solvent and solute can be applied to magmas. In conclusion, then, it may be stated that no concentration of any substance toward the cool margin could occur in appreciable. amount in the time available for such action in a cooling mass of completely molten rock. DIFFUSION TOWARD MARGIN DURING CRYSTALLIZATION There is, however, another case of diffusion of material toward the cool boundary for which we know the nature of the material that should move in that direction. Harker lays stress on the fact that in any cooling mass of magma there should be a time DIFFUSION IN SILICATE MELTS 313 when crystallization of an early-formed mineral A takes place ‘only near the border, the rest of the mass being still above the temperature of crystallization. The greater concentration of the substance A in the interior of the mass where none has yet pre- cipitated should constitute a driving force tending to cause that material to diffuse toward the margin. This case may be stated fairly simply as a definite diffusion problem. When the tem- perature of a thin layer at the margin has fallen to such a value that a certain fraction of the amount of substance A in that thin layer has precipitated, then the magma in this layer is saturated with A. If this condition is maintained for an infinite period of time, the whole body of magma will eventually acquire the same concentration in A as this marginal saturated solution’ and all of substance A in excess of this concentration will be precipitated at the margin. If we assume the contact surface plane as in a tabular mass, this is essentially the same problem as the heat- conduction problem involved in the cooling of a sheet of metal one face of which is kept at constant temperature. A solution of the problem is given by the equation: which gives the concentration c in terms of the original concentra- tion ¢, at any point at distance x from the margin after the time it, the concentration of the marginal saturated solution being taken =o. Values of c for various values of x and ¢ have been calculated for a diffusivity 0.25 in cm. per day, approximately the highest experimental value, and are plotted as concentration curves in Figure 7. The figure shows that after two-thirds of a year the precipitating effect has been felt for a distance of 0.33 m. from the margin, all the rest of the magma being entirely unaffected. After sixty-four years the precipitating effect has been felt for a t Neglecting the Soret effect. Binal N. L. BOWEN distance of 3.3 m. while all the rest of the magma is unaffected, and similarly for the other concentration curves. We can, moreover, determine from the figure the amount of material that would be precipitated at the margin at the end of any period of time. At the end of infinite time the concentration curve would correspond with the axis of abscissae, that is, the concentration at all points is zero of the scale, or equal to the concentration of marginal saturated solution. The whole area a Re i NAS! OS Fic. 7.—Curves of concentration in an igneous mass showing diffusion of material toward a cool boundary. of the rectangle between the extreme ordinates of the figure repre- sents, therefore, on a certain arbitrary scale the amount of material which would be removed in bringing the whole magma to the concentration of the marginal saturated solution. On the same scale the area lying above and to the left of the concentration curve for any time represents the amount of material removed during that time. We may take a specific case and imagine that the mineral precipitated at the margin is, say, amphibole, which occurs in the magma to the extent of 20 per cent, and that the DIFFUSION IN SILICATE MELTS Raa cooling of the marginal layer had proceeded until one-fourth of the amphibole contained in that layer (or 5 per cent of the layer) had been precipitated. If this condition were maintained the precipitation of amphibole at the margin would proceed by diffusion from the parts not yet cooled below the temperature of precipitation of amphibole. At the end of two-thirds of a year a proportion of the excess amphibole would be precipitated equal to the area between the one-year curve and the axis of ordinates. Regarding this area as a triangle of base sensibly 0.3 m. and assigning any arbitrary total thickness « to the mass, then the thickness of the amphibole deposit in meters would be © 8 Se I Waa | ico That is, a deposit 0.0075 m. or 2 cm. thick would be formed on the margin in two-thirds of a year. Its thickness is independent of the total thickness of the mass of magma and all of it would be derived from a layer of magma 33 cm. thick. After sixteen years the deposit would be about 2 cm. thick, all coming from a layer of magma about 1.5 m. wide. After two hundred and fifty-six years the deposit would be about 8 cm. thick, all from a border portion less than 7 m. wide. It is apparent that the possible results of diffusion after the manner postulated are exceedingly small. A mass of magma large enough to remain in the necessary condition for two hundred and fifty-six years would have a border phase 8 cm. thick. By necessary condition is meant that the margin should be cooled within its crystallization range and the main portion of the mass be not yet so cooled. The indications are that the mass would require to be at least 300 m. thick and be intruded under special conditions of temperature of magma and of wall rock, and the border phase would then be insignificant. Even by making more liberal assumptions as to the amount of chilling at the margin, say, a chilling sufficient to precipitate 25 per cent of the magma solution, the possible border phase would be increased in magnitude only five times. Moreover, as one increases the extent of marginal chilling, a stage is soon reached where so much precipitation takes 316 NV. L. BOWEN place in the marginal phase that no diffusion into that region can occur. One then arrives at a method of formation of a border phase that has been suggested by Daly, who regards the border phase as a chilled phase having the composition of the original magma.* FORMATION OF REACTION RIMS We have seen in the foregoing that the movement of large quantities of material through long distances by diffusion in a magma cannot be credited when the relatively rapid rate at which — the magma must cool is considered. On the other hand, diffusion through short distances is to be expected, and such phenomena as the formation of reaction rims about foreign inclusions are readily to be attributed to diffusion. At the same time it should be noted that a rather wide reaction border will require a very considerable period of time for its formation if diffusion alone is active. Figure 7 enables one to form an idea of the period of time required for the diffusion of material from an inclusion to various distances in the surrounding medium if the scale of concentrations is reversed, that is, if zero is placed at the top and one at the bottom. The solution is by no means a rigid one for a small inclusion, but for a large slablike inclusion is sufficiently good to enable one to draw general conclusions. The figure shows that after sixty-four years the effect of the inclusion is barely felt for about 3 m. and is strongly felt (one-half saturation) for not more than 1 m. These con- siderations suggest that the formation of reaction rims up to 2 m. thick, such as those described by Ussing, about inclusions of quart- zite in augite syenite at Kangerdluarsuk would require a period of time of the order of magnitude of one hundred years if diffusion alone were operative.” The growth of crystals is itself largely dependent upon diftu- sion, but no quantitative estimate of the rate of growth is possible without some knowledge of the concentration gradient along which flow of material takes place, that is of the degree of super- saturation possible in the liquid interstitial to the crystals. The t Igneous Rocks and Their Origin, p. 237. 2 Geology of the Country about Julianehaab, Greenland, p. 362. DIFFUSION IN SILICATE MELTS . By fact that rocks are normally millimeter-grained rather than centi- meter- or meter-grained even in large masses is, however, a tribute to the slowness of diffusion in magmas. The fact that certain conclusions are reached above on the assumption that diffusion acts alone should not be taken as indicating that the writer believes that no other processes could occur. To account for the extremely coarse grain of many pegmatites, for example, it seems necessary to assume circulation of solutions, and in many other cases cited circulation (convection) would be inevitable. SUMMARY The rate of diffusion in certain silicate melts has been deter- mined experimentally by permitting diffusion against gravity of a heavy liquid into a lighter liquid. The concentration curves found are not coincident with any theoretical curves calculated on the basis of a constant value of the diffusivity, but can be inter- preted on the assumption that the diffusivity varies with concen- tration and is less for concentrations corresponding to more viscous liquids than for those corresponding to less viscous liquids. Taking as representative of the “average diffusivity”? the amount of material which penetrates into the upper layer, the following values of the average diffusivity (k) were found: for diopside into Ab,An,, k=o.015; for diopside into Ab,An,, kR=o.14 to 0.3, depending on the proportions; and for diopside into Ab,An., k=o.2, all in cm.” per day. The value 0.25 (close to the maximum experimental value) is taken as probably representing a fair estimate of diffusivity in magmas, and with this as a basis it is shown that such phenomena as the formation of border phases about large bodies of igneous rock by diffusion cannot be considered possible in the time available for such action in a cooling magma. On the other hand, the forma- tion of reaction rims about inclusions may be attributed to diffusion, though for very wide rims a considerable period of time will be required. THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION AND MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS J. H. L. VOGT TRONDHJEM, NORWAY INTRODUCTION In a later paper of this treatise (I-IV) are given the physico- chemical laws which govern the crystallization of igneous rocks. Subsequently, it will be shown that the same laws can be applied to the explanation of the chemical composition of igneous rocks, and consequently also of magmatic differentiation. As I shall often refer to my earlier publications on the problems here discussed, I give a list of those of most importance: ““Studier over slagger,’”’ Svenska Vet.-Akad. Handl., 1884. (Stockholm, 1885.) “‘Beitrige zur Kenntnis der Gesetze der Mineralbildung in Schmelzmassen und Ergussgesteinen,” Archiv for Mathem. og Naturv., Vols. 13 and 14. (Kristia- nia, 1888-90.) “Die Silikatschmelzlésungen,” I and II. Kristiania Videnskabs-Selskap, 1903, 1904. “Physikalisch-chemische Gesetze der Krystallisationsfolge in Eruptivge- steinen,”’ Tschermaks min. und petrogr. Mitt., Vols. SOM Coe 5), XXV (1906), and XXVII (1908). “Uber anchi-monomineralische und anchi-eutektische Eruptivgesteine,” Kristiania Vid.-Selsk., 1908. “On Labradorite-Norite with Porphyritic Labradorite-Crystals: a Contri- bution to the Study of the ““Gabbroidal Eutectic,” Quart. Jour. Geol. Soc., 1909. “Uber das Spinell: Magnetit-Eutektikum,” Kristiania Vid.-Selsk., 1910. “Die Sulfid: Silikatschmelzlésungen” (a review, 97 pages); Norsk Geologiee Tidsskrift (Kristiania), IV (1917). “Die Sulfid: Silikatschmelzlésungen. Part I. Die Sulfidschmelzen und die Sulfid: Silikatschmelzen.” Kvristiania Vid.-Selsk., 1919. Later will appear Part II. Die Nickel-Magnetkies-Lagerstatten. For geological surveying, chemical analysis, photographs, etc., for this publication I have had contribution from Den Tekniske Hoiskoles Fond (The Foundation of the Technical University of Norway). 318 ERRATUM In the article by J. H. L. Vogt, p. 318, in the first line, delete the words “a later paper of.”’ MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 3109 IL REVIEW OF THE PHYSICAL CHEMISTRY OF THE CRYSTAL- LIZATION OF IGNEOUS MAGMAS In the examination of these laws two different methods can be used: (a) the synthetic, in which there is an opportunity for precision-determinations, especially of temperature. Previous investigations on the crystallization of silicates from melts have been, nearly without exception, conducted at the pressure of one atmosphere; (0) the analytic, mainly based on the study of the structure of the rocks. In this manner we may examine the sequence of crystallization, and so also the “‘individualization- fields” of the minerals, further the mix-crystal systems, the chemical composition of eutectic intergrowths, etc.—all under the physical conditions, especially with regard to pressure and time, present during the solidification of the different igneous rocks. The synthetic method forms the important base. The analytic method gives us, in particular, information as to the extent to which the results of investigations conducted chiefly at atmospheric _ pressure and during short periods of time, can be transferred to apply to the physical conditions under which the crystallization of magmas took place. The two methods, therefore, go hand in hand and complete each other. REMARKS ON THE STRUCTURAL CRITERIA FOR THE SEQUENCE OF CRYSTALLIZATION The sequence of crystallization in igneous rocks may usually be determined by the complete, partial, or wanting idiomorphism of the minerals, by the inclusions, by deposition on a solid body (Fixkorper-Absaiz), by ‘together-swimming”’ structure (synneusis- strukiur, see below), by law-governed intergrowths, etc. The complete zdiomorphism of a primary mineral A against all the other minerals shows that its crystallization was finished before the commencement of the solidification of the others. From the partial idiomorphism of the primary mineral A against the primary mineral B whose idiomorphism is wanting, we can infer that the crystallization of A had commenced at an earlier stage than the crystallization of B. But we must not draw the more extensive 320 Jee de aVOGE conclusion that A in its entirety had crystallized before the com- mencement of the crystallization of B. As to the conceptions allotriomorphism, hypidiomorphism, and panidiomorphism, we refer to the petrographic textbooks. _ Inclusions of an idiomorphic, primary crystal A in B implies that A had crystallized earlier than the surrounding parts of B. But if A only appears in the exterior zone of B, the interior part of B may have crystallized earlier than A. And even if inclusions of idiomorphic crystals of A appear evenly distributed over the whole of B, also in the kernel of B, it may be that part of A also may have crystallized at a later stage. Asan example, idiomorphic crystals of apatite, as is known, in many cases appear in the oldest silicates and in the ore minerals, indicating that the apatite crystal- lized before the commencement of the solidification of the iron ore and the silicates. But I warn against the conclusion, which is often drawn, that the apatite in its entirety crystallized during the earlier stage. Further, it must be taken into consideration that small portions of the mother-liquid occasionally may be inclosed or included in a mineral during its growth. As example we refer to the well-known zonally arranged glass-inclusions in leucite, sanidine, etc., in many dyke and effusive rocks. If corresponding magma- inclusions occur in deep-seated rocks, a complete crystallization of this material will take place. Thus the result will be inclusions in the host of a later-crystallized mineral. Inclusions of mineral A in mineral B may furthermore be due to the fact that A originally, at high temperature, occurred as a solid solution in B, and that afterward, owing to reduced solubility by decreasing temperature, A separated from the solid solution. As an example we refer to the well-known inclusions of perthitic albite or albite-oligoclase in the microcline of granites, etc. The microcline (or orthoclase) dissolved about 28 per cent Ab+An, the greater part of which later separated during refrigeration. Further may be mentioned the secretion of lamellae of monoclinic pyroxene in orthorhombic pyroxene’ and conversely also of ortho- «Cf. the general account in my publication in Tscherm. min. u. petrogr. Mitt., Vol. XXIV (1905), pp. 537-42. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 321 rhombic pyroxene in monoclinic.t In the same manner the well- known microscopic inclusions, often with idiomorphic contour, of titanic iron ore in hypersthene, diallage, and plagioclase of gabbros may be explained.? The latter inclusions were often interpreted by earlier investigators as older than the host-mineral, but in their present form they must be explained as later secretions from an originally solid solution. As may be understood from this account concerning the inclusions we must take into critical consideration a great number of momentums in determining the successive age of the minerals. When a substance is segregated from a solution, it often, as is well known, adheres to a solid already present (solid body or Fixkér per). The result of this is the deposition on a solid body (Fixkorper-A bsatz), which is also very important in the solidification of igneous rocks. We may here, for instance, refer to Figure 34, illustrating the depo- sition of spinel on pyrite; to Figure 33, illustrating the deposition of titanomagnetite on olivine; and to Figure 35, where in one place pyrite has been deposited on apatite while in another apatite has been deposited on pyrite. The individuals of a mineral, segregated from a magma at an early stage, frequently swam together to assemblings or aggre- gates, the result of which is a structure, for which I propose the term together-swimming structure or synneusts structure. This together-swimming may occur very rapidly. I refer to my publication “Die Sulfid: Silikatschmelzlosungen” (I, 1919), Figure 11, illustrating assemblages of octahedrons of magnetite in a bessemer-matte, consisting chiefly of Cu.S, and to Figure 28, a, illustrating assemblages of small individuals of zincblende in a slag. The solidification period of the two molten masses just mentioned, respectively molten sulfide and molten slag, needed only a very short time, at most half an hour. W. Wahl, ‘Die Enstatitaugite,”’ Tscherm. min. u. petrogr. Mitt., Vol. XXVI (1906). 2In this connection we refer to a treatise by A. Johnsen, ‘‘Regelmissige Ein- lagerung von Eisenglanz in Cancrinit,” Centralbl. f. Min., Geol. und Pal., 1911, and by O. Andersen, “‘On Aventurine Feldspar,” Amer. Jour. Sci., Vol. XL (1915). 3 Composed of civ, syn=together and vedo.s, neusis=swimming. 322 JHE: ViOGE The phenomenon here discussed may, as to igneous rocks, be illustrated by Figure 1, representing a dunite from the Hestmando- field in the northern part of Norway, with an average of only about 1 per cent chromite. In some parts of the thin section chromite is entirely or almost entirely lacking, but in other places we may find aggregates of ten to twenty small octahedrons of Fic. 1.—Dunite from the Hestmandé-field, northern Norway. Groups of octahedrons of chromite, illustrating “together-swimming” or synneusis structure. (Photo. 25:1.) (Black=chromite, white =olivine.) chromite which, as well with reference to the idiomorphism against the olivine as with reference to the together-swimming structure, must have crystallized while the olivine was still in a molten condition. From this and other dunite rocks with a little chromite we may draw the conclusion that the chromite commenced crystal- lizing earlier than the olivine, when there was at least 1 or 0.5 per cent, perhaps only 0.33 per cent, chromite present. But MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 323 this does not exclude that the olivine may have commenced crystallizing earlier than the chromite when the latter only amounted, for example, to 0.1 or 0.05 per cent. The earlier silicate minerals, for example, olivine in olivine-rich gabbros, and hypersthene or diallage in hypersthene- or diallage-rich gabbros, also often show the together-swimming structure, as in Figures ro, fone 20 and 21, and 23. The relative commencement of the solidification, especially of the minerals that commence crystallizing at a somewhat early stage, may often quite easily be decided by the structure. On the other hand, the allotriomorphism of a mineral C, against the minerals A and B, shows that C only commenced crystallizing after an often quite essential part of A and B had already solidified. Especially where the later mineral C is present in a small quan- tity, its allotriomorphism in connection with its appearance as Zwischenklemmungsmasse (or mesostasis) presents an easily recog- nizable criterion that it belongs to.a very late stage of the crystalliza- tion. But at this late stage the minerals A and B will in many cases have continued forming. We refer to the explanation given in connection with Figures 17 and 18. The simultaneous crystallization of two or more minerals may be manifested in various ways. With two simultaneously crys- tallizing minerals, each may grow until the individuals of A happen to collide with the individuals of B. Or some of the segregating mineral A may be deposed on the already solidified crystals of B, and some of the simultaneously segregating mineral B on the already solidified crystals of A. In this manner we may observe crystals of plagioclase with quite good idiomorphic contour against the hypersthene or diallage and, further, crystals of hypersthene or diallage with quite good idiomorphic contour against plagioclase in the same thin section of an anchi-eutectic norite or gabbro. In the deep-seated rocks it may in such cases often be quite impossible to decide which of the two minerals first commenced crystallizing. It may also often occur that the two minerals crystallize in an intimate intergrowth. In this manner simultaneously crystallizing 324 IEE. VOGE feldspar and quartz, as is well known, sometimes produces micropegmatitic or granophyric structure, as in graphic granite. Corresponding structure, which gives an evidence of a crystallization along a eutectic boundary curve, or exceptionally by a binary (or ternary or still more complex) eutectic, is also sometimes found with other minerals, for instance, between olivine and magnetite (see Fig. 28). In many, possibly in most, cases the crystallization of the minerals A, B, C, etc., takes place in the following manner: Each mineral begins crystallizing at its proper stage, and continues to grow until the entire magma has solidified. As an extreme example we may choose apatite. This phosphate is only slightly soluble in silicate magmas at temperatures just above that at which the silicates commence to crystallize. If there is 0.20 per cent apatite present, the essential portion, perhaps 0.18, 0.10, or 0.195 per cent, has already crystallized before the silicate minerals have commenced segregating. But we may be pretty certain that a trifle phosphate, 0.02, 0.01, or perhaps only 0.005 per cent, still exists in solution at this stage and little by little solidifies later. It has, however, not been possible for me to substantiate this by observation with respect to the apatite; but I have been able to establish that spinel, when present only as ©.o1 or at most 0.02 per cent, only commenced crystallizing after a great part of the silicate mineral A had solidified. (See Fig. 33 and the chapter on spinel.) Fe,O, and the different ferromagnesian silicates are only slightly soluble in acid—or granitic—magmas, and therefore commenced crystallizing at an early stage. We find, however, as is discussed below, a small remnant of magnetite and ferromagnesian silicate in the final product of the solidification. We may consequently draw the conclusion that the essential part of the magnetite and the ferromagnesian silicate was certainly solidified during the first stage of the crystallization, but that a little remnant stayed in the solution and was solidified later. In a binary system, type IV, of two discontinuous mix-crys- tals—A, melting at relatively high temperature (for example, FeS,, MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 325 at high pressure), and B, melting at relatively low temperature (for example, FeS)—idiomorphic crystals of B never appear in A, but, on the contrary, idiomorphic and usually somewhat resorbed crystals of A appear in B. Here the sequence of crystallization, without regard to the proportion of weight between A and B, is first A and later B, and this to be understood thus, that the crystal- lization of A was completed before that of B began. (See Fig. 36.) In the usual silicate eruptives, most frequently consisting of a whole series of components, we may also meet corresponding crystallization of a certain mineral, completely solidified at a relatively early stage. See Figures 20 and 21, representing crystals of hypersthene imbedded in diallage. Another case of crystallization completed at an early stage is illustrated by Figures 8 and 9 (and a theoretical explanation given below) with crystallization at the beginning stage of hypersthene, while the Fe-Mg silicate at a later stage entered into biotite. In this treatise (Part I) we are only going to consider the solidification of the rocks (the transition from liquid to solid phase). We, however, also discuss the continued change of the minerals, which may be founded on the later crystallization of a substance originally in solid solution, and furthermore we are going to deal with the reactions which appear in the solid phase on the boundary between two minerals, and which are an immediate ‘result of the cooling of the rocks after completed crystallization. We shall, however, not discuss the later changes, which are not a direct result of the solidification of the rocks, but are founded on exterior incidents, as, for instance, dynamo and contact meta- morphosis, chemical actions, etc. INTRODUCTORY REMARKS CONCERNING THE APPLICATION OF THE PHYSICO-CHEMICAL LAWS TO THE CRYSTALLIZATION OF MAGMAS Magmas usually consist of a whole series of components, which entail a complication of the equilibrium existing in the magma and consequently also of the laws of the crystallization. In many cases, however, an essential simplification of these complications takes place, as, according to H. E. Boeke,’ the ™ Grundlagen der physikalisch-chemischen Petrographie (1915), p. 104. 326 J. H. L. VOGT following assertion is applicable: ‘The saturation-boundary between binary mix-crystals as well as in general the equilibrium between two solid phases of a binary system do not change by contact with other phases and components, when these new entering components do not form solid solutions or stoechiometric com- pounds with the former solid phases.” The proportion between An and Ab in the segregated plagioclase mix-crystal will in this manner be the same whether the crystallization takes place in a pure An+Ab melt or in a silicate melt (or magma) which besides plagioclase also delivers, for instance, magnetite, olivine, etc. In order to investigate the laws of crystallization of the principal components of the magma, we may generally leave the components which are present only in subordinate quantity out of consideration, provided that the latter do not form solid solutions or enter into mix-crystal combinations with the principal components. We must, however, take into consideration that when A, B, and C form a ternary eutectic, and C only is present in small quantity, the simultaneous crystallization between A and B, along the eutectic boundary between A and B, will not be identical quanti- tatively with the composition of the binary eutectic between A and B. If C, however, is present in minimal quantity, the dif- ference between the point in question on the eutectic boundary between A and B and the binary eutectic A:B will be so incon- siderable that it practically may be left out of consideration. THE FELDSPARS, AB: AN, OR: AB-+-AN. The binary system Ab: An (with melting-point Ab = 1100+ 10° and An=1550°+ 2°) belonging to mix-crystal type I has been studied in detail by N. L. Bowen’ of the Geophysical Laboratory of the Carnegie Institution of Washington at the pressure of one atmosphere (and with chemically pure substances). We reprint Bowen’s graphic exhibit as Figure 2, where the great horizontal difference between the liquidus and solidus curves is shown. As an example we may mention that from a molten mass, Ab,An,, the first mix-crystal, separating at 1450° (without super- t Amer. Jour. Sc., Vol. XXXV (1013). MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 327 saturation), has a composition nearly exactly Ab..Ang., or Ab,;An,. Even before Bowen’s investigation (1913), I had (see especially Tscherm. Mitt., Vol. XXIV [1905]), on the basis of the well-known zonal structure of the plagioclases, and further on the basis of the proportion between Ab: An in the total plagioclase calculated from the analysis of the rocks and in the first segregated crystal, decided that the system Ab:An belongs to type I, and, furthermore, that in the crystallization of plagioclase in eruptive rocks, especially in t00An 80 60 AO 20 oAn OA 20 40 60 80 100A6 Fic. 2.—The melting-diagram, An: Ab, after Bowen dike and effusive rocks, there is a very great horizontal difference between the liquidus and the solidus curves. The binary diagram for Ab:An, at the pressure of one atmos- phere and for too per cent Ab+An, may with unessential modifica- tions of the horizontal difference between the two curves (see a following chapter) be transferred to magmas, crystallizing at high pressure, which besides Ab and An contain other components. The investigation of the mix-crystal system and the proportion between the liquidus and solidus curves for Ab:An may conse- quently also be applied to rocks, which besides plagioclase also 328 fee SVOGE contain a great quantity, 40 per cent and still more, of foreign components, as quartz and Fe-Mg silicates. Petrographic experi- ence proves that in such rocks we can detect no difference in the horizontal distance of the two curves, and this is in best accordance with the general law cited on page 326. Or:Ab or Or:Ab+An.—Because of the extreme viscosity which characterizes melted KAISi,Os and NaAISi,Os (without or with only a small quantity of CaAlLSi,Os), the synthetic study of the system Or:Ab or Ab+An is connected with exceptionally great difficulties. We may therefore here use the analytic method, cf. my earlier publication in Tscherm. Mitt., Vol. XXIV (1905). When Or is predominant, orthoclase first crystallizes, and when Ab-+An is predominant, plagioclase first. The boundary (“indi- vidualization-boundary’’) is decided by the following method of investigation: In a number of rocks, where the proportion Or:Ab:An in the entire rock was determined on the basis of the rock analysis, we find crystallized as No. I: plagioclase at 32, 32, 32.5, 32-5, 33-5, 3A, 36, 36, 37-5, 39-5, 40, and 4x Or:Rest Abita,; orihoclaye (or microcline) at 42, 43, 43-5, 43-5, 46, 47, 47, 50; 50, 50-5, 52, and 52.5 Or:Rest Ab+an.' Ab+an here represents Ab with a small but variable quantity of An, consequently albite, oligoclase, and andesine. Between the two “‘individualization-fields”’ lies the boundary for orthoclase: albite, oligoclase, or andesine at about 0.4 Or:0.6 Ab or Ab4an, perhaps nearest 0.42 Or:o0.58 Abian, applying to magmas at high pressure and with predominant Or+Ab+An with some mixture of other components. This boundary must be interpreted as an eutectic boundary curve. ; When orthoclase (or microcline) crystallizes at high temperatures from granitic magmas, which besides predominant Or also contain a good deal of Ab+an, there enters in the orthoclase up to about 30 per cent (or 28 per cent) of Ab+an, which is partially separated by the cooling of the solid solution as albite or albite-oligoclase- «Two uncertain determinations with 38 and 36 Or are here left out of consideration. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 329 perthite. By the crystallization of acid plagioclase, the latter absorbs up to about 10 per cent (or 12 per cent) Or, which on cooling may give anthiperthite. Basic plagioclase (labradorite- bytownite) seems to absorb a somewhat smaller quantity of Or. Concerning this matter we refer to the chapter on the anorthosites in Part IT. By the above-mentioned crystallization in the acid magmas of Or and Ab (or Ab+,,) there result two minerals: orthoclase about 0.7 Or + 0.3 Ab (or Ab+a,), and acid plagioclase, about o.9 Ab (or Abya,) + 0.1 Or. Orthoclase and albite have almost exactly the same melting- point, and almost exactly the same atomic weight, probably also about the same latent melting-heat, etc. Granted a binary eutectic system (type V), the eutectic must lie almost exactly midway between 0.7 Or + 0.3 Ab and o.9 Ab + o.1 Or, consequently at o.4 Or:o0.6 Ab, just as we found above. As An has a far higher melting-point than Or (and also than Ab), it is likely that the eutectic boundary between orthoclase and plagioclase with decreasing Or is characterized by increasing An in the plagioclase. In this manner we shall probably find the individualization-boundary between orthoclase and labradorite at Bens | OLIO455 Ons AD an. This explanation is, however, not explicit, as we have not taken into consideration that in certain rocks or under certain conditions, the details of which we are not acquainted with, the two independent minerals, orthoclase and albite (or some other acid plagioclase), do not appear, but instead we find the mineral anorthoclase. In my treatise in Tscherm. Mitt., Vol. XXIV (1905) I pointed out the fact that a whole series of analyses of anorthoclase shows telations about o.4 Or:o0.6 Ab (or about 0.42 Or:o0.58 Ab+an) and I set forth the hypothesis that the anorthoclase might be defined aS a microscopical or submicroscopical eutectic intergrowth of orthoclase (microcline) and albite (or some other acid plagioclase). This hypothesis is, however, quite dubious, and the physicochemical interpretation of the anorthoclase is still an open question. 330 TE VOGE QUARTZ (QU) AND FELDSPARS. QU:OR, QU:AB, QU:AN, QU:AB-+AN, QU:OR:AB+AN The binary system Si0,:CaAl,Si,Og (An) has been examined by G. A. Rankin and Olaf Andersen’ (at the Geophysical Labora- tory, Washington) with the result: An, melting-point =1550°= 2°; Eutectic SiO,:An= 52 per cent An:48 per cent SiO., melting-point ag 322 According to I. B. Ferguson and H. E. Merwin? (1918) the melting-point for tridymite is 1670+10°C; for cristobalite r7ro#10°C. K. Endell and R. Rieke* (1912) decided for cristoba- lite 1685+ 10°. N.L. Bowen‘ decided a somewhat lower tempera- ture, and C. N. Fenner’ thought he might fix the melting-point of cristobalite at 1625°, which, however, according to the latest precision-investigations, must be a little too low. It is a matter of course that Qu and Or, as well as Qu and Ab, in the same manner as Qu and An, must form a binary eutectic. Because of their extreme viscosity the binary eutectics Qu:Or and Qu: Ab are not experimentally determined. We may therefore here use the analytic method. We shall commence with graphic granite, the structure of which, as already established many years ago by W. C. Brégger,® is due to a simultaneous crystallization of quartz and feldspar. That is to say, the crystallization took place at a eutectic point or along a eutectic boundary curve. Referring to my earlier publications’ on the problem in question, we are going to give a collocation of all the hitherto published usable or at any rate somewhat usable analyses of graphic granite from pegmatitic granite dikes. t The System Anorthite-Forsterite-Silica,” Amer. Jour. Sci., Vol. XX XIX (1915). 2 Amer. Jour. Sci., Vol. XLVI (1918). 3 Zeitschr. f. anorg. Chemie, Vol. LX XIX (1913). 4 Amer. Jour. Sci., Vol. XX XVIII (1914). 5 [bid., Vol. XXXVI (10913). 6 Geol. Foren. Forh., Vol. V (1881), and Zeitschr. f. Kryst. Min., Vol. XVI (2800), I, pp. 148-59. 7 “Silikatschmelzlés.,” II (1904), and Tscherm. Mitt., Vol. XXV (1906). MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 331 TABLE I No. | SiO. | ALO; Fe.0; FeO ‘Meo CaO |Na.0} K:O | H.0 | Total Microcline Graphic Granite T../74.04|14.40| Nil Nil | Nil Jo. 33/2.01] 9.36] Nil |r00.18 Norway........|| 2--|74-99/14.31 Nil Nil | Nil Jo. 39/2.42} 9.02] Nil |100.14 3..173.82|14.44| Nil Nil | Nil Jo.35/2.45] 8.90} Nil] 99.96 4../74.47|15.13| Nil ING PONDS os lls o colle owe 5 Bones tebe seat 5. -173.70\04.11| Niall Nil | Nil Jo. 30/3.04] 8.72] Nil] 99-06 Sweden......... [ ©. .\\72.anlta, sn ©2230 ...| Nil Jo.13]2.1q]10.09]/0. 28] 99.91 7..|74.58|13.37| 0.24 |....] Nillo.32/1.16] 9.80]0.57|100.04 8. .|73-89|13.75| 0.26 |....| Nil} Nil |2.10| 9.00]0. 24] 99.24 United States... .|] 9--|73-92|14.26] 0.30 |....| Nil] Nil]2.06/ 8.99/0.11| 99.64 % 10. .|72.76|15.44/(InAl,O,;)|....| Nil Jo. 19]2.35| 9.28/0.15|100.20 II..|72.80]15.07] 0.26 |o.21| Nil} Nil |3.35] 7.92/0.30! 99.91 Oligoclase Graphic Granite Womway.24.....|.12..|76.8 \r4.2 Nil NM ISM eo POs WeSS ooo OOS Swedenes 2. | .13../76.67|/14.20] 0.14 |..../0.04|/2.67|5.33/0.52 |o.48|100.04 EXPLANATION Nos. 1-5 and 12, see “‘Silikatschmelzlés.,” II. No. 1 from Arendal; Nos. 2-3 from Hitteré; No. 4 from Raade; No. 5 from Arendal is industrially pulverized graphic-granite, in which a trifle surplus feldspar is not excluded. In all these analyses a small loss (0.1-0.3 per cent) caused by ignition is deducted. The precipitate of Al.O, was in all cases entirely white, and on account of this a special analysis of iron was not made. Some iron, less than ©. per cent Fe,O;, is, however, not excluded. No. 12, approximate analysis from Evje. Nos. 1 and 5 were analyzed by A. Gronningsater and E. A. Dalset, assistants at the time. Nos. 2-4 and 6 were analyzed by students. K,0,Na.0 and CaO in No. 4 have not been included on account of less accuracy. Nos. 6, 7, and 13 from A. Bygdén, Bull. of the Geol. Inst. of Upsala, Vol. VII (1906); from Elfkarleé, Skarpé, and Ytterby. Nos. 8-10 from Edson S. Bastin, U.S. Geol. Surv. Bull. 420 (1910); from Topsham, Me., and Redford, N.Y. (No. 10, a little Fe,O, [n.d.] in A1,O,.) No. 11 from Hiriart Hill, Cal., analyzed by W. T. Schaller, cited by H.S. Wash- ington, ‘Chemical Analysis of Igneous Rocks, 1884-1913,” U.S. Geol. Surv. Prof. Paper 99 (1907). (Cited below as Wash.) In the above table I have not included: Analysis of oligoclase graphic granite from the West Indies in Bygdén’s treatise (Wash., p. 267, No. 5, with 68.12 per cent SiO,), as this specimen is greatly decomposed, with con- siderable new-formed epidote, etc. This analysis can therefore not be used in the determination of the proportion of quartz and feldspar. Analysis No. 4 in E. S. Bastin’s treatise (Wash., p. 108, No. 4, with only 71.00 per cent SiO,), as Bastin informs us of this specimen: “Some small Bo) SH E VOGE areas of pure feldspar were associated with the graphic granite in this specimen, so that the silica percentage shown in the analysis is lower than it would be for graphic granite alone.”’ The same is probably also the case of an analysis, by A. W. Howitt, 1888, from Victoria (Wash., p. 112, No. 30). With respect to the other analyses which are noted in Washington’s Index as graphic granite we remark: “The graphic microgranite,” page 73, No. 2 (with 3.74 per cent Fe.O;, 2.81 per cent FeO), represents a rock, and not graphic granite. The same also applies to No. 19, page 94 (with 0.73 per cent Fe.O,;, 0.78 per cent FeO, 0.99 per cent MgO). In the treatise, above cited, by Bygdén, as also in a treatise by H. E. Johansson,? an analysis by P. J. Holmquist? has been taken as an example of an albite-graphic granite, showing 77.32 per cent SiO., 0.34 TiO, 11.62 ALO;, 1.57 Fe.0;,0.69 FeO, o.10 MnO, 0.62 CaO, 0.80 MgO, 0.99 K.O, 5.81 Na.O, 0.65 H.O, total 100.51. This was computed by Holmquist as: 39.0 per cent Qu, 49.3 Ab, 4.6 Or, 1.9 An, also 2.4 chlorite, 2.3 magnetite, 0.8 titanite, 0.3 water, and a little calcite. The analysis is from a thin dyke in diabase (R6d6) and does not permit any exact determination of quartz: albite in graphic granite. The precision-determination of the quantitative proportion of quartz and feldspar is complicated partly because of the inevit- able errors in the analyses, of which more below, and partly because we cannot always be certain that the analyses represent absolutely pure intergrowths of the two minerals. In granite-pegmatite dikes we sometimes meet specimens of which one part consists of pure feldspar, free from quartz, and the other part of graphic granite, retaining the crystallographic orientation of the feldspar. ‘That is to say, some feldspar first crystallized alone, and later, having reached an eutectic boundary curve, it continued its growth simultaneously with quartz. Sometimes we may find in the center of a large specimen of graphic granite small parts of pure feldspar. In such cases, the analyses of course cannot be used for precision-determinations of the relative proportions of quartz and feldspar. We may here refer to Bastin’s remarks concerning his analysis No. 4. On the basis of the analysis, we are going to calculate the quantitative proportions of quartz and feldspar in our microcline- graphic granites, according to the following methods: * Geol. Foren. Firh., Vol. XXVII (1905). 2 Sveriges Geol. Unders., C. 181 (1899). peste MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 333 a) Originating from K,0, Na,O, and CaO we calculate the quantity KAISi,Os (Or), NaAISi,Os (Ab), and CaALSi,Os (An), and when this is done the sum of feldspar is deducted from the analytically determined sum of SiO,, Al,0;, K.0, Na2O, and CaO. The difference is quartz. This method contains a very great source of error, as an error in the determination of K,0, Na.O, or CaO in the calculation of the amount of feldspar will be doubled respectively 6, 8.5, or 5 times. To this may be added the inaccuracy in the SiO, determination. b) Originating from the proportion K.O:Na.0:CaO we calculate the percentage of SiO. in the feldspar (ex. 65.28 per cent in No. 1) and then we calculate the proportion between quartz (mz) and feldspar (1—7), ex. for No. 1.:”. 100+ (1-).65.28=74.04. A relative error in the determination of K.O, Na.O, or CaO will in this manner be eliminated. But an error in the determination of SiO, will be doubled nearly three times. ! We have a control of the calculation, in method (a), in the calculated percentage of Al,O; in the feldspar, compared with the percentage of Al.O, found in the analysis. As an example, No. 5 shows 1.06 per cent too much A1,O; in the calculation, consequently also too much feldspar, and so too little quartz. The calculated 21.58 per cent quartz must consequently be increased. On the other hand, No. 9 shows 1.12 per cent too little Al.O, in the calculation, consequently too little feldspar or too much quartz. With combined consideration of both methods of calculation, and of the sum of the analysis minus ignition and Fe.0O;, we have under C written the probable percentage of the quartz. The rest is feldspar. TABLE II A B c No. Sea ceee Poe Qu Or Ab An Al.O; Qu Qu itt.6 qlee 25.60 55-51 OS 1.64 =O. 34 DG 22 WRoc! 2 Sve ete eae 24.34 53-50 20.52 1.04 +o.16 24.90 |24.6 26 leseueinete 24.54 2.78 20.78 it WG —=©, 02) 24.25 124.4 Soo aie 21.58 S472 25.78 1.04 ++-1.06 23.52 |23.5(+?) Osa dee 20.55 59.84 18.57 0.65 +0.28 19.28 |20.3(+?) Tc Gee 20.43 58.12 9.84 T.59 —0.25 27.53 |28.5(+?) Ss beeen 27 .O1 53-38 SP CoH) alll see eA R —0.53 MB 57 Bos Os aes DN), BD Bee LAN Ty tienen sia —1.12 23.67 |25.5 TO), 2 Steen 22.04 55.04 IQ .94 0.95 —1.18 20.79 |21.9 (?) iis 6 ee 22.70 460.07 ZO WANTING |e au —0.07 19.19 |21.0 (?) The specimen No. 6 is an erratic block (with 0.30 per cent Fe,O; and 0.28 per cent ignition). Here a minimal, hardly perceptible, decomposition is not excluded, so the determination 20.3 per cent quartz (or according to Bygdén 20.81 per cent) is probably too low. For No. 11 I have no supply of the original literature. The analysis shows 0.26 per cent Fe.O;, 0.21 FeO, and 0.30 ignition, and to the calculated result is added an interrogation point. ‘ B3A. INA TE. VOGI Arthur Holmes’ has used another method in order to determine the quantitative proportions of quartz and feldspar in graphic granite from dikes of granite-pegmatite (in Mozambique), viz., Rosiwal’s planimetric method. In this manner, from different localities, he found quartz amounting to 27.9, 27.1, 26.3, 25.6, 25.3, and 24.2, average 26.1 per cent (calculated in percentage by weight) and rest, 73.9 per cent of microcline with the ordinary perthitic admixture of albite-oblioclase. Consequently we have the following determinations of the quantity of quartz in micro- cline graphic granite: Calculated from the quantitative analyses: 28.5 (too high ?), 25.5, 25.4, 25.3, 24.6, 23.5 (?), 21.9 (too low?), 21.0 (?), and 20.3 (too low ?). By the Rosiwal method: 27.9, 27.1, 26.3, 25.6, 25.3 and 24.2 per cent of quartz. The majority of these determinations are subject to great sources of error, which may amount to several per cent. If we take this into consideration, I think I am justified in drawing the conclusions that the proportions of quartz and feldspar in microcline graphic granite from dikes of pegmatite are subject only to small variations or are practically constant, and that we may fix the proportion pretty closely at: 26 per cent quartz:74 per cent microcline.? The graphic granite in granite-pegmatite dikes crystallized at a relatively late stage, viz., after the essential part of the mica and, most frequently, also a part of the feldspar had solidified. Only a trifle mica was left at the time for the forming of the graphic granite. But in addition to this, besides the components of the feldspars and the quartz, there was surely some H.O present, possibly partly connected with SiO, in forming a separate com- ponent (as H,SiO, [?]). The graphic granite will thus have crystallized from a solution, which consisted predominantly of the components of feldspar and quartz, but also of a little mica and a small quantity of a component, as H.O and H,SiO, (?). The graphic granite has in this manner crystallized at a eutectic boundary t Quart. Jour. Geol. Soc. London, Vol. LXXIV (2919), p. 77- 2In “Silikatschmelzlés.,” IL (1904), I gave the proportion 25.75:74.25, which is practically the same. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 335 curve, located quite close to the eutectic between the microcline components and quartz. As microcline (from dikes of granite-pegmatite) always contains considerable Ab;an, most often 25-30 Abian: 75-70 Or, it almost certainly has a somewhat lower melting-point than pure Or, and we must therefore assume that the eutectic Qu:Or contains a little more Qu than the eutectic Qu: microcline. As the binary eutectic we shall assume 28 Qu:72 Or. As pure Ab has nearly the same melting-point as pure Or, nearly the same molecular weight (Or=279.4, Ab=263.3), and possibly also almost exactly the same melting-heat, it must be supposed that the eutectic Qu:Ab holds about the same percentage of Qu as the eutectic Qu:Or. As an approximation we may consequently assume 28 Qu:72 Ab as the binary eutectic. QU:AN AND QU:AB, QU:OR For the pressure of one atmosphere we have the synthetic determination: Eou-an=48 Qu’: 52 An, at 1353°. Further we refer to the determinations just mentioned (for a very high pressure): EQu-Or= ca. 28 Qu: 72 Or EQu-ab= ca. 28 Cu:72 Ab. Even if an error of a few per cent may be found in the latter statements, it is evident in any case that the binary eutectic ~Qu:Or or Qu:Ab contains much less quartz than the binary eutectic Qu:An. This is in accordance with Ab (as well as Or) having an essentially lower melting-point, in round numbers 450°, than An. The course of the melting-curve (see Fig. 3) on the Qu side in the neighborhood of Qu will be about the same, whether the second compound is An or Ab (or Or). If we extend the curve, experimentally determined for Egu-an on the quartz side, and draw the curve on the feldspar side for Equa, (respectively Ho.-o;) about parallel with Eou-an, a binary eutectic Eouap (respectively Egu-or) will appear with composition about 25-30 t Qu here signifies christobalite from the melting-point to 1470° and tridymite from 1470° to 1353°. 336 JOH. Ey VOCE Qu:75-70 Ab (or Or), just as we have in reality derived from the analyses of the graphic granites. As the melting-points as well as the binary eutectics (as will be shown in a following chapter) are only very little displaced by pressure, we are justified in drawing a parallel between the eutectic SiO,:An, determined for low pressure, and the eutectic SiO,:Ab (or SiO,: Or), calculated for high pressure. The case is somewhat 100An OAn or Ab or Ab OSi0, 100 SiO, Fic. 3.—Melting-diagram, An:SiO, (after Bowen), and Ab:SiO, (schematic after Vogt). ; complicated, however, by the fact that SiO, in one case is tridymite but in the other quartz. | The melting-point for the binary eutectic Equa» (or Eou-or) must, according to the nature of the case, lie considerably lower than the melting-point of pure Ab (or Or), consequently consider- ably lower than 1100° and certainly somewhat lower than 1000”. As an estimate we set it at 975°, which should be approximately correct. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 337 Qu:Ab+An.—In a ternary system consisting of two components (as, for instance, Ab and An), which form a binary mix-crystal system of type I, and a third component independent of the former (as Qu or CaMgSi,0¢), there appear, according to F. A. H. Schreine- maker’s theoretical investigations,’ two melting-surfaces (Fig. 4), Tou ca. 1675° Qu Fic. 4.—The ternary system An:Ab:Qu which intersect in a curve, viz., ‘‘eutectic boundary-line’’? or briefly a “eutectic line or curve.” Three subcases may occur, accordingly as the eutectic boundary-line (Equ-an to Equ-ap on Fig. 4) has a continuous decline, a minimum, or a maximum. In the ternary system Diops:Ab:An the eutectic line according to Bowen’s experimental investigations (see Fig. 6) has a continuous decline from Epjops-an (at 1270°) to Epjiops-ap (at a little below t Zeitschr. f. physikalische Chemie, 1905, Vols. 50, 51, and 52. 2 This term I have used in my earlier treatise. Boeke (loc. cit.) uses the shorter term “‘eutectic line.” 338 EEE VOGL T100°), consequently with a difference of ca. 200° between the two points. For the analogous system Qu:Ab:An, where the difference between the two points (Egu-an at -1350° and Egu-ap at probably a little below 1000°) is still greater, certainly at least 350°, we may also suppose a continuous decline for the eutectic line. This line, on account of the steep decline near Ab of the binary liquidus curve between An and Ab, will probably assume the course outlined on the horizontal projection, Figure 5. The crystallization be- tween Ab+ An and Qu has consequently the same course as between Ab+An and diopside (see below). Even if the curve between Eou-an and Egu-ap, contrary to our conjecture, should show a maximum in the vicinity of Egu-an or a minimum in the vicinity of Eou-ap, this would in no degree worth mentioning modify the course of the curve in the horizontal projection. We calculate the chemical composition of the end members and of a pair of intermediate compositions. TABLE III EQu-Ab+An by Percentage of EQu-Ab ee EQu-An = Ab:} An 2 An:3 Ab Quay Mane teres 28 30 42 48 {Ab 72 48 20) ikea ea sce aVa Beatin ot Ale doll Bedell Hi 8 cape aie 16 209 52 SiO ate Dh AS 75-95 74.5 70.5 NIE Ores Acetate b 13.95 i 52 16.25 19.0 CaO ecg se Sema ceenta a BoB 5.85 10.5 Nat © eeeniie 8.50 5.65 2.4 |) Ge That the calculation here given of the eutectic between quartz and albite, oligoclase, etc. (which is supported by the theoretical argument on the eutectic Qu: An at the pressure of one atmosphere, and by analogy conclusions according to the composition of micro- cline graphic granite) is essentially correct, is confirmed by the close conformity between the two analyses, Nos. 12 and 13, of oligoclase graphic granite and the compositions here calculated, especially for 2 Ab:% An. For the system Quartz:Orthoclase (microcline, with about 72 Or+28 Abian):Albite (with about 88 Ab or Ab+an and 12 Or) we must have three individualization-fields, with partial eutectics MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 3309 respectively about 26 Qu:74 orthoclase (72 Or+28 Ab), about 26 Qu:74 Albite (Ab with little An and about 12 Or) and about 40 Or:60 Ab+an (or 42:58). If we leave the inconsiderable St0,,1625° Ab, ca.1100° An, 1550° Fic. 5.—The ternary system Ab: An:SiO, (horizontal projection) admixture of An out of consideration, we must suppose for Qu:Or: Ab a ternary eutectic, with about 26 Qu and about 42 Or:58 Ab, consequently about 26 Qu:31 Or:43 Ab (or Abia,). We calculate the composition of these eutectics (see Tscherm. Miit., Vol. XV [1906], p. 385) as shown in Table IV. TABLE IV APPROXIMATE CALCULATION OF EUTECTICS QUARTZ: ORTHOCLASE (MICROCLINE): ALBITE (WITH LITTLE AN) Eutectic Qu: Ortho- “Ternary Eutectic”’ Batestic - Albit Percentage of SE Gar pou Oriecase Mite (88 Fis es AOE OS ils eee 26.0 |26.0 |26.0 |26.0 |26.0 |26.0 |26.0 |26.0 |26.0 |28.0 |32.0 (Oi? 5:56 ae $828 |S8.B S854 [Stew leew anise [Bien So 4 8.6 |) B77 | B.6 IND oo alts ieee pene BOP Ze LOn 2a 77 A2eA a IAEA 2OKOM|BOLON|O4e ON |O3HON|OORSmI5Gea JANOS 9 Bae O.§ || £5 || 2 9.5 | ¥o8 | BO | ©2© || ©c§ | Bog | B.© || ©.© [0.35 74-7 |74-45174.05]75-55|75-3 |74-9 |74.15]76.45|76.2 |76.45|76.9 ALO, ../13.85/14.0 |14.25/14.1 |14.25/14.55]15.05/14.35114.5 |14.4 |14.15 IKO somes 9.0 | ©. || ©. || G25] Kea5] Fas S225) wes | ug | wig! mAs NOR ts seks 285] QBo2S) Qo |) S$sO© |) Aw | A597 | ABs WoO || oR || Wee || One CAO a Qu || © O29 |) HO |] Ose GO |] WB 1] Cott!) On8 || Wale || 132 340 TEE VOGT TABLE V A = SPHER- a Quarrz- NDESITES ACITE wae ACITE PORPHYRY PERCENTAGE OF rs Emmons (Idd.) |(Lag.) | (Idd.) | (Lag.) | (Lag.) | (Lasp.) |(Streng) 14a 15a 16a 17a 18a roa 20a 21a 22a SiO Seer 59.06] 60.39] 62.00*|62.54| 69.36] 76.48] 75.07/72.24 | 74.11 PAIK O Fea rocsierty cir ae 8 16.40] 16.96] 17.84); 16). 23] 12.00] 12a u5|EsnOam momo ARGH Os eat nude AXo|| Ws SOloaosoa 225014) OnSS) | CO) O)5|\aelO 7] een EOP a heel aeptane AS) BAA) AAw T5352. 06 Soe 3.051] Legs IMgOe, cateetscce 2.03] 3.61) 2-04 | T.15) 1234 (oO. 30]) Onr4|sOsO0N | monog Ca Ose ions see AGA Bodin Beg | AWS) Bou O04) ©.) ©.05 1.38 INFO) Seria pesca aos 5540) BLS AAG) || Bat) ALCO Anti] 4.2) 2.5 1.54 K,O.. Sa acer a TE eAO|| Qo WA || Bove) B.Cx) B78) 4.57) 5-24) 5.67 H,O, ign ALO! AOR uO || 2.75) OAs) CaF woBdi| 7.20 0.56 UO coscoccs 98.31] 99.I1|100.13 |99.35|100.04] 99.96] 99.92/100.11] 99.90 PHENOCRYSTS (14b) (150) 166 17) 18) 19) 20b 226 SIO. ee Hee ee 56.25| 55.92|/56.41 | 55.42|(65.771)| 62.14] 65.40]...... 61.80 A OR cree N59) Ad. G37 .40) || AI.Cu|| Qe. Ful) QH=.34o)| ws.7aill. .- 55} 19.28 Hes Ose cen aera ©.77| 1.00] 0.691] 1.09] tr. tr. tr eee 2.02 IIIs OP ee ola. Gstaies I.06] 0.55] 0.09 | tr. ©. 60] (trs | se aa ae 0.01 CaOr An eens Os! O05) O37 |} Catal) GoW) B26) O.BOllooascaiioccu-- Naz Ont cance te 5A 5.60] 5.28 Soi] 5.OZ) TOG] So58ilo.0¢.- 0.68 KOM Seiscnaieen O-Oul| OC.00| O.4o || ©-76]) O.83]] W.00]) O-A5].....- 12.18 IBIAO), Ws, sooo coe O88) OcOGGllo 00055 On 521 Oogvil OBE] w.BOloo- 0 - 0.25 Tor ete 99.54] 99.71|100.24|100.05|100.09]/100.20] 98.90]...... 100.10 Grass Basis GROUNDMASS I4¢ I5C 16¢ 17¢ 18¢ IQ¢ 206 21C 226 SIO scarce 68.11| 68.60|69.94 | 70.19] 76.75] 74.50] 74.96] 74.41] 74.44 EO eee 15.56] 17.27/15 .63 (r2'.32] 12).88)) Ten67/|e seo] neers IN One somo case urd ©5@60) B@-CO)) 20st] BH7oUOH socec ©).86| 1.60] e el aanieee LES OVI ais ec cults DUD Oe SS] i 0) THEO Rena alors |G 0's 2).08|| eae INCOME sek ieee TEATO| OAC MOn2 Oil OMs52) | ONOO|NNORGO|MNnEL: 0.50] 0.01 CaO eps aee A Oil] 172) QoA@ 2.50) 1.28) ©. 70) Ono? te e'S | amram INGOs cod cago 055 Bofle| eee) Bins || Bye Bohol) Bose] 2.76] S27) FB. ae UA Oe etary ner te 2 Hitl| AL. Oil) Qatds 2.80] 3.08] 5.35) 4al4 4a o| mye 1BLO) Ho ono coc A2A|| 2.C3] 3.2 2°30) (6.54 i 103||) eS 2) eet O- | erences Motalle neers 99 .62|100.65|100.16] 99.91] 99.68} 99.01] 99.41/101.55] 90.45 * +-9.17 TiOz, o. 29 P.O; in the rock. 7 In the rock 0.13 and in the groundmass o.30 per cent MnO. fis FeO. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 341 EXPLANATION Nos. 14 and 15: H. Emmons, “Island of Capraja, at Elba,”’ Quart. Jour. London, 1893. ‘The two feldspars 14) and 150 isolated by density 2.67—No. 16: A. Hague and J. P. Iddings, “Volcanoes of Northern California, Oregon, and Washington Territory,” Amer. Jour. Sc., 3, Vol. XXVI (1883). Hyper- sthene-andesite.—No. 17: A. Lagorio, ‘‘Uber die Natur der Glasbasis sowie der Krystallisationsvorgainge in eruptiven Magma,” Tscherm. min. u. petrogr. Mitt, Vol. VIII (1887); from Hliniker Valley, Hungary.—No. 18: Hague and Iddings (loc. cit.); California. The plagioclase is andesine-oligoclase. The determination of SiO, in the plagioclase is too high, owing to impurity.— No. 19: Lagorio (Joc cit.). Spherulitic rock; from Alausi, Ecuador.—No. 20: Lagorio (Joc. cit.); from Summit County, Colorado. With phenocrysts of quartz and two feldspars, one monocline (analysis No. 200) and the other tricline—No. 21: Laspeyres (see Zirkel’s Textbook of Petrography, 1894, Vol. II, p. 177); from Halle, Germany.—No. 22: A. Streng, Neuer Jahrb. f. Min., Geol. u. Pal., 1860, from the Harz Mountains. In order to establish that the crystallization in the eruptive magmas of the different feldspars and of quartz is in conformity with the physicochemical details which we here have developed essentially on the basis of the analysis of graphic granite, we refer inter alia to my earlier statement in Tscherm. Mitt., Vol. XXV (1906). On pages 340 and 342 we give a small selection (analyses Nos. 14-29) from the numerous analyses, compiled from the literature, partly of porphyritic rocks, with special analyses of (a) the whole rock, (6) the porphyritic feldspar, and (c) the glass or ground- mass, and partly of some granites with special analyses of (6) the basic concretions (or orbicules), and (c) the inclosing rock. Granites with basic concretions (Nos. 24-260), or basic orbs (Nos. 276-29b) are shown in Table VI on p. 342. | We call special attention to the following: 1. In the intermediate and the acid eruptive rocks, which contain the ordinary admixture of ferromagnesian silicates and iron, or titanic iron, ore (especially magnetite and ilmenite), an essential part of these minerals crystallizes at an early stage. A small quantity of Fe.O,, FeO, and MgO, however, is left in the remaining magma. ‘This appears in the solidified rocks as the glass basis or groundmass in the porphyritic rocks, or as the inter- vening mass between basic concretions or orbicules in granites. 342 JE SEE OV OGLE In the final, very complicated eutectic, chiefly consisting of feld- spars and quartz, there usually seem to appear o.5 per cent Fe.O,, ©.25 per cent FeO, and o.1-0.25 per cent MgO; thesaeunes: TABLE VI Basic Concretions Basic Orbs Percentage of 23b 2401 25): 25b2 26b 276 285 20): 202 SIO Mere essere 53.80] 54.73] 560.01] 56.53] 64.30155.72 | 65.57) 61.10} 68.02 AMOS seen Me eee Ou'77|| (its To} a“) eR OSS V5.6 0.06 OS Til ecu A Ose eine Ce IQ.20| 14.02] 15.19] 16.47] 15.909/21.35 | 17.46] 15.55] 15-31 Hes Ose ee bas kc te 7 60|. 2434 DVM Atl APSA So oe « 4.1s|L 2-20 0.59 EO sees 4.92] 4.89] 5.40] 5.98] 8.81 1 22a) arenes Wir). ene Fe meni | eis eee [fee ee o.40| 0.20] tr. OMIM Iho llaob oS olloddc co - IMO) lat eaka-csero ote AKO GoAC| AO7) 2.07] 2.67) ©.0% 2053) OngolmeseAn CaO eran 557} UO.2O)| ABS A.@el] B57) R10 240 | ae eee INAS Oe a rome nines 2.16] 2.98] 5.66] 5.50} 4.06] 5.71 eal Aa) 2 ths IRAQ Eee eevee Jn Foes] DaO7|| Asad) Becskoll AAO 1 93 || “asl @.38]| S-67 IPO a eonon cena. 1.20] tr O53) O27 “tre! | 2. Be] | ee Tice US Ree ae eed: Te? S| 2 3|| 2 0 ORO? ||NNOHOS|NOn4O 1.26] 1.80] 0.46 Mota eee IOT.59|100.99] 99.21) 99.98]100.44/99.94 | 99.83|100.58] 99.98 Intervening Mass 23¢ 246 25 Cx 2502 26¢ 276 28¢ 20C SM"Orscccscsoccas 74.40| 70.44] 71.90] 73.60] 73.70] 70.05 |68.27] 71.7I]...... ABIL @ Fresco neon eral Ieeoreeeresel|senret too ©.35] ©.23)) we. OBE MMe loecc sollac sca c TN O etal cone aren epete 13.91| 15.63] 14.12] 12.46] 14.44] 14.78 |15.50] 15.05]...... Hel O easyer Nt : TAA TA) WAT Oa.ABloooss< ‘i rea eee HORS aes f “39 x 1a) ©.85| woG/s| BLO] 3.37 3} 0.20 BE sc IY Bol OGD rea cmnariee Peet cael lest ara ©.©§] O.i15)) we O22 ||. ascent See Mig @ ya anne P23 | OMS 5 ONS S| ME OMEL/7]|eNt ©: 44) ||) a0] Ons 0] peer CaOh etl estes ees OQoOn|) LOS} eons! CoAO) aocks)| B71 || WOB) #-ABacooc- Na,O He eM t ate ALO ACR Asgall AeA) Mall Bone |) B@n\| 4. 30)\occ55- AO eoetne tte Seo. 5 AGRO! Sows) | Anni) Ao) ALA) aot |) 5-87) SHAG .200¢ 1D © Feige oe eer meal meet ei om til eer o VECO gee MMA Iyer IBA Blalhe'o.6 dia allo ov 0.¢ hemac.b elke uate HOS) O55] O09) C.gs)| O.0n] Oa || 1.55) @.O7||,...-. Mota eae vac LOO. 25/100 .82|100.35|100.00/100.30|100.12 |9Q.25] 99.77|.-.-.-- EXPLANATION No. 23: See Rosenbusch, Elemente der Gesteinslehre, 1901; from Pelvoux.— No. 24: Graber, Jahrb. d. k.-k. geol. Reichsanstalt, Wien, 1897; from Topla in Carinthia.—No. 25: Clarke, U.S. Geol. Survey Bull. 168, 24; from Mount Ascutney, Vt.—No. 256, and ¢2, granite porphyry.—No. 26: J. A. Phillips, Quart. Jour. London, 1880; from Peterhead, Scotland.—No. 27: H. Backstrém, Geol. Foren. Férh., Stockholm, 1894; from Kortfors, Sweden.—Nos. 28-29: K. yv. Chrustschoff, ‘Uber holokrystalline makrovariolithische Gesteine,” Mém. de Vacadémie des sc. de St. Pétersbourg, 1894. No. 28 from Altai; No. 29 from Fonni, Sardinia. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 343 however, especially for MgO, depend somewhat upon the Mg- bearing mineral component in question (biotite, hypersthene, etc.). 2. As previously mentioned and as illustrated by analyses Nos. 14-19 and 20-22, plagioclase crystallizes when there is a surplus of Ab++An; orthoclase, on the contrary, when there is a surplus of Or in the original solution. The boundary lies, as previously explained, at about 0.4 Or:0.6 Ab+An. In the plagioclase which crystallized first, relatively much An appears, consequently relatively much CaO (and ALO,) (cf. the analyses Nos. 140-19)). In consequence, the remaining magma shows a decreasing percentage of CaO, and this in the “granite eutectic” falls to 0.25, 0.5, or 1 per cent CaO, or, with predominating plagioclase in the eutectic, not quite so low. If we leave magmas with only a trifle of Na,O (Ab), or of K.O (Or), out of consideration—where the crystallizing orthoclase absorbs practically all of the Ab or the crystallizing albite or albite- oligoclase the Or—the contents of Or (or K,O) in the magma remnant increases by the crystallization of plagioclase (cf. the analyses Nos. 14c-19¢ and 24c, 27c),and the contents of Ab (or Na,O) in the magma remnant increases by the crystallization of ortho- clase (cf. No. 21c). But we especially emphasize that this relative imcrease has a limited course, and that the limit of about 0.4 Or:0.6 Ab or Ab+An ts not exceeded, or only perhaps now and then some- what exceeded because of supersaturation. In this matter we especially refer to the analyses of the glass basis or groundmass of porphyritic rocks. In judging these analyses, however, we must take into consideration that the glass, as is shown, for example, by the water content, is always or nearly always somewhat decom- posed, whereby especially a little alkali will be extracted. We further refer to the intervening material between basic concretions or orbicules in granites. As examples these intervening masses from casually chosen granites show: In Nos. 28 and 209, where orthoclase crystallized first, res- pectively 0.45 Or:0.55 Abyan and 0.46 Or:0.54 Ab+an. In No. 23, where both orthoclase and plagioclase seem to have crystallized at an early stage: 0.37 Or:0.63 AD+an. 344 TE VOR In Nos. 24, 25c% and c?, 26, and 27, where plagioclase (or perhaps predominant plagioclase and some orthoclase) crystallized at an early stage, respectively: 0.40, 0.38, 0.41, 0.38, and 0.36 Or. The remainder is Ab+an. These values, calculated from the analyses, for the proportions Or: Ab+An need, however, a small correction, as we have taken for granted that the whole quantity K.0, Na.O, and CaO form respectively Or, Ab, and An, while in reality a trifle K,O (and a still smaller amount of Na.O) in several cases enters into biotite. 3. The glass basis, or the groundmass, in andesites (with at least about 56 per cent SiO., that is to say, with at least so much SiO, that a little of the independent component quartz entered into the melted magma) and also in dacites, trachytes, rhyolites, quartz-porphyries, etc. (with max. about 72 per cent SiO.) without exception shows an increased percentage of SiO, as compared with the entire rock. Gradually as the crystallization, for example, of an andesite with s9 per cent SiO, (No. 14a) advances with solidi- fication of magnetite, ferromagnesian silicates, and medium-basic plagioclase, the temperature drops, and simultaneously the mother liquor becomes richer in SiO,. By rapid cooling the viscosity increases and causes the cessation of the crystallization; in other words, the fluid remnant stiffens into glass, with varying percent- ages of SiO, according to the time at which the crystallization ceased. And this point of time may lie even considerably lower than the stage of the final eutectic. This we may illustrate by giving the percentages of SiO, in the entire rock and in the glass basis (or in some cases the groundmasses); the latter, however. always contains a little H,O, showing that it is somewhat decom- posed. (See my treatise in Tscherm. Mitt., Vol. XXIV [r1905].) ANDESITES Percentage of SiOz in ROC a ielt Lacnss okane 56.8 §7.8 58.1. 50.1 60.1 60.4) (6270 G2) suycemm Glass basis....... 64.5 65.1 70.8 68.1 68:7 68.6 60.0)9607 ON jome DACITES Percentage of SiO: in ROCK oc Ae eoheosateet ere O56) 10S 43) 00h4) s O5mS O5.5 67.3 Glass. basis )?\ae% 302 sees 41.9 74.8% 76.75 70.2 70.2 ald) 73-0) age * Goundmass. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 345 Further we include a series of rhyolites, dacites, spherulites, and quartz-porphyries, in which phenocrysts of feldspar (orthoclase or plagioclase) as well as of quartz usually appear. Gl.=glass basis; Grm.=groundmass; Sph.=Spherulite. TABLE VII PERCENT- SPHERULITIC AGS OS DacITES RHYOLITES AGES Sph. Grm. | Grm. |Sph.and GI. | Gl. Sph. Gl. Gl. Sph. Gl. HOG) 4... . SiO, 174.6 |75.07|/72.5 Wha N7BcO 71.4 76.5 72.8 Gl., Grm., | {SiO, [76.05*|74.96/77.51| 72-7 |72-6 |74.5|72-5]74.6 |73.7|70-7 or Sph...| |H,O | 22 iS | woz ©. || ASS TA ALAl uO | BoA 8.7 * Only ro.24 per cent Al.O;. Somewhat decomposed. { Only 1.52 per cent Al.0;. Somewhat decomposed. Percent- Spherulitic Rocks with Quartz-Porphyries with age of |Spherulites and Glass Basis Groundmass Sollee oe as. neo: 73-2 174-4 175.4 |73-4|Qu-porph.. .| 72.0 he, TAP ERs WW, 172.4 |72.7 |73-9 |73-1 74-4]74-4](70.05 Cl... ron Ae glee la onl AS a The glass basis, or the groundmass, in the Bonnin rocks consequently shows: a) In rocks with about 60 per cent SiO,, a sometimes very considerable increase in the percentage of SiO, (for example, from 58.1 to 70.8 per cent SiO.) and in rocks with about 65 or 65-70 per cent SiO, a smaller increase, though in undecomposed state not above 75 per cent SiO.. b) In rocks with about 73-75 per cent SiO, we find, on the other hand, about the same percentage of SiQ, in the glass basis or groundmass es in the entire rock. The analyses show some slight variations, partly in one and partly in the other direction. But this is certainly caused in some cases only by slight inaccuracies in the relatively old analyses, and in others by the groundmass and especially the glass basis (probably without exception) being a little decomposed, as shown by a little H,O. These law-governed relations, which are established by nu- merous analyses, may depend on the fact that in a magma consisting chiefly of Qu and Or, Ab and An components, with a 346 Svs Es VOCE surplus of feldspar components, the crystallization of feldspar may continue without a simultaneous secretion of quartz until the eutectic boundary-line between quartz and the feldspar components has been reached. When this has occurred, however, a simultaneous crystallization of feldspar and quartz commences, with only a quite inconsiderable change of the SiO, percentage of the magma remnant, while we constantly more and more approach the “ternary” eutectic: Qu:Or:Ab+An (with a trifle magnetite and ferromagnesian silicate). The groundmass in the quartz porphyries and the closely related rocks consists, as is well known, in some cases of microfelsite and in others of granophyre, and these structural forms indicate a simultaneous crystallization of quartz and the feldspar in question. The final crystallization consequently took place also with regard to the structure at a eutectic or eutectic boundary-line. Especially acid quartz porphyries (with more than 75 per cent SiO,) show phenocrysts of quartz and feldspar in about equal amounts, but groundmasses of normal microfelsite or granophyre, that is to say, with relatively less quartz than among the pheno- crysts. The groundmass, consequently, here must have grown a little more basic than the original rock. I lack material, however, to prove this by chemical analysis. If we now turn to the deep-seated igneous rocks, we find that the quartz-norites and quartz-gabbros (with about x to 5 or 6 per cent quartz), the quartz-diorites, the quartz-syenites, etc., pre- vailingly show that the quartz first began crystallizing at a relatively late stage. As we shall explain later (Figs. 17 and 18) when treating of the quartz-norites, this crystallization of quartz at a late stage took place, not by itself, but simultaneously with the final crystallization of the feldspar (the plagioclase) and the ferromagnesian silicate in question. In the granite porphyries, which contain but little ferromagnesian silicate and magnetite but are especially rich in feldspar, with about 66-70 per cent SiO,, the crystallization commenced with the solidification of some feldspar. The crystallization in ordinary granites usually commenced with a solidification of some magnetite and ferromagnesian silicate, MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 347 while the feldspar first commenced crystallizing at a somewhat later stage. The sequence of the commencement of the crystalliza- tion in the granites is in most cases (1) pyrite, zirkon, apatite, etc.; (2) iron ore and ferromagnesian silicate; (3) feldspar; (4) quartz. But it appears from the structure that the ferromagnesian silicate, especially biotite, continued crystallizing after the commencement of the solidification of both feldspar and quartz, and that the feldspar continued crystallizing also during the segregation of the quartz. In most of the granites, however, we are unable to deter- mine with accuracy from the structure, the quantitative propor- tions of feldspar and quartz during the intermediate and later stages of the crystallization. The case is complicated by the fact that the granite magma contains, besides the usual ferromagnesian silicate, Or, Ab, An, and Qu components, some H.,O, probably partly entering into a SiO, combination, for example, as H,SiO, (?), and the latter was not split up until a later stage of the crystallization period. If this supposition is correct, the consequence will be a somewhat reduced quantity of the independent quartz component during the first part of the crystallization period—that is to say, during the first part of the crystallization the feldspar was rela- tively more abundant than that corresponding to the proportion calculated from the relation between feldspar and quartz in the resulting solid rock. We have an instructive orientation on the composition of granite magmas at a late stage of the solidification in the composition of the intervening masses between basic concretions, or orbicules, in granites, which show these structural elements. (See analyses Nos. 23¢-29¢.) These intervening masses prove throughout that during the crystallization a displacement of the composition of the magma remainder took place in the direction of the—in other ways determined—‘‘granite eutectic,’ and we especially empha- size that the analyses of the intervening masses Nos. 2 BG, QEGoy Biol 26¢c almost exactly correspond with the “‘granitic eutectic.” Above we have only considered granites with relatively basic concretions, or orbicules. But from strongly acid granites, with about 78-80 per cent SiO, in the whole rock, we know a couple of 348 JH. EVOGH examples’ of orbicular structure, the orbs chiefly consisting of quartz, and in addition some sillimanite and tourmaline (!). The intervening mass between the orbs here contains less SiO, (and less quartz) than the orbs, therefore reckoned from the acid pole there here appears a displacement of the residual magma in the direction of the eutectic, quartz:feldspar. In these cases we have, however, the complication that the orbs contain much tourmaline and sillimanite, the latter being very little soluble in acid magma. Finally we compile a series of analyses of glass bases (Gl.), respectively groundmasses (Grm.), and spherulites (Sph.) from porphyritic rocks, and intervening masses from granites with basic concretions, etc. These analyses represent approximately the composition of the residual magmas resulting from far-advanced TABLE VIII ANALYSES OF THE FINAL PRODUCT OF THE CRYSTALLIZATION OF ACID ROCKS—THE GRANITIC EUTECTIC SiO; No. Without SiO. Al.0; Fe.0; FeO MgO CaO Na20 K.0 H.0 Total FO GISiSpheeeeee 30 oft W72eOl\o 5 ooo 15.04|....] tr. jo. 25/1. 75/8. 8510.94] 99.52 TMM Se eevee Bir HbeY W7i2bo@) lao dgocllacanc easieles sls oe «lOOOl7n5(7]| eres | ene 32 AQ oas Ou oc oc 16.28]... ./0. 20l0. 59/2.12/6.40|/1.35| 99-47 : 23 Tok 73037 2Bo8Ollc ooo 1.54|0. 26/0. 99/3 .00]/5. 74/1 .08/100. 07 Glass basis... .| 434 73-3 |72-35|13-97| I-20 0.46\0. 72/3. 58/5.38!1.37| 99.12 35 | 73-7 |73-05|14.67| 0.89 0. 26/0.97/3-99/5.11]0.91| 99.85 9c | 75.3 |74-59|12.88] 0.80 0. 30/0. 76/3. 30/5. 35|1.03| 99-01 Intervening 2562| 74.0 |73.69|12.46] I.21|1.75]0.17/0.3614.47/4.92/0. 38/100. 09 mass of 23€ | 74.9 |74.40|13.91| 1.39]... .|0.28)0.61/4.65/4.36|0. 65/100. 25 granites....| |26¢ | 74.1 |73.70|14.40| 0.43|1.40] tr. |r.08]4. 21/4. 43/0. 61|100. 39 36 74.0 |73.21|12.90| 2.10]....]0.27/0.88/4.8314.75|1.04] 99.98 Spherulites...| 437¢r | 75-1 |74.52|12.97| 2.02]... .]0.2510.92I4. 26/4. 53/0. 83/100. 30 38 74.7 |173.72|12.91| 1.37]....]0.25|1.37/4.0214.45/1.30] 99-45 Ge rere 30 73.5 |72.70|13-79| I.01],...|0.65|2.07/4.93|4. 33/1. 10|/100. 48 Sph {3762 | 74.0 |73.42|14.20] 1.01]... .]o.43/1.00/5.61/3.19]0.84] 99.79 Pataseest er \40 74.8 |74.36/14.46] 1.62]....]0.44/1.49/6.11|1.490/0.57|100.54 t From Krageré and Modum in Norway and from Pine Lake in Ontario (lecture. by W. C. Brégger on the Krageré locality, in Kristiania Vidensk. Seisk, 1901, and Frank D. Adams, Bull. Geol. Soc. Amer. No. 9, 1898, see résumé in my treatise in Tscherm. Mitt., Vol. XXV [1906]). MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 349 EXPLANATION No. 30: Spherulitic glass basis from liparites—No. 31: groundmass from quartz-porphyry.—-Nos. 32, 33: Glass basis from trachytes.—Nos. 34, 35, 30: Glass basis from spherulitic rocks—Nos. 36, 37¢: and C2, 38, and 40: spplucinulittes from spherulitic rocks. Nos. 30, 32, 34, 35, 30-40 reprinted from the above-cited excellent treatise of Lagorio, 1887.—No. 31, see Zirkel’s textbook—No. 33, Williams, Neues Jahrb. f. Min., Geol. u. Pal., Beil. eo V, 1887 (see also my treatise in Tscherm. Mitt., Vol. XXIV [1905]). crystallization. In judging these analyses we must take into consideration that throughout a little alkali was probably extracted from the glass basis, so that the determined percentages of alkalies may bea trifle toolow. The analyses are generally arranged accord- ing to decreasing K,O (Or) or increasing Na,O (or Ab+An). We especially direct attention to the close accordance between these analyses of the residual magmas resulting from the solidifica- tion—partly from dike and surface rocks, and partly from deep- seated rocks'—and the eutectic Qu:Or:Ab+An, calculated on the basis of the graphic-granite analyses and the theoretical explana- tions. (See the table, p. 339.) The accordance is especially pro- nounced when we take into consideration that the percentage of SiO, in the analysis of graphic granite will be reduced about 1 per cent when r or 2 per cent of ferromagnesian silicate and magnetite is added, and that the analyses of groundmasses, etc., which are rich in plagioclase, contain a little more CaO (or An) than the graphic granites calculated in the table, page 339, where only a small admixture of An is presupposed. The analyses in the table, page 348, of the residual magma represent the granitic eutectic, consisting of predominant Qu, Or, and Ab+An, with the addition of quite small admixtures of iron oxide (Fe,O,) and ferromagnesian silicate. If we leave the latter quite subordinate admixtures out of . consideration, the analyses Nos. 25¢2, 23c, 26c, 36, 37c, and 38 almost exactly represent the “ternary”? eutectic Qu:Or:Ab (or Ab+An) with nearly exactly 0.4 Or:o.6 Ab (or Ab+An). And this we may by a short catchword name the “‘ternary”’ granitic * Regarding the inconsiderable influence of the pressure on the composition of the eutectic we refer to a following chapter. 350 Jd. ES VOGR eutectic. This term strikes the essential point, since the eutectic in question consists practically only of Qu, Or, and Ab. To this must, however, be added a quite small admixture of An, iron oxide, and ferromagnesian silicate, so that the eutectic in reality is more complicated. In order to avoid misunderstanding I have therefore put the term ‘“‘ternary”’ in quotation marks. [To be continued] RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA? CHESTER K. WENTWORTH University of Iowa INTRODUCTION The nature of the northeastern termination of the great over- thrust block of the earth’s crust, bounded on three sides by the Pine Mountain fault, the Hunter Valley fault, and the Jacksboro cross fault of Tennessee, has long been an unsolved problem to students of Appalachian structural geology. Many geologists have noted the rather abrupt ending, near the breaks of Big Sandy River, of the imposing barrier of Pine Mountain and its replace- ment to the northeastward by the irregular ridges and valleys of the unbroken coal field, but the manner in which the great anti- clinal fold and the resulting thrust fault died out has not until recently been satisfactorily solved. In 1916 Hinds,’ in his report on the Clintwood and Bucu quadrangles, called attention to a zone of disturbed rocks nearly at right angles to the general lines of disturbance in this region and extending partly across the trough of coal-measure rocks from Big A Mountain to Skegg Gap on Pine Mountain. Hinds attrib- uted the disturbance in this zone to the same forces that produced the Hunter Valley fault on the southeast and the Pine Mountain fault on the northwest, but he failed to perceive its significance, for he thought it was limited to certain areas and did not extend entirely across the synclinal block. In April, 1920, Mr. M. R. Campbell, in charge of geologic work in this coal field for the United States Geological Survey, called attention to the possibility of the belt of disturbed rocks mapped * Published by permission of the Directors of the U.S. Geological Survey and the Virginia Geological Survey. The illustrations were prepared for the Virginia Geological Survey. _ ? Henry Hinds, “The Coal Resources of the Clintwood and Bucu Quadrangles, Virginia,’’ Virginia Geol. Survey Bull. 12 (1916). 354 352 CHESTER K. WENTWORTH by Hinds being but part of a continuous fault or zone of faulting from the Hunter Valley fault at Big A Mountain to Skegg Gap in Pine Mountain, and the author was requested to examine the region as carefully as the limited time at his disposal would permit, in order to establish the character and extent of the movements that produced the disturbance. ‘The result of his examination was the establishment of the presence of an overthrust fault entirely across the great crustal block, thus showing that it is bounded on all four sides by overthrust faults and that it has moved bodily to the northwest a distance of many miles. The results of his studies and their application to the mechanics of the problem of the overthrusting of this great mass of strata for at least six miles are here set forth. The fault bounding the crustal block on the northeast, which, on account of its general agreement with the course of Russell Fork, is here called the Russell Fork fault, was mapped in connection with coal investigations carried on co-operatively by the Virginia Geological Survey and the federal Geological Survey. The areas mapped as undifferentiated buckled and faulted rocks by Hinds,* on the Clintwood and Bucu quadrangles, were subjected to careful study by the writer to determine whether or not there was a | continuous break across the coal-measures trough from the vicinity of Big A Mountain to Skegg Gap, but in the two weeks spent on this study there was not time to cover much of the area lying on either side of this zone and the structure contour maps of the report by Hinds furnished many data in compiling the sections shown below and in deducing the amount of displacement. The writer is indebted to Mr. M. R. Campbell for many helpful suggestions and much assistance in the course of the study. Hinds, in his report on the coal resources of the Clintwood - and Bucu quadrangles, describes the structure of the northeastern end of the Middlesboro syncline in considerable detail. His studies here and farther northeast in Buchanan County? have shown that Op. cil. 2Op. cit. .3 Henry Hinds, “‘Geology and Coal Resources of Buchanan County, Virginia,” Virginia Geol. Survey Bull. r8 (1918). RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 353 the great overthrust of Pine Mountain suddenly becomes very much less severe at Skegg Gap, and from there northeastward the structure is essentially a low anticline broken by a minor over- thrust which decreases rapidly in extent of thrust and comes to an end a few miles into Buchanan County. He considered that the principal Pine Mountain overthrust was cut off at the north- east end by the Skegg Gap fault which he mapped as far as Russell Fork. Between this point and Big A Mountain he has mapped a number of narrow areas of faulted and buckled rocks which he describes in some detail and in explanation of which he postulates lateral shearing with the southwest side moving northward with some overthrusting and buckling against the northeast side. He states that succeeding this movement there was normal faulting along this line in which the southwest side was downthrown. His evidence for this belief is not clear, and his several areas of dis- turbed rocks are separated by areas in which he found no evidence of movement. DESCRIPTION OF THE CUMBERLAND BLOCK’ The structure of the area concerned in this paper has been described in considerable detail at many points by previous writers.” It is not the writer’s purpose to give here a thorough description of the structure or topography but rather to point out briefly their alient features. The “remarkable quadrilateral block’? whose southwestern extremity was first recognized by Safford and described in detail by Keith extends from the valley of Cove Creek in Campbell County, northeastern Tennessee, northeastward for one hundred and twenty-five miles to the valley of Russell Fork of Big Sandy River in Dickinson and Buchanan counties, Virginia. It is sur- prisingly uniform in width, averaging about twenty-five miles, is bounded on the northwest by the Pine Mountain fault and on the southeast by the Hunter Valley fault and the closely associated Wallen Valley fault, which is developed only from near Big Stone t This name is here applied for the first time. 2 J. M. Safford, Geology of Tennessee (1869); M. R. Campbell, Geologic Folios 12 and 59, Arthur Keith, Geologic Folios 33 and 75, G. H. Ashley and L. C. Glenn, Prof. Paper 49, all of the U.S. Geol. Survey. CHESTER K. WENTWORTH 354 i ‘y pue € soinsiy JO suUOT}DeS oANjONI}s ayy Jo SUOT]LIOT PUL sassoUUAy, PUL “AYoNUDY “VIULSITA JO So}e}S OY} UL OIRO] S}I SuUIMOYS Yo[q purpraquing ay} Jo deur jeory—t “oly ureruenyASuuay-04 tre1ues|keuusg (SS SS Sod RSS - A _ i i Bc ngsert ais Sea pare aN fe i ,0f,28 RUSSELL FORK FAULT. OF SOUTHWEST VIRGINIA = 355 Gap southwestward. The southwest end is terminated by the Jacksboro cross fault and the northeast end by the Russell Fork cross fault to be described below in more detail. The general relations of these boundary faults may be more clearly seen by reference to the map and diagram, Figures 1 and 2, and to the structure sections, Figures 3 and 4. From Norton, Virginia, northeastward, coal-measure rocks are exposed at the surface throughout the entire width of the block; but from Norton southwestward the block may be divided into two parts, that part lying northwest of Stone and Cumberland mountains being synclinal in structure and composed of coal- measure rocks, whereas that part lying southeast of these moun- tains is anticlinal in structure and composed of rocks of very much greater age. ‘The syncline, which is now generally known as the Fic. 2.—Outline diagram of Cumberland block showing the bounding faults Middlesboro syncline, is a broad, flat-bottomed trough at the northeast end of the block, but farther west in the vicinity of Dante an arch appears which develops rapidly westward into the Powell Valley anticline that constitutes the southern part of the block. Both the Middlesboro syncline and the Powell Valley anticline are characterized by steep dips on the northwest limbs and gentle dips on the southeast limbs, as shown in the sections. The erosion by Powell River and its tributaries of the rising crown of the Powell Valley anticline accounts for the exposures of pre-Carboniferous rocks in the southern portion of the block and the narrowing of the coal-measure portion on the north. The general relation of these different structural units and their expression in the areal relationships of the Pennsylvanian and pre-Pennsylvanian rocks may be seen by reference to Figure 1. The topography of the block is intimately related to the struc- ture. Pine Mountain throughout its entire length is a conspicuous CHESTER K. WENTWORTH 356 (‘AaAING “Joay “SQ ay} Jo Te “f 17S uyayng pure ‘OF sadvg wuorssaforg ‘SZ pun ‘6S ‘EE ‘ZI souoy I180]004 WO} pazl[eiouas JeYMWOS puv po[idur0d Useq sARY SUOT}IES BSoy) “I IINSLY vas suOT}eIO] JOY “ysvayjI0U SUTYOO] SB SIay}O oy} “JSaMyY}IOU BULY{OO] SB UV—aS SI P—Y ‘99G “YO purploquing oy} Jo sanyonazjs ;ersues Surmoys suosegs—'f “ory ‘dq e19 ud 9) u3 i SM) aD SM) uo #q rad *“uI9 ae ag}? SRS EE FASS FESS eS Tee 2 eta == Leehed He Se SRR Rey ee Ee Ser ee eee Z s 74a) PAS ins | 9S 1 t tud5} MSD Bip 4s 4S t 5S 3a 21D TU Bmp RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 357 Cle. Beye Cle | _-7-Cle Sea /eve/ Fic. 4.—Sections showing the character of the Russell Fork fault at several Sections are shown as looking north. places. For locations see Figure 1. 358 CHESTER K. WENTWORTH barrier especially on its northwest face, where at many points it rises in less than a mile one thousand to two thousand feet above the streams which parallel it. For nearly ninety miles no stream crosses it and in the entire distance of one hundred and twenty-five miles not over half a dozen roads afford passage from one side to the other. Its crest is the resistant conglomerate of the Lee forma- tion, which marks the edge of the overthrust block. Cumberland Mountain and Stone Mountain are composed of the same forma- tion, which is steeply upturned in that part of the fold common to the syncline and anticline mentioned above. Black Mountain and parts of Sandy Ridge are residual mountains left in the dissection of the nearly horizontal coal measures. Big A Mountain, accord- ing to Hinds,‘ is composed of resistant sandstones of the Rockwood formation, overthrust on the coal measures. The surface features of that portion of the block within the coal field are those of a maturely dissected plateau with sharp- crested ridges and V-shaped valleys, for the most part without valley flats. The surface of the pre-Carboniferous portion is rolling, with sink holes in the limestone portion and some, though not at this particular point very striking, allineation of ridges and valleys with the northeast-southwest trend of the Appalachian structure. The surface configuration of the area and the control of topography by the great structural features 1s admirably shown on the contour maps of the United States Geological Survey, to which the reader is referred for further detail. THE RUSSELL FORK FAULT The Russell Fork fault differs from the faults which bound the Cumberland block on the other three sides in that it is not a low- angle overthrust and that in it the greatest displacement is in a horizontal direction with comparatively little vertical movement. Its trace? is closely followed except at a few places by Russell tHenry Hinds, “The Geology and Coal Resources of Buchanan County, Vir- ginia,” Virginia Geol. Survey Bull. 18 (1918), pp. 58-59. 2“Trace’’ of a fault is here used in its mathematical sense of the line of inter- section of one surface with another, i.e., the intersection of the fault plane with the surface of the earth. RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 359 Fork of Big Sandy River, and even at those places it is marked by the allineation of minor drainage lines or surface nenieues which would not otherwise be easily explained. Erosion of crushed and weakened rocks along the fault trace has produced the low saddle at Skegg Gap and the saddle in the point of the spur west of Russell Fork and one mile north of B.M. 1221" on the Clintwood quadrangle. From a point one-half mile upstream from B.M. 1282 to B.M. 1221 Russell Fork flows in a course somewhat farther northeast- ward than in adjacent parts of its course up and down stream. In the high land which lies southwest of this part of the river and northwest of the village of Haysi are cut two short cleftlike hollows which are closely aligned with the fault trace as located to the north and south, and have without doubt been determined by the presence of the weaker rock in the zone of deformation adjacent to the fault. One of these hollows enters the river valley just at the railroad bridge east of B.M. 1221 and the other extends from near Haysi south and just to the west of B.M. 1380. These hollows are somewhat straighter and narrower than most of the ravines of similar size which erosion has cut in the rocks of this region, but their most distinctive characteristic is their location where they cut off in part the narrow strip of high land between them and the river. Taken together and with the other topo- graphic features which show alignment, they are very significant, and their locations are not to be explained as accidental. Between McClure River and Russell Fork, at the close approach before they join, a low saddle in the spur owes its position to the weakness of the rocks along the fault line. The very straight course of Fryingpan Creek from elevation 1,311 feet to its mouth is determined by the fault, and it is interesting to note that this creek has a very slight fall in this part of its course and its bed is graded for the entire distance with ripple-marked sand. Russell Fork leaves the fault trace at a number of points and because of its cutting across the undisturbed and more resistant rocks at t The area crossed by Russell Fork fault is shown in detail on the Regina, Ky., and the Clintwood and Bucu, Va., sheets of the Topographic Allas cf the United States. Frequent reference is made to points on these maps in locating the features described. 360 CHESTER K. WENTWORTH these points is not so perfectly graded. But the weakness of the disturbed rocks close to the fault has apparently enabled Fryingpan Creek, though a comparatively small stream, to grade its lower course to the temporary base as determined by the rocks over which Russell Fork flows. The main line of the fault passes somewhat to the north of Abners Gap; from elevation 1,424 feet southeast to the mouth of Carroll Presley Branch it follows Russell Fork for most of the distance, and thence southeast to the point where it is truncated by the main overthrust fault; in the north face of Big A Mountain its trace lies somewhat to the north of the channel of Russell Fork. A branch leaves the main fault at elevation 1,424 feet, and extends northwestward along the course of Russell Fork to a point in Little Pawpaw Valley about a mile north of Cannady Post Office, and is here named the Little Pawpaw fault. Along most of its course the rocks northeast of the fault are horizontal or nearly so and undisturbed. The fault plane, or, better perhaps, the planes of movement, for the most part dip at high angles, 75° to go° to the southwest. That there has been intense compression is shown by the mashed condition of the shale and jointed condition of sandstone on the southwest side of the fault. At numerous exposures in the zone of faulting, slicken- sides indicate considerable vertical movement which has resulted in lifting the beds on the southwest above those on the northeast, displacing the coal beds by from 50 to 200 feet. Because of the shearing which has brought anticlines into contact with synclines and vice versa, it is difficult to determine the true amount of differential vertical movement, but the essential point is that the vertical movement is slight and that the hanging wall has moved up as a result of thrust. At many points there are planes other than those which bear the vertical slickensides, a series of hori- zontal slickensides trending closely in the direction of the fault, and usually these surfaces are rubbed and planed much smoother and more nearly plane than the others, indicating, it seems to the writer, that these surfaces are the result of more extensive movement along the fault line than the other planes along which RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 361 a slighter movement has taken place, to accommodate the thrust in a direction lateral to the main Russell Fork fault line (Figs. 5 and 6.) At a few points, notably at Skegg Gap (Fig. 7), the slickensides and planes of movement within the zone of faulting indicate a com- _ bination of the main southeast-to-northwest thrust with the side or southwest-to-northeast thrust, making the direction of movement a resultant of the two. ‘The slickensided surface here shows the result of pronounced movement and the white quartz pebbles which are so numerous in the Lee formation are planed off flush Fic. 5.—View of river bank looking southeast near B.M. 1227. Massive and undeformed sandstone on left of fault trace with deformed shale at right. Strong horizontal slickensides are formed on the face of the sandstone at this place. with the matrix. It is important to mention that a short distance northwest of Abners Gap the writer saw the most abundant evidence of thrust in the mashing and crushing of shale in a fine exposure and that here the slickensides indicate movement at an angle of 45° to the horizontal in a due north direction. At this point the fault trace is more nearly athwart the main direction of thrust, and here, if anywhere, would be expected evidence of strong overthrust. In the Little Pawpaw fault there was seen little indication of shearing but abundant evidence of compression and slight over- thrusting. | 362 CHESTER K. WENTWORTH MECHANICS The history of the deformation (Fig. 8) is conceived to be as follows: The rocks of the Cumberland block were subjected to strong lateral compression applied from the southeast. The thicker sedi- mentary rocks west of A (Fig. 2, p. 355) seem not to have yielded as did those to the east, and acted as a buttress against which the rocks to the east were deformed. ‘The compressional stress was much more intense at the Tennessee end of the block and the first result of the stress was the folding of the Powell Valley anticline and of the lateral anticline which later broke and formed the Jacksboro cross fault. Between A and B Keith™ found evidence of this now broken anticline which was the result of deformation of the rocks of the Cumberland block against the more competent buttress on the west. After the Powell Valley anticline had been in large measure formed and the Jacksboro cross anticline had probably reached its full development, the stresses were then transmitted across the block, and yielding farther northwest resulted in the folding of the rocks into the Pine Mountain anticline. It is probable that by” this time overthrusting and shearing to the northwest had com- menced at the southern end of the Jacksboro cross anticline, for the movement of the rocks of the Powell Valley anticline north- westward differentially with respect to the nearly undisturbed rocks on the west had already been very considerable. With the continued crumpling of the Pine Mountain anticline, the Jacksboro cross fault developed progressively toward the northwest, and, when it reached the then position of the corner C of the block, initiated the great Pine Mountain fault. There had by this time been considerable skewing of the entire block which was pivoted at or near its north corner, with the result that the corner of the block at A had been thrust more extensively on the rocks to the west than had the corner at C. The overthrust to the west in the Jacksboro cross fault is, how- ever, believed to be only the smaller movement incidental to the skewing of the block, while the main movement in this fault was t Arthur Keith, U.S. Geol. Survey Geol. Atlas, Briceville Folio No. 33 (1896). "wre RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 363 the shearing by which the block to the east moved several miles to the northwest. The writer does not believe that there was in the original stress any distinct southwesterly component, but Fic. 6.—Slickensided shale near mouth of Lick Creek south of Birchleaf considers the Jacksboro thrust to be solely the result of the twisting of the block. The Pine Mountain fault is compound at C, consisting of four distinct overthrusts, but becomes more simple northeastward. The faulting which commenced at the southwest developed pro- gressively toward D as the stress continued, but naturally the 364 CHESTER K. WENTWORTH total displacement was less at D than at C. It seems likely that there was some slight displacement beyond Skegg Gap and into Dickinson County along the Pine Mountain fault before it was intersected by the Russell Fork cross fault. In following chronologically the development of the Jacksboro— Pine Mountain line of faulting the Russell Fork cross fault was temporarily omitted. The history of its development is corre- lated with the events described above, as follows: Only after there had been considerable development of the Pine Mountain anticline at C, and some shearing along the Jacksboro cross fault, was the skewing of the Cumberland block felt at the northeast end. Its first expression was the development of a tension or normal fault starting at F (Fig. 2) and extending toward E with continued twisting of the block. The presence of the Little Pawpaw fault, which appears to be primarily the result of such tension incident to twisting, leads the writer to believe that the point, or, perhaps more correctly, the area of pivoting, was somewhat to the north of E. On the other hand, evidence of somewhat more pronounced compression along the fault from D, part of the distance toward E, seems to indicate that compression was even at first dominant in that part of the line. It seems therefore probable that the region of pivoting is located between D and E but somewhat nearer the latter. After the extension of the Pine Mountain fault beyond Skegg Gap and the extension of the Russell Fork cross fault beyond E as a normal fault, the accumulation at the northeast end of the block of the northwestward-trending stresses, which had long been operative at the Tennessee end of the block, reached the critical point, and the northeast end was broken loose along the line largely determined by the pre-existing normal fault. The line of this break intersected the Pine Mountain fault at Skegg Gap, stopped farther movement in that fault east of that line, and permitted the Cumberland block to be thrust not over two miles northwestward at this end. Since the Russell Fork fault line forms an angle of over 90° with the line of the Pine Mountain overthrust, the overthrusting of the east end of the block brought about compression along the whole extent of the Russell Fork RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 365 fault, reversing the condition of tension which produced the normal fault, and producing overthrusting and considerable crumpling of the shale and crushing and jointing of the sandstone adjacent to the fault plane. The net amount of overthrust is very slight, probably at no point reaching 500 feet, and the rocks on the south- west side of the fault are nowhere over 250 feet above those on the northeast. The shearing loose of the northeast end of the block, its overthrust along the already established Pine Mountain Fic. 7.—View of Skegg Gap looking north along fault line. At this point the resistant basal conglomerate of the Lee on the overridden side (right) is adjacent to the weak Pennington rocks of the overthrust side (left). fault, and the compression and slight thrusting along the Russell Fork cross fault were the closing events in the history of the Cumberland block as a unit. There are many especial features which are particularly in accord with this interpretation of the movement of the Cumber- land block as a progressive skew with final release of the east end. The skewing of the southwest end of the block first with the Skegg Gap corner remaining longest in place explains admirably the otherwise anomalous facts of rather strong overthrust in the Jacksboro fault, the trace of which is nearly at right angles to 366 CHESTER K. WENTWORTH RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 367 Schematic structure lines of sections. Present fault lines placed with reference to the block. — — — — — Reference line of block. Displacement lines of the sections. Sea-level lines of the sections. SRE SC ae Lines showing present position of fault boundaries and reference line of block. Fic. 8.—Serial diagram showing history of the deformation and displacement of the Cumberland block as interpreted by the writer. In sketch No. 7 of the series is shown the present condition in outline. The heaviest lines show the structure in the sections schematically. The lightest lines are the “‘sea-level” lines of the sections and the fault lines of the sections stil! referring to sketch 7 of the series above. The solid lines of medium weight are the present traces of the various faults. The medium-weight dotted line is an arbitrary axis line the positicn of which is the same with reference to the block in each sketch of the series. In sketch 1 of the foregoing series the present fault traces are shown by the light dotted lines of the background and the present position of the axis line by the light and straight dotted line. The then position of the fault traces and of the axis line of the block is shown in sketch 1 by the medium-weight solid and dotted lines respectively. The light ‘‘sea-level”’ lines and fault lines of the sections and the very heavy lines of the schematic sections are the same as in sketch 7. The arrows which point upward in the sketches indicate the direction of the main thrust; the arrows pointing down and to the right at the left-hand end of each sketch indicate the resisting stress of the buttress southwest of Jacksboro as described in the text. The first sketch assumes the prior formation of the Hunter Valley. The second shows some slight buckling of the southwest end of the block. The third shows more intense buckling here. The fourth shows more intense buckling and extension of the folding eastward along the line of the Pine Mountain fault. In No. 4 also is shown the beginning of the Jacksboro overthrust fault. In No. 5 this fault has extended far around the north side of the block and the Russell Fork fault has been initiated as a normal fault. Sketch 6 shows further extension of the faulting and only a small corner of the block near the northeast corner remains attached. In sketch 7 this small attachment is broken and the block has been thrust into its present position. The relative movement of the block at different stages has been shown by the gradual migration of the heavy dashed axial line toward its final position as shown by the light dotted axial line of each sketch. 368 CHESTER K. WENTWORTH the trace of the Pine Mountain fault and of only slight overthrust and more restricted compression in the Russell Fork fault with its trace at a much greater angle with the main overthrust. The stronger development of the Powell Valley anticline and the presence of the Wallen Valley fault only at the southwest are also strongly in accord with this suggested interpretation. The greater intensity of stress implied by the compound char- acter of the Pine Mountain overthrust at C (Fig. 2) and the prob- able development of the Powell Valley anticline before the rocks of the block were competent to transmit the stresses to the Pine Mountain fold which later broke in a fault, point strongly to the initiation of the faulting at the south end of the Jacksboro cross fault to allow the necessary shortening of the strata on the northeast side of that fault. The four faults as interpreted by Keith, and corroborated by the displacement of the north limb of Powell Valley anticline in the Briceville quadrangle, give clear evidence of a movement of at least ten miles to the northwest. At Skegg Gap the evidence does not indicate over two miles of over- thrust at the most. In the course of his meditation on this study the writer has made very briefly a few computations, based on extremely general and only very approximate assumptions, which are given below. Their value is solely to indicate orders of magnitude, and it is hoped that they may serve, as they did in the case of the writer, to visualize the immensity of forces involved. FORCE TO SHEAR AND FORCE TO THRUST ASSUMPTIONS Block 125 milesX 25 miles} mile Density 170 lbs. per cubic foot Coefficient of friction, mean between rough and smooth granite, o. 60 Shearing strength 200 pounds per square inch Average extent of overthrust, 6 miles RESULTS Force to shear block loose over entire area= 25X10" pounds Force to move block against friction on horizontal plane= 23 X 105 pounds Work done in moving block 6 miles at angle of 5 degrees=85 X 10” foot pounds Equivalent to 420,000 horse-power working for 100,000 years Estimated coal in block = 50X 10? tons RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 360 Burning of this coal would produce power enough to move the block 2.2 feet, assuming the usual engine efficiency. It has actually moved an average of at least six miles. It is especially interesting to note that the force required to shear the block loose over the whole area is only about one-tenth of that required to produce motion against the resistance of friction. Since both forces are proportional to area and only one—that of motion against friction—proportional to thickness, we find that for a block of any area and of a thickness of 287 feet, according to the conditions assumed, the shearing force is just equaled by the force to overcome friction, and as thickness is greater than this amount the latter force is greater in proportion. It is evident, then, that in the case of most overthrust faults the motion of the rock involved against the resistance of friction is more impressive than the production of the break which separated it. STUDIES OF THE CYCLE OF GLACIATION WILLIAM HERBERT HOBBS! University of Michigan I. THE CYCLE OF MOUNTAIN GLACIATION WITHIN MODERATE LATITUDES In a general discussion of glacial sculpture in mountains,” the writer has made use of the terms grooved or channeled upland and fretted upland to describe respectively the early and the late effects of the erosional action of mountain glaciers. The Bighorn Mountains in Wyoming and the Swiss Alps were chosen as type examples of these contrasted erosion surfaces, the characteristics of which are, that in the former large areas of the preglacial upland still remain (Figs. 1, 2, and 3), while in the latter its complete dissection by cirque recession and enlargement has resulted in a system of main and secondary rock palisades described as comb ridges. Between these contrasted land surfaces many gradations exist, though examples of the former are relatively uncommon. Similar to the channeled upland of the Bighorn Mountains, though with less of the preglacial surface retained, are portions of the Uinta and Wasatch mountains, of which excellent illustrations have been supplied by Atwood. All the best examples are furnished by the Rocky Mountains in the interior of the American continent, where the moist westerly winds have been robbed of their moisture in crossing the high Sierra Nevada and Cascade ranges. In a visit to the Glacier National Park, the writer was impressed with the fact that a type of topography is there represented which indicates a still later stage of sculpture by mountain glaciers than t [llustrations from photographs by the author. 2““The Cycle of Mountain Glaciation,” Geogr. Jour., Vol. XXXV (1010), pp. 147-53, Figs. r-19. Also, “‘Characteristics of Existing Glaciers,” pp. 25-40, RISsie Sor 3 “Glaciation of the Uinta and Wasatch Mountains,” U.S. Geol. Survey, Prof. Paper 61 (1909), maps of Pl. 8A. 37° STUDIES OF THE CYCLE OF GLACIATION 371 does the fretted upland as exemplified by the Alps. The most striking peculiarities of this type are found in the unusual number of isolated sharp peaks of monumented aspect (Figs. 4 and 5), and this is combined with a general absence of the comb ridge (a rare example is shown in Fig. 6) and a frequency of unusually low cols or passes (Fig. 7). Unlike the true horns of the Matterhorn type, which in the fretted upland are relatively few in number and may perhaps represent by their summits points near the original surface of the upland, the monuments of the northern Rocky Mountains show a tendency to appear in pairs, and in many Fic. 1.—View of Mt. Mathews, Bighorn Range, taken from the southeast and showing the character of the preglacial surface. At the left in middle distance is a cirque. instances at least they are remnants of lower portions of the preglacial surface (Figs. 5, 8). Both in the Bighorn Range and in the Glacier National Park the glaciers have today nearly or quite disappeared, being now represented by small horseshoe or cliff glacierets only. The earlier conditions of nourishment were, however, as we know from more or less extended studies, notably different from those of today. In the Bighorn Range the glaciers of Pleistocene time extended far down the valleys, where strong terminal moraines are found to mark the limits of their advance." tR. G. Salisbury, ‘‘Cloud Peak—-Sheridan Folio,” U.S. Geol. Swurvey; also, N. H. Darton, U.S. Geol. Survey, Prof. Paper 51, pp. 71-91, Pls. 37-36. Be WILLIAM HERBERT HOBBS Fic. 2.—View of cirque above Seven Brothers Lakes, seen from the ridge south of Trail Lodge, Bighorn Range. Fic. 3.—Nearer view of the cirque shown in Figure 2. Characteristic surface of preglacial area in foreground. STUDIES OF THE CYCLE OF GLACIATION Be In the Glacier Park district the Pleistocene glaciers occupied the entire valleys within the range and spread out eastward their aprons of Piedmont type. They also extended westward a long distance down the valley of the Flathead River." It is here proposed to use the term monumented upland to describe the extreme type of mountain sculpture which is repre- sented in the Glacier National Park and which is believed to be due to continued glacial action upon a fretted upland like that of the Alps. Cirque enlargement carried to this stage has sapped the Fic. 4.—View of Reynolds Mountain, a characteristic monument of the Glacier National Park region. View taken from the trail to Piegan Pass. main comb ridge so as to largely obliterate the azgualle type of crest or aréte (Fig. 6 shows one of the remaining comb ridges). Matterhorns have in the process been reduced in size as the cols are progressively lowered and widened and are transformed into arétes. ‘The last remnants of the upland to be removed by this continued cirque enlargement are found away from the original divide and outward toward the flanks of the upland, for the reason that in their later stages cirques enlarge excessively on their lateral walls. A good illustration of this tendency is supplied by the t Wm. C. Alden, ‘‘Pre-Wisconsin Glacial Drift in the Region of Glacier National Park,” Bull. Geol. Soc. Am., Vol. XXIII (1912), Pl. 37. See also by same author, “Glaciers of Glacier National Park,’ and especially the map opposite p. 32. 374 WILLIAM HERBERT HOBBS = Fic. 5.—A pair of monuments of monumented upland seen from’the Piegan Trail, Glacier National Park. Fic. 6.—View of a comb ridge looking across the Piegan Pass, Glacier National Park. STUDIES OF THE CYCLE OF GLACIATION 375 gently sloping summit plane of Quadrant Mountain in the Yellow- stone National Park at an elevation of between 9,000 and 10,000 feet' (Fig. 9), since Antler Peak and Bannock Peak guard the entrance to the cirque. It is especially because the comb ridges in the highest levels are precipitous and correspondingly thin that a continuation of Fic. 7—Gunsight Pass, seen from Gunsight Chalets looking across Gunsight Lake, Glacier National Park. the process removes their pinnacles while the broader ridges some- what farther out and just below the mother-cirques are being sharpened into peaks, both alike through sapping from the cirques. To bring together the extremes of mountain glacier erosion which are represented by the Bighorn Range and the Glacier National Park with the intermediate stages which connect them, the four generalized plans of Figure 1o have been prepared. In order, these are: T. The youthful channeled or grooved upland II. The adolescent early fretted upland III. The fretted upland of full maturity IV. The monumented upland of old age “The Cycle of Mountain Glaciation,” Geogr. Jour., Vol. XXXVI (1910), Figs. 8, 14. 376 WILLIAM HERBERT HOBBS These four stages are perhaps best illustrated by the Bighorn Range, the mass of Snowden in the Welsh highland, the Alps, and Glacier National Park (Fig. 10). Fic. 8.—Monuments on either side of entrance to cirque above Ptarmigan Lake, Glacier National Park. (Photograph purchased of Northern Pacific Railway.) STUDIES OF THE CYCLE OF GLACIATION Bf y| The two districts which are here contrasted, the Bighorn Range and the Glacier National Park, furnish also the opportunity to contrast the effects of rock structure in modifying the forms of relief shaped by mountain glaciation. Whereas the high upland of the Bighorn Range has a core of massive rock, thus resembling the Wasatch and Uinta ranges and the Alps, the rocks of Glacier National Park are sediments and dominantly shales and limestones. It was to be expected that the characteristic structures of these J Fic. 9.—Cirque at head of Panther Creek, Yellowstone National Park, with pair of monumented peaks at entrance—Antler Peak and Bannock Peak. sediments, their bedding planes and their joint system, should exert a strong influence upon the topographic forms produced, as indeed they have. The influence of the bedding planes is displayed in the Glacier National Park in the accentuation of the rock terraces at the upper ends of valleys within the cirques themselves. As in the Canadian Rockies across the international boundary, this character reaches an extreme (Fig. 11). An excel- lent instance is shown also in an illustration by Alden.? t Glaciers of Glacier National Park, Fig. 11. 378 WILLIAM HERBERT HOBBS To the well-developed jointing found in the rocks of the Glacier National Park must be ascribed the well-marked checkerboard pattern displayed by the park valleys, a pattern which strikes one at once when the topographic map is examined. A number of observations of the bearings of master-joints which were made by a *e (Cirque ¥\ Cirque Y C Irque “ Ge Cirque GrooveD UPLAND EARLY FRETTED UPLAND (Youth) (Adolescence) Bighorn Range Welsh Highland Cirque (mature) Cirque & a (mature) Horn FRETTED UPLAND MoNUMENTED UPLAND (Maturity) (Old Age) Alps Glacier National Park Fic. 10o.—Stages of sculpture by mountain glaciers the writer indicated a rather general correspondence between them and the trends of the valleys in which they were found. In some instances the lower spurs which have been less extensively sapped by glacial erosion indicate very clearly the dominating influence of the joint planes in shaping them. ‘The cirques themselves also display this tendency by their approach to rectangular outlines. SELUDIES OF THE CYCLE OF GLACIATION 379 II. THE TRANSITIONS BETWEEN THE MOUNTAIN GLACIER AND THE ICE CAP _ From the standpoint of the sculpturing of the lithosphere, the ice cap is sharply set off from all types of mountain glacier through its inability to accomplish a sapping of rock surfaces due to rapid frost-weathering. Its sculpturing processes are therefore restricted to plucking, abrasion, and to a very limited extent frost-weathering on flattish surfaces—processes which in combination leave the rock rounded and presenting surfaces which are flatly convex ogo Fic. 11.—View of terraced cirque above Lake Grinnell, Glacier National Park skyward. ‘That these processes combined play but a subordinate role to frost-weathering in the case of all the types of mountain glaciers, would seem to be sufficiently attested by the sharply accented features which are brought about with their concavities toward the sky." Since the mountain glacier owes its very existence to a rock container within the lithosphere surface, the inclosing rock walls t Hobbs, Earth Features, etc. (1911), p. 379, Fig. 405. 380 WILLIAM HERBERT HOBBS must in general project above the ice of the glacier. The rock surface will also be reached by air and water wherever crevasses descend through the ice of the glacier to the bed upon which it rests. The conditions essential to the sapping process are a supply of water on the rock surface and oscillations of temperature about the freezing-point. These conditions are not realized either in the case of ice caps or of continental glaciers, save only where nunataks emerge from beneath the ice near to the glacial margin. When during an advancing hemicycle of glaciation a mountain glacier is so amply nourished that the rock walls of its containing basins become entirely submerged (ice-cap stage), a profound and immediate transformation takes place in the sculpturing processes. Sie ee - eS = == SS eee = 7 SS =a ta DICED. eB aes Sag Se == ae ase: ness Fe St mae =, Fic. 12.—The northern cirque (Kjedel) on Galdhépig in the glaciated surface a Norway. (After E. Richter.) Up to this time, under the dominating influence of the sapping process, the effect of the glacial sculpture has been to sharpen all projecting features of the relief as the glacial basins and channels are carved deeper and extended outward from each individual locus. Now, however, under the plucking and grinding processes alone, which have usurped the functions of the frost-weathering, the pinnacles and horns within the comb ridges are truncated and ground down, with the result that above the shallowed cirques and the largely obliterated U-valleys there extends a flatly convex surface like that which is fashioned by the same processes beneath a continental glacier. The sharp relief which was inherited from the period of mountain glaciation is thus gradually ironed out into a flatly convex surface which is everywhere ground and polished by abrasion. The U-valleys are first effaced, beginning at their STUDIES OF THE CYCLE OF GLACIATION 381 lower extremities, and the last of the hollowed features of the inherited surface to disappear are the increasingly truncated remnants of the cirques, which in their later stages take the form of an armchair-like depression. Such features are well displayed in Norway where the continental glacier has similarly ironed out the inherited grooved or fretted upland (Fig. 12). Such a surface as succeeds to a fretted upland under the sculptur- ing action of either an ice cap or a continental glacier will resemble in form a grooved glacial upland of extreme youth such as is illustrated by the Bighorn or Uinta ranges of the Rocky Mountain region, but it has less pronounced relief and, unlike such a pre- glacial remnant (‘‘biscuit-cut”’ surface), the upwardly convex surfaces are here planed and polished by abrasion. In the receding hemicycle of glaciation which succeeds to the culmination of glacial alimentation, the flat dome of the ice which constitutes the ice cap will have its surface progressively lowered until the stage is reached at which the rims of-the buried cirque remnants begin to emerge from beneath their mantle of ice. In West Antarctica, near the winter quarters of the Swedish Antarctic Expedition of 19g01~3, ice caps now blanket both James Ross and Snow Hill islands, and, like all Antarctic glaciers, they are in a receding hemicycle of glaciation. On the first-named island the rims of the cirques have emerged from beneath their cover along the eastern and southern margins of the island (Fig. 13). The Gourdon and Rabot glaciers are already apparently in large part detached from the dome of the ice cap, which here rises to its highest point in the Haddington berg. In the largest of the cirques lies the Hobbs Glacier, which is still in part fed by two ice cascades situated near the middle of the rim. Except that the continental glacier, and not an ice cap, has been the modeling agent, Mount Washington in the White Mountains? * Otto Nordenskjéld, ‘“‘Die schwedische Siidpolar-Expedition und ihre geogra- phische Tatigkeit,”’ Schwedische Siidpolar-Expedition, to01-3, Vol. I, Lieferung 1 (1911), pp. 154-55, Map 3 and Pl. 13, Fig. 1. ‘ 2 J. W. Goldthwait, ‘‘ Following the Trail of the Ice Sheet and Valley Glacier on the Presidential Range,” Appalachia, Vol. XIII (1912), pp. 1-23 (reprint), Pls. 1-9; “Glacial Cirques Near Mt. Washington,” Am. Jour. Sci., Vol. XXXV (1913), pp. I-19; “Remnants of the Old Graded Upland on the Presidential Range of the White Mountains,” 7bid., Vol. XXXVII (1014), pp. 451-53; ‘‘Glaciation in the White Mountains of New Hampshire,” Bull. Geol. Soc. Am., Vol. XXVII (10916), pp. 263-94, Bier. 382 WILLIAM HERBERT HOBBS and Mount Ktaadn in Maine’ would appear to supply near parallels to the sculpture just described, since the “gulfs” of the districts have been clearly recognized as cirques. Tarr has claimed that the mountain glaciers which sculptured the cirques on Mount Ktaadn 64-10 Fic. 13.—Map of portions of the James Ross and Snow Hill Islands of West Ant- arctica. (After Otto Nordenskjéld.) were subsequent to the continental glaciation of the region. This is disputed by Goldthwait, who brings forward evidence to prove that in the White Mountains the mountain glaciers were antecedent to the continental glaciation which shaped the higher and flatter rock surfaces. We hardly see how there could fail to be glacial ~R.S. Tarr, ‘The Glaciation of Mt. Ktaadn,” Bull. Geol. Soc. Am., Vol. XL (1900), pp. 433-48. STUDIES OF THE CYCLE OF GLACIATION 383 remnants in occupation of the cirques for at least a brief period while the continental glacier was withdrawing from the region. These would presumably develop in much the same manner as those already described on James Ross Island, but with differences which will be pointed out in the next section of this paper. Gold- thwait is no doubt correct in believing that the mountain glaciers had a much longer life during the advancing hemicycle of glaciation and that the cirques were shaped at that time. It is even doubt- ful if any appreciable work of erosion or deposition was accom- plished in the later period of mountain glaciation, and _ this interpretation would be in harmony with Goldthwait’s observations. Ill. THE GLACIAL CYCLE ON THE MARGINS OF THE CONTINENTAL GLACIER OF ANTARCTICA It is a fundamental and prerequisite condition for the sequence of stages through which mountain glaciers pass during a receding hemicycle of glaciation that the areas of alimentation and ablation should be sharply separated from each other. The former is restricted to the upper levels, and alimentation is augmented in amount toward the top, whereas the area of wastage is found in the lower levels and the losses are increased toward the bottom. Such a distribution results principally from two conditions: (1) mountain glaciers are nourished by upwardly directed air currents which deposit their moisture as a result of progressive adiabatic refrigeration; and (2) they are wasted by contact with warm-air layers whose temperature rises progressively toward the bottom. It is a direct consequence of the combination of these conditions that mountain glaciers during a receding hemicycle of glaciation become reduced in area through withdrawal of the glacier foot up the valley, and even in its expiring stage the glacier head occupies essentially ithe same position thai it did at the beginning (Fig. 14). Were these two conditions affecting the size of mountain gla- ciers not realized, the results would be quite different. When we examine the glaciers on the margins of the inland ice of the Antarctic, we find they differ widely from those of moderate latitudes, which are the ones that are well known and have formed the basis of our classification. Within the Antarctic air temperatures do not rise 384 WILLIAM HERBERT HOBBS above the freezing-point even in the summer season, save only during short intervals at the termination of the fierce Antarctic blizzards. Furthermore, these marginal glaciers to the inland ice are nourished, not by inwardly and upwardly directed air currents, ; is “9 ~ I Hie PIEDMONT EXPANDED-FOOT DENDRITIC TNE Sey Rte Wee Malaspina Glacier, Taku Glacier Early Stage Alaska Baltoro Gl, Himalaya Mts DENDRITIC RADIATING HORSE =shOB TYPE TYPE TYPE Late Stage Alpine Glaciers of Tasman GI.,N.Z. Glaciers Glacier Nat Fark GLACIER TYPES OF RECEDING FAEMICYCLE Progressive Withdrawal of Glacier Foot , Fic. 14 as are the mountain glaciers of moderate latitudes, but by down- wardly and outwardly flowing currents which bring drift snow from the inland ice and often carry it beyond the marginal glaciers to be dissipated upon the surface of the sea. Separate areas of STUDIES OF THE CYCLE OF GLACIATION 385 H tl We 8 VED AN : WZ a) ; a¢ } = ¢ o 3 1 4 +> £ rg =D 2a6 KoA 39 "Uo i SS === ‘7 fe : / i Anes p ace EAS J aX ' too f { fi f! Vee ‘ \ [\\\\ee { 1 ———— > \ SSO Fic. 15.—Map of an area in South Victoria Land near the winter quarters of the last Scott Expedition, showing waning glaciers which are withdrawing at both their upper and their lower margins. (After Griffith-Taylor and others.) 386 WILLIAM HERBERT HOBBS nourishment and waste in distribution with reference to altitude are thus not realized, and the otherwise universal law of exclusive drawing in of the foot of the glacier during its waning stages does not hold. That this is true is particularly well shown in the area of waning glaciers described as “‘ice-slabs” by Ferrar, the glacialist of the first Scott Expedition to the Antarctic, and fully mapped by Griffith-Taylor, Debenham, and Wright of the last Scott Expe- dition’ (Fig. 15). Ona far larger scale and related to a continental glacier rather than an ice cap, these dying glaciers represent a later stage than those marginal types which have already been referred to from West Antarctica—the Gourdon, Hobbs, and Rabot glaciers of James Ross Island. By examination of the map (Fig. 15) it will be noted that these glaciers must in an earlier stage have been connected together as a piedmont which was then a part of the parent area of inland ice lying to the westward. From that continental glacier when detachment occurred the rims of the battery of remodeled cirques which rise west of the existing glaciers must have emerged from the ice mantle in forms not unlike those now seen on the margins of James Ross Island. Their subsequent diminution in size has gone on through withdrawal both from the cirques and from the lower portions of their valleys—from both extremities toward a central position at a moderate altitude, where the last stand will be made before final extinction. The usual law of ablation regulated with respect to altitude here plays, therefore, no part, and it is evident that the reflection and consequent intensification of solar heat radiation in the neigh- borhood of exposed rock walls has here been the controlling factor in localizing the wasting process. This effect of exposed rock surfaces has been recognized for high latitudes by the observation of moats surrounding nunataks? and of the lateral streams beside glacier tongues? t Robert F. Scott, Scott’s Last Expedition, Vol. II, map opposite p. 198. 2 “Characteristics of Existing Glaciers,” pp. 169, 257, Pl. 33B. 3 [bid., Pl. 25A. — REVIEWS Two Gas Collections from Mauna Loa. By E. S. SHEPHERD. Bull. Hawatian Observatory, Vol. XIII, No. 5, May, 1920. This is a brief report by Dr. Shepherd of the Carnegie Geophysical Laboratory at Washington on two gas samples collected by Dr. T. A. Jaggar, Jr. The samples were taken near the edge of a flow of incan- descent rough pohoehoe lava on the south slope of Mauna Loa. The gases were collected in vacuum tubes from a depth of 2 feet in a 2-inch crack in the lava surface. The lava at a depth of 3 feet was glowing and the estimated temperature at the point of collection was 300° C. A condensation of water within the tube was noted immediately upon collection. The analyses showed that about 70 per cent by volume of the gas (computed at 1200° C. and 760 mm. pressure) was water, in which respect the gases closely resemble those of Kilauea. About 16 per cent was nitrogen and the remainder mainly SO,, SO, and CO.. The water cannot be explained as the result of oxidation of hydrogen by admixed air, as is shown by the nitrogen percentage. Ii all the nitrogen were assumed to come from admixed air, the oxygen in such a quantity of air would be insufficient to account for the observed water. The evidence of these samples accords with the classic work of Day and Shepherd at Kilauea in demonstrating the abundant presence of water in certain volcanic gases. The gases of Mauna Loa show a high degree of oxidation, i.e., they have been almost completely burned. In general, they show a high degree of similarity to the Kilauea gases although the latter are rather variable. Especially noteworthy at Mauna Loa is the abundance of 5O,—2 to 8 per cent. E. S. BASTIN The Geology and Ore Deposits of the Virgilina District of Virginia and North Carolina. By Francis BAKER LANEY. Virginia Geological Survey, University of Virginia. (Prepared jointly by the Virginia Geological Survey and the North Carolina Geological and Economic Survey.) 10917. Pp. 176. The Virgilina district which lies partly in Virginia and partly in North Carolina is one of the copper districts in the eastern United States 387 388 REVIEWS that has produced considerable tonnages of ore. The investigation covers an area of approximately 550 square miles, including parts of Charlotte, Halifax, and Mecklenburg counties, Virginia, and parts of Granville and Person counties, North Carolina. The area is made up almost wholly of igneous and highly meta- morphosed rocks. They include ancient metamorphic gneisses and schists, the origin of which is unknown; a sequence of volcanic rocks, both basic and acidic, and volcanic clastics of each type, together with much volcano-sedimentary material; intrusive rocks of both basic and acid types, such as gabbro, diorite, granite, and syenite; and different varieties of dike rock, especially diabase. There is a small area of red or brown sandstone of Triassic-Newark age. Except the intrusives, the sandstones, and the dikes the rocks are all highly schistose and gneissoid in texture. This prominent schistosity of the rocks is probably the most obvious structural phenomenon of the district, although jointing is prominent and there is conclusive evidence of folding. There is little direct evidence of faulting, but the intense dynamic metamorphism of the district could hardly have occurred without causing a certain degree of faulting. With the exception of a few mineralized areas in more or less epidotized zones of the true basic schist, where deposits of native copper or of cuprite occur, the ore deposits are found in well-defined fissure veins, which occupy fractures in the rocks—in some instances possibly fault planes. The rock in which the veins occur is basic in character—the Virgilina greenstone—having the mineralogical and chemical nature of andesite; but it is thought that the vein material, both ore and gangue, was derived from the granitic magma of the region. The gangue minerals, exclusive of included fragments of schist, named in the approximate order of their abundance, are: quartz, calcite, epidote, chlorite, hematite, sericite, albite, and possibly other plagioclase feldspars in small amount, and pink orthoclase. The ore minerals, named in the approximate order of their abundance are: bornite, chalcocite, native copper, malachite, azurite, cuprite, chalcopyrite, chrysocolla, klaprothite (?), pyrite, argentite, silver, and gold. Of these minerals, bornite (in part), chalcocite (in part), chalco- pyrite (in part), pyrite, klaprothite, argentite, native copper, and gold are regarded as hypogene or primary; while a part of the chalcocite, bornite, and chalcopyrite, and all the native silver, cuprite, malachite, azurite, and chrysocolla are held to be supergene or secondary. REVIEWS 389 The author gives a description of individual mines and prospects. A good geologic map of the district is appended to the report. 155 Jaks Wo The Stratigraphy and Correlation of the Devonian of Western Ten- nessee. By Cart O. Dunsar. State of Tennessee, State Geological Survey, Bull. No. 21, Nashville, Tenn., 1919. This volume is a detailed statement of the stratigraphy and correla- tion of the Devonian rocks of the western valley of the Tennessee River. The long sequence of the Devonian strata exposed in this region, especially the presence of the Upper Oriskany, and the abundance of fossils, probably will make this the standard section of the Lower Devonian of the entire Mississippi Basin. The important paleontological aspects of the problem are well treated. Following is the sequence of the Devonian formations of western Tennessee, as given by the author: Series Group Formation Neo-| Chautauquan Chattanooga shale devonian Hardin sandstone member Senecan Break Erian - Meso- devonian Pegram limestone —————— Break Camden chert —— Break Harriman chert Oriskanian ———— Break Quall limestone Ulsterian Break Decaturville chert ———————— Break Birdsong shale ——_—_———— Break ————_—_—_—_ Flat gap limestone Bear Branch limestone | Pyburn | Ross limestone —————— Break Rockhouse shale Paleo- devonian Helderbergian or Linden 10n Olive Hill format 390 REVIEWS The Geology and Coal Resources of the Coal-bearing Portion of Tazewell County, Virginia. By T. K. HARNSBERGER. Virginia Geologi- cal Survey, University of Virginia, Bull. No. 19. Prepared in co-operation with the U.S. Geological Survey. 1919. Pp. 195. This report deals with the coal resources of Tazewell County in southwestern Virginia. The surface rocks in the coal district belong to the Devonian, Mississippian, and Pennsylvanian systems. All the commercially valuable coal is in the Pennsylvanian. The most prominent structural feature of the area is the Dry Forks anticline. The Pocahontas syncline and other folds occur in the region. The coal area is bounded on the southeast by a series of thrust faults. The Tazewell County coal field originally extended to the southeast far beyond its present limits but folds and faults lifted the coal-bearing rocks of the region to the southeast far above those of the present field and they have been removed by erosion. The total area of coal land is 696.5 square miles. The total thickness of the coal-bearing formations is about 2,800 feet, every portion of which is exposed in some part of the area. At least fifteen coal beds are 30 inches or more in thickness over territory of sufficient extent to justify mining. In general the coal is of good coking quality and has a high ~ fuel value. Because of the extreme variability of the coal beds, plans for development should be preceded by careful geological examination. Complete descriptions of the various coal beds are given. Included in ~ the report are both a topographic and a geologic map of the coal area. Re Aca POSITIONS OF ALL KINDS| Never was the demand so great for qualified teachers and specialists. For ten years we have given our time and energy to this work. Write fer our free literature. State qualifications briefly. Co-operative Instructors’ Association Marion | - - - Indiana Bee caotubs Ones by iy BAILEY WILLIS Compilation Edited by \GISTS and all readers of geologic erature will welcome the publication, -The.Summer Quarter Courses are the same in educational and credit value as those offered in other quarters of the year. The colleges, the graduate schools and in pro-. fessional schools provide courses in Arts, Literature, Science, Commerce and Administration, Educa- / tion, Law, Divinity, Medicine, and Social Service Administration. Ideal place for recreation as well as study, golf, tennis, rowing, etc. Two great parks a Lake Michigan within walking distance. Students may register for either term or both. 1st Term—June 20-July 27 2d Term—July 28-Sept. 2 Write for complete announcement Che Unitersity of Chiragu BOX 523--FACULTY EXCHANGE CHICAGO, ILLINOIS ae the present state of Enuleiee i ion concerning many of the funda- U iversity of Chicago Press . Illinois By SAMUEL WENDELL WILLISTON Professor of Paleontology in the University of Chicago necessary scientific Sas of our recone ne concerning the ou of eas, lakes, and rivers of past and present times. While no attempt has been €, Save incidentally, to discuss the strictly terrestrial reptiles, twelve of the four- ucture, habits, geological distribution, etc. Unlike most of the books for general reader treating of “ancient monsters,” the present work is by an author has ee a years in the a and laboratory in the collection and study numerous sinisreitions. in tree part fron the pen or brush of the author , include not only living types and twenty-four restorations of extinct forms, 0O many figures elucidating the structure and habits of the animals. viii++252 pages, royal 8vo, cloth; $3.00, postpaid $3.20 : | UNIVERSITY OF CHICAGO PRESS ILLINOIS | FINE INKS AND , ADHESIVES ie For those who KNOW Drawins Inks Eternal Writing Ink Engrossing Ink Taurine Mucilage Photo Mounter Paste Drawing Board Paste Liquid Paste : Office Pasie- eee a Vegetable Glue. Etc. 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One colored map shows the topography of Vancouv range and the other is a geological map of Vancouver an vicinity. 112 pages, paper covers; $1.50, postpaid $1.60 THE UNIVERSITY OF CHICAGO © CHICAGO - EDITED BY : THOMAS C. CHAMBERLIN AND ROLLIN D. SALIS - With the Active Collaboration of ‘WELLER, Invertebrate Paleontology ALBERT JOHANNSEN, Petrology : EDS 0. NS. ASTIN, Eeonawiie Geology ) __ ROLLIN T. CHAMBERLIN, Dynamic Geology Fane EDITORS -JOHN C. BRANNER, Leland Stanford Junior Uiveriey RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. WILLIAM H. HOBBS, University of Michigan FRANK D. ADAMS, McGill University CHARLES K. LEITH, University of Wisconsin _ WALLACE W. ATWOOD, Clark University Leland Stanford I unior University WILLIAM H. EMMONS, University of Minnesota : ithsoni ituti ARTHUR L. DAY, Carnegie Institution. JULY-AUGUST a THomaAS C. CHAMBERLIN 301 — I. THE INTIMATION Ss OF Pare DEFORMATION - Rotttn T. CHAMBERLIN 416 GEORGE ROGERS MANSFIELD 444 OF “SUMMARIES OF PRE-CAMBRIAN. LITERATURE OF NORTH BY EDWARD STEIDTMANN- - - - - TERENCE T. QUIRKE 469 ys OF A SPECIES IN PALEONTOLOGY AND A NEW KIND OF TYPE = - - - - - - - - EDWARD L. TROXELL 475 esta es ie ie ie 2 hs es = - = ae - - 480 Cate AGO ILLINOIS, U.S. A. _ - ‘THE CAMBRIDGE UNIVERSITY PRESS, Lonpon eee eu SBne eaISHs, Toxyo, Osaka, Kyoto, FuKvoKA, oo THE MISSION BOOK sO SHANGHAI THE JOURNAL OF EDITED BY THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY ae With the Active Collaboration of | Set STUART WELLER _ ALBERT JOHANNSEN- oh mt Invertebrate Petcontaloey ; EDSON S. BASTIN ROLLIN T. . CHAMBERLIN Economic Geology The Journal of Geology is published by the University of Chicago at the aoe of 5750 Ellis Avenue, Chicago, Illinois, semi-quarterly, on or about the following dates: Febru May 1, June 15, August 1, September 15, November 1, “December 15. § The subscriptio per year; the price of single copies is 65 cents. 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While the cost of printing remains as now, the Journal cannot supply rey : reprints will be furnished authors at cost, if ordered in advance of publication. — ~ Entered as second-class matter, March 20, 1893, at the Post-office at Chicago, Ill., under tl Acceptance for mailing at special rate of postage provided for i in Section 1103, Act of | Oc! daly x 15, 1918. } Among articles to appear in early numbers of the Journal of Geology are the following: The Physical Chemistry of the Crystallization and Magmatic Differ- entiation of Igneous Rocks. land IV. By J.H.L. Vocr. Cycles of Erosion in the Piedmont Province of Pennsylvania. By F. BAScoM. Theoretical Considerations on the Genesis of Ore- Deposits. By Reve RASTATN, ; Note on a Possible Factor in aeons of Geological Climate. By HARLOW SHAPLEY. The Horizontal Movement of Geanticlines and the Fractures near Their Surface. By H. A. BROUWER. Field Observations in Northern Norway Bearing on Magmatic Differentiation. By STEINER FOSLIE. Geologic Reconnaissance in Baja California. By N.H. Darron. Pleistocene Mollusca from Northwestern and Central Illinois. By FRANK COLLins BAKER. ; The Character of the Stratification of the Sediments in the Recent Delta of Fraser River, British Columbia, Canada. By W. A. JOHNSTON. Outline of Pleistocene History of the Mississippi Valley. By FRANK LEVERETT. Pennsyloanian Stratigraphy of North Central Texas. By RAYMOND C. Moore and FREDERICK B. PLUMMER. Suggestions as to the Description and Naming of Sedimentary Rocks. iby A. J. Tieje. The Marine Tertiary of the West Coast of the United States: Its Sequence, Paleogeography, and the Problems of Correlation. By Bruce L. CLARK. The Relation of the Physical Properties of Natural Glasses to Their Chemical Composition. By Wititam O. GEORGE. Notes on a Recent Collection of Fossil Plants from the Dalles Group of Oregon. By RALPH W. CHANEY. THE GEOGRAPHY OF ILLINOIS By DOUGLAS C. RIDGLEY Professor of Geography in the II]linois State Normal University Published in May 1921 A Geography as readable as the most interesting novel The author’s aim is to provide an authoritative study of the state, presenting in compact form the wide range of physical influences which make up the geographic env ironment in which men live. The natural features and natural resources of the state are treated in some detail. The great occupations of mankind— agriculture, mining, manufacturing, transportation, and trade —are discussed with sufficient fulness to give an adequate idea of their development and present importance within the state. The population census of 1920 for Illinois is treated in a final chapter. The book is designed to be of interest to the busy citizen who wishes to know his state as a unit in its present-day activities; to teachers and pupils who would know Ilhnois well enough to interpret other regions in com- parison with the home state; to all who wish to learn the reasons for the high rank of [llinois in many lines of human endeavor. The reader will find among the references listed in the bibli- ography a wealth of material with which to continue a study of the state and its resources as fully as may be desired. Abounds with drawings, maps, illustrations, and colored insert maps xviit+ 385 pages, r6mo, cloth; $2.50, postpaid $2.65 THE UNIVERSITY OF CHICAGO PRESS CHICAGO - - ILLINOIS ley VOLUME XXIx NUMBER 5 THE BeORNAL OF GEOLOGY JULY-AUGUST sr921 DIASTROPHISM AND THE FORMATIVE PROCESSES XIV. GROUNDWORK FOR THE STUDY OF MEGADIASTROPHISM PART I. SUMMARY STATEMENT OF THE GROUNDWORK ALREADY LAID: THOMAS C. CHAMBERLIN Research Associate, Carnegie Institution of Washington PART If. THE INTIMATIONS OF SHELL DEFORMATION ROLLIN T. CHAMBERLIN é The University of Chicago PART I. SUMMARY STATEMENT OF THE GROUNDWORK ALREADY LAID INTRODUCTION As set forth in the first of this series of articles, it has been their main purpose to develop into more explicit form the basal ideas that logically belong to an earth built up of planetesimals. Inevitably, the alternative ideas that have been based on the older concept of an earth of gaseo-molten origin have been more or less constantly compared with them. The whole of the field has not yet been covered, but as the study now passes to a new and difficult phase, it is felt that it will be serviceable to assemble in brief, rather categorical statements such of the basal ideas already « Published by permission of the Carnegie Institution of Washington. 391 3092 THOMAS C. CHAMBERLIN developed as will form the groundwork for this next stage of the work, which will be a study of the megadiastrophism of the earth. The introduction of the new term, megadiastrophism, calls for an explanation, if not an apology. It has already been found desirable by dynamic geologists to introduce the word “dias- trophism,’’ as a term of more comprehensive meaning than ‘“‘deformation.”” The latter, while general enough in its etymo- logical sense, has come to have a rather special meaning, by reason of its long usage to designate folding, faulting, and similar declared distortions of strata. It is not usually understood to denote those more intimate changes of form that take place in the deeper interior of the earth’s body. To try now to make it include these would be at the risk of misinterpretation. But the new term diastrophism may be used to cover any form of distortion of solid bodies, and thus meet an imperative need. There now arises a need for a still more comprehensive term which shall denote the diastrophism of the earth as a whole—or of large parts of it, such as continents and suboceanic segments— in a collective way without regard to the various special modes by which the diastrophism is effected. In the very nature of the case, the diastrophism of these great units will be composite and very complex, but we need to deal with them in a unitary way despite this complexity. The term megadiastrophism seems suit- able for this purpose. One of the most formidable obstacles in the way of bringing into actual use a new set of concepts where an old set has long had full possession of the thought, lies in the difficulty of really clearing the mind of all the incidental factors of the old concept, and of putting in their place a full set of the new. The difficulty does not lie so much with the main bold features of the new view as with the less obvious progeny of derivative concepts that must, in consistency, go out with the parent concept. In the study of any basal subject that has run far back into one’s past thinking, a large brood of derivative concepts is quite sure to have been drawn out, but their connection with the parent idea is quite likely to have become obscure or to have passed entirely out of consciousness, so that the setting aside of the parent idea does not automatically DIASTROPHISM AND THE FORMATIVE PROCESSES 393 take them with it. It is not at all surprising, therefore, that, even in the most sincere endeavor to give true shape to a new issue under a new view for the purpose of a candid and hospitable test, some of the derivatives of the old view should unconsciously slip in and be treated as though they were offspring of the new. This, in reality, vitiates the whole test. The problem that has thus actually been fashioned and put to trial is a hybrid; it is not a true problem under either the old or the new view. For example, under the theory of a gaseo-molten earth, it was logically assumed that each spherical layer of the earth’s interior was homogeneous; and hence it had a definite ‘““melting-point.”* There followed closely the inference that the material of such a layer must have a common state, either liquid or solid. From these logical derivatives of the primary assumption, far-reaching inferences were drawn in perfect consistency, and these, by years of association, have been woven into the web and woof of current thought with little consciousness that they are only dependencies of a cosmological postulate. But if, on the other hand, the material of all such layers is very heterogeneous chemically, because it is an intimate mixture of planetesimal débris laid down at random, the logical inference is that each layer embraces a wide-ranging group of solution tempera- tures and has no single point of liquefaction. If it is subjected to a rising temperature, this would, at any given stage, cause the lique- faction of only that fraction of the material which was susceptible of liquefaction at the temperature reached, not the ‘‘melting”’ of the whole layer. ‘This fraction would naturally be scattered throughout the mass of the layer and would give rise only to interstitial liquidity. The solution temperatures of the larger portion of the layer would not yet be reached, and this portion would remain solid. Now if the working mechanism of the body is so actuated by the joint force of internal and external stresses that graded pressures are brought to bear, greater below than above, and more or less intermittent, the disseminated liquid is likely to be kneaded out of the layer in the direction of least resistance and so leave the residue solid. ‘In revised terms, as applied to interior conditions, a solution temperature or a narrow group of temperatures at which the constituents enter into mutual solution. 304 THOMAS C. CHAMBERLIN Now it will be seen that this second chain of derivatives is very different from the preceding chain and that the two are mutually exclusive. The links of the two cannot be mixed without the loss of all logical force. Ti mixed, the terms of the problem become a hybrid of incompatibles; such a chain does not exist in nature; it is not a real problem at all; it is merely a supposititious combina- tion of incongruities. The concept of isostasy gives rise to one set of derivatives, if based on the hypothesis of a crust floating on a liquid substratum inclosing a centrosphere of concentric homogeneous layers; and to quite a different set of derivatives on the hypothesis of a solid elastic earth whose internal material is heterogeneous and has suffered internal distortion. The problem of the saltness of the sea has one set of subconcepts if the hydrosphere, at the outset, was as great as now or even greater, and quite another set if the hydrosphere started from a minimum and has grown steadily ever since and is growing still. To mix these throws the whole effort out of court. And so of not a few other earth problems of the more complex order. In view of the difficulties of meeting the imperative require- ments of consistency in working out such complex problems as the inner diastrophism of the earth, it is hoped that the following effort to reduce to brief convenient form the essential concepts already reached in the study of planetesimal accretion will be found service- able. They are not a formal summary of the preceding articles nor drawn exclusively from them; some of them even have no dependence on the planetesimal hypothesis; they are merely found to be tributary to a satisfactory concept of megadiastrophism under the conditions of accretion. To make the statements brief and convenient, qualifications have been largely neglected and some statements may seem somewhat too baldly affirmative, but recurrence to the fuller discussions will, it is hoped, show that reasonable recognition has been made of legitimate grounds of doubt and needs of qualification. It is quite impossible here to accredit these propositions to those who have done most to develop them; they are merely assembled as propositions tentatively accepted as groundwork for further study. ~ DIASTROPHISM AND THE FORMATIVE PROCESSES 395 GENERAL PROPERTIES OF THE EARTH 1. The solid elastic nature of the earth is accepted as having been put beyond serious question by the concurrent testimony of seismic waves, the body tides, the polar nutations, and collateral evidences. . 2. The outer and major part of the earth is held to be minutely heterogeneous in chemical and physical composition, but yet, in a mechanical sense, sufficiently homogeneous to transmit seismic vibrations in legible form. 3. Some questions remain respecting the earth core; it is held to be dense and rigid; but earthquake waves traversing it do not, as yet, tell an unequivocal story. Possibly it is formed of concentric zones rather homogeneous in themselves but varying from one another; possibly, also, segregation of metallic matter toward the center has gone far enough to give a higher ratio of density to elasticity in this inner part than in the accretional zone above, and thus introduced seismic anomalies. 4. So far as now deducible, elasticity and rigidity increase toward the center faster than density. Since simple density segregation would scarcely carry a relative increase of rigidity and elasticity, this seems to imply a dynamic cause. The mean rigidity of the earth seems to be distinctly higher than that of steel." 5. The major pulsation-period of the earth seems to be of the order of an hour but it is not yet precisely deducible from elastico- rigid-density data;? nor is the naturalistic evidence conclusive. It may be near enough to commensurability with the semi-diurnal tide to strengthen it by resonance, but this is not certain. 6. The mode of motion of the poles in the earth is an index of high elastic rigidity. Schweydar has recently determined the t Schweydar’s recent determination is two and one-half times the rigidity of steel; he gives the rigidity of the central part of the earth as ten times that of the surficial part. ‘‘On the Elasticity of the Earth,” Naturwissenschaften (1917), Potsdam, Germany, Part 38. 2 On the influence of gravity on elastic waves, and in particular on the vibrations of an elastic globe, see T. J. A. Bromwitch, Proc. Lond. Math. Soc., Vol. XXX (1899). 3 Nagaoka tried to deduce it from the eruptions of Krakatoa, Nature (May 26, 1907), pp. 89-91. 306 THOMAS C. CHAMBERLIN nutation period as 432.8 days.’ This is so nearly commensurate with the fortnightly tide that there may be a resonance relation between them. The subnutation that has an annual period is probably the result of the seasonal shift of solar effects north and south of the equator. s 7. The elastic nature-of the body tide is accepted as practicnlia demonstrated by the researches of Michelson, Gale, and Moulton, added to those of previous investigators.” 8. The water tides are held to spring in part from the body tide and in part from the direct attraction of the tide-producing bodies; their rise to notable value depends on the resonance of their basins. THE TIME FACTOR g. The arguments once urged against any great age of the earth because of the sun’s short life, tidal action, etc., are held to be wholly invalid. An age somewhere between one billion and several billions of years seems best to fit in with astronomical and biological considerations. Ample time should be allowed for the evolution of star-clusters and the stellar galaxy, as well as life-evolution. 10. Estimates of the earth’s age, based on current geological processes, require large corrections for the accelerating effects of present high reliefs and soil cultivation; in particular, for (qa) increased vertical circulation, (b) more rapid cycles of evaporation and precipitation, (c) greater instability of vegetal clothing, (d) more rapid run-off, (e) deeper penetration of solvent action, (f) greatly increased soil waste, and (g) the much greater length of the low-relief periods than of the high-relief periods. The required corrections are probably great enough to reconcile the geologic with the radioactive estimates. tr. For the computations used in the articles here summarized, a range wide enough to cover the uncorrected geologic as well as the radioactive estimates was used, as follows: (a) for the time since *W. Schweydar, Naturwissenschaften (1917), Potsdam, Germany, Part 38. 2 A. A. Michelson and Henry G. Gale, ‘‘The Rigidity of the Earth,” Jour. Geol., Vol. XXVII (1919), pp. 585-601. 3“‘The Quantitative Element in Circum-Continental Growth,” Article VIII, Jour. Geol., Vol. XXII (1914), pp. 516-28. DIASTROPHISM AND THE FORMATIVE PROCESSES 397 the beginning of the Paleozoic, from one hundred million to four hundred million years; (b) for that since the beginning of the Proterozoic, from three hundred million to twelve hundred million years; (c) for that since the earliest Archeozoic whose age has been estimated, from four hundred million to sixteen hundred million years.* 12. The time occupied in the evolution of terrestrial life is regarded as one of the most dependable evidences of the earth’s age, though its testimony is of a rather general nature. Since the evolution from the early Paleozoic to the present is confessedly only a small part of the whole evolution, it was taken as 1/10 in the computations.” 13. Combining biologic, geologic, and radioactive estimates, the total period of life-evolution is taken roundly as lying between one billion and four billion years. 14. The length of the period during which the rate of planetesi- mal infall was compatible with life, previous to the earliest deter- mined Archean, is thus made to range between six hundred million and twenty-four hundred million years.4 15. Making allowance for the formative stage that precedéd life-evolution, the whole age of the earth is taken tentatively as falling somewhere between three billion and five billion years. All these estimates are of course only intended to serve working purposes in the light of the latest evidences; the whole matter is to be kept sub judice awaiting further light. CONSIDERATIONS ADVERSE TO HIGH ESTIMATES OF DIASTROPHISM 16. Great thicknesses of shallow-water sediments do not necessarily imply great sinking of the crust. Measured in the usual way, thicknesses much greater than any observed may be laid down in the normal process of continental outgrowth with- out necessarily involving any crustal sinking at all. Very thick 1 “The Bearings of the Size and Rate of Infall of Planetesimals on the Molten or Solid State of the Earth,” Article XIII, Jour. Geol., Vol. XXVIII (1920), pp. 675-77. 2 Tbid., p. 675. 3 [bid., p. 676. 4 Ibid. s 5 “Foreset Beds and Slope Deposits,” Article VI, Jour. Geo_= Vol. XXII (1914), pp. 271-72. 398 THOMAS C. CHAMBERLIN sediments, however, usually carry evidences of actual sinking, but such sinking must be determined on its own specific grounds. 17. Great sea-transgressions of the land do not necessarily involve great continental depressions; they are more or less due to general denudation, to shore cutting, and to sea-rise caused by sediment from the land.* 18. Effective base-leveling implies an absence of crustal move- ment while it is in progress.?_ It thus bears on the promptness and completeness of isostatic readjustments. PERIODICITY OF DIASTROPHISM 19. Effective base-leveling is evidence that the earth-body is strong enough to stand the strain of ordinary loading and unloading for a long period without essential yielding; it thus implies that isostatic adjustments are periodic rather than continuous, and that diastrophism, in so far as it is assignable to such loading and unloading, is similarly periodic. 20. The stages occupied in base-leveling and sea-transgression were probably much longer than the intervening stages of active deformation. 21. The mechanism of isostasy implies that great basins once formed tend to remain basins permanently, and that great pro- tuberances tend to remain protuberances except as worn down. Isostasy is not hospitable to great inversions of sea and land. To this there may be regional exceptions where great erosion is closely paralleled by great deposition. 22. Since the present isostatic status follows a period of great diastrophic readjustment, it is an open question whether the present degree of compensation is essentially a consequence of that dias- trophism, or is a normal state approximately maintained at all times. “The Lateral Stresses within the Continental Protuberances and Their Rela- tions to Continental Creep and Sea-Transgression,”’ Article III, Jour. Geol., Vol. XXI (1913), p. 585; ‘‘Rejuvenation of the Continents,” Article IV, Jour. Geol., Vol. XXI (1913), Pp. 673-75. 2 “The Rejuvenation of the Continents,” Article IV, Jour. Geol., Vol. XXI (1913) . pp. 676-81. ———— DIASTROPHISM AND THE FORMATIVE PROCESSES 399 23. The data assembled by Rollin Chamberlin’ imply that prolonged loading and unloading are either (a) sufficient to cause diastrophism at long intervals, or else (b) act as triggers to set off other forces that were steadily accumulating during the quiescent periods. Whether loading and unloading are in themselves suff- cient causes of deformation or not is a question on which studies in megadiastrophism are expected to shed decisive light. THE TOTAL AMOUNT OF SELF-COMPRESSION OF THE EARTH 24. In spite of the foregoing conservative considerations, the amount of unequivocal deformation shown in Paleozoic and later strata alone is so large as to overtax all resources of diastrophism safely assignable to the cooling of the earth, and yet the very com- plex diastrophism of the earlier areas greatly exceeded this later diastrophism. The deformations since the middle of the Miocene are so great—ain view of their lateness in the history of the earth— as to suggest that the causes of diastrophism are very persistent and very profound. 25. If we try to measure the total diastrophism by comparison with the total life-evolution, a result more than ten times that of the Paleozoic and later ages is implied, for diastrophism should have been very active in the formative ages and declined afterward, while life-evolution appears to have been accelerated as time went on. 26. If the intimate crumpling, close folding, and faulting of the Archean and Proterozoic terranes is made the basis of estimate, the total diastrophism must have been very great, but just how great it is impossible now to say. 27. If the present continents be looked upon as the outcome of a contest between sea-shelf outbuilding, on the one side, and inthrust from the oceans, on the other, the total diastrophism is clearly large, but very difficult to estimate definitely. 28. If the early earth be supposed to have been segmented ‘in a natural mechanical way, and the existing continents and ocean basins interpreted as derivatives from these segments by t“Periodicity of Paleozoic Orogenic Movements,” Article VII, Jour. Geol., ‘Vol. XXII (1914), pp. 315-45. 400 THOMAS C. CHAMBERLIN outgrowth, thrust, and deformative shift, the total diastrophism appears even greater than that inferred from the preceding data. The concurrent import of all lines of evidence is that the total diastrophism of the earth was very great, but a more compre- hensive and quantitative mode of estimate is needed, and especially one that covers the diastrophism of the formative stages, in respect to which these lines are very weak. ‘DIASTROPHISM ESTIMATED BY PLANETARY COMPARISON 29. A comparative study of the volumes and densities of the earth and its neighbors affords an entirely independent mode of estimate and gives definite quantitative results." 30. If the moon were built up of moon-stuff—having its present mean density of 3.34—to a sphere whose mass equaled that of the earth, it would have a volume of 430,353,000,000 cubic miles, while the actual earth’s volume is only 259,924,000,000 cubic miles. To reduce the hypothetical moon-earth to the volume and density of the real earth would require a shortening of the radius of 725 miles and of the circumference of 4,555 miles.’ 31. If Mars were built up of Mars-stuff at its present mean density of 3.58 to a spherical body of the mass of the earth, it would have a volume of 401,502,000,000 cubic miles. To compress this to a body of the density and volume of the earth would involve — a shortening of its radius of 618 miles and of its circumference of 3,883 miles. 32. If Venus were similarly built up of its own material to a mass equal to that of the earth, its volume of 289,506,000,000 cubic miles would have to be shrunk radially 177 miles, and cir- cumferentially 1,r12 miles, to have the volume and density of the earth. . 33. The earth, moon, Mars, and Venus revolve in a tract whose total width is less than 3 per cent of the radius of the planetary system. They were, therefore, probably formed under much the same dynamics, of much the same kinds of material, and in much «The Order of Magnitude of the Shrinkage of the Earth Deduced from Mars, Venus, and the Moon,” Article X, Jour. Geol., Vol. XXVIII (1920), pp. 1-17. 2 [bid., p. 13. DIASTROPHISM AND THE FORMATIVE PROCESSES 401 _ the same way, and hence are closely comparable. There might _have been some gradation of material, but since the earth is com- pared with the next outer and the next inner planet, any such gradation is largely equated in combining the results. These four bodies may therefore be taken as representing four stages of growth of a single body under the conditions that prevail at their mean distance from the sun, which is substantially the earth- distance. 34. The giant gaseous planets cannot properly be compared with these solid: bodies without radical qualification, for the giants were probably gaseous from the start and never underwent the _ sifting necessary to cull out material unsuited to form the earth and kindred bodies (see 48 below). The evolution of the giant planets belongs to a distinctly different category. 35. Comparing the densities of the earth, Venus, Mars, and the moon as they now are, a marked increase of density with increase of mass is shown: to wit, the moon, with mass 0.0122 (earth=1), has a density 3.34 (water=1); Mars, with a mass 0.1065, has a density 3.58; Venus, with mass 0.807 (?), has a density 4.85 (?); and the earth, whose mass is unity, has a mean density 5.53.7 . ; 36. Closer inspection shows not only an increase of density with mass but an accelerated rate of increase of density for each increment of mass. ‘This clearly implies that their densities arise from their Own massiveness, an inference in harmony with No. 4 above.’ 37. Under the kinetic theory of gases, the larger the mass, the greater its ability to hold light molecules. Greater proportions of intrinsically light matter were therefore almost certainly gathered into the more massive bodies. They became dense in spite of a larger proportion of intrinsically light material.s 38. Let it be noted that the acquirement of high density is not held to be a matter of simple mechanical compression; this was a conditioning factor in the process, but only that. There were probably added (a) progressive rearrangements and reorgani- zations of the material into denser forms as the stresses grew, including not only simple physical readjustments but the formation «Tbrd., p. 10. 2 Tbid., pp. 16-17. 3 [bid., p. 17. 402 THOMAS C. CHAMBERLIN of new compounds, new minerals, and possibly even new molecules and new atoms; (0) an increase of endothermic compounds as the - temperature increased; (c) the removal of liquefied material and its heat of liquefaction; as also (d) the self-heating radioactive sub- stances that helped to produce the liquidity. This process was essentially a metamorphic one, but in the deep interior the stresses and temperature rose to a much higher order than in the zone of observation, and the metamorphism is assumed to have been more radical.* 39. Simple pressure experiments are incompetent to , determine the limit of self-compression in this broader sense. Such experi- ments cannot even cover the full range of metamorphic reorganiza- tion recorded in the observational zone. They are much less competent to set metes and bounds to the higher order of meta- morphism under immensely greater stress conditions. At best they can only indicate how much of the observed effect is to be assigned to simple mechanical compression and how much to metamorphism in the interest of higher density. 4o. To set the new view into sharp contradistinction to the olf let it be noted that the planetesimal earth is looked upon as a profoundly metamorphic earth with a minor igneous accessory, while the traditional earth is commonly regarded as a profoundly igneous earth with a minor surficial metamorphic subsidiary. RESCRUTINY OF THE FORMATIVE PROCESSES The importance of the results of planetary comparison made it seem imperative to rescrutinize the whole chain of postulates that lies back of them. Chief among these were the tenets of the planetesimal hypothesis itself. This hypothesis was therefore reconsidered from the ground up. At the same time competing hypotheses were retested, for these lie, in a similar way, back of the older views of diastrophism, whether their authors are aware of it ornot. This restudy brought forth not only evidence confirmatory of previous views but supplementary considerations which need to be summarized here as part of the groundwork on which further study is to proceed. * The Origin of the Earth (1916), chap. ix, ‘‘The Inner Organization of the Earth,” — pp. 226-40. DIASTROPHISM AND THE FORMATIVE PROCESSES 403 41. To follow accurately the logic of the planetesimal hypothesis, it is necessary to keep clearly in mind that its point of view is pre-eminently dynamic. Its type ideas are not based on material forms, but on dynamic organizations. ‘This was set forth in explicit terms in the earliest full statement of the hypothesis.t In spite of this, the hypothesis has come to be more or less unconsciously regarded as dependent on a special interpretation of spiral nebulae, because these nebulae have been much used as illustrations of the type of deployment supposed to have been involved in the genesis of our planetary system, but the theory is not thus dependent, as was urged from the outset. It will stand or fall solely on its ability to explain the remarkable characteristics and relationships of our planetary system. The requirements imposed by these are so many and so exacting that no theory but the true one has any chance of fully meeting them. We may, of course, think they are met when they really are not. We hold it is quite sure, how- ever, tha® in time the “vestiges of creation”’ will give convincing tests, and the essentials of the whole history of the earth will be read from beginning to end. 42. Two distinct types of planetesimal organization are recog- nized: In one, planetary nuclei, serving as collecting centers, revolve among the planetesimals and gather them in, forming bodies of notable size. In the other, there are no such collecting nuclei, and the formation of bodies of planetary size is a practical impossibility; the planetesimals remain small and constitute a multitude of minute secondaries. 43. Two distinct classes of planetesimal secondaries are recog- nized: (a) those which the parent body may develop by its own genetic resources, 1.€., monoecious secondaries, and (6) those which can be developed only by the co-operation of another body serving as a second dynamic parent, i.e., dioecious secondaries.’ ‘In outlining the planetesimal hypothesis for the use of students in Chamberlin and Salisbury’s Geology, Vol. II (1905), pp. 38-40, it was specifically pointed out that the planetesimal condition may arise in different ways, as from a gaseous nebula of the Laplacian type, or a meteoritic swarm of the Lockyer type. An origin from a spiral nebula was made the leading type for reasons specified, but to this was added: ‘‘ While this will be followed as the type view, let it be distinctly noted that the planetesima | doctrine of accretion does not stand or fall with this particular conception” (p. 40). 2 The Origin of the Earth (1916), “Celestial Kinships,” pp. 1o1-2. 404 THOMAS C. CHAMBERLIN Monoecious secondaries may arise as:orbital ultra-atmospheres’ do, and in similar ways, and are normally minute. They thus probably attend all stars in prodigious numbers but small mass. Dioecious secondaries are assigned to the dynamic action of a passing body on an eruptive sun by first stimulating an effective outburst and then drawing the projected matter into orbits about the mother- body, a purely dynamic function. ‘The eruptions are quite sure to take place by successive impulses, and a part of each belch is likely to remain under self-control and act as a collecting center for the more scattered planetesimal part. Thus the main mass will be gathered into a comparatively few, rather large planets. 44. While thus monoecious systems may be nearly universal, dioecious systems can arise only when the necessary dynamic encounter takes place. Close approaches of stars are rare events; there is, therefore, no reason to suppose that planetary systems /zke our own are common in the heavens. Only one such is known. Still, rarely as one star closely approaches another, the multitude of stars and the great length of celestial time makes possible a fairly large number of even this very peculiar class of secondaries. It is a logical error to base arguments on the assumption that planetary systems of this type are universal or even necessarily frequent attendants of stars. 4s. The cosmological rescrutiny brought out in ies: terms than before the extreme improbability that a planetary system like our own would arise from any form of centrifugal action in a condensing gaseous or quasi-gaseous nebula. The principles of the kinetic theory of gases, combined with the dynamic considera- tions that lie back of the Roche limit? and of the new criterion of Moulton, indicate that all such action would result in minute secondaries without effective collecting nuclei, a condition which practically inhibits the formation of large planets.* 1 The Origin of the Earth (1916), ‘‘Celestial Kinships,” p. 21. 2F. R. Moulton, ‘On the Application of Roche’s Limit and a New Criterion of Somewhat Similar Character,” Astrophys. Jour., Vol. XI (1900), pp. 120-26. 3 Ibid. 4“‘Selective Segregation of Material in the Formation of the Earth and “Tts Neighbors,” Article XI, Jour. Geol., Vol. XXVIII (1920), pp. 137-44. -DIASTROPHISM AND THE FORMATIVE PROCESSES — 405 46. Planetary generation by the dioecious method is not confined to the approach of one star to another; bodies less massive than stars, if they make sufficiently close approaches, are compe- tent to develop planetary systems. Only 1/745 part of the mass of the sun was required to form the planets of our system. The critical point in such cases is the ability of the small passing body to impart the requisite revolutionary momentum.' 47. New potentialities of projection from the sun have recently been disclosed. Twice during 1919, Pettit observed that erupted calcium vapor ascended by a@ succession of accelerating impulses. In one case, the ejected calcium vapor increased its outward velocity from 5.5 kms. per second to 60 kms. per sceond; in the other, from 37 kms. per second to 163.9 kms. per second. In both cases, the calcium was moving at its highest velocity when it ceased to be visible, high above the sun, probably either from cooling or from scattering, or both.’ DIVERGENCIES IN THE MODES OF PLANETARY CONDENSATION 48. The planetary nuclei diverged into two lines of descent almost as soon as they emerged from the sun. The nuclei that were massive enough to remain hot and gaseous at all stages, and to hold practically all molecules that came within their control, naturally grew to be giant planets. Nuclei that were not massive enough to hold all the solar gases, but in the main only the heavier ones, such as later made up the stony and metallic bodies, followed a much more selective career. This was a very vital matter, for the solar gases, constituted as they were, could not condense directly into bodies of the composition of the earth; a preliminary sifting was indispensable. 49. A further divergence soon followed in this sifted class. A few of the larger nuclei were only incompletely sifted; so that they retained relatively small amounts of gases of the atmospheric ”) t“Multiple Phases of the Planetesimal Hypothesis,” zbid., pp. 149-50. 2 Recent Disclosures Bearing on the Solar Parentage of the Planets,” zbzd., Pp. 145-49; Edison Pettit, “‘The Great Eruptive Prominences of May 29 and July 15, 1919,” Astrophys. Jour., Vol. L (October, 1919), pp. 206-19. 3 “The Physical Phases of the Planetary Nuclei during Their Formative Stages,” Article XII, Jour. Geol., Vol. XXVIII (1920), pp. 481, 487-80. 406 THOMAS C. CHAMBERLIN type. In most cases, however, the nuclei were unable to hold any appreciable amount of such gases. Some of the smallest could not hold the hot vapors of even stony and metallic substances. This would have led to their complete dissolution but for the fact that, on emergence from the sun into interplanetary space, they promptly condensed into clouds of precipitates and these soon gathered into precipitate aggregates. ‘These, being much larger and less active than the molecules of the previous vapors, were held under control and collected into planetoids.* 50. The formation of precipitates and precipitate aggregates of stony and metallic substances apparently played an important part in the condensation of planetary nuclei. As such precipitates appear to be forming. now in the photosphere of the sun, it is assumed that they would be formed freely in solar gases projected into planetary space. Such aggregates would act as Brownian particles and the condensation would not be strictly gaseous. If the nuclei later passed into the liquid state, crystalline and con- cretionary aggregates would probably form and give rise to a solid-liquid Brownian mixture. The descent was therefore that of Brownian mixtures of different types rather than simple gaseous condensation.” 51. Each planetary nucleus must have inherited internal motions from its solar state and from its ejection, and this must have promoted cooling and precipitation during the first critical stages. Later, convectional movements were added and con- tinued the precipitation. The inherited motions must have been more or less asymmetrical and this tended to give asymmetry to the core as it solidified. 52. The inherited motions and the sifting processes were sources of hazard to each small nucleus. Probably the smallest plane- toids and satellites now seen were the smallest that could be formed in this way. The spheres of control of even small nuclei were, however, surprisingly large, and this was doubtless the saving « “The Physical Phases of the Planetary Nuclei during Their Formative Siaee Article XII, Jour. Geol., Vol. XXVIII (1920), pp. 492-08. 2““The Formation of Precipitates and of Brownian Mixtures,” zbid., pp. 489-92. 3“‘The Motions Inherited from the Solar Eruption,” zbid., pp. 483-87. DIASTROPHISM AND THE FORMATIVE PROCESSES .407 factor. The sphere of control—as against the sun at the distance of the earth—of a mass 1/20 of that of the earth is 458,000 miles in diameter. A mass no greater than 0.000,000,296 of the mass oi the earth has a sphere of control 8,200 miles in diameter (MacMillan). The functions of spheres of control in the genesis of planets are a feature that has been too much overlooked.* 53. Asymmetry in a planetary core was likely to make itself felt in the distortion of the mass later built upon it. The inherited notion that the earth core, if liquid, would be merely a melt whose solidification would have to await the progress of freezing from its surface downward belongs to an old order of thought. The solidify- ing process should rather be studied as progressive supersaturation and precipitation. The formation of the solid core doubtless depended chiefly on the order in which the various possible minerals were formed and the extent to which they settled toward the center. The inherited and convectional motions doubtless affected the lodgment of the precipitates and rendered the growth of the core more or less asymmetrical. 54. Such external forces as gave rise to changes in the rate of rotation, or produced tides or nutations, must have played their part in distorting the forming core. THE SIZE OF THE PLANETESIMALS 55. At the outset, the planetesimals were merely the molecules of the scattered solar gases or the minute precipitates formed from these or else the molecules which escaped later from the nuclei. Starting thus small, and conditioned by their wide dispersion,. their growth was necessarily not only slow but precarious.* 56. The Zodiacal Light is reflected by minute particles that probably have orbits of the planetary type, and hence are in fact “Table of Dynamic Properties,” ibid., p. 478. For function of spheres of control, see Article XI, zbid., pp. 128-34. - 2 ““The Critical Conditions That Controlled the Passage of the Nuclei into Collect- ing Cores,” Article XII, ibid., pp. 477-500. 3 “Exterior Agencies That Affected the Planetary Cores during Their Formation and Afterward,” ibid., pp. 500-504. 4“The Nature of the Planetesimals at the Start,” Article XIII, zbzd., pp. 666-71. 408 THOMAS C. CHAMBERLIN planetesimals. If they are planetesimals left over from the forma- tion of the planets, they are surely old enough to have grown to the largest practicable sizes; but, in spite of this, they are certainly quite small. If they have had a more recent origin, they should still represent normal growth. In any case, they add their testimony to the smallness of planetesimals.* 57. The restudy of cosmological processes led. to a new view as to the relations of the erratic elements of the solar system, the meteors, meteorites, and comets, to the normal elements, the planets, planetoids, planetesimals, satellites, and satellitesimals, to the effect that they were all formed by the same type of dynamic action, save that the former were given erratic orbits, while the latter were given concurrent orbits. The ways in which this difference arose are given in the original discussion. These give a unitary view to the whole solar system. Now under them, chon- drules are interpreted as aggregates of stony and metallic precipi- tates from solar gases, growing in a manner similar to that of the planetesimals. If so, the sizes of chondrules and planetesimals should be about the same. Chondrules vary in size from walnuts down to dust particles, millet seed being mentioned as representa- tive. Their history differs in some features from that of meteor- ites, and they might be called meteoresimals to distinguish them. They seem to be immensely more numerous than meteorites; probably at least a hundred million of these meteoresimals “burn out” in the upper air as “‘shooting stars” for every meteorite that reaches the ground. 58. Meteorites proper are interpreted as fragments of erratic bodies disrupted by the extremes of heat and cold they suffer at the two ends of their very elliptical orbits. The parent bodies are held to have been formed by the aggregation of precipitates in a way similar to the aggregation of planetoids (No. 49 above), except that the original material was given very diverse and elliptical orbits.2 Under this view, it is not the meteorites but the chondrules that are analogous to the planetesimals. «“The Zodiacal Planetesimals,” Article XIII, Jour. Geol., Vol. XXVIII (1920), DaO72. ; 2 “The Testimony of the Aberrant Bodies of the Solar System,” zbzd., pp. 696-701. 3 [bid., pp. 697-08. DIASTROPHISM AND THE FORMATIVE PROCESSES 409 59. The size of planetesimals is not really important in con- sidering the heating effects of their infall, for what might be gained by their aggregation into large sizes would be lost by the lessened frequency of their infall, but to round out the inquiry a study was made of the effects of the infall of supposedly large bodies. 6o. The great pit and the scattered material of the famous Meteor Crater (Coon Butte), Arizona, the greatest of accessible examples of its kind, show that the impact of a large body— probably meteoritic or cometic—caused extremely little melting but yet great excavation, much upturning and wedging aside of strata and wide scattering of débris. Its lesson is that the impact of such bodies converts their energy of motion mainly into new forms of mechanical work, and very subordinately into melting.’ - 61. On the supposition—not in fact accepted—that the craters of the moon are pits formed by infalling bodies, the proper inference from the observable effects is the same as that from Meteor Crater. The steep-walled pits and the great radiating lines of débris are direct evidence of great mechanical effects. The lofty walls of the pits show no signs of the collapse that should attend great melting. The evidence of lava flows, in proportion to the number and size of the craters, is not remarkable on the most favorable interpretation. The level tracts, interpreted as lava, may be merely débris plains. The lunar Alps, Apennines, and other mountain ranges of the moon imply at least a crust stout enough to sustain such great elevations. Nothing observable definitely implies a holo-molten state.’ MELTING EFFECTS OF PLANETESIMAL INFALL 62. The planetary nuclei undoubtedly picked up some planetesi- mals that had wide-ranging orbits, but the larger portion specially related to the earth were distributed in the form of a ring around the sun about 55,000,000 miles in breadth, 58,000,000 miles in depth, and 292,000,000 miles in length. All these planetesimals had their own independent orbits, and were sustained by their own revolutionary energy. Their condition was antithetical to the collisional and collapsing habit of molecules in a gaseous 1“The Testimony of Coon Butte or Meteor Crater,” zbid., pp. 686-89. 2“The Questionable Intimations of the Craters of the Moon,” zb7d., pp. 690-94. 410 THOMAS C. CHAMBERLIN organization. They were subject to collision with one another, indeed, but their sparse distribution made the contingency less immanent than might easily be imagined. When collisions did occur, rebounds were more probable than mutual coherence; permanent unions were only probable when their relative speeds and other conditions were specially favorable. After union, they were liable to be driven apart again by succeeding collisions. Ultimate capture by the earth core was probable, but even in this case only as their orbits favored. ‘They were perturbed by all the attractions of the solar system. ‘These, on the whole, favored capture by the earth nucleus but not in all cases. The process of collection was intricate, indirect, and slow." 63. As a step toward realizing the sparseness of the plane- tesimals and the time required for their collection, let them be supposed to stand still as at first distributed, while the earth nucleus, taken as a net 6,000 miles in diameter, sweeps through them at 18 miles per second. To expedite the work, let the path of this net be so shaped and shifted by some demon that it will clean up an entirely new swath at each revolution. Even then it would take about 100,000,000 years to sweep up all the planetesimals.’ 64. To try, as a next step, the most rapid natural way, let the planetesimals act as though particles of a gas, collapsing on the track of the nucleus after each sweep—though that is far from their habit—and let each sweep, as before, clear a path 6,000 miles in diameter at the rate of 18 miles per second. It would then take about 260,000,000 years to gather in go per cent of the plane- tesimals; to sweep up all would require an indefinite period. 65. The real case was much less favorable. The planetesimals and the nuclei were moving in the same general direction, at some- what similar speeds. They could thus come together, as a rule, only as one overtook the other, or as their paths converged, a relatively slow method. They attracted one another but this did not necessarily bring them together, for their orbits might become so adjusted to one another that they merely became traveling companions, like the earth and the moon. Mutual attractions 1““The Intimations of the Planetesimal Mechanism,” Article XIII, Jour. Geol., Vol. XXVIII (1920), pp. 677-78. 2 Ibid., p. 678. 3 [bid., p. 679. DIASTROPHISM AND THE FORMATIVE PROCESSES 4it would constantly give rise to perturbations and these, on the whole, would favor the gathering in of the planetesimals, but just how fast is beyond the reach of rigorous computation. It can only be reached roughly by approximations. A period some- where between one billion and three billion years seems most probable.* 66. While the total heat of planetesimal infall was great, the melting effects were conditioned by the rate of fall and by the extent of atmospheric surface into which they fell. At the stage when the earth was one-third grown, there would remain to fall in 4X107° planetesimals averaging 1/50 lb.; or about 3 X10% planetesimals per square foot of earth surface. Taking the radio-bio-geologic estimate (2.410% years), as the time, the average rate of plunge into the upper part of each one-square-foot air-column would be one planetesimal in 6.7 days. Even at the time estimate based on the older geologic scale—probably much too short—a planetesimal would fall into the top of each square- foot air-column not oftener than once in 40 minutes. Now in the upper air, half the heat would be promptly radiated outward, and the rest would act at a disadvantage in heating the earth’s surface some miles below. The mean intervals between falls would quite surely be much too great to produce the melting of the earth’s surface.” _ For a cross test, let the method be reversed by selecting a rate supposed to be sufficient to produce melting and testing this by the time. Let it be supposed that one fall per second per square- foot air-column would have been a melting rate. All the plane- tesimals would, at this rate, have been collected in a little over 4,000 years, an impossibly short time. It can scarcely be held that a rate of one fall per hour per column would produce surface melting; and yet, at that rate, all would have fallen in less than 15,000,000 years. We found that the impossibly speedy demon- directed method required 100,000,000 years. The melting of the surface during the last two-thirds of the earth’s growth seems out of the question.’ t Tbid., pp. 679-81. 2“The Rate of Planetesimal Infall,”’ zbid., pp. 681-86. 3 Ibid., pp. 683-86. 412 THOMAS C. CHAMBERLIN 67. The protracted rescrutiny of the cosmological postulates back of the planetary comparison was made to see if the results of that comparison needed any serious modification or qualification. It was found that the total self-compression should be placed somewhat higher than the high figures given by the comparison, on account of the larger proportion of light material which the larger bodies gathered in. As the amount had previously been found to be unexpectedly large, the correction may be treated simply as a margin of safety. The trustworthiness of the compara- tive method seemed to be greatly strengthened by the rescrutiny; the deductions from the planetary comparison are therefore regarded as firm groundwork for further study. THE DIASTROPHIC RESOURCES OF A PLANETESIMAL EARTH 68. An earth built of planetesimal dust settling from the air in a mixed state would retain, to an almost ideal degree, its latent resources for subsequent chemical combination and _ physical reorganization. It would retain also about as much as possible of its potential energy of position, for the accessions would be very loose as first laid down. In strong contrast to this, the resources of a molten earth would be dissipated in large measure while still in the fluid state. The molten globe spent its energies in a hot youth; the cooler planetesimal earth conserved its resources for its later life. 69. In a molten earth, the high heat would be the master factor; its rate of dissipation would set the pace of progress. In a planetesimal earth, the strength of the solid material would be the ruling factor. Self-compression would take place only as this was overcome. Heat, of course, would be developed, but only as the yield of the solid matter permitted; it would be merely an incident of the process and would help to stay the process until it was taken care of. 70. In a dust-built earth, self-compression began as soon as a new layer was laid on an old one. Thereafter, compression con- tinued by stages and intervals as long as loading continued. Dias- trophism therefore ran through the whole formative history and was doubtless more active during the stages of growth than since. DIASTROPHISM AND THE FORMATIVE PROCESSES 413 71. It seems inconsistent with the ultimate constitution of matter, as now understood, to assign any limit to compression so long as pressure increases and there is any way by which the energy of organization can escape or take a new form of greater density. As energy continues to escape from the earth, and as there still remain resources of reorganization into denser forms— at least in the outer part of the earth—diastrophism is probably yet far from the end of its career. It is probably competent to rejuvenate the continents for eras yet to come. VULCANICITY AS A DIASTROPHIC AUXILIARY 72. Our planetary system, embracing nearly a thousand bodies all told, presents a great graded series in which the largest mass is many million times that of the smallest.t There is also a grada- tion in physical state. At the upper extreme, Jupiter is domi- nantly fluid; at the lower extreme, the planetoids and satellites are atmosphereless solids; in the middle, gases, liquids, and solids are combined. The earth is near the middle but dominantly solid. The dividing line, where fluids and solids might be sup- posed to be critically balanced, lies considerably above the earth in the series. The subordination of the fluid element in the earth is assignable to certain restraining factors imposed by the rigidity of the material. These give rise to a partition of the energy set free by self-compression, so that only a portion of it manifests itself as temperature. A portion becomes refixed in endothermic compounds; a portion is consumed as latent heat of liquefaction and is forced into higher horizons where a solid state is resumed and the liquefying energy is again set free and readily discharged, while a third portion is probably consumed in physical and, perhaps, even atomic reorganization. The joint effect is the persistent removal of liquidity and the conservation of solidity. The whole is a profound metamorphic process. The special processes of vulcanism are thus looked upon as subsidiary to the metamorphic- diastrophic processes, but still as important auxiliaries. “The Physical Phases of the Planetary Nuclei during Their Formative Stages,”’ Article XIT, Jour. Geol., Vol. XXVIII (1920), table I, p. 476. 4I4 THOMAS C. CHAMBERLIN 73. Since the formation of mutual solutions of rock material within the earth affects only such part of the mixed matter as becomes soluble under the contacts and conditions present, and since the solid state is resumed at or near the surface, magmatic generation is the matter of primary moment in the history as well as the philosophy of magmatization. Magmatic differentiation belongs essentially to the reverse process, and is dependent on the generative process. 74. The temperature curve of the interior, under this view, does not depend primarily on cooling from the surface or on the arrest of a convectional circulation, but on dynamic action within the body itself, starting with the restraints of inherited solidity in the clastic matter and adding new restraints at intervals later, by transformations of such a nature as to fit a part of the mixed material for a higher state of solidity, while a part was liquefied and sent to the surface to resume solidity there. 75. The gases and gas-producing substances entrapped by the burial of minutely mixed planetesimal matter should have been well-nigh amaximum. Subsequent processes of partial liquefaction and extrusion should have set these gases free, and they should have joined whatever liquid material was in process of formation. The magmas should thus have been rich in gases; sometimes becoming explosive. A large gaseous factor is-therefore held to be characteristic of the vulcanism of an earth so built. : 76. On the other hand, during the protracted boiling of a gaseo-molten earth, potential gases should have been set free to a maximum, and all gases should have been brought to the surface by convection, whence they should have escaped to the fullest extent consistent with gravity, because of their hot state. There should have remained in the boiled liquid merely the equilibrium quantity required to balance the partial pressures of the atmos- phere.t Laboratory melts of like material under like conditions should indicate the limited amount of this. The cooled mass could scarcely have carried those abundant supplies of gas that have been so amply manifested by the extrusive action of all the * Rollin T. Chamberlin, ‘“The Gases in Rocks,” Jour. Geol., Vol. XVII (1909), pp. 565-68. ee DIASTROPHISM AND THE FORMATIVE PROCESSES 415 geologic ages. If the lunar craters are volcanic, as we assume, the evidence against a molten moon becomes still more imperative, for even in its cold, mature state the moon cannot hold free volcanic gases. All such gases should have escaped while the moon was still hot and boiling, and it should later have cooled to a smooth, gasless globe, singularly unfitted for the explosive action which its surface implies.* CLUES FROM SURFICIAL DIASTROPHISM 77. The diastrophism of a solid earth should have been a unit, in all its great essentials. The deformations of the shell should have been intimately related to the diastrophism within the shell, if indeed not largely dependent on it. The mode of junction of the under surface of the shell with the upper surface of the interior mass should be especially instructive. One of the newer methods of study has disclosed the important fact that very notable down- ward protrusions are developed. ‘These are defined by plunging zones of accommodation that are at least suggestive. The intima- tions of these are herewith added as Part II, since these seem to belong with this résumé of ground work for megadiastrophic study. Other studies in the zone of observation offer clues of great value; indeed, no line of inquiry lacks them. Two of these are very specially related to the study of inner diastrophism: the experi- ments of Adams? that point toward a higher degree of rigidity than was accepted previously, and the contributions of Van Hise? and Leith? to the methods of metamorphism, especially the selective and rejective phases of anamorphic action, which point toward methods of reorganization very like the more radical selective and rejective metamorphism assigned to the deep interior of the earth. t Article XIII, Jour. Geol., Vol. XXVIII (1920), pp. 694-95. 2 Frank D. Adams, ‘‘An Experimental Contribution to the Depth of the Zone of Flow in the Earth’s Crust,’’ Jour. Geol., Vol. XX (1912), pp. 97-118; F. D. Adams and J. A. Bancroft, ‘‘On the Amount of Internal Friction Developed in Rocks during Deformation, and on the Relative Plasticity of Different Types of Rocks,” Jour. Geol., Vol. XXV (1917), pp. 597-637. 3C. R. Van Hise, “‘A Treatise on Metamorphism,” Monogr. 47, U.S. Geol. Survey (1904). 4 Leith and Mead, Metamorphic Geology (1915). 416 ' ROLLIN T. CHAMBERLIN PART II. THE INTIMATIONS OF SHELL DEFORMATION As the result of field studies in Pennsylvania in 1905, the crustal shortening involved in the folding of the Appalachian Mountains west of Harrisburg was found to have been fifteen miles. Computations based upon this shortening and the con- ~ sequent upbowing seemed to indicate that the shape of the deformed section was that of a triangular prism or wedge pointed down- ward.t Two sides of the wedge were found to converge beneath the mountainous tract till they came together under the middle portion of the deformed belt at a depth of thirty-two miles. No consideration whatever of stress-and-strain relations entered into the deduction of this wedge-shaped form. It came out directly from the graphic treatment of the field measurements, quite irrespective of any theory of mechanics, or of the nature of diastro- phism; in fact, the result came as a distinct surprise. It was soon seen, however, that the shape was in harmony with the principles of fracture under essentially uniform horizontal com- pression. The key to the method used in this inquiry lay in the axiomatic proposition that there must be a definite relation between the thickness of the deformed shell, the horizontal shortening which this shell has suffered, and the amount of resulting vertical bulge, on the assumption that there has been no notable compacting of materials.2 When the horizontal shortening and the consequent upswelling have been determined from field measurements, the thickness of the deformed shell can readily be calculated. The most important outcome of the method is that it gives the under- configuration of the deformed shell in addition to other qualities. It is from this feature that the most important intimations relative to the deeper diastrophism are drawn. Besides the under-configuration of the deformed shell, the following generalizations seem to be warranted: (a) Sharp folding ‘Rollin T. Chamberlin, ‘‘The Appalachian Folds of Central Pennsylvania,’ Jour. Geol., Vol. XVIII (1910), pp. 228-51. 2 For a discussion of this and other possible qualifying factors, see Jour. Geol., Vol. XVIII (1910), pp. 236-37, and also ‘‘The Building of the Colorado Rockies,” Jour. Geol., Vol. XX VII (19019), pp. 235-38, 244-47. ~ DIASTROPHISM AND THE FORMATIVE PROCESSES 417 and much crustal shorting indicate the deformation of a thin shell; otherwise the resulting upward bulging would be enormous. (6) Open, gentle folding may signify either a thin shell or a thick shell, according as there has been little or much upbowing. (c) For a given crustal shortening, the greater the vertical uplift the thicker the shell which has been actively deformed. The detection of the wedgelike shape of the deformed mass led to a consideration of the nature of the plunging planes which out- line the block. If the wedge was defined by fracturing, the borders, as shown by the Daubrée experiment,’ by the familiar crushing- strength tests upon building stones, as also by an analysis of the stress-strain relations, should be fault planes dipping beneath the de- formed block at angles in the general vicinity of 45° though in most cases somewhat less. ‘This is the result to be expected in a case of non-rotational strain, in which the axes of strain do not change posi- tion with respect to the axes of stress. If the developing strain be rotational in character, the angles of the shearing planes will be lowered from 45°, in proportion to the extent of the rotational element.? If, on the other hand, definite shearing planes do not develop, and the deformation is largely by folding, it is possible that the folding dies out below by affecting successively narrower and narrower belts. Though such a process would make the deformed block taper downward, just as in the preceding case, the borders would be much less sharply defined. With increas- ing resistance to deformation with increasing depth, in accordance with the results of the experimental work of Adams and his colleagues,? it seems mechanically logical that folds should die out in this fashion. But whatever the nature of the bordering zones of accommodation may be, the results of the computations in the case worked out show that the folded tract becomes narrower *G. A. Daubrée, Etudes synthéliques de géologie expérimentale, T.1., p. 316, Plate II. 2R. T..Chamberlin and W. Z. Miller, ‘‘Low Angle Faulting,” Jour. Geol., Vol. XXVI (1918), pp. 1-44. 3 Frank D. Adams, ‘‘An Experimental Contribution to the Depth of the Zone of Flow in the Earth’s Crust,” Jour. Geol., Vol. XX (1912), pp. 97-118; F. D. Adams and J. A. Bancroft, ‘‘On the Amount of Internal Friction Developed in Rocks during Deformation, and on the Relative Plasticity of Different Types of Rocks,” Jour. Geol., Vol. XXV (1917), pp. 597-637. 418 ROLLIN T. CHAMBERLIN below, thus maintaining the general wedge shape. The deepest folds lie beneath the middle of the belt. In the broader study of diastrophism it is to be recognized that movement is not wholly confined to the strongly deformed masses, but takes place in some less degree in the masses that lie below them and at their sides. Associated movements are to be recog- nized which extend deep into the earth and possibly even through- out its whole mass. ‘The relations between the distinctly distorted section and the environing portion are various. For example, the bordering thrust-faults on the margins of the southern Appalachi- ans and of many other strongly compressed mountain ranges indicate that actual fracturing and shear take place very commonly between the strongly deformed and the slightly deformed blocks near the surface. Where the deformation has not been so intense, a sharp upturn of the strata in a great fold may mark the borders of the mountainous belt, as at Tyrone, Pennsylvania, and in the Colorado Rockies. Here actual fracture has not developed to any important extent, though there has been an approach toward fracture on the outer limbs of such folds. The adjustment between the more movable, more deformed portion and the less movable, less deformed region has been accomplished, partly by shearing and partly by mass rearrangements taking place in the folding process. With increasing depth below the surface, actual fault- ing should diminish, though distributive shear should presumably descend much deeper. No limiting depths can be assigned, for the time element plays an important part, though not easy to evaluate. To quick-acting stresses the earth reacts as an elastico- rigid body; under long-continued stresses it yields to slow mass movement. With greater depth molecular rearrangement and recrystallization should presumably take precedence. ‘These might be manifested by folding, or by cleavage, or perhaps-only by rock flow. Under such conditions the deformed block would probably not be sharply bordered. While the border belts near the surface are in places actual fault planes, nevertheless, throughout most of their extent they probably constitute zones, perhaps of considerable breadth, sepa- rating the more deformed mountain mass from the less deformed DIASTROPHISM AND. THE FORMATIVE PROCESSES 410 mass to the side of it and belowit. The accommodation may take place by differences in the amount of wrinkling on the two sides, differences in the extent of elongation and schistosity, or differ- ences in some less clearly defined type of rock flow. It may be merely a zone in which, as one goes from the undeformed region into the crumpled mountain belt, folding rapidly becomes more pronounced and an increase in schistosity becomes marked. The dividing belts may be vaguely outlined or they may be more sharply defined. Because the wedge shape seemed to be in general harmony with certain recognized facts and principles, it was natural enough to suspect that it might prove to be a type of diastrophism of wide application. Wedge dynamics might prove to be characteristic of other mountain systems, and might be applied perhaps also to the elevation of plateaus, and those movements which control the rise of continental masses. A plateau-forming movement, if the outcome of lateral thrusting, would be assigned to a thick shell gently wrinkled. Very little shortening of a still deeper section would suffice to elevate a mass of continental dimensions. In ro10, following the publication of the Appalachian paper, an attempt was made to apply these principles to continental dias- trophism. Cross-sections of the globe were drawn with border planes dipping inward beneath the continents at 45°. Because of the curvature of the earth, each plane, in order to carry out the principle consistently, was drawn to cross the different radii of the globe at 45°. The result of such a treatment is shown in Figure r. Outlined thus, the continents appear as shallow units very subordinate to the oceanic segments. The latter are truly the master segments, which squeeze the continental wedges periodi- cally outward, as well as laterally, when the materials of the contracting globe become strained beyond their yielding point. This suggested extension of the wedge principle to the continents was not pushed farther at the time, for the reason that certain possible objections quickly came to mind, so that it seemed advis- able to allow the question to lie fallow for a while and await develop- ments. After a wait of ten years, during which time this principle was discussed with several successive classes of graduate students, 420 ROLLIN T. CHAMBERLIN and during which time geologic thought has progressed more and more toward the conception of a solid globe, it now seems worth while to put the ideas briefly into print: Objection will, of course, at once be made by many to any prolongation of shearing planes S. iy t. 0 ©. RoE AMICON Fic. 1.—The continental wedges. Section through Washington, Chicago, Salt Lake, Mt. Shasta, Australia, and South Africa. Surface relief exaggerated and diagrammatic. Drawn in gto. deeply beneath the surface. The tenacity of the idea of easy flow- age in the depths is remarkable. The idea of a molten interior, though disclaimed in name by most of those who follow closely the development of geologic science, is yet followed unconsciously, in fact, to some degree at least by most of them. On the other hand, DIASTROPHISM AND THE FORMATIVE PROCESSES 421 it is the view of those inquirers who accept the modern evidences of increasing solidity with increasing depth that the idea of easy movement in the interior is to be scrupulously avoided, whatever may be the form or mode of movement. It is held by them that differential motion of all kinds in the solid interior takes place only in response to high differential stress. If the term flow is to continue in use, it should carry simply the idea of an intimate distributive method of deformation and be shorn entirely of all suggestion of easy movement, for that belongs to liquidity. Strong support for this view is found in the experimental work of Adams, which extends the zone of cavities to greater depths than formerly supposed possible, owing to the increasing strength of the rocks under cubical compression. There are increasing grounds for the view that the various special methods of deformation have a more intimate association with one another than has been generally recognized, and that processes heretofore confined to the upper zones may have applica- tion to greater depths. The true status of present knowledge of the movements in the deeper unseen zone has been most judi- ciously and trenchantly stated by Leith in his vice-presidential address before Section E of the American Association :! Notwithstanding these and other considerations, any conclusion as to the existence of a deep zone in which all rocks flow when deformed is hypothesis, not proved fact, and perhaps will always remain so. The environmental condi- tions are not accurately known, and even if each of the factors were measured, their conjoint effect is still speculative. Variations in the time factor alone may determine whether a rock flows or fractures. Rock flowage which has occurred in rocks now accessible to our observation fails to indicate increase _ with depth with sufficient clearness and definiteness to warrant confident downward projection. The general purport of the under-configurations developed by these shell studies carries at least an intimation that the principles of surficial diastrophism are to some extent applicable to the deeper problems of the continents as well. When the ocean basins sink and the continents are uplifted, some adjustment necessarily takes ™C. K. Leith, “‘The Structural Failure of the Lithosphere,” Sczence, Vol. LIIL (1921), pp. 195-208. 422 ROLLIN T. CHAMBERLIN place between the segments. Near the surface the adjustment may well be accomplished by thrust-faulting; by movement along distributed shear planes where clear-cut faulting does not develop; by accommodative folding, and in part by the deformation involved in the formation of flow cleavage, and possibly in other ways. It is important to understand clearly that the border zones between the more deformed blocks and the less deformed blocks are not held to be simple fault planes, though thrust-faults do so commonly emerge at the surface in these situations, but instead are held to be zones of more composite nature in which adjustments between the larger units are accomplished in the various ways outlined above. In full agreement with the view that the results of deformation, in the zone of observation, afford perhaps the best criterion we have at present for judging of the behavior in the deeper zones, we may make the projecting planes, or zones, between the oceanic and continental segments the line of approach to the deeper problem, éspecially if the problems of the deeper horizons are as strictly the deformations of solids as are those of the surface, how- ever different their conditions of temperature and pressure. With depth, fracture should become an exceptional phenomenon, though certain types of shear should be more persistent. And it would seem likely that deformation by distributive movement involving crumpling, cleavage, and general rock flow, should predominate in the deeper horizons. Cleavage, by recrystalization, develops parallel to the elonga- tion of the mass, whatever be the nature of the compressive stress which produces it.1 From the experiments of Daubrée* upon the orientation of mica flakes under various conditions of compression, we learn that the direction of elongation is determined by the direction of least resistance. The deformation is controlled by the difference between the stresses along greatest and least axes of stress, and in this the least axis of stress is most important. By chan- ging the position of the orifice (direction of easiest relief) in a com- pression cylinder, such as that used by Daubrée, without changing ™C.K. Leith, Structural Geology (1913), pp. 84-87. 2 Op. cit., pp. 407-22. ne DIASTROPHISM AND THE FORMATIVE PROCESSES 423 in the least the direction of applied force, the mica flakes could be made to assume any desired orientation. This illustration is introduced here for the reason that geologists so commonly refer to the direction of applied force as though it of itself determined the result, and relate everything to this direction without con- sidering with equal care the lines of resistance. But it is the differential stress which is all important in deformation, and in this the axis of least compressive stress plays a part of the most critical importance. In fact, “lines of least resistance’? might well be made the topic of a hortative sermon: When a region is subjected to strong compressive stress under ordinary conditions, the axis of least stress is the vertical one, and the easiest relief is upward. Elongation then presumably takes a vertical direction, as does also whatever flow cleavage develops. But under conditions of special burden, lateral elongation may be a condition precedent to a final vertical one. A notable shear zone extending obliquely downward on the 45° principle, such as is here postulated between segments, might be expected to exert an orienting effect on the direction of most ready relief for some distance beyond the point where actual shear ceases, though it is uncertain how far beyond. After that influence ceased to be effective, the inclination of the ensuing schistosity should theoreti- cally become more nearly vertical. If, in the deeper parts, the border planes between the segments come into parallelism with the elongation under recrystallization, and so with the schistosity, they should become steeper below. But it is not certain that the parallelism exists, nor do we know the controlling conditions sufficiently well to be certain that the elongation of the mass will be straight upward. In the actual drawing of the border planes, the angle of 45° serves largely as a convenient average inclination, suggested by the planes of no distortion in the ellipsoid of strain.t It is, however, recognized in engineering practice that the angle of fracture under compressive stress, even where the strain is entirely non-rotational, varies widely from 45°, depending upon the nature of the tC. K. Leith, Structural Geology (1913), pp. 16-20. 424 ROLLIN T. CHAMBERLIN substance.t This variation applies especially to rupture, and it is uncertain how deep rupture may be safely projected. In rock flowage other conditions obtain. The inclination might be expected to become steeper. If it be true that in the lower reaches the border zones should become more steeply inclined than here drawn, that would amount to projecting the roots of the continental masses to greater depths. But the real significance of the location and — behavior of the bordering zones in the deeper portions of the globe can only be satisfactorily treated by tracing the diastrophic phenomena throughout the stages of the earth’s growth. Under the plantesimal view, each of the stages involved at first surficial diastrophism and later underwent the various deeper diastrophisms involved in the upgrowth of the continental and oceanic segments. This will be taken up in a later paper of this series. According to the general philosophy of which the wedge theory is a part, condensation of material in favor of greater density takes place throughout the deep interior under the influence of gravita- tive force. This causes shrinkage and the development of strong lateral thrusting in the outer portion of the globe. When the growing stresses reach and exceed the strength of materials under the conditions obtaining within the earth, a period of diastrophism sets in. The vaster and heavier oceanic segments take the lead in descending and as they do so, the continents, several or all, are wedged upward. Some may be wedged up more than others, or one side of a continent uplifted more than the other sides. A moderately thick shell forming only part of a continent may suffer notable shortening and be pushed up into a plateau. A thinner shell along the borders of a continent, yielding more readily and suffering much greater shortening, may be folded and faulted into a mountain system. The tangential compressive stresses necessarily extend throughout the outer portion of the globe and are not to be thought of merely as thrusts from an active oceanic _ mass against a passive continental mass, but the actual deformation into mountain systems is, for reasons to be brought out later, a t Walter H. Bucher, ‘‘The Mechanical Interpretation of Joints,” Jour. Geol.,. f Vol. XXVIII (1920), pp. 707-30; Vol. XXIX (1921), pp. 1-28. DIASTROPHISM AND THE FORMATIVE PROCESSES 425 distinctly localized phenomenon occurring where the compressive stresses exceed the strength of materials. In conclusion, it is felt that the shell-deformation is intimately related to the less obtrusive diastrophism of the subshell mass. The plunging zones that form the common border of the interlock- ing tracts give suggestive intimations that may have wider applica- tion and more profound significance. These form a line of approach to the larger diastrophic problem and so properly con- stitute a part of the groundwork for the study of megadiastrophism. THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION AND MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS | JH ks VOGr Trondhjem, Norway jut CaMgSi.O. (DIOPSIDE) AND SiO, The binary eutectic between CaMgSi,O,; (melting-point = 1391°) ~ | and SiO, (melting-point =ca. 1700°, see p. 330) consists, according to Bowen (Amer. Jour. Sci., Vol. XX XVIII [1914]) of 84 per cent Diops:16 per cent SiO., with melting-point =1362°, and at this temperature SiO, forms tridymite. THE TERNARY SYSTEM CaMgSi,0¢:Ab:An AND THE SEQUENCE OF CRYSTALLIZATION BETWEEN THE PYROXENE MINERALS AND PLAGIOCLASE The ternary system between the chemically pure substances CaMgSi,0¢, NaAlSi,O3, and CaAlL,Si,03 has recently been explained by N. L. Bowen in a treatise “The Crystallization of Haplobasaltic, Haplodioritic, and Related Magmas” (Amer. Jour. Sci., Vol. XL [x915]). The melting-points are: CaMgSi,Os, diopside=1391.5°. CaALSi,03, An=1550+2°. NaAlSi,0s, Ab=1100+ 10°. The binary eutectic Diops:An was determined as 58 per cent Diops:42 per cent An, with melting-point at 1270°. On account of the extremely high viscosity of the melting masses, consisting of predominant Ab, the binary eutectic between Diops and Ab could not be determined, but by extrapolation © (see Fig. 6) it lies at only a few per cent of Diops to nearly roo per cent Ab, and naturally at a temperature lower than the melting-point of Ab, consequently somewhat lower than 1100", thus a couple of hundred degrees lower than the eutectic between Diopsand An. From the theoretical explanation of Schreinemaker 426 MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 427 it appears (cf. p. 337) that the ternary system Diops:Ab:An must form two melting-planes, one for Diops and the other for Ab+An. This is verified by Bowen’s experimental investiga- tions, whereby further is proved that the eutectic line between Diops, 1394 Ab, ca 1100" ~— An,1550° Fic. 6.—The ternary system An: Ab: Diops (horizontal projection), after Bowen Epiops-An. 2Nd_ Epjops—ap. has a continuous decline, without a maxi- mum ora minimum. From Bowen’s diagram (Fig. 6) I calculated some points on the eutectic line. Melting-Points 58 Diops:42 An: o Ab or 58 Diops: 42 AnyooAbo 1270° 51 Diops:39 An:10 Ab or 51 Diops:49 An gAbz ca. 1260° 44 Diops:36 An:20 Ab or 44 Diops:56 An ¢;Ab,;; ca. 1238° 38 Diops: 32 An:30 Ab or 38 Diops:62 An »Abs ca. 1235° 33 Diops:27 An:40 Ab or 33 Diops:67:An pAb ca. 1225° 28 Diops: 22 An:50 Ab or 28 Diops:72 An jAby ca. 1215° 23 Diops:17 An:60 Ab or 23 Diops:77 An2Ab;3 ca. 1205 18 Diops:12 An:70 Ab or 18 Diops:82 An;;Abs; ca. 1185° The sequence of crystallization is illustrated by some examples taken from Bowen’s treatise, to which, however, I add a few comments. In a melt (m) of 50 per cent CaMgSi.O, and 50 per cent Ab,;.An;,—accordingly with a surplus of CaMgSi,O.—diopside 428 We Teh, be OCI commences to crystallize, if subcooling is left out of consideration, at atemperature of 1275°. After the separation of a certain amount of diopside the melting mass arrives at a point (”, ca. 38 per cent Diops:62 per cent Ab,.An;, and at a temperature of 1235°) on the eutectic boundary curve. Then a simultaneous crystallization of diopside and plagioclase commences, the latter in the beginning with a composition Ab,.Ango, but on continued cooling with continually less An. On the presumption of a complete equilibrium between the solid and liquid phases, accordingly between the already crystallized plagioclase and Ab+An in the solution, the composition of the crystallized plagioclase is continuously displaced. The quantity of the liquid grows continually less by continuous crystal- lization. The last remnant of liquid is spent at a point (O on Fig. 6) 23 Diops:60 Ab:17 An, at the melting-point 1200° and with a separation of a minimal quantity of plagioclase Ab;.Ansp. If, on the other hand, we choose a melt of 15 per cent CaMgsSi,0. and 85 per cent Ab,oAn;o.—consequently with a surplus of Ab+An— plagioclase of the composition Ab,.Ang, commences crystallizing at the temperature 1375° (see point F on Fig. 6). On continuous cooling at first only plagioclase crystallizes. On the presumption of a complete equilibrium the composition of the already separated plagioclase is continuously changed. In this manner a plagioclase Ab,An, appears at 1300°. The eutectic boundary curve is reached at G, at a temperature of 1216°. Now a simultaneous crystalliza- tion of diopside and plagioclase commences, as the Ab: An relation in the separated plagioclase little by little is displaced. At 1200° the last remnant of liquid is spent, the entire mass of plagioclase having the composition Ab;.Anso. With lacking equilibrium the first separated plagioclase remains unchanged. By the continuous separation of relatively An-rich plagioclase, the quantity of Ab in the solution increases continually, and even more strongly than by the equilibrium. In this manner, by the simultaneous crystallization of diopside and plagioclase along the eutectic boundary-line we here at last obtain a plagioclase with a very high percentage of Ab. Bowen emphasizes—and rightly—that in a ternary system Ab:An:Diops (as in the analogous system Ab:An:Qu) there is MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 429 no eutectic point. But an eutectic boundary curve exists with simultaneous crystallization of plagioclase and diopside. This simultaneous crystallization does, however, not take place at a constant temperature, but continues some distance at decreasing temperatures. With complete equilibrium this drop of tempera- ture 1s, however, relatively small, as by the examples above chosen: for 50 per cent Diops: 50 per cent Ab,An,, from 1235 to 1200°, and for 15 per cent Diops:85 per cent Ab,An, from 1216 to 1200°. In deep-seated igneous rocks (with very slow solidification) a com- plete, or in special cases not quite but only approximately complete, equilibrium takes place between the solid and liquid phases (see a separate chapter in the following). And even in the more quickly cooled effusive rocks no completely lacking equilibrium appears, but an imperfect equilibrium, where the degree of imperfection is of a somewhat changeable nature. With regard to the relations of crystallization, especially in the deep-seated igneous rocks, it is of subordinate importance whether the simultaneous crystallization of the final components takes place at a constant temperature (eutectic point) or—by a small dis- placement of the quantitative proportion between the components —within a smal] interval of temperature for a short distance along a eutectic boundary curve. Bowen disputes the justification of the term “‘gabbroidic eutectic’? which I have previously used. I find, however, supported by Bowen’s experimental investigations, that this term must be maintained, when we emphasize the fact that here we have to do with a short distance on a curve and not with a point. We shall now examine the crystallization of the pyroxene minerals and plagioclase, especially in gabbros and norites, and shall commence with the well-known orbicular norite (orbicular quartz-norite) at Romsaas, in the Archaen formation, 50 km. south- east of Kristiania, described by several earlier investigators, especially C. Bugge.* Romsaas, which is a small hill rising about 60m. above the surrounding gneiss, consists chiefly of quartz-norite, with which t Kristiania Vidensk. Selskab, 1906. Here the earlier works of L. Meinich(1878), Th. Hiortdahl (1878), and K. v. Chrustschoff (1897) are cited. 430 I Si EO VOCE is connected another quite subordinate variety of norite (see the analysis in the chapter on norite in Part Il). The entire MINERALS AND ROCKS FROM ROMSAAS aa z = ORBICULAR Pas oy 5 = NoriTtE ze SSIGRES | fa PLAGIOCLASE 8 Bin Z Ay a | & 2 | gs EE Bie 3 | FF | aer jamal 22 No 41 42 43a 43 43¢ 44 45 46 48 SiOw: 2084: 53-3.| 37-04] 52-33) 50: 25) 57-55) 542 55on. 25mg 75 |52.86 MI Ose pests |e Sao | eeaeteell icant ed | eter rs Gi 0.58] 0.40] 0.5 an ALO;..... 3.0/| 20.15] 29190] 27.03 27-20) 4. 45|) 21556) 9 1086 os FesOy ee T.2\ g2| 0-51] 0-45] 0.32] 0.50) 0.22] 0.4 35 |(0.47) COW ne: cA CAE Aaie sk| SeaUaN AG ae aN 24 14.50] 1.509] 10.6 g2 |11.81 MnO ie ©: 30/2 (O40) oes eee eel leet ©.50| 0.20] ©.4)|" (0.46/22 eee WIEO) so oo oe 23.7 | 16.44] 0.97] ©.19| ©.14| 22.08) 1.85] 16.0 61 |I7.15 CaOe eee 20) |) O70] LE.04|) 19 4)50| sO ROS | 2 On| ames mere 4.58 NazO\2 22 dee 2.57| 4.80] 5.40] 6.01] 0.64) 4.44] 1.7 1428 KOZ eae ence 7.01] 0.42] 0.16] 0.26] 0.56] 0.74] 0.65 1.03 BLO cas cl Reet T2312. ceet hal een | otencnee ©. Ll (On5 2 Or23 0.53 le eee eereme lato rralore TsO) ee tare 0422.0 2710) 1252 On40|tO 0.99 Total .|100.0 | 99.93|100. 66/100. 28/100. 43] 99.36|100.77| 99.6 |100.57 |99.87 EXPLANATION No. 41: Hypersthene (somewhat changed to hornblende) from the orbs, the medium of four well-conformable analyses, by Hiortdahl (2), Meinich, and Chrustschoff.—No. 42: Biotite from the orbicules by Bugge.—No. 43a-c: Different analyses of plagioclase from the intervening mass between the orbs, No. a (from the kernel of the plagioclase) by Meinich, Nos. 8, c (fromthe exterior parts of the plagioclase) by Chrustschoff.—No. 44, the orb, and No. 45, the intervening mass between the orbs, by Bugge.—No. 46, the coarse-grained orbic- ular norite, calculated by myself, and originating from 70 per cent orbs (No. 44) and 30 per cent intervening mass (No. 45).—No. 47 orbicular norite with smaller orbs, analyzed by Bugge.—No. 48, the ordinary norite, analyzed by Bugge, whose analysis applies to a rock with 3.88 per cent pyrrhotite which I have deducted from the analysis, and recalculated to the sum found. igneous field' has an area of only ca. 65,000 sq. m. In some places, in part directly at and in part quite close to the boundary of the norite field, the rock has an orbicular structure. We may « See map and profile in Beyschlag, Kreusch, Vogt, Erzlagerstdtien, I (2d ed., 1914), Fig. rot. ks eee MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 431 distinguish a variety (see Figs. 7 and 8) with large orbs of a diameter from about 4 to 8 or 9, mostly about 5 cm., and another variety with small orbs, of a diameter of about 2cm. These orbs consist in the interior exclusively of hypersthene (0.28 FeSiO,:0.72 MgSiO,, if we leave out of consideration the links containing CaO and Fic. 7.—Orbicular quartz-norite from Romsaas, Norway Al.O,) which, however, has been changed in part to hornblende. In the exterior part appears also some biotite which partly lies as shells or scales on the outside of the orbs (see Fig. 8). According to the detailed calculation which I effected on the spot, the rock with the large orbs consists (according to weight) of 70 per cent of orb substance (analysis No. 44) and 30 per cent of intervening - mass (analysis No. 45). On this basis, the composition, No. 46, of the entire rock is calculated by myself. The close conformity 432 SAE VOGH between analyses Nos. 46-48 prove that the orbicular rock, Nos. 46 and 47, is only a facies near the boundary of the ony quartz- norite (No. 48). The mineralogical composition of the orbs (with some biotite in the exterior part) of the intervening mass (by Bugge), and of the entire quartz-norite (from my own calculation) are as follows: Orb Intervening Total Quartz- ass Norite Hypersthene snes sayasenon cee ane 92.0 ° 63.0 Biotite, s....5:2. eee Ae ee TB 10.0 8.0 Plasioclase.23 cas tare: tana sea ae arenes tr. 1B 05 23.8 16 1 OE) oy Aner reer trocar nat MERI ern one eallicen ciate -c Bua a aa I5.0 4.0 Raat is 2.2 Ayan Oe ah eee rn 0.55 0.39 0.5 Apatité..../ .2tkasvednaaacemae cree 0. 23 1.18 0.9 Total: iste See eee ee eee 100.0 100.0 100.0 In addition there sometimes appear in the intervening mass small individuals of garnet, exceptionally also of pyrrhotite (‘nickel pyrrhotite’) in very small quantity. The plagioclase of the intervening mass, which on the average may be placed at 47 Ab:3 Or:50 An, therefore almost exactly Ab,An, varies between Ab,,An;s (with a little Or, analysis No. 43a, with extinction 7°rs5’ on oor according to Bugge) in the kernel of the individuals and Ab,;An,, (analysis No. 43c, with extinction 3°30’) in the exterior zone. Locally the plagioclase contains still more Ab, according to Bugge with extinction on oor of 1°15’, corre- sponding to about Abs.An;s (in both cases with a little Or). Con- cerning this matter we refer to some remarks in a following chapter. Naturally the orbs are first formed, and only later the inter- vening mass solidified. Near the center of the orbs, the hypersthene individuals are to a great extent radially arranged. In the exterior part of the orbs‘ we find, on the other hand, an indication of con- centric structure (see Fig. 8). If we leave apatite and rutile out of consideration, we may distinguish between the following stages of crystallization (Fig. 9): * Some inclusions of the material of the intervening mass also appear here and there in the orbs (cf. p. 320). ‘MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 433 (t) hypersthene in great quantity (stage i); (2) biotite, in the exterior part of the orbs simultaneously with the crystallization of the rest of the hypersthene (stage ii), then followed some biotite alone, as scales or thin shells on the outside of the orbs (stage iii); CS — ——— —— . a= Ve Ba ey Ai it l Zz i aa jen Yo Fic. 8.—Section through the orbicular quartz-norite from Romsaas, Norway. Natural size. Lightly shaded mineral is hypersthene, dark is biotite, and white is feldspar and quartz. ' if (3) after the orbs were formed, the intervening mass had the composition of a biotite-quartz-diorite (analysis No. 45), and the crystallization became that usual in these rocks, viz., at first biotite (the close of stage iii), then plagioclase (stage iv), and finally also quartz (stage v) solidified. 434 7 He LaVOGT We especially call attention to the following: In the magma, so extraordinarily rich in ferromagnesian metasilicate, only hyper- sthene crystallized at the beginning. Then the formation of this mineral stopped, as the ferromagnesian silicate still remaining in the magma entered into biotite. The change from hypersthene to biotite was probably caused by the quantity of H.O present in the magma, and this quantity had been relatively enriched by the separation of the large quantity of hypersthene. We shall return to this matter in a following chapter. Hynersthene Ih ITH WV V Fic. 9.—Diagram illustrating the different stages of the crystallization of the orbicular quartz-norite from Romsaas. We further emphasize that from the original magma, so rich in ferromagnesian metasilicate, a quartz-dioritic magma was sepa- rated at a far-advanced stage of the solidification and there resulted, by continuous solidification, at last even a magma for special ‘“‘oligoclase (or andesine) granite dikes,” consisting of biotite, andesine (AbssAn,.), and quartz. We refer to a special chapter in Part IT. The normal quartz-norite (Fig. 10) from Romsaas, consisting of ca. 63 per cent hypersthene (included a little secondary horn- blende), 8 per cent biotite, 24 per cent plagioclase (Ab,An,), 4 per cent quartz, and, in addition, a little apatite, rutile, and pyrrhotite, in part shows accumulation (together-swimming or synneusis struc- ture) of hypersthene individuals which often have a well-developed idiomorphic contour on their boundary toward the plagioclase or quartz. The hypersthene, therefore, must have crystallized com- pletely or in a great measure before the plagioclase and the quartz. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 435 _ The biotite appears partly in the exterior portions of the hyper- sthene individuals, and partly—and chiefly—grown on to these. The hypersthene in several places shows idiomorphic contours also against the biotite, the latter must accordingly chiefly have been formed at a latter stage than the former. The labradorite Fic. 10.—Quartz-norite from Romsaas. (Black=pyrrhotite) and the quartz form an intervening mass between the accumula- tions of hypersthene, and accordingly crystallized at a somewhat later stage. The investigation of the Romsaas rocks is in certain respects very instructive, but does not fully inform us of the relation of the crystallization between the hypersthene and the plagioclase, as _ the ferromagnesian silicate of the later stage entered into biotite instead of into hypersthene. We are therefore going to investigate some rocks where this phenomenon does not appear. | 436 I ES VOGM In the norites and gabbros, especially rich in orthorhombic or in monoclinic pyroxene (hypersthene-norites and diallage- gabbros) with relatively little, say 10, 20, or 25 per cent of labra- dorite, the pyroxene individuals to a very great extent show an idiomorphic contour against the plagioclase. Further, the pyroxene individuals are often to some extent accumulated in aggregates, consequently showing synneusis structure. On the other hand, the plagioclase shows no signs of idiomorphism but only appears as ery Fic. 11.—Photomicrograph (18:1) Fic. 12.—Drawing (11:1) Norite from Meseel, Norway. Hypersthene with spredominant idiomorphic outlines against the labradorite. The photograph represents the lower part of the drawing. The shaded mineral in small quantity is hornblende, the black pyrrhotite. mesostasis (Zwischenklemmungsmasse) between the pyroxene indi- viduals. This may be explained by the fact that an essential part of the orthorhombic or monocline pyroxene in question had solidified even before the commencement of the crystallization of the plagio- clase. We must not draw the conclusion, however, that the pyroxene individuals in their entirety had crystallized at an earlier stage than the plagioclase. On the contrary, in some of the pyroxene individuals, we find the idiomorphic contour against the plagioclase lacking, and this must indicate a simultaneous crystalli- zation of both minerals during the last stage of the solidification. As an example we refer to the photograph (enlarged 18:1) and MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 437 drawing (enlarged 11:1) of a hypersthene-norite from Messel (about 10 km. from Arendal, Norway), containing about 20 per cent labradorite (Ab,An,), nearly 80 per cent hypersthene (consisting, according to the determination of the optical character and of the Fic. 13.—Norite from Skougen, Norway. The hypersthene, to a great extent, has idiomorphic outlines against the labradorite, and the hypersthene in several places shows synneusis structure. The biotite is inclosed in the exterior parts of the hypersthene individuals or is grown on these. (24: 1.) optic axial angle, of about 0.25 FeSiO,:0.75 MgSiO,), a little horn- blende, and a little pyrrhotite, but no biotite, diallage, and oxidic iron ore. . In gabbros and norites, containing somewhat more plagioclase (labradorite), say 30, 35, 40, or 45 per cent, and correspondingly less pyroxene, we still find the pyroxene individuals to greater or lesser extent with idiomorphic contours against the plagioclase, while the latter lacks idiomorphism. 438 J. H. L. VOGT As an example we refer to Figures 20 and 21 and to Figure 13, microscopic photograph of a norite from Skougen in Bamle, Norway. This rock consists, according to microscopic investigations supported by chemical analysis (see Part II), of about 47 per cent hypersthene (according to the analysis of the Frc. 14.—Anorthosite from Hitteré, Norway. The labradorite (Ab,An,, light) has idiomorphic contours against the hypersthene. (25: 1.) rock and the optical determination calculated as 0.32 FeSiQ;: 0.68 MgSiO,), a trifle secondary hornblende, 48 per cent labra- dorite (ca. 38 Ab, 4 Or, 58 An, or about Ab.An;), 3 per cent biotite, and 1-2 per cent magnetite-ilmenite, see later (Figs. 31-32), 0.07 per cent apatite, and a little pyrite (o.24 per cent S). On the other hand, in rocks especially rich in plagioclase we find throughout the idiomorphism more or less well developed by the plagioclase, but not by the pyroxene. This applies to all anorthosites which I have investigated, where the ferromagnesian er APES See GS MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 439 silicates, indifferently whether hypersthene (Fig. 14), diallage, or olivine (see Figs. 23 and 24), for an essential part form a mesostasis between the plagioclase (labradorite or sometimes bytownite). The question in hand, I have to some extent considered in a paper, accompanied by several analyses, published in the Quart. Jour. of the Geological Society, 1909, on labradorite-norite with porphyritic labradorite crystals from Flakstadden in Lofoten. Referring to the quantitative analysis of this rock, given in the chapter on anorthosite-norite in Part II, I shall here give a short résumé. ‘The entire rock consists of: ca. 70.65 per cent labradorite, 55 An, 39 Ab, 6 Or Coe One = per cent He,O), Gis O70) per cent Heli; ca. 10.0 per cent hypersthene ca. 10.0 per cent diallage ca. 2.3 per cent biotite ca. 0.9 per cent apatite My. 2 per cent “‘titanomagnetite” Relatively to the entire rock, there first crystallized 23 per cent prophyritic labradorite of a composition 61 An, 33 Ab, 6 Or, and occurring as very large, up to 15-18 cm. long and 6-8 cm. wide, crystals, thick tabular along (o10). The remainder, 77 per cent, form a coarse-grained groundmass, consisting of: ca. 61.9 per cent labradorite, 52 An, 42 Ab, 6 Or per cent Fe,O, per cent FeTiO, i \o per cent “‘titanomagnetite”’ 9 ca. 13.0 per cent hypersthene ° fo) per cent diallage .O per cent biotite ca. ©.12 per cent apatite In this groundmass the labradorite continued crystallizing, and some magnetite (or ‘‘titanomagnetite””) commenced to form, of which more below. Only at a somewhat later stage, after the quantity of hypersthene and diallage had risen somewhat above 30 per cent, the pyroxene commenced forming. The hypferitic (or ophitic) texture of plagioclase crystals, with tabular development along (o10), appears in the gabbro rocks only when the latter contain at least about 55 per cent plagioclase (labradorite). The laths of plagioclase show partial idiomorphism 440 SEE VOGE against the hypersthene (see, for example, Figs. 15 and 16) or the diallage, which ordinarily entirely lack idiomorphism. We may consequently draw the conclusion that the crystallization of labra- dorite in these plagioclase-rich rocks must have commenced before the beginning of the solidification of the pyroxene. The idio- morphism of the labradorite, however, is often only quite slightly developed, as illustrated in Figures 15 and 16. This tells us that Fic. 15.—Photomicrographic (19:1) Fic. 16.—Drawing (35:1) Hyperitic- (or ophitic-) structured norite from Erteli, Norway. Consists of about 56 per cent labradorite (ca. Ab,;An,), 41 per cent hypersthene (0.31 FeSi0,: 0.69 Mg SiO,), a little magnetite and pyrrhotite (0.07 per cent S, see Fig. 46), 0.09 per cent apatite; see analysis in Part II. The drawing (35:1) represents the central and lower parts of the photograph (19:1). only a certain small part of the labradorite, in this case, had solidi- fied before the pyroxene began forming. Rosenbusch‘ emphasized, and rightly, with regard to the gabbros, that in rocks rich in plagioclase, the plagioclase, and in the varieties rich in diallage, the diallage, develops in idiomorphic individuals, and further, that in rocks rich in diallage, the idiomorphism of the diallage is the more prominent the greater its quantity. “Man wird also scheinbar gen6tigt ein gewisses Schwanken in der Reihen- folge der Ausscheidungen anzunehmen.”’ But Rosenbusch did not engage in the physicochemical interpretation of the phenomenon. * Mikroskop. Phys. d. Mass. Gest. (4th ed., 1907), II, 1, p. 364. ey ce MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 441 According to my own extensive investigations of the question in hand (whereof I have only given a very short résumé) the facts previously set forth by Rosenbusch are confirmed, and we draw the conclusion that the sequence of crystallization of plagioclase and pyroxene depends upon the relative quantity of the two minerals, for the crystallization commences with the solidification of the mineral present in excess of a certain limit. I have bestowed much labor on the determination of this limit, trying to get it as exactly as possible, by structural investigation on a series of norites, and partly also of gabbros, of which we have numerous quantitative analyses, so that the proportion by weight between the plagioclase (with a determined Ab:An relation) and the pyroxene may be quite exactly calculated. We then find that the limit essentially depends on the composition of the plagio- clase. As an example may be mentioned that in a diallage-bearing quartz-norite with about 58 per cent SiO. (and standing on the boundary near quartz-hypersthene diorite), containing about 28 per cent pyroxene (hypersthene with a little diallage), about 60 per cent plagioclase (andesine, AbssAn,.), a trifle orthoclase, about 4 per cent biotite, about 1 per cent iron ore, and about 5 per cent quartz, the hypersthene in a great measure appears with idiomorphic contour against the plagioclase (andesine). In rocks with basic labradorite, as Ab,An., the plagioclase on the other hand partly shows an idiomorphic contour against the pyroxene, even when there is as much pyroxene as 35~-40 per cent present. The individualization boundary, determined by the sequence of crystallization, amounts approximately in the deep-seated rocks to: With Ab,.Any» about 45-50 per cent ae With AbsAns about 40-45 per cent pyroxene With Ab;An; about 35-40 per cent pyroxene With AbsAny about 25-30 per cent pyroxene With Ab,;,An;. probably 15-20 per cent ered the remainder plagioclase By pyroxene we here understand, with regard to the norites, hypersthene of the common composition of these rocks, viz., 6.3-0.35 FesSi0;:0.7-0.65 MegsiO,, and, with regard to the gabbros, diallage with about the corresponding iron content. 442 Umlélsxiba AOI We are able to obtain a more detailed determination of the individualization boundary by studying a series of porphyritic rocks, with phenocrysts of plagioclase when this mineral is in excess, and of pyroxene when the ferromagnesian silicate is in excess. I am, however, far from having sufficient material for such a precision-determination. Because of physicochemical considerations we must draw the conclusion that the individualization boundary here shown is a eutectic boundary between pyroxene and plagioclase. And our individualization boundary shows almost exactly the same course as the eutectic boundary curve between diopside and Ab+An, determined by Bowen at a pressure of one atmosphere. If we imagine a quartary system, consisting of Ab, An, CaMgSi,O¢ (Diops), and CaF eSi,O. (Hed), this system will sepa- rate into two fields, each consisting of a continuous mix-crystal. Ab+ 04 RSIS eR ee eet ein » it 8.1 EEL IWIZO). 2 scofdiointae sees ae ces 9.3 8.3 1 (CRO). 3 o ode Gteten ete nae eee eee aes 9.9 8.7 sli IN BAO), 5: 6 o-g GneRaREE arNeeae 2 DB 3.6 TSAO), 0 9 Sg Oa ee eae 0.4 0.6 On, For the diallage of the gabbros we assume: 40 CaMgSi.O¢, 36 CaFeSi,0¢ 7 MgAl,sSi0¢, 11 FeFe.SiOe, 6 MgSiO;=47.5 per cent SiO., 3.8 Al,O;, 6 Fe.0,, 73.1 FeO, 11.1 MgO, 18.5 CaO. And for the augite of the diorite: 52 CaMgsi,0¢, 34 CaFeSi.0¢, 3 MgALSiO«, 6Fe Fe2SiOe, 5 Mg SiO;= 50.45 per Gent S107, 1.65 AlLO,, 3.3 Fe.0,, 11.3 FeO, 12.2-MgO, 21.1 CaO. GABBROIDIC EuTEcTIC LINE BETWEEN Drioritic Eutectic LINE BETWEEN PLAGIOCLASE AND DIALLAGE PLAGIOCLASE AND AUGITE a) Ild IIIb IV V VI 45 Diallage: 4o Diallage: 35 Diallage: 33 Augite: 27 Augite: 20 Augite: 55 AnzsA bisOr, | 60 AnezAb320re | 65 AngzAby7Ore |67 Ang2zAbgsOrz0]73 An3zA bs6Orz2}80 Anz2A besOrra Sire 48.0 50.5 83.2) Be eit Ba 5 60.2 ALO; . 19.9 OE] 19.3 18.5 18.5 18.8 Fe,0,. 2G 2.4 DW 1.1 1.0 ©o7/ FeO. . 5-9 52 4.6 BaF B50) QB MgO. 550 4.4 2.9 4.0 BoB 2.4 CaO.. 16.9 I4.9 12.6 12.6 10.4 7.8 Na.O. Toe Bok BHO 3.8 4.8 6.2 K.O.. 0.4 0.6 0.7 1.1 ToS 1.6 In the anchi-eutectic norites or gabbros with rather basic plagioclase (Ab,An,), the simultaneous crystallization of plagio- clase and pyroxene will commence at about II (or IIb) and be finished at about III (or III0), or at still less pyroxene and more of a relatively Ab-rich plagioclase, about equal to IV. And in the medium-basic anchi-eutectic diorites the simultaneous crystalliza- tion of plagioclase and pyroxene will commence about at IV (or between IV and V) and finish at about VI. [To be continued] TYPES OF ROCKY MOUNTAIN STRUCTURE IN SOUTHEASTERN IDAHO™ GEORGE ROGERS MANSFIELD U. S. Geological Survey, Washington, D.C. INTRODUCTION GENERAL STRUCTURAL FEATURES SPECIAL STRUCTURAL FEATURES Noteworthy unconformities ““Swallowtail” folds The Bannock overthrust The Blackfoot fault Drag folds Fan folds The Meadow Creek graben NoTES ON THE DEFORMATION OF SOUTHEASTERN IDAHO Epochs of deformation Rocky Mountain geosyncline Favorable formations Horizontal thrusting ; Factors in deformation Later deformative epoch Relaxation and readjustment INTRODUCTION Since 1909 the United States Geological Survey has been making detailed studies of portions of the western phosphate field, chiefly in southeastern Idaho. This region contains a series of sedimentary rocks 40,000 feet or more thick, including large bodies of high-grade phosphate rock that will prove of great economic importance for the future, if not for the present. There is interesting geologic structure and a variety of problems covering a wide range of geologic and geographic phenomena. « Read before the Geological Society of America, December 30, 1919; published by permission of the Director of the U.S. Geological Survey. 444 ROCKY MOUNTAIN STRUCTURE IN IDAHO 445 The area included in the detailed surveys is nearly 3,000 square miles comprised in the Fort Hall Indian Reservation, and in the Montpelier, Slug Creek, Crow Creek, Lanes Creek, Freedom, cco re a Be 43° Oh: YUU, Gua el VE o - CECA Eos (1.G a tay, 112° Fic. 1.—Index map of southeastern Idaho: A, Fort Hall Indian Reservation; ~ B-H, seven quadrangles; B, Cranes Flat; C, Henry; D, Lanes Creek; E, Freedom; F, Slug Creek; G, Crow Creek; H, Montpelier. The areas lettered 2-10 are those illustrated by the corresponding figures. Henry, and Cranes Flat quadrangles, all of which are fifteen- minute quadrangles except the Montpelier, which is a thirty- minute quadrangle. The location of these areas is shown on the accompanying map, Figure 1. 446 GEORGE ROGERS MANSFIELD Three semi-detailed reports’ and a number of shorter papers have already been published and a fourth report? is now in press. An additional, more extended report is well advanced in preparation and includes a discussion of the geography, geology, and mineral resources of the seven quadrangles named. The purpose of this paper is to present in advance of the detailed report some of the striking structural types of the region and to discuss briefly certain conditions that attended the development of these structures. The maps used in illustration of the structural features are extracted from the detailed geologic maps of the quadrangles mentioned. Their locations are shown on the index map, Figure tr. GENERAL STRUCTURAL FEATURES The stratigraphic series in southeastern Idaho includes more than sixteen recognized unconformities. Most of them do not appear to record great crustal disturbances, but a few indicate changes of considerable magnitude. Several are very striking, both as seen in the field and in cartographic representation The region is traversed by many folds, some of which exceed 50 miles in length. The more important folds are synclinoria with relatively narrower intervening anticlines or anticlinoria, usually unsymmetrical and inclined or even overturned eastward or northeastward. ‘The axes for long distances are nearly horizontal or slightly undulatory, due to the presence of relatively broad and low transverse folds, and the pitch is gentle, generally toward the north or northwest. The trend of the folds is convex toward the northeast, bending from a little east of north in the Montpelier quadrangle to northwest in the Lanes Creek quadrangle and beyond. This arrangement gives rise to long nearly parallel folds t See especially H. S. Gale and R. W. Richards, “Preliminary Report on the Phosphate Deposits in Southeastern Idaho and Adjacent Parts of Wyoming and Utah,” U.S. Geol. Survey Bull. 430 (1910), pp. 457-535; R. W. Richards and G. R. Mansfeld, ‘“‘Preliminary Report on a Portion of the Idaho Phosphate Reserve,” U.S. Geol. Survey Bull. 470 (1911), pp. 371-451; R. W. Richards and G. R. Mansfield, “Geology of the Phosphate Deposits Northeast of Georgetown, Idaho,” U.S. Geol. Survey Bull. 577, 1914. 2G. R. Mansfield, ‘‘The Geography, Geology and Mineral Resources of the Fort Hall Indian Reservation, Idaho, with a Chapter on Water Resources, by W. B. Heroy,” U.S. Geol. Survey Bull. 773. ROCKY MOUNTAIN STRUCTURE IN IDAHO 447 somewhat similar to those of the southern Appalachians. The Idaho folds, however, appear to be less regular in form than those of the Appalachian region. The intensity of the folding may be judged by the fact that within the region of the seven quadrangles mapped there are forty-two folds or groups of folds that have been considered of sufficient importance to receive names and to merit individual treatment in a detailed description of the region. The influence of the transverse folds is seen chiefly in the widen- ing or constriction of the longitudinal folds, in the production here and there of canoe- or cigar-shaped folds, and in the zigzag outcrop of certain formations, which cross the axes of the longitudinal folds. The principal faults of the region are reverse and are doubtless chiefly associated with the Bannock overthrust, which has a length probably greater than 270 miles and a horizontal displace- ment certainly not less than 12 miles and perhaps greater than 35 miles. Normal faults are numerous and have produced a wide range of effects upon the pre-existing structures. Possibly some of the faults now regarded as reverse may prove to be normal. The intensity of the faulting is suggested by the fact that about sixty faults or groups of faults are sufficiently noteworthy to receive individual consideration in a detailed description of the region. About half of these are thrusts associated with the Bannock over- thrust. SPECIAL STRUCTURAL FEATURES The structures to which attention is especially directed in this paper are (1) noteworthy unconformities; (2) ‘‘swallowtail”’ folds; (3) the Bannock overthrust; (4) the Blackfoot fault; (5) drag folds; (6) fan folds; and (7) the Meadow Creek graben. ‘These will be described in the order named. Noteworthy unconformities—A very marked unconformity occurs in the southeastern part of the Montpelier quadrangle, where strongly folded Triassic and Jurassic beds pass beneath gently folded or nearly horizontal beds of the Wasatch formation (Eocene). This unconformity is the most striking of all the unconformities of the region. It represents at least the great post-Cretaceous mountain-building epoch of the northern Rocky EXPLANATION SEDIMENTARY ROCKS 6 i em IN NS ty Ne Vie —Aensivne —tvisaltroao vi gua j snoaswisa> ————_sisevaar. sass he, 2 DISSWIDJ- 4 ott : Twin Cr linet i. Wood shale. Higham grit 74 fe Cosy: ZZ) Prose eis BA 57) ® I RQY te 2? 448 GEORGE ROGERS MANSFIELD SS (QUAL, WES ) 5 5 c R wo — cw . x = 2oo c 63 ae 2 es sSah SIS. a = AoGN 5 OL EcR YN [PS ES | 28 Js SS\ 2] Dk yc ne ETS dk Se || EUS i no) ze | SOORSEN] ao v9 Le fPN\SN 58 Be {lS <5 yesh BW Ee yes Bele AS = a +] oS 3) 09 eS adh 08 x os Fad BSSV 3g A ells fe xa ae BS eee OS 5 Ww g 0 = Gor Oo = a) = F gos = ao Juavagy ausao0.s197q ry rs} A LL SL MLSS: Neer SS 2 Miles <4 fi | NS § N WW a ATA AN Os Dox a A SELL: Fic. 6 4.—Map of part of the Preuss Range, Lanes Creek quadrangle, showing the Blackfoot fault and associ- ated features. cover. The dip of the thrust plane as a whole in the St. Charles district is probably gentle. A measurement in Paris Canyon about 6 miles north shows the dip at that locality to be 23°. ROCKY MOUNTAIN STRUCTURE IN IDAHO 455 The Blackfoot fault.—This fault, which is illustrated in part in Figure 6, takes its name from the Blackfoot River, which it crosses at the upper entrance to the Narrows. ‘The relations of the fault at this point indicate that its plane dips about 33° south. It is /acktoot Fiver Or "y Ridge C | | mm ee ae Overthrust Section MM Section KK Fic. 6B.—Sections along the lines K—-K’ and M-M’ of Fic. 64 (eo) ) (2) {9) (oe) Ct regarded as a thrust fault and is supposed to have originated in a transverse anticline, located near the line along which occurred the maximum yielding of the rocks to the compressive earth stresses of the region. ‘The anticline broke and the southeast limb, which became the upper fault block, swung northward about a pivot located in the vicinity of Timothy Creek in the Freedom 456 GEORGE ROGERS MANSFIELD quadrangle, about 5 miles east of the area illustrated. The rock formations cross this creek without apparent displacement by the fault, but they make a pronounced bend, which is favorably located ~ to mark the position of the unbroken portion of the axial zone of the anticline. The Blackfoot fault has a known length of about 13 miles west- ward from the point of origin above indicated. At this distance it disappears beneath basalt. The variations in dips on the flanks of the big anticline in the upper thrust block, some of the strata being locally overturned, and the presence of minor folds, make it difficult to determine the throw of the fault. The maxi- mum observed effect is produced where it cuts the big anticline. The fault is probably offset beneath cover by one of the normal faults shown in the southwestern part of the area. The upper fault block affords an unusually fine example of the manner in which the outcrops of a formation, such as the Phos- phoria, occurring on opposite limbs of an anticline, are spread by the uplift and erosion of a fault block, in which the anticline is included. In the lower block to the north the corresponding outcrops are much nearer together. The structure section along the line KK’ crosses the Blackfoot fault. It shows the anticlinorial and synclinorial character of the folds, their overturning toward the northeast, and the manner in | which some of the subsidiary thrusts are thought to have originated. The thrusts are presumed to pass into the Bannock overthrust, which underlies this district. Drag folds.—Folds of this type, usually sharp and unsymmetri- cal, occur at a number of places in the region. Several of them are shown in Figure 6 in connection with the big anticline in the upper fault block. One of them is crossed by the line of structure section MM’. Here upper beds of the Brazer limestone (Mississippian) in small sharp folds are locally exposed by the erosion of the over- lying Wells formation. Drag folds on a somewhat larger scale occur on the hills south of Montpelier Canyon about 3 miles east of Montpelier. In Figure 7A, a view south along the west flank of Waterloo hill shows an anticline overturned eastward in which curving beds of ROCKY MOUNTAIN STRUCTURE IN IDAHO 457 B Fic. 7—A, View south along west flank of Waterloo Hill about 3 miles east of Montpelier, Idaho, showing drag folds accompanying large unsymmetrical anticline; B, view of same folds northward from different viewpoint, showing west flank of the same anticline. Cw, Wells formation, Pennsylvanian; Cpa, phosphatic shale member of Phosphoria formation, Permian; Cpb Rex chert member of the Phosphoria forma- tion, Permian; Trw, Woodside shale, Lower Triassic. 458 GEORGE ROGERS MANSFIELD the Rex chert member of the Phosphoria formation (Permian) form the east limb of the anticline and indicate the occurrence of a syncline farther east. In the middle of the view is a sharp anti- clinal fold inclined eastward. Near the base of the slope at the right (west) is another drag fold, very sharp and inclined eastward. These drag folds are composed of upper beds of the Wells formation and of the overlying phosphatic shales of the Phosphoria formation, which are relatively less competent than the Rex chert above or the bulk of the Wells formation below. The effect of the drag folds is to duplicate the outcropping beds of phosphatic shales, which appear as separate bands on the hillside. The anticline, of which the drag folds form a part, is largely eroded, but its west limb is exposed in a branch canyon to the north. Figure 7B is a view north from a somewhat different viewpoint. It shows the westerly dipping beds of the west flank of the anticline and the same two drag folds illustrated in the previous view. Fan folds —Folds of this type have been recognized at several places in the region. Usually they are so deeply eroded that only their stumps remain, or they are broken by faults. In the vicinity of Sugarloaf Mountain, however, in the Cranes Flat quadrangle, see Figure 8, there is a fine example of an inverted fan fold. The rocks immediately involved belong to the Homer limestone member of the Wayan formation (Lower Cretaceous ?). Sugarloaf Mountain was selected by St. John" of the Hayden Survey years ago as a station, and he drew a geologic structure section through it, in which he shows a southwesterly dipping series of strata, overlapped on the west by basalt at Sheep Mountain just west of the area shown in the figure, and arched into a promi- nent anticline at Sugarloaf Mountain by anigneousintrusion. The structure of the area near Sugarloaf Mountain is not so simple as figured by St. John. The limestone, which he did not differentiate from the other strata, is there thrown into a series of relatively sharp folds, among which narrow folds of the underlying sandstone rise to the level of erosion here and there. Southwest of Sugarloaf Mountain the dips of the limestone and the related strata are Orestes St. John, “‘ Report of the Geological Field Work of the Teton Division,” U.S. Geol. and Geog. Survey Terr. (1877), 1879, pp. 351-60. ROCKY MOUNTAIN STRUCTURE IN IDAHO 459 135° R42E. Ho Coens seh cocos, - A ~ f ~ Sugarloaf Min eee “x, : ; I] “Pothetical structure 1 Yn (9) 1Mile ——- EXPLANATION QUATERNARY CRETACEOUS Alluvium Hill-wash and older Wayan formation alluvium Inclu ing Homer limestone QUATERNARY? TERTIARY Ss atTSS7S y ~ EG Basalt Andesite — Dikes Strikeanddip Fic. 8.—Map with geologic structure section of the Sugarloaf district, Cranes Flat quadrangle, showing an inverted fan fold. 460 GEORGE ROGERS MANSFIELD southwest, but northeast of the mountain the dips of the limestone are northeast. Thus the structure of the limestone is a syncli- norium, having the general form of an inverted fan fold. The intrusion at Sugarloaf Mountain is a thickened sill or incipient laccolith, arching with the strata in the northwestern extension of the mountain but eroded on the southwest limb beneath the summit and southeastward. The fold which forms the mountain is one of the subordinate folds of the synclinorium rather than a major structural feature supported by a relatively large intrusive body, as postulated by St. John. The geologic structure section AA’ illustrates the features described above. Its line forms an angle of about 30° with that of St. John’s section. An example of what is believed to be the stump of an upright eroded fan fold is found in the western part of the Crow Creek quadrangle, see Figure 9.- Snowdrift Mountain, part of one of the most persistent anticlines of the region, is flanked on either side by synclines which are in general inclined eastward. The Webster syncline on the east is markedly unsymmetrical, the west limb being steep and locally overturned eastward, but the east limb has a gentle westerly dip. The Georgetown syncline along the west side of Snowdrift Mountain is deeper and the limbs are steeper. The east limb is locally vertical or even overturned. Thus the intervening Snowdrift anticline is with little doubt an eroded fan fold. The structure sections along the lines SS’ and TT’ illustrate the features cited. At the line SS’ the axis of the fan fold is some- what inclined eastward and the Webster syncline is broken by a local thrust. Although the Snowdrift anticline does not every- where show a tendency toward fan folding it is closely folded throughout most of its length and here and there exhibits that tendency, as shown on the west flank of Pelican Ridge, see structure section GG’, Figure 10B. The Dry Valley anticline, west of the Snowdrift anticline, see section SS’ Figure 9B, locally has similar tendencies. Other instances which may not be figured here are illustrated in the forthcoming detailed report. The Meadow Creek graben.—-Perhaps the most striking effect of normal faulting in the region is the production of horst and graben structure in the northwestern part, see Figure 1c. The Saige Oe a ROCKY MOUNTAIN STRUCTURE IN IDAHO 461 valley of Meadow Creek in the southern part of the Cranes Flat quadrangle is a fault trough or graben. This structure may be traced about 15 miles southeast into the Lanes Creek quadrangle, where it apparently dies out. The northward extension of the graben is concealed by basalt and Quaternary deposits. The bounding ridges are composed of Carboniferous rocks and are conspicuous topographic features. Two transverse normal faults intersect the graben, one near the south boundary of the Cranes Flat quadrangle and the other in the northwest corner of the Lanes Creek quadrangle, down-faulting the portion between them. The bounding ridges in the down- faulted area are farther apart than in the portions to the northwest or southeast. In the southeastern part of the graben beds of the Thaynes group (Lower Triassic) are exposed and in the widened, down-faulted portion both the Woodside shale (Lower Triassic) and Thaynes appear, though most of the area is underlain by basalt and Quaternary deposits. The structure of the rocks within the graben is probably synclinal, as shown in structure sections EE’ and GG’. It is with little doubt the continuation of synclinal structures observed farther southeast. The fault which lies along the northeast side of the graben is concealed for much of its length, but in the southeastern part of the area here shown is represented by two faults, separated by a narrow strip of the Phosphoria formation, but together bringing Lower Thaynes into proximity with the Wells. Northwest of the area illustrated the fault doubtless continues for some distance beneath the basalt. Its stratigraphic throw is not known but is estimated at 3,000 to 4,000 feet. The transverse normal fault that passes between Limerock Mountain and Pelican Ridge causes the mountain to stand nearly a mile northeast of the line of continuation of the ridge. Similar effects in reverse order are produced where the fault intersects Little Gray Ridge. Neither the amount of the downthrow nor the hade of the transverse fault is known, but from the effects and assumed values for the dips of the lateral faults that bound the graben it is thought that 5,200 feet may represent the order of magnitude of the vertical displacement. 462 GEORGE ROGERS MANSFIELD The other transverse normal fault intersects the graben south- — east of Pelican Ridge. Its generally east-northeasterly course is ; largely concealed by basalt, but it cuts across Little Gray Ridge £ = © ce 3 5 @ 539 x 3 y 2 6 y a Bx = aR Boe. 63 c # ZB e S&P §& 8 & 8 om] Soom FEMS 3 &ppa 8 ao O8 F 43 INE TE MMS A? FAlees sl 71 2 2 (fe | E> ts Ras OS ® PLZ Tas|,. /E 9 € [rlj y= 4 to =| & > &| < QP H|] DS BEEK S710 $ = | Zé #/® 8G 8]? [88 eee § Seas) 9 J § © 0]; ¢ | 33 cy ‘aD 0 d ou yy i aE JE ek as 00 § ats ® Eee 8 7\2 8 ZA ® |i 2 | wap e gs 28 é ZS eo gs 0 9 | iQ = - x ia = ! n 35 ; = x LET TRE SE 2 Siig | NE 5 Lp Pe U 0 4 Zp Ms P10" VSS 'S pizd Bere’ =a) iS “i yy CG /LL9 REN SEZ PIERMN NS his yg ERENT SL BOSONS Le 5 =e Bs : = Li OSes S YG, ZS = Frc. 9A.—Map of parts of the Preuss Range, Slug Creek, and Crow Creek quadrangles, showing eroded fan folds and associated features. 463 ROCKY MOUNTAIN STRUCTURE IN IDAHO and there offsets the boundary between the Brazer and Madison limestones (Mississippian). Basalt has outflowed on the west The downthrow flank of the ridge along part of the fault trace. V6 ‘SIq JO ,J-J pure ,S-G sour] ay} Buoye suonsas—'g 6 ‘OI 782}0S2 JPAsequ! snozU0D Bi: i | Z ANI H3auD Yaq07NCS y31SE3amM t \ ANIMDILNY “LAW 44l4PMousS ASTIVA Awa 1 OLLNY BNITONAS | | t ‘ ‘ | SNIMDSILNY 141dGMONS: ANMSNAS .NMOLS9YORS | ANIMDILNY ASZTIWA AUG {aa / ANITONAS HaLSEaM = ¢ espiy Kag is to the northwest and, employing similar assumptions to those noted for the last described fault, an estimate of 3,500 feet may be made for the vertical displacement at this locality. GEORGE ROGERS MANSFIELD 464 The faulted ridge including Limerock Mountain and Pelican Ridge, a continuation of the Snowdrift anticline mentioned above, is here a horst, for it stands between the Meadow Creek graben on Suse) UOSsipeW pa 9 EU} SIEM _Z GHG 7 WY 4} c vogewigs e14oydsouy meys 2ynytoou, e maou mor o[ 1 | ALMAHOALNOINIT BIEYS epispoom 4 a () OC, dnou8 seukeyy > ALIWMOSNOINTT @ug|seu| 42D 9 [ese] ALIWHQINGONT @ugjspues ssnaig ajesawofguco wiesydg ie, FI h a @ugspues duinys ALIWHOSINOINGG Wee g _ WINIAN||e 4pjO Pus YSeEMj)I(4 QR ; 3 J SHD0H AUVLNAWIGaS NOILVNVW1dx3 OP ‘sojsuevipenb yooig soue'y pur ‘Aruoy ‘VepJ sourto ‘sginjeoy pozelosse pue uoqris yoo Mopeay 94} Jo deyy— pol ‘oly "922fOS2 |PAIejuU! tnoyUOD a ee aera dl Pe pe ee er ae saIIW & Zz i) 0 Red If at wets, 02 a pas Ae Tig3 ay, Sh Nf SAMO, YAY We KGS Hoe Ws 4 z ; | ZS ye Me « Gy (2402 BAe] PUB 2I13Se/2014y 9)/eseg d aaa CSS SxAS0y SNO3INOI the northeast and another down-faulted area on the southwest. The relations of this horst to the adjoining areas, both northeast and southwest, are shown in structure sections EE’ and GC’. ROCKY MOUNTAIN STRUCTURE IN IDAHO 465 The ridge northeast of the graben is not clearly a horst though it is much broken by faults and has suffered extrusion of rhyolite. NOTES ON THE DEFORMATION OF SOUTHEASTERN IDAHO Epochs of deformation.—Although southeastern Idaho was profoundly affected by crustal disturbances at the close of the Jurassic, the observed mountain structures appear to be the result of two later epochs of mountain building. The earlier of these occurred after the deposition of the Wayan formation and before the deposition of the Wasatch formation. It probably corresponds Limerock Mt.Chorst) Meadow Creek (E ra en Yo ~ ~ ner, SECTION. E-E’ Pelican Ridge Khorst) Meadow Cr ° (graben) 90 Py == 2 2 ; cw IRS > } "SECTION 6-G’ Fic. 10 B.—Sections along the lines E—E’ and G-G’ of Fig. 10A with the interval between the Adaville and Evanston formations of Veatch’ or the epoch which, according to Ransome,’ “‘appears to have begun at the close of the recognized Laramie or possibly even earlier, and to have attained its maximum between the Fort Union, which chiefly on the basis of its plant remains is generally classed as basal Eocene, and the mammal-bearing lower Eocene Wasatch.” The second mountain-building epoch occurred after the depo- sition of the Salt Lake formation which, on the basis of present rather unsatisfactory evidence, is tentatively assigned to the Pliocene. This formation locally has steep dips thought to-have been produced by deformation in late Pliocene or post-Pliocene time. . 1 A.C. Veatch, ‘‘ Geography and Geology of a Portion of Southwestern Wyoming,” U.S. Geol. Survey Prof. Paper 56 (1907), p. 75- 2F. L. Ransome, “The Tertiary Orogeny of the North American Cordillera and Its Problems,” Problems of American Geology, pp. 287-376, p. 322, New Haven, rots. 466 GEORGE ROGERS MANSFIELD Rocky Mountain geosyncline.—Southeastern Idaho forms a part of a great geosyncline in which sediments were deposited with few interruptions of magnitude from early Cambrian to Upper Cretaceous times. This great trough extended from the Arctic Ocean southward through the Great Basin and was in general an area of subsidence or a negative element’ on which the sediments had accumulated in great thickness. On the west during the same interval a relatively persistent land mass or positive element had separated the geosyncline from the Pacific Ocean, and on the east a less persistent barrier at times had separated it from the interior sea. The geosyncline served to localize the deformation and had a directive influence upon it. The tangential pressure which pro- duced the folds and overthrusts was normal to this structure and, in southeastern Idaho, came from the west southwest. Initial dips within the geosyncline and differences in the charac- ter of the sediments doubtless tended still further to localize the folds and thrusts and to determine their character. Favorable formations —Many of the Paleozoic formations are massively bedded and would act as competent strata under deformation. A number of formations, however, contain shaly members. Some of the limestones, too, are thin bedded. Such for- mations exposed to deformation in the zone of fracture would furnish horizons in which thrust planes might originate. The Bannock overthrust zone is complex and no one formation has yet been identified as the source of the thrust plane. The Mesozoic formations are generally weaker and less well consolidated than are the Paleozoic rocks. Lying with favorable initial dip and in great thickness athwart the direction of maximum compression, the Mesozoic rocks crumpled under the accumulating compressive stress and permitted the more or less folded Paleozoic rocks with some accompanying or overlying Mesozoic rocks to override them. They thus generally form the basement over which the great thrust block of the Bannock overthrust moved and on which it now rests. Although it has been customary in « Bailey Willis, “A Theory of Continental Structure Applied to North America,” Bull. Geol. Soc. America, Vol. XVIII (1907), pp. 389-412. ROCKY MOUNTAIN STRUCTURE IN IDAHO 467 the discussion of overthrusts to regard the lower block as passively overridden by the upper or thrust block, it is probable that both participate in the movement, the separated parts moving past each other, as suggested by Barrell. Horizontal thrusting.—The original nearly horizontal attitude of the Bannock thrust plane has been modified by subsequent compression and folding, but it indicates that the effective deforma- tive forces acted horizontally and were not the surface expression of obliquely emerging, deep-seated shear, such as was postulated by Willis? for the fault zone along the east side of the Sierra Nevada Mountains. Factors in deformation.—Chamberlin and Miller}? have shown from their own experiments and from the earlier work of Cadell, Willis, Adams, and others that many factors are involved in the production of low-angle faulting, such as is exemplified in great overthrusts. Among these may be mentioned: (1) rotational strain; (2) increase in resistance to deformation with depth; and (3) a relatively large ratio of thrust to weight. (1) Rotational strain as a factor in the deformation of south- eastern Idaho is clearly indicated by the frequency of inclined or overturned structures. (2) Although no data are available regarding conditions in depth, it is clear from the horizontality of the thrusting previously mentioned and from the locally fractured and generally unmeta- morphosed condition of the strata, that the deformation took place at no great depth. The visible structures at least were developed in the zone of fracture. No evidence of flowage has been found. (3) The great horizontal displacement produced by the Bannock overthrust shows that the thrust was enormous. The weight, on the other hand, could not have been very great because of the apparent shallowness of the deformation. t Joseph Barrell, ““The Upper Devonian Delta of the Appalachian Geosyncline,”’ Am. Jour. Sci. (4th ser.), Vol. XX XVII (1914), p. 107. 2 Bailey Willis, “Structure of the Pacific Ranges, California,” Bull. Geol. Soc. America, Vol. XXX (1919), pp. 84-86. 3R. T. Chamberlin and W. Z. Miller, ‘““Low-Angle Faulting,” Jour. Geol., Vol. XXVI, No. 1 (1918), pp. 1-44. 468 GEORGE ROGERS MANSFIELD Later deformative epoch—rThe later epoch of deformation was marked by broad uplift rather than by intensive folding. There was, however, some folding, involving locally steep or overturned dips, but generally of an open character. The folding of the plane of the Bannock overthrust is of this type and is thus structurally more akin to the later than to the earlier deformative epoch. Relaxation and readjustment.—The vigorous compression of the earlier deformative epoch was succeeded by relaxational phases involving normal faulting and gradual readjustment to new con- ditions of equilibrium. Some normal faults along the Bannock fault zone represent with little doubt the fracture and jostling of blocks under light load near the margin of the fault block. Other normal faults partly concealed by Tertiary beds also may be referred to the interval of relaxation following the earlier defor- mative epoch. Many of the normal faults, however, including those that produced the horst and graben structure, are not asso- ciated with Tertiary beds. ‘There is a single doubtful exception to this statement. On the other hand there is definite evidence that Tertiary beds have been displaced by normal faults. Thus these faults have been considered as later than the Tertiary beds but earlier than the earliest Quaternary, and hence associated with the relaxational interval succeeding the later deformational epoch. The horst and graben structure is more or less intimately associated with extrusions of ryholite and basalt. The basalt in particular has flooded the valleys in the vicinity of these structures and has emerged along some of the fault lines. On the other hand, sufficient erosion had occurred after the faulting to produce prac- tically the present topography before the extrusion of the basalt. That event, therefore, probably accompanied a relatively late reopening of some of these faults, together with the development of new fissures. At present no definite evaluation of the parts to be assigned to the two relaxational intervals may be made. DISCUSSION OF “SUMMARIES OF PRE-CAMBRIAN LITERATURE OF NORTH AMERICA,” BY EDWARD STEIDTMANN TERENCE T. QUIRKE University of Illinois The recent ‘““Summaries of Pre-Cambrian Literature of North America” by Professor Edward Steidtmann’ bring up points to which certain contributions and corrections should be made. He states that, (1) . . . . the pre-Cambrian rocks northeast of Lake Huron show one con- spicuous unconformity, and above the conspicuous unconformity are two series of slightly metamorphosed, dominantly clastic sediments, separated by an inconspicuous unconformity. (2) The lower one, the Bruce series, locally contains tillites. The upper series is generally known as the Cobalt series. (3) At Killarney, on the north shore of Lake Huron, Collins has found that the Bruce and possibly the Cobalt series are intruded by Killarney granite, and in this locality they assume many of the characteristics of the older series, the Timiskaming.” In regard to the so-called ‘inconspicuous unconformity,”’ Steidtmann followed Coleman (1915) and Miller and Knight (t915).4 Later work by Collins’ shows that in the region southeast of McCabe Lake, north of Cutler, erosion removed the Serpent quartzite, the Espafiola group, the Bruce conglomerate, and part of the Mississagi formation, amounting to the greater part of the Bruce series, previous to deposition of the Cobalt series. In 1914 he showed that the erosion interval amounted to thousands of t Jour. Geol., Vol. XXVIII (1920), pp. 643-58. 2 [bid., p. 643. The numbers have been inserted by the writer. 3 A. P. Coleman, Problems of American Geology, pp. 81-161. Quoted by Edward Steidtmann, op. cit., p. 647. 4W. G. Miller and C. W. Knight, Jour. Geo!., Vol. XXIII (1915), pp. 585-909. Quoted by Edward Steidtmann, oP. cit., p. 656. 5 W. H. Collins, report in preparation. 469 470 TERENCE T. QUIRKE feet over large areas.* And in the Espanola’ area there is a prob- able difference of erosion of more than 5,000 feet within ro miles. It is largely a matter of opinion as to how important such an unconformity should be considered, but it is surely not inconspicu- ous. Erosion intervals of thousands of feet in Paleozoic series are considered notable, and they should not be undervalued in pre- Cambrian series. However, in fairness to all it must be added that there are places where the unconformity appears to be slight; generally only by tracing it over large areas, as Collins has done, can the observer recognize its true greatness. Furthermore, in comparison with the great unconformities beneath and above the Huronian formations the Bruce-Cobalt unconformity is much less conspicuous. ‘This at least may be said, the unconformity locally is inconspicuous but nevertheless important. Second, in saying that the Bruce series, the lower Huronian series of Ontario, locally contains tillites, Steidtmann appears to follow Coleman (1915).3 The fact is that the tillites are charac- teristic only of the upper group, the Cobalt series, or specifically the Gowganda formation of Collins.4 The glacial origin of at least part of the Cobalt conglomerate is held by Wilson (1913),5 Collins (1914),° and by Coleman himself first (1907)? and last (1920). The Bruce conglomerates in certain phases are distinctly different in character from the Cobalt tillite. They lack the thinly laminated slates and the well-bedded slate layers carrying scattered = W. H. Collins, Canada Geol. Survey, Museum Bull. No. 8 (1914), p. 21. 27, T. Quirke, Canada Geol. Survey, Mem. No. 102 (1917), p. 42. Apparently through errors in copying, the summary of Quirke’s classification on p. 657 of Steidt- mann’s paper differs considerably from the work summarized, both by omissions and by faulty arrangement. See Canada Geol. Survey, Mem. No. 102, pp. 6 and 7. 3 Quoted by Steidtmann, op. cit., pp. 646, 648. 4W. H. Collins, Canada Geol. Survey, Mem. No. 95 (1917), p. 10. 5 Morley E. Wilson, Jour. Geol., Vol. XXI (1913), pp. 121-41, and Canada Geol. Survey, Mem. No. 17 (1912). Quoted by Steidtmann, op. cit., p. 658. 6 W. H. Collins, Congrés géologique international, X1Ith Session (1914), pp. 399- 407. Quoted by Steidtmann, op. cit., p. 650. 7A. P. Coleman, Am. Jour. Sci., Vol. XXIII (1907), pp. 187-92; Jour. Geol., Vol. XVI (1908), pp. 149-58; Bull. Geol. Soc. Am., Vol. XIX (1908), pp. 347-66. 8 A. P. Coleman, Economic Geology, Vol. XV (1920), No. 6, pp. 539-41. ae oe hae PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 471 pebbles and bowlders. They have little argillaceous matrix about the inclusions in massive conglomerates. They are not ‘“‘slate” conglomerates, as part of the Gowganda formation is. Their character is essentially that of a basal or alluvial deposit. In part they are well sorted and in part they are massive, but the massive material is composed characteristically of dark-colored graywacke, or of gritty or bowldery conglomerate. In no instance is there record of striated, glacially soled bowlders being found in Bruce conglomerates. Nowhere to our present knowledge is there a polished basement beneath them. The matrix of the Bruce con- glomerates is commonly dark-colored, arkosic, and graywacke-like; the matrix of typical Cobalt tillite is green, pale-green on weathered surfaces, and looks like metamorphosed clay. So clear is the difference in the character of certain phases of the matrices that members of Collins’ parties from 1914 to 1918, with some practice, were able to tell from a glance at typical hand specimens whether or not the rock was Bruce or Cobalt conglomerate. This difference in the character of the matrix is a genetic difference, connected with the glacial origin of the one and the non-glacial origin of the other. Those phases of the Bruce conglomerate which are very similar to the less characteristic phases of the Cobalt conglomerate conceivably may be of an obscure, glacial origin. It is not altogether surprising that a reviewer of the literature should fall into confusion. Originally, in accordance with the best nomenclature of the day, Coleman’ referred to the Cobalt con- glomerate as Lower Huronian, as is quite clear from his writings in . 1907 and 1908; whereas the only locality of distinctly glacial deposits he cited is the Cobalt silver-producing district which is underlain by the upper, Cobalt series, not by the Bruce con- glomerate now known as the lower series. Furthermore, Coleman, himself, at one time seems to have been confused by the similarities between the Bruce and the Cobalt conglomerates. Indeed Pro- fessor Willmott? previously (1901) had written: ‘“‘The two slate t A. P. Coleman, “‘The Lower Huronian Ice Age,”’ Jour. Geol., Vol. XVI (1908). p- 149; Bull. Geol. Soc. Am., Vol. XIX (1908), p. 355; Am. Jour. Sci., Ser. 4, Vol. XXIII (1907), pp. 190-01. 2 A. B. Willmott, American Geologist, Vol. XXVIII (1901), p. 19. 472 TERENCE T. QUIRKE conglomerates of Murray are so much alike that they cannot be distinguished. Where the limestone band is absent, as it often is, they join, and Murray himself confesses that he could not draw the dividing line.’”’ Nevertheless, in some such places the dividing line may be, and has been, drawn. On the other hand, there are phases of the Bruce conglomerate so similar in character to phases of the Cobalt conglomerate that no distinctions have yet been recognized. Another factor which may have caused confusion in the reports of Coleman is the fact that he found at Cobalt the Cobalt conglomerate to be the basal conglomerate of the Huronian formations, the entire Bruce series being wanting. Thus, carrying his correlations westward from Cobalt, he supposed the basal conglomerate of the original Huronian area to be the same as that at Cobalt, whereas it is actually the base of the Mississagi formation of the Bruce series. However, all this was clearly put straight by Collins in 1916, and it seems a pity to have confusion again after the known facts have been published. So far as is now known, the Bruce conglomerates, certainly for the main part, are not of glacial origin, but some of the Cobalt conglomerates are agreed to be tillites. Regarding the last topic, the work of the writer carried on this summer near Lake Geneva, 20 miles northwest of Sudbury, Ontario, shows that syenitic masses intrude the Cobalt series, thus confirm- ing and complementing the work of Collins (1916)? on the age of the Killarney granite. Collins found that the Bruce series certainly and possibly the Cobalt formations, are intruded by an acid intru- sive in an area north of Lake Huron, from 15 to 25 miles southward from Sudbury. Now it is known that what might have been con- sidered a local phenomenon of little consequence in pre-Cambrian classification and correlation must be regarded as probably a widespread and considerable intrusion. The age of these intrusions having been determined and confirmed in areas 40 miles apart, it becomes necessary to scrutinize carefully those local correlations and distinctions which are based largely upon different periods of orogenic movement and acid intrusions. Almost certainly it will 1 W. H. Collins, Canada Geol. Survey, Mus. Bull. No. & (1916). 2 Ibid. No. 22 (Feb. 5, 1916). Quoted by Steidtmann, op. cit., p. 650. PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 473 be recognized, as it has been found already, that some masses of supposedly pre-Huronian and Sudburian rocks will be identified as Huronian sediments intruded by these late pre-Cambrian granites and syenites. However, there are surely some areas which are not subject to such a revision, in which the reality of the pre-Huronian sediments seems to be beyond dispute; so that we may bring our ideas of the pre-Cambrian succession of events north of Georgian Bay more up to date, as follows (the events being listed in chronological order from the bottom up): SEQUENCE OF PRE-CAMBRIAN EVENTS IN THE TIMISKAMING REGION Proterozoic Era Intrusions of Killarney and Geneva granites and syenites, accompanied by severe faulting, mountain folding, and extensive warping. Injections and extrusions of basic rocks (Keweenawan) Deposition of Whitewater series Chelmsford sandstone Onwatin slate Onaping tuff Trout Lake conglomerate (not tillite) Hiatus, relations unknown Deposition of Cobalt series White quartzite Cherty quartzite Lorrain quartzite and conglomerate (not tillite) Gowganda formation—including tillites Considerable interval of erosion, the resulting sediments being unknown Deposition of Bruce series Serpent quartzite and conglomerate (not tillite) Espafiola limestone Espanola graywacke Bruce limestone Bruce conglomerate (probably not tillite) Mississagi quartzite and basal conglomerate (not tillite) Great interval of erosion, the resulting sediments being unknown Archeozoic Eras Time of orogenic diastrophism accompanied by acid intrusions (Algoman) Deposition of pre-Huronian sediments (Sudburian and others), quartzites, graywackes, conglomerates 474 TERENCE T. QUIRKE Great interval of erosion, the resulting sediments being generally unknown, but represented in part by Sudburian and other pre-Huronian sediments Time of granite intrusions and diastrophism (Laurentian—all inferred from the presence of granite bowlders in pre-Huronian conglomerates) Deposition of products of Keewatin weathering, accompanied and inter- rupted by volcanic extrusions and intrusions of undetermined order and distribution Note:—The above communication has been made with the permission of the Director of the Geological Survey of Canada. teen THE NATURE OF A SPECIES IN PALEONTOLOGY, AND A NEW KIND OF TYPE SPECIMEN EDWARD L. TROXELL Yale University In discussing the subject, ‘“What is a species ?”’ with my asso- clates in vertebrate paleontology, many points of interest have come up which will bear repeating. The great complexity of the problem and the number of elements which enter into its com- position make the solution difficult—perhaps impossible—for certainly no single definition can apply in every case. Professor Schuchert has said that he finds it best to let the man who is studying a given specimen decide what a species is, after he has read several definitions, and that even for students who are writing their first papers it is best left to the individual; he may stand or fall on his ingenuity in handling this unanswerable question. The more deeply we go into the study of a specimen, not only do the features multiply, but they increase in importance, and thus we raise a variety to a species, or a species to a genus. Along with the personal element and the extent of our knowledge, very important factors indeed, we must in paleontology consider another, viz., the actual worth of the specimen, and it is undeniable that a new species should not be made on fragmentary material unless its characters are unique and of such importance as to reveal the existence of a new and strange group of animals, hence scarcely less than a genus or even a family. As an example of such speci- mens, whose discovery gave us the first knowledge of strange groups, there may be mentioned Archaeotherium mortoni Leidy, Ammodon leidyanus Marsh, or Rhinoceros occidentalis Leidy. It is a well-known fact, and yet one not fully recognized, that new types based on good specimens are and should be made, to supplant older inadequate ones. We may cite as an example the mammal Dinohyus Peterson, which Matthew says is not different 475 476 EDWARD L. TROXELL from Daeodon (D. shoshonensis Cope), or the dinosaur Tyranno- saurus, which may be identical with Manospondylus (M. gigas Cope). Many such examples might be given showing where a more complete skeleton has usurped the taxonomic position of a type based on a mere fragment, thereby forcing the latter to fall into a group of historic relics, which merely mark the progress of our science. A new kind of type specimen.—lt is here suggested that we recognize this principle of substitution as a useful one, and we propose the name of protype (=for the proterotype) for such a supplanting type specimen. A protype should be based only on a very complete skull or skeleton. It is based on supplementary material, and as such is one of the forms of apotypes. In other words, a protype may be said to be a proxy, for it operates with full authority under a special designation. The material of a protype should, it would seem, be given a new name, preferably as a subspecies, for in such a humble position under the original pro- terotype it preserves the name of the older species replaced; it has the taxonomic advantage of linking the two together, and further- more, if it is later found that the new specimen is not identical with the older, the subspecies can be raised to the full rank of a species. Thus it is evident that a new species name should only be given, first, in the case of a protype, as just set forth, and second, to any specimen, fragmentary or perfect, which clearly possesses distin- guishing features or some unusual or unique morphologic character — that may be accentuated by its stratigraphic occurrence and that needs to be published and made widely known, thereby adding to the sum total of human knowledge. Making of species —Many naturalists have ventured an opinion as to what limit of variation should constitute the bounds of a species. This has, by common consent, been determined by the limits of interbreeding, where procreation is impossible or the offspring is sterile. In the natural state, it is interesting to note that, though the barrier is usually a physical or physiological one, yet it may be purely psychological, and as such may break down in captivity, giving rise to most unusual hybrids capable of repro- ducing their kind. A SPECIES AND A NEW KIND OF TYPE SPECIMEN 477 There exist the so-called geographical species which show no marked morphologic differences. Modern biologists make such distinctions in color or in the nature of the hair or muscles; these are, however, criteria necessarily lost to the paleontologist, who generally has only the hard parts for study. The student of vertebrate fossils is further limited, as a rule, in his making of species, to a single specimen, while the student of modern biology has many cotypes or paratypes. Considering the vastness of geological time, it is easy to see that two specimens of the same geological formation may have been separated by tens of thousands of years, during which impor- tant changes not only of habitat but as well of form and habits may have occurred. In such a case, nothing of the resulting trivial changes in the soft parts may be discerned. Relativity of species.—Generally the small unit characters have value only when grouped; not always does a specific feature fully determine the bounds of a species. But on the other hand, no matter how trivial this character may appear to be, if we find it occurring constantly in one set of specimens and not in others, it is to be regarded as specific in value and typifies the individuals of that group. Furthermore, not all the varieties referred to a species may have all of the so-called specific characters. Two specimens under the same species may have but one or two only of the several secondary features supposed to characterize the group as a whole. Distinguishing characters are of two classes, relative and absolute. Sometimes a relative difference may be advanced so far that it appears to be, or may actually become, absolute; for example, the growth of a bone until it meets a joint and develops a facet, or the reduction of a premolar to a point where it becomes obsolete. The so-called absolute characters may be thought of as unit characters. Relative characters may be illustrated by the familiar variation in size, and then we wonder what the limits of size are beyond which animals cannot interbreed, for we realize how positive a barrier this may become. Im this respect it seems safe to say that in mammals a size difference of 30 per cent would 478 EDWARD L. TROXELL tend to separate two groups, based on our general conception of the ultimate barrier between species. Carrying the subject to a further quantitative analysis, what range of “‘ratios” can we allow within a species? Two skulls of approximately the same size may vary in proportions, one part being r5 per cent larger, another part as much smaller. Here is a range of 30 per cent; is it a specific difference? The personal equation of course enters into all such questions and the resulting taxonomy. Presenting specific characters.—A glance through the literature shows many instances where an author at the beginning of an original description states that the “specific characters” are so and so, and proceeds to list a number of features which, taken singly, might not even be subspecific, or, on the other hand, might be generic. In addition, these “specific characters” which may serve to distinguish two species do not show a contrast with a third or fourth; for instance, characters “‘a’’ may show the distinction from species ‘‘A,” but we must look for another group of criteria, ““b,”? to separate our species from ““B.” A species depends not only upon its own features as selected by its author, but upon the features of the other species in the genus as well; the specific characters marking the boundary in one direction may not be such as to show it in another. New discoveries are constantly being made which contradict general statements of distinctions, and overturn our nicely adjusted taxonomy, unless we carefully indicate the type material compared when we draw our contrasts. The safer course, then, seems to be to limit one’s self to the “distinguishing characters,” as so many paleontologists do, without making a guess as to what the undis- covered or unknown specimens may show; or to indicate just what species are being distinguished by certain characters, in which case the description resolves itself into as many parts as there ‘are species to be compared, and each description shows definite contrasts and possibly a wholly different set of distinguish- ing features; all of this is again subject to one’s learning and powers of contrasting the variables. A SPECIES AND A NEW KIND OF TYPE SPECIMEN 479 Summary.—tin the last analysis it rests with each author what his specific differentiations will be, and their validity will often depend upon and be rated by the general standard of his work. _ It is proposed here that, in accordance with a recognized need, we employ protypes to serve as “proxy types” to substitute for inadequate existing type material. The practice is criticized of attempting to classify the characters © of a type specimen as specific or generic without knowing all the related forms or without specifying the relation to each neighboring species or genus. It is suggested that contrasts be drawn with each other species separately, or that the noncommittal term “distinctive features”’ be used. REVIEWS Het Verband tusschen den plistoceenen Ijstujd en het Ontstaan der Soenda-Zee (Java- en Zuid-Chineesche Zee) en de Invloed daarvan op de Verspreiding der Koraalriffen. ... . (The Sunda Sea and Its Barrier Reef.) Door G. A. F. MoLencrAAre) 2 K. Akad. Wet. Amsterdam, Verslag der Afdeeling Natuurk., Vol. XXVIII, 1919, pp. 497-533- The shallow Java Sea between Java-Sumatra and Borneo and the confluent shallow southern part of the China Sea are united by Molen- graaff under a single name, the Sunda Sea. He accounts for the flat sea floor, nowhere more than 40 fathoms deep, by supposing that, so far as its area was already a lowland of weak rocks in pre-Glacial time, it was worn down still lower during the Glacial epochs of lowered sea surface, and that its deeper, sea-covered parts were in the same epochs filled up to a corresponding level. Large rivers, fed by the heavy rainfall of the region, are thought to have been active agents of gradation. The degraded and aggraded lowland was submerged and the present sea created when the ocean finally rose to its normal level in post-Glacial time. The margin of the submerged lowland is assumed to lie at the present 40-fathom line, outside of which a relatively rapid descent is made to deep water. Evidence of submergence is found not only in the embayed mouths of tributary rivers not yet filled with deltas, but also in the occurrence of detrital tin ore in the extension of river courses a mile or more from the shore of certain tin-bearing islands. The fine sediments by which the sea bottom is now covered are regarded as river deposits laid down while the sea was rising to its present level. Although the’ region here considered—Sundaland, as Molengraaff calls it—is known on the basis of abundant geological evidence to have long been much more stable than the disturbed region of the several deep seas and many islands farther east, the essential exclusion of crustal subsidence and the restriction of the Sunda sea-floor origin to so short a geological interval as the Glacial epochs of the Glacial period seem open to question. Moreover, the present margin of the shallow floor near the 40-fathom line, which Molengraaff regards as the built-out border of the worn-down lowland when the Glacial ocean was lowered, 480 ... g |40.04| 0.07 Garmeteae eee |i Aange|onno Ilmenite....... if 0.76/49.32 0,07 oooo°o MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 533 No. 49: Websterite, composed only of bronzite and diallage. Hebbville near Baltimore. G. H. Williams, Amer. Geologist, Vol. VI; F. W. Clarke, U.S. Geol. Sur. Bull. 228 (1904), p. 51.—No. 50: Pikrite. Schwarzenstein, Fichtelgebirge. In the rock 0.09 percent CO,. Giimbel, Geogr. Beschreibung Bayeryns, 1879. (Rosenbusch, Gesteinslehre, p. 352.)—No. 51: Hypersthene diabase. Twins by Rapidan, Virginia. Campbell and Brown, Bull. Geol. Soc. Am., II.—No. 52: Diallage-hornblende-gabbro. Veltein, in the Alps. Kiich- ler, Chemie der Erde, I, 1914. The analysis of the rock from Hecker, Neues Jahrb. f. Min., Geol. u. Pal., Beil., Bd. XVII (1903).—No. 53: Hypersthene- biotite-dacite. Upway, Victoria. Skeats, Quart. Jour. Geol. Soc. London | 1910. In the rock 0.16 per cent S. In the biotite H.O+, 3.20 and H,0+, 0.43 per cent.—No. 54: Kimberlite, strongly decomposed. From Elliot County, Kentucky. In the rock 8.92 per cent H.O, 6.66 COn, 0.28 SO;, 0.05 NiO. Diller, U.S. Geol. Soc. Bull. 38, and Amer. Jour. Sci., 3d Series, Vol. XXXII; Clarke, U.S. Geol. Soc. Bull. 228, p. 66. OLIVINE AND ORTHORHOMBIC PYROXENE We shall commence with some analyses of olivine and bronzite from olivine nodules in basalts. We shall base the calculations for the bronzite, often somewhat decomposed, on the entire quantity of iron found analytically. In reality a little, but only very little, iron in the bronzite will appear as Fe.O,. The stoechiometric relation between MgO and FeO in olivine and bronzite for olivine nodules: Sidi 55a ~_—{ Olivine 1 MgO:o0.11 FeO (Kappenstein, etc.).... 550 | Bronzite t MgO:o0.14 FeO : 56a Olivine t MgO:o0.11 FeO 565 Bronzite t MgO:o0.10 FeO 57a ee t MgO:0.09 FeO Dreizer Weiher, Eifel.... ) 578 Bronzite 1 MgO:o. 11 FeO : 58a Olivine 1 MgO:o0.11 FeO 586 Bronzite t MgO:0.10 FeO St 1, Marb Fu toes 598 Olivine t MgO:o.10 FeO ace aes { 59) Bronzite t MgO:o.11 FeO Kaiserstuhl, Baden ..... Gog Olivine 1 MgO:0.09 FeO 5 606 Bronzite t MgO:o.10 FeO Reihenweiler, Alsace .... one Olivine 1 Mg0:0.14 FeO ; 616 Bronzite 1 MgO:0.14 FeO Nos. 55a, 6, and c: Kukurzenkezel near Kappenstein: Schadler, Tscherm. Miti., Vol. XXXII (1914).—No. 56a and 6: Schiller, Becke, Tscherm. Mitt., Vol. XXIV.—Nos. 57a and b: Th. Kierulf, Bischof’s chem. Geol., and Pogg. 534 THESE VOGAs Ann., Vol. CXLI.—Nos. 58a, 6, and c: Philipp, Neues Jahrb. f. Min., etc., 1871, and Rammelsberg, Pogg. Ann., Vol. CXLI.—Nos. 50a, 5, and c: Bauer, Neues Jahrb. f. Min., etc., 1891, II1.—Nos. 60a and b: Knop, Neues Jahrb. f. Min., etc., 1877.—Nos. 61a and 6: Linck, Zettschr. f. Kryst. u. Min., Vol. XVIII. The value of FeO in the bronzite should be reduced a little throughout, probably about one-tenth in most cases. Joh. Schiller has discussed the question in hand in a special treatise,t partly on the basis of several of the analyses here given of olivine nodules in basalts, and partly on the determi- nation of the chemical composition of the two minerals on the basis of the axial angle and optical character. He comes to the result that MgO and FeO in the feldspar-free rocks are quite evenly distributed in the olivine and orthorhombic pyroxene, and this conclusion is confirmed by my own investigations. But with regard to the rocks containing feldspar he supposes a relative, sometimes even a relatively extensive, enrichment of MgO in the olivine. The observations on which he bases this last construc- tion, however, are few, and in my opinion rather dubious.” Olivine and orthopyroxene,’ isolated from a series of peridotites poor in iron (saxonites, olivine-schists, etc.) with only very little Al,O,, Fe.0;, and CaO, show: Olivine, 1 MgO:0.08, 0.08, 0.08, 0.10, 0.11 FeO; Orthopyroxene, 1 MgO:0.07, 0.07,.0.07, 0.10, 0.12, 0.12, FeO. In peridotites, a little richer in iron, and at the same time carrying somewhat more Al,O,, Fe,O,, and CaO, we find: Olivine, 1 MgO:0.15, 0. 21 FeO; Orthopyroxene, 1 MgO:0.13, 0.15, 0.16 FeO. As well with. regard to the short report above as to Schiller’s investigations, MgO and FeO in the feldspar-free rocks in question are quite evenly divided between the two minerals. The various lesser differences—which would indicate a small relative enrichment 1 Tscherm. Mitt., Vol. XXIV (1905). 2 Especially for the extremely low FeO-contents in olivine from an olivine-gabbro from Tilai, Ural. 3 Enstatite-bronzite-hypersthene deserves, in the same manner as anorthite- bytownite-labradorite-andesine-oligoclase-albite, a common term, and as such I will use “orthopyroxene,” that is to say, pyroxene belonging to the orthorhombic system. I believe I occasionally have heard or read this term before, so the proposition is not originally mine. MAGMATIC DIFFERENTIATION .OF IGNEOUS ROCKS 535 of MgO in the olivine in some cases, and in the orthopyroxene in others—approximately balance, and probably depend chiefly on the source of errors connected with the determinations. If we pass on to the gabbroidic rocks, we find that hypersthene, in the common anchi-eutectic norites, usually shows—as well on the basis of the analysis of isolated hypersthene, as by the determina- _ tions of the axial angle undertaken by earlier investigators and by myself—a composition between about 30 and 38-4o per cent FeSiO, (stoechiometric). And the olivine shows, as well on the basis of the analyses of isolated material, as on my own determina- tions of the axial angle, about 32-35 per cent Fe,SiQ,. ‘In the hypersthenite-norites (with only relatively little plagio- clase) we usually find, however, a relatively lower percentage of iron, as well in the rock as in the separated silicate minerals. ‘This is discussed more elaborately in Part II. We shall include a couple of separate determinations: The thin secretion of a hypersthenite- norite, above mentioned (Fig. 25), consisting chiefly of hyper- sthene, augite, hornblende, and olivine shows: Hypersthene, optically negative, 2V=ca. 80°, gives 25-30 (about 27) per cent FeSiO;; Olivine, optically negative, 2V=ca. 83°, gives about 30 per cent Fe,SiO,. Olivine-carrying norite with only about 4o per cent labradorite from Skjeekerdalen (Figs. 20-21): Hypersthene, optically negative, 2V=ca. 80-85°, gives about 25 per cent FeSi0,; Olivine, optically negative, 2V=ca. 85-88°, gives 20-25 per cent Fe,SiO,. Also in the igneous rocks containing feldspar, we find approximately the same MgO:FeO proportion in both minerals. Any relative enrichment of MgO in the olivine is usually not to be found. ORTHORHOMBIC AND MONOCLINIC PYROXENE As special Fe,O, determinations are lacking in several cases and in others are little instructive on account of a later oxidation, we in both minerals originate from the entire percentage of iron, this giving a quite true image of the relative proportions of MgO and FeO in the two minerals. On account of the small percent- age of Fe,O, the statements for FeO ought, however, for the 536 TOE VOGT orthorhombic pyroxene to be reduced about one-tenth, and for the monoclinic pyroxene, which throughout contains a little more Fe,O;, about one-eighth. This correction, however, is of small extent. The stoechiometric proportion between MgO and FeO in orthorhombic and monoclinic pyroxene from the same rock: Marburg...... Te Bronzite t MgO:o.11 FeO Weeier 59¢ Diopside 1 MgO:o0.07 FeO Ola nen oHlee Vee ae 58 Bronzite 1 MgO:o.11 FeO fee aaa 58¢ Diopside 1 MgO:o0.14 FeO a ML ah ce Bronzite t MgO:0.10 FeO 60¢ Diopside t MgO:o0.16 FeO | Kappenstein. . . {559 ee t MgO:0.14 FeO ( 55¢ Diopside 1 MgO:o0.18 FeO ‘ 49) Bronzite 1 MgO:o. 20 FeO WiebSteritie td se ininicr ccceee one ee re Rs 1 MeOro. 46 eG ie 50) Bronzite 1 MgO:o. 20 FeO Pikrites ec eat tegen tance eee nee 1 MgO:.22 He@ : 510 Hypersthene 1 MgO:0.40 FeO Hypersthene Diabase.........:-.. Ba ; eie 1 MeOrelee FeO If we deduct the small, analytically found figures for Fe,O, from the last rock (No. 51), we have: Hypersthene, No. 510, 1 MgO: 0.39 FeO; Augite, No. 51¢, 1 MgO: 0.35 FeO. The seven double analyses of orthorhombic and monoclinic pyroxene give approximately the same proportions between MgO and FeO, in some cases a little difference in one, and in some in the other direction, but there is no:particularly constant enrichment of one component in either of the two minerals. A series of analyses shows that where orthorhombic and monoclinic pyroxene appear as primary formations in the same rock, the monoclinic is charac- terized by a somewhat higher percentage of TiO., Cr,O;, ALO; —and probably also of Fe,O;—than the orthorhombic. DIALLAGE AND PRIMARY BROWN HORNBLENDE Kiichler’s two analyses from a diallage-hornblende gabbro (No. 52) show, the total quantity of iron being reckoned as FeO: Diallage, No. 52c, 1 MgO: 0.57 FeO; Hornblende, No. 52d, 1 MgO: 0.48 FeO. MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 537 If we deduct 3.88 per cent Fe,O, in the diallage and, at an estimate, 2.0 per cent Fe,O, in the hornblende, we get: Diallage, No. 52c, 1 MgO: 0.46 FeO; Hornblende, No. 52d, 1 MgO: 0.42 FeO, consequently, as emphasized by Kiichler, about the same MgO: FeO proportion in both minerals. HYPERSTHENE AND BIOTITE The two analyses from a dacite (No. 53) show: Hypersthene, No. 530, 1 MgO: 1.00 FeO; Biotite, INOM53¢, te Vig@ rir o2) He®: consequently exactly the same MgO:FeO proportion in both minerals. | . If for the Romsaas quartz-orbicular-norite, which in the entire rock only contains about 0.5 per cent Fe,O,, we assume as an estimate 1 per cent Fe,O, in the biotite, we get: Hypersthene, No. 41, 1 MgO : 0.39 FeO; Biotite, No. 42, 1 MgO: 0.27 FeO. Even if the last figure is not quite exact, relatively somewhat less FeO appears in the biotite than in the hypersthene. When there is a simultaneous appearance of biotite and hyper- sthene in the same rock, the biotite seems throughout to carry considerably more TiO, than the hypersthene. The summary, here briefly stated,. verifies the earlier con- clusion, especially by A. Merian? (1884), W. Wahl? (1906), and Kiichler (Joc. cit., 1914), viz., that the composition of the ferro- magnesian silicates depends quite simply upon the composition of the entire rock or magma, and further that the relations between MgO and FeO (or Mg-and Fe-silicate) in two from the same magma crystallizing ferromagnesian silicates such as olivine: orthorhombic t “Analysen gesteinsbildender Pyroxene,”’ Neues Jahrb. f. Min., etc., Beil., Bd. IIT (2884). 2 Die Enstatitaugite (dissertation), Helsingfors, 1906; Tscherm. Miit., Vol. XXVI (1907). 538 Jo Hea VOG Ts pyroxene, orthorhombic:monoclinic pyroxene, diallage:primary hornblende, hypersthene:biotite, are not subject to extensive variations. We may find a little variation sometimes in one and sometimes in the other direction, but this may be due in part to inaccurate determinations. But all in all, we here have approxi- mately the same MgO:FeO proportions in both minerals. We especially emphasize that no mineral is characterized by a constant relative enrichment either of MgO or FeO. Lesser variations, with regard to the MgO:FeO proportion, by two or still more ferro- magnesian silicates, crystallizing from the same magma, may be due to a ‘series of factors, of which we may mention the horizontal distance between the liquidus and solidus curves (or the difference between the a:b proportion in the first crystallized mix-crystal and in the liquid phase); the degree of equilibrium between the solid and liquid phases; the electrolytic dissociation. A small horizontal difference between the liquidus and solidus curves, and a nearly complete equilibrium between the solid and liquid phases will cause nearly the same MgO:FeO proportion between the segregated ferromagnesian silicates and the magma, and consequently also between the ferromagnesian silicates mutually. As well in olivine as in orthopyroxene and diopside-heden- bergite, the Meg-silicate is concentrated in the first mix- crystal. By more or less incomplete equilibrium between the liquid and solid phases—as in the dike and effusive rocks—we may expect a relative enrichment of MgO in the mineral which first commenced crystallizing. With two ferromagnesian silicates we may generally expect a more evenly distributed MgO: FeO pro- portion among deep-seated rocks with complete or nearly com- plete equilibrium between liquid and solid phases than among dike and effusive rocks. Addition.—Also in ilmenite a little MgO enters, viz., as MgTiO . In this manner ilmenite No. 54f from a peridotite (with about 0.12 FeO:0.88 MgO in the entire rock) shows not less than 8.68 per cent MgO, or 0.64 FeO:0.36 MgO. The ilmenite from the labradorite rock near Ekersund (with about 0.5 FeO:0.5 MgO MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 539 in the entire rock) usually contains 3 to 4, up to 5.14 per cent MgO (the last analysis equivalent to 0.78 FeO:0.22 MgO). In the dacite No. 53, with 0.53 FeO:0.47 MgO in the whole rock, the hypersthene as well as the biotite carries almost exactly 0.50 FeO:0.50 MgO and the ilmenite 0.96 FeO:0.04 MgO. The proportions of FeO and MgO in the ilmenite, from observa- tion of the three rocks just mentioned, must be a function of the FeO: MgO proportion in the entire rock or in the original magma, but in such a manner that the ilmenite throughout shows a very extensive relative enrichment of FeO (as FeTiO,) and consequently vice versa an extensive, relative decrease of MgO (as MgTiO,). [To be continued] CYCLES OF EROSION IN THE PIEDMONT PROVINCE OF PENNSYLVANIA* F. BASCOM Bryn Mawr, Pennsylvania Since 1912, when Professor Barrell brought to the attention of the Geological Society of America some conclusions opposed to the earlier interpretation of the erosion history of certain portions of the Appalachian highlands, the writer has had in mind the possible application of similar conclusions to the erosion history of the Piedmont province of Pennsylvania. The results of this intention are presented in this paper. The conclusions reached are not precisely in accord with those enforced with so much originality by Professor Barrell, nor do they involve much that is new in the interpretation of the erosion history of eastern Pennsylvania, but they are presented as a record of the present stage of the study of the peneplains of the Piedmont province of Pennsylvania. ' Jt is the purpose of the paper to call attention to the fact that — the erosion history of eastern Pennsylvania as indicated by altitudes and by the record of sedimentation must have been complex, that it was made up not of one or two or three cycles of prolonged erosion, but of many interrupted cycles, and that vestiges of nine of these cycles testify to their reality. Other cycles may have existed and probably did exist, but too briefly for permanent record. Six of these nine cycles are thought to belong to post- Cretaceous time and three to Cretaceous time. The question of the subaerial or marine origin of these pene- plains is debated, but decisive criteria are lacking for a final pro- nouncement. t Published by permission of the Director of the United States Geological Survey. The writer takes pleasure in acknowledging her indebtedness to M. R. Campbell and G. W. Stose of the United States Geological Survey for helpful comments and queries made on the subject-matter of this paper, and to Professor W. M. Davis for valuable specific suggestions. 540 _ CYCLES OF EROSION IN PENNSYLVANIA 541 In any investigation of the cycles of erosion, complete or incom- plete, that collectively constitute the erosion history of the Pied- mont province of the Appalachian highlands, the stratigraphic record preserved on the margin of the province must furnish the data by which the succession and age of such erosion cycles stand or fall. That aerial and marine erosion has taken place ever since continental plateaus and oceanic basins came into existence is unquestioned: only such interaction of air, water, and land masses is conceivable. ‘The character and rapidity of erosion will be con- trolled by altitude, rock, and climate, but the duration of an erosion period will be dependent upon the stability of the strand line: a long period of quiescence will permit prolonged aerial and marine erosion with reference to a given base-level, and a period of uplift will interrupt and renew erosion with reference to a new - base-level. The evidence of such movements of the strand line inaugurating erosion is furnished by the stratigraphic register. In the Piedmont province of Pennsylvania with the beginning of Cretaceous sedimentation the stratigraphic record seems to indicate a succession of such erosional conditions maintained by an alternation of periods of continental quiescence with periods of movement. That these periods of stability have been of different durations is an obvious deduction from the sedimentary record. The stratigraphic record on the Atlantic plain which is the submerged margin of the Piedmont province is as follows: . Recent deposits . Unconformity Pleistocene deposits Aialinota (Cape lVilay)ianclayaisand sand\eravelia meee ya aera 30 feet Unconformity Wicomico (Pensauken): clay, sand, and gravel................ 25 feet Unconformity Sunderland (Bridgeton): clay, sand, and gravel............... 25 feet Unconformity BelatesP randy wine: sand andygravellenit. yates esl iyore olerore 1+ foot Pleistocene (or Late Tertiary ?) deposits Unconformity ? Barly randywane-sandranducrayelanm mir ret termine ye 50 feet 542 F. BASCOM Miocene deposits Unconformity St. Mary’s: sandvand tela... 203. yee tne | ae ee 280 feet Choptank: sand, clay; and marl). 099). ee eee 175 eee Unconformity Calvert: sand andiclay 73. ¥ es vss' cs aa celle Sate ee 310 feet Unconformity Eocene deposits Nanjemoy: Sand.) .4ccc0l 54 oho eiee ee ee oe 125 feet Aquia: greensand \ 35.502 e Oana oe dene ee 100 feet Unconformity Upper Cretaceous (Cretaceous) deposits Manasquan: clay andisands{22.7,..402.0 0. 00000 tee 50 feet Rancocas; preensand . 2 &.v 4202 a ehean oie ee Ue 80 feet Monmouth:/sand)sniio.5.. nae beh oe ae oe a ee 100 feet Unconformity Matawan: micaceous sandy clay....... lo 70 feet Unconformity Magothy: sand and clay...... diene specs ee 100 feet Unconformity Raritan: clay andysand < 2.0.00. 54040 oe 2 eee 350 feet Unconformity Lower Cretaceous (Comanchean) deposits Arundel: clay and sand........ Ten R IRONS 66 ooo 3 125 feet Unconformity Patapsco:\clay and isand?)).2 7.) 25.51.50: 440 eee 200 feet Unconformity Patuxent: sand:antid arkose. © 02.0.0 0)0 oi ean a ee 350 feet Unconformity Crystalline formations With the Cretaceous, Tertiary, and Pleistocene registration of continental movements before us, it is no longer possible to believe that the erosion history of this region is told in two cycles of erosion, producing two peneplains: the Kittatinny,' or Schooley, of Cretaceous age, and the Shenandoah, or Somerville,? of Tertiary age. That there is topographic evidence of more than two erosion t Bailey Willis, ‘‘The Northern Appalachians,” Nat. Geog. Mon., 1895, pp. 1690- 202. C.W. Hayes, “The Southern Appalachians,” Nat. Geog. Mon., 1895, pp. 305-36. C. W. Hayes'and M. R. Campbell, “‘Geomorphology of the Southern Appalachians,” Nat. Geog. Mon., 1894, pp. 63-126. 2W. M. Davis and J. W. Wood, Jr., “The Geographic Development of Northern New Jersey,” Proc. Bost. Soc. of Nat. Hist., Vol. XXIV (1889), pp. 365-423. CYCLES OF EROSION IN PENNSVLVANIA 543 periods in the Appalachian highlands has been recognized by Keith,* Campbell,? and others. In the sedimentary sequence of the Atlantic plain there are ten significant unconformities—that is, ten intervals of erosion alternat- ing with intervals of deposition. Not all of the deposits are known to be marine, so that ten submergences cannot be postulated. There are six less significant unconformities. The time represented by the deposits and unconformities has been estimated at 56,500,000 years. There could not conceivably be conditions more favorable for a succession of erosion cycles falling so far short of completion as to leave permanent traces of the sequence, nor a stratigraphic record more compelling for the acceptance of such traces as evidences of erosion cycles. It is not probable that traces are preserved of every incomplete erosion cycle. The topography of erosion cycles early interrupted would be obliterated by subsequent erosion cycles of longer duration. Small beginnings, if they existed, might be quite similar to the three most recent terraces, which are being modified and will in time be completely obliterated by subsequent erosion. Such incomplete, obliterated cycles may be registered only in the lesser unconformities of the stratigraphic record, which is easily more complete than the topographic record. Did the geologist base his expectations on stratigraphy alone, he would look for a series of more or less discontinuous and more or less warped benches or terraces facing the sea, or, in the case of the lower terraces, following inland the river valleys, and not perfectly stairlike because each terrace will have its peculiar angle of slope. The terraces are the topographic record of the succession of inter- rupted erosion cycles of which the unconformities in the strati- graphic sequence are the geologic record. t Arthur Keith, “Some Stages of Appalachian Erosion,” Bulletin Geol. Soc. America, Vol. VII (1806), pp. 519-24. “‘Geology of the Catoctin Belt,” Fourteenth Annual Report, U.S. Geol. Sur., Part II (1892-93), pp. 285-395. 2M. R. Campbell, “‘Geographic Development of Northern Pennsylvania and Southern New York,” Bulletin Geol. Soc. America, Vol. XIV (1903), pp. 277-96. 3 Joseph Barrell, ““Rhythms and the Measurement of Geologic Time,” Bulletin Geol. Soc. America, Vol. XXVIII (1917), pp. 745-904. ' 544 F. BASCOM Nature is less obvious and more complex in her methods and evidences than such expectations would imply, but a detailed and careful study of approximately level tracts and benches seems to justify the following series of peneplains and terraces related to the major unconformities of the stratigraphic record. Altitude Name West ia Sediments Age Preserved Peneplains Kittatinny..... 1800-1600-1100|] Patuxent. Jurassic and Quartzite Lower Creta- ceous Schooley...... 1300-1000-900 | Patapsco- Lower Creta- | Granite Arundel. ceous Honeybrook...| 860-800-700 | Raritan- Upper Creta- | Granite Manasquan. ceous Harrisburg.... 800-500 Aquia- Tertiary Shale St. Mary’s. Early Early Pliocene Shale, etc. Brandywine....| 500-400-390 | Brandywine. (Pleistocene) Terraces Late 400-300-200 | Late Pleistocene Mica gneisses Brandywine... Brandywine. Sunderland. ...| 300-180-100 | Sunderland. Pleistocene Mica gneisses Wicomico..... 90-45 Wicomico. Pleistocene Mica gneisses WADE. sooo oo 45-40-09 Talbot. Pleistocene Mica gneisses The oldest peneplain, the highest inland from the sea and the lowest near the sea where it is preserved under sedimentary rocks, . is the Kittatinny. This peneplain surface is so strikingly upheld, Kittatinny 2000° : eco Peneplain 1500° 1000' 500° , Sea-level Fic. 1.—Section across Godfrey Ridge and Kittatinny Range, at the Delaware — Water Gap, Delaware Water Gap quadrangle, Pennsylvania—New Jersey. i | Honeybrook Schooley Schooley |, ,,eybrook Harrisburg Delaware Watergap quadrangle although not preserved unmodified, by the indurated sandstone of Kittatinny Mountain (1,600 feet), its type locality (see Fig. 1), and in other resistant ridges of the Appalachian highlands that it was early recognized and an effort was made to fit it to the lower summits of Schooley Mountain and still lower surfaces in the CYCLES OF EROSION IN PENNSYLVANIA 545. Piedmont province. In order to secure continuity between adjacent and discordant levels, it was necessary to assume abrupt and steep warping of the peneplain in some localities, and when accordance was secured it left unexplained the remarkable preserva- tion throughout the Piedmont province of so ancient an erosion surface, and failed to explain why no records were preserved of the later continental movements and erosion cycles which are recorded in the sedimentary succession. Fic. 2.—Kittatinny, Schooley, and Honeybrook peneplains in the Reading quadrangle. The summit of the ridge on the left at 1,140 feet represents the Kit- tatinny; the ridge in the middle at 1,000 feet, the Schooley; the ridge on the right at 700 feet, the Honeybrook peneplain. West Reading in middle distance, looking east. The Kittatinny peneplain has been traced northeastward from the type locality to the base of the Catskill Mountains in New York, and westward and southward into Maryland and West Virginia. In the Blue Ridge province, surfaces which are probably remnants of the Kittatinny have altitudes of 1,800 feet in southern Pennsylvania (South Mountain), 1,300 on Blue Mountain to the northeast, and 1,200 feet in the quartzite ridge east of Reading (Penn Mountain, the dominating highland of the area, designated the Reading Prong of the New England upland). (See Figs. 2 and 3.) Whether the Kittatinny peneplain is anywhere preserved in the Piedmont province of Pennsylvania is questionable. Reduced remnants of it may appear on Welsh Mountain (Honeybrook 546 F. BASCOM quadrangle), a quartzite ridge, but, as is to be expected in a region so near the sea, erosion in subsequent cycles has probably modified the Kittatinny surface, notwithstanding the resistant character of the rock. Near the “‘fall-line”’ where the lowest and oldest formations (Patuxent formation) of the coastal plain lie directly upon a pene- plained surface of crystalline rocks, the floor which bears them may be that part of the Kittatinny peneplain which was submerged, was buried beneath sediments, and was thus preserved without modification while far inland the peneplain was still developing. Fic. 3.—Schooley peneplain in the Reading quadrangle. The summit of Irish Mountain in the distance at 1,000 feet represents the Schooley peneplain, as seen from a point on the Early Brandywine peneplain, two miles east of Shoemakersville, looking south 45° east. The Patuxent formation once overlapped the margin of the Piedmont province to a distance inland considerably greater than is now covered by it; but wherever it has been removed by erosion, the surface of the peneplain has been attacked so that the old surface cannot be found except perhaps in the immediate vicinity of the remnants of the formation. ‘This surface lies at about 180 feet above sea-level, rising in Maryland to 280 feet. ‘This first peneplain, carved on a dissected highland or possibly on uplifted peneplains, obliterated in this region all pre-existing erosion sur- faces except those that were protected by a cover: an example of such a surface is to be found on Paleozoic rocks, where they are covered by Triassic formations (see Fig. 4). The later peneplains, carved on uplifted peneplains or terraces, never completely obliter- ated pre-existing erosion surfaces. CYCLES OF EROSION IN PENNSYLVANIA 547 That the Kittatinny erosion cycle exceeded in duration any of the subsequent cycles must have been the case not alone because no subsequent cycle has been coextensive with it, but also because no subsequent cycle has succeeded in wearing down the most resistant rocks leveled by Kittatinny erosion and located well within the area of subsequent peneplanation. The next oldest peneplain, the Schooley, with its type locality the granite summits of Schooley Mountain (1,300 feet), New Jersey, has been traced northward to the Mohawk Valley, westward to Fic. 4.—Section of erosion surface on the Paleozoic, protected by a cover of Triassic rocks. Port Kennedy, Montgomery County, Pennsylvania. Syracuse,’ and southward to the Potomac.? In western New York it appears to coalesce with the Kittatinny, suggesting that in that region there was no uplift separating the two erosion periods. At the Delaware Water Gap the ridge between Godfrey Ridge and Kittatinny Range with summit areas at 1,000 feet may represent the Schooley peneplain, and Wind Gap between Blue Mountain and Kittatinny Mountain in Northampton County at the same elevation may have been a water gap during the early part of the Schooley erosion cycle. In the Blue Ridge province remnants are preserved in summit areas of 1,200 and 1,000 feet ™ Memorandum by M. R. Campbell. 2 Mem. by the writer. 3 G. W. Stose, ‘‘Text of Delaware Water Gap Sheet,” U.S. Geol. Survey. 548 F. BASCOM altitudes. East and northeast of Reading there are many flat- topped granite and quartzite hills rising to a height of 1,000 feet, the Schooley level in that locality (see Figs. 5-10). On one of these, the Schooley remnant is separated from the adjacent Kittatinny Fic. 5.—Schooley peneplain above the Honeybrook peneplain in the Boyertown quadrangle. The summit of Long Hill in the distance at 1,040 feet represents the Schooley peneplain, as seen from a point one-half mile southeast of Shanesville, looking south. The summit of the ridge in the foreground is a remnant of the Honeybrook peneplain. Fic. 6.—Schooley peneplain in the Reading quadrangle. The higher parts of the past-maturely dissected upland one and one-half miles southeast of Fleetwood represent the Schooley peneplain at an altitude of 940-1,000 feet. Hill road, looking south on hills south of Princeton. remnant by a steep slope (Fig: 9). In the central Piedmont province the Schooley peneplain descends to an altitude of 800 feet (Coatesville quadrangle). If the Schooley peneplain reappears near the “‘fall-line,” it is found on the border of, and passing CYCLES OF EROSION IN PENNSYLVANIA 549 beneath, the Patapsco formation, which rests upon eroded Patuxent, at an altitude of roo feet, rising to 130 feet in Maryland. The next movement of uplift not only raised the Schooley peneplain Fic. 7.—Schooley, Honeybrook, and Late Brandywine peneplains in the Boyer- town quadrangle. High hills in the background are remnants of the Schooley pene- plain, altitude 1,000 feet; hil! in center in middle distance is at the level (660 feet) of the Honeybrook peneplain; and the foreground is on the Late Brandywine peneplain, altitude 420 feet. Looking west from Palm Station. Fic. 8.—Schooley and Honeybrook peneplains in the Boyertown quadrangles. Hills in the background are remnants of the Schooley peneplain, altitude 1,000 feet; foreground on the Honeybrook peneplain, altitude 800 feet. Devil’s Hump, looking south 30° west. and remnants of the Kittatinny peneplain to a considerable height, but warped them.* The next younger peneplain, the Honeybrook, appears in God- frey Ridge, northwest of Kittatinny Mountain, at the Delaware 1 Bailey Willis, op. cit., pp. 189-90. C. W. Hayes, op. cit., p. 330. 550 F. BASCOM Water Gap, and on the hill summits southeast of Kittatinny Mountain at an altitude of 800 feet. This altitude is a very persistent one in the Appalachian Valley from this region to Susquehanna River. The Hamburg and Slatington quadrangles show the Honeybrook peneplain dominating the interstream areas. It retains an altitude of 800 feet in the Blue Ridge province and is well shown east of Reading, where Neversink Mountain, Guldin Hill, and the southeastern spur of Penn Mountain preserve its surface (see Fig. 9). The Honeybrook and the Schooley are here Schooley Late B i ae dane Her (oy Reading quadrangle Ess] Granite Quartzite Frc. 9.—Section across Penn Mountain and Guldin Hill, showing remnants of the Kittatinny, Schooley, and Honeybrook peneplains, and of the Late Brandywine and Sunderland erosion surfaces. Reading Prong of the New England upland, Reading quadrangle, Pennsylvania. Schooley 1000' Honeybrook 750° Harrisburg E 500’ Beer 250' o' Boyertown quadrangle Fic. to.—Section across Long Hill, Devil’s Hump, and Gabel Hill, showing the Schooley, Honeybrook, and Harrisburg peneplains. Boyertown quadrangle, Penn- sylvania. found adjacent, are both cut in granite, and are separated by-a steep slope (see also Figs. 11 and 12). In the Piedmont province the Honeybrook descends to 700 feet. On the divide between Susquehanna and Schuylkill rivers the North and South Chester Valley Hills preserve this peneplain. The most extended remnant of it is found on the granite about Honeybrook, 16 miles south of Reading, and from this type locality CYCLES OF EROSION IN PENNSYLVANIA 551 the peneplain is here named the Honeybrook.’ It is not claimed that this plain has been traced throughout the Appalachian high- lands division, but in the Piedmont province of Pennsylvania it ! Fic. 11.—Honeybrook peneplain below the Schooley, Boyertown quadrangle. The upland in the distance represents the Schooley peneplain, altitude 1,000 feet, and that in the middle distance, altitude 800 feet, the Honeybrook peneplain, as seen from a point one-fourth mile southwest of Shanesville, looking north 15° west. Fic. 12.—Water gap in a ridge whose summit is a remnant of the Honeybrook peneplain, Boyertown quadrangle. Upland in background, altitude 1,000 feet, represents the Schooley peneplain. Summit of ridge 800 feet and stream in the water gap, 440 feet. View from a point one-fourth mile southwest of Shanesville, looking north 45° west. seems to represent a distinct erosion level between the Schooley and Harrisburg. The Honeybrook peneplain has been completely 1 The Schooley peneplain was traced from Pennsylvania to the Potomac Valley in Maryland to surfaces (Green Ridges) which have been ascribed to the Weverton peneplain (Maryland Geol. Survey, Vol. VI [1906], pp. 87-88). Elsewhere in the central Piedmont of Pennsylvania the Weverton as defined corresponds to a lower peneplain than the Schooley. A new name has therefore been introduced for a redefined Weverton. 552 F. BASCOM removed at the “‘fall-line.” It passes under the Raritan (Upper Cretaceous) formation near the junction of the Coastal Plain and Piedmont. Fic. 13.—Harrisburg peneplain in the Coatesville quadrangle. The dissected peneplain at an altitude of 600 feet, as seen one-third of a mile northwest of Humphrey- ville, looking southwest. | i Fic. 14.—Harrisburg peneplain on Schuylkill River, in the Reading and Honey- brook quadrangles. The summit of the hill in the middle distance in which the river is cutting a steep bluff represents the Harrisburg peneplain, altitude 600-660 feet. Gibraltar Hill, 900 feet, a monadnock, on the left, and the Sunderland plain in the foreground, as seen from Lookout Point, Neversink Mountain, looking south. The Harrisburg peneplain‘’ has been restricted, with the approval of its sponsor, at its type locality northeast of Harrisburg to upland 1M. R. Campbell, Bulletin Geol. Soc. America, Vol. XIV (1903), pp. 277-96. CYCLES OF EROSION IN PENNSYLVANIA 553 surfaces on the Ordovician (Martinsburg) shale which reach 600 feet, and to corresponding altitudes on Delaware and Potomac rivers (see Figs. 13 and 14). At this altitude it is widespread in ® « Fic. 15.—Honeybrook and Harrisburg peneplains in the Boyertown quadrangle. The Honeybrook peneplain corresponds with the surface of the upland on the left at an altitude of 800 feet and the Harrisburg with the upland in the distance on the right at an altitude of 600 feet, as seen from the northeast end of Long Hill, looking north 55 east. Fic. 16.—Remnants of Early Brandywine and Harrisburg peneplains in the Harrisburg quadrangles. The surface of the upland represents the Early Brandy- wine peneplain upon which the hills, rising to an altitude of 740 feet and perhaps to the level of the Harrisburg peneplain, stand as monadnocks. The view is from a point on the Sunderland level one-half mile northwest of Maiden Creek, looking southwest. the central Piedmont province and descends on the border of the upland to 500 feet (North and South Chester Valley Hills in the Schuylkill Valley). The Harrisburg does not appear in the “fall- line” zone, but probably descends below sea-level beneath the 554 F, BASCOM Aquia Greensands (Tertiary), which lie far out on the Coastal Plain. The Early Brandywine, the youngest and most widely preserved of the five peneplains, is found on Ordovician shale at the 500-foot level, northeast of Harrisburg. It contains at this altitude in the Fic. 17.—Early Brandywine peneplain in the Boyertown quadrangle. The peneplain corresponding with the surface of the upland in the distance at an altitude of 560 feet is seen from a point one-half mile west of Eschbach, looking south 45° east. Fic. 18.—Early Brandywine peneplain in the Reading quadrangle. The pene- plain at an altitude of 500 feet is represented by the surface of the upland in the distance, as seen from a point on the Sunderland level one-half mile northwest of Maiden Creek, looking south 65° west toward Leesport. valley of the Delaware at the Water Gap and in the Schuylkill and Potomac valleys (Antietam quadrangle). It ranges from 400 to 450 feet in the Piedmont upland of Pennsylvania where, as is to be expected, because it is the most recently formed peneplain, it is the most pronounced upland level (see Figs. 15-18). CYCLES OF EROSION IN PENNSYLVANIA West Chester is located upon this peneplain sur- face, which is the dominant altitude throughout the West Chester quadrangle (see Figs. 19 and 20). At 4oo feet it carries Early Brandywine gravel and sand, 10 miles west of the ‘‘fall-line” zone. It has been named the Early Brandywine from the formation which is found at this altitude and on the seaward continuation of the slope. This peneplain is corre- lated with the so-called Lafayette" terrace recognized in Maryland? but a more widespread extension is claimed for the Early Brandywine peneplain. The Early Brandywine peneplain is everywhere submaturely dissected. The summits of the inter- stream areas preserve the peneplain, gentle slopes from these summit remnants lead to the gorges (Pleistocene) of the main streams and of the larger tributaries or form the U-shaped valleys of head- water streams. These slopes have an elevation inland from 300 to 4oo feet and in the “fall-line” zone from 200 to 300 feet (see Figs. 21-25). Following the deposition of the Early Brandywine formation and before the deposition of the Sunder- land formation, the whole continental shelf was brought above the sea and master-streams of the Atlantic plain were extended to the edge of the con- tinental shelf. To this period, which may have been well within Pleistocene time, is attributed the forma- tion of the Late Brandywine benches and slopes. Few formations can be correlated with it, as the * Owing to the change of the name Lafayette to Brandywine, the more recent name has been given to the peneplain. The‘’name Brandywine is taken from a village of that name in Prince George County, Md., where the formation is reported to be characteristi- cally developed. The position and level of the gravel of this type locality at 233 feet seem to indicate that it is the low-level Brandy- wine or Late Brandywine gravel as it is provisionally named in this paper. 2 Maryland Geol. Survey, Vol. VI (1906), pp. 59-60. West Chester Quadrangle Harrisburg 750° E Brandywine Gravels 00" Early Brandywine 555 Sunderland E. Brandywine ate Brandywine 500’ 250' ¢ Fic. 19.—Section in West Chester quadrangle 556 F. BASCOM marine sedimentation of the period took place, mainly at least, beyond the continental shelf. Gravel, which has been included in the ‘‘Brandywine” (Early and Late Brandywine), but which lies at all places at a lower level than the Early Brandywine gravel, is thought to be a terrestrial deposit of Pleistocene streams. Such gravel is found on the Chester quadrangle at an altitude of 300 feet and on Elk Neck, Elkton quadrangle, between 200 and 300 feet. The records of this period of erosion are the dissection of the Early Brandywine peneplain, producing the stream terraces and Harrisburg ' Earl : oe Brandywine ate Brandywine 250' i rol Norristown quadrangle Fic. 20.—Section in Norristown quadrangle 1000! Chester Quadrangle Early Brandywine 25 Late Brandywine Sunderland Wicomico Talbot Fic. 21.—Section in Chester quadrangle the slopes which separate the Early Brandywine peneplain and the Sunderland terrace, and the submerged valleys on the continental shelf. Late Brandywine slopes are well defined on the Chester .quadrangle, and furnish a commanding site for the buildings of Swarthmore College. The Sunderland, Wicomico, and Talbot terraces have been recognized and defined in Maryland.t In Pennsylvania a scarp separates the Late Brandywine and the Sunderland. This scarp, which the central building of Swarthmore College fronts, represents either the old estuarine shore cliff or the escarpment of the wide meander belt of Delaware River. Erosion truncated the Late Brandywine slopes and dissected them and the Early Brandywine peneplain along drainage ways. What has been called the Somerville peneplain seems to the writer ¥ Maryland Geol. Survey, Vol. VI (1906), pp. 61-67. CYCLES OF EROSION IN PENNSYLVANIA to be such an inland extension of erosion during the Sunderland cycle. In general the Sunderland extends from faeroo to the 180 contour limes: the Wicomico from the 80 to the 90 contour lines, and the Talbot, where it does not coalesce with the Wicomico, from the 40-foot contour to sea-level. These three terraces are conspicuously developed in eastern Pennsylvania parallel to Delaware River. In Maryland the Wicomico and Talbot terraces are in some places oblit- erated and the Sunderland reaches the edge of the beach with a cliff 100 feet high, but this is not the case in Pennsylvania where the terraces are not seacoast features. The Wicomico terrace wraps about the Sunderland as the Sunderland does about the Late Brandywine, with usually a well- marked break between the two, except in the gorges of the tributary streams. The Talbot terrace borders the Wicomico, which it penetrates along drainage ways, and in some places parallel to Delaware River coalesces with the Wicomico. It has not proved practicable to show by graphic means the distribution of the remnants of the peneplains and terraces in the Piedmont province of Pennsylvania. It may be stated that in general the oldest peneplain is farthest inland and the young- est nearest the shore, with those of inter- mediate age ranging between. If this region had been one of uniform resistance to weathering, there would have been a perfect operation of this law of areal Cross Section Schuylkill River and Wissahickon Creek Early Brandywine Brandywine Late Brandywine 500° Early Brandywine Q' 1000’ 750' 250' Fic. 22.—Section crossing Schuylkill River and Wissahickon Creek 557 558 F. BASCOM distribution: the areal succession of peneplains from interior to coast would exactly accord with the chronological succession. The region is, however, one of varied structural and lithologic resistance to weathering and the peneplains are not therefore so simply spaced; younger peneplains on relatively weak rocks are found inland at higher altitudes than the marginal remnants of older peneplains. This fact would be still more apparent if the extreme margins of the older peneplains, now buried beneath sedimentary formations, were shown. The question of the origin of these neneetiie that is, of the nature of the dominant erosive agent, is open to debate. The Cross Section 1000 Brandywine River 750° MERE Pa ‘ 500 Bee ry peace randywine 250° 0 Fic. 23.—Section crossing Brandywine River three youngest terraces, in Maryland presumably-of marine origin, are in this region of fluvial-estuarine or of fluvial origin; that is, they were developed on the borders of the Delaware estuary or on a shrinking meander belt of Delaware River. That the Late Brandywine is of subaerial origin is concluded from the evidence of the valleys, now submerged, which extend across the continental shelf and which it is believed were excavated in Late Brandywine time. That the five peneplains are in part of marine and in part of subaerial origin seems a warranted conclusion. Each peneplain was partly submerged and carries marine sediments, but there does not seem to be sufficient proof that any one peneplain was com- pletely submerged. They parallel the coast line as would be the case were they of marine origin, but this may also be true of sub- aerial peneplains, and the great inland extension of the Kittatinny CYCLES OF EROSION IN PENNSYLVANIA 559 and Schooley peneplains with an indefinite thin margin is indicative of subaerial eresion. ‘The contact of the Honeybrook and Schooley peneplains, on the Reading quadrangle, on the other hand, suggests _ Fic. 24.—Late Brandywine peneplain in the Reading quadrangle. The pene- plain at an altitude of 400 feet, as seen from the hillside south of Oley Furnace, looking south toward Friedensburg. - a Fic. 25——Honeybrook and Late Brandywine peneplains in the Reading and Boyertown quadrangles. The surface of the upland in the distance at an altitude of 800 feet represents the Honeybrook peneplain, and the level land in the middle distance at altitudes ranging from 400 to 440 feet, the Late Brandywine peneplain, as seen from a point one-fourth miles southwest of Oley Furnace, looking south 45° east toward Shenkel Hill. a sea cliff. In the case of the Harrisburg and Early Brandywine peneplains definite proof of subaerial or marine origin has not been found. THE HORIZONTAL MOVEMENT OF GEANTICLINES AND THE FRACTURES NEAR THEIR SURFACE H. A. BROUWER Delft, Holland - Most islands of the arcs which lie to the east and the southeast of the Asiatic continent show proof of an uplift of the land rela- tively to the sea-level, which is amply demonstrated in tropical regions by the presence of upheaved fringing reefs. In the East Indian Archipelago there exists a striking difference between the western and the eastern parts as regards the rising islands and the submarine topography. If the sea-level were to be lowered 200 m., Sumatra, Java, and Borneo would form one mass of land with the peninsula of Cambodia and Siam, just as Australia would form a single mass with the Aru Islands through the vast tract now occupied by the shallow Arafura Sea and the Bay of Carpentaria to New Guinea and the islands Misool, Waigeu, Batanta, and Salawati to the west of New Guinea. Between these two near-land-masses lies an area in which deep sea basins alternate with upheaved islands. From a geological point of view Verbeek! first drew attention to this remarkable fact, of which a more satisfactory discussion has been made possible because of the new deep-sea chart of the Siboga Expedition.2 In Verbeek’s opinion the elevation of the islands surrounding the Banda Sea is the result of folding at greater depth. The active forces first began compressing near the surface, and as the geosyn- clines were formed they became active at greater depths. Later Molengraaff* expressed similar ideas, and for the southeastern por- tR. D. M. Verbeek, “‘Rapport sur les Moluques,” édition frangaise du Jaarb. v. h. Mynwezen in Ned. O. Indié, Vol. XX XVII (1908), pp. 833, 834. 2G. A. F. Tydeman, “Hydrographic Results of the Siboga Expedition,” Chart 1, in M. Weber, Siboga-Expeditie, Part III, Leyden, 1903. 3G. A. F. Molengraaff, “Folded Mountain Chains, Overthrust Sheets and Block- Faulted Mountains in the East Indian Archipelago,” Compte rendu du XIIe congrés géologique international, Toronto, 1913, p. 6990. 560 HORIZONTAL MOVEMENT OF GEANTICLINES 561 tion of the Malay Archipelago he distinguished two types of mountain-building: (1) the overthrust type of Miocene age, cul- minating in overthrusts of great magnitude, which was the expres- sion of a very powerful, but not deep-seated compression, and (2) the block-faulted type of Plio-Pleistocene age, consisting of ranges of elevated islands alternating with deep sea basins, these being the expression of a deeper-seated, but perhaps less energetic com- pression. It is only the vertical movements of the rows of islands that have been considered by these authors. In some recent publi- cations’ I have pointed out that: 1. The youngest crustal movements in this region are a younger phase in the same process as the older and an exact continuation of the mid-Tertiary crustal movements. Of the mid-Tertiary phase we know only the folds and overthrusts which represent action at greater depth; of the youngest phase only the fractured and faulted crust which represents action near the surface; but the two phenomena are mutually complementary and the rows of uplifted islands indicate the spots where the folding process con- tinues at the greater depths with the same tendency to form overthrusts. 2. From the outline of the rows of islands we may conclude that they have a large movement in a horizontal as well as in a vertical direction. The horizontal movements of the curving rows of islands are expressed by several of their characters. 1. The striking fact that the Tenimber Islands and the Kei Islands have an outlying position in the row and both are’situated opposite a depression in the Sahul bank which constitutes the Australian continental shelf. Opposite these depressions the gean- ticline met with less resistance. =H. A. Brouwer, “On the Crustal Movements in the Region of the Curving Rows of Islands in the Eastern Part of the East Indian Archipelago,” Proceed. Kon. Akad. v. Wetensch. Amsterdam, Vol. XXII, pp. 772-82; “On Reef Caps,” ibid., Vol. XXI, pp. 816-26; “Fractures and Faults near the Surface of Moving Geanticlines,” ibid., Vol. XXIII, pp. 570-76; ‘Uber Gebirgsbildung und Vulkanismus in den Molukken,” Geol. Rundschau, 1917, p. 197; ‘‘Uber die horizontale Bewegung der Inselreihen in den Molukken,”’ Nachr. d. Gesellsch. der Wiss. su Gottingen, 1920. 562 H. A. BROUWER 2. The coincidence of asymmetrical reef caps with marked outward bends of the row of islands, instances of which are found in the island Rotti to the southwest of Timor and in the island Jamdena of the Tenimber group. 3. The faults and fractures near the surface demonstrate differ- ences in rate of horizontal movement between adjacent parts of the moving geanticlines. In the following pages the above-described faults and fractures will be dealt with in connection with the vertical and horizontal movements of the geanticlines near the surface of which they occur. Because the geanticlines have risen from the sea and were in conse- quence exposed to eroding influences during a much shorter time than those of the continental mountain ranges, the outer form is not in the main controlled by erosion, but by the crustal movements themselves, and the latest phase of mountain-building manifests itself clearly in the shape of the geanticlines near the surface. CRUSTAL MOVEMENTS AND MORPHOLOGICAL STRUCTURE When crustal movements take place they generally cause the strata to break near the surface and to fold at greater depths. An extension of the geanticlinal axis is here obtained through gaping fractures, or by movements parallel to fault planes which must be inclined to the geanticlinal axis. Shortening of the geanticline is possible by faulting along fault planes which are not perpendicular to the geanticlinal axis. Similar relations prevail for a lengthening or a shortening of a section of the geanticlinal surface with a plane perpendicular to the geanticlinal axis. In addition to the control by the direction and the rate of the movement, the position of the fault planes is determined by a great many other factors, e.g., by stratification and by the composition and distribution of the rocks near the surface. Leaving out of consideration those local areas within which the anticlinal axis shows an important pitch, the morphological aspect of the surface will be controlled chiefly by the more or less horizontal transverse faults, the gaping transverse fractures, the more or less longitudinal faults, and the gaping longitudinal fractures. We are here considering those regions only of the geanticlinal surface where the faults, through their more or less equal position HORIZONTAL MOVEMENT OF GEANTICLINES 563 and their more or less equal direction of movement, bring about considerable alterations in the broad outlines of the morphological structure. Zones of constant lithological characters will generally be separated near the surface by planes which are parallel to the geanticlinal axis. If these planes are more or less vertical, this will chiefly influence the distribution of the vertical longitudinal fractures and the longitudinal faults. If these planes are prin- cipally more or less horizontal, this will chiefly influence the dis- tribution of the faults along horizontal planes, but they will be of little importance for the major morphological structure and will here be left out of consideration. Whether these planes are nearly vertical or nearly horizontal, the lithological character is of little importance for the distribution of the transverse faults and frac- tures which strongly influence the morphology at the surface of the geanticline. Thus we find that the outline at the surface is mainly controlled by the direction and the rate of the crustal move- ments in so far as the transverse fractures are concerned. OLDER FOLDS CUT OFF BY THE PRESENT COAST LINE The surface and the deeper parts of moving geanticlines will generally not move in the same direction and at the same rate, because: 1. The intensity and likewise the direction of the forces which cause the movement near the surface will generally be different from those which obtain at greater depth. 2. The transmission of directed forces will decrease from the surface to the zones of higher plasticity at greater depth. If the forces which cause the movement are deep-seated, and the crust near the surface does not respond to the direct influence of the compressional or tensional stress, the displacements near the surface will be the result of the movement at greater depth. In forming a judgment on the genesis of fractures and folds this should be borne in mind. A result of the difference between the movements at greater depth and those near the surface is that, if at greater depth the movement has a horizontal component, those points which were originally on the same vertical line will in a later stage of evolution of the geanticline form an irregular curve, the form of which will 564 H. A. BROUWER depend upon the direction and the rates of movement at different depths. If a geanticline is elevated above the sea, the deeper- seated parts will gradually be uncovered by erosion and the surface of the geanticline will in time consist of rocks which were in the zone of flow during an earlier stage of the mountain-building process. As they are approaching the earth’s surface, the rate and the direction of the motion may differ more and more from those at greater depths on the same vertical line. That older folds terminate abruptly against the present coast lines is a phenomenon which is well known from Japan and from several islands of the East Indian Archipelago (Fig. 1). Particu- larly on Ceram this fact is very strikingly exemplified. In the Gas oy ws Fic. 1.—Older folds terminating abruptly against the present coast lines of the island of Ceram. (East Indian Archipelago.) Scale 1:3,000,000. __.... Approximate Tertiary strike. greater part of the island the strike of the Tertiary mountain range is NW.-S.E., whereas the present coast line has for the middle part an east-west direction, so that the ridges of the high moun- tains terminate abruptly near Taluti Bay on the south coast and near Savai Bay on the north coast. Similar facts have been explained by von Richthofen™ as a result of tensional stress on a large scale, and he believed that the mountain arcs of eastern Asia, although bearing a great resem- blance to the Alps and the Himalayas, have been formed by ten- sional, and not by compressional stress. Various authors have pointed out that this conception is not exact, and particularly because the fractures resulting from tensional stress are generally straight, whereas the ranges which lie to the eastward of the « F. von Richthofen, “‘Geomorphologische Studien aus Ost-Asien, IV,”’ Sztzungsber. der Berlin. Akad. der Wiss., XL (1913). HORIZONTAL MOVEMENT OF GEANTICLINES 565 Asiatic continent are arcs which present their convex sides to the oceanic areas. ‘The tension hypothesis of Von Richthofen has been applied by some authors to the East Indian Archipelago, but the numerous fractures which without doubt exist near the surface can be explained in a simpler manner by the action of compres- sional stress. It is not necessary to distinguish two periods of folding with different directions of the compressive forces, if we have regard for the fact that the older folds are cut off by the present coast lines. If the strike of the older folds is independent of the outlines of the present rows of islands, this may be in part a result of a change in the direction of the compressive forces; but it can be entirely a result of the fact that the folds which now appear at the earth’s surface have been formed in a much earlier stage of evolution of the geanticline, and that during their elevation the horizontal com- ponent of the rate of movement was different for neighboring parts of the geanticline, while the transmission of the directed forces has increased and the intensity and the direction of the forces has changed, whereas at greater depths the plastic deforma- tion has continued. GROUPS OF SMALL ISLANDS WITH HIGH REEFS In many rows of islands the breadth of each island is in direct proportion to the amount of elevation. In the Timor-Ceram range the long and broad island of Timor shows elevated reefs at the altitude of 1,300 m. in its central part, whereas in the short and much narrower island of Rotti elevated reefs are known at an alti- tude of but 470m. This will generally be true wherever the vertical motion prevails. ‘The increase in breadth results from the fact that the vertical component of the rate of movement has generally been in the same direction near the coast as it has near the axis of the geanticline. If the distance of the geanticlinal axis from the coast line be considerable, the vertical component of the movement need not be the same for longitudinal and for transverse coasts. The length of the island may still increase though the breadth decreases, or both may decrease and the island get shorter and narrower, while the top is still moving upward. However, if the geanticline shows 566 H. A. BROUWER a normal evolution, high reefs will always be found on large islands. If this is not the case, and if adjacent small islands show ele- vated reefs at high altitudes, this points to the existence of frac- tures. This case is illustrated by the islands of the Babber group (Fig. 2). Some fine specimens of terraced islands are found in this group. In Babber the uppermost elevated reefs are found at an altitude of 650 m.;* the small island of Dai with a steep coast has fif- teen terraces, the highest at 620 m. above sea-level; the small island = of Dawera has probably six- oooh, ie teen terraces; and Daweloor é lawera, 328 has fourteen entirely covered ee. “Shaweloorl with reefs. On Dawera the Baier es highest point is at an alti- ~t : tude of 328 m., and on Dawe- IMaseta, loor of 280 m. On Wetan, i which also consists entirely ee ne of upheaved reefs, there are “Cag six or seven terraces in the Fic. 2.—The islands of the Babber group. (Southeastern Malay Archipelago.) Scale 1:3,000,000. 320, etc., altitude of the upper- most elevated reefs in meters. 200, 500, 1,000, submarine contours in meters. southern part with a maxi- mum altitude of 320 m. Wetan is separated from Babber by a narrow and deep strait without reefs.’ Kisser, a small island of the Sermata group shows the same char- acteristics, having a fine terraced appearance with the highest reefs at an altitude of 147 m., though in the neighborhood of its coasts the sea bottom falls off rapidly to great depths. THE EVOLUTION OF PARALLEL ROWS OF ISLANDS AND LONGITUDINAL FRACTURES STUDIED IN THE PROFILE If we consider the evolution of geanticlines in a direction parallel to the geanticlinal axis, we find long and high islands where they are highest, and small and low islands at the depressions of the tF. A. H. Weckherlin de Marez Oyens, ‘‘De Geologie van het Eiland Babber,” Handel. v. h. XIVe Nat. en Geneesk. Congres 1913, pp. 403-68. 2R. D. M. Verbeek, op. cit., p. 458. HORIZONTAL MOVEMENT OF GEANTICLINES 567 axis. In the present-day stage of mountain-building this fact is illustrated by the Timor-Ceram row of islands, where a well-marked culmination occurs in the central part of Timor and well-marked depressions are found to the east and to the west of it. Secondary culminations and depressions are also found. Sometimes two more or less parallel ranges of islands have the same direction as the geanticlinal axis. An example in the East Indian Archipelago is supplied by the islands of the Tenimber group, where the row which includes the main island Jamdena is a ellesaaaty a fa) Maru2io 9 25 Laibobary rr Cee Wotanmic? Wulteru SelujsoCD (A) Ga) Sjer ray yo ay & Fic. 3.—I, The islands of the Tenimber group. (Southeastern Malay Archi- pelago.) Scale 1:3,000,000. 225, etc., altitude of the uppermost elevated reefs in meters. II, The axes of the two secondary geanticlines (schematic representation). accompanied by another row including the islands Selu, Wuliaru, Wotar, Laibobar, Maru, and Molu. The latter row differs from that of Jamdena in that it consists of smaller islands, although the elevated reefs are known at higher altitudes. On Wotar they are found at an altitude of 225 m., whereas on the main island Jamdena of the southern row the greatest height is at most r50m. ‘The reef cap, which covers Jamdena nearly continuously, is asymmetric, rising gradually from the northwestern coast in the direction of the main watershed of the island and thence descending rapidly toward the southeastern coast. I have explored portions of the coast of 568 H. A. BROUWER the gently sloping northwestern part of this asymmetrical geanti- cline and found drowned river valleys which were observed far inland from the coast. Thus the upheaved island Jamdena is separated from the row of upheaved islands to the northwest by a zone which is covered by the sea, and in which during the youngest evolution of the geanticline positive movements have prevailed. These facts can be explained in much the same way as has been done by Escher’ for the group of islands southeast of Celebes which are known as the Tukang Besi Islands and which consist of four rows. ‘Two of these rows consist of islands with elevated reefs which mark the anticlinal axes, whereas the two remaining show barrier reefs and atolls which mark the synclinal axes (Fig. 3). === FF Fic. 4. One of the possible evolutions of two parallel rows of islands, of which different phases are represented in the southeastern Malay Archipelago (schematic representation). IV, Stage with elevated central basin. We suppose that the geanticline at the Tenimber Islands is developing as two secondary geanticlines with an intermediate secondary geosyncline. The greater breadth of the islands in the southeastern secondary geanticline (although the reefs are not elevated to higher altitudes) may be the result of the prevalence of horizontal movements which caused the development of the asymmetrical reef cap (Fig. 4). 1B. G. Escher, “Atollen in den Nederlandsch Oost-Indischen Archipel: De Riffen in de Groep der Toekang Besi Eilanden,” Meded. Encyclop Bureau, Afl. XXII (1920). HORIZONTAL MOVEMENT OF GEANTICLINES 569 The further evolution of the geanticline can take place in differ- ent ways. If we suppose that in the next stage the plastic defor- mation of the northwestern secondary geanticline at greater depth causes chiefly a movement in a horizontal direction, the region of strongest upheaval will be displaced to the southeast. We may suppose that the rows of islands move in the direction of Australia, which for our considerations is the same as if Australia moved in the direction of the row of islands. Hobbs‘ has pointed out that ‘mechanical difficulties disappear if the principal active forces in- volved in the folding of the Alps are considered as directed from the northwest toward the southeast. So far as our general con- clusions are concerned, we may consider these movements as rela- tive and not as absolute. The upper parts of the secondary geanticline do not move at the same rate and the higher parts of the folds were originally above the downward-moving secondary geosyncline. In a later stage of evolution these may be above the rising northwestern secondary geanticline and will be elevated above the sea. Though differing in details, the geanticline of Timor may repre- sent a later stage of geanticline evolution than the Tenimber Islands. In Pliocene time the geanticline near Timor was sub- jected to prolonged denudation and almost entirely disappeared below the sea. The crustal movements resulted in the develop- ment of two geanticlines and an intermediate, in part subdivided, geosyncline (cf. Molengraaff, op. cit., p. 694), which became throughout fairly well filled by an accumulation of late Tertiary sediments deposited during a period of slow subsidence. Flexures and faults of considerable horizontal extent occur in the limbs of the geosyncline, which have caused the Pliocene strata within the basin to become bent abruptly upward near the edges. These longitudinal flexures and faults, which are essentially the same phenomenon, are the surface expression of an earlier, more plastic _ deformation at greater depth. Reefs and other littoral deposits spread over a great area, and after a certain period of evolution a great portion of Timor must have been covered by a sea full of ™W. H. Hobbs, “Mechanics of Formation of Arcuate Mountains,” Journal of Geology, Vol. XXII (1914), p. 85. 570 H. A. BROUWER coral islands and reefs, from which the islands emerged which are now the higher mountain groups of the present much enlarged island. A similar stage of evolution is now to be observed in the same range of islands more to the east. The islands of the Sermata group clearly illustrate the movements of reefs in the period of develop- ment of the geanticline in which only its highest parts emerge from the sea as a group of smaller islands. The island of Luang has an altitude of 260 m. and, according to my observations, is built up entirely of Permian rocks. Together with two small islets at its southeastern extremity, it is fringed by a very broad reef, extending far to the east in the direction of Sermata and far to the west as well. Green islets far from the north coast, and barren, dry portions far from the south coast, mark the limits in northern and southern direction; beyond them the sea floor declines rapidly. Luang as well as the two small islets close to it rise up steeply from this broad reef, and no trace of elevated reefs was detected; the islands impress us as having originally formed one continuous whole and as having been separated by a positive movement, which may also account for the formation of the broad encircling reef. In its eastern part the island of Moa consists of a low, very broad plateau of coral limestone, which rises scarcely more than 10-20 m. above the sea. From this plateau rises the steep Kerbau Mountain to an altitude of 4oom. Elevated reefs are lacking on the slopes of this mountain, and if the eastern part of Moa were a little lower, this region would present an aspect similar to that of Luang. The Island of Lakor, between Luang and Moa, consists of a low coral plateau, and Meaty Miarang forms the southern part of a large atolliform reef on the northern part of which lie the two low Ukenaé Islands. To the east of Luang and to the west of the eastern part of Moa the reefs are elevated to much greater altitudes and the group of the Sermata Islands shows a well-marked depression of the geanticlinal axis of the Timor-Ceram row. ‘This part is much disturbed by transverse fractures and no sufficient data are available for judging whether the submersion observed on some islands is the consequence of the pitch of the geanticlinal axis only, or whether this region has passed, or will in the future HORIZONTAL MOVEMENT OF GEANTICLINES 571 pass, through a stage with a secondary geosyncline between two secondary geanticlines. After the Plio-Pleistocene reefs had been formed, a general elevation of the island of Timor took place. The elevation of the land has been somewhat greater at the edges of the secondary geosyncline than in the geosyncline itself, but the general movement resulted in the formation of a large anticline with the highest ele- vated reefs in the central part of the present island. In this latter stage of evolution the horizontal movements near the surface may have had a much smaller rate of movement than those at greater depth, while the central basin was gradually upheaved above the sea. The horizontal movement at greater depth may have pre- vailed in one of the secondary geanticlines only, but this is not a necessary condition. In our Figure 4 one of the possible modes _of upheaval is represented. DIFFERENT TYPES OF GEANTICLINAL MOVEMENT The movement of a geanticline can be broadly described in the first place, in terms of the movements of the projections of the geanticlinal axis on the horizontal plane and on a vertical plane approximately parallel to the part of the geanticlinal axis under consideration. It is next of importance to take note of the move- ment of the section of the surface of the geanticline with a vertical plane at right angles to the geanticlinal axis. At the beginning of the movement we consider the geanticlinal axis to be a straight line; in a later stage this line will not be the geanticlinal axis, but for an approximate judgment this method is sufficient. ‘The pro- jections would undergo no changes in form if the geanticlinal axis was displaced parallel to itself. In general the vertical as well as the horizontal projection will develop a curved form. Some general types are given in Figure 5. In Diagram I of Figure 5 the differences of plasticity and rate of movement between the surface and the deep-seated parts will have an influence on the development of longitudinal fractures only. The deformation of the sections perpendicular to the geanti- clinal axis will be influenced by the place, the speed, and the duration of these fracture movements. 572 H. A. BROUWER In Diagram II of the same figure the same considerations apply. The bending of a; will be much less than that in the figure, and the distinct traces of transverse fracture movements on the islands will disappear rapidly through erosion, although they may be percep- tible near the transverse coasts. In Diagram III of the figure more or less longitudinal fractures may develop, which in connection with the deformation of the I z ZZ Le on ———— a , QQ a —— —— & re ea Senet A Be Fic. 5.—I, Displacement of the geanticlinal axis parallel to itself. II, Hl, and IV, Displacements in which the vertical or the horizontal projection, or both, have obtained a curved form. f= ai a, See? ee? : 4 a oe ee g Fic. 6.—Deformations of the horizontal and vertical projections of the geanticlinal axis neglecting any displacements parallel to themselves. sections perpendicular to the geanticlinal axis will be more or less important. ‘Transverse fractures may be observable especially at the straits between the different islands of a row. Diagram IV is a combination of Diagrams II and III. If in Diagram II, a: has one or more bending-points, which is equivalent to the development of transverse folds normal to the geanticlinal axis, then the place, rate, and duration of the trans- verse fracture movements near the surface may be strongly influ- enced by these folds. The same considerations are applicable to IITaz, and to [Va; or IVa, or to both of them. HORIZONTAL MOVEMENT OF GEANTICLINES 573 If we consider Diagram IV, the more general type of defor- mation, supposing that the horizontal and the vertical projections of the geanticlinal axis have an equal number of bending-points, then two different types can be distinguished according as the bending-points of the horizontal and vertical’ projections alternate or do not (Fig. 6). In this figure the combinations p-r, g-s, g-r, and p-s are different curves in space to which the originally rectilinear geanticlinal axis a has been distorted. The displacement of the geanticlinal axis parallel to itself and the distortion of the sections perpendicular to the geanticlinal axis are left out of consideration. The bending of p and q is much less than that which is shown in the figures. It would be more important if a strong compression had been acting in the direction of the geanticlinal axis from which would result a deformation to transverse folds normal to the geanticlinal axis. In this connection it is necessary to consider the geanticline over sufficiently long distances to obtain a judgment concerning the deformation of the vertical projection of the geanticlinal axis. APPLICATION TO THE TRANSVERSE FRACTURES OF THE TIMOR-CERAM ROW OF ISLANDS If considered over large distances it might seem that the geanti- cline, Sumba-Rotti-Timor-Sermata Islands, represents approxi- mately TypeI. The uppermo:t elevated reefs are found in Central Timor at an altitude of 1,300m.; in West Timor, southeast of Kupang, they are at a height of 500 m., on Rotti at 470 m., and on Savu at 300m. In East Timor the altitude is estimated at 600 m., on the islands farther to the east such reefs are known at altitudes of 140 m. on Letti and of 20m. on Lakor, while on Luang no ele- vated reefs are found (cf. also Fig. 7). , Thus the part, Sumba- Savu-Rotti-West Timor of the geanticline, would represent approxi- mately I p-r, and the part, Central and East Timor-islands farther to the east, would represent I q-s. If considered in detail the deformation is much more compli- cated. The deformation of the vertical projection is very slight and in many cases it is not exactly known. If the motion of the geanticlinal axis parallel to itself be neglected, this projection 574 H. A. BROUWER nearly coincides with a, and the distinction between I a,-r, I a,-s and II a,-r, Il a,-s disappears. Between Rotti and West Timor (Fig. 7) II p-r may be represented, but the deformation of the horizontal projection is the only important one. The strait betweeh Timor and Rotti coincides with a bending- point of the horizontal projection of the geanticlinal axis. In seeking an explanation for the existence of this strait, we might suggest the pitch of the geanticlinal axis on both sides of the strait, while at the place of the strait the axis could disappear below sea- level. But if considered in detail, this explanation alone is not applicable. On Rotti we find between the main island and the peninsula of Landu a narrow strait which only recently has been & if Sar de y , I 43 POR 20 “OS CoP Sermatheiciatas \ Ee a 0 1 ye. k y ae Landu ¢ Va ee 2H Fic. 7.—I, Rotti, Timor, and the Sermata Islands. 470, etc., altitude of the upper- most reefs in meters. Straits and transverse dislocations are near the bending-points of II. II, Geanticlinal axis with bending-points between Timor and Rotti and between Timor and the Sermata Islands. . Rottl, yzo. filled up by a mud bank still inundated at spring tide. At both sides of the narrow strait high walls of elevated coral limestone occur, and during an exploration of the island I found a small isolated rock composed of coral limestone which emerges from the mud in the middle of the strait. These facts point to the existence of transverse gaping fractures formed by a movement with a com- ponent normal to the fracture plane. We have already mentioned similar facts in connection with the groups of small islands having high reefs. Another example of the same sort is found to the east of Timor (Fig. 7). Considering the large bendings only, a bending-point is located between East Timor and the Babber group; but if con- sidered in detail bendings of smaller amount may also be observed. We note a bending of the geanticlinal axis between East Timor and HORIZONTAL MOVEMENT OF GEANTICLINES 575 Letti of the Sermata group, and in the neighborhood of the bending- point we observe the northern, non-harmonic position of the small island Kisser which is covered by elevated reefs and surrounded by deep seas. ‘There is here again the evidence that bending of the geanticline at greater depth is accompanied by transverse fractures near the surface. The fractures which occur farther to the east and their connection with the sharp bending in the 200 m. contour line of the Sahul shelf has already been discussed in earlier papers. - Still another example is found between Ceram Citta, ee Se very striking irregular- 5 a SSN : ity in this portion of the a ge Ee SM 2 geanticline is the narrow an % “arp ae Manipa Strait, nearly e 5,000 m. deep between Ceram and Buru, here also near the bending- point of the horizontal projection of the geanti- Rue, Via JOE clinal axis. If this bending-point is not so clearly visible in the Fic. 8.—I, The deep Manipa Strait (+4800 m.) present topography for between Ceram and Buru. 1000, 4000, submarine p) ih Th Wat contours. IJ, Geanticlinal axis with strong trans- the reason that the 1rac- ors dislocations near the bending-point. ture movements are very strong, it may be inferred from the strike of the Tertiary moun- tain range. In West Buru and in the greater part of Ceram this strike is about NW.—SE., whereas in West Ceram and in the islands between Ceram and Buru it is E. NE., and NE. strikes also have been observed (cf. Fig. 1).2 Thus the Tertiary mountain range displays a considerable bending from Ceram to Buru. As we have pointed out, the strike of the folds and the overthrusts of the Tertiary phase of crustal movement, and =H. A. Brouwer, Joc. cit. 2L. Rutten and W. Hotz, “ De geologische Expeditie naar Ceram,” T'ydschr. Kon. Ned. Aardr. Gen., Vol. XXXVI (1919), 9° Verslag. 576 H. A. BROUWER the fractured and faulted crust near the surface of the youngest phase of the deformation are the result of different stages of the same process. As has been stated above, p and gq will nearly coincide with az, and the bending of # and q is much less than is represented in the figures. In the case of the Strait of Manipa a. compression may even have been acting in the direction of the geanticlinal axis so that the origin of the strait may in part be due to transverse folding. But no sufficient data are as yet available for an exact judgment on the problem of deformation in space. We have seen that in the large bendings of the geanticlinal axis a distinction between Types I and II can be made, though if considered for bending at relatively small distances, these two types are very similar for the reason that p and q nearly coincide with a,. In the Timor-Ceram row the following rule seems to be approximately applicable: Considerable transverse fractures near the surface of the moving geanticline coincide with bending-points of the horizontal projection of the geanticlinal axis. In most cases it is clearly observable that the fractures near the surface have been formed by a movement having an important component normal to the fracture plane, and that the fractures near the bending-points are the surface expression of differences in rate of movement of neighboring points in the horizontal projection of the geanticlinal axis. RELATIVE AND ABSOLUTE HORIZONTAL MOVEMENT If we neglect the displacement of the geanticlinal axis parallel to itself, as has been done, we find evidence only for the relative horizontal displacements of different points in the geanticlinal axis. The absolute horizontal movement may be considerable, but it cannot be inferred from the surface characters of the present geanticlines. If our interpretation of the evolution of the central basin of Timor is correct, important absolute horizontal movements must have taken place at greater depth, while the superficial parts moved at a slower rate. This conception agrees with the inter- pretation of the evolution of the Western Alps, as this has been HORIZONTAL MOVEMENT OF GEANTICLINES 577 demonstrated by Argand.t. Here likewise we see that in Mesozoic time geanticlines formed, separated by geosynclines, and that these have been moved in a horizontal direction. It may be that the southeastern Indian Archipelago will in the future arrive at the same stage as was long before reached in the Alps. As the hori- zontal movements proceed, the sea basins will narrow, and even- tually the masses of the deeper parts of the present rows of islands will be pushed over the present Australian continent and the Sahul shelf which extends its borders. For a judgment, whether the active force tending to produce movement is directed to the south- east or to the northwest, as would follow from the conceptions of Hobbs? and Wegener,’ no sufficient data are available. tE. Argand, “Sur l’arc des Alpes occidencales,” Eclogae Geol. Helv., Vol. XIV (1916), p. 179. . 2W. H. Hobbs, op. cit., p. 91. 3 A. Wegener, “‘Die Entstehung der Kontinente und Ozeane,”’ Die Wissenschaft, 1920. REVIEWS The Oil and Gas Resources of Kentucky. By WiILLARD ROUSE Juttson, Kentucky State Geologist. Department of Geology and Forestry, Frankfort, Ky. 1919. Pp. 630. This volume is a review and summary of the known oil and gas resources of Kentucky, found in strata ranging from the Ordovician to the Pennsylvanian. The “‘Calciferous’? (Lower Ordovician) con- tains a small amount of oil and gas, but it is doubtful if it will ever be important commercially. The “Trenton” formation (Middle Ordo- vician) is an important source of oil, and the Cincinnatian has yielded some. The “Niagaran” (Silurian) is productive, but the Onondagan formation (‘‘Corniferous,’”’ Middle Devonian) holds first place in the production of petroleum and natural gas in the state. The ‘Black Shale” (Upper Devonian) does not yield petroleum in commercial quantities, but some gas has been derived from it. The Waverly series (Lower Mississippian) contains oil and gas sands of importance. The St. Genevieve (=St. Louis limestone, Middle Mississippian) has not yielded petroleum on a commercial scale, but has yielded much gas. The Chester or Mauch Chunk formation (Upper Mississippian) is a good producer in eastern Kentucky. Both oil and gas occur in the Pottsville Conglomerate (Pennsylvanian). Neither petroleum nor gas is known to exist in formations younger than the Pennsylvanian, within the state. Roe Aae Descriptive Mineralogy. By W. S. BAytEy. Appleton & Co., HO. IOs FAA= prem, 8.5. To quote from the Preface, “The following pages are presented with the purpose of affording students a comprehensive view of modern mineralogy rather than a detailed knowledge of many minerals. .... The volume is not a reference book. It is offered solely as a textbook.” Bearing these statements in mind, the work is one which may be highly commended, especially since it does not invade an overcrowded field. As the title indicates, it is almost purely a descriptive mineralogy, lacking a discussion of crystallography, but containing material on the 578 REVIEWS Ts70)) composition, classification, formation, and alteration of minerals, as well as the principles and methods of blowpipe analysis. Appendices contain lists of minerals arranged according to their principal constitu- ents and to their mode of crystallization, a list of reference books, and an abbreviated “‘key to the determination of minerals.” The key is a device which classifies minerals according to*luster, streak, color, and hardness and gives merely the pages in the main part of the text, where the detailed descriptions of the minerals may be found. Perhaps the most noticeable defect is the paucity of photographs (less than forty), which probably accounts for the low price of the work. However, there are numerous drawings which remedy this deficiency to a large extent. Minerals are classified according to their chemical composition. The arrangement of the silicates, a most difficult problem, is especially worthy of favorable comment. The book should be of great value as a text for advanced work in descriptive mineralogy. De Jey Detailed Report on Webster County. By D.B. REGER, West Virginia Geological Survey, Morgantown, W.Va. 1920. 671-+xvi pages, 35 halftone plates, and 24 zinc etchings in the text, ac- companied by a separate case of topographic and geologic maps. Price, including case of maps, charges prepaid, $3.00. Extra copies of topographic map, 75 cents, of the geologic map, $1.00. Webster County contains the northward extension of the famous New River Coal group, as also the Kanawha group and the lower mem- bers of the Allegheny Series in its northern portion. RECENT PUBLICATIONS —HENpDERSON, C. W. Gold, Silver, Copper, Lead and Zinc in Colorado in 1918. Mines Report. [U.S. Geological Survey, Mineral Resources of the United States, 1918. Part I: 27. Washington, 1920.] Gold, Silver, Copper, and Lead in South Dakota and Wyoming. [U.S. Geological Survey, Mineral Resources of the United States, 1017. Part I: 11. Washington, 1o19.| —Hess, F. L. Cobalt. [U.S. Geological Survey, Mineral Resources of the United States, 1916. Washington, roro.] Cobalt, Molybdenum, Nickel, Titanium, Tungsten, Radium, Uranium, and Vanadium in 1917. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part I: 29. Washington, 1920.] —Hit, J. M. Arsenic, Bismuth, Selenium, and Tellurium in 1o1g. [U.S. Geological Survey, Mineral Resources of the United States, ro19.° Part I: 3. Washington, 1920.] Bauxite and Aluminum in 191g. [U.S. Geological Survey, Mineral Resources of the United States, 1919. Part I: 5. Washington, 1920.] —Ho.puHavus, Kart. Sobre alguns Lamellibranchios Fosseis do Sul do Brasil. [Monographias do Servico Geologico E Mineralogico do Brasil. Vol. II. Rio de Janeiro: Imprensa Nacional, 1918.] —Hopxins, O. B. Structure and Oil and Gas Resources of the Osage Reserva- tion, Oklahoma. T. 25 N., Rs. rr and 12 E. [U.S. Geological Survey, Bulletin 686-H. Washington, ror9.] —Jones, E. L. A Deposit of Manganese Ore in Wyoming. [U.S. Geological Survey, Bulletin 715-C. Washington, 1920.] Some Deposits of Manganese Ore in Colorado. [U.S. Geological Survey, Bulletin 715-D. Washington, 1920.] —Kartser, E. Bericht iiber geologische Studien wiahrend des Krieges in Siidwestafrika. [Abhandlungen der Giessener Hochschulgesellschaft, II. Verlag von Alfred Tépelmann in Giessen, 1920.] Der Eldolithsyenitlakkolith der Serra de Monchique im siidlichen Portugal. [Separat-Abdruck aus dem Neuen Jahrbuch fiir Mineralogie, Geologie und Paliontologie, Beilage-Band XXXIX (Festband Bauer), Seite 225-267. Stuttgart, 1914. E. Schweizerbart’sche Verlagsbuch- handlung. Ndagele and Dr. Sproesser.| Studien wahrend des Krieges in Siidwestafrika. [Sonder-Abdruck aus der Zeitschrift der deutschen Geologischen Gesellschaft, Band 72, Jahrgang 1920, Monatsbericht Nr. Bell 580 RECENT PUBLICATIONS 581 —Kartz, F. J. Abrasive Materials in t918. [U.S. Geological Survey, Mineral Resources of the United States, 1918. Part Il: 33. Washington, 1920.] —Kwnopr, ApotFr. A Geologic Reconnaissance of the Inyo Range and the Eastern Slope of the Sierra Nevada, California. With a section on the Stratigraphy of the Inyo Range, by Edwin Kirk. [U.S. Geological Survey, Professional Paper 110. 10918. Washington, ror9.| Geology and Ore Deposits of the Yerington District, Nevada. [U.S. Geological Survey, Professional Paper 114, 1918. Washington, | 1919.| —LarsENn, E.S., anp Livineston, D.C. Geology of the Yellow Pine Cinnebar Mining District, Idaho. [U.S. Geological Survey, Bulletin 715-E. (Pre- pared in co-operation with the Idaho State Bureau of Mines and Geology.) Washington, 1920.| —LEFFINGWELL, E. pE K. The Canning River Region, Northern Alaska. [U.S. Geological Survey, Professional Paper 109. Washington, r1919.] —LeITH, C.K. International Control of Minerals. [U.S. Geological Survey, _ Mineral Resources of the United States, Part I: B. Washington, ror19.] —LesHerR, C. E. Coal in 1918. Part A. Production. [U.S. Geological Survey, Mineral Resources of the United States, 1918. Part II: 27. Washington, 1920.| —LINDGREN, WALDEMAR, AND LouGHIIN, G. F. Geology and Ore Deposits of the Tintic Mining District, Utah. With a Historical Review by V. C. Heikes. [U.S. Geological Survey, Professional Paper 107. Washington, 1910.] —Liverpool Geological Society, Proceedings of the. Vol. XII, Part IV. Sessions the Fifty-Eighth, Fifty-Ninth, and Sixtieth, 1916-1919. [Liver- pool: Royal Institution, Colquitt Street, 1920.] —Liovp, E. R. Petroleum in 1018. [U.S. Geological Survey, Mineral ; Resources of the United States, 1918. Part II: 32. Washington, 1920.] —Liovp, E. R., anp Martuer, K. F. Structure and Oil and Gas Resources of the Osage Reservation, Oklahoma. T.20N.,R.11 E. [U.S. Geologi- cal Survey, Bulletin 686-J. Washington, 1919.] —Loso, B. O Museu Nacional durante o Anno de 1919. [Museu Nacional do Rio de Janeiro. Rio de Janeiro, Imprensa Nacional, 1920.] —Lovucuiin, G. F. The Oxidized Zinc. Ores of Leadville, Colorado. [U.S. Geological Survey, Bulletin 681, 1918. Washington, 1919.] —McCasxkey, H.D. Quicksilver. [U.S. Geological Survey, Mineral Resour- ces of the United States, 1916. Part I: 24. Washington, r919.] —McLennan, J. C. Report on Some Sources of Helium in the British Empire. [Canada Department of Mines, No. 522. Mines Branch, Bulletin No. 31. Ottawa, 1920.] —DE Marceriz, Emm. Atlas établi sous la Direction de, Enquete sur les Richesses Minérales du Nord-Est de la France et des Régions voisines. [Paris, 1918.] 582 RECENT PUBLICATIONS —MarsHatL, R. B. Results of Triangulation and Primary Traverse, 1911 and 1912. [U.S. Geological Survey, Bulletin 551. Washington, r910.] —MeEtnzer, O. E. Bibliography and Index of the Publications of the U. S. Geological Survey Relating to Ground Water. [U.S. Geological Survey, Water-Supply Paper 427. Washington, ro109.] —Munition Resources Commission, Canada. Final Report of the Work of the Commission, November, 1915, to March, 1919, inclusive. [Toronto, 1920.] —Norturop, J.D. Natural Gas Gasoline. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Washington, 19709.| —Ospson, C. C. Peat. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part II: 20. Washington, ror10.] —ParDEE, J. T. Geology and Mineral Deposits of the Colville Indian Reser- vation, Washington. [U.S. Geological Survey, Bulletin 677. Wash- ington, rorg.| —Philippine Division of Mines of the Bureau of Science. The Mineral Resources of the Philippine Islands for the Years 1917 and 1918. [Manila, 1920.] —Ransome, F. L. Our Mineral Supplies: Quicksilver. [U.S. Geological Survey, Bulletin 666-FF. Washington, 1o109.] Quicksilver; with a Bibliography by I. P. Evans. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Washington, rgr9.| —Ransome, F. L., BurcHarp, E. F., AnD GALE, H. S. Contributions to Economic Geology, 1917. Part I. Metals and Nonmetals except Fuels. [U.S. Geological Survey, Bulletin 660. Washington, 1919] POSITIONS OF ALL KINDS Never'was the demand so great for qualified teachers and }j’ specialists. For ten years we have given our time and -energy to this work. Write for our free literature. State tributions. fem Rvatbe, ‘Wiaseuny 7 a. Vol. I. No. 5 Fe a The Flora of the Eagle Creek Formation By RALPH W. CHANEY qualifications briefly. Co-operative Instructors’ Association Marion é ; : soc8 Indiana WEATHER CHARTS Prepared by J. PAUL GOODE - Professor of Geography in the University of Chicago The Eagle Creek formation’ is posed along the bottom of the olumbia River gorge on the Ore- © 20 side. It is the oldest forma- in the region. | The monograph ance a num- ber of drawings, tables, and nearly hundred halftones. Monthly weather chart, 15104 inches, 3 cents Climatic chart (yearly), 8X 10% inches, 13 cents aher, $1.00, postpaid $1. I 0 E “UNVERSITY OF CHICAGO PRESS ! Soca - ~ ILLINOIS The University of Chicaye Press. Chicago - - - Illinois "HE ORIGIN OF THE EARTH By THOMAS Cc. CHAMBERLIN Head of the Department of Geology, The Unwersity of Chicago \ é x ! - One of “The University of Chicago Science Series” xii-| 272 pages, 8vo, cloth; $1.75 net, postpaid $1.90 © FROM THE PREFACE . “In telling tie story of this search for the mode by which the eaeth came ‘into being, we have let the incidents that led the i inquiry on from one stage to nother fall in with the steps of the inquiry itself. It is in keeping with the purposes of this series of booklets that the motives which set researches going should have their place with the quests that arose from them. .... 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Source Book for the Bec Geograp 1 7 of North America By CHARLES C.COLBY Assistant Professor of Geosmanhyy#! ene University of Chistes The fundamental idea of the book is to a available to the ts tae material on this subject which is scattered widely through literature. — The course’ has been given for a number of years at the University of Chicago has resulted collection and organization of the material most suitable for the purpose. — ie The book contains short magazine articles and materials from Canadian, Mexi and United States government publications. The selection of the material has b collection of materials will make possible the re “af courses in| this i impo subject. 8vo, cloth; 410 soils $4.00, postpaid $4. 5 hats it Go, THE UNIVERSITY af CHICAGO PRE CHICAGO NUMBER 7 sEOLOGY © OT sae TPES abe “THE ne EDITED BY a THOMAS C. CHAMBERLIN AND ROLLIN D. With the Nera Collaboration- of ART WELLER, Invertebrate Paleontology ~ ALBERT J OHANNSEN, Pétrology ON s S. BASTIN, Economic Geology ’ ROLLIN T. CHAMBERLIN, Dynamic ‘Geology ASSOCIATE EDITORS \ CHIBALD. GEIKIE, Giese Britain : JOHN C. BRANNER, Leland Stanford Junior University R ES BARROIS, France RICHARD A, F. PENROSE, Jr., Philadelphia, Pa. CHT PENCK, Germany aii WILLIAM H. HOBBS, University of Michigan S REUSCH, Norway FRANK D. ADAMS, McGill University i DEGEER, Sweden , CHARLES K. LEITH, University of Wisconsin —_, EDGEWORTH DAVID, Australia WALLACE W. ATWOOD, Clark University V , Leland Stanford Junior University WILLIAM H. EMMONS, University of Minnesota WALCOTT, Smithsonian Institution ARTHUR L. DAY, Carnegie Institution . OCTOBER-NOVEMBER 1921 HE MaxINE TERTIARY oF THE WEST COAST OF THE UNITED STATES: ITS Bruce L. CLARK 583 INE OF PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY - FRANK LEVEREIT 615 *h . PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION AND MAGMATIC DIFFER- TIATION OF IGNEOUS ROCKS - HPL NR aha) ie tenia bet CER i OGL Oz SBSTIO! NS. AS TO THE: DESCRIPTION AND NAMING OF SEDIMENTARY gh ay ramet a Re ee te | AL fooling) 05g POLO TVONG 2) ne eee Ae ee inden aah a Opn THE UNIVERSITY, OF CHICAGO PRESS > CHICAGO, ILLINOIS, U.S. Ae ey J THE CAMBRIDGE UNIVERSITY PRESS, Lonpon THE MARUZEN-KABUSHIKI-KAISHA, Toxyo, Osaka, Kyoto, FUKUOKA, SENDAI THE MISSION BOOK COMPANY, SHANGHAI Re eas eg eg Ee eR aes le EDITED BY 1m : a ee THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY With the ete Collaboration of , ; STUART WELLER : reilh ALBERT plo SEN ete Invertebrate Paleontology ; Se Petrology EDSON S. BASTIN ; eee a a ROLLIN T. CHAMBERLAIN — ! Economic Geology The Journal of Geology is published by the University of Chicago at the University of Chi 5750 Ellis Avenue, Chicago, Illinois, semi-quarterly, on or about the following dates: February riod) May 1, June 15, August 1, September 15, November 1, December 15. {] The subscription pric per year; the price of single copies is 65 cents. 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Abounds with drawings, maps, illustrations, and colored insert maps xvitit+ 385 pages, r6mo, cloth; $2.50, postpaid $2.65 THE tie) Seance OF riage PRESS CHICAGO” - - ILLINOIS VOLUME XXIX NUMBER 7 THE POURNAL OF GEOLOGY OCTOBER-NOVEMBER 10921 THE MARINE TERTIARY OF THE WEST COAST OF THE UNITED STATES: ITS SEQUENCE, PALEOGEOGRA- PHY, AND THE PROBLEMS OF CORRELATION: BRUCE L. CLARK University of California, Berkeley INTRODUCTION Considerable work has been done on the marine Tertiary deposits of the West Coast during the past ten years, and some of the discoveries that have been made have greatly modified many of our previous conceptions. The purpose of this paper is to review some of the most salient facts concerning the stratigraphic _ divisions, paleogeography, and correlation of these horizons in order to give the reader some idea of the present status of this knowledge. The paper includes a correlation table of the marine West Coast Tertiary. The construction of such a table is a very difficult task, and it will undoubtedly be a good many years before a table can be made which will be satisfactory to everyone working in this field. None of the West Coast Tertiary horizons has been thoroughly studied: there is a notable lack of detailed mapping, and most of the faunas have been inadequately monographed. The West Coast Tertiary still offers some of the most important problems for stratigraphic and paleontological research in the United States, and if the reader can obtain from this paper some t Read before Geological Society of America, December, 1920. 583 584 BRUCE L. CLARK - idea of the problems involved, the task will have been well worth while. There are several factors, aside from the lack of a sufficient number of trained workers, that have hindered the progress of correlation of West Coast horizons. These factors may be con- sidered under the following headings: (1) temperature differentia- tion (2) geographical isolation, and (3) poor preservation. 1. It is well known that the marine faunas living on the Pacific Coast can be separated into distinct faunal and geographical provinces. In this respect the West Coast of North America is typical of the whole Pacific border. For example, the fauna found off the coast of Panama is very different from that living along the coast of southern California, and the latter has very little in common with that off the coast of Alaska, while faunas from some of the intermediate areas are almost equally distinct.t It is generally recognized that Pleistocene and Recent times mark one of the maximum periods of emergence of all the continents. While this is not true in so great a measure of all the periods of the Tertiary, it is well known that the North American continent was sub- merged only on its borders, and that during a large part of this time the Pacific and Atlantic oceans were disconnected. ‘The study of the Tertiary faunas along the coast discloses marked evidences of temperature differentiation; the faunas of the north having a more boreal aspect than those of the south. This differentiation was most exterme in the Pliocene and Upper Miocene, and it is undoubtedly because of this that there has been so much confusion in the past in the correlation of the deposits from various sections along the coast now referred to those horizons. There is good evidence of temperature differentiation during the Oligocene and Middle Miocene, and what is more interesting is that accumulated evidence seems to show that this differentiation of the faunas had its effects even as far back as Eocene times. «W.D. Dall, Summary of the Marne Shellbearing Mollusks of the Northwest Coast of America, Bulletin 112, United States Natural Museum (1921), pp. 1-213. 2 J. P. Smith, ‘‘Climatic Relations of the Tertiary and Quaternary Faunas of the California Region,” Proc. Cal. Acad. Sci., Fourth Series, Vol. IX (1919), No. 4, pp. 123-73. THE MARINE TERTIARY OF THE WEST COAST 585 2. The second factor, that of geographical isolation, was very probably an important one, and if so was the result of numerous partially isolated local basins of deposition. The sediments of the Tertiary were for the most part laid down in geosynclinal troughs which paralleled the present Coast ranges. ‘The number and position of these troughs has varied through the different periods and epochs of deposition. ‘There was therefore a condition similar to that which existed in the Appalachian geosyncline dur- ing the Paleozoic. Great thicknesses of clastic sediments, in aggregate exceeding 40,000 feet, were deposited on the West Coast during Tertiary time. These Tertiary basins existed either as large embayments or long inland seas, some of the latter of which were comparable in size to the Mediterranean and were probably nearly as well separated from the main ocean basin. ‘These con- ditions produced marked local environments, with corresponding local changes in the faunas. It is very probable that the faunas in each basin derived certain peculiar characteristics due to isolation alone. 3. Still another factor that has brought about difficulties in correlation in the West Coast Tertiary has been the rather general poor preservation of the fossil material. The Tertiary beds have been extensively folded and tilted, and this deformation has resulted in the leaching of the original material of the shells, especially in the sandstones and shales. Intensive collecting will in time remedy this difficulty as well as bring to our knowledge a larger number of localities where the fossils are in a better state of preservation. In presenting a correlation table of this kind, one of the first things that will be asked is the author’s point of view in attacking the problems. The point of view accepted is that diastrophism is the fundamental basis for differentiating geologic divisions. In other words, the divisions recognized in this paper have been made on the basis of stratigraphic breaks which are believed to be more than local. It is important to note that every stratigraphic unit thus recognized is also represented by a distinctive fauna. The paleogeographic maps presented in this paper are not accurate in detail and will undoubtedly be modified by future work. The present knowledge of the geology of the Coast ranges 586 BRUCE L, CLARK does not enable one to show the exact location of the shore lines of all the seas that have occupied this general area. However, it is believed that the plates show the approximate distribution of the seas and will give the reader some conception of the location of the most important land masses, the degree of isolation of the basins, and the present known extent of the Tertiary horizons in California. TERTIARY DIVISIONS There are at least five major divisions of the Tertiary of the West Coast which in the writer’s estimation might be recognized as representing true periods. Each one of these five major divisions is composed of more than one epoch of deposition. Each epoch is represented by distinct faunas which lived in distinct seas. The deposits belonging to each of the major divisions will be referred to as a “‘series’’; thus, the Eocene, Oligocene, Lower- Middle Miocene, Upper Miocene, and Pliocene series. To the deposits of each epoch of deposition the term “group” has been applied. The term “formation” is reserved for the lithologic member within the group. EOCENE During the Eocene period of the West Coast there were at least three epochs of deposition, as indicated in the correlation table, and there is a suggestion that there were four and possibly five. At the present time, however, only three distinct stratigraphic divisions have been definitely separated. ‘These are the Martinez (Lower Eocene), Meganos (Middle Eocene), and the Tejon (Upper Eocene). Crustal movements of considerable magnitude separated these epochs. In the region of Mount Diablo, middle California, there is a difference in dip and strike between the Martinez and Meganos groups. Over large areas along the coast where beds of the Meganos and Tejon deposits occur in contact, there is an angular unconformity separating the two. ‘The marine faunas of these three divisions of the Eocene differ greatly from each other, further substantiating the stratigraphic evidence of marked OF THE Catrounts Const VAscrs saccos mcr PLIOCENE MARGARITA Crespo Saw Panto Urere Miocene ‘axes TeMnLOe Lowne Miocenr VAQUEROS | OL00CRNE Saw Lowenro Sew North of Mount Diablo ONDEDAN® to! Mocet Diasto Restos Santa Cuz West of Mount Diablo Cartium meckionum Gabb nodomas skamboni rinowx yor* aa! HANTA MARGARITA A Wadia tumidus Pesos raymond Clark Trophon fonderorum Gabb) crn Dosinia merriamk Clark Modictus gsbbi Clark Calypiraes diadenst Seat pabbl Rémond Santa Margarita and Clee! KIER Cordium Lerensanwm Aroold Acila cettyiburgenits Reagan aso" C and tol wis Clark pachardl Clark Stimpson Warm Water Fauna Pecten kealyi Pecten exeni Aroold Dendraster cidtri Rémond Dendraster perrind Weaver SANTA MARGARITA Pecten crassicarde Conrad Pecten raymondi Clarke Peten pableansis Conrad Astrodapiis tumidus SANTA MARGARIT Astrodapris whitneyil imond Astroda pris antiselli Connd Re soo" Astrodapris whitneyih ss and sh Rémond 1250" CHEERO Peten crassicarde Connie Palen raymendi Clarke Peclen wesverk Clark Seulella abhi Rémond ‘Aitrodapsts clerbornrls Kew meiones Pecten raymondi Clark, nh. Pecten tolmant Wall te mbrose Astrodapsis brewerianur Rémon Acanihina sinuatim ) ayco" ry "Oursin Sandstone" Arnold “Claremont Shale" “Sobrante Sandstone”? Pecten andersoni Arnold Ostres titan 0. var? Acasoma barkerianum Cooper Turritella ocoyana Conrad Pecten peckhami Gabls Pecten peckhami Gabb sooo" $000! weand sh organic shale 3790" ss, eh, and cons! BAN LoRnNz0 Acita dali Arnold Conlium lorensanum Arnold aE fine! tuts Fusinus hecoxt “SAN JAON Mel age phonas SNicole 6 Ancila Sokit Gabb Aras ravidum Gabb} Puriewr Aeonti Aroold sro" s asoo" ss and ah BUTANO (Oligocene ?) Dendraster orecememsis Dendroster interlinestus TEMDLO TEMDLOR deo Shale" “Monterey Shale" lambre Sandstone” Pecten andersoni Arnold “Tee Shale" Agavoma santacrusana G Chiene ecurit Shemard Nastia moramians Martin Turritella ocoyana Conrad — non} Solariuen lorencana Arnold MARINE TERTIARY OF THE WEST COAST Dorinis jacalitosensis Attrodapsis radite-| (aps Gott Resell Arai Iumidus Aitrotapiis antiselli Conrad Aitrodapris ormatus Kew 3900! TEMBLOR “Salinas Shale" Pecten andersoni Arnold Pecten peckhami Gabb Pecten discus Conrad Turritells ocoyana Conrad) s000") organic shales VAQUEROS v. ERO Turritella imesana Conrad) Turritelia inesana Conrad Pecten magnolia Conrad Pecten perrini Arnold $00" ey SAN LORENZO Telling lorensonum Arnold 2. JACAUITOS Pecten terminus Arnold) Derins fecalitsrensis . Los Axcetss ro Saxta | Gexrmaimes Steno Verrexa Demockr “ANA MOUNTAINS Gerson, Wasser Sexerr-Moowar Fernando Grup PONT ‘ Nom! “ Astrodapris tumidus mondi Turritela freya Nomland Rémond Astroda prix coalingensis Kew Astrodapiit major Kew no SADE CEDAR MOUNTADX eth ewart Valet ypeai Pi q pas nmadeare 2 Merriam é 2 2 TORTORIAN a a a i) z TEMBLOR 3 TEMBLOR TEMBLOR 7MoDELO? TEMBLOR TEMELOR TEMBLOR VIRGIN VALLEY MERVCHIFPUS f0N wry ‘ “Big Blue” (Sandstones and Turritela ecoyana Conrad; (Sandstone I Sole lnrd and Shales) “ Wahkiakum’? a a Joo" Shales) i ‘and Shale) | Pecten pecthami Gabty “Tiakeley” (in part) " F Pecten andersoni Chione temMorensis Area devincta Conrad Merychippus coalingensis mold Anderson Turritelia eregonensis ’ fauna ects peckhami Turritella eceyana Conrad Pecten XGA Chione temMorensis ‘clen andersoni Arnold nderson. Evstolinm petrorum MASEALE Chione temblorensis Conus oweni ippar (ome rs Anderson "Anderson, Sent are i ad Trophon hernensis Trophan kernensis ndenon Anderson A gasoma barkerianum Turritella ocoyana Cooper Conrad Turritella ocoyana Conrad] = Seutella merriami Pack “PUENTE Haniel tl 900" ry FITLire's || Stactoaine Waser) a Primitine MeryohpPar VAQUEKOS VAQUEROS vA‘ : [> Dt Peeten perrini Arnold Pecten respeensis Arnold | Pecten saprenni P Arnold Turritella teestna Conrad couaeeaaval x ai Turritella inesana Conrad) Pecten eppeenais Amold,| Peeten ses, it Arnold, var hyde ar A Pecten magnolia Conrad | Pecten aia! Conrad Turritela inesana Conrad | Turritella i) a Conrad} (Including Oligocene) ‘banksi Arnold | Scutella Joirbankit Arnold Jo0o" Jooo" ed aoe) eer ES SS a eee RATT AROORH) fanglomerate, ss, sh JS00! a. Lanenze ae AQUITANIAM ts wan "Aste Vai ncaa, and] ‘ " Acila pettyaburgenals ouarnae > zone Acila ¢eldysburgenris Reagan ie washington ana Dall aC XREVENIAGEN SHALE Leda lincolnensis Weaver. Mac it Weaver Gabby var, Turrtiele wr “eee Conrad Turritella diversilineata ran Se _KWEYENIAGEN “Stok (Shales and Sandstones) p bepress sone” t Opp, shales a San Emigdio Mountains nit Teeritelis forerensis jimeya Conrad Jurca rinmata Cabb | Tari N: biecha caloric Detecmsl ‘Cite : | 4 sth ooppaael Wit aale adie Mow “3H tative > As ee we? ; ey sch od | i ray 7 bi hha Ca, 7 We . ) aaa haa : Ng sf K i ' wy ia * — me PR Me frien tern eet cer fe nar i i i iy 4 i i H i! j ; i F 2] a2 ee nae ae biog in ; | | i any | Seah ne . a bie } j i nite =) ‘r } ; 1 Whit See ag foe al § ea * fl i; «7 { : THE MARINE TERTIARY OF THE WEST COAST 587 hiatuses.t Thus, on the West Coast, the Eocene period may be definitely stated to be made up of at least three epochs of deposition and should be recognized as a true period rather than as an epoch. Correlation of Eocene deposits.—The evidence for the correlation of the West Coast marine Eocene with that of the Gulf and East Coast provinces and through them with Europe is based upon the identity of species or the presence of closely related forms common to the two regions. The evidence appears to be much better for the correlation of the Meganos and the Tejon (Middle and Upper Eocene) than for the Lower or Martinez group. There can be little doubt but that during those epochs of time there was a direct connection between the Gulf of Mexico and the Pacific Ocean. Climate-——The climate during the Eocene of the West Coast was subtropical or possibly warm temperate rather than tropical. The arkosic character of the Meganos deposits, a character very general on the West Coast, strongly suggests that we are dealing with deposits which were derived from an arid coast, while Tejon sandstones, at most localities in California, are composed almost entirely of pure quartz grains, indicating humid climatic conditions at that time.” (Fig. 1.) Paleogeography.—As indicated by the paleogeographic maps, the deposits of the Martinez group (Lower Eocene, Fig. 2) were laid down in much more limited basins than those of the Meganos and Tejon groups. Apparently there were at least four separate basins in California, the connections between which were indirect. It seems very probable that when the faunas obtained from these four areas have been more fully described, we shall find that the geographical factor has caused considerable difference between them. =R. E. Dickerson, ‘Fauna of the Martinez Eocene of California,” Bull. Dept. Geol., Univ. Cal., Vol. VIII (1914), No. 6, pp. 61-180. B. L. Clark, ““The Meganos Group, a: Newly Recognized Division in the Eocene of California,” Bull. Geol. Soc. Am., Vol. XXVIII (1018), pp. 218-96; “Stratigraphy and Faunal Relationships of the Meganos Group, Middle Eocene of California,” Jour. Geol., Vol. XXIX (1921), No. 2, pp. 125-65. 2R. E. Dickerson, “Climatic Zones of Martinez Eocene Time,” Proc. Cal. Acad. Sci., Fourth Series, Vol. VII (1917), No. 7, pp. 193-96. 588 BRUCE L. CLARK The Meganos and the Tejon seas (Figs. 3 and 4) were some- what similar in outline. In middle California the deposits of these epochs were laid down in a great trough of which the present Great Valley of California is a remnant. The Meganos sea was the wider of the two Eocene seas that occupied this depression Fic. 1.—A key map showing the general distribution of positive and negative areas in California during the Tertiary. Of all the positive areas outlined, No. 6, the Santa Monica Mountain area, is the most problematical. (1) Sierra Nevada area; (2) Klamath Mountain area; (3) Coast Range area; (4) Tehachapi Peninsula; (5) Sierra Madre, San Bernardino, San Jacinto Mountain area; (6) Santa Monica Mountain area. and was connected with an east and west trough in southern Cali- fornia in the region of the present Santa Ynez Mountains. These two general areas of deposition existed throughout the Tertiary. They were bordered by areas or zones of uplift which have also been more or less permanent. East of the great north and south trough was the Sierra Nevada block which dates back to the Upper THE MARINE TERTIARY OF THE WEST COAST 589 Jurassic. To the west there was a positive area covering the present western side of the Coast: ranges of middle California. Apparently throughout the entire Eocene period this area to the west was a positive block; the absence of either continental or marine deposits in this area is the chief basis for the conclusion. Fic. 2.—Martinez (Lower Eocene) The smaller positive and negative areas which existed during the Oligocene, Miocene, and Pliocene in this western area were not differentiated during the Eocene. One of the most permanent positive areas of the Tertiary was that which existed in the region now occupied by the Tehachapi and San Emigdio Mountains (Fig. 1). This area formed the east— west peninsula which separated the northern from the southern basins. The extent of this peninsula varied considerably during the different epochs of deposition. The area now covered by the 590 BRUCE L. CLARK Santa Monica Mountains was apparently part of an early positive block bordering the east-west: trough mentioned above. To the north of this trough the region now occupied by the Santa Ynez Mountains constituted the westward extension of the old Tehachapi peninsula. Po Fic. 3.—Meganos (Middle Eocene) Some of these old positive areas were so persistent throughout the Tertiary time that they might well be given names, following, on a smaller scale, the example of Schuchert in his Paleogeography of North America. Certain of these positive and negative areas have been persistent throughout all Tertiary time, while others were formed at a later date. Also, it is worthy of note that the old positive and negative areas had the same trend as the present mountain ranges. THE MARINE TERTIARY OF THE WEST COAST 591 OLIGOCENE Accumulated evidence appears to show that there were at least two distinct epochs of deposition on the West Coast during the Oligo- cene. ‘The two epochs are represented by the Molopophorus lin- colnensis and Acila gettysburgensis zones of Dr. C. E.Weaver. They Fic. 4.—Tejon (Upper Eocene) will be referred to as the Lincoln and San Lorenzo.*. Recent field work of the writer has shown fairly conclusively that there were at least two distinct epochs of deposition in Oregon and Washington. He believes that this will prove to be the case in California when more detailed work on the stratigraphy of the Oligocene over wider areas has been completed. It is certain that the two faunas first tC. E. Weaver, ‘‘Tertiary Formations of Western Washington,” Wash. Geol. Surv., Bull. 13 (1916), pp. 1-21; “‘ Preliminary Note on the Paleontology of Western Washington,” Wash. Geol. Surv. Bull., 15 (1912), pp. 1-80; ‘‘Tertiary Faunal Horizons of Western Washington,”’ Pub. Univ. Wash., Vol. I (1916), No. 1, pp. 1-67. 592 BRUCE L. CLARK differentiated in Washington are also represented in the same sequence in California, and tentatively we may consider the Oligocene series as being made up of two distinct parts, referred to in the correlation table as the San Lorenzo series. The aggregate thickness of the marine beds of the Upper and Lower Oligocene of the West Coast exceeds 10,000 feet. A large part of these sediments consists of shales and shaly sandstones. Correlation.—The evidence for the correlation of the West Coast marine Oligocene deposits is indirect. No molluscan species or even apparently related forms have been recognized as common to the Oligocene of the West Coast and the Gulf province. ‘The faunal evidence at hand seems to show that after the close of the Tejon epoch (Upper Eocene) there was no direct connection between the Atlantic and the Pacific Coast basins. Dr. Ralph Arnold was the first to announce the presence of Oligocene in California. The type section of the San Lorenzo is in the Santa’ Cruz Mountains of the Santa Cruz Quadrangle, California. Dr. Arnold concluded that this formation is of Oligo- cene age because of its stratigraphic position between beds generally recognized as belonging to the Upper Eocene and Lower Miocene (Vaqueros) age. He observed that the fauna of the San Lorenzo appeared to have both Eocene and Miocene affinities.‘ Later studies of the faunas of the Lincoln and San Lorenzo horizons have borne out Arnold’s original conclusions.? At the time Arnold did his work the Lincoln horizon had not been differentiated. The fauna of this horizon shows a,much closer relationship to that of the Tejon (Upper Eocene) than to that of the Lower Miocene, while the fauna of the San Lorenzo horizon, equivalent to Weaver’s Acila gettysburgensis zone, has a Miocene aspect, a fairly large number of the genera and species being common to the two. =R. Arnold, “Tertiary and Quaternary Pectens of California,” U.S. Geol. Surv., Prof. Paper 47 (1906). J. C. Branner, F. G. Newsom, and R. Arnold, U.S. Geol. Surv., Folio 163, Santa Cruz Folio. 2B. L. Clark, “Occurrence of Oligocene in the Contra Costa Hills of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. IX (1915), No. 2, pp. 9-21; “San Lorenzo Series of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. XI (1918), No. 2, pp: 45-234. B.L.Clark and R. Arnold, “Marine Oligocene of the West Coast of North America,” Bull. Geol. Soc. Amer., Vol. XXIX (1918), pp. 297-308. ‘THE MARINE TERTIARY OF THE WEST COAST 593 Climate.—The temperature conditions during the Oligocene time were fairly uniform along the West Coast as far north as Alaska. The waters of the Lower Oligocene, judging from the molluscan fauna, were subtropical to warm-temperate, while those of the Upper Oligocene sea were more temperate.t Thus the fauna of the San Lorenzo horizon (Upper Oligocene) is more closely related to that living off the coast of California, Oregon, and Washington at the present time than it is to that of the Lincoln. Paleogeography.—The distribution of the Lower Oligocene deposits (the Lincoln horizon) (Fig. 5) corresponds closely to that of the Tejon (Upper Eocene). In California there was a long inland trough corresponding closely to, though somewhat wider than, the present Great Valley of California. The presence of great thicknesses of organic shales of the Kreyenhagen formation, from which the oil of the Coalinga field is derived, indicates that the deepest portion of the Lower Oligocene trough was along the western border of the present San Joaquin Valley. The distribution of the Upper Oligocene (San Lorenzo) is very different from that of the Lower Oligocene. In California there were two limited basins of deposition, one in middle California in the vicinity of San Francisco, and one in the region of the southern end of the San Joaquin Valley. A fauna referred to the Oligocene which may be Upper Eocene.— The fauna of the Tejon (Upper Eocene) of the West Coast, as has already been stated, can probably be correlated with the upper Claiborne horizon of the Eocene of the Gulf province, but whether or not there is a fauna on the West Coast that is equivalent to the Jackson horizon of that province can only be proved by further detailed study. ; The possibility of referring the Molopophorus lincolnensis zone, now considered Lower Oligocene, to the Jackson stage has been considered. Dr. C. E. Weaver has listed a number of species tR. E. Dickerson, ‘‘Climate and Its Influence upon the Oligocene Faunas of the Pacific Coast,” Proc. Cal. Acad. Sci., Fourth Series, Vol. VII (1917), No. 6, PP. 157-92. 2R. W. Anderson and R. W. Pack, ‘‘Geology and Oil Resources of the West Border of the San Joaquin Valley North of Coalinga, California,” U.S. Geol. Surv. Bull. 603 (1915), pp. 74-78. 4 594, BRUCE L. CLARK in the former horizon that are also found in the Tejon and the generic assemblages are notably similar. The Molopophorus lincolnensis zone, however, shows a closer relationship to the Acila gettysburgensis zone (Upper Oligocene) than to the Tejon, and subsequent work on the fauna of the Molopophorus lincolnensis Fic. 5.—Lower Oligocene zone has shown that there are fewer species common to the Tejon than Dr. Weaver supposed.’ There is still another fauna which may represent an Eocene stage higher than the Tejon. During the year 1912 a collection was made by Mr. F. M. Anderson and Mr. Bruce Martin, at that time curator and assistant curator in the department of paleontol- ogy of the California Academy of Sciences, from the Greise Ranch tC, E. Weaver, “Preliminary Report on the Tertiary Paleontology of Western Washington,” Wash. Geol. Surv., Bull. 15 (1912), p. 16; “Tertiary Formations of Western Washington,” Wash. Geol. Surv., Bull. 13 (1916), p. 167. THE MARINE TERTIARY OF THE WEST COAST 505 near the town of Vader in southern Washington. This collection came from beds unconformable on the Tejon, and below strata containing a typical Molopophorus lincolnensis fauna. The fauna was aescribed by Dr. R. E. Dickerson and consisted of forty-eight species, of which thirty-six were considered new and thirteen were determined as common to the Molopophorus fauna.t The writer has had the opportunity of making larger collections from the Greise Ranch locality, and with Dr. G. D. Hanna, present curator of the department of paleontology of the California Academy of Sciences, has re-worked the fauna listed and described by Dr. Dickerson. ‘The results of this work show quite conclusively that there is very little, if anything, in common between this fauna and that of the Molopophorus lincolnensis zone. The fauna at the present time consists of about seventy-five species and is very distinct from any other known fauna on the Pacific Coast. None of these species have been definitely determined as common to either the Tejon or the Molopophorus lincolnensis zone, and the stratigraphic position of the horizon renders it possible that these beds are equivalent to the Jackson of the Gulf province. MIOCENE The marine Miocene of the West Coast is divisible, both on the basis of stratigraphy and fauna, into two major series each of which contains minor horizons. The portion of the geological section referable to the Monterey series (Lower-Middle Miocene) contains two fairly distinct faunas and two epochs of deposition, at least in certain areas of the state of California. The lower of these two divisions is the Vaqueros group, sometimes referred to as the “Turritella inezana”’ zone. The upper division of the Monterey series is herein referred to as the Temblor group and is represented by the fauna of the “Turri- tella ocoyana”’ zone. The deposits of the Vaqueros Sea covered a much more limited area than those of the Temblor, and have not been found in Oregon or Washington. tR. E. Dickerson, ‘‘Climate and Its Influence upon the Oligocene Faunas of the Pacific Coast with Descriptions of Some New Species from the Molopophorus Lin- colnensis Zone.” Proc. Cal. Acad. Sci., Fourth Series, Vol. VII (1917), No. 6, pp. 157-92. 596 BRUCE L. CLARK The upper major Miocene division constitutes the San Pablo series, which is composed of three minor stratigraphic and faunal divisions, the Briones, Cierbo, and Santa Margarita groups. As will be brought out later, each one of these groups represents a distinct sequence of deposition and possesses a fairly distinctive fauna. Monterey series —Whether or not the Vaqueros and Temblor represent separate stratigraphic units has been the source of con- siderable disagreement in time past.t Nearly everyone, however, who has studied the fossils obtained from these beds has agreed that there are two fairly distinct, though closely related, faunas, one the fauna of the Turritella inezana zone, the other that of the Turritella ocoyana zone. Recent stratigraphic and paleontological work, the results of which are still unpublished,? appears to show that at certain locali- ties in California there were crustal movements of considerable magnitude between the deposition of the Vaqueros and the Temblor. The proper valuation of this hiatus is, in the writer’s mind, still an open question. ‘The faunas appear to be fairly closely related, and because of the obscure stratigraphic relations at various localities and the general similarity of the faunas the groups have usually been thrown together. The United States Geological Survey, in its more recent publications on Coast Range geology, applies the name ‘‘Monterey group” to these deposits, but the writer con- siders the Monterey a “‘series’’ because, at least in certain localities, it is composed of two epochs of deposition, the Vaqueros and the Temblor. Stratigraphic relations of the Monterey series to the Upper Oligo- cene.—There is no conclusive evidence that there were any great 1G. D. Louderback, ‘‘Monterey Series of California,” Bull. Dept. Geol., Univ. Cal., Vol. VII (1913), No. 10, pp. 177-241. F. M. Anderson, “‘Stratigraphic Study of the Mount Diablo Range of California,” Proc. Cal. Acad. Sci., Third Series, Vol. II (1905), No. 2, pp. 161-248. F. M. Anderson, ‘‘ Further Study of the Mount Diablo Range of California,” Proc. Cal. Acad. Sci. Fourth Series, Vol. III. 2 Mapping by Dr. Kew of the United States Geological Survey shows an important unconformity in southern California between the Temblor and the Vaqueros. Mr. Wayne Loel, formerly of Leland Stanford University, is working on a monograph of the Vaqueros. He believes that the faunas of the Temblor and the Vaqueros represent two distinct horizons. THE MARINE TERTIARY OF THE WEST COAST 597 crustal movements just previous to the deposition of the Vaqueros. However, that there was an important hiatus following the deposi- tion of the San Lorenzo is brought out by a comparison of the San Lorenzo and Vaqueros faunas. Very few of the species of the San Lorenzo (Upper Oligocene) have been found in the Vaqueros, _ while a very large percentage of the species of the latter horizon are common to the Temblor. It is this great faunal change between the San Lorenzo and Vaqueros that is most significant and indicative of one of the major breaks.t Correlation of the Temblor and Vaqueros.—As in the case of the Oligocene, very little direct evidence has been obtained for the correlation, on the basis of the invertebrates, of the divisions of the Monterey series with the Lower-Middle Miocene of the eastern province and Europe. ‘These deposits of the Monterey series were first referred to the Miocene by Conrad? as early as 1837. This determination was made chiefly on the general sim- ilarity of the generic assemblages to the faunas of the Atlantic Coast Miocene. Following Conrad, the beds here referred to the Monterey series were determined by Whitney and Gabb, both of the old California State Geological Survey, as Miocene. No attempt was made by these pioneers to recognize any sub- divisions in the Miocene. Beds now recognized as Upper Miocene (San Pablo) were called Pliocene by Whitney and Gabb. The first announcement of a correlation which gave a fairly definite position to the Temblor group appeared in a paper by Professor J. C. Merriam, entitled “‘Tertiary Vertebrate Faunas of the North Coalinga Region of California.”4 Previously the Temblor had been referred by some geologists to the Lower Miocene and by others to the Oligocene. In the region of North Coalinga «B. L. Clark, ‘San Lorenzo Series of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. XI (1918), No. 2, p. 105. 2T. A. Conrad, ‘‘Fossils from Northwestern America,”’ Geol. U.S. Ex. Exped., Vol. I (1849), App., pp. 723-29, Pls. 17-20; Proc. Acad. Nat. Sci. Phila., Vol. VII (1837), Pp. 441. 3W. M. Gabb, California Geological Survey: Palaeontology, Vols. 1 and IL (1864-69). 4]. C. Merriam, ‘‘Tertiary Vertebrate Faunas of the North Coalinga Region of California,” Trans. Am. Phil. Soc., New Series, Vol. XXII (1915), Part III, pp. 1-44. 598 BRUCE L. CLARK the land formation locally known as the Big Blue was interca- lated with beds of Temblor age from which good marine faunas have been obtained. The Big Blue was first considered by Arnold and Anderson? as a part of the Santa Margarita (Upper Miocene). Later mapping, however, showed that it is more closely connected to the Temblor (the so-called Vaqueros) than to the Santa Margarita of that section. In describing the fauna obtained from the Big Blue, Merriam Says: In terms of the vertebrate series of Western North America the fauna of the Merychippus zone in the north Coalinga region is clearly later than lower Miocene and not later than upper Miocene. The fact that the Big Blue comes in a section where the Temblor deposits are very thin, and we think are only the top of that section, makes it seem reasonable to believe that the Temblor deposits as a whole belong to the middle Miocene rather than to a part of the upper Miocene.3 A clue to the age of the Vaqueros deposits was obtained very recently by the discovery of land-laid deposits near the south end of the San Joaquin Valley which are intercalated with marine deposits of the Vaqueros age. Beds containing a Vaqueros fauna are found immediately below these land-laid beds, and the marine beds immediately above are believed to represent the same horizon. The announcement of the discovery of a vertebrate fauna obtained from these land-laid beds associated with the Vaqueros was recently made by Dr. Chester Stock.4 These land-laid deposits, referred to as the Tecuja beds, were tentatively correlated by Dr. Stock with the John Day horizon of Oregon. He reports the presence of the genus Hypertragulus, a form related to the early camels and deer. Hypertragulus occurs both in the Upper Oligocene and the Lower Miocene, but the species from the Tecuja beds seems more t The beds mapped as Vaqueros in the Coalinga field by the United States Geo- logical Survey belong to the Temblor horizon rather than to the Vaqueros. 2R. Arnold and R. Anderson, ‘‘Geology and Oil Resources of the Coalinga District of California,’ U.S. Geol. Surv., Bull. 396 (1909), p. 99. 3 J. C. Merriam, ‘‘Tertiary Vertebrate Faunas of the North Coalinga Region of California,” Trans. Am. Phil. Soc., New Series, Vol. XXII (1915), Part III, p. 20. 4 Chester Stock, ‘‘An Early Tertiary Vertebrate Fauna from the Southern Coast Ranges of California,’ Bull. Dept. Geol., Univ. Cal., Vol. XII (1920), No. 4, pp. 267-76. THE MARINE TERTIARY OF THE WEST COAST 599 closely related to the John Day (Upper Oligocene) form than to that found in the lower Rosebud (Lower Miocene) of the Great Basin region. While Dr. Stock has indicated the relationships of of the Tecuja vertebrate fauna to that of the John Day, he has also stated that it may occupy a position in the Tertiary transi- tional between Oligocene and Miocene. It seems to the writer that the stratigraphic and paleontologic evidence favors the Lower Miocene age of these beds rather than the Upper Oligocene. The most important evidence in favor of this last conclusion is that the invertebrate fauna of the Vaqueros as already stated, is very closely related to that of the Temblor, a very large percentage of the species being common to the two faunas. Some of the forms listed are types of considerable ornamentation and complexity and are known to have a fairly short geological range. On the other hand, the known fauna of the Vaqueros is very different from the known fauna of the San Lorenzo, and there is here a much greater faunal break than between the Vaqueros and the Temblor. In the section at the south end of the San Joaquin Valley, where the Tecuja beds occur, the Vaqueros rests directly and unconformably upon the San Lorenzo. . Climate.—The conditions of temperature during the Vaqueros and Temblor epoch seem to have been between warm-temperate and subtropical. The generic assemblages of the two horizons are very similar. The large lyropectens and dosinias are found in both, and a fairly high percentage of the species are common to the two horizons. This close relationship of the faunas indicates a similar temperature of the waters. One of the puzzling problems in connection with the origin of the Temblor deposits is the great thickness of organic shales that is found all along the coast of California and especially in the southern part of the state. In some localities these organic shales, a very large proportion of which are composed of the frustules of marine diatoms, have a thickness exceeding 5,000 feet. Diatomace- ous oozes in any considerable quantity are now only found in Arctic and Antarctic waters, and from this we might judge that these shales were deposited in cold water. However, the fossil molluscan faunas found in these shales, or closely associated with them, 600 BRUCE L. CLARK indicate a moderate temperature. Dr. J. C. Branner, in a paper read before the Cordilleran Section of the Geological Society of America, suggested an explanation of this apparent disagreement between the floral and faunal evidence.t The great thickness of diatomaceous shales in southern California is to be explained by the hypothesis that cold currents carried the diatoms southward along the coast and finally into the partially land-locked basins of southern California where they were killed by the change in temperature. The continuous supply from the north resulted in great thicknesses of deposits composed largely of the tests of these minute plants. This hypothesis may also be an explanation of the origin of the diatomaceous shales of the Oligocene and Upper Miocene. | Paleogeography—The Temblor deposits have much wider dis- tribution than those of the Vaqueros (Figs. 6 and 7) and are found on the eastern as well as the western side of the Coast ranges. On the eastern side from the vicinity of Coalinga northward these deposits are composed of coarse clastics, while to the west organic shales cover the larger part of the section. The comparison of the area covered by this sea with that which existed during Lower Oligocene time (when the Kreyenhagen shales were deposited), (Fig. 4) shows a marked change. As has already been stated, the Oligocene sediments were deposited in an inland north-south trough very similar to that which existed during the Eocene. The deepest part of the Oligocene trough was on the eastern side of the present Coast ranges, a very large part of the western side at that time apparently having been subject to erosion. On the other hand, the deepest part of the Temblor sea was on the western side of the present Coast ranges; the areas which had been land during the Oligocene were inundated, while to the east, where the Oligocene trough had been deepest, the strand-line deposits indicate shallow water conditions. At this time the interior Diablo range prob- ably formed an archipelago of islands. San Pablo series—The San Pablo series is recognized as the second major division of the Miocene. Like the San Lorenzo and J.C. Branner, “Influence of Wind on the Accumulation of Oil-bearing Rocks,” Proc. Thirteenth Ann. Meeting of the Cordilleran Section of the Geol. Soc. of Am., Bull. Geol. Soc. Am., Vol. XXIV (1913), pp. 94-95. THE MARINE TERTIARY OF THE WEST COAST 601 Monterey series, the San Pablo is divisible into minor units on the basis of stratigraphy and fauna. The faunal changes and discon- formable relationships of the beds indicate that the sea advanced and retreated three times in middle California, during this period. It is only in middle California that we find the complete sequence of the Upper Miocene series. The two lower divisions of the San Fic. 6.—Vaqueros (lower Monterey, Miocene) Pablo series, the Briones and Cierbo groups," have been recognized only in the general region of San Francisco Bay. Stratigraphic relationships.—In certain sections immediately east of or in the Salinas Valley, a distance of not more than too miles 1 The use of the term San Pablo for the Upper Miocene series of deposits on the West Coast makes it necessary to dispense with the term San Pablo within the group. The name Cierbo is therefore used in this paper in referring to the middle group of the San Pablo series. The type section of the Cierbo is in the south side of the Canada del Cierbo near Carquinez straits. Santa Margarita is a name in common use for the upper portion of the section in the southern part of the state of California and will be applied as a general name for the upper member of the San Pablo series. 602 BRUCE L. CLARK from the San Francisco Bay area, we find evidences of crustal movements between the Temblor and the Santa Margarita which have been described as mountain-making. In the Salinas Valley region it is not uncommon to find the difference in dip between the Temblor and the Santa Margarita as much as 30° to go°, together Fic. 7.—Temblor (upper Monterey, Miocene) with marked difference in strike. Dr. Arnold, in describing the movements that caused this unconformity, says: One of the most widespread and important periods of diastrophism in the Tertiary history of the Pacific Coast was that immediately following the deposition of the Monterey or lower middle Miocene. Its effects are visible from Puget Sound to southern California It is marked as much by readjust- ment, by local faulting and folding as by general movements of elevation and subsidence. In some regions the folding and faulting were intense, the greatest disturbances accompanying the uplift of the mountain ranges to an altitude of thousands of feet. In other regions low broad folds were formed during the THE MARINE TERTIARY OF THE WEST COAST 603 post-Monterey disturbance, and the strata were not upheaved to a great altitude. Faulting on a most magnificent scale took place along the earth- quake rift and certain other fault-zones, especially that in the Salinas Valley, and along these lines of displacement, masses of granitic rocks, which during the preceding epoch had been subject to little or no erosion, were suddenly thrust upward and left exposed to the ravages of streams that assumed the proportions of torrents in certain regions, as for instance adjacent to the Carrizo Plain in south-central California. The post-Monterey diastrophic movements in the Puget Sound province also produced sharp relief as is evi- denced by the coarse sediments immediately following the disturbance. The localization of movements during the period is exemplified at numerous locali- ties in the Coast Ranges. Throughout much of the coastal belt, and probably likewise in the interior, great volcanic activity took place during the middle Miocene, this being the last epoch of volcanism in the Coast Ranges, south of San Francisco Bay. It is interesting to note that only a comparatively short distance to the east of the southern Salinas Valley area, where the great unconformity between the Temblor and the Santa Margarita deposits is best seen, it has been very difficult to find the line separating these two horizons. Both the Santa Margarita and Temblor deposits to the southwest of the San Joaquin Valley are composed of organic shales, and in consequence of the diffi- culty in separating the two horizons the United States Geological Survey has applied the name Maricopa shale to the deposits as a whole.2, No marked stratigraphic break has been found in middle California between the deposits of the lower San Pablo series (Briones group) and those representing the Temblor. Here the beds of these two horizons are parallel, the chief basis for mak- ing the separation being irregular contacts and the difference between the faunas. In middle California, therefore, we have no evidence of crustal movements immediately after the deposition of the Temblor. Correlation.—Direct evidence for the correlation of the San Pablo series with the eastern and European sections is lacking. The writer has presented the evidence for the correlation of this tR. Arnold, ‘‘The Environment of the Tertiary Faunas of the Pacific Coast of the United States,” in Willis e¢ al., Outlines of Geology (1910), p. 241. 2 R. W. Pack, ‘Geology and Oil Resources: Sunset-Midway Oil Field, California,” U.S. Geol. Surv., Prof. Paper 116 (1920), p. 35. 604 BRUCE L. CLARK group ina former paper. The correlation is based upon an analysis of the molluscan fauna by the percentage method and the evidence afforded by the occurrence of vertebrates in beds immediately above and below the San Pablo. The following quotations are taken from the above-mentioned paper: The percentage of Recent molluscan species in the San Pablo of middle California as listed by the writer is 23 plus; as based upon the gastropods the percentage is only 11 per cent. If we use the percentages as applied to the east coast Neocene and if we can rely upon the equal refinement in the determination of the species, the San Pablo may be considered to be upper Miocene in age, possibly lower Pliocene. Probably the best evidence showing the age of the uppermost beds of the San Pablo of middle California comes from vertebrate material obtained in the fresh-water beds which in middle California overlie unconformably the San Pablo group. This material was described by Professor J. C. Merriam in his paper ‘‘ Vertebrate Fauna of the Orinda and the Siesta Beds in middle California.”* His conclusions as to the age of these beds as shown by the vertebrates are as follows: “‘The mammalian remains known from both the Orindan and Siestan up to the present time all represent forms such as might be expected in the late Miocene or in the earliest Pliocene, but it will be neces- sary both to have better material from the Orindan and Siestan and to have well known faunas of western Miocene and Pliocene for comparison before the last word on the age determination can be pronounced. ‘‘Considering the indefiniteness of all the factors concerned, one would not seem justified in being more definite than to state that the Orindan and Siestan faunas are near a late Miocene stage. When the faunas of the two formations are better known, it may appear that more than one stage is represented.”? The reader will remember from the discussion of the age of the Temblor that the vertebrate fauna obtained from the Big Blue formation, which is apparently intercalated with the Temblor deposits in the north Coalinga region, was determined by Dr. J. C. Merriam as being not earlier than Middle Miocene. In this same section the Santa Margarita formation is found unconformably above the Big Blue and marine Temblor beds. Also, as will be brought out in the discussion of the Pliocene, a Lower Pliocene vertebrate fauna was found in land-laid beds which rest unconform- «J. C. Merriam, “‘ Vertebrate Fauna of the Orindan and Siestan Beds in Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. VII (1913), No. 19, pp. 373-85. 2B.L. Clark, ‘The Fauna of the San Pablo Group of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. VIII (1915), No. 22, p. 439-42. THE MARINE TERTIARY OF THE WEST COAST 605 ably upon the top of the Santa Margarita. Thus it would seem that if we can trust the correlation on the basis of the vertebrates, there can be very little doubt as to the age of the Santa Margarita group which comes between two vertebrate horizons, one not earlier than Middle Miocene, the other not later than Lower Pliocene. Climate.—The paleontological evidence seems to show that, beginning with the Upper Miocene, there was a temperature differentiation on the West Coast that was even more marked than that existing today. The Briones and Cierbo groups (lower and middle San Pablo) are not found in southern California, and because of their limited dis- tribution give us very little evidence of temperature differentiation. The fauna obtained from the Santa Margarita (upper San Pablo) in middle California may be regarded as approximately warm-temperate, and if it were now living it would probably not be found south of Santa Barbara County. This conclusion is based upon the large percentage of recent species found in the faunal assemblage and common to the fauna now found living between San Francisco Bay and Santa Barbara. The presence of certain recent species and the absence of certain genera found at northern localities indicate that the Santa Margarita horizon in southern California represents a warmer facies than that found in middle California. There is, however, a sufficient number of distinctive species common to the two horizons to establish their correlation, though the faunas are on the whole very different. The fauna of the Montesano formation of Washington, described by Dr. C. E. Weaver,’ is apparently Upper Miocene in age, but just what part of the San Pablo series it represents has not been established. This fauna, judging from the recent genera and species in the assemblage, is boreal and consequently very different from that of the San Pablo. If the correlation of the Montesano formation with the San Pablo series is correct, there was at that time a temperature differentiation comparable to that found between the recent faunas of middle California and Alaska. tC, E. Weaver, “‘Tertiary Formations of Western Washington,” Wash. Geol. Surv., Bull. 13 (1916), pp. 1-327. 606 BRUCE L. CLARK Paleogeography.—The sediments of the Briones and Cierbo groups were deposited in a limited arm of the sea confined to the San Francisco Bay region (Figs. 8 and g). Where all the groups of the San Pablo series are present, erosion contacts are found separating them. With the opening of the Santa Margarita there was a great inundation, somewhat comparable to that of the Temblor, though Fic. 8.—Briones (lower San Pablo, Miocene) the basins of the former were more local. Beds of Santa Margarita age are found from a little north of San Francisco to the region just north of Los Angeles (Fig. 10). The deposits are found on both the eastern and the western sides of the Coast ranges, and over large areas to the south of San Francisco these deposits are composed very largely of organic shales of considerable thickness. THE MARINE TERTIARY OF THE WEST COAST 607 PLIOCENE The marine Pliocene of the West Coast as now recognized is divisible into at least three distinct horizons: the Jacalitos, the Merced, and the Saugus. Continental deposits are found in different parts of the Coast ranges representing all three of these horizons, and in some instances to which reference has already been made these continental deposits are found closely associated with the marine beds. Stratigraphic relationship of the Pliocene and Upper Miocene.— — No evidence of folding between the beds of the Santa Margarita (Upper Miocene) and the Jacalitos (Lower Pliocene) has been obtained in southern California, though good erosion contacts are found separating them and the faunas are on the whole very differ- ent. In the vicinity of Mount Diablo, just to the east of the San Francisco region, the Santa Margarita beds were folded before the 608 BRUCE L. CLARK Lower Pliocene deposits were laid down. The Lower Pliocene in this region is composed of the Pinole tuff and the Orinda forma- tion which are of continental origin. Faunal relationships of the Pliocene-—Dr. Nomland’s study of the faunas of the Jacalitos (“lower Etchegoin’’) and Santa Margarita has shown that they are very distinct, and that the hiatus between them was more than local.t. None of the highly ornamented gastropods, pelecypods, or echinoids has been found common to the two, and the percentage of recent species in the Santa Margarita is much less than that in the Jacalitos (lower Etchegoin) of Nomland. An unconformity has been found in the Fernando series in the region just north of Los Angeles which is probably the largest and most important stratigraphic break in the West Coast marine Pliocene? Over a fairly large area in that region there is a marked difference in div and strike between the lower and middle Fernando, now referred to the Pico formation by the United States Geologi- cal Survey, and what has previously been referred to as the upper Fernando.3 The beds of this upper horizon contain a very large percentage of recent species. The Geological Survey proposes to use the name “Saugus formation” for the upper Fernando sec- tion. It is herein referred to as the Saugus group. The faunal break between the Saugus and the Pico indicates a great lapse of time. Indeed, the difference is so great that the question may be raised as to whether the Saugus does not belong to the Pleistocene J. O. Nomland, “Fauna of the Lower Pliocene at Jacalitos Creek and Waltham Canyon, Fresno County, California,” Bull. Dept. Geol., Univ. Cal., Vol. IX (1916), No. 14, pp. 199-214; ‘‘Fauna of the Santa Margarita Beds in the North Coalinga Region of California,” Bull. Dept. Geol., Univ. Cal., Vol. X (1917), No. 18, pp. 293-326. 2G. H. Eldridge, and R. Arnold, ‘Santa Clara Valley, Puente Hills and Los Angeles Oil Districts of California,” U.S. Geol. Surv., Bull. 309 (1907), pp. 1-250. R. Arnold and R. Anderson, “‘ Geology and Oil Resources of the Santa Maria District, California,” U.S. Geol. Surv., Bull. 322 (1907), pp. 1-157. W.S. W. Kew, “Struc- ture and Oil Resources of the Simi Valley, Southern California,” U.S. Geol. Surv., Bull. 691 M (1919), Pp. 323-55. 3 A paper by Dr. W. S. W. Kew of the United States Geological Survey is now in press in which the Fernando is considered a group composed of the Pico and Saugus formations separated by an unconformity. THE MARINE TERTIARY OF THE WEST COAST 609 rather than to the Pliocene. If so, the West Coast Pleistocene formations have been generally folded. Correlation.—There has been considerable confusion in times past as to the proper sequence of the Pliocene. The difficulties appear to have been due to two factors: first, the basins of deposi- tion during the Pliocene were more local and isolated than they had Fic. 10.—Santa Margarita (upper San Pablo, Miocene) been during previous periods; and second, conditions of tempera- ture varied from one locality to another. These factors were undoubtedly the cause of the great differences found between the faunas of the various provinces; however, the latter factor, temperature, was the most important. The great confusion as to the proper sequence of the Pliocene formations is reflected in the numerous names which have been given to them. Beginning 610 BRUCE L. CLARK in Oregon, we have the “‘Empire” formation;? in northern Cali- fornia, the ‘Wildcat’’;? in the region of San Francisco Bay, the ““Merced”’;3 and a little to the south of this, in Santa Cruz County, is the “‘Purissima.’’4 Still farther to the south are the “‘Jacalitos” and “‘Etchegoin”’ formations in the Coalinga region;> the ‘‘Pico”’ and “Saugus” formations of the Fernando group® in the Ventura region and the “San Pedro’ and “‘San Diego”’ Pliocene deposits on the southern California coast. The “Purissima,”’ ‘ Jacalitos,” “Etchegoin,’’ and ‘“‘lower Fernando” have in the past been referred to the Upper Miocene and correlated with the San Pablo of middle California. During the last few years our ideas of the sequence of these various formations have been very radically revised as the result of more detailed studies of vertebrate and invertebrate faunas. The writer’s study of the San Pablo series convinced him that the faunas of that series belong to an older horizon than those of the Jacalitos-Etchegoin, Purissima, and Pico, and that the percentage of recent species in the latter beds indicate Pliocene age. Later work by Dr. Nomland on the Jacalitos and Etchegoin of the Coa- linga region corroborated these conclusions. =W. H. Dall, ““The Miocene of Astoria and Coos Bay, Oregon,” U.S. Geol. Surv., Prof. Paper 59 (1909), pp. 1-261. 2A. C. Lawson, “‘The Geomorphogeny of the Coast of Northern California,” Bull. Dept. Geol., Univ. Cal., Vol. I, No. 8 (1894), pp. 24-272. Bruce Martin, ‘“ Pliocene of Middle and Northern California,’ Bull. Dept. Geol., Univ. Cal., Vol. IX (1916), No. 15, pp. 215-59. 3 A. C. Lawson, U.S. Geol. Surv., Folio 193, San Francisco Folio; ‘‘Post-Pliocene Diastrophism of the Coast of Southern California,” Bull. Dept. Geol., Univ. Cal., Vol. I, No. 4 (1893), pp. 115-60. 4J. C. Branner, F. G. Newsom, and R. Arnold, U.S. Geol. Surv., Folio 163, Santa Cruz Folio. 5 J. O. Nomland, “‘ Fauna of the Lower Pliocene at Jacalitos Creek and Waltham Canyon, Fresno County, California,” Bull. Dept. Geol., Univ. Cal., Vol. IX (1916) No. 14, pp. 199-214; ‘‘Etchegoin Pliocene of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. X (1917), No. 14, pp. 191-254. R. Arnold, “Palaeontology of the Coalinga District, California,” U.S. Geol. Surv., Bull. 396 (1909), pp. 5-169. 6W. A. English, “‘Fernando Group near Newhall California,” Bull. Dept. Geol., Univ. Cal., Vol, VIII (1914), No. 8, pp. 203-8. 7R. Arnold, “Palaeontology and Stratigraphy of the Marine Pliocene and Pleisto- cene of San Pedro, California,” Cal. Acad. Sci., Memoir IIT (1903). THE MARINE TERTIARY OF THE WEST COAST 611 The most conclusive evidence of the Pliocene age of the Jacalitos and Etchegoin was the discovery of fossil land vertebrates in these formations in the Coalinga district. These vertebrate remains were obtained from three distinct horizons: one in what has been mapped as Jacalitos, another in the middle of the type section of the Etchegoin, and the third at the top of the Etchegoin. Pro- fessor J. C. Merriam, to whom this material was referred for study, has determined these faunas to be of Pliocene age. Merriam referred to the lowest fauna, that of the Neohipparion zone, as being not older than Lower Pliocene. The next horizon is fairly well up in the Pliocene, and the fauna from the upper Etchegoin is _ referred to the Upper Pliocene. It is interesting to note that stratigraphically above the Plio- hippus proversus beds of Merriam (uppermost Pliocene), in the above-mentioned section, there are several thousand: feet of land- laid deposits which are folded and are older than the Pleistocene terraces of that region. No evidence of the exact age of these beds has been obtained, but it is suggested that they may be the land-laid equivalent of the Saugus of that vicinity. Climate.—One of the most interesting results of the study of the invertebrate faunas of the Pliocene is the evidence they give of temperature differentiation. The fauna from the Wildcat and Merced of northern and middle California is essentially boreal in character. In southern California the Pliocene is for the most part represented by a fairly warm-temperate fauna. These two faunas, the boreal and the warm-temperate, have very little in common, and consequently it was a long time before their con- temporaneity was recognized. ‘The solution of the problem was obtained from the fauna of an intermediate area. The fauna of the type section of the Purissima in the Santa Cruz Mountains of California is in part warm-temperate and in part boreal, and cer- tain species very common in the north, some of which are fairly highly ornamented forms, were found in this section. Paleogeography.—It was noted in the first paragraph, under the discussion of the factors causing the differentiation of faunas of 1J. C. Merriam, ‘‘Tertiary Vertebrate Faunas of the North Coalinga Region of California,” Amer. Phil. Soc. Trans., N.S. Vol. XII, Pl. III (1915), pp. 26-43. 612 BRUCE L. CLARK the Pliocene, that the basins of deposition were local. Crustal movements appear to have been more frequent during the Pliocene than during the other periods of the Tertiary, and we may therefore expect to find a large number of stratigraphic breaks which do not represent any very great time break. At the opening of the Pliocene the Jacalitos sea was much more limited than the seas of ee ee. ee SS Fic. 11.—Merced (Middle Pliocene) the Middle or possibly the Upper Pliocene, and the principal localities where deposition took place were probably in the region of the southern and western side of the San Joaquin Valley and the southern part of the Salinas Valley. This sea was probably con- nected with the ocean in the vicinity of the upper Salinas Valley. With the opening of the middle Pliocene a great change had taken place. The sea transgressed over wide areas to the north. Areas which had previously been subject to erosion were covered. THE MARINE TERTIARY OF THE WEST COAST 613 This great incursion is referred to here as the Merced sea and represents the time of deposition of the Purissima, Etchegoin, and Jacalitos (Fig. 11). The condition was similar to that which existed in the Upper Miocene, when there were restricted basins at the opening of the period, a great incursion during the later part of the period, and then a retreat shown by widespread unconformities. Fic. 12.—Saugus (Upper Pliocene) The Saugus formation, of which little is known, appears to have been formed after the folding and consequent retreat of the sea from the great inland basins in the San Joaquin and Salinas Valley districts. ‘The Saugus beds are largely confined to the coast, but in the Ventura district an embayment extended a considerable distance inland (Fig. 12). The Pliocene period was followed by great crustal movements which folded the formations in great anticlines and synclines, some © 614 BRUCE L. CLARK of which were overturned. In certain localities the Cretaceous has been thrust over the younger formations. Though this great period of crustal movement had its culmination at the close of Saugus time, it was well under way even prior to the deposition of these beds. In the past geologists have generally assumed that the division between the Pliocene and Pleistocene occurs after this great folding, that is, at the end of Saugus time. But the very large percentage of the Recent species in the Saugus suggests that the group may possibly be of Pleistocene age, and therefore a large part of the folding which created the Coast ranges may have taken place during Pleistocene time. OUTLINE OF PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY! FRANK LEVERETT Ann Arbor, Michigan RELATION OF PRESENT STREAM TO PREGLACIAL VALLEYS The headwater portion of the Mississippi River, above St. Paul, Minnesota, is in a region so thickly covered by glacial deposits that the present streams are entirely independent of the preglacial valleys, and their history begins with the recession of the ice in the last, or Wisconsin, stage of glaciation. ‘The courses of preglacial drainage lines in this region have been only partially traced by means of deep borings. This paper deals, therefore, mainly with the part below St. Paul. For 15 miles below St. Paul the Mississippi follows the valley of a small tributary of the preglacial river, the course of the main valley being a few miles to the west, passing beneath Lake Min- netonka, and across the lower end of Minnesota Valley, and continuing through Dakota County to the present stream at Pine Bend, 6 miles above Hastings. From Hastings to Clinton, Iowa, the river practically follows the course of the preglacial valley, though it cuts off projecting points from the west bluff at ‘Trem- pealeau, Wisconsin, and in the north part of Clinton, Iowa (Fig. 1). ‘At the mouth of the Wapsipinicon River, below Clinton, the Mississippi leaves the preglacial valley, passes into a rock gorge, and flows across rapids to Rock Island. Two courses for the preglacial valley have been suggested, one to the southwest from the lower part of Wapsipinicon Valley to Muscatine, on the present Mississippi, the other to the southeast.to Hennepin, on the Illinois Valley. Along both lines the glacial deposits are very thick, and the rock surface much lower than the present streams. The southeastward course seems on the whole more likely to have been t Published by permission of Director, U.S. Geological Survey. 615 616 FRANK LEVERETT Fic. 1.—Map of Mississippi River PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY 617 followed by the preglacial Mississippi, for other preglacial valleys converge toward it, one followed by the lower course of the Rock River, reversed, and another by Duck Creek, and its continuation in an abandoned valley to the east of Rock Island. The breadth of the valley leading to the Illinois also appears to be greater than that of the one to the southwest. &W ND «There is a further educational value in detailed, systematic description. In courses in advanced general geology, students are often required to make vertical sections based on folios, etc., and even to describe formations in class. If descriptions are so meager that students cannot visualize the sections they describe or construct, is not the value of such work greatly vitiated ? DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 653 B. INoRGANIC— 1. Of chemical origin a) Calcareous rocks b) Ferruginous rocks c) Siliceous rocks d) Halides e) Sulphates f) Rarer types 2. Of mechanical origin a) Uncemented b) Cemented It is highly doubtful whether new names under all these head- ings are needed. Whoever is not enamored of the emptiness of Hegelian dichotomy and its dread of intercrossing rubrics will rest satisfied with the use under A of such stand-bys as bacterial limonite (A 2), diatomaceous earth (A 3), or coal (A 4). Like granite, dio- rite, gabbro, these names mean something to the geologist who cannot be forever searching his Greek or Latin lexicon for prefixes and suffixes. Nor can any inventor of systems hope to force aside such terms as travertine (B 1a), clay ironstone (B 10), novaculite (B 1c), salt (B 1d), gypsum (B te). THE NAMING OF ROCKS UNDER AI AND B2 Obviously, if such familiar names as onyx, sinter, uintaite gypsum are not readily replaceable, there must be a deeper reason even than their establishment in usage. The analysis of this reason results, it is believed, in the discovery that, preponderantly, such names have three great values: (1) they are simple; (2) they aid clear visualization; (3) they suggest a dominant mode of origin. Dissatisfaction with such terms as limestone and sandstone prob- ably arises, it may be unconsciously, from the failure of these names to meet the two latter criteria. Does not, then, the renaming of organic calcareous rocks and of “clastics” involve, as its pivotal motive, the use of a terminology which shall at least approach the suggestiveness of ‘“‘coal,’”’“‘tripoli,”’ etc.2 Furthermore, should not the new names be simple or at least readily comprehensible? Should they not aid visualization ? And should they not somehow unveil the complex of conditions under which a given rock originated ? 654 A. J. TIESE In pursuance of this belief, the renaming of rocks falling under A 1 and B 2 is considered from five points of view: (1) rock source of the sediment; (2) size and shape of grain; (3) degree of cementa- tion; (4) mineral composition; (5) fossil content. 1. Rock source.—Seemingly, there is much satisfaction in speak- ing of aqueous and eolian sediments, perhaps even more in men- tioning an anemopotamoclast. Nevertheless, such terms do not aid the sedimentary-rock student in the same way that peridotite or bostonite aids the igneous-rock student. True, one cannot. assert that sands from gabbro rock sources will look different from sands from granite rock sources; of the requisite thin sections there are too few descriptions to tell. In the writer’s own experience, nevertheless, Cambrian sands derived from the microcline granites of Wyoming do seem to have a characteristic appearance under the microscope. Would it not secure definiteness of description if sedimentary rocks, mainly, of course, sandstones, had prefixed to their colorless names such terms as granitogene, gabbrogene, quartzitogene ? 2. Size and shape of grain.—Shape of grain should be an essen- tial part of a microscopic description. The terms angular, near- angular, subrounded, rounded might constitute a useful series. Size of grain is, under the microscope, capable of exact delimi- tation. The scale used in connection with igneous rocks is not, however, subdivided enough. Accordingly, the adoption of a modification of the New York City Aqueduct standard is advocated. The terms suggested chiefly, perhaps, apply to sandstones, some- what to limestones, little to shales. Sedimentary rock very coarse-grained...................- grains over I mm. Sedimentary rock coarse-grained.............. grains between o.5 and 1 mm. Sedimentary rock medium-grained.......... grains between 0.25 and 0.5 mm. Sedimentary rock fine-grained.............-. grains between o.1 and 0.25 mm. Sedimentary rock very fine-grained........ grains between 0.05 and o.1 mm. Sedimentary rock superfine-grained.................. grains below 0.05 mm. By this scale, of course, arkoses, graywackes, conglomerates, breccias, even most grits would be very coarse-grained. It will be observed, however, that the terms in the table refer to measure- ments under the microscope and for sandstones, limestones, and DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 655 shales only. For field use in connection with arkose, graywacke, conglomerate, breccia, and grit, other sizings for fine-grained, medium-grained, coarse-grained are advocated. Fine-grained arkose (graywacke, etc.)........... grains to % in. in diameter Medium-grained arkose (graywacke, etc.)....grains from % to } in. in diameter Coarse-grained arkose (graywacke, etc.)........ grains over % in. in diameter In most cases, particularly in conglomerates, maximum and mini- mum as well as average sizes should be noted. Some term seems needed to denote a rock which is mainly an even-sized matrix, but contains a few pebbles over { in. in diameter. Pebbled sandstone, pebbled limestone, pebbled shale are advanced. 3. Degree of cementation.—The microscope permits the abandon- ment of the field terms to denote hardness: soft, subsoft, hard, superhard. The following incomplete table is tentatively offered for criticism. Rock Not Well Cemented Cemented; Grains Not Cemented; Grains (Primarily field terms) Interlocked Interlocked Sandrock Sandstone to quartzite ee sandstone Paraquartzite Limerock Limestone [eee Magnesian limerock Dolomite eens Clay Shale Slate Arkose Arkosite to quartzite Arkositite arkose Glauconite sandrock Glauconitite (existent ?) Existent ? Ferrite sandrock Ferrite Existent ? Gravel Conglomerate Quartzite- conglomerate Most of these terms are comprehensible at a glance. By ortho- quartzite is meant rock cemented only through infiltration and pressure. By paraquartzite is meant quartzite mainly originating through contact metamorphism. Doubtless it is illogical to remove quartzite, marble, and slate from the category of metamorphic rocks. However, quartzites, slates, and marbles are universally given a place in vertical sections and in geologic folios are described under sediments. Schists and gneisses are not so treated. Practical exigencies would seem to override Aristotelian ‘“‘laws.” 656 A. J. TIEIE 4. Mineral composition.—In the field, proportions between minerals cannot be determined. Under the microscope they can. The only true problems which arise concern the existence in given rocks of varying proportions of quartz and feldspar, of quartz and calcite, of quartz and glauconite, of calcite and siderite, of calcite and glauconite, of siderite and glauconite, or of various analogous but rarer combinations. At present, arenaceous limestone is used to denote a rock preponderantly calcareous and, may we say, one- fourth arenaceous. Calcareous sandstone is employed when such percentages are reversed. But in the writer’s experience a number of rocks exhibit percentages of minerals close to 50:50. It is suggested that such terms as calarenite, sidarenite, glaucarenite would prove useful and not uneuphonious names for 50:50 com- binations of calcite and quartz, siderite and quartz, glauconite and quartz. Limestone-glauconite, limestone-ferrite are examples of similarly compounded names for other mineral mixtures in rocks. Simple field names suggested on page 655 would thus be used only in emergencies. The presence of glauconite involves a minor problem. Except when calcite is a cement, its presence in small percentage in a sandstone would hardly warrant the use of the name calcareous sandstone. Glauconite, however, throws some light upon the conditions of deposition. It is advocated that glauconitic as an adjective be employed even if the percentage in a sandstone or limestone be as low as 5 per cent. 5. Fossil content.—Fossiliferous sandstone, shale, and limestone are names already familiar. However, every geologist should recognize at sight the various invertebrate phyla and the main classes. And many geologists could thus characterize fossil- bearing rocks as predominantly graptolitic, coralline, vermicosic, pelmatozoic, bryozoan, brachiopodic, molluscan, trilobitic, etc. Coquina seems to be a term for pelecypodic limerock. ILLUSTRATIVE DESCRIPTIONS OF SEDIMENTS With some hesitation, the present discussion is closed with illustrative descriptions of hand specimens of Cambrian rocks and DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 657 of thin sections made therefrom. Each description, though brief, includes (1) locality, position in stratigraphic column, and descrip- tion of field specimen; (2) texture, list of constituents, and relative proportion of more important minerals; (3) description of the chief mineral or minerals; (4) brief description of minor miner- als; (5) name. Items (12), (2), (3), (5) are never omitted, and here each is given a separate paragraph; (4) is sometimes omitted. Terms logically connected are hyphenated, as fine- grained; glauconite-limestone; ferruginous-calcareous. The descriptions are, as stated, purely illustrative. It may be added, however, that the writer has ventured partly to base upon these and other thin sections certain conclusions upon Middle Cambrian paleogeography rather at variance with the accepted account for Wyoming. Wy 16: Taken in unnamed creek on south side of Duncom Mountain, + mile east of Devil Canyon Road, 20 ft. above granite. Description of hand specimen: Arkose, massive, gray-yellow to buff, weathering brownish-white to gray, with grayish-pink irregular lenses, cross-bedded; pebbles quartz and feldspar, usually about + in., in finer but arkosic matrix; quartz angular to sub- rounded, iron-stained; feldspars pink angular cleavage fragments; traces of basic material ? Texture granular-fragmental. Constituents quartz, 80 per cent; microcline and a plagioclase near oligoclase, 15 per cent; small amounts of orthoclase, biotite, ilmenite, zircon, apatite, as acces- sories, and kaolin, sericite, leucoxene, muscovite, limonite as alteration products. Liquid and gas inclusions in quartz. Cement quartz and limonite. Two marked groups of quartz grains. Larger average 0.4 to 0.5 mm. in diameter, smaller 0.06 to0o.o8 mm. Larger grains sub- rounded to oval, smaller angular. Vein quartz suggested by wavy extinction. Microcline fragments quadrangular, fresh; plagioclase same. Either rarely over 0.08 mm. Muscovite apparently not primary. Classed: granitogene fine-grained arkose. 658 AL J.P Wy 23: Taken at Middle Fork of Crazy Woman Creek, 15 ft. above granite. Description of hand specimen: Arkose, quartzitic, red-brown, massive, coarse, interbedded with finer-grained non-arkosic sand- stones. Quartz grains rounded, uniform-sized, up to ¢ in. in diameter. Feldspars fresh cleavage fragments, about 33 per cent of rock, up to¥ in. in diameter. General vitreous luster. Texture granular-fragmental, secondary growth of quartz grains. Constituents quartz, 95 per cent; small amounts of biotite, limonite, apatite, zircon (?). Cement quartz and limonite. One quartz fragment 3 mm., largest otherwise 0.15 mm.; 50 per cent of quartz 0.8 to 1 mm.; rounded and oval. Classed: granitogene fine-grained arkosite. Wy 27: Taken at Johnson Creek, 4 ft. above granite. Description of hand specimen: Sandstone, massive, buff, with ys In. bands of chocolate-brown, doubtless iron stain along obscure bedding planes; contains rare pebbles of purplish shale and yellowish quartz, minimum § in., maximum 1 in. In general medium-grained, friable, subvitreous luster. . Texture granular-fragmental. Constituents quartz, 60 to 80 per cent, dependent on amount of feldspar, now altered to sericite; small amounts of microcline, biotite, hornblende, plagioclase near oligoclase, zircon, apatite, magnetite, ilmenite (inclusion) as acces- sories, and kaolin and sericite as alteration products. Liquid and gas inclusions in quartz. Cement limonite, kaolin, sericite, possibly chalcedony. Quartz vari-sized, largest grain 0.55 by 0.25 mm., no grains below o.05mm.; near-angular to subrounded, frequently oval. Feldspars suggest secondary growth, averaging 0.05 to o.o8 mm. Classed: medium-grained pebbled sandstone. Wy 50: Taken % mi. from mouth of tributary, flowing west- ward from Hunt Mountain to South Beaver Creek, in float not far above granite; similar material in place, 30 ft. south and 1o ft. above granite. t The belt of exposure is so narrow that this locality seems sufficiently identified. DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 659 Description of hand specimen: Arkose conglomerate, pebbles not ranging above ? in. Prevailing color where fresh dark-brown shot with gray, the pink of feldspar sharply contrasting; from dirty grays and whites of weathered surface pebbles stand out in relief. Fucoid markings on what may be bedding-planes 1 in. apart. Pebbles 90 per cent quartz, faintly brownish-green, rounded to near-angular, breaking with matrix; feldspars pink fresh cleav- age fragments. Matrix 90 per cent of rock, sand, fine-grained, dull to earthy luster. Texture granular-fragmental, large grains showing micrographic intergrowth. Constituents quartz 90 per cent; microcline, ortho- clase, uncertain plagioclase 8 per cent; small amounts of biotite, apatite, zircon as accessories, and sericite, kaolin, limonite, and chlorite as alteration products. Liquid and gas inclusions in quartz. Cement a sericitic-kaolinic-limonitic ‘“‘mess.”’ ~ Quartz vari-sized, 1.5 by 0.8 mm. in larger grains, perhaps vein quartz, to judge by wavy extinction; average grains, 0.08 mm.; rounded to near-angular. Microcline fresh, 0.03 to 0.04 mm. Orthoclase same size, largely sericitized. Much organic material, seemingly chitinous. Classed: ferruginous arkose-conglomerate. Wy 93: Taken on Willow Creek at Burgess Ranger Station, - 2 ft. above granite. Description of hand specimen: Shale, thin-bedded, green when fresh, sparsely specked with glistening mica flakes and containing lenses of whitish-green sandstone, 1 in. long; on weathered surface bluish-black. Hard, arenaceous, fracturing irregularly, fresh sur- face of dull luster, bedding-planes subvitreous luster and slightly wavy. Texture granular-fragmental, pilitic through alteration and with parallel arrangement of minerals, excluding quartz. Constituents chlorite, sericite, epidote, presumably alterations from biotite, muscovite, feldspar; small amounts of quartz, plagioclase, mag- netite, zircon. Glauconite indeterminable. Cement a sericitic- chloritic felt. 660 A. J. TIEJE Quartz rarely 0.06 mm., very angular. Plagioclase rare. Buio- tite bleached. Classed: micaceous-arenaceous shale. Wy 2: Same locality as Wy 16, 40 ft. above granite. Description of hand specimen: Sandstone, thin-bedded, alter- nating and interleaved with fissile green shales; now banded green and white, subsoft, micaceous, now reddish-green, medium-grained, calcareous. Texture granular-fragmental. Constituents quartz go per cent; glauconite 8 per cent; magnetite, ilmenite, calcite as accessories, limonite and leucoxene as alteration products. Cement calcite. Quartz averages 0.05 mm., angular; rare vein quartz. Glau- conite in aggregates (0.16 mm. diameter) oval to rounded, bordered and veined by limonite; no seeming relation to shell fragments. Classed: glauconitic superfine-grained sandstone. Wy 9: Same locality as Wy 16, 170 ft. above granite. Descripton of hand specimen: Glauconite sand, massive, emerald green, crumbling, coarse-grained, subvitreous luster. Texture granular-fragmental. Constituents glauconite 95 per cent; limonite as alteration product 5 per cent. Cement limonite. Various chitinous fragments and rods. Glauconite aggregates as in Wy 2, but fresher. Classed: glauconite sandrock. Wy 25: Same locality as Wy 27, approximately 30 ft. above granite. Description of hand specimen: Sandstone, massive, buff where fresh, weathering dark-red. Superhard, fine-grained, dull luster. Weathering exhibits buff nodules in relief. Texture granular-fragmental. Constituents quartz 85 per cent; small amounts of biotite, muscovite, glauconite, magnetite, ilmenite, plagioclase, microcline, zircon, apatite as accessories, and leucoxene, kaolin, limonite, sericite, chlorite, epidote as alteration products. Liquid and gas inclusions in quartz. Cement quartz and limonite. Quartz averages o.2mm., angular; fragments often elongate; one grain 0.6 by 0.35 mm; vein quartz rare. Plagioclase a mass DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 661 of sericite needles, microcline much fresher; average size for both, 0.05mm. Biotite shredded. Muscovite largely secondary, but some long twisted primary fibers. Classed: granitogene glauconitic-ferruginous fine-grained sand- stone. Wy 58: Taken at Turkey Creek, + mi. south of Steamboat Point, 9 ft. above granite. Description of hand specimen: Limestone, 1-in. lenses, reddish- brown, coarse-grained; occurring in shale, fissile, paper-thin, green with silky luster, trilobitic on bedding-planes (Ptychoparia), brachiopods rare. Texture granular-fragmental. Constituents calcite 95 per cent; small amounts of quartz, magnetite, zircon, siderite as accessories. Cement calcite and a trifle limonite. Calcite averages 0.75 mm.; often in rods and rectangular blocks, obviously fossil fragments, the remaining calcite due to solution and redeposition. Within shell fragments a finely comminuted mixture of quartz, calcite, siderite; limonite, probably from siderite, out- lines fragment edges. Quartz 0.02 to 0.03 mm., angular. Classed: coarse-grained brachiopodic limestone. Wy 99: Taken at Deer Creek, 1 mi. northwest of Sheep Moun- tain, 100 ft. below the persistent sandstone described as Wy 8g. Description of hand specimen: Limestone, massive 1-in. bed between thick green shales; gray-blue, weathers dirty-brown, crossed by veins of calcite; hard, medium-grained, subvitreous luster, brachiopodic and trilobitic. Texture granular-fragmental. Constituents calcite 90 per cent; small amounts of quartz, magnetite, glauconite, pyrite, zircon, ilmenite as accessories, and leucoxene as alteration product. Calcite 0.35 mm., anhedral; at times within shell fragments and then comminuted; ‘“‘rods” clearly from genal spines of trilobites, fragments 4 to 5mm. long. Quartz mainly in shells, angular, seldom as much as 0.o4mm. Glauconite seemingly developed in shells. Much chitinous material. Classed: medium-grained brachiopodic trilobitic glauconitic limestone. 662 Al EEE: Wy ro5: Taken at Horse Creek No. 3, 1 mi. west of Sheep Mountain, 125 ft. above granite and at much the same horizon as Wy 99. Description of hand specimen: Limestone, massive, light-green and specked with hard black grains, weathering dirty brown-red. | Hard, fine-grained, subvitreous luster, fractures conchoidally. Texture granular-fragmental. Constituents calcite 60 per cent; glauconite 30 per cent; quartz 10 per cent. Magnetite and zircon as accessories, and limonite as alteration product. Calcite 0.15 mm., probably due to recrystallization. Calcite in shell fragments as in Wy 99. Quartz grains surpris- ingly large, average 0.1 mm. and up to 0.5mm.; very angular. Glauconite in large aggregates, even 2 by 8mm.; some curved, as if by replacement of whole fossils; indifferently near to or remote from magnetite; fresh. Limonite mainly from magnetite. Classed: fine-grained brachiopodic trilobitic glauconite- limestone. Wy 89: Taken from massive bench on south side of Bald Mountain. Description of hand specimen: Sandstone, massive, pinkish buff, slightly splotched with brown, weathering dull grayish-white; subsoft, fine- to medium-grained, traces of shale streaks, earthy to subvitreous luster, brown splotches interpreted as oxidation of trilobite shields. Cross-bedded? Most persistent bed in the Cambrian below the flat pebble. Texture granular-fragmented. Constituents quartz 95 per cent; small amounts of biotite, magnetite, ilmenite, plagioclase, glauco- nite as accessories, and leucoxene and limonite as alteration prod- ucts. Cement quartz and calcite. Quartz averages o.15mm.; largest grain 0.3 by 0.15 mm.; angular, sufficiently cemented to suggest quartzite; few indications of secondary growth. Grains limonite-rimmed. Biotite much shredded. Chitinous rods. © Classed: fine-grained quartzite-sandstone. Wy 63: Same locality as Wy 58, 150 ft. above granite. Description of hand specimen: Limestone, massive, gray-green, weathering reddish-brown to gray. Subsoft, medium-grained, argillaceous, arenaceous, glauconitic, ferruginous. _ DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 663 Texture granular-fragmental. Constituents calcite 50 per cent; quartz 333 per cent; siderite 10 per cent; small amounts of mag- netite, ilmenite, zircon, muscovite, Blnucenite as accessories, and limonite as alteration product. Calcite o.2mm., giving evidence of recrystallization; also as rods. Glauconite aggregates about 0.5 mm., much replaced by limonite. Limonite also edges siderite rhombs. Quartz 0.04 mm., near-angular to subrounded. Classed: arenaceous trilobitic medium-grained glauconite- limestone. Wy 13: Same locality as Wy 16, 260 ft. above granite. Description of hand specimen: Limestone, thin-bedded, blue- gray, subsoft but brittle, the 1-in. layers separated by micaceous shale; subvitreous luster, black-specked, seemingly unfossiliferous. Texture granular-fragmental. Constituents calcite 50 per cent; quartz 25 per cent; glauconite 20 per cent; small amounts of ilmenite, zircon, apatite as accessories. Liquid and gas inclusions In quartz. Cement calcite. Quartz o.t mm., angular. Calcite grains 0.5 mm.; main occur- rence as rods. Classed: arenaceous trilobitic medium-grained glauconite- limestone. Wy 57: Same locality as Wy So, 4 ft. above that horizon. Description of hand specimen: Sandstone, massive, gray-green with irregular-bedded effect due to light-colored stretches between dark-brown bands, with parallel orientation of Dicellomus shells; soft, coarse-grained, calcareous, particularly toward top, glau- conitic, dull to subvitreous luster. Cross-bedded. Texture granular-fragmental. Constituents quartz 80 per cent; glauconite ro per cent; calcite 5 per cent; small amounts of biotite, muscovite (?), apatite as accessories. Cement calcite. Liquid and sas inclusions in quartz. Quartz o.2 to 0.5 mm., subrounded to oval, at times subhex- agonal. Glauconite seldom related to shell interiors, not even in an admirable cross-section with quartz fragments in shell and cemented by calcite and limonite. Classed: glauconitic medium-grained sandstone. 664 Ae Die Wy 84: Taken at Cambrian Creek, tributary to East Fork of Little Bighorn River, long. 10° 45’ W., lat. 44° 50’ N., 34 ft. below the flat-pebble conglomerate. Description of hand specimen: Limestone, massive, gray-green, weathering gray to reddish brown, subsoft, coarse-grained, arena- ceous, glauconitic, subvitreous luster, with calcite veins and 1-in. crystals; breaks in smooth angular blocks; presents corrugated surface where calcite has dissolved on weathering. Texture granular-fragmental. Constituents quartz 47 per cent; calcite 47 per cent; small amount of glauconite as accessory, and limonite as alteration product. Cement calcite. Quartz averaging o.3mm., largest grain 0.5 by 1.8mm.; sub- rounded; grains broken and healed by calcite; slight traces of secondary growth; small grains seemingly fragments of larger ones cemented. Calcite shows recrystallization. Glauconite aggre- gates about 0.1 mm., rounded. Bryozoan-like fragments. Classed: glauconitic trilobitic medium-grained calarenite. Wy 66: ‘Taken on south side Tongue River, directly opposite mouth of Sheep Creek, 60 ft. below the Cambrian-Ordovician contact. Description of hand specimen: Limestone, ;/,- to 1-in. beds, greenish-white to buff, hard, fine-grained, argillaceous, arenaceous, subvitreous luster, slightly ripple-marked, raindrop-pitted (?). Texture granular-fragmental. Constituents siderite 30 per cent; calcite 30 per cent; quartz 30 per cent; glauconite 5 per cent; a little apatite, ilmenite, muscovite, magnetite, and plagioclase; muscovite and plagioclase very rare; a little limonite as alteration product. Calcite averages o.t mm., larger grains 0.2mm. Quartz 0.05 mm., near-angular to subrounded; inclusions of hematite scales ( ?). No fossils. -Classed: glauconitic-arenaceous medium-grained siderocalcite. Wy 72: Taken on East Fork of Little Bighorn River, 2 mi. northeast of Little Bald Mountain, at base of Ordovician “ Bighorn dolomite.” DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 665 Description of hand specimen: Limestone, 4- to 6-in. beds, gray-green, weathering light buff with yellow stains, hard, fine- grained, slightly dolomitic, argillaceous, dull luster, lower beds much jointed, causing weathering in subquadrate slabs; flat-spired gastropods abundant. | Texture granular-fragmental. Constituents calcite 99 per cent; a little accessory glauconite, limonite as alteration product. Cement calcite. | Calcite grains often recrystallized, largest 0.3 mm. and appar- ently cavity-filling. Classed: gastropodic medium-grained limestone; traces of glauconite. Wy 69: Same locality as Wy 2, 2 ft. below Cambrian-Ordovician- contact. Description of hand specimen: Conglomerate, massive, greenish- gray; pebbles limestone, distinguishable with difficulty on weath- ered surface, greenish, subsoft, fine-grained, glauconitic, flattened, elongated, length { in. to 2 in., often loose ochreous earth, lining cavities, when fresh breaking with matrix, and usually aligned in parallel planes, constituting 50 per cent of rock; matrix limestone, greenish, fine-grained, dull luster. (This is the famous flat-pebble conglomerate of Dakota and Wyoming.) Texture conglomeratic. Constituents calcite go per cent; small amounts of glauconite, quartz, pyrite, magnetite as accessories, and hematite and limonite as alteration products. Cement calcite. Calcite either as pebbles, merely fragmentary in slide, or as interlocked crystals in matrix. Pebbles characterized by calcite, criss-crossed and specked with glauconite (percentage from 25 to 333), and interlocked with quartz grains below 0.01 mm. diameter. Quartz very rare in matrix. Pyrite altering to hematite and limonite. Classed: glauconitic flat-pebble limestone-conglomerate. Wy 24: Same locality as Wy 23, 20 ft. below Cambrian- Ordovician contact. Description of hand specimen: Seemingly sandstone (and so described by one observer), massive, pink, weathering fainter pink. 666 As Sc TEESE Superhard, fine-grained, argillaceous, and seemingly slightly cal- careous, subvitreous luster. Texture granular. Constituents dolomite 90 per cent; quartz 5 per cent; small amounts of magnetite, glauconite, ilmenite as accessories, and limonite as alteration product. Cement dolomite and limonite. Dolomite in rhombs, inclusions of limonite; averages 0.07 mm. Quartz rarely over 0.02 mm., very angular. Glauconite altering to limonite, latter also between dolomite crystals. Classed: glauconitic superfine-grained dolomite. REVIEWS The Cost of Mining. By James R. Fintay. Third’ edition (en- tirely revised, enlarged, and reset). McGraw-Hill Book Co., 1920. The new edition of this standard work on mining costs in addition to amplifying and bringing up to date the data on mining costs found in the earlier editions, contains a considerable amount of material of broader economic interest relating to mineral resources. Chapter I, for example, discusses mineral wealth as a source of national power, chapter III treats of the nature and use of capital. The cost of mining data is presented seriatim by mineral commodities and covers coal, iron, copper, lead, silver-lead, zinc, gold, and silver. The chapter dealing with each of these is commonly prefaced by some general discussion and by statistics of production. Cost data for iron-mining relate only to the Lake Superior region. In the chapters devoted to copper occur such paragraph headings as ‘Geologic Unconformities at Jerome,” ‘“‘Characteristics of Belt Rocks,” “Theories of Formation of Jerome Deposits,” etc.; the book is therefore somewhat broader in scope than its title would suggest. The book commends itself not only to the engineer but to the economist, geologist, or geographer concerned in the réle of mineral resources in the industrial life of the United States. E. S. BASsTIN Extracts from “The Mining Handbook,” Geological Survey of West- ern Australia, Memoir No. 1, 1919. A series of advance sepa- rates of chapters from the ioneone Handbook. This mining handbook is a worthy attempt to furnish to ie interested in mining in Western Australia a large amount of varied information likely to prove of service to them in the exploitation of mineral deposits. The handbook includes chapters on the relations of physiography and of petrology to the exploitation of mineral deposits, chapters expounding the mining regulations and explaining various methods of governmental assistance to prospecting and mining. Then follow chapters dealing with the major base metals, with the various 667 668 REVIEWS steel-alloying metals, with the minor metals, and, finally, with coal and a few non-metallic mineral resources. The chapter on physiography in its relation to prospecting and mining is mainly a discussion of the influence of topography on the discovery and development of ore bodies and, reciprocally, the influence of ore bodies on topography. The chapter on minerals of economic value lists the composition and the principal physical properties of economic minerals and cites their main utilizations. The chapter on petrology and its application in industry is an exposition of petrology in its simplest form, defining the principal rock- forming mineral and the principal groups of igneous, sedimentary, and metamorphic rocks and expounding the application of petrology in geologic surveying, in the study of ore deposits, in engineering, architec- ture, and agriculture. The chapter on the relation of the law to prospecting and mining covers the legal restrictions governing the location and development of mineral deposits in Western Australia. Three points of contrast between the Western Australian mining laws and those of the United States are noteworthy. In the United States, discovery must precede the staking out of mining claims and ground cannot be validly held until there has been an actual discovery of mineral. In Western Australia ground can be marked out and held, even though no minerals have been discovered. In the United States title in fee simple to a mining claim is acquired by patent, subject to extra-lateral rights of adjoining claim-owners. In Western Australia the crown does not part with the title to the land. Leasehold is the rule, coupled with labor conditions. In Western Australia the principle of extra-lateral rights, which has resulted in so much troublesome litigation in the United States, does not apply but the holder of a mining lease is only entitled to such portions of the lode or lodes as occur within the boundaries of his lease extended vertically downward from the surface. An interesting feature of the Western Australian mining law is the provision for a reward of up to one thousand pounds, offered for the discovery of payable gold at a place distant more than two miles from any place where payable gold has up to then been discovered. Several other forms of governmental assistance to mining include advances for the purpose of pioneer mining and prospecting, the establishment and subsidizing of plants for ore treatment, assistance for drilling, including the purchase or hire of drilling plants, the advancement of money for REVIEWS 669 drainage, shaft sinking, and for development of transportation facilities to assist mining operations. A separate chapter, entitled “Assistance to Prospecting and Mining,” explains these various forms of governmental assistance in detail and also lists geographically and by minerals all available government reports and maps covering mining districts. Another chapter is a glossary of common terms in mining and geology. Among the major metals, iron ores though widely distributed in Western Australia have as yet been developed only on a small scale for the production of flux for copper and lead smelting and no detailed geologic surveys have been made of any of the iron deposits. Copper deposits, though widely distributed, have been developed only to a minor degree. The production of lead ores has been small. Among the steel hardening metals, the production of manganese, tungsten, and molybdenum has been so small as to be essentially neg- ligible. Among the rare metals, there has been little or no development. The small tin production has come mainly from alluvial deposits, but in the Wodgina tin field, tin and tantalum occur in pegmatite dikes which, together with granite, intrude metamorphic sedimentary rocks. The chief constituents of these pegmatitic dikes are albite and quartz, with occasionally scaly lepidolite and tourmaline; in addition, ortho- clase, mangano-tantalite, and tin occur in varying quantities, as well as some of the rare radioactive minerals. In the vicinity of and along the margin of many of the pegmatite dikes are bands and bunches of tourmaline, sometimes to such an extent as to make up fully one-third of the entire rock. One of the most conspicuous of the pegmatite veins, about half a mile in length and 30 feet in width, has proved to be suffi- ciently rich in tin and tantalum to be worked. The tin ore, cassiterite, is concentrated along certain lines in the pegmatites and does not appear to be generally disseminated in minute quantities throughout its mass. The tin occurs in all shapes, from | minute grains up to pieces weighing as much as roo pounds. The coal deposits of Western Australia range in age from Carbon- iferous, through Permo-Carboniferous to Mesozoic, Tertiary, and post- Tertiary. The only deposits which have been extensively mined are those of the Collie field of Permo-Carboniferous age. In this field the total thickness of the coal seams is about 137 feet. The coals are semi- bituminous, non-coking coals which are dirty to handle and deficient in volatile materials. It is interesting to note that the coals appear to be mainly of drift origin and to have been deposited by current action on an extensive basin or river valley. The banded appearance of most 670 REVIEWS of the coals and their relatively high percentage of ash is probably a result from this mode of origin as is also the absence of fire clays beneath them. The available reserves of the field are estimated at three and one-half billion tons. E. S. BASTIN Summary Report, Canadian Geological Survey. Ottawa, 10109. Pari C. Alberta-Saskaichewan Region. Pp. 52. 1. “Cretaceous, Lower Smoky River, Alberta.” By F. H. McLeEarn. 2. “Geology of the Swan Hills in Lesser Slave Lake District, Alberta.’’? By JoHn A. ALLEN. 3. “Northern Part of Crowsnest Coal Field, Alberta.” By BRUCE ROSE. 4. “‘Gasoline in Natural Gas. Experiments on Alberta Gas.” By D. B. Dow tne. 5. “Surface Deposits of Southeastern Saskatchewan.” By , J. STANSFIELD. The annual Summary Report of the Canadian Geological Survey for 1917 and since is issued in parts and each part is designated by a letter of the alphabet. Before 1917 the whole annual Summary was bound in one large volume. 1. The Cretaceous begins with the Lower Cretaceous and extends into the Montana group of the Upper. Marine and non-marine forma- tions alternate and the total thickness represented is about 4,470 it. The Dakota sandstone cannot be recognized in its normal subaerial development. The beds dip to the south from 12 to 60 ft. per mile and represent the north limb of a very broad, shallow syncline. The structure is not favorable for oil. 2. The Swan Hills lie south of Lesser Slave Lake, have a maximum elevation of 4,320 ft. above sea-level, and represent remnants of a once more extensive, maturely dissected upland. The Cretaceous is repre- sented by the Montana group. The basal member is marine, and the upper two members, the Sawbridge and Edmonton, are of fresh-water origin. ‘The early Tertiary is represented by the Paskapoo formation but there is no marked unconformity between the Upper Cretaceous and the Tertiary. 3. Formations ranging from Devono-Carboniferous to Upper Cretaceous, probably Tertiary, in age, are described. Coal seams of REVIEWS 671 economic importance are found only in the Kootenay, and a large reserve of bituminous coal occurs within the Rocky Mountains. 4. This is a description of the apparatus and the results of a number of experiments carried on at various gas wells. At the pressures under which the tests were made the amount of gasoline in the Alberta gases per 1,000 cu. ft. varied from o.1 pints to 3.7 pints. 5. This area is covered with glacial drift, averaging from 4o to 70 ft. in thickness. ‘Two main terminal moraines cross the area but ground moraine covers most of the area. The residual alkali material formed in the dried-up sloughs contains only a very small per cent of potash, and is of no economic importance. The waters from the drift are hard and contain calcium and magnesium carbonates and sulphates while the waters from wells reaching the Tertiary strata are soft and contain considerable sodium chloride. Part D. Manitoba Region. Pp. to. 1. ““Athapapuskow Lake District, Manitoba.” By E. L. BRUCE. 2. “The District Lying between Reed Lake and Elbow Lake, Manitoba.” By E. L. Bruce. 3. “Reed-File Lakes Area, Manitoba.” By F. J. Alcock. 4. “‘Wekusko Lake Area, Manitoba.” By F. J. Alcock. 5. ‘Superficial Deposits and Soils of Winnepegosis Area, Manitoba.” By W. A. JOHNSTON. 6. ‘‘Gold-Quartz Veins and Scheelite Deposits of South- eastern Manitoba.” By E. L. BRUCE. 1. Chalcopyrite was discovered along joint or fracture zones in fine-grained, massive greenstone. Some distance from these occurrences the greenstone is intruded by granite and these deposits are directly related to these intrusions. With the present conditions of transporta- tion mining conditions are not favorable for this area. 2. The geology of this area is much simpler than that of other nearby areas in northern Manitoba as the pre-Cambrian is represented by the Amisk series of greenstones and derived schists, and intrusive granites. The younger pre-Cambrian formations are absent, and since the crest of a large anticline crosses this area these younger formations have probably been removed by erosion from the crest of this anticline. Ordovician dolomites and Glacial and Recent deposits are noted. No 672 REVIEWS economic deposits have yet been discovered and on the whole con- ditions for the formation of ore deposits have not been as favorable as in nearby areas. 3. The pre-Cambrian rocks of this area are divided into an igneous complex consisting of altered volcanic and intrusive rocks, a sedimentary complex of granite-gneiss and staurolite-schist, and batholithic intru- sives. Ordovician dolomites occur and Pleistocene and Recent deposits are abundant. fs 4. The geology of this area is very similiar to that of the Reed-File lakes area. A number of productive gold-bearing quartz veins occur near the borders of the granite masses. 5. Because of the practical exhaustion of homestead prairie land in easily accessible areas, a map of an area of about 1,500 sq. mi. around Lake Winnipegosis was prepared. This map will show the character of the soil and forests and will also indicate the land that can be readily cleared. 6. The gold-quartz veins in the pre-Cambrian rocks of southeastern Manitoba were sampled and assayed for both gold and platinum. Most of the assays showed a very small amount of gold present but in no case was platinum detected. In a fine-grained, massive, roughly sheeted, hornblendic rock scheelite occurs in small vuggy lenses not in all cases parallel to the sheeting. The returns from a shipment of the ore to the Ore Dressing Laboratory, Ottawa, were not encouraging. Part F. Maritime Province Region. Pp. 36, figs. 3. 1. ‘‘Investigations in Western Nova Scotia.” By E. R. FARIBAULT. 2. “‘Investigations in Western Nova Scotia and New Bruns- wick.” By ALBERT O. HAYES. 3. “Peat Investigations.”” By A. ANREP. t. A description of a number of small manganese deposits in Nova Scotia and notes on the occurrence of platinum in the scheelite and gold veins of the gold-bearing series. 2. The drift over the Carboniferous rocks of the Sydney coal basin contains bowlders of rocks which outcrop to the south of the basin and this with the general direction of glacial striae proves that the direction of ice movement in this part of Cape Breton Island was northward. This report is almost entirely economic and gives many details concern- ing the structure and extent of a number of coal horizons. The New REVIEWS 673 Ross, Lunenburg County, manganese deposits are described as occurring along a fissure in granite. Calcite and manganese oxide was deposited in this fissure and later movements broke up this vein and formed a frault breccia. Secondary enrichment from surface waters has con- centrated the manganese oxide into bodies of workable size and high- grade ore. 3. A few preliminary results are given of investigations of peat bogs near St. John and Moncton, New Brunswick. Part G. The Platinum Situation in Canada, 1918. By J. J. O'NEILL. Pp. 19, map. The chief platinum-producing areas in Canada are in Ontario, British Columbia, and Yukon. In the nickel-copper ores of Sudbury, Ontario, platinum occurs as sperrylite, the platinum arsenide. In British Columbia platinum is found both in the solid rocks and the gravels. In the solid rocks three distinct types of deposits are recog- nized—first, in association with chromite in peridotite-pyroxenite rocks; second, in association with chalcopyrite deposits; third, in shear zones in typical granite. In the gravels of Yukon platinum is widely distributed but not in large enough quantities to be profitably exploited for this metal alone. Canada appears to have possibilities of becoming one of the great producers of platinum. In 1918 only one hundred ounces of platinum were recovered, but probably more than 50,000 ounces of the platinum metals were contained in ores mined in Canada, but not recovered. J. F. W. The Silurian Geology and Faunas of Ontario Peninsula, and Mani- toulin and Adjacent Islands. By M. Y. WititaMs. Canadian Geological Survey, Ottawa, Memoir 111, 1919. Pp. 195, appendices III, pls. XXXIV, figs. 6, maps 2. In this memoir the author gives his conclusions, based on five seasons of detailed field work, on the general Silurian problems of south- western Ontario. Detailed sections, descriptions, notes on origin and correlation, and complete fossil lists for the various members of the Silurian system are given. Nine diagrams are given to illustrate the conditions of sedimentation during various stages of the Silurian period. Nine new species of brachiopods and one new variety are described. The three appendices contain descriptions of a new species of brachiopod 674 REVIEWS by Foerste,’a new species of crinoid by Springer, and two new species of corals by Chadwick. The important physiographic feature of the area is the Niagara escarpment which is formed by the outcropping edge of the Niagara dolomite. The Silurian formations classified on the basis of lithology fall into the following three groups in ascending order: (1) Alternating sand- stones, shales, and limestones represented by the Medina-Cataract, Clinton, and Rochester formations and indicating changing conditions of land in respect to the sea. (2) Massive dolomites represented by the Lockport and Guelph formations and suggestive of widespread seas of moderate depth. (3) Saline sediments containing lenses of salt, gypsum, and impure clastic dolomites represented by the Cayugan group which were formed in shallow, practically isolated interior water basins. ' The disconformity between the top of the Ordovician represented by the Richmond and Queenston shale, and the base of the Silurian represented by the Whirlpool sandstone is distinct. The Bass Island group of the west and the Akron dolomite of the east are put at the top of the Silurian and the disconformity between these formations and the basal Devonian is also well marked at a number of localities. Breaks in sedimentation occur at the base of the Lockport and Salina. Chapter vi contains notes on the salt, gypsum, petroleum, natural gas, and other materials of economic importance found in the area. The report is well illustrated and is a careful, detailed, and concise statement of the Silurian geology of southwestern Ontario. JES We The Geography of Maryland. By WrtitAM BULLOCK CLARKE. The Surface and Underground Water Resources of Maryland, Including Delaware and the District of Columbia. By Wm. BuLLock CiarkE, E.B. MatTuews, and E. W. Berry. Mary- land Geological Survey, Vol. X, 1918. Pp. 553, figs. 96. © Part Lis a brief discussion of the geology and physiography, including the Coastal Plain, Piedmont Plateau, and the Appalachian physiographic provinces, climate, flora and fauna, and the natural resources of the state. Among the chief resources may be mentioned coal, clays, building and decorative stones, limestone products, agriculture, and timber. A number of suggestions for physiographic and geologic excursions within the state are included. REVIEWS 675 Part IT is a more detailed discussion of the geology and physiography of the region. The geology is dealt with by physiographic provinces and includes sedimentary, igneous, and metamorphic areas, and, stratigraphi- cally, rocks from pre-Cambrian to Recent. ‘The discussion of the under- ground water resources, which forms the greater part of the paper, includes an explanation of the general principles involved and local de- tailed descriptions of the resources by counties. There are appended to the report eleven tables of statistics of various sorts. A. C. McF. William Smith, His Maps and Memoirs. By T. SHEPARD, M.Sc., F.G.S. Proceedings of the Yorkshire Geological Society, N. 5., Nol exe Part Lil yep. 273: William Smith was one of the pioneer English geologists in strati- graphic and areal work. The report consists of descriptions of his various maps and writings, the first produced in 1799 and the last in 1827. It includes many reproductions of the original diagrams and charts. A. C. McF. Upper Cretaceous of Maryland, Systematic Report. Maryland Geological Survey, 1916. Pp. 1022, pls. 7 (general), go (pale- ontological). I. The Upper Cretaceous Deposits of Maryland,’ by W. BuLtock CLARKE.—Under this heading is included a discussion of the general geology of the Coastal Plain region of the state, to which the Cretaceous deposits are limited, including the physiography, stratigraphy, structure, and conditions of sedimentation. A bibliography and table of distri- bution of the fauna and flora are also given. II. “‘Petrography and Genesis of the Sediments of the Upper Cretaceous of Maryland,’ by Marcus I. Gorpman. Based upon petrographic and field evidence-—The author finds three types of sediment present, (1) delta type, (2) lagoon type, and (3) open-water glauconitic type. A brief discussion of the origin of glauconite and the methods of petrographic examination is given. III. ‘‘The Upper Cretaceous Floras of the World,” by E. W. Berry. —No attempt at detailed correlations of these widely scattered floras is made. A discussion of the place of origin and subsequent migrations of the great dicotyledonous flora, which makes its sudden and dominating 676 REVIEWS appearance in the Upper Cretaceous, is given. ‘The flora shows great modernization compared with the Lower Mesozoic horizons. Extensive floral lists are given. IV. “Correlation of the Upper Cretaceous Formations,’ by W. BULLOCK CLARKE, E. W. Berry, and Jur1a A. GARDNER.—Complete accordance between the faunal and floral evidence seems to be lacking. The problems involved are discussed in detail. V. “The Systematic Paleontology of the Upper Cretaceous Deposits of Maryland,” by R. S. Basster, E. W. Berry, W. B. CLARKE, JULIA A. GarRpDNER, H. A. Pitspury, and L. W. STEPHENSON.—Some 325 species and varieties are described of which approximately one-fifth are fossil plants. The majority of these are figured. The volume contains ninety plates of excellent figures. The report is of importance to the stratigraphic and paleontologic world. A. CosMick? RECENT PUBLICATIONS —Ross, C.S. Structure and Oil and Gas Resources of the Osage Reservation, Oklahoma. Tps. 20 and 21 N., R. 12 E. [U.S. Geological Survey, Bulletin 686-N. Washington, 1919.] —SCHALLER, W.T. Micain1o18. [U.S. Geological Survey, Mineral Resour- ces of the United States, 1918. Part II: 26. Washington, 1920.| Thorium, Zirconium, and Rare-Earth Minerals in 1919. [U-S. Geological Survey, Mineral Resources of the United States, r919. Part II: 1. Washington, 1920.] —SCHOFIELD, S. J. Geology and Ore Deposits of Ainsworth Mining Camp, British Columbia. [Canada Department of Mines. Geological Survey, No. 99, Geological Series. Memoir 117. No. 1773. Ottawa, 1920.] —Scuuttz, A. R. A Geologic Reconnaissance, for Phosphate and Coal in Southeastern Idaho and Western Wyoming. [U.S. Geological Survey, Bulletin 680, 1918. Washington, ror1g.| —SCHWENNESEN, A. T. Ground Water in the Animas, Playas, Hachita, and San Luis Basins, New Mexico. With Analyses of Water and Soil, by R. F. Hare. [U.S. Geological Survey, Water-Supply Paper 422. Wash- ington, ror9.| —SCHWENNESEN, A. T., AND MeErtnzER, O. E. Ground Water in Quincy Valley, Washington. [U.S. Geological Survey, Water-Supply Paper 425-E. Washington, ror9.| —Scientia, Vol. XX VII, N. XCVI-a Series II. [Bologna: Nicola Zanichelli, 1920.] —SmitH, GEORGE Otis. Economic Limits to Domestic Independence in Minerals. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part I: A. Washington, 1o19.| U.S. Geological Survey, Thirty-ninth Annual Report for the Fiscal Year Ended June 30, 1918. [Washington, ror9.] —STANTON, T. W., AND VAUGHAN, T. W. The Fauna of the Cannonball Marine Member of the Lance Formation. [U.S. Geological Survey, Professional Paper 128-A. Washington, 1920.] —STEBINGER, Eugene. Oil and Gas Geology of the Birch Creek-Sun River Area, Northwestern Montana. [U.S. Geological Survey, Bulletin 691-E. Washington, ror9.| —STEPHENSON, L. W. A Contribution to the Geology of Northeastern Texas and Southern Oklahoma. [U.S. Geological Survey, Professional Paper 120-H. Washington, ro19.] 677 678 RECENT PUBLICATIONS —Stone, R. W. Magnesium. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part I: to. Washington, ro19.] Sand and Gravel. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part II: 25. Washington, 1919.] —Tuomson, J. A. The System of Animate Nature. Vols. I and II. [New York: Henry Holt & Co., 1920. (Price $6.00 net.)| —Universo, L’. Anno 1, Num. 2. Marzo-Aprile, 1920. Institute Geo- grafico Militare. [Firenza, 1920.] —Voct, J. H. L. Om Manganrik Sjgmalm. I Storsjgen, Nordre Odalen. [Den Tekniske H¢giskoles Geologiske Institute, Meddelelse Nr. 6. Norges Geol. Unders.. Aarbok, rors, VI! Kristiania.|) ~ Die Sulfid: Silikatschmelzlésungen. ([Saertryk av Norsk Geologisk Tidsskrift. Bind IV. Kristiania, 1917.] Die Sulfid: Silikat Schmelzlésungen. I. Die Sulfidschmelzen und die Sulfid: Silikatschmelzen. Videnskapsselskapets Skrifter. [I. Mat.-Nat. Klasse. 1918. No.1. Kristiania: I Kommission Hos Jacob Dybwad. t1or10.] —Warinc, G. A. Ground Water in Reese River Basin and Adjacent Parts of Humboldt River Basin, Nevada. [U.S. Geological Survey, Water- Supply Paper 425-D. Washington, roz9.] —WASHBURNE, C. W. The Capillary Concentration of Gas and Oil. [Re- printed from Bulletin No. 93, September, 1914, American Institute of Mining Engineers. New York, 1914.] Oil-Field Brines. [Reprinted from Mining and Metallurgy, No. 164, August, 1920, American Institute of Mining Engineers. New York, 1920.| —WE Its, R.C. Sodium Salts. [U.S. Geological Survey, Mineral Resources of the United States, 1917. Part II: 23. Washington, 1919.] —WestTcaTE, L. G. Deposits of Iron Ore near Stanford, Montana. [U.S. Geological Survey, Bulletin 715-F. Washington, 1920.] —WuitE, Davin. Contributions to Economic Geology, 1917. Part II. Mineral Fuels. [U.S. Geological Survey, Bulletin 661. Washington, 1919.| Shorter Contributions to General Geology, 1917. [U.S. Geological Survey, Professional Paper 108. Washington, ro19.] Structure and Oil and Gas Resources of the Osage Reservation, Oklahoma: Introduction. [U.S. Geological Survey, Bulletin 686-A. Washington, rgr1q.| —YAaLE, C.G. Gold, Silver, Copper, Lead and Zinc in California and Oregon. 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THE UNIVERSITY OF CHICAGO PRESS CHICAGO - - ILLINOIS VOLUME XXIX NUMBER 8 THE JOURNAL OF GEOLOGY NOVEMBER-DECEMBER 1921 DIASTROPHISM AND THE FORMATIVE PROCESSES XV. THE SELF-COMPRESSION OF THE EARTH AS A PROBLEM OF ENERGY T. C. CHAMBERLIN The University of Chicago The discoveries of the last three decades have led to new views of the constitution of matter, new evaluations of cosmic energy, new estimates of evolutionary rates, and new concepts of the time factor generally. Nearly all the fundamental concepts of geology need some degree of revision in the light of these radical advances. Among the rest there is need to rectify the concept of the earth’s compression. THE CONCEPT OF COMPRESSION IN THE LIGHT OF NEW CONCEPTS OF MATTER So long as matter was supposed to be formed of minute irre- ducible atoms, it was logical to assume that when these atoms were pressed into contact there was an end of compression. It was also quite natural to build upon this mechanical concept a merely mechanical notion of the process of compression. The new discoveries, however, lead to the view that the atom is a highly dynamic organization, a complex revolutional system, carrying within itself prodigious stores of energy and a structure as open as a planetary system. The materialistic factors—if 679 680 T. C. CHAMBERLIN indeed they are really materialistic at all—recede to minute points, and do little more than play the part of carriers of electric charges. The prodigious energies of the atoms seem to be stored in the extremely rapid revolutions of these charged integers and in the fields of force and the polarities which arise from them. The atom itself and all the combinations into which it enters are therefore to be regarded as theoretically compressible to an undefined degree. The old assigned limit vanishes, and no new one takes its place. For aught that is now known, even the nuclei, or protons, and the electrons may themselves be composite dynamical organizations and subject to compression. The fact that a nucleus has a mass 1,800 times that of an electron suggests that perhaps analysis has yet one or more steps at least to take. Compression is therefore to be pictured as the struggle of one phase, or one set of phases, of energy against another phase, or set of phases, of energy, both sets being embodied in motion. While perhaps it cannot yet be said to be strictly proved that the positive and negative charges are in revolution about one another, there seems to be no other way in which the prodigious energies associated with them can be stored without giving such evidences of themselves as characterize the non-revolutionary activities, distinctions to be considered later. Moreover, the notable successes of the revolutional hypotheses in accounting for observed phenomena leave little room for doubt that they are substantially true, and may be taken as a fairly safe working-basis. In addition to the evidences of the atoms themselves, the analogies of the larger units of the cosmos lend support to the view that the atoms are revolutionary organizations. THE CONTRASTED MANIFESTATIONS OF ENERGY In the great stellar field, where the largeness of things makes visualization easier than in the hidden ultra-microscopic world within, energy manifests itself in two rather distinct kinds of activity. ‘The one is continuous motion in cyclic orbits or spiraloid revolutions running on indefinitely without loss of energy. It is, therefore, conservative and singularly undemonstrative. In the other, the motion is habitually interrupted by reversals and so is DIASTROPHISM AND THE FORMATIVE PROCESSES 681 discontinuous and disjunctive, giving rise to diversions and scatter- ings of energy in oscillatory radiations. This vibratory phase of activity is at once dissipative, agitative, and demonstrative. It has a general destructive tendency, while cyclic motion has a general constructive tendency. However, by weakening old structures, vibratory action prepares the way for new construction. The two types are therefore co-operative as well as antagonistic. The vibratory type has its chief manifestation in the heat-light- X-ray series; the cyclic type, in the planetary-stellar revolutional systems, and in atomic, molecular, and crystalline organizations. In application to material substances, the revolutional type is predominant in atoms, molecules, crystals, and true solids generally; the vibratory type is most manifest in the true fluidal states; in a special sense it may be said to dominate gases. THE RELATIVE ENERGY-VALUES OF THE TWO TYPES In the ultimate analysis of all the cosmic states taken together, the revolutional type greatly preponderates in energy-value. This is not in accord with our sense-impressions. It is a rather singular fact that the values of these contrasted phases of energy are inversely proportional to their obirusiveness. Neither rotations nor revolutions are notably demonstrative, while potential energy of position is only visualized by a mental effort, if visualized at all. The rotation of the earth involves a motion of a fraction of a mile per second; its revolution involves a mean motion of 18 miles per second, while its potential energy of position has a value of 356 miles per second. In this only relations to the sun are included; relations to the rest of the cosmos, in which further great, but only partly known, stores of energy are involved, are neglected. Over against these great but unobtrusive forms of the earth’s energy, stand the very impressive vibratory energies of the heat- light-X-ray series, the specially obtrusive and spectacular energies of the cosmos. While the precise sum total of these cannot be given for lack of adequate data, an excessive estimate may easily be made, and this will serve as a limiting value. According to Lane’s law the highest temperature of a condensing body occurs at the stage when it is passing from the gaseous to the liquid state. 682 T. C. CHAMBERLIN Let this stage be assigned the earth in its early history to give it a maximum value of the agitative type of energy. It must then of course have extended far outside its present solid surface. The parabolic velocity—the velocity that carries to infinity—at the present surface, is 6.95 miles per second. It is obvious therefore that the mean velocity of the molecules of the earth-substances could not have been so high as this without dissipating the earth. The maximum mean velocity of the earth-molecules must, therefore, always have been appreciably lower than 7 miles per second. We have, therefore, as the respective mean velocities, something less than 7 miles per second for the vibratory energy, something more than 18 miles per second for the revolutional velocity and—neglecting the rotational velocity altogether—356 miles per second for the potential energy. As the mass is the same in all cases, the energy-values are as the squares of these figures. Reduced and combined, the ratio of the vibratory energy of the earth, on the most generous allowance, cannot be more than 1/2600 of that of the revolutional energy, even when a large factor isneglected. The purpose of this comparison is to show the exagger- ated importance that has been given to the agitative phases of energy, as also to the gaseous state, in the study of the earth’s energy-values. In this, however, we have only considered the megascopic motions of the earth. We have yet to consider the ultra-microscopic phases in which prodigious energies are even more unobtrusively concealed. To approach the ratio between the dissipative and the con- structive classes of energy in the earth-matter itself, let the familiar case of a bowlder on the surface be taken. Let it have the mean temperature of the earth’s surface, say 15°C. Its absolute tempera- ture will then be about 288 centigrade degrees. ‘This represents a linear extension of about .0057. All the rest of its extension represents the work of constructional energy—here interpreted as revolutional energy—except the space occupied by the atomic nuclei and the revolving electrical charges. While the total value of the energy of the revolving constituents of the atoms is undeterminable at present, it is certain that it is almost incompar- ably greater than that of the 288° C. temperature. DIASTROPHISM AND THE FORMATIVE PROCESSES 683 When this atomic energy, which is even more unobtrusive than the energies of celestial revolution, is added to the macroscopic energies, the disparity mounts up to a very high figure. The agitative energies that so deeply impress our senses are really little more than trivial, relatively, in the true cosmic scale. Now, the resistance that is offered to the compression of an earth made up of solid matter springs mainly from the forces that determine the constitution of this matter. The analysis of these constitutional energies, as now interpreted, involves the electronic revolutions, together with the fields of force and the polarities that spring from them. These may not be all the forces involved— very likely they are not all—but they form the truest picture now available and they may be taken as representative. They are herein made a working-basis, subject to correction as additional light is disclosed. THE RELATIVE ENERGY-VALUES OF THE POSTULATED EARTH-FORMING NEBULAE In estimating the potential energy of the nebular matter which, by hypothesis, was condensed to form the earth, in each of the two representative views, the planetesimal and the gaseous or quasi- gaseous, it is assumed, in both cases, that the earth was formed in essentially its present position and relations in the solar system. In Article XIII of this series, a conservative estimate of the belt occupied by the planetesimals that were later to form the earth, gave it a space-value of 9 X10” cubic miles. The gaseous nebula that was to form the earth, measured at the time it first came into self-control and was most extended, had a volume less . than 3.5 x<10'° cubic miles. The ratio is about 250,000 tox. The vastly superior space occupied by the planetesimals, however, does not carry proportional value in potential energy. Its importance chiefly lies in the mode of support of the planetesimals and in their modes and rates of assemblage. CONTRASTED MODES AND RATES OF ASSEMBLAGE The modes of concentration were radically different. The planetesimals were sustained in their orbits by velocities of a t Jour. Geol., Vol. XXVIII (1920), p. 678. 684 T. C. CHAMBERLIN mean value of about 18 miles per second. Thus sustained, they only joined the collecting nucleus as variations in their orbits brought them into conjunction with it—a slow process, occupying perhaps two or three billion years.t The intervals between the infalls of the planetesimals were, therefore, such that nearly all the heat of their impact with the atmosphere and with the earth’s surface was lost before they were buried by added material. The growth of the earth was thus made by the slow accumulation of essentially cold, solid particles mixed at random. On the other hand, if the earth-forming nebula be assumed to have been gaseous and to have descended along the gaseous line, its volume was sustained by collisions and rebounds of the con- stituent molecules, and it contracted as fast as the loss of this interaction, i.e., the loss of heat, permitted. Under Lane’s law the maximum temperature was reached at the stage when the gaseous body passed into the liquid state. As radiation follows the law of the fourth power, the collapse was relatively rapid; at the most it cannot be assigned more than a few million years. Enormous losses of energy would be suffered in either the plane- tesimal or the gaseous mode of assemblage, and so we must take up the question of the earth’s primitive energy presently from the opposite point of view: What energy-values were /efét for the evolution after the earth was able to make a record of its own compression? ‘The point of most importance here is the radical difference in the respective factors that controlled the self- compression which followed the nebular concentration. It is obvious that the gaseous descent was controlled by heat and that this remained the master factor in the shrinkage of the earth after it became a white-hot molten globe. In the self-compression of the earth built of solid planetesimals, or planetesimal dust, solidity was the primary resisting-factor that held the compression in check. The energy-factors in this case were those to which the solidity was due. These are herein interpreted as revolutional phases of energy together with their derivatives. Heat in one case and solidity in the other were then the master factors in the t See ‘‘The Rates of Planetesimal Infall,”’ Article XIII, Jour. Geol., Vol. XXVIII (1920), pp. 677 ff. DIASTROPHISM AND THE FORMATIVE PROCESSES 685 compression process. In the latter case, the heat generated in the course of the compression was secondary to the revolutional energies. The special courses taken by both the primary and secondary energies become therefore vital elements in the com- pressional process; to these we shall presently turn. As indicated above, to form estimates of the energy-values that were inherited by the earth at the stages when it began to make its automatic record of self-compression, it is necessary to enter into a more specific analysis of its status in the two hypothetical cases. COMPARATIVE VALUES OF ENERGY AVAILABLE FOR DIASTROPHISM The deformations of the earth are the most available test of the energies that entered into its self-compression, though by no means the only test. There is now no ground to doubt that the diastrophism was large, whatever estimate may be made of its precise value. There must have been enough energy in an avail- able form to actuate the distortions involved, and this energy must have been properly distributed in time and place. The sources of this energy need therefore to be considered in respect to their availability, as well as their adequacy. Fortunately, the problem for all cosmological lines of descent seems to center in the alterna- tive: Was the earth assembled in a fluidal condition dominated by heat, or was it built up gradually by accessions of small frag- mental matter in a cool, solid, highly mixed state? If there are tenable hypotheses of an intermediate sort, the considerations that apply to these type-views can easily be adapted to them. The chief energies available for the evolution from this point on, are (1) the residue of the potential energy of position, except of course what still remains potential; (2) chemical and physical combinations, readjustments, and reorganizations, so far as condi- tions permitted them to take place during the compression; and (3) the disintegration of radioactive substances, including any other changes in atomic constitution that may have taken place, if any. ‘These atomic factors may possibly have some relation to the extreme stresses that arose from compressional action, but as 686 T. C. CHAMBERLIN there is now no evidence of this, they must be treated under a head of their own. 1. The period of compression.—lf the earth remained fluidal until all its rock-substance was condensed into a globe, none of the energy lost in the assembling was available for making the observed diastrophic record, since this could only begin after consolidation began. If, on the other hand, the earth was built up of small solid accessions loosely laid down, these must have begun to suffer com- pression and distortion as soon as one layer was laid on another. The distortional process must in this case have run on thence through the whole history of growth. The compressional and distorttonal actions were furthermore brought on very gradually and great lapses of time were available to meet the growing stresses by the resources of readjustment, reorganization, metamorphism, and diastrophism. 2. Availability of the main compression.—lf the earth was assembled in a fluid state, the interior underwent the full measure of fluidal compression from gravitative action before it could make any diastrophic record; little more than the effects of cooling remained available for deformative work after solidification took place. If, on the other hand, the material of the earth was added slowly in a loose, solid state, the main compressive effects entered into the record; for while the distortions in the deep interior would never be accessible, they must have been at all stages the foundation on which the later accessions were built and hence they gave direction to, as well as participated in, the stress effects that arose at every subsequent stage in the increase of mass. They must still continue to participate in the effects of all the more general changes in gravity. 3. Chemico-physical combinations, readjusiments, and reorgant- zations.—If the earth remained fluid and convective until fully assembled, almost ideal opportunities for chemical combination and physical adjustment, as well as chemico-physical reorganization, would have been offered, except in so far as the heat itself may have restrained such action. To this extent the chemico-physical resources should have been exhausted before they became available for diastrophism. But if the earth were built up of solid particles DIASTROPHISM AND THE FORMATIVE PROCESSES 687 of various sorts mixed by the chance of infall, it would offer almost ideal conditions for recombination, readjustment, and reorganiza- tion, which, in this case, would run hand in hand with diastrophism and contribute to it. 4. Relative exhaustion of potential energy by segregation.—li the earth was fluid until fully assembled, there should have been facilities for the arrangement of the earth substances in concentric layers according to specific gravity. This would have been a special means of reducing to the lowest terms the potential energy that might otherwise have remained available for deformative work after solidification made a diastrophic record possible. If, on the other hand, the matter remained a heterogeneous mixture so far as intrinsic heaviness was concerned, a corresponding amount of potential energy remained available for the diastrophic record. In so far as segregation by gravity took place during the com- pression of the mixed solid mass, it co-operated with other deforma- tive processes and left its effects in the record. 5. Relative exclusion or retention of gaseous consittuenis.—lf the earth remained fluid and convective until fully assembled, the gaseous constituents should have had favorable opportunities for escape and should have been impelled to escape by the very high heat, so that only such quantities as were required to balance the partial pressures of the same constituents in the atmosphere should have remained to take part in vulcanism later. On the other hand, if the earth was built up by solid particles added slowly to the surface and subjected to weathering and to mixture with air and water, as it was gradually buried, the complex should have | afforded almost ideal conditions for the evolution of volcanic gases when it was later subjected to heat and pressure. The phenomena of the moon are especially instructive in this respect, for the gravity of the moon is insufficient to hold free volcanic gases even in its present cold state; much less then in a hypothetical molten state. No equilibrium factor should have been retained in this case. But the evidences of vigorous explosive action on the moon are very pronounced. * Rollin T. Chamberlin, ‘‘The Gases in Rocks,” Jour. Geol., Vol. XVII (1909), Ppp- 565-68. 688 T. C. CHAMBERLIN 6. The distribution of the radioactive substances —If the earth were assembled in a fluid state, the radioactive substances should have settled toward the center because of their high specific gravity, or else, if convection prevented this, they should have been dis- tributed sub-equally through the whole mass. There should at least have been no concentration of such heavy material in the upper layer. But the special investigators of the subject agree that if the whole earth were as rich in radioactive substances as its accessible portion is, the heat generated would be many times greater than the heat now conducted to the surface and radiated away. Were this true, the earth should have been growing hotter all through its history and no shrinkage at all could be assigned to cooling. On the other hand, if the earth were built up of hetero- geneous clastic matter that carried its chance portion of radio- active particles, and if these, by their heating action, liquefied the most susceptible matter immediately enclosing them, and if such liquid matter were then squeezed to or toward the surface by the powerful extrusive agencies that belong to a solid earth, the radio- active substances would be concentrated in the zone of lodgment of these igneous portions. ‘This limits the radioactivity to a degree that seems to fit the observed facts and the theoretical intimations of the case. It is quite obvious that, so far as deformative effects assignable to cooling are concerned, the hypothesis of a molten earth is seriously embarrassed by this newly discovered source of heat superposed on an already embarrassing inheritance of heat from its earlier history, while under the hypothesis of a cold-grown solid earth, it is a welcome agency. The combined import of all the preceding considerations leaves the fluid earth embarrassingly short, if not fatally short, of resources of energy available for making the observed diastrophic record, while the planetesimal earth is much more amply, and apparently quite adequately, supplied with such energy, and this becomes available in such a slow way as to give great allowances of time for the increments of compressive stress to work out their adjustments and easements along metamorphic and diastrophic lines. DIASTROPHISM AND THE FORMATIVE PROCESSES 689 THE INTERCHANGES BETWEEN THE TWO BASAL TYPES OF ENERGY Before taking up the special modes of the compressional process, the interchanges between the two basal types of energy need con- sideration. ‘The constructional and the agitative phases of energy are not only interchangeable but interchanges are persistently taking place on the surface and within the earth, and these inter- changes play a vital part in the process.of the earth’s self- compression. ‘The proper recognition of these is indispensable. Exchanges between thermal and mechanical energy are too familiar to need notice; they are a basal feature in modern industry. But exchanges between agitative and organizing energy, Le., between vibratory and revolutional energy, as such, though they may not differ in essence from the well-recognized interchanges, need a word of emphasis. Some of these changes from the agitative to the constructive are even more familiar than the mechanical changes, but interpretation has not given them the value to which they are entitled. We know that the grass and the trees grow, but we easily overlook the fact that such growth is a widespread and important endothermal process. It belongs to the unobtrusive class and does not enforce attention. The prairie fire and the holocaust of the forest, the complementary exothermal process, command our lively attention. The unobtrusiveness of endo- thermal action is likely to deceive us as to the balance between interchanges of energy in nature. The problem in any special case is to determine the balance between opposing actions. ‘There is, however, no doubt as to a real preponderance of exothermic action on the earth’s surface. When lavas come up from below, they usually undergo exothermic reactions to a greater extent than endothermic reactions. This in itself raises the question whether endothermic reactions are not preponderant in the region whence the lavas come. Van Hise,’ Leith,? and their associates, have shown by the extensive collection and study of data from the full tC. R. Van Hise, ‘‘A Treatise on Metamorphism,” Monogr. XLVII, U.S. Geol. Surv. (1904). 2C. K. Leith and W. J. Mead, Metamorphic Geology, Henry Holt and Co. (1915). See particularly the chapters on ‘“‘Katamorphism” and ‘‘Anamorphism”’ in both works. 690 TI. C. CHAMBERLIN range of the accessible terranes, that while exothermic reactions preponderate in the outer or katamorphic zone, the preponderance is reversed below and endothermic reactions take precedence in the anamorphic zone. It is to be noted that while katamorphic action, exothermic action, and the lowering of density, commonly go together, as also anamorphic action, endothermic action, and rise of density, they do not invariably coincide; and further, that none of these necessarily excludes the others from any horizon. The essential question is not one of exclusive action but of pre- ponderant action.’ It is in the natural order of things that in the great contact zones between the atmosphere, the hydrosphere, and the lithosphere, there should be a trend of energy toward its agitative phases, and that in the stabler solid zones below there should be a compensating trend toward the constructional and the persistent, without limiting either zone to one type of action. In terms of the two basal types of energy, exothermic action, on the average, involves a change of organizing or revolutional energy into vibratory-dissipative energy; while endothermic action is commonly the reverse. THE CONDITIONS THAT DETERMINE THE INTERCHANGES In a very broad sense, open conditions and freedom from pressure or other forms of restraint, favor exothermal reactions, while confinement and pressure favor endothermal reactions. Any form of crowding, even self-stress, naturally tends toward divergence of energy into the various paths available to it, since this affords relief. ‘The higher the stress, the more it forces itself into unoccupied paths. Concentrative stress, therefore, favors the passage of a portion of the energy along endothermic lines and the formation of dense substances; while dispersive stress, low stress, and no stress, are less compulsory and give exothermic action freer scope. Apparently crowding is not confined to imposed stresses, but arises from what may be styled the self-stress of the activity. A small mass of gas in open space exerts little interior stress upon tCompare C. K. Leith, “‘The Structural Failure of the Lithosphere,” Vice- Presidential Address, Geol. Soc. Amer., Science, N.S., Vol. LIII (March 6, 1921), pp. 205-7. DIASTROPHISM AND THE FORMATIVE PROCESSES 691 itself, and only the larger vibrations are in evidence, but if the mass grows indefinitely the internal self-stresses increase and there appear in succession the shorter and more intense vibrations ranging up through the whole gamut of vibrations to the X-rays and doubtless beyond. There is, in this, increased pressure, of course, but the activity itself is increasingly divergent as well as increased in amount. However this may be interpreted, there is a growing complexity of vibration, and it seems to bea safe generaliza- tion that growing mass and growing internal pressure are attended by increase in the diversity of phases assumed by the compressional energy; in other words, there are more varied partitionings of the energy and it takes a larger number of paths, including more fre- quent interchanges between the endothermic and exothermic phases. As there is thus crowding in various directions for ease- ment, the direction that gives greatest relief from the stress imposed by the environment naturally becomes a predominant trend. Where there is high pressure and it is unescapable, the line of relief is the passage of energy into a constructional form that gives additional density. Where the pressure is weak or absent, an expansional or dispersive form of energy may be more efficient in giving relief. Both forms are likely to be present and to co-operate with one another in any pronounced case. THE TESTIMONY OF PRESENT INTERNAL STATES AS TO THE DOMINANT DIRECTIONS TAKEN BY ENERGY IN THE INTERIOR Tidal* and nutational? evidences concur in indicating a higher degree of rigidity and elasticity in the interior, taken as a whole, than in the outer shell. Seismic waves add very specific confirma- tory evidence, so far as the outer seven-eighths of the volume of the earth is concerned. The seismic evidence for the remaining central part is as yet obscure, and is differently interpreted by the special students of the subject. Ina general way, the whole of the interior is covered by the tidal and nutational evidences. These favor the interpretation of the central part as highly rigid and elastic, since t A. A. Michelson and Henry G. Gale, ‘‘The Rigidity of the Earth,” Jour. Geol., Vol. XXVII (1919), pp. 585-601. 2W. Schweydar, “Die Elasticitat der Erde,” Naturwissenschaften, Part 38. Potsdam, Germany (1917). 692 T. C. CHAMBERLIN these qualities fit the general import of the evidence, but for the present it is prudent to leave the question of the state of the center to be settled in the future. It is to be observed that the increasing density of the interior tends to dampen the speed of the seismic waves, and that correction for this effect must be made in deducing the inward increase of rigidity and elasticity from the seismic records. When allowance is made for this, the generalization that rigidity and elasticity are notably higher in the interior than in the outer shell is put beyond serious question. ‘This means that in the partition of the compressional energy between those phases that increase the rigid elastic attachments of the molecules to one another and those phases that weaken or destroy these attachments, the former have been favored in a marked degree. This is testi- mony of a most cogent sort. By interpretation, this signifies that only a minor part of the compressional energy took the vibratory form in the interior, the major part taking the revolu- tional or constructive form, and that in doing this it served to promote compactness and the strength of hold of the constituents on one another. DID THE ORGANIZING ENERGY EFFECT CHANGES IN THE CONSTI- TUTION OF THE ATOMS? Lest the seeming needs of the case bias us toward one con- clusion rather than another, let us hasten to note that the mean density of the earth, compared with the probable density of the original matter, is such as to offer no real ground for bias in favor of atomic construction, for the higher density of the interior is fully accounted for by the density gradients that arise from meta- morphism in the zone of observation. Atomic construction, if invoked as an aid, might as easily render the interior mass too dense, as to help explain the density as it is. Nor is there more than un- certain evidence bearing on the atomic question; but the matter is too important to be ignored in a discussion of the effects of com- pressional energy. The most remarkable of known exothermal effects connected with rock-substance springs from the spontaneous atomic disinte- gration of the heaviest known elements. No evidence that this DIASTROPHISM AND THE FORMATIVE PROCESSES 693 disintegration has anything to do with relief of pressure, such as might be assigned to their rise from the interior, is now available. Any such possibility must be left to the revelations of the future. It is logically necessary, however, for one who believes in the indefi- nite cyclic persistence of the cosmos, to suppose that the present exothermic action is the reversal of an endothermic process that gave these elements the stores of energy they are now so persistently and systematically discharging. ‘The place and time and condi- tions of this storing action are altogether open questions. By interpretation, the energy now being given out springs from intense revolutional action, for revolutional motion seems to be the only probable way in which such prodigious energies can be stored in so unobtrusive a state and given out so regularly and systematically and in such concentrated forms. ‘The storing process must prob- ably have involved somewhat similar forms and intensities of action. One of the most common speculations as to the place and condi- tions of this storage process, locates it in some center of great stress where pressure and heat co-operated. This should perhaps be amended by recognizing that the more intense vibratory agencies of the X-ray end of the series were even more probable agencies, because their motions were more nearly commensurate with the minute and swift revolutions that are supposed to store the energy in question. The center of the earth is possibly a place of the right type, but it belongs to an inferior order compared with the centers of stars, unless solidity counts for something. In this case the center of the earth might have a preferred place, since our planet is among the largest of known solid bodies. An alternative specula- tion places the origin of the radioactive substances in the outlying regions of space. There is perhaps a suggestion of general atomic change in the remarkable phenomena of thermionic emission, contact potentials, and photoelectric action. These seem to imply that there is some kind of commensurability between the extremely intense oscilla- tions of the vibratory activities and the orbital periods of the electrons, so that effective interaction and perhaps interchange takes place between them. Commensurability is perhaps the property by which interchange is effected between the minutely 694. T. C CHAMBERLIN vibratory and the minutely revolutional phases of energy. These speculative suggestions have little value beyond helping to make it clear that it is by no means safe to assume that atomic con- struction and destruction are not common functions of the interior. WHAT AMOUNT OF COMPRESSION IS IMPLIED BY THE MEAN DENSITY OF THE EARTH ? As already noted, there is no need to push appeal to the organiz- ing functions of energy in the interior so far as to assume the building up of atoms for the sake of explaining the higher density of the earth’s interior; indeed, if there is any constructive work of that sort, the increase above the decrease of density cannot go very far without making the mean density too great to fit the evidence. If we assume that the primitive matter had the meteoric density of 3.69 adopted by Farrington, and compare this with 5.53, the mean density of the earth adopted by Moulton, the mean increase in density due to compacting, reorganization, atomic change, etc., is only about 50 per cent. Or if, to assume an improbably low figure for the density of the earth’s original matter, we take the moon’s mean density, 3.34—assuming that the effects of com- pression at the moon’s center are offset by the porosity of its outer part—the increase in the earth’s mean density would be only a little over 65 per cent. In either case, or on any plausible assump- tion, some part of the compacting must be assigned to mechanical compression, so that the increase of density assignable to reorganiza- tion under the special conditions of the interior is not very large. THE INTIMATIONS OF THE DENSITY GRADIENT IN THE ZONE OF OBSERVATION Geologists have been at great labor to compile thermal data from mines and deep borings that they might deduce from these a temperature gradient that would throw light on interior conditions, but the same line of attack on the rising density of the interior seems to have been overlooked. It is to be recognized, of course, that neither of these gradients can be projected to the center of the earth without reservation, for both curves probably fall off notably in the interior, but the density curve is probably as trustworthy a DIASTROPHISM AND THE FORMATIVE PROCESSES 695 guide as the temperature curve, for, in the planetesimal earth, both arise from compression and its direct and indirect conse- quences. After an elaborate study of the most reliable data, Dr. H. S. Washington thus sums up his conclusions in a recent paper: ‘“‘T am inclined to place the average density of the crust at about 2.75 at least for the uppermost shell, while that of 2.80 would prob- ably be nearer the truth for an average of any considerable depth, say 20 miles or more.’”’ The mean depths of these two shells can scarcely be more than 8 or 10 miles apart. The rise of density in this little difference of depth, if projected to the earth’s center, would give a density there nearly twice that computed from the classic law of Laplace, or from the law of Roche specially formu- lated to meet the astronomical requirements. No account is here taken of mechanical compression, for the specific gravities adopted as the basis of the estimates were all taken under atmospheric pressure. Much less was any account taken of hypothetical quantities of metals or other specially heavy material, for both these shells are formed of common rock. The two elements most common in the zone of observation, oxygen and silicon, often unite to form tridimite in the outermost shell but not in the plutonic rocks, where the same elements appear as quartz. This distribution is commonly assigned to differences in the physical conditions of the two horizons, especially differ- ences in pressure. The specific gravity of tridimite ranges from 2.28 to 2.33, while that of quartz is 2.65. There is thus a rise of density of 15 per cent, so far as these minerals are concerned, between two horizons both of which lie in the limited zone made accessible by deformation and denudation. The most instructive and suggestive data, however, are found in the progressive stages of increase in density developed in several different kinds of silts as they pass into various kinds of schists, and thence, in part, into.a group of heavy minerals of which the garnets may be taken as types. The compression of the silts into shale may be neglected since a notable part of the increased density tH. S. Washington, ‘‘The Chemistry of the Earth’s Crust,” Jour. Franklin Inst., December, 1920, p. 804. 696 T. C. CHAMBERLIN of the shale was due to the mechanical elimination of porosity. In forming the schists there was true reorganization with increased density, and still later there was further partial reorganization into much heavier minerals of the garnetic group. In the case of the garnets there is a rise in density from the schist minerals which formed them of 36 to 84 per cent, as pointed out by Van Hise.* In these cases the rise of density is unequivocally the result of the metamorphic reorganization of very common and representative kinds of material. It is to be noted further that one reorganization follows another even in this limited zone. These specific cases show the possibilities and the actual tenden- cies to increase of density by metamorphism, quite apart from mechanical compacting. They point to the very significant fact that the chief density effects are to be sought in metamorphism rather than mere mechanical compacting by pressure. The crux of the compressional problem therefore lies in metamorphic reorgani- zation, in selective liquefaction, and in the extrusion of magmas. The concrete task thus imposed is the tracing of the paths followed by the compressional energy and the study of the kind of work each phase of energy does. The specific phenomena to be explained are (1) the rising density, rigidity, and elasticity of the interior material; (2) such a distribution of density as to satisfy the intimations of the precession of the equinoxes and the nutation of the poles; (3) the amounts and kinds of diastrophism recorded in the structure of the earth; and (4) the protrusion and persistent maintenance of the continents and the complementary depression of the ocean basins, as well as the special configurations of the earth. It is not suffi- cient to explain these separately by isolated postulates. unless these are shown to be mutually compatible and connected with a common origin; these features are to be explained as a co-ordinate group of phenomena arising from a common origin and a common line of dynamic procedure. THE SPECIFIC PATHS OF THE COMPRESSIONAL ENERGY In the following analysis, it is to be understood that the earth is assumed to be, and to have been at all stages after the formation ™C.R. Van Hise, of. cit., pp. 205 ff. DIASTROPHISM AND THE FORMATIVE PROCESSES 6097 of its collective nucleus, a solid body, built of heterogeneous plane- tesimal matter at the start, that it was subjected to a slowly growing gravitative pressure whose total accession was spread over a period of the order of two or three billion years, and that there were large allowances of time for metamorphism and for the adjustment of the strain arising from each increment of pressure. On these assumptions the chief partitions and paths of the com- pressional energy, seem logically to have been as follows: The first step was the partition of the initial increment of energy by the passage of a part of the stress into strain, while another part took the thermal form. So long as the strain lasted, its energy was stored or latent. A varying but large measure of energy seems thus to have been stored all through the geologic ages. It appears from stratigraphic evidence that the strain- limit in the sub-surficial material has been high enough at several periods to permit the accumulation of stored energy sufficient to - actuate declared deformative ‘‘revolutions”’ in spite of such partial easement as may have taken place, during the stages of accumula- tion, from the milder forms of idiomolecular action about to be described. The second step in the compressive process was the co-operative action of the stored energy of strain and the agitative thermal energy. The latter aided change by loosening the fixed elastico- rigid attachments of the molecules, the essential properties that gave rigid symmetry to the crystals and solidity to the amorphous fragments. The hold of crystals and of clastic fragments upon their constituent molecules was unequal because some of them lay at the angles, or on the edges, or on sharp curves of the little masses, where fewer other molecules supported them. So also the strains arising from pressure upon the interlocking crystals or fragments was greatest on these outlying molecules. The particular molecules thus least securely held and those most severely strained, yielded first. This eased the strain on these particular points and threw the stress on new molecules; from this, new action of like order arose and the process was essentially repeated.* tIn many cases, especially near the surface, solution and chemical reaction greatly aided molecular transfer and crystalline reorganization, but these are ac- cessory agencies rather than factors of the compressional process, as such. 698 T. C. CHAMBERLIN The detached molecules of this first action, responding to such stress as was then felt by them in their relatively free state, took the lines of least resistance until they reached some point where the organizing force of some crystal, so situated as to be able to grow, brought them under control and reattached them with due orientation. Such reattachments were obviously conditioned by the balance of strength between the crystalline force and the weakest phase of the general pressure. As a result the growing crystal extended itself most in the line of least resistance and co-operated with other crystals of like situation in developing parallelism of structure. It is obvious that such individual actions on the part of single molecules acting by themselves, and acted upon by special stresses, could take place while as yet the general strain was far below the strain limit and general detachment could not take place. If the pressure came on slowly enough, the whole crystal or fragment might be broken down in this piecemeal way while the general strain was below the mean strain-limit. As only a few molecules were in transit at any given time, the mass as a whole would remain solid throughout the process. The action was thus the special work of individual molecules, each suffering its own strain and playing its own part in its own way, ie., it was zdiomolecular. ‘The process is sharply distin- guishable from the common movement of all molecules, such as usually takes place when liquids flow. The process may be studied to advantage in the granulation of snow at temperatures that inhibit liquefaction.* So long as the stress and strain were mild, the foregoing action was obviously slow and had rather narrow limitations. But with notable increase of stress, giving rise to increase of strain, and increase of heat, the process appears to have been hastened and given a tendency to collective action in parallel lines, planes, or belts, doubtless because resistance was less effective against such tC. S. Peet and E. C. Perisho, working with the writer in the winter of 1894, found by daily micrometric measurements of many granules, that the larger ones grew every day whether the temperature was at, above, or below o° C., the growth appar- ently taking place at the expense of the smaller, more sharply curved granules. DIASTROPHISM AND THE FORMATIVE PROCESSES 699 united action. This apparently went so far in some cases as to verge toward general simultaneous molecular action, but analysis seems to show that it remained idiomolecular in actual method. From such quasi-collective but really idiomolecular action, cleav- age, schistosity, and other forms of structural parallelism arose. In glaciers this idiomolecular type of action seems to range from snow granulation to the point where fracture takes place and the movement becomes a massive shear.* By further increases of pressure, the strain limit along selected lines was reached and definite fracture and shear took place, or else the whole mass was crushed to fragments. In either of these cases, the action became diastrophic rather than metamorphic. It has been very generally held that when such depths and pressures are reached as to inhibit fracture, general movement of the molecules over one another after the manner of liquids must take place. The original idea of “rock flow” seems to have sprung from this notion. In a highly rigid body, such a general movement of molecules upon one another requires the breakage of all the bonds between molecule and molecule and so involves a maximum of force. Moreover, this force must come from dzffer- ential stress, since balanced stress, within limits, forces the mole- cules into closer and stronger relations. The supposed “‘flow”’ seems improbable except when true liquidity, which destroys the special rigid bonds of the solid state, is brought about. So far as differential stresses affect the solid matter of the interior, ease- ment by idiomorphic action, either collective or isolated, seems to require much less energy and is hence more probable. With the open structure now assigned matter, and with the abundant evi- dence that molecules really collect into crystals in the midst of rock that seems quite solid, and with other phenomena giving evidence that in some way molecules creep through solid matter, there seems no substantial ground for excluding idiomolecular action from the deep interior. t Tt was from the study of the granulation of snow, the growth of glacial granules, and the development of schistosity in the glaciers of Greenland, in 1894, that the method and importance of this individual action of molecules, while the mass remained solid, was first realized by the writer. ‘‘Glacial Studies in Greenland,” Presidential Address, Geol. Soc. Amer., Vol. VI (1895), pp. 209-14. 700 T. C CHAMBERLIN In tenacious solids where impact takes place at high velocities and with prodigious force, as in the case of a steel target struck by a solid shell, there is no time for idiomolecular action, and very little for any form of selective or metamorphic action, and so all molecules are apparently caused to move over one another in a way that is scarcely distinguishable, if at all, from real flow. The slowness of the increases of stress in the interior of the earth, how- ever, is thought to put deep-seated diastrophism in a quite different category from this velocity-stress action. Although fluidal action is placed in a secondary order in the evolution of a planetesimal earth, the formation and extrusion of magmas play a very important function in its compression, but that must be left for a later article. FIELD OBSERVATIONS IN NORTHERN NORWAY BEARING ON MAGMATIC DIFFERENTIATION STEINAR FOSLIE! Norges Geologiske Unders¢kelse, Kristiania, Norway Recent years have witnessed a marked development in the understanding of differentiation processes. Petrographers have tried to get away from the purely theoretical considerations about the matter and to harmonize the conclusions from the field observa- tions with the results of synthetical experiments on silicate minerals and their crystallization obtained by the Geophysical Institute at Washington and others. Although these experiments are still far from covering all subjects involved, and although the multitude of field observations from most parts of the earth are often contra- dictory, there seem to be certain lines of development which prove _ promising. The results now generally converge toward the conclusion that an ordinary fluid silicate magma, without concentrated mineralizers, is not capable of splitting up into two magmas mutually insoluble or with limited solubility. Accordingly, the main part of the differentiation processes is transferred to the period of crystalliza- tion, resulting in considerable restriction of possibilities. According to this supposition, naturally the first mode of separation to be considered was that of the heavier crystals from the lighter ones and from the still fluid magma by gravitative settlement. In a number of instances this sort of differentiation has definitely been proved to occur. But it also became obvious that this sort of separation could not alone account for a great many differentiation processes actually observed. The newer theory of squeezing differentiation—quite as well in accord with the latest results—seems to be capable of a more general application in those very frequent instances where lateral 1 State mining geologist, Geological Survey of Norway. 701 702 STEINAR FOSLIE pressure prevailed during crystallization. This theory was sug- gested by A. Harker, and especially after it had been formulated and developed by N. L. Bowen* it has aroused keen interest among petrographers. In the present paper I have attempted to give some examples from northern Norway bearing on this sort of differentiation. SHORT REVIEW OF THE GENERAL GEOLOGY OF CALEDONIAN As known, the Caledonian mountain chain—of late Silurian to early Devonian age—traverses the whole length of Norway from SSW. to NNE., in its northern part occupying nearly the whole breadth of the country. The axis of the chain forms a marked geosynclinal depression of the old Archaean Scandinavian shield and of the pre-Cambrian peneplain. The depression is filled with a very thick series of sediments, strongly folded and metamorphosed. It is supposed that the whole of Scandinavia has been covered by the Cambro-Silurian sediments, remnants of which are found at many places above the Archaean rocks. They are unmeta- morphosed and unfolded wherever well beyond the Caledonian folding region. Toward this old mountain chain, first folding sets in, then we meet an increasing degree of metamorphism, which attains its maximum at the axis of the chain. In the same direction the thickness of this series of sediments increases markedly. In the eastern, unfolded zone (especially in Sweden) the total thick- ness is generally only some few hundred meters, in the folded but unmetamorphosed Kristiania region the thickness surpasses 1,000 m., and toward the mountain chain it reaches several thousand meters. Although no determined fossils have yet been found in the metamorphosed sediments of northern Norway, there are several reasons to believe that the sediments here represent the same Cambro-Silurian series. The original sediments in the geosyncline were extensive layers of slates, marls, limestones, and dolomites, with subordinate sandstones. The metamorphism has been tN. L. Bowen, ‘‘The Later Stages of the Evolution of the Igneous Rocks,” Jour. Geol., Vol. XXIII (1915), suppl.; ‘‘Crystallization-Differentiation in Igneous Magmas,” ibid., Vol. XXVII (1919), pp. 393 fi. FIELD OBSERVATIONS IN NORTHERN NORWAY 703 very strong throughout the region, producing garnet-mica schists, marbles, quartzites, etc., but generally not with the development of lime-silicate minerals. In this region there has not been disclosed any unconformity or discontinuity in the sedimentation. During the Caledonian folding this series was intruded by great masses of eruptive rocks, especially in the axial part of the chain. Most of these are ordinary granites. There also occur in considerable quantities femic eruptives, very intimately intruded in the schists, and like these thoroughly metamorphosed to amphibolites and different eruptive gneisses. _ They are much differentiated, the last products generally being soda-rich granites (or Trondhjemites). Moreover, we find as isolated fields more extensive areas of gabbroidic eruptives, less metamorphosed, sometimes nearly fresh. They were also intruded during the Caledonian folding period and are chemically nearly associated with the former group, from which they do not differ very much, if at all, in age. They also show marked differentiation and on account of their relative freshness afford very interesting petrographical material. All the eruptives seem to belong to the same cycle of orogenic intrusions. Original lavas or tuffs, which occur in great masses in the Trondhjem district farther south, have not yet been identified among them. The intrusions have everywhere occurred under a heavy load of sediments, and the whole complex must have been heated to a considerable temperature. Outside the real root of the mountain chain all eruptives seem to have been intruded completely parallel to the schists. Espe- cially in the district to be considered here, we nowhere find crossing eruptive dikes or other eruptive bodies cutting the schistosity (excepting the pegmatite dikes). It seems to have been a general rule that the magmas, after intrusion, were subject to magmatic migration between heated schists, from the root of the mountain chain outward. In this way the magmas may have wandered for very considerable distances from the places where they broke into the schists, until they came to rest. The moving force was induced 704. STEINAR FOSLIE by the orogenic folding itself, and lateral pressure existed through- out the crystallization period. / THE RAANA NORITE FIELD (FIGS. I AND 2) This eruptive body, to be especially considered here, is situated at the south side of the Ofoten Fjord, west of the known ‘harbor of Narvik, at 68°20’ latitude. It has recently been closely inves- tigated by the author on account of discoveries in the last years of extensive but poor deposits of nickeliferous pyrrhotite. In the very rugged country with steep mountains rising directly from the sea and differences in height of more than 4,000 feet, the whole eruptive mass is exceedingly well exposed, and the results of differentiation can be followed in all details. The norite is injected into a thick series of garnet-mica schists with some interstratified layers of marble. The injection is parallel to the schistosity and forms a lens-shaped body with a thickness of about 3,500 m. and a diameter of about 12km. ‘The relative thickness of the lens is greater than is generally the case in this sort of intrusions, and is supposed to be due to the influence of some E.—W. folding axes. The section of the eruptive body with the present surface has an area of 67 km.?, 3 km.? of which has been cut off by erosion. No offsets or crossing dikes occur in the surrounding schists. The very first investigation teaches that the norite field is not homogeneous throughout, but is composed of a central mass of quartz-norite and a very considerable and continuous marginal zone of more femic normal norite, occupying the border against the schist all around the field to a width of one-third to one-fourth of the whole diameter. To understand the reason for this, it is important to know the tectonic position of the eruptive mass. One might of course be tempted to believe that the field after differentiation might have been thrown down in an inverted fold, and that the basic border zone accordingly should represent the lower part of the magma basin, separated out by gravitative differentiation in the same way as is the case in the Sudbury field. This, however, is definitely FIELD OBSERVATIONS IN NORTHERN NORWAY 705 proved not to be the case. The norite body forms a lens in its normal position between the schists, with a mean dip of about 30°, and the basic border zone is quite as well developed in the upper as in the lower part of the lens. Between the quartz-norite and the normal norite there is no eruptive contact, but a gradual though rapid transition. As a third group we may unite all the olivine-bearing rocks: lherzolites, troctolites, and olivine-norites, generally very rich in olivine and sometimes nearly of dunitic composition. ‘They occur as very numerous, greater or smaller bosses and bands in the normal olivine-free norite of the marginal zone, but never in the central quartz-norite. In the marginal zone they occur irregularly distributed all through it from the outer contact with the schists toward the inner border against the quartz-norite and all around it, quite as nu- merous in the upper as in the lower part of the eruptive. They strike one as being swimming bodies in the norite magma. It is easily proved that they are neither younger intrusions nor older inclusions, but are very nearly of the same age as the environing norite. While the above-named groups of rocks are all very nearly related and form a stepwise but nearly continuous series without definite eruptive contacts, there is chemically and tectonically a gap between these and a last group of eruptive rocks. The latter form well-defined dikes, cutting all the former rocks and consisting of an aplitic soda-rich granite. ‘That they belong to the same eruptive series, with their source in the central part of the lens, is proved by the fact that they are confined to the norite field and occur in greatest quantity in the central field of quartz-norite. Here they form a network of narrow dikes, occupying about 7 per cent of the total area. The dikes are well defined, but with slightly blurred contacts. The same dikes occur also in the marginal norite, here only occupying about 3 per cent of the area, but more regularly and with razor-sharp contacts. The very last products of volcanic action are some irregular veins of pegmatitic potash-granite and of snow-white, pure quariz, both carrying black tourmaline. STEINAR FOSLIE oR SSS Sooo 000001: I ‘aTvaG ‘PPy A110u veurey oq} Jo dep[—r ‘org am! es y” yal UK puvvyl | O - 000001:1 :¥9 ayyouyssdd Snosasiiay2iN & {S1Y9S- DIU - {aUsoy =| {5142S -DIIW - aul] JO Uozltopy aul] fo uozl4oyy ses afl4OU-Z10NF) (0) 94 1/0{204] afl(oZsay7 Af [JOU-AUIAI/ FIELD OBSERVATIONS IN NORTHERN NORWAY 707 The distinct gap between the main mass of massive norites and the dike-formed acid differentiation products of aplitic granite reminds us very much of corres- a ponding features of the “red rock” Sora in the Duluth gabbro.t YY The deposits of nickeliferous V// pyrrhotite—which will not be treated here—are in their occur- rence confined to the olivine- bearing rocks and to the marginal normal norite, and no traces of them are found in the quartz- norite or the younger dikes. In the olivine rocks the sulphides occur only as impregnations, but are relatively very rich in nickel. While the percentage of sulphur generally is between 1 and 2 per cent, the nickel content neverthe- less reaches 0.7 per cent, of which not more than o.1 per cent seems to be in the form of silicate in the olivine mineral. In the norite, the sulphides occur partly as impreg- nations, partly as segregated richer masses, but the nickel content in the pure sulphide is lower, gener- ally between 1.5 and 4 per cent. The mineralogical composition of the rocks is as follows: The olivine-bearing rocks consist / of olivine, rhombic and monoclinic 3 Vj pyroxene, and plagioclase, but gen- 3 —} erally no primary biotite. Acces- sory constituents are picotite, green spinel, magnetite, and pyr- rhotite. The rhombic pyroxene is an enstatite with less than 500 N SS Irs SX \ N) | ll 0 (ea ff Pte NS f | Raaneggen 11 It tala Pat a TS ~ s I | (anany Thy “Uf SS TEEN se ee X\ I verheld | q Suiiill SS S Fic. 2.—Section through the Raana field. Vertical and horizontal scale, 1:100,000 SS S S t Frank F. Grout, “A Type of Igneous Differentiation,” Jour. Geol., Vol. XXVI, pp. 626 ff. 708 STEINAR FOSLIE , 17 per cent ferrous silicate, the monoclinic pyroxene is a diallage. The plagioclase is always bytownite, about Ab.,Ang,, nearly with- out zones, and occurring interstitially between the other minerals. In the normal norite the minerals are: Hypersthene with abous 25 per cent ferrous silicate, poikilitic diallage in big individualt with hypersthene inclusions, labradorite-bytownite, often with zonal structure and varying composition, from 50 to 80 per cent An, and some primary biotite. Uralitization occurs in many parts of the field, but has no interest in this connection. The consid- erable variations in this rock seem exclusively to be due to quan- titative variations in the relative proportion of pyroxene and plagioclase. ‘The monomineralic rocks, anorthosite or pyroxenite, are however, never developed. The quartz-norite contains iron-rich hypersthene with 40-45 per cent ferrous silicate, much diallage, which is here not poikilitic, but crystallized at the same time as the hypersthene, and further generally plenty of biotite. The plagioclase is labradorite with marked zonal structure, from 40 to 55 per cent An. Besides, this rock always contains small amounts of microcryptoperthitic ortho- clase, rich in natron, and of free quartz. The aplitic granite dikes contain quartz, oligoclase with 26 per cent An, non-perthitic microcline, and a little muscovite and biotite. The pegmatitic granite dikes contain cntarens perthitic microcline with a faint green color, and black tourmaline. The perthite consists of 87 per cent microcline and 13 per cent albite-oligoclase with the composition Abs,An,,. The chemical composition of the differentiation products is seen from the analyses in the following table, where the calculated norm is also given. The mode of these rocks naturally differs somewhat from the norm, even apart from secondary processes. So the potash, instead of forming orthoclase, partly enters into plagioclase and biotite, while part of the lime together with aluminia enters pyrox- enes instead of plagioclase. After an exact microscopic measurement, I give the mineral composition actually found in the normal norite and the quartz- norite, corresponding to the analyses No. 3 and No. 6 respectively. FIELD OBSERVATIONS IN NORTHERN NORWAY 729 ANALYSES (0) I 2 3 4 (4a) 5 6 7 ————/ SHO} oseeaneode 52.6 41.74 41.32 52.30 50.10 50.10 50.70 55-90 72.20 ARO} a seams 0.4 0.12 0.08 0.62 0.5 0.25 0.45 0.30 INKOB so ooo cute 16.2 2.87 agit 10.28 18.5 20.70 17.50 I4.22 les @aiys esc cers 0.5 2.06 1.83 Ons; 26.75 0.5 ©.50 0.08 0.55 INGO cee 6.7 10.66 8.63 8.80 6.3 4.38 6.97 2.00 1 bol O aegis o.1 0.35 0.13 0.20 0.10 0.07 0.09 0.03 INDO), adooupoe 12.2 38.70 35.26 18.30 12.54 12.54 9.43 5.98 I.03 (CAO Nani vec: 7 1.48 2.22 6.06 8.65 8.65 10.35 8.32 2.60 IBA OSES eesti: HEracenie illnesses None BRT ACE all wanihein (emeen eer Trace Trace 0.02 Naz2O voteaes 1.8 0.63 0.54 THA TVA Willers 5) r.58 2.75 2.55 KOO Nessie I.0 0.24 0.08 ONS Om [aces crsen as 0.6 0.69 I.50 3.78 IBoO EA ey aniaiclcs 0.05 0.09 0.06 OROSH| nee 0.05 Trace 0.05 0.08 Widsostn uence ORO ON mle eye MA eye Mca eta ae 0.09 OHOOH [Pa eye oie orarrerneaae ter tcc cliyarie Soo. songs emee 0.06+ 0.12 0.28 MsGO) Ilan cas ot 0.05 0.05 0.05 Trace INT oy 5g Sta a usametetall ieee tee tes 0.14 CV LOMA eral tee Mca areel BeI Le aaa tan a IPN ain Te ter Meal lel lam URE a (GOR aan ope s 0.06 0.05 OMEO || epae ets 0.09 0.09 0.08 INonel|Peneeoee CI tose ciel | euctet is cus etns|(awetawesteten al asc ncycihy ital at antes casted Pueea tas teh SecuTtc Meta eed vezi ot RITE SE HRD ea a Trace TB 5 5.0600 OHI (OD EET teat | lhiaceebcs revel grater Real (A ic eves | sie Clatsratelheucrctrel ool laeoeticnaroneley| Petcare rete Trace (COD MRRAR re Peis ce oo. None None ONOTE [eee el eras are 0.40 0.20 0.06 EHO) nS pdoeadien o.7+ 0.25 0.39 ORSOn Eee 0.7 I.12 0.50 0.65 SEOs) atone ee Bees (0.03)} (0.04) OLO6s | SE eae |e eee 0.07 0.07 0.06 aotaleere 100.13 100.40 OORSON | TOORS Zi Meee | Mera 100.37 | 100.41 | 100.22 Ee ONOTSemiaiecee!| sc a's ae sace 0.06 ©.14 OF OS) When a lero 0.03 0.03 ° BG taller svar le.s:cvsiostcrave 100.34 @Qj/s75 || LOOO7 Nacooscscllacavsace 100.34 | 100.38 | 100.22 CALCULATION OF THE NORM 1 (0) I 2 3 4 (4a) 5 6 7 (OYE EA. SB als al DEER | Ritts cle ierad Crete Aer (acai eti| (Sener cen |eiceas aan 0.66 4.48 34.74 Orthoclase...... 6.12 I.30 6.12 BU Aw |e kane: 3-34 3.890 8.88 22.24 WA bitewe eis cin se i520 5.24 3.90 O54) Noaesins ne 12.58 13.62 23.23 21.48 Anorthite...... 33.08 4.31 I1l.12 211A estes 41.98 47.26 30.88 12.79 INetelamenascss aya \casoures al armoesie OAc {o)! laeretetaiaal la aews ool emiooaaie | aeoineletal la pee el secrociads (ConMMGhNsococdllooosvaonlloocnooce TEAS Bt Neuere oa ives eecveweeas aay | tes ee wa | Paar cea | aC a 1.22 2 Els soosnall Gyvbocike) 10.94 2B (TG Eves looodocon 57.90 65.43 67.47 Q2.47 Diopside....... 2.44 THO Gellert: RoGWiltomonen are 0.89 1.33 WE QO\. | eee sees Hypersthene....} 39.82 G30 la igedaib.e FOoAD list odoeoe 31.05 30.20 23.45 5.84 liane pee 0.62 70.52 W272 THOS MIhedereee (jetta Seer Beal Iateety aie | leteronaaior Magnetite...... 0.70 4.41 2.55 OnA GH Remermaee 0.70 0.70 0.12 ©.70 Ilmenite........ 0.76 0.22 0.15 Te 22h eusiees eearese 0.91 0.46 0.85 0.76 Apatite mer nies srsemesecs ONgiaylanecine Ryovaade) Genin La cell lass eco oat ro Se ooo On Ta: | sae eae Ghromitey neal a ae onions OPTIC YAW aie | Wbenstoaaa cen 0.15 ORT OM ee ay Sia piney a IByriterr pyar es ack seciunjck 0.22 0.53 Cyst / | [uso siione 0.10 0.10 OMTOMIE setae = fem 44.34 89.28 76.10 (oc ty fy fil hn ee Olas 41.58 32.92 31.83 7.30 MgO: FeOmol Best Fs aout 7 ata eT (ige Ale aie Es ah Ape cea eae ee Awe: 1 KS EA KEY TO THE TABLE OF ANALYSES No: Petrographic Name Symbol Locality Analyst (0). .} Calculated composition : of undifferentiated magma TST S GPa AR hs etter te lla iach tee ae pevseoncoaddollaaca loodasssooud RS siu|| CHAS bo deocbbo6.S (IV) V.r.%5.2. (@) 2 Raanbog River Naima Sahlbom 2....| Troctolite Weireisieerestn 2) Tverfjeldet : Naima Sahlbom 3....| Normal norite (QU) IEW 3 Te 5 Source of Raanbog River | Olaf Réer 4....| Normal norite Partial analysis Source of Raanbog River | Naima Sahlbom (4a) .| Calculated after 4 JP VOL aA A A Mey sts Settat IIE payai coaster ells MES Nes am cD rey 5....| Uralitized norite RE ecaneenert Arnesskaret Olaf Réer 6....| Quarts-norite 1”. YS o(ByaA Stemnes in Raana Olaf Réer 7....| Aplitic granite I’ .3 (4). 21 Qpe3 Sepmolvarre Olaf Roer 710 STEINAR FOSLIE While the composition of the quartz-norite is pretty uniform throughout, the marginal norite varies considerably. The analysis No. 3 represents a type very rich in hypersthene, analysis No. 5 a type rich in plagioclase, and analysis No. 4 a mean type. Normal Norite, No. 3 Quartz-Norite, No. 6 Weight Percentage Weight Percentage Quartz ee caer ee eee None DT Orthoclase.ke sc eee None 4.7 Plasioclase este nl eee 20.0 (ca. AbyAngo) | 59.2 (Ab;;An,) Diallagem. ee wee re 4.6 16.6 Fy PersEnenene ee ane Aree 50.2 II.o Bigtitehes seus con eee ee ee age Buon Amphibole, secondary....... 16.1 Very little Magnetite+-ilmenite........ 1.4 Dth IPAViaAOONEDs cao po noose aeons 0.2 0.2 Rutiless Fs.) sae eee oes 0.2 None AMO tala) A Spore) ony nat 100.0 100.0 * A special case. Generally there is more biotite in the quartz-norite than in the normal norite. By the aid of the seven analyses stated above we may calculate the mean composition of the four main groups of .rocks occurring in the field. They are as follows: Oltiethenins Marginal Zone | Central Field litic Grani Rocks of Nommal Oh uaz 2 Dikes Ee SION Secu Reo wes ere 41.53 50.80 55-90 72.20 il Bt @ eran eatin CET 0.10 0.44 0.45 0.39 PeN 0 Se ete Ny ne neem 5-20 16.99 17.50 Tne 1 GH O ger eS AG AH eel wet Ocoee 2.40 0.44 0.08 0.55 DO Te ie ato apie te rela ee 9.64 6.47 6.97 2.00 IMO 5 ore par TaN 0.24 0.14 0.09 0.03 NEC O) Aue Oi. Cert san aap Fhe 36.08 13.20 5-98 I.03 CaO ees Sie Rae ee ete 1.85 8.43 8.32 2.60 Ba@eeiinin coed ine enews peas None Trace Trace 0.02 Nat OR a neraatetn ieee 0.59 1.43 Dis 2. Isis RON at a tae ero eS ©.49 o.60 1.50 3.78 a Beis cbakercr comet ae epee 0.07 0.03 ©.05 0.08 PAO eae Neen Msi ccm arty cts Has ae alee se @569) . eaunone idee eee a eres Cay ae rata ee eh ©. 20 0.06 0.05 Trace Cie OF ona Se raccaatie te malar 0.08 0.09 None. | {235 aeanee These variations are illustrated in sae diagram of the differ- entiation processes (Fig. 3). FIELD OBSERVATIONS IN NORTHERN NORWAY 7II By measurement from the exact geological maps of the field we find the areal distribution of the main rock groups to be the following: km.? OuartZ noritern Vom aoe eee 30.0 Normal monites ec as eeenee $3.2 35% 35% 30% 50% 25% 25% 20% 20% 15% 10% 0% 40% Sid2 50% 60% 70% 75% Side Peridorites Norite, Qu norite Granite Undifferentiated mai gma Fic. 3.—Differentiation diagram, based upon the mean composition of the main rock groups. ; After the modifications necessary for the topographical irregu- larities, after recalculation to weight percentage, and after intro- ducing the calculated quantity of the granite dikes, the proportion becomes the following: Weight Percentage IADlIGICzeranite scene suey celle sence 3.9 Owartz noriter ew micn seen Bile INormalmoritem sae hea een Bi) adi Olivine-bearing rocks.............. 7 702 STEINAR FOSLIE It may be noted here that these percentages are not propor- tional to the areal distribution of the rocks at the surface, because the quartz-norite and the normal norite without doubt are concen- trically arranged throughout the field, so that any section through the center of the field will give the same image. Accordingly, their relative volume is with the greatest approximation calculated here after the formula for concentric spheres. After the results given above, stating the quantitative propor- tion and the chemical composition of the main groups of rocks, we are in the rare and happy position to be able to state—with a fair amount of accuracy—the mean composition of the whole field, representing the original, undifferentiated eruptive magma. This composition is given in column o, in the table of analyses. The most remarkable feature is the very high magnesia content in connection with the relatively high silica content. It is an ideal norite magma. The regular, stepwise development of the differentiation products ° is best seen from the norm calculations. Not only the chemical composition changes along definite lines, but also the composition of the individual minerals, especially the plagioclase and the rhombic pyroxene. Very remarkable is the regular change in the ratio MgO™!:FeO™! in the silicates, decreasing markedly toward the salic rocks. The reason for this appears from what follows. THE DIFFERENTIATION PROCESS Starting with a study of the known forsterite-diopside-silica and the forsterite-anorthite-silica diagrams,’ we learn that in the mean, undifferentiated magma of the Raana field, nearly without olivine in the norm, olivine will anyway be the first mineral to crystallize— beginning at the border toward the surrounding schists—and it will continue to separate out until the eutectical line olivine- pyroxene isreached. By further cooling, rhombic pyroxene crystal- lizes out at the same time as the already formed olivine crystals are resorbed. If this process continued undisturbed, the final result would be an olivine-free rock, corresponding to the norm. However, we find *See N. L. Bowen, Jour. Geol., Vol. XXIII, suppl., pp. 20 and 29. FIELD OBSERVATIONS IN NORTHERN NORWAY 713 in the field, as stated above, a series of olivine-rich rocks all around the marginal zone of the eruptive body and of nearly the same age as the environing norite. These olivine rocks can only have been individualized during that period of the crystallization process when free olivine crystals were really suspended in the crystallizing marginal zone. It is obvious that even the slightest accumulation of these crystals at certain places would at once bring them in excess, and prevent them from being wholly resorbed. _ Consequently, olivine would become a part of the norm compo- sition at those places. We may conclude, therefore, that such an accumulation of olivine crystals has really taken place. That gravity separation has not played a prominent part in it is obvious from the distribution of the olivine rocks. We are forced to con- clude that convection currents and other movements in the magma have been able to bring about such an accumulation, possibly also a conglutination of already formed crystals. The explanation is confirmed by two facts: First, in all the olivine rocks the olivine crystals are surrounded by resorption rims, proving that resorption has been going on, but stopped at a certain point. Secondly, the olivine crystals in all these fields have nearly the same proportion, Mg,SiO,:Fe,Si0,, quite independ- ent of the quantity of olivine, and consequently have all separated out from a uniform magma. We get a natural explanation of the varying quantities of plagioclase and pyroxenes in the olivine rocks and of their “swim- ming” character in the norite magma. After the termination of the resorption period and the con- solidation of the olivine rocks, the rest of the magma in the marginal zone should obviously be expected to have become poorer in mag- nesia and richer in silica than the magma of original composition in the still fluid central part. Field observations teach us, however, that the olivine-free norite in a very thick marginal zone has a more basic composition than the central part. To explain this, we might think of compensating currents in the magma prior to the resorption period, which in combination with the resorption of some of the olivine might produce this basic 714 STEINAR FOSLIE composition. Chemical calculations, however, show this to be impossible, and the differences in the composition of the plagioclase point in the same direction. We might also consider another sort of differentiation, according to which the marginal zone of the massive represents the mean, original composition of the magma, intruded by younger femic and salic differentiation products. ‘The evident field observations, however, contradict such a supposition also. Here, therefore, the squeezing theory, as developed by N. L. Bowen (loc. cit.), turns out to be a very natural and obvious explanation. It must be remembered that the process takes place under orogenic pressure. While the segregated crystals are only sus- pended in the magma, the pressure is static and has no influence on the differentiation. From the moment when the outmost shell forms a fixed crystal mesh, this shell has eventually to take up the mountain pressure, but, of course, at first is not able to do so. The pressure then will be dynamic. Following Bowen, this stage is supposed to occur when about 80 per cent of the mass is crystal- line and only 20 per cent liquid. As the volume of the magma is diminished by cooling and still more by crystallization, the outer shell will be compressed and its remaining interstitial liquid squeezed out. At that advanced stage of crystallization, this liquid will contain plagioclase, richer in soda, and pyroxene, richer in iron and lime than the already segregated crystals, moreover eventually potash-feldspar, free quartz, and magmatic water. These components accordingly will move, and the direction of movement will be inward from the z»nz of dynamic into the zone of static pressure, because in the crystallizing zone with still static pressure the diminishment of volume will have the effect of releasing the pressure. It is important to note that the process is here not supposed to be a squeezing for a long distance through a crystal mesh. Prob- ably it is mainly a differential movement, restricted to the narrow transition zone, which moves inward at the same rate as the crystal- lization proceeds. 3 FIELD OBSERVATIONS IN NORTHERN NORWAY 715 “The wave of crystallization is followed by a wave of squeezing.” Continually the squeezed material will mix with the more fluid magma inside, which gradually becomes more acid. The next step is the abrupt transition from the normal norite of the marginal zone to the quartz-norite of the central part. This is very easy to explain. It represents the stage when the outer, solid shell has grown sufficiently thick and strong to resist the compressing forces. From that moment no more compression and no more squeezing takes place. The remaining magma crystallizes quietly without further differentiation to a unitorm quartz-norite, carrying a little free quartz and orthoclase, more acid plagioclase, and much biotite on account of enrichment of the magmatic water. Of course, the contraction or the release of pressure continues as the crystallization proceeds, and finally results in a general formation of fissures in all directions, when the resulting stresses have grown sufficiently strong. This fissuring obviously occurred at a stage when the quartz- norite was consolidated, with exception of the very last interstitial liquid, containing a considerable proportion of magmatic water and mineralizers. The liquid was drained into the fissures, forming aplitic dikes of soda-rich granite. This last separation is obviously not due to squeezing; but whether the liquid was really sucked out into the fissures or driven out by the pressure of the enriched gaseous mineralizers is difficult to tell. At any rate, there resulted a direct connection between these dikes and the last consolidated minerals in the quartz-norite, producing the slightly blurred con- tacts mentioned above. ' Also in the marginal normal norite, clefts were formed at this period and filled with the same dikes of granite. Here they have razor-sharp contacts because the marginal rock at that time was completely solid. The mineralizers once more separated out, carrying with them much potash and silica and giving rise to veins of pegmatite and pure quartz, both rich in tourmaline. From the foregoing we see that the theory as modified covers all the observed facts. It is beyond doubt that the differen- 716 STEINAR FOSLIE tation proceeded quite 7 sziw and is confined to the crystal- lization period. Further we have learned that the squeezing differentiation gives results which in many respects are similar to those of gravitative separation. In the one case the liquid is removed from the crystals, in the other case the crystals from the liquid. There is only one point left which cannot yet be explained in detail: how some of the primary olivine crystals could be brought to accumulate or conglutinate at certain places. On this point, therefore, we can only state the fact and remark that any suppo- sition of immiscibility of olivine with the rest of the magma, resulting in the formation of fluid drops of olivine, would not help us, but be in opposition to several of the above-stated facts. As will easily be seen, the special differentiation type of Raana is not apt to occur very frequently. It presupposes the following conditions: (1) the crystallization must take place under lateral pressure; (2) it must take place completely in situ, in a closed room without supply of new magma during the process; (3) gravi- tative separation must not play a prominent part. In the same district we meet a somewhat different type of squeezing differentiation, one which will prove to have a more general occurrence in orogenic folding zones. It occurs when the central part of the eruptive mass is not—as in Raana—protected against lateral pressure. A short statement of it is here added. THE AMPHIBOLITE SERIES As mentioned in the first part of this paper, in the folded mountain region there occur considerable quantities of femic eruptive rocks very intimately injected into the schists and com- pletely recrystallized through dynamic metamorphism. They do not occur in so extensive individual masses as do the fresher . rocks of the Raana type, but they are much more widely distributed. Chemically they have diabasic composition and differ from the corresponding rocks of the Raana field in carrying more iron in proportion to magnesia, more sodium, titanium, and phosphorus. On account of the total parallelism of all eruptive injections in the district, direct observations of relative ages can generally not FIELD OBSERVATIONS IN NORTHERN NORWAY Asal be made. The amphibolites might be somewhat older than the Raana norites and accordingly have been subject to a longer period of dynamic metamorphism during the Caledonian folding. But the reason for the different degree of metamorphism might also be that the process has been able to act more severely upon these rocks on account of their lesser thickness and their very intimate injection in the surrounding schists—which show about the same degree of metamorphism—while the massives of the Raana type have resisted better. The mineral composition of the ordinary amphibolites is: amphibole and acid plagioclase as the predominant minerals, more or less epidote or clinozoisite, quartz, leucoxene, and often garnet and biotite. None of the primary minerals are left. The original basic plagioclase has been more or less Arbitized producing plagioclases from albite to oligoclase composition, while part of the lime enters the epidote minerals. The original pyrox- enes have been changed to amphibole, and it is noteworthy that while in the Raana field the uralitization has produced a nearly colorless amphibole of actinolitic composition, poor in aluminia, the metamorphic rocks contain common green amphibole, rich in aluminia. The differentiation shows Ueeee features analogous to those just described from the Raana field, but also significant differences. In many of the amphibolite zones we find a number of small bosses of serpentine rocks, very irregularly distributed. The dozens of such bosses observed nearly always occur within the amphibolite rock and are not separately injected into the schists. They are all of relatively small dimensions, rounded or lens-shaped, and sharply.defined from the surrounding amphibolite. They obviously correspond to the peridotitic rocks of the Raana field, but are always completely metamorphosed to serpentine and talc minerals. In numerous cases the amphibolite itself is nearly homogeneous, and without intermediate steps there is a wide gap over to soda-rich granites which occur nearly everywhere in intimate connection with the amphibolite series in such a way that there can be no doubt of their mutual relation as differentiation products. 718 STEINAR FOSLIE Here the granite does not occur as crossing dikes, but as small lenses or bands arranged parallel to the schistosity of the amphibo- lites. In some cases they occur as separate sheets of considerable thickness in or at the border of the amphibolite. Only in rare cases, when the amphibolite has retained a more massive structure without marked schistosity, do they occur as numerous crossing, irregular stringers cutting the femic rock. The contrast between the two rocks is still more marked than was the case in the Raana field on account of the great difference in chemical composition and the different degree of metamorphism. ~ While the amphibolites are completely recrystallized, the granitic differentiates have retained much of their original structure, partly because they are somewhat younger, partly because the granitic mineral association has by far not the same tendency toward mineral readjustment under new conditions as is the case with the femic rocks. These granites are characterized as soda-rich, but their compo- sition may differ somewhat. Sodium may be quite predominant among the alkalies, the rock becoming a Trondhjemite as described by V. M. Goldschmidt from the Trondhjem district farther south. By increasing potash they get a more granodioritic composition, up to a limit with about equal molecular amounts of soda and potash. In all cases they are poor in dark minerals and generally have aplitic structure. They are distinctly different from the ordinary granites in the district, which occur as great independent eruptions, and where the potash is always predominant. The chemical composition of the ordinary amphibolite and its granitic differentiate is seen from the analyses on page 719. In some cases we meet intermediate rocks between the amphibo- litic and granitic extremities, mainly of dioritic composition. The variations are never regular, but form ‘“‘schlieric”’ or banded alternations in the schistose rock. From the foregoing we have seen that the characteristic regular differentiation step between the normal norite and the quartz- norite in the Raana field has no correspondent in the amphibolite series. This is a natural consequence of the fact that here the FIELD OBSERVATIONS IN NORTHERN NORWAY 719 central part of the mass was not protected against the lateral pressure which prevailed during the whole crystallization. The result of the differential squeezing during the crystallization period has therefore been the ultimate separation of the last con- solidating constituents of the magma. During the continued pressure after crystallization of the main rock mass, this last residue has been squeezed into the already consolidated rock as ANALYSES CALCULATION OF THE NORM No. 8 No. 9 No. 8 No. 9 SHOh.ooascapoansese 49.12 67.40 Quartzierase cee 3.36 19.74 BIR @ open ets ita. sctevs 2.46 0.20 Orthoclase......... 2.22 II.12 JNUOF nanoeSuae oe 13.70 16.52 Al biteyaeencamcnie + 22.01 48.73 eo Oakecrere cise gif scores I.05 2.33 Anorthites peer 24.46 13.34 HE ORS oie cicero bs 10.67 0.34 GCorundumeeeeeee ee eee 0.10 INTO). CoB Se naees 0.18 0.03 IW O) Cag nomena 5.19 0.88 SDR Sale yerpeiere 52.05 93.03 (CHO) S peo Deere II.02 3.43 | BAO Pair, an 0 Trace Trace Diopsidesneee eee Lett SEG | ayete tepals NEY Oveeaig ne ceemeae 2.58 5.76 Hypersthene....... 18.17 2.20 1KG{0) se Renee 0.44 I.03 iFlematitenee acetone 2.24 IPOs G SRO eee 0.22 0.50 Magnetite......... Bir Sie will [tates crane reds ey Vaverereiersi shes cis « 0.19 o.10 Ilmenite.......... 4.71 0.61 COD eacrneaias: I.21 None iApatitenpeen cers 0.52 1.34 MO af essyee joi seiaie, oe T8317) 0.81 Ay nite seers 0.36 0.19 To Ore iSaack evels 0.01 (0.05) ZS Hempenee nee 44.05 6.58 ho tall nyaya are 100.31 100.41 = OOS ACs 0.10 0.05 MgOol: FeQmol FR QAvL tl Sopseevaterteners Total 100.21 100.36 KEY TO THE TABLE OF ANALYSES No. Petrographic Name Symbol Locality Analyst 8....| Garnet-amphibolite Uae Mere AS Bjérkaasen Mine Olaf Réer 9....| Aplitic granite 1D? Ag eec Brugsaas Naima Sahlbom lenses, stringers, and bands of aplitic granite. Their ordinary arrangement parallel to the schistosity shows that this must have been partly developed already during the consolidation as crystal- lization schistosity. The magmatic water and mineralizers with their dissolved substances seem to have taken their own path, having a more active power of motion contrary to the purely passive motion of the ordinary squeezed material. They have given rise to replacement phenomena and formation of ore deposits, still younger than the aplitic granite. GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA N. H. DARTON United States Geological Survey, Washington, D.C. During 1921, I spent several months in Baja California deter- mining geologic relations of a large area, and as there is but little on record regarding the geology of this region, it is desirable to set forth such of my observations as appear to be of general interest. I journeyed with pack train from near Ensenada south- ward to beyond Comondu and later extended the reconnaissance to beyond La Paz as far as Todos Santos. Most of the observations were in the district between the high sierra and the west coast, but the peninsula was cross-sectioned near San Ignacio and Santa Rosalia, from Mulegé to La Purisima, and in the La Paz region (Fig. 1). Most of my attention was given to the sedimentary rocks of Cretaceous and Tertiary age, but the limits and character of some of the crystalline rocks were ascertained. Many of the general relations are set forth in the cross-sections of Figures 2, 3, and 4. PRE-CRETACEOUS The lowest strata observed were schists and other metamorphic rocks, invaded by white, massive, coarse-grained granite. At most places this granite includes fragments of schist, and many clearly intrusive contacts were noted. The granite constitutes the high Sierra San Pedro Martir in the north central part of the peninsula, where one or two summits reach an altitude of about 10,000 feet. It also outcrops north of Ensenada, north of Santa Caterina, and in many high central ridges from latitude 29° 30’ to latitude 28°, and is prominent in the sierra at the lower end of the peninsula southeast of La Paz. I crossed the schists in several broad zones between Santa Caterina and Calmalli and noticed that a few miles 720 GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 721 7 s BO SAN LUCAS: 110° Fic. 1.—Map of Baja California showing principal shore features and settlements, and location of cross-sections in Figures 2, 3, and 4. south of the latter place they are overlapped by younger sediments. They appear again in high ridges in Cerros Island and from Punta San Lorenzo to Isla Santa Margarita on the west side of Bahia 722 N. H. DARTON Se Ss allel Te /GNEOUS AND METAMORPAIE Sines eae AanraNe ess 2A Vor NAN == 7 Ocean = basa Pant ei a ZN Ls ars zi X> Ar L- 1G RAN ale (eedione MAIZE! Gi NITE ee fi 77 = conglomerate, < and igneous ClaraDesert an "Smiles Vert. 3000 aoe Fic. 2.—Sections across parts of northern and central Baja California Q=Quaternary Scale: de la Magdalena.‘ They also occur in the south end of the pen- insula in the Triunfo mining district south of La Paz. * According to J. Ross Brown, Resources of the Pacific Slope, San Francisco, 1869, P. 143. Areutayend etud ee tusojijeg wleg [e1]Ue0 ssor9e suoTIIEgG—'E *oIg = § === 8 P24 MONLBA E390 IOV 2124S ESS, UPIIQ DITO. weed Tj9eT 2 - quoqs Pues ESO RYTTHT EASA snoaus/ OPEL DSE aS as ee Teu0yspues ese 37 THIN N. H. DARTON a erusojyeD eleg urayynos yo jared sso1se suorjsas—"h *o1,7 SST eS ZAG B99. ye WiGa Sa a DUS204 pugs esayis— a auo}spues PSN. a ewe SY \ 5 oe | GF “8397 SSS SS 2 Sa Ee Serer as ss Wa ly > WW Ss oY ros eps SE ES U0336 gt GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 725 Lindgren’ has given considerable information regarding schist and granite in the northern part of the peninsula; Gabb? noted some features observed on his trip north from La Paz; and Emmons and Merrill’ described relations of the schists near the onyx quarries east of Santa Caterina. I obtained no evidence as to the age of these granites and schists, but my general impression was that they were much older than Cretaceous. I did not observe any overlap by Cretaceous rocks, but I saw Tertiary deposits lying on or against them at various places. In the region east of La Paz, the granite appears to be the floor on which the Tertiary volcanic series lies. This relation is strongly indicated at Cabo Lobos on the east side of Isla Espiritu Santo where a separating fault appears hardly possible. A similar relation is indicated in Sierra Giganta, northwest of Loreto. The structural relations of the ridges of schists in Isla Santa Margarita and extending northward to Cabo San Lazaro is not known, and it is not unlikely that these rocks are the uplifted floor of the strata farther east. CRETACEOUS SYSTEM General relations—A large part of the peninsula is underlain by Cretaceous rocks which outcrop extensively in the central and northern parts. ‘Two principal series are present, both of late Cretaceous age and separated by an uncomformity, the older series, of unknown correlation, having been uplifted, flexed, and cut by large igneous masses before the younger series, Chico, was deposited. ‘The Chico beds were not observed in contact with the ‘older series, and the principal evidence of the separateness of the two is the difference in attitude of the beds throughout the area and the fact that the older series is uncomformably overlapped by / t Lindgren, ‘‘ Notes on the Geology of Baja California, Mexico,” Proc. Cal. Acad. Sth, Second Series, Vol. I (1889), pp. 173-96, and Vol. II (1890), pp. 1-17; and “Geology and Petrography of Baja California, Mexico,” Proc. Cal. Acad. Sci., Second Series, Vol. III (1890), p. 26. 2 W. B. Gabb, Geol. Survey of Cal.: Reports, Vol II (1869), Appendix, pp. 1-20. 3S. F. Emmons and G. P. Merrill, “Geological Sketch of Lower California,” Bull. Geol. Soc. An., Vol. V (1894), pp. 489-514. 726 N. H. DARTON the Eocene strata which lie with apparent conformity on the Chico beds. This relation is shown in the following section (Fig. 5). These pre-Chico Cretaceous rocks consist of conglomerates, quartzites, tuffs, and agglomerates with large bodies of inter- bedded eruptive rocks. They are also cut by dikes and large stocks of igneous rocks of vari- ous kinds. In many localities theigneous rocks predominate over the sediments or pyroclas- Frc. 5.—Sketch section showing rela- .. és F tions of Eocene and Chico beds to meta- tics, and in places there is much morphic and igneous rocks of Cretaceous metamorphism. Unaltered or (pre-Chico) age, near latitude 30°, west but little altered sandstone and coast, Baja California. a, exposed overlap 4 aes oo shales appear in places, notably near old San Domingo Mission, 25 miles north of San Quintin, where they contain many large oyster shells, and in the Arroyo San José, 40 miles southeast of Santa Caterina. Limestone alsooccurs. It is conspicious north and northeast of the ruins of Mission San Fernando, 30 miles due east of Rosario, where the relations shown in the following sketch section (Fig. 6) are presented. The limestone is filled with fossil oysters of upper Cretaceous age. Part of the outcrop is shown in Figure 7. These metamorphic Cretaceous rocks appear to extend as far south as latitude 28° 40’, and possibly the schists and other rocks, Fic. 6.—Sketch section showing relations of Cretaceous limestone north of the ruins of San Fernando Mission, 30 miles east of Rosario, Baja California. although apparently of an older series and extending to Calmalli, may be metamorphosed Cretaceous sediments. Apparently they are so regarded by Bosé and Wittish in their brief statements regarding relations in the Northern District. *“Memoria de la Comisién del Instituto Geolégico de México que exploré la regién Norte de la Baja California,” Inst. Geol. de México, Perergones, Vol. XIV (1913) PP. 397-533- ? GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 727 CHICO FORMATION The upper part of the Chico formation (late Cretace ous) rises above sea-level a few miles north of Rosario, and it remains in view along the ocean bluffs and lower parts of the valleys of the Rio Rosario, Arroyo San Vicente, Rio San Fernando, and Arroyo Santa Caterina as far as latitude 29° 24’, a few miles southeast of Punta Canoas. ‘The rocks are soft sandstone and shale of light-gray to buff color with round concretions at most places. Emmons and Merrill* noted the occurrence of Upper Cretaceous fossils at a point about 3 miles north of Santa Caterina Landing. Specimens collected from massive sandstone near sea-level and in calcareous layers 200 teet above were determined by T. W. Stanton as follows: Arca breweriana Gabb; Baculites chicoensis Trask; Tessarolax distorta Gabb; and Inocerami, not determinable. Fic. 7.—Fossiliferous Cretaceous sandstone northeast of San Fernando Mission, twenty miles east of San Rosario, Baja California. The fossils which I obtained in these beds at the Arroyo Hondo, 15 miles north of Rosario, were identified by Dr. T. W. Stanton as follows: Ryhnchonella sp.; Ostrea sp.; Inoceramus whitneyt Gabb; Yoldia nasuta; Nemodon vancouverensis Meek?; Dentalium sp.; Gyrodes sp.. Anchura sp.: Cinulia obliqua Gabb; Baculttes chicoensis Trask; and Baculites occidentalis Meek. All these fossils are typical of the Pacific Coast Chico which is of Upper Cretaceous age. Lindgren? has reported the occurrence of the Chico fossil, Coralliochama orcutti, in sandstones appearing in a small area among the volcanic rocks a few miles southwest of Ensenada. = Op. cit., Pp. 501. 2 Op. cit., p. 176. 728 N. H. DARTON TERTIARY SYSTEM EOCENE General relations.—1 found that on the shore of the Pacific Ocean the earlier Tertiary deposits began at the mouth of the Rio San Vicente in latitude 31° 30’ and extended south to about latitude 29° 25’, south of Punta Canoas. In this area they underlie a narrow coastal plain mostly from 8 to 10 miles wide, but broadening to nearly 20 miles near latitude 30°. They lie on the Chico beds but to the east abut against the metamorphic pre-Chico rocks, and the termination of the crop to the north and south is due to the west- ward extension of the latter to the ocean shore. Throughout the area the strata dip at very low angles to the west, and local flexures are rare and slight. Some general relations are shown in sections Tt, [SON ES) 2 The rocks are shales and sandstones mostly of light-gray to buff color, 1,200 feet or more thick near latitude 30°. To the south they contain fossils of the Martinez or Middle to Lower Eocene age, but in the region north of latitude 30° most if 10t all of the beds appear to be Tejon or Upper Eocene, although the lower formations may exist beneath the surface. There are promiient exposures along the lower parts of the valleys of the Rio Rosario, Arroyo San Vicente, Rio San Fernando, and Arroyo Santa Caterina, near Rosario and southward, and in bluffs along the ocean near Cabo Colnett and Punta Camalu and near Bluff Point and Punta Canoas, the two latter southwest of Santa Caterina. As shown in section 5, Figure 2, the thick succession .of beds exposed in the bluffs along the ocean at Bluff Point is preserved by the heavy lava cap of Mesa San Carlos. Fossils—Fossils were collected at several localitities in the Eocene beds and kindly determined for me by Dr. Julia A. Gardner. On the shore of the Pacific Ocean, a mile south of the mouth of the Arroyo San Antonio, latitude 31° 05’, the following were found: Cylichna sp.; Turritella n.sp., cf. T. uvasana Conrad; Amauropsis sp.; Leda sp.; Cucullaea matthewsont, Gabb?; Cardium, cf. C. brewert Gabb; Cardium sp.; Tellina? sp.; and Semele sp. On the ocean shore a half-mile south of Colnett Creek, 5 miles southeast of Cabo Colnett, were collected Omphalius sp.; Cardium GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 729 n.sp., cf. C. guadrigenarium Conrad. In the bluffs 5 miles east of San Quintin there were collected Turritella n.sp.; Leda sp.; Tel- lina sp., cl. T. hornit Gabb; and Spisula sp., cf. S. merriami Gabb. In the drift at the mouth of San Simon Cafion, 6 miles southeast of San Quintin, Cardium cooperi Gabb ? was collected. These are all regarded as Tejon or Upper Eocene in age. The following fossils were collected from two horizons in the slopes about 5 miles northeast of Santa Caterina Landing: Upper beds, Surcula sp.; Heierotoma gabbi Stanton: Turritella pacheococensis Stanton ?; Turritella sp.; Natica (gyrodes), cf. N. lineata Dickerson; Amaurop- sis sp.; Dentalium coopert Gabb?; Leda sp. 2; Glycimeris n.sp. aff. G. veatchit var. major Stanton; Cucullaea matihewsoni Gabb; Ostrea sp. ind.; Pecten sp.; Pinna sp.; Crassitellites sp.?; Veneri- cardia planicosta Lamarck var.; Phacoides sp., cf. P. diegoensis (Dickerson); Phacoides sp.; Cardium cooperi Gabb?; Cytherea sp. aff. C. hornit Gabb; Spisula sp. The age of these is regarded as Upper Martinez or Middle Eocene. From beds 200 feet lower were collected: Cucullaea matthewsonti Gabb?; Ostrea sp. ind.; Anomia sp.; Lima multiradiata Gabb; Phacoides sp.ind. The age of these is regarded as Lower or Middle Martinez or Lower Eocene. Emmons and Merrillt reported fossils from rolled pebbles of impure limestone obtained along the beach of the south of Santa Caterina Landing “‘which had evidently fallen from the cliffs above, and from a bed of similar composition in place at what was assumed to be about 1,200 feet higher in horizon, at San Carlos anchorage (collected by Mr. A. E. Foote), 8 miles north of Bluff Point.” These were determined by Dr. T. W. Stanton as follows: Cardita planicostata Lam., Leda gabbi Conrad, Urosyca caudita Gabb, and undetermined species of Nucula, Pectunculus, Tellina, Turritella, Dentaliuwm, and Crassatella. ‘These forms were regarded as Tejon (Upper Eocene). EOCENE (?) WEST OF LA PAZ Sandstones which appear to be of Eocene age outcrop in an oval area of about 200 square miles in the valleys of arroyos Liebres, Colorado, San Hilario, and Guadalupe, all west of San Hilario. 1 Op. cit., pp. 501-2. 730 N. H. DARTON Smaller exposures are revealed by the Arroyo Salado and its branches near Rancho Sauce, 35 miles west-northwest of San Hilario, and by arroyos Conejo and Datilari, 20 miles south- southeast of San Hilario (or about 40 miles due west of La Paz). The rocks are light-gray sandstone, mostly soft, but with harder layers and hard concretions. Some argillaceous members are included and some of the sandstone layers have a greenish tint. In the extensive expo- sures on the Arroyo Colorado about Rancho Tepetate where dips are from 3° to 5°, the thick- ness is not less than 3,000 feet unless the strata are duplicated by faulting. The dips are somewhat steeper near Rancho Sauce where more than 2,000 feet of beds are exposed. One of the largest outcrops in this vicinity is just south of Rancho Santa Rosa where sandstone Fic. 8.—Sandstone (Eocene?) on the Arroyo ledges extend along the Colorado one mile below Rancho Tepetate, ten panks of the arroyo miles west of San Hilario, Baja California. for several miles. It is capped by late Tertiary or Quarternary conglomerate and lime- stone. In the valley of the Arroyo de los Liebres, ledges of sand- stone and sandy clays are exposed in a wide area of low ledges and buttes which extend to, or nearly to, the mouth of the arroyo. All the dips are at low angles to the north. The exposures in the valley of the Arroyo Colorado extend from the junction with the Arroyo de los Liebres nearly to the mouth of the Arroyo Caracol. Prominent cliffs of the sandstone occur at 2. ae GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 731 Rancho Tepetate and for. some distance above and below that place (Fig. 8). Dips are all to the north or north-northeast at low angles. The outcrop of the formation extends up the Arroyo San Hilario to within about 4 miles of Rancho San Hilario where the top member is massive, pale buff sandstone with irony layers and many fragments of oyster shells. It dips north and apparently passes beneath the Monterey beds. There sandstones have yielded few fossils, but echinoid spines and foraminifera are abundant at Rancho Tepetate, and oysters and a shark’s tooth near Rancho Santa Teresa all appear to indicate Eocene age. Evidently the exposures are due to a mound of the formation extending southeast and southwest with altitude sufficiently great in the region from the Arroyo Salado to the Arroyo Conejo for the strata to be revealed by erosion in the deeper valleys. MIOCENE Monterey beds.—In the vicinity of the oases of La Purisima and San Hilario I found small exposures of strata so closely resembling the Monterey formation of southern California that tentative correlation seems desirable. Outcrops extend along the Arroyo de la Purisima from 2 miles above tidewater to within 6 miles of La Purisima and a small one appears in the upper part of the latter village. Outcrops also extend along the valley of the Arroyo San Gregorio for a mile or more about to miles southwest of La Purisima, and smaller crops appear above Purisima Vieja and San José, respectively 15 and 20 miles northwest of La Purisima, and on the Arroyo San Raimondi, 35 miles northwest of La Purisima. The same beds appear in an area of 2 or 3 square miles a short distance west of San Hilario, and in small crops of steeply upturned beds appearing at intervals from a point a mile northwest of San Luis to a point 15 miles southeast of that place. The relations are shown in sections 13, 14, 17, 18, and 19, Figures 3 and 4. At most places the beds are more or less tilted and flexed, and various younger formations overlie them unconformably, as shown in Fig- ures 9 and 10, although at several places where the strata are not flexed there appears to be gradation into the overlying “yellow beds.” 732 N. H. DARTON The principal rocks of the Monterey beds are gray to pale- buff, fine-grained sandstone and sandy shale with abundant fish scales. They include interbedded diatomaceous deposits mostly from 1 to 3 feet thick and some thin layers of glassy silica from buff to black in color. More than 500 feet of beds are exposed along the Arroyo de la Purisima where for several miles the forma- tion extends from 5 to 20 feet above the bed of the arroyo and there are many flexures. The southernmost exposures in the Arroyo de la Purisima in the west end of the Big Bend about 2 miles above tidewater contain much fine, white diatomaceous earth interbedded in fine-grained, yellowish sandstone and compact shale with many fish scales. In Fic. 9.—Flexures in the Monterey beds on the Arroyo de la Purisima, ten miles below La Purisima, Baja California. this region the dips are to the west or southwest, so that higher beds rise as the valley is ascended. In about 3 miles the lowermost beds appear in the crest of a low anticline, and from these were obtained the following fossils, determined by Dr. Julia A. Gardner: Scutella andersoni; Chrysodomus sp.; Turritella sp., cil. T. mar- garitana Normand; Vermetus sp.; Leda sp.; Arca, cf. A. medio- impressa Clark; Pecten, cf. P. crassicardo Conrad; Crassitellites (Crassinella) sp.; Cardaum sp. nov.; Chione sp.; Cytherea sp.; Mecoma sp.; Solen sp.; and some corals and bryozoa. The age is regarded as probably Vaqueros, equivalent to Lower Monterey. These lowest beds appear again in another small uplift a short distance north, beyond which the strata descend in apparent regular order showing extensive exposures in the banks of the arroyo as shown in Figure9. Here the formation is overlain by conglomerate. GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 733 Yellow beds.—Under this title I shall group deposits of late Miocene age exposed extensively along the western slope of the peninsula trom latitude 29° to latitude 24°. They are mostly soft, loamy sandstone and sandy clay of pale straw-yellow tint with local limy beds, the latter generally full of fossils. Some contemporaneous igneous rocks are included in places. The yellow beds have an ag- gregate thickness of 500 feet near the coast be- tween San Ignacio and La Purisima, although at the latter place the amount is not more than 120 feet. Asstated above, there is uncon- formity between this formation and the Mon- terey beds, only notice- able, however, where the latter Pare flexed; the overlying “‘mesa_ sand- stone” is also uncon- formable, as shown in Figures 11 and 12. The yellow beds are first noticeable in thin, scattered bodies lying on Fic. 10.—Steeply tilted Monterey beds on east the metamorphic menice side of the Arroyo de la Purisima, six miles below . La Purisima, Baja California. near latitude 29°. Near Jatitude 28° 30’ they have the relations shown in section 7, Figure 2, having been preserved from erosion by a thick cap of basalt. This relation continues for many miles south, as shown in sections 8-15, although the floor of metamorphic rocks sinks out of sight a short distance south of Calmalli. In the La Purisima region and near San Hilario the underlying formation is the rather uneven surface of the Monterey beds. Probably the formation thickens consider- ably under the Santa Clara Desert, where doubtless it is underlain 734 N. H. DARTON by Jater Cretaceous rocks such as those reported to appear in the southern portion of the Santa Clara Mountains. Local features.—About 150 feet of the yellow beds are exposed under the lava cap at Cerro Angel, 17 miles west of San Ignacio (see sec. 10, Fig. 2). The principal material is gray to pale greenish- yellow sandstone with beds of volcanic ash. ‘There are extensive exposures about San Ignacio where the strata are capped by basalt, as shown in Figures 13 and 14. Fic. 11.—Upturned “yellow beds” overlain unconformably by Mesa sandstone, six miles west of La Purisima, Baja California. Fic. 12.—“Yellow beds” overlain unconformably by Mesa sandstone on Arroyo de la Purisima, five miles below La Purisima, Baja California. The yellow beds in the exposures extending from the Arroyo Valle to beyond Rancho Quarente, about 40 miles south of San Ignacio, present a uniform succession, about 500 feet thick, of soft, yellowish sandstone and loam. The base is not exposed and the top is eroded. Some beds contain considerable clay, and others are nearly pure sand. ‘The principal color is a pale greenish-yellow. A few thin layers of hard sandstone and conglomerate are included. A hard fossiliferous layer occurs in places in the middle of the beds exposed. ee GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 735 The yellow beds outcrop for many miles along the Arroyo San Raimondi (or San Miguel), 30 miles northwest of La Purisima, from a point a half-mile above Rancho las Tules to its mouth, with relations shown in Figure 16. Pale-yellow, loamy sands or soft sandstone prevail, and at several points, notably at Rancho San Antonio and near the outcrop of the Monetrey beds 6 miles southwest of Caije (a very small settlement 35 miles northwest of La Purisima), a basal limestone member is exposed filled with Fic. 13.—San Ignacio, Baja California, from thesouth. MHighsierra in distance; later Miocene capped by basalt in middle distance. Fic. r4.—Upper Miocene strata capped by basalt at San Ignacio, Baja California fossils (not determined). Agglomerate and sheets of lava occur in the lower part of the formation in this valley, as shown in Figure 16. In the valleys of the Arroyo Juanico and the Arroyo Mesquital, 20 to 25 miles west of La Purisima, the yellow beds are exposed in slopes and cliffs of considerable extent, overlain in part by lava and to the east by gravell beds at the base of the mesa sandstone. The principal material is soft, yellowish sandstone, in part contain- ing some clay. The top beds have been eroded and the base is 736 WN. H. DARTON not revealed, although probably not far below the bottom of the valleys. In the Arroyo San Gregorio, about 6 miles southwest of La Purisima, the yellow beds are exposed lying on Monterey beds and overlain by mesa sandstone. ‘They thin rapidly to the north where the surface of the Monterey beds rises rapidly. ‘The yellow beds outcrop prominently again near Purisima Vieja, 1o miles north- west of La Purisima, exhibiting all the strata down to the richly fossiliferous limestone bed which occurs at the base of the forma- ‘tion near La Purisima. The following section is exposed in this vicinity. Probably Monterey beds are not far below the bottom of the arroyo at this place. The stratigraphic relations of the yellow beds are extensively exposed in the Arroyo de la Purisima from near its mouth to a point Fic. 15.—Section in the Arroyo San Raimondi northwest of La Purisima, Baja California. a, agglomerate; m, Monterey beds; g, Quaternary. about 6 miles above La Purisima where the base of the overlying mesa sandstone crosses the canyon. ‘The principal features are shown in the cross-section 14, Figure 2, and the columnar sections in Figure 17. For some distance near the 1,360-foot boring the yellow beds may either thin out or give place horizontally to a massive bed of conglomerate which lies on the Monterey beds; the precise relations are obscured by talus which breaks the continuity of the outcrops. The view, Figure 18, shows high cliffs ro miles below La Purisima where the yellow beds include a massive member of impure limestone filled with fossils, apparently the same bed as the first: one rising above tidewater several miles southwest (see Fig. 17, sec.1). This bed appears either to thin out or to give place to sandstone and agglomerate farther north near the drill hole. There are extensive exposures of yellow beds about La Purisima (see Fig. 19) with a basal member of limestone filled with ee oe ee GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 7737 fossils. This member is conspicuous in the bed of the arroyo a short distance below the village where it is crumpled in a series of small but closely appressed flexures: It also outcrops almost continously for several miles below this place, mostly in the bed of the arroyo. At two localities about y 4 miles below La Purisima, small arches i reveal the top of the underlying Monte- rey beds with contact clearly exposed. The yellow beds and this basal limestone disappear a short distance farther down- : stream or near the 1,360-foot boring, as ., ree ae nt ae shown in Figure 17. The precise strati- opposite Purisima Vieja, ten graphic conditions at this place could ae eee ee not be ascertained, but the basal lime- stone, at least, appears to abut against the old slope of a mound of Monterey beds, probably a local shore line. The igneous rocks included in the yellow beds are the products of contemporaneous volcanic action, and it is not unlikely that to the east they grade into the lower part of the great succession of TWO MILES NEAR g ABOVE 2ND AT TIDEWATER OUTH END NORTH SIDE NORTH OF rd ABOVE DRILL © LA PURISTMA BIG BEND BIG BEND BIG BEND _ ee HOLE MESA SANDSTONE Fic. 17.—Columnar sections along the Arroyo de la Purisima, showing the stratigraphical relations of the yellow beds. m, Monterey beds; a, agglomerate; Js, fossiliferous limestone at base of yellow beds; s, massive sandstone at La Purisima; b, sheet of basalt in mesa sandstone. a agglomerates, etc., which constitute the base of the high sierra. In the valley of the Arroyo San Raimondi (or San Miguel), 40 miles northwest of La Purisima, there are many exposures of large bodies of agglomerate included in or displacing the upper members of the yellow beds (see Fig. 20). One notable outcrop is near the Rancho San Antonio, where the rocks have the relations shown in 738 N. H. DARTON Figure 15. Another similar mass is exposed in the Arroyo Valle, 8 miles west of Rancho San Antonio, and on the Arroyo Vaca, 50 miles west by north of La Purisima, there is an exposure high in the canyon walls showing the relations indicated in Figure 21. This section shows that the agglomerate was erupted during the time of deposition of the yellow beds and prior to the lava flow which now caps the mesas. A small amount of agglomerate lies on the Mon- terey beds in the Arroyo San Gregorio, 11 miles W. 10° S. of La Purisima, and also near the 1,360-foot bore hole on the Arroyo de — la Purisima. Fic. 18.—Yellow beds and overlying Mesa sandstone on the Arroyo de la Purisima, ten miles below La Purisima, Baja California. Massive fossiliferous bed near center. Thin sheets of basalt are included in the yellow beds in the lower part of the canyons of the Arroyo San Raimondi and the Arroyo Caije. The relations of the old lava flows are well exposed just south of Caije and along the west side of San Juanico Bay, 7 miles due west of La Purisima. ‘The exposure near Caije is due to a low dome, and here the lava is overlain by a 30-foot, soft, gray sandstone, a Jocal member of the yellow beds. A thin sheet of basalt caps the yellow beds at Purisima Vieja, as shown in Figure 16, and halfway between that place and Poza Honda an igneous Mass occupying the bottom of the valley was probably a vent from which this sheet was erupted. GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 1739 Fossils —The yellow beds contain fossils at various places which indicate later Miocene age, Carrizo Creek horizon. Possibly the sediments represent a still longer epoch. Fossils collected from the prominent limestone bluff at the head of tidewater at the mouth of the Arroyo de la Purisima were determined as follows by Dr. Julia A. Gardner, of the United States Geological Survey: Scutella Fic. 19.—La Purisima from the south. Pilon in center is Mesa sandstone capped by basalt; slopes of yellow beds below. Lava-capped mesas in the distance. Fic. 20.—Conglomerates, agglomerates, tuffs, and igneous flows near San Miguel, fifty miles northeast of La Purisima, Baja California. gabbi Remond: Terebra sp., cf. T. simplex Carpenter; Conus sp., cf C. vittatus Hwass; Oliva sp. aff. O. angulata Lamarck; Oliva n.sp. aff. venulate Lamarck; Phos. sp.; Turritella sp., cf. T. coopers Carpenter; Turritella n.sp., cl. T. ocoyana Conrad; Turritella n.sp. aff. T. goniostoma Valenciennes; Turritella sp., ci. T. inezcana Conrad; Cancellaria n.sp., ci. C. verusta Gabb; Macron merriamt Gabb?; Natica pablonesis Clark?; Chlorostoma (Omphalius) sp. 740 | N. H. DARTON aff. C. dalli Arnold; Glycimeris n.sp. (2); Arca n.sp., cf. A. samu- loensis Osmont: Arca sp.ind.; Ostrea veatchit Gabb; Pecten (Pecten) carrizoensis Arnold; Pecien (Lyropecten) crassicardo Conrad; Pecten n.sp. A.; Cardium sp.; Phacoides sp.; Mytilus trampaensis Clark ?; Chione sp. Chione sp., cf. diabolensis Clark; Semele n.sp. and Balanus concavus Brénn. ‘The relations of this limestone are shown in the sections of Figures 1 and 2. Apparently it is at the base of the formation, and the same bed appears again along the arroyo a mile above the 1,360-foot bore hole and outcrops almost continuously to beyond La Purisima. In the vicinity of the latter place Dr. Kew collected the following fossils, determined by Dr. Julia A. Gardner, of the United States Geological Survey: Strombus n.sp.; Turriiella n.sp.; T. n.sp., ci. T. ocoyana Conrad; T. sp., cf. T. margaritana Normand; Calyptraea costellata Conrad ?; Arca sp. Ostrea sp. ind. Pecien n.sp.; Venericardia sp., cf. V. calt fornica Dall; Chione sp. Chione n.sp.: Bala- nus concavus Bronn—a Carrizo Creek fauna. Fic. 21.—Section in east wall of canyon of the ; : i : Arroyo de la Vaca, show- The higher limy member occurring in ing relations of agglom- erate in the yellow beds. a thick ledge near the middle of the yellow beds along the walls of the Big Bend and for some distance above it along the Arroyo de la Purisima, as shown in Figures 17 and 18, contain many fossils. A large collection was made from this horizon, but it has been misplaced in the National Museum in Washington. ‘The basal limestone of the yellow beds outcropping in the bed of the Arroyo San Gregorio near Purisima Vieja yielded the following forms: Purpura, cf. P. vaquerosensis Arnold var.; Turritella, cf. T. ocoyana Conrad; T., cf. T. meziana Conrad; T., cf. T. margaritana Normand: Arca sp. ind. Pecten sp., cf. P. estrellanus Conrad; Pecten crassicardo Conrad; Pecten sp., cf. Pecien n.sp., Cytherea n.sp. ind.—a Carrizo Creek fauna. Only a few fossils were found in the extensive exposures of yellow beds north of the Arroyo Valle, 45 miles northwest of La Purisima. In a lower sandy member are abundant Ostrea bour- geosit Gabb and Turritella, cf. T. jewettt. At another place were collected, from the middle beds, Chione, cf. C. elsmerensis English and many specimens of Tellina, cf. T. ocoides Gabb. ‘These fossils GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 741 were determined by Dr. Julia A. Gardner, who regards them as probably Upper Miocene. Fossils collected near the top of Cerro Angel, 18 miles west of San Ignacio, were identified as follows: Oliva-sp.; Turritella, cf. hoffmant Gabb; Calypiraea, cf. diegoana Conrad; Glycymeris sp.; Arca sp.; Pecten n.sp.; Pecten sp.; Dosiniaarnoldt Clark: Chione elsmerensis English?; Chione sp.; and Macoma vaulecki Arnold. They are regarded as San Pablo or late Miocene. These yellow beds outcrop extensively under the Java mesas about San Ignacio (see Fig. 14) and in the arroyo below that village. Fossil oysters are abundant in some of the layers. Yellow beds similar to those of the La Purisima region over- lie Monterey beds in the slopes east of Rancho Tepetate. The steeply uplifted strata just northwest of Rancho Platana include a richly fossiliferous limy ledge which yielded fossils of the same fauna as those occurring in the basal hmestone member of the yellow beds in the Arroyo de la Purisima. The following were identified by Dr. Julia A. Gardner: Strombos n.sp. (same as one at La Puri- sima); Zurritella n.sp. 1 and 2; Codakia sp.; Cardium sp. ind.; chione sp., ci. C. fernandoensis English; C. sp. ind. and Psephidea ? sp.—a Carrizo Creek fauna. It is reported that later Miocene beds outcrop on the gulf shore near Cayote Point and Agua Verde Bay, doubtless coming up from under the agglomerate series, as shown in section 4. From a bed in the base of the formation or not far below it, 8 miles northwest of El Pilon, were collected the following fossils: Turritella sp., cf. T. margaritana Normand; Turritella N. sp.; Yoldia sp.; Ostrea sp. ind.; Mytilus? sp.; Modiolus? sp.; Pecten? sp., Chione sp., ci. C. fernandoensis English and Mactra? sp.; deter- mined by Dr. Julia A. Gardner who regards them as late Miocene or early Pliocene. MESA SANDSTONE AND THE GREAT LATER TERTIARY VOLCANIC SERIES Gabb recognized the fact that the widespread mesas of southern Baja California consist of a thick mass of west-dipping sediments and conglomerates. He named it the ‘‘mesa sandstone,” and while he erroneously extended the application of the name to other 742 N. H. DARTON formations constituting mesas in the central and northern parts of the peninsula, the name may be useful in the southern part of the region until a more definite classification is practicable. I find that the formation presents two phases: a massive, gray sandstone, several hundred feet thick in the western portion of the mesa region, rapidly merging into conglomerates with thick bodies of agglomerate and tuff to the east. This relation is shown in sections 11-20, Figures 3 and 4, and is a most striking feature. The coarse sediments are in hard, massive beds, 4,000 feet thick in places, constituting the high sierra extending continuously southward ————— i Fic. 22.—Mesa sandstone on the Arroyo San Andres, twenty-one miles south of Comondu, Baja California. Shows massive bedding. from near latitude 28° to beyond La Paz. Sheets of contemporane- ous igneous rocks are included and the succession is penetrated by many intrusions, notably in the Mulegé and Santa Rosalia regions. To the east it lies on schist, granite, etc., and to the west on sand- stone of earlier Tertiary age. South of the latitude of La Paz the thickness diminishes, volcanic rocks are not present, and at Todos Santos the underlying rocks reach the shore of the Pacific Ocean. I did not observe its relations north of latitude 28°. In the area of the great, lava-covered mesas extending along the Pacific slope past San Ignacio, La Purisima, and Comondu, the yellow beds are overlain by massive, gray, mesa sandstone. This sandstone is conspicuous in the walls of many canyons which GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 743 cut the mesa zone, notably in the arroyos San Raimondi (or Miguel), San Gregerio, De la Purisima, Comondu, San Benancio, San Andres, and Santo Domingo. The greatest thickness observed in this area is 30c feet; to the west the beds are thin out to nothing under the lava caps of the mesas, and to the east they merge into the great formation of detrital and volcanic rocks constituting the central sierra. ‘This transition is well exposed in the Arroyo San Raimondi near the little pueblo of San Miguel, in the canyon of the Arroyo San Gregorio-above Purisima Vieja, in the Arroyo de la Purisima above Huerta Vieja (a few miles above La Purisima), and it is also evident in the canyons from the Arroyo Comondu southward. The change to the east consists in the sediments becoming coarser and thicker, and probably over- lying beds also are present under the great sheets of the basalt which cover the high mesas. Close scrutiny would probably reveal the presence of several formations in the sandstone series. A feature of this kind is clearly apparent about La Purisima where a harder, massive sandstone appears as a huge wedge in the other sediments (see Fig. 17). It thins out to the southwest, a mile or two below La Purisima. The eastern extension of the mesa sandstone was not studied in detail, but was observed in various trips across the high sierra. The predominating rock is conglomerate in thick beds and consist- ing largely of bowlders of igneous rocks together with some quart- zite. Sheets of lava, layers of tuff, and thick bodies of agglomerate are included. Some of the lavas are basalt, others are light- colored, more acidic rocks. To the east many intrusive masses are present. These are conspicuous west of Santa Rosalia, near Mulegé, east of Conception Bay, and southward past Loreto. A fine exposute of mesa sandstone on the west bank of the Arroyo San Gregorio, 10 miles slightly north by west of La Purisima, shows the following strata: ILANvay SINGEE BVETMOID OP WANES, boos sasdaancedo og sou oo-++ Sandstone, gray, massive, compact.:-..-.........- 60 - Samed stoneywiiteSOltmc yt bite -ko eater eer 120 Bowldersvand: Sandi: 36 7/5. Sic ea etre sere sleepers ater 40 Agglomerate, reddish in places..................-. 4 Yellow beds in bottom of the arroyo............. 30 744 N. H. DARTON Near Purisima Vieja, on the Arroyo San Gregorio, 12 miles northwest of La Purisima, a thin lava bed lies at the base of the formation (or in the top of the yellow beds, see Fig. 16), and a short distance above that place is a vent from which this lava flow probably came. Near San José, 7 miles farther up the same canyon, conglomerates predominate in the high canyon walls and two sheets of basalt are included. The later are well exposed in Arroyo Caije near Rancho Nombre de Marie, 25 miles south- south west of Mulegé. In the extensive exposures of the agglom- erates and conglomerates near San Miguel on the Arroyo Raimondi (or Miguel), as shown in Figures 15 and 20, a thin sheet of trachyte is included in the succession. Near Rancho los Angeles in the same canyon the cliffs 500 feet high consist of coarse conglomerate, some beds being harder than others and consisting largely of bowlders of igneous rocks in gray sand. Below Rancho Mescal, a few miles above Rancho las Tules, underlying gray sandstone appears lying in yellow beds and in places on agglomerate which displaces the upper members of the yellow beds. These features are also well exposed halfway between Rancho las Tules and Rancho San Antonio. Not far below the latter place the basalt beds of the mesa sandstone thin out and the lava of the mesas lies directly on top of the yellow beds. The relations in this arroyo are shown in Figure 15. Mesa sandstone appears at San Vicente, a ranch 5 miles south- east of La Purisima, where a fine spring issues from it, and is well exposed along the arroyos San Antonio and Pabellon 10-20 miles south of La Purisima. In the latter there are high bluffs of it at Rancho Pabellon and extending to beyond Coyote hole a watering- place as head of tidewater, 20 miles south-southwest of La Purisima. Here the rock is soft sandstone, in part pale greenish and yellowish. It is capped by caliche and conglomerate of Quaternary age which covers the adjoining mesas. The yellow tint may be due to a mixture of detritus from yellow beds which do not outcrop in this vicinity. In the deep canyon of the Arroyo Comondu, near the village of Comondu, there are high walls of mesa sandstone capped by basalt .of the lava sheets which cover the adjoining mesas, and there is also a cap of basalt at lower level, apparently a remnant GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA — 745 of a sheet which flowed down the canyon when it had only about half its present depth. It is possible, however, that this lava sheet is included in the mesa sandstone, for such a relation is shown on the south side of the canyon 12 miles below Comondu, just above the mouth of the Arroyo Belamote. In that vicinity also there are some conglomerate members showing strong unconformity at base, and a few conglomeratic layers are included in the gray sandstone. It is probable that the mesa-sandstone outcrop extends down the Arroyo Comondu to Rancho San Andreta, near the mouth of the arroyo, where the following section is exposed: onplomerdteni sais cars ceases welermeonare tera 30 feet, many fossils Grayssand, paleyereenish’ tints. ac. -s cece 20 feet imypledre yellowish. Majer omits 5 feet exposed Fossils collected from the basal bed in this exposure were determined by Dr. Gardner as Turritella, Cytherea and Balanus ?, and a Chione resembling C. elsmerensis English, probably Pliocene in age. This is the only paleontologic evidence that I obtained as to the age of the formation and it is not conclusive. Extensive exposures of mesa sandstone are presented in the Arroyo San Benancio and the Arroyo San Andres south of Com- ondu, one of which is shown in Figure 22. The sandstone, con- glomerate, agglomerate, tuff, etc., constitute cliffs and high canyon walls along the west side of the Gulf of California from Agua Verde Bay to Canyon de los Reyes. A section in the latter shows the following features: GENERALIZED SECTION OF STRATA IN CANYON DE LOS IES Feet Soft, gray sandstones, some beds conglomeratic (top (OVE TaKGhod®) uate Ar a MEN I RR aetna oR Ee ene 220 Rhy olite tows (bench-maker)) ae catei-ete rae ake 40 Soft, gray sandstone, several beds of conglomerate TOMEMOVEASESiscic ersten Amie a atonslecdohetateeneuee oretons 260 Conglomerate with pawliees of igneous and other crystalline nOCKSH wr miarqamiee sr eetsyeretensereaniok 40 Rhyolite flow (making wide bench)............. 30-70 Gray conglomeratic sandstone with agglomerate Ney Chasers at hooey PRE REP RSEIS OGIO ort casos 200 Agglomerate, tuff, volcanic ash, igneous sheets. Bedding massive extends to tide-level......... 650 Motalithickmesswe sd) Getic cise see cls cters 1,650 746 N. H. DARTON The thin sheets of rhyolite included in this section thin out to the south, but thicken considerably to the north, and the mass of agglomerate also thickens greatly in that direction. This series appears again in ridges passing just east of La Paz and on the islands of Espiritu Santo, Partida, and San Josef, where it consists of thick bodies of agglomerate, tuff, and eruptive rocks. ‘The thinning and fining of the formation to the west is plainly visible in many canyons on the west slope of the sierra west and northwest of La Paz as far north as the Arroyo Santa Cruz near latitude 25° 20’. In this part of the peninsula the soft, gray, massive, typical ‘‘mesa sandstone” appears extensively in the highlands of the sierra as a component of the complex; but to the west, as the beds thin and fine, it becomes the dominant feature. It is exposed in the arroyos Conejo, Datilar, Guadalupe, 35-60 miles west of La Paz, and in the cliffs just below San Hilario; but at all of these places layers of pebbles are included and scattered bowlders occur. About El Pilon, to miles north of Hilario, it appears in many cliffs, some of which show limy layers. Farther north the conglomerate admix- ture increases notably in the vicinity of Rancho Jesus Maria, 35 miles north-northwest; but at the north side of Cerro Nombre del Dios, 6 miles southwest of that ranch, the gray sandstone contains only a few thin beds of conglomerate and widely separated bowlders. The westernmost exposure observed in this portion of the peninsula was on the Arroyo San Luis, 17 miles northwest of San Luis, where the sandstone is fine-grained and massive. PLIOCENE TO POST-PLIOCENE Along the wide belt of lower lands adjoining the Pacific south of latitude 26° 30’, there is a cover of sand and gravel with lime- stone members which doubtless include not only the Quaternary, but probably also a formation of Pliocene age. These deposits vary in thickness from a few feet to 200 feet or more, and they extend far up the slopes, and on the higher mesas they are probably represented by a thick cap of bowlders. The latter are especially well exhibited on the high mesas north and northeast of San Hilario and on the long sloping mesas crossed by the main trail north from San Luis. The wide, low plains of the Magdalena region are EE ——— =< eel GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 747 covered by a deposit of sand of Quaternary age which to the south extends to the Arroyo Salado and beyond, and some distance up the slopes to the east. At Aqua Verde near Rancho Salado (latitude 24° 31’, longitude rrr° 31’), underlying yellow sands with hard, limy ledges appear in a cliff 40 feet high. The section at this place is as follows: : Feet Gray conglomerate sandstone, which floors the adjoining low Slope pmaesawh fateie:.. icc planer Leen Can ee oiake I5 Yellow, fine-grained sandstone somewhat harder beds above, and at base a wedge-shaped, hard, limy ledge with fossils’ 25 The fossils collected here were determined by Dr. Julia A. Gardner-as follows: Turritella sp.; Arca aff. A. microdonta Conrad; Mytilus sp.; Modiolus sp.; Periploma (Halistrepta) sulcata, Dall?; Tivela n.sp.; Chione latilamenosa, A and M? C. sp.; Metis aff. M. alta Conrad and Balanus,sp. ‘‘ Age post-Miocene.”’ These yellowish beds extend for 3 or 4 miles up the Arroyo Salado and lie on the Eocene (?) sandstone. Probably -their yellow*color is due to material from the Monterey formation or yellow beds underlying them. They extend up the adjoining plateaus to an altitude of 500 feet or higher. Limestones of this formation are conspicuous in the Cerrito Flor de Melba, 60 miles west of La Paz, and on the walls of various arroyos from Datilar to Cuafio, as well as in cliffs along the ocean (60-40 miles west of La Paz). These limestones occur at several horizons, and they are highly fossiliferous at most places, but the fossils of the upper beds at least appear to be post-Pliocene. The dip is to the south- ‘west at a low angle, and the beds extend far up the west slope of the sierra where they overlie the mesa sandstone. THE GREAT UPLIFT ‘It has been found that much of the peninsula of Baja Califor- nia has been uplifted out of the sea in very recent geologic times. Deposits of modern sea shells occur at many places in regions up to altitudes of 1,000 feet and they are reported as high as 3,300 feet. Old belts of sand dunes and shore lines are conspicuous far above 748 N. H. DARTON sea-level in several areas. Wittich' has presented many interest- ing facts relating to this subject with illustrations of shell deposits and shore features up to altitudes of 1,052 meters in a divide near Mission San Borja; this was as high as he ascended in his explo- rations. : The emergence has occupied a somewhat long time and may still be in progress. The shore erosion has been effected at vari- ous stages, although some of it may have been developed during the preceding submergence. The latter was apparently relatively transient, and most of the present configuration of the land was developed prior to this event. tErnesto Wittich, ““La Emersion Moderna de la Costa Occidental de la Baja California,” Soc. Cien. Antonio Alzate (Mexico) Memoirs, Vol. XXXV, pp. 121-44. InpEx To Votume XXIX PAGE Alcock, F. J. The Reed-Wekusko Map-Area. Northern Manitoba. Review by J. F. W. : rine ste hwo Alling, Harold L. The Mineralose be at the Beldseaes Part I. 194, 205, 213, 242, 254, 258, 275, 279 Alton, Illinois, The Pleistocene Succession Near, and the Age of the Mammalian Fossil Fauna. By Morris M. Leighton : 505 Anderson, Carl B. The Artesian Waters of North Eastern Minera, Review by A.C. McF. .. APE Die othe ete dh EOS Anne Arundel County, The Physical Reaenres at By Homer P. Little and Others. Review by R. D. S. : 90 Anorthosite-Gabbro in Northern New York, Beattnes of a “TRedhy a ByaWalliany | Mallen). 29 Artesian Waters of North Eastern Taner The. By Carl B. Andee son. Review by A. C. McF. . CMe S Iles Ney AD OO Atollen in den Nederlandsch- Goreindmenen Arehinel De Riffen in de Groep der Toekang Besi-Eilanden. (Atolls in the Dutch East Indies.) Door Dr. B. G. Escher. Review by W. M. D. Li Wea Baja California, Geologic Reconnaissance in. By N. H. Darton. 720 Bascom, F. Cycles of Erosion in the Piedmont Province of ee Vania) |). 54° Bastin, E. S. Review of Betraets fom “The Wining endnook se Geological Survey of Western Australia. ee OG/7; Review of The Cost of Mining, by res R. F nally Sela OOF, ———. Review of Two Gas Collections from Mauna Loa, by E. S. Shepherd . . SUE Nef Bayley, W. S. Diesedineire itaeneosy, Revie by D. ie F, Meare 570) Berkeley, Morgan and Jefferson Counties [West Virginia], as on. By G. P. Grimsley. Review by A.C. McF. . 96 Berry, E. W., Clarke, William Bullock, Matthews, E. i, saad ‘The Surface and Underground Water Resources of Maryland, Including Delaware and the District of Columbia. Clarke, William Bullock. The Geography of Maryland. ReviewbyA.C.McF. .. 674 Black Lake Area, Quebec, Contributions to the Mineralogy of. By Eugene Poitevin and R. F. D. Graham. Review a cE Wise ts 92 Bowen, N. L. Diffusion in Silicate Melts. SHARIA, Settee LOG Braxton and Clay Counties [West Virginia], Repent on. By Ray V. Hlenner es Review sbyrA (©. Vickie = aren si ele eice rae 93 749 750 INDEX TO VOLUME XXIX Brouwer, H. A. The Horizontal Movement of Geanticlines and the Fractures Near Their Surface Browning, Iley B., and Russell, Philip G. Coale anal SEracHine a Magoffin County, Kentucky. Review by R. D.S. . ; Bucher, Walter H. The Mechanical Interpretation of Joints II. . Cady, Gilbert H. Geology and Mineral Resources of the Hennepin and La Salle Quadrangles.. Review by A. C. McF. ; Camsell, Charles, and Malcolm, Wyatt. The Mackenzie River Baste Review by J. F. W. : : A : ; : Canadian Geological Survey Shonrneneia? enone Part C. Alberta- Saskatchewan Region. Part D. Manitoba Region. Part F. Maritime Province Region. Part G. The Platinum Situation in Canada. Review by J. F. W. : ; Chamberlin, Rollin T. Diastrophism and the Honmative Processed XIV. Groundwork for the Study of Megadiastrophism. Part II. The Intimations of Shell Deformation , Vulcanism and Mountain-Making: A Suopletmentar Note : Chamberlin, Thomas C. Diastrophism and the Formative Processes. XIV. Groundwork for the Study of Megadiastrophism. Part I. Summary Statement of the Groundwork Already Laid XV. The Self-Compression of the Earth as a Problem of Gealoey Changes of Geological Climate, Note on a Possible Factor in. By Harlow Shapley : Clark, Bruce L. The Marine Tertaye of the West Coast of He United States: Its Sequence, Paleogeography, aunt the Problems of Corre- lation The Siiniieranslite andl [Sarena Relmonenine 6 ie Meganed Group, Middle Eocene of California Clarke, William Bullock. The Geography of Maryland) Oras William Bullock, Matthews, E. B., and Berry, E.W. The Surface and Underground Water Resources of Maryland, Including Dela- ware and the District of Columbia. Review by A. C. McF. Coal-bearing Portion of Tazewell County, Virginia, The Geology and Coal Resources of the. By T. K. Harnsberger. Review by R.A. J. Coals and Structure of Magoffin County, Kentucky. By Iley B. Brown- ing and Philip G. Russell. Review by R. D. S. Collins, W. H. Onaping Map-Area. Review by J. F. W. Colorado Bureau of Mines, Fifteenth Biennial Report, for r9r7 and 1918. Review by D. J. F. : Contributions to the Mineralogy of Black ihalke nee Suchen. By Eugene Poitevin and R. P. D. Graham. Review by J. F. W. Cost of Mining, The. By James R. Finlay. Review by E.S. Bastin , PAGE 94 670 416 166 391 679 502 “583 125 674 390 89 gt 88 Q2 667 INDEX TO VOLUME XXIX FSI PAGE Crystallization and Magmatic Differentiation of Igneous Rocks, The Physical Chemistry of the. By J. H. L. Vogt . 5 | Bile, AAO, Bis, OaG Cycle of Glaciation, Studies of the. By William Herbert Hobbs. . 370 Cycles of Erosion in the Piedmont Province of Pennsylvania. By F. Bascom : : : ‘ : : : : : ‘ : a ASO Dake, C.L. The Sand and Gravel Resources of Missouri. Review by REEDS S... 5 3 : 5 3 go Darton, N. H. Geologic Reconranhestines 4 in nBare Galltinanta nate mami 2X) Davison, Charles. Volcanic Earthquakes. . . 97 Description and Naming of Sedimentary Rocks, Supmentions® as for ah. Biv Aen elaiey eur: : O50 Descriptive Mineralogy. By W. S. Barley Review opr 1D i, im, J BGS Devonian of Western Tennessee, The Stratigraphy and Correlation of the. By CarlO. Dunbar. Review by R. A.J. . 389 Diastrophism and the Formative Processes. XIV. Cronadwiaok foe the Study of Megadiastrophism. Part I. Summary Statement of the Groundwork Already Laid. Thomas C. Chamberlin . 2 BOR Part II. The Intimations of Shell Deformation. Rollin T. Chamberlin : ; 416 XV. The Self-Compression of the Farth as a Problem 6 Gasoer sien Can @hramaberiine : MARRS ea iis ie : e070 Diffusion in Silicate Melts. By N. L. Bowen US 1 ey ie fn Cae att Rae 2G Diplocaulus, A New Form of. By M.G. Mehl. .. ew ua AS Discussion of ‘‘Summaries of Pre-Cambrian Literature of North America,” by Edward Steidtmann. By Terence T. Quirke. . 469 Dunbar, CarlO. The Stratigraphy and Correlation of the Devonian of Western Tennessee. Review by R.A. J. . ‘ : : ‘ 5 BSC) Hanshieuakess volcanics. By, Charles Davison). 2 800). ae 07 Editorial Note. . 87 Emmons, William Harvey. Geolony at Berroleumn: Review by 1B), Do. Bae AAO ih Ulan ee es eee Mone Tm HRS a) C4 Wire niga obra We aL(O)N Escher, Dr. B. G. Atollen in den Nederlandsch-Oost-Indischen Archipel. De Riffen in de Groep der Toekang Besi-Eilanden. (Atolls in the Dutch East Indies.) Review by W.M.D. . EA S2 Examples of Squeezing Differentiation from Northern Norway. By Stemar Koshie 7 ; ; 701 Extracts from “The Mining Handbook i Grolereal ISuevey af Western Australia, “ IRGnen lel Spleen a Po Bee Ra 807 Features of a Body of Anorthosite-Gabbro in Northern New York. By Walliame |i. Miller 7) 3". PA ORE Oa Fog. MO CN A 29 Feldspars, The iltiere Hosea iey7 of he: Part I. By Harold L. Alling 752 INDEX TO VOLUME XXIX Fifteenth Biennial Report, Colorado Bureau of Mines, for 1917 and 1918. Review by D. J. F. 4 Finlay, James R. The Cost of Mining. even te E. S. nee Foslie, Steinar. Examples of Squeezing Differentiation from Northern Norway Gas Collections from Mauna Loa, Two. By E.S. Shepherd. Review by E. S. Bastin Geanticlines, The Higeeaveal Movement oe =A the Beers mee Their Surface. By H. A. Brouwer . ee Genesis of Ore Deposits, Theoretical Dysiee eae of the. By R. H. Rastall : fae hee Geologic Psvieneer nee 7 in Bare Garrats: By N. H. Darton Geological Climate, Note on a Possible Factor in Changes of. By Harlow Shapley Geology and Coal Resources of the @eabeerane ame cf Tazewell County, Virginia, The. By T. K. Harnsberger. Review by RAS je : Geology and Mineral Easiness a the Fiestas a be Salle Quad- rangles. By Gilbert H. Cady. Review by A. C. McF. . Geology and Ore Deposits of the Virgilina District of Virginia and North Carolina, The. By Francis Baker Laney. Review by R.A. J. . Geology of Petroleum. By William Harvey Emmons. Review by E. S. B. : Glacial Gravel Seam in Epes von at eon Wicca A. By F. T. Thwaites : Glaciation, Studies of the tik ne By William Eferhert Eancee , Graham, R. P. D., Poitevin, Eugene, and. Contributions to the Mineralogy of Black Lake Area, Quebec. Review by J. F. W. Grimsley, G: P. Report on Berkeley, Morgan and Jefferson Counties [Wes Viren} ‘Review by A.C. Mck. 2° 2. (23 ae Harnsberger, T. K. The Geology and Coal Resources of the Coal- Bearing Portion of Tazewell County, Virginia. Review by R.A. J. Haynes, Winthrop P., Moore, Raymond C., and. Oil and Gas Resources of Kansas. Review by R. D. S. nee Henner, Ray V. Report on Braxton and Clay Counties [W ae Naess 1 Review by A. C. McF. Het Verband tusschen den pReneconen eed en en Outstaat a Soenda-Zee (Java-en Zuid-Chineesche Zee) en de Invloed daarvan op de Verspreiding der Koraalrifien..... (The Sunda Sea and Its Barrier Reef.) Door G. A. F. Molengraaff. Review by We Me Donec ot 108) at ee ey eee PAGE 88 667 701 387 560 487 720 502 39° 189 387 IQI 57 37° 92 96 390 93 480 Eee INDEX TO VOLUME XXIX We PAGE Hobbs, William Herbert. Studies of the Cycle of Glaciation 370 Horizontal Movement of Geanticlines and the Fractures Near Their Surface, The. By H. A. Brouwer 560 Igneous Rocks, The Physical Chemistry of the Crystallization and Mag- matic Differentiation of. By J.H.L. Vogt . . 318,426, 515,627 Jillson, Willard Rouse. The Oil and Gas Resources of Kentucky. Review by R. A. J. Ee ayes. Ger) Seana a (0 Joints, The Mechanical iniesnaintion ae oie By Walter H. Bucher I Kraus and Hunt. Mineralogy. Review by D. J. F. 188 Laney, Francis Baker. The Geology and Ore Deposits of the Virgilina District of Virginia and North Carolina. Review by R. A. J. 387 Leighton, Morris M. The Pleistocene Succession near Alton, Illinois, and the Age of the Mammalian Fossil Fauna 5 505 Leverett, Frank. Outline of Pleistocene History of } Vneeiecion Valley 615 Little, Homer P., and Others. The Physical Features of Anne Arundel County. Review by R. D.S. : : Se ae go Logan, W. M. Petroleum and Natural Gas in Randa Review by RDS: 90 Mackenzie River Basin, The. By Charles Camsell and Wyatt Malcolm. Review by J. F. W. 5 04 Malcolm, Wyatt, Caco hanes aah The Mack enne WEE Bei Review by J. F. W. 04 Mansfield, George Rogérs. Types a Rocky Terre ea Sinnetine fi Southeastern Idaho ea Marine Tertiary of the West Const a the Umted States, ‘The: Its Sequence, Paleogeography, and the Problems of Correlation. By Bruce L. Clark 583 Maryland Geological Survey. Upper Geeraccous os Wereband Syste matic Report. Review by A. C. McF. 675 Maryland, The Geography of. By William Bulloce Cini: "The Surface and Underground Water Resources of Maryland, Including Delaware and the District of Columbia. By William Bullock Clarke, E. B. Matthews, and E. W. Berry. Review by A.C. McF. 674 Matthews, E. B., Clarke, William Bullock, and Berry, E.W. The Surface and Underground Water Resources of Maryland, Including Delaware and the District of Columbia. Clarke, William Bullock. The Geography of Maryland. Review by A. C. McF. Oza Mauna Loa, Two Gas Collections from. By E. S. Shepherd. Ree by E. S. Bastin. eS ee 387 754 INDEX TO VOLUME XXIX PAGE Mechanical Interpretation of Joints, The. II. By Walter H. Bucher I Megadiastrophism, Groundwork for the Study of. Diastrophism and the Formative Processes. XIV. Part I. Summary Statement of the Groundwork Already Laid. By Thomas C. Chamberlin . 3091 Part II. The Intimations of Shell Deformation. By Rollin T. Chamberlin : 416 Meganos Group, Middle HoCene or Calton, “The Setieranine al Faunal Relationships of thes By Bruce L: Clark.) .) ene Mehl, M. G. A New Form of Diflocaulus : 48 Miller, William J. Features of a Body of Anorocires Gabbed in Northern New York : : ; ? : é ! é : 29 Mineralography of the Feldspars, The. Part I. By Harold L. Alling 194, 205, 213, 242, 254, 258, 275, 270 Mineralogy. By Kraus and Hunt. Review by D. J. F. ; 0 Ess! Mineralogy, Descriptive. By W.S. Bayley. Review by D. J. F. Bane StS7 3) Mineralogy of Black Lake Area, Quebec, Contributions to the. By Eugene Poitevin and R. P. D. Graham. Review by J. F.W. . Q2 “Mining Handbook, The,” Extracts from Geological Survey of Western : Australia. Review by E.S. Bastin . —. 4) OG, Mining, The Cost of. By James R. Finlay. Rewew by E. S. Basta ae 1007, Mississippi Valley, Outline of Pleistocene History of. By Frank Leverett . ; 615 Molengraaff, G. A. F. Het Verband fieechen den plistoceenen sail: en het Ontstaan der Soenda-Zee (Java-en Zuid-Chineesche Zee) en de Invloed daarvan op de Verspreiding der Koraalriffen..... (The Sunda Sea and Its Barrier Reef.) Review by W. M. D. : 480 Moore, Raymond C., and Haynes, Winthrop P. Oil and Gas Resonces of Kansas. Rewer byiReDiSae (ya (eh ae 89 Nature of a Species in Paleontology and a New Kind of Type Specimen, The. By Edward L. Troxell . : % f : so ANAS North America, Summaries of Pre-Cambrian Wieratare ot By Edward Steidtmann ; 3 81, 173 Northern Norway, Examples of Squeeze IDiterenteion from. By Steinar Foslie . : 5. 12 70m Note on a Possible HactOne in Changes af Grolonienl icine By Harlow Shapley 6 ! 5 d aad ; i : é ww e502 Oil and Gas Resources of Kansas. By Raymond C. Moore and Winthrop P. Haynes. Review by R. D. S. Be a 89 Oil and Gas Resources of Kentucky, The. ‘By Willard Rome qilleont ewe wa bya eens A) Hie ey RS Onaping Map-Area. By W. H. Collins Review be ale F. W. Rei QL Ontario Peninsula, and Manitoulin and Adjacent Islands, The Silurian Geology and Faunas of. By M. Y.‘Williams. Review by J.F.W. 673 a INDEX TO VOLUME XXIX 5 PAGE Ore Deposits, Theoretical Considerations of the Genesis of. By R. H. Rastall i : : ; : ; ; ; 4 , ; ; AGT Paleontology, The Nature of a Species in, and a New Kind of Type Specimen. By Edward L. Troxell . . 475 Pennsylvanian Sandstones of Osage County, Otome ‘Chand Nee WAFS Thy ay, MIBiasatehoveny IOs. gh ge 5 oe ous a ue ve 66 Petroleum and Natural Gas in Indiana. By W. M. Logan. Review by RODS ae : : ‘ 5 : 5 . f ‘ : , go Petroleum, Geology of. “EY William Harvey Emmons. Review by Kos Bs. : IQI Physical Chemistry of ime Gravion and Magenertte lngderentintion of Igneous Rocks, The. By J. H. L. Vogt 5 6 4BISy AAG, Srey O27) Physical Features of Anne Arundel County, The. By Homer P. Little and Others. Review by R. D. S. EE : go Piedmont Province of Pennsylvania, Cycles of Birositorn in “hie, By F. Bascom ‘ ; : b 540 Pleistocene History of “Mississippi Vv Allee Ounine of By Frank Wevereriun : 615 Pleistocene Succession Nedr Alton Tine The! and ane Age of fhe Mammalian Fossil Fauna.. By Morris M. Leighton ; 505 Poitevin, Eugene, and Graham, R. P.D. Contributions to the Mineral. ogy of Black Lake Area, Quebec. Review by J. F. W. . : ‘ 92 Powers, Sidney. Strand Markings in the Peony Sandstones of Osage County, Oklahoma... . 2 Jae 66 Pre-Cambrian Literature of North Aracicns Sammanes of By Edward Sreidimanm . ss) 4 Satyr ae : , : f F 3 81, 173 Quirke, Terence. T. Discussion of “Summaries of Pre-Cambrian Literature of North America” by Edward Steidtmann . . . 469 Rastall, R. H. Theoretical Considerations of the Genesis of Ore Deposits ... ; Pee A : : : : : 4 : 4 MSI Recent Publications : , 5 SSO, O77 Reed-Wekusko Map-Area, Northern Mentitabe, The. By F. J. Alcock. Review by J. F. W. : 5 ea vite) Reger, D. B. Detailed Report on weneten County: Nbstrat : : 579 Report on Berkeley, Morgan and Jefferson Counties [West Virginia]. ByACe Py Grimsley.) Reviewsby, AC Vichy lesen) enn) seco Report on Braxton and Clay Counties [West Virginia].. By Ray V. Henner. Review by A. C. McF. : : : 93 Ikeviews =. eee 88, 188, 28, ARS, 578, 667 Rocky Mountain neues in Southeatina Idaho, Types of. By George Rogers Mansfield RE FE ha se arity rane ota ee TMI os eafe. W 2t 756 INDEX TO VOLUME XXIX PAGE Russell Fork Fault of Southern Virginia. By Chester K. Wentworth . 351 Russell, Philip G., Browning, Iley B., and. Coals and Structure of Magoffin County, Kentucky. Review by R.D.S.. 89 Sand and Gravel Resources of Missouri, The. By C.L. Dake. Review by ReDss: ; 90 Sedimentary Rocks, Shrgwestions as to the Tererntion mad Nanas af By A: J. Tieje . oo aoe Self-Compression of the Earth as a role af Gealone The. Dias- trophism and the Formative Processes. XV. By T. C. Cham- berlin . 679 Shapley, Harlow. Nate! ona Peas Hey aver in au Ghanees of Geclenenl Climate re 502 Shell Deformation, The Tmpniaee of Diaceophion and thie Forma- _ tive Processes. XIV. Groundwork for the Study of Megadias- trophism. Part II. By Rollin T. Chamberlin «ate Oh Feet ARE Shepard, T. William Smith, His Maps and Memoirs. Review by A. C. McF. : . 5) SRG Shepherd, E. S. Two Gas @olectane fron Manns Lon Review by E. S. Bastin : ; 387 Silicate Melts, Diffusion in. ne N. iu, Bowed ‘ 205 Silurian Geology and Faunas of Ontario Peninsula, and Manitoulin ane Adjacent Islands, The. By M. Y. Williams: Reviewby J. F.W. 673 Smith, William, His Maps and Memoirs. By T. Shepard. Review by A. C. McF. : ie day) Species in Paleontology, The Nature oF a, and a NeW Kind of ye Specimen. By Edward L. Troxell ; pane wa, Squeezing Differentiation from Northern Norway, Bvamaples of By Steinar Foslie : 701 Steidtmann, Edward. Shortnineniss of Bae Garten eens of N arth America é St, 172 Strand Markings in the Bermey ierceere Sanidstonee’e of Osawe coun Oklahoma. By Sidney Powers : 66 Stratigraphic and Faunal Relationships of the Niceanos Grows Middle ' Eocene of California, The. By Bruce L. Clark 125 Stratigraphy and Correlation of the Devonian of Western Tennesse The. By CarlO. Dunbar. Reviewby R. A.J. . : 389 Studies of the Cycle of Glaciation. By William Herbert Hous : 370 Suggestions as to the Description and Naming of Sedimentary Rocks. By A. J. Tieje . ee 650 Summaries of Pre-Cambrian Literyeure of North engi, By Edward Steidtmann ; a ae Si, 173 “Summaries of Pre- Chico IWteriensnize of North Arneticn” by Edward Steidtmann, Discussion of. By Terence ia Onnke = ee 469 SS Williams, M. Y. The Silurian Geology and Faunas of Ontario Penin- sula, and Manitoulin and Adjacent Islands. Review by J. F. W. INDEX TO VOLUME XXIX 757 ; PAGE Summary Report, Canadian Geological Survey. Part C. Alberta- Saskatchewan Region. Part D. Manitoba Region. Part F. Maritime Province Region. Part G. The Platinum Situation in Canada. Review by J. F. W. 3 670 Summary Statement of the Groundwork trendy, Teta Sinenraniion and the Formative Processes. XIV. Groundwork for the Study of Megadiastrophism. Part I. By Thomas C. Chamberlin 391 Theoretical Considerations of the Genesis of Ore Deposits. By R. H. Rastall 487 Thwaites, F.T. A Given Gravel Sean in avian af epee “Hits: consin 57 ines, ke Iie Semaesiene as io the Description ar Nawiiag of ‘Sadi mentary Rocks 650 Troxell, Edward L. The Neen of a Nonceeen in paleantalony anil ¢ a New Kind of Type Specimen Sue Oa mine 0207/5) Two Gas Collections from Mauna Loa. E, S. shepherd: Review by E. S. Bastin Ba Weyoh7/ Types of Rocky iio Siaaainee in . Seutiheaston ithe By George Rogers Mansfield SAM ei ee ee Upper Cretaceous of Maryland, Systematic Report. By Maryland Geological Survey. Review by A.C. McF. . . . 675 Viala, Dr. M. Les Iles Wallis et Horn. (The Wallis and Horne Islands, Pacific Ocean.) Review by W. M. D. 483 Virgilina District of Virginia and North Carolina, The Geslony anal Ore Deposits of the. By Francis Baker Laney. Review by R. A. J. 387 Vogt, J. H. L.. The Physical Chemistry of the Crystallization and Magmatic Differentiation of Igneous Rocks . . 318, 426, 515, 627 Volcanic Earthquakes. By Charles Davison , sti diccanteRe 97 Vulcanism and Mountain-Making: A Supplementary N oe By Rollin T. Chamberlin atari tintin athe hf ap oe 166 | Wallis et Horn, Les Iles. (The Wallis and Horne Islands, Pacific Ocean.) Par le Dr. M. Viala. Review by W. M. D. é 483 Water Resources of Maryland, The Surface and Underground, Taciecine Delaware and the District of Columbia. By William Bullock Clarke, E. B. Matthews, and E. W. Berry. The Geography of Maryland. By William Bullock Clarke. Review by A.C. McF. 674 Webster County, Detailed Report on. By D.B. Reger. Abstract. 579 Wentworth, Chester K.' Russell Fork Fault of Southern Virginia 351 673 me By EDWARD M. J. 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