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THE 

iNClENT VOLCANIC ROCKS 

SOUTH MOUNTAiN, PENNSYLVANIA 



A UlSSliRTAllON 

'^ft^BNTBD TO THE naASU Olt UNiVEKSlTV SW^MS Mf THfi 

juiiKS llorKitis t;N:vEHsi'rr for the psurkk of 
DOCTOR OP rliiLosopity 



FLORENCE BASCOB 



IU9A 



DEPABTMENT Ol' TllK IX'J'KIilOU 



BULLETIN 



UNITED STATES 



GEOLOGICAL SURVEY 



ISTo. 136 




WASHINGTON 

aOTBBNMEHT PRIMTING OFFICE 

1890 



'■ "V. ^ 




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1r.,>i6 /6r/./t^«-4. 



'IC fu, iM^t^uMy 



UNITED STATES GEOLOGICAL SURVEY 

CEAKLES I>. AViLCWT'J, IIIRECTOR 



JsTOIENT VOLOANTO ROCKS 



SOUTH MOUNTAIN, PENNSYLVANIA 



FLOEENOK HASf!OM 




WASHINGTON 

GOVERNMENT PRINTING OFl'ICK 

180 



CONTENTS. 



Pago. 

.etter of transmittal 11 

utroduction 13 

JhapterI. An account of geological surveys in the South Mountain below 

the Susquehanna 14 

Historical review 14 

Bibliography 19 

Jhapter II. Geological relationshi}) of the rocks of the Monterey district 20 

General description of the area studied 20 

Extent and character of the three rock types 20 

Sedimentary rocks 1 21 

Areal distribution 21 

Structural features 21 

Thickness 21 

Age and superposition 21 

Acid eruptives 23 

Areal distribution 23 

Character 23 

Previous descriptions 23 

Economic value -. 24 

Basic eruptives 24 

Areal distribution 24 

Character 24 

Previous descriptions 25 

Ore deposits of the Monterey district 25 

Comparative age of sedimentary and igneous rocks 27 

Contacts described 27 

Conclusions 28 

Comparative age of acid and basic eruptives 29 

Field relations 29 

Probable expLination 29 

Summary 30 

Chapter III. Petrographical description of the Cambrian rocks 31 

Macroscopical description 31 

Microscopical description ^ 31 

Qnartzite 31 

Slates 33 

Chemical analysis 33 

Summary 34 

Ha.i>ter IV. Petrographical description of the acid eruptives 35 

Nomenclature 35 

Quartz-porphyries 39 

Distribution 39 

Macroscopical description 39 

Microscopical description 39 

Phenocrysts 39 

Feldspar 39 

Quartz 40 

B 



6 CONTENTS. 

Chapter IV— Continued. Page. 

Quartz-porphyries — Continued. 

Microscopical description — Continued. 

Groundmass 41 

Accessory constituents 41 

Aporhyolites 42 

Distribution " 42 

Macroscopical description 42 

Microscopical description 44 

Phenocrysts 44 

Feldspar and quartz 44 

Other porphyritic constituents 45 

Groundmass 46 

Fluidal structure - - . - - 46 

Micropoikilitic structure 47 

Spherulitic structure 51 

Chain spherulites 53 

Axiolitic structure. 54 

Rhyolitic structure " 54 

Lithophysal structure - * 55 

Micropegmatitic structure * 55 

Perlitic structure * ^ 55 

Amygdaloidal structure ^-* - 55 

Taxitic structure * 57 

Summary of proof of devitrification • 57 

Opinions of petrographers 59 

Chemical composition of the acid eruptives 61 

Acid volcanic breccia 63 

Distribution 63 

Tuff 63 

Flow breccias 63 

Tuffaceous breccias 64 

Metamorphosed acid eruptives : sericite-schists and slates 64 

Summary 66 

Chapter V. Petrographical description of the basic eruptives 68 

Nomenclature 68 

Melaphyres and augite-porphyrites. -. 69 

Distribution 69 

Macroscopical description 70 

Microscopical description 72 

Original structures 72 

Secondary structures 73 

Original constituents 73 

Secondary constituents 74 

Accessory minerals 77 

Discussion of chemical analyses 78 

Basic slates 79 

Distribution and description 79 

Basic pyroclastics 80 

Distribution and description 80 

Crushed porphyrites SO 

Tuffaceous breccia SO 

Ash 80 

Summary 81 



CONTfiNtS. 7 

Page 

CttAt*Tt:ft VI. Summary of concluftioriH 82 

Evidence of the eruptive character of the two rock types * . 82 

Field evidence » . . 82 

Schistosity .*. 83 

Lamination ...* '..^ i. ■. ....^...i — i^^ — 82 

The slates 82 

Absence of gradation between igiieous and clastic rocks . ^ i ^ . . * . . . 83 

Surface-flow features *i ^i..*.. 83 

Petrographical evidence ii *...i ^ 83 

Structural i ...i-.i i ^ 83 

Mineralogical ..^^ i ^ ii.i....i....-.ii».-- 84 

Chemical -.i..i..ii 84 

Original rock types ■.. i.... .i. 84 

Acid igneous rocks i.. ...... ..^ 84 

Basic igneous rocks i - 85 

Similar rocks in other regions ■... . .*.... 86 

Literature ..&!. ^... .«.. ^.m a.^, :. ■. .... it. .^t. .<.<.. 87 



ILLUSTRATIONS. 



Page. 

*LATE I. Map showing location of Monterey district 13 

II. Panoramic view of mountains of the Monterey district 16 

III. Geologic and topographic map of the Monterey district 20 

IV. Crumpling of sandstone at the east end of the tunnel through Jacks 

Mountain on the Gettysburg Railroad 22 

V. Sections through the Monterey district 24 

VI. Junction of felsite and sandstone on the old Tapeworm Railroad 28 

VII. Flow structure in aporhyolite, Monterey district 42 

VIII. Flow structure in aporhyolite. Raccoon Creek 44 

IX. Spherulitic aporhyolite, Monterey district 46 

X. Aporhyolite with spherulit^s in layers 48 

XI. LithophyssB, Raccoon Creek 54 

XII. Flow breccia 62 

XIII. Acid breccia, Raccoon Creek 64 

XIV. Sericite-schist, Gettysburg Railroad 66 

XV. Thin sections : a, quartzite ; 6, feldspar crystal 96 

XVI. Thin sections : a, perthitic structure in feldspar; 6, stretched feld- 
spar crystal 98 

XVII. Thin sections: a, quartz-albite mosaic filling crack in feldspar crys- 
tal ; 6, micropoikilitic structure 100 

XVIII. Thin sections : a, flow structure in an aporhyolite ; h, chain spherulites 

in an aporhyolite 102 

XIX. Thin sections: a, 6, augite-porphyrite in ordinary and in polarized 

light * 104 

XX. Thin sections: a, h, perlitic parting in an aporhyolite in ordinary 

and in polarized light 106 

XXI. Thin sections: a, perlitic parting in an aporhyolite; ft, axiolites in 

an aporhyolite 108 

XXII. Thin sections : a, 6, altered and unaltered spherulites in aporhyolites . 110 

XXIII. Thin sections : a, altered spherulites in ordinary and in polarized 

light; 6, chain spherulites with phenocryst 112 

XXIV. Thin sections : a, 6, rhyolitic structures in aporhyolites 114 

XXV. Thin sections : a, rhyolitic structure in an aporhyolite ; 6, piedmont- 

ite 116 

XXVI. Thin sections : a, ft, amygdaloidal aporhyolites 118 

XXVII. Thin sections: a, &, tridymite spherulites in an aporhyolite 120 

^XVIII. Thin sections : a, augite-porphyrite ; ft, melaphyre 122 

9 



DEPABTMBNT OK Tllli l.NTHlllOll 



I5ULLET1N 



"UNITED STATES 



GEOLOGICAL SURVEY 



N"o. 13 G 




WASHINGTON 

GOTKRNMEMT PEINTIHG OPFIOK 

1800 



THE ANCIENT VOLCANIC ROCKS OF SOUTH 

MOUNTAIN, PENNSYLVANIA. 



By Flobbnob Basoom. 



INTRODUCTION. 

The moontaiu range known in Vermont as the Green Mountains, in 
Massachusetts as the Taconic Mountains, and in New York and New 
Jersey as the Highlands, is the South Mountain in Pennsylvania and 
Maryland, the Blue Eidge in Virginia, and the Smoky Mountains in 
North Carolina. The South Mountain in Pennsylvania lies just east 
of the middle of the State, and stretches from Maryland north and 
ea«t in a sickle-shaped curve toward the Susquehanna. While in the 
New England States the mountains of this range rise to heights from 
3,000 to 4,000 feet above sea level, and in its southern extension from 
4,000 to 7,000 feet, in Pennsylvania its summits rarely exceed 1,500 feet. 

South Mountain is 50 miles in length, and 10 miles wide at its broad- 
est point. It covers from 150 to 175 square miles, and abundantly 
exposes distinct and interesting rock types. These rocks, prevailing 
throughout the entire South Mountain range, have long been known 
to geologists, although their true character was not recognized until 
recently. 

In December, 1892, as the result of field work on the part of Dr. G. 
H. Williams in the northern and of the writer in the southern portion 
of South Mountain, there appeared a preliminary description^ of two of 
the rock types, in which their identification as ancient volcauics was 
announced. In this bulletin it is proposed to substantiate that identifi- 
cation with more detailed proof. It is further proposed to show :that 
these ancient igneous rocks were, at the time of their consolidation, 
identical in character with their recent volcanic analogues, and that 
their present differences are due to subsequent changes, chief among 
which has been devitrification. It is also proposed to recognize these 
facts in the nomenclature. 

The petrographical features of the third and only remaining rock 
type of the South Mountain will also be described in some detail. 

A brief report upon the previous geological work accomplished in 
the South Mountain, and some account of the structural features of 
the mountain and the age of its rocks, will precede the petrographical 
fliscnssion. 

'The voleanio rocks of the South Mountain in Pennsylvania and Maryland: Am. Joor. Sci., 3d 
Series, VoL XLIV, Deo., 1892, pp. 482-496, PI. I. Kepriuted in the Scientifio American for Jan. 14,1893. 

13 



CHAPTER I. 

AN ACCOUNT OF GEOLOGICAL SURVEYS IN THE SOUTH 
MOUNTAIN BELOW THE SUSQUEHANNA. 

HISTORICAL REVIEW. 

The first topographical description of South Mountain appeared as 
early as 1755.^ It was made by Lewis Evans, of Philadelphia, who 
describes the South Mountain with a fair degree of accuracy, as "not 
in ridges like the Endless Mountains, but in small, broken, steep, stony 
hills; nor does it run with so much regularity." He continues: "In 
some places it gradually degenerates to nothing, not to appear again 
for some miles, and in others it spreads several miles in breadth." 

In a publication^ which appeared in Germany in 1787 several pages 
are devoted to a general description of South Mountain. Two of the 
type rocks (the sandstone and the porphyry) were noted and aptly 
described, as the following quotation shows: 

One finds here and there gray laminated sandstones ivith quartz veins; fragments 
of coarse ferruginous quartz. At one spot on the road [from Sharpshurg to Freder- 
ioktown] I found blocks of gray-reddish porphyry with little transparent quartz 
grains intermixed, and milk-white opaque feldspars. * * * The South Mountain 
in its entire extent contains rich crevices, gangnes, and nests of ore, especially of 
iron and copper. * * » i have still to add, from the observations made upon 
this journey, that the eastern slope is gentler and more gradual than the western. 

The most important publications on the South Mountain have 
appeared under the auspices of the various surveys of Pennsylvania 
and Maryland. The First Geological Survey of Pennsylvania was 
organized in 1836 under the distinguished geologist, Henry D. Kogers. 

The difficulties encountered, however, were so great that it was not 
until 1858 that the two quarto volumes of the survey were issued. 
Professor Eogers deals somewhat cursorily with the South Mountain 
region. He says :^ 

In its geological constitution, this tract is without much variety, for it pontainB 
scarcely any rocks except those of the Primal series. It is doubtful if the trne 
gneissic rocks anywhere reach the surface within its borders, and only in one or two 
localities have even the lowest members of the Auroral limestones been met with 
covering the upper Primal slates. Even of intrusive igneous rocks, it embraces » 
singularly small amount, those met with being chiefly greenstones and trap rocks. 

'Apalysis of a Map of the British Colonies in America, 1755. 

^Scbopf 's Beytrago zur mineralogischen Kenntniss des ostlichen Thells von Nordameiika VJw 

seine Gebiirge, chapter 30, pp. 96-101, Erlangen, 1787. A translation of this work has been madefy 

Prof. John M. Clark, of the New York State Museum of Natural History, to whose kindness ths 

writer its indebted for a copy of the unpublished manuscript of the pages relating to Sou^ HoilBtl|i|l> 

» Geology of Pennsylvania, Vol. I, Part II, pp. 203-209, 185S. 

X4 



BASCOM.] HISTORICAL REVIEW. 15 

The geological stractoro or mode of stratification of this belt is equally simple. 
The whole tract consists of two or three groups of higli, narrow, nearly parallel 
anticlinal ridges, expanding and subdividing toward the southwest. These are 
composed of the Primal white sandstone. Between them are high parallel valleys 
and plateaus of the Primal upper slates, which, from being softer and more fissile, 
have been worn and trenched by the plowing force of waters to somewhat lower 
levels than the more resisting, better cemented sandstones. The crests of the ridges 
are therefore stony and rugged, their flanks usually smoother, being formed chiefly 
of the 8laie^ 

That Professor Eogers did not include in the "singularly small 
amount" of igneous rocks the rocks forming the valleys, the mountain 
flanks, and even the summits of the mountains, is further indicated by 
the following: 

Another section across the mountaiu more to the nouthwest extends from south to 
north along the Baltimore and Carlyle turnpike. The first important stratum of the 
hills is the usual gray siliceous altered rock, so common along their southern side. 

North of this, about 3 miles from Petersburg, occurs the dark green slate, with its 
epidote and white intrusive quartz. Succeeding this is an extremely compact sili- 
ceous altered slate, and beyond this a reddish gray rock, of the same series, containing 
specks of reddish feldspar and small veins of epidote, and near this the fissile tal- 
cose rock, several times mentioned before. * * * The summit of the ridge exhibits 
a dark blue and greenish blue indurated rock, weathering a dark brown, and evi- 
dently very ferruginous. It appears to be a band of the Primal slate in a highly met- 
amorpMo condition, approaching jasper. * * * These lower Primal slates are 
highly indurated, and even decidedly crystalline, containing in some of their layers 
segregated specks, and even half-formed geodes of epidote and other minerals. 

On page 206 of the same report Professor Rogers has figured a sec- 
tion across South Mountain, along the Gettysburg Railroad, from Fair- 
field to Monterey Springs, This section shows stratified rocks lying 
in a series of anticlinal flexures, which accord rather with Professor 
Bogers's conception of "rock waves" than with his observed dips. 
These dips are, with a single exception, to the southeast. Accompany- 
ing the section is the following description, which bears directly upon 
the portion of South Mountain that is particularly discussed in this 
bulletin : 

Passing now to the eastward of the Green Ridge axis we cross a high slope of 
alate, apparently the upper Primal, in a synclinal fold, and then traverse a succes- 
sion of outcrops of the Primal white sandstones and shites to the eastern base of the 
Jjiigh land called Jack's Mountain, at the foot of wLich the older rocks disappear 
pnder the Mesozoic red sandstone of the plain of Adams County. 

The exposures in the sandstone near the tunnel opposite Jack's Mountain indicate 
fk probable thickness of 1.000 feet. Near the tunnel at the northwest side of the 
piountaln there is a hard epidotic rock, and not far from it a highly altered greenish 
slate, a rock found in several other localities farther west and containing layers of 
gray slate, sppited tvith epidote. Farther west occurs epidote with asbestus. Near 
Minie Branch search was jnade many years ago for copper ore, but nothing was found 
to justify the expectation of finding a productive vein of that mineral. A small 
quantity of copper ore was once obtained and a furnace built for smelting it in a 
small ridge north of Jack's Mountain, but the exploration was abandoned. The 
metal occurs in the form of a green and blue carbonate, with a little native copper. 
Evidently the ore is not abundant. 



* The it-fUics m this and thf- following quotations are the writer's, 



16 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

Many of the characteristic features of the South Mountain rocks were 
aptly described by Professor Eogers, yet, phiinly, their nature was not 
fully understood nor their importance appreciated at the close of the 
First Geological Survey of Pennsylvania. In 1860 the Primal series 
of Pennsylvania, as it occurs in Maryland, was thus characterized by 
Tyson :^ 

1. A hard saudstone, made up of grains of (xuartz^ with occasionally grains of feld- 
spar and kaolin. 

2. A slate, varying in color from gray to brownish and greenish. It is ranked as 
an argillite, but portions of it assume a marked talcose appearance, especially in the 
Catoctin Mountain, and in parts of Middletown Valley, where it has been much dis- 
turbed and altered by proximity to intrusive rocks. These last consist of amphibo- 
lites (trap), porphyries, amygdaloid, serpentine, and epidote. This last-named rock 
is extensively developed, both in large masses and intercalated between the slates. 

The Second Geological Survey of Pennsylvania was organized in 1874 
under Prof. J. P. Lesley, who had been an assistant to Professor Rogers 
in the earlier survey. 

With improved facilities for scientific work, and with more accurate 
methods of mapping, a new survey of South Mountain was undertaken 
by Dr. Persifor Frazer, with A. E. Lehman as topographical assistant. 
Five sections (Nos. 7, 8, 9, 10, 11), more or less incomplete, were made 
through the mountain, and as the result of his investigations Br. Frazer 
says:^ 

It is apparent that the great South Mountain is composed essentially of two 
groups of rocks, the lower (and along this line, the northwestern) consisting of 
various modifications of the quartz conglomerate above spoken of, and in which 
quartz occurs in various forms. 

The upper and southeasterly group is felsitic in character, but contains, also, large 
beds of hydromica and chlorite schists intersected by veins of milk quartz, while 
the orthofelsite presents every variety of appearance, from a sandy and earthy slate, 
in which the crystals of orthoclase are very much decomposed, indeed, are some- 
times almost clay, through the jasper-like variety to the massive and coarsely por- 
phyritic structure in which it is suited to be used as an ornamental building stone. 

In the same year (1877) Dr. Frazer published a further account^ of 
the nature and origin of these '^orthofelsites." This account was in 
substance twice repeated by him, in 1879^ and in 1880:^ 

The rocks of this region [South Mountain] may be divided into two great series — - 
a western (underlying), of which the characteristic strata are composed of quartzit9 
and of arenaceous schist containing quartz pebbles (Mountain Creek rock), and th© 
eastern (overlying) of hydromica and chlorite schists, and orthofelsite, both porphy— 
ritic and unporphyritic. Both these series show indications of having been pene^ 
trated by dikes of plutonic character within this area. 

The porphyry, which carries the copper in this region, shows no character of igneous 
actionf hut occurs in coarse and thin beds, more or less disintegrated, and in certain. 



»Reporton the Geology of Maryland, Jan., 1860, pp. 34, 35. 

^Report of Progress in the Counties of York, Adams, Cumberland, and Franklin, 1877, CC, pp 
285-295. 
■The copper ores of Pennsylvania: Polytechnic Review, Nos. 16 and 17, Vol. Ill, April, 1877, 
4Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 338. 
<Seoond Geol. Survey, Pa., CCC, Appendix, 1880, pp. 312-313, 



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BA8COM.] HISTORICAL REVIEW. 17 

localities reduced almost to the state of kaolin. Nothing which might correspond 
to the term sandstone was observed, though all the above sediments were free of 
grit and sandy particles. * * * It seems fair to conclude that the region of the 
copper-bearing rocks belongs to the Huronian cycle. 

With these views Dr. T. Starry Hunt expressed entire concurrence.^ 
Dr. Hunt had previously made some study of the South Mountain 

rocks, and published, at various times, his observations concerning 

them. In 1876 he said : 2 

In the southern part of Pennsylvania, to the west of Gettysburg, this moun- 
tainous belt, rising between the Mesozoic on the east and the great limestone valley 
on the west, presents an immense development of a peculiar type of crystalline 
rocks which I detected there last year, and which has a considerable geological 
importance. It is a bedded petrosilex, grayish, reddish, or purplish in color, some- 
times granular but more often jasper-like in texture, and frequently porphyritic from 
the presence of small crystals of orthoclase-feldspar or of glassy quartz. There is 
found here a great breadth of this rock distinctly bedded, presenting different varie- 
ties, and alternating with dioritic, or diabasic, epidotic, and chloritic rocks, with 
argillites, in which are sometimes included thin beds of the petrosilex, the strata 
generally dipping at high angles to the east. 

These rocks Dr. Hunt provisionally referred to a position near the 
base of the Huronian division, adding: 

This petrosilex is identical in its lithological character with the halleflinta, or 
stratified flint rock of the Swedish geologists, which is by them assigned a similar 
position, i. e., above the most ancient gneisses. 

In 1878 Dr. Hunt expressed essentially the same views,^ and in 1879* 
he opposed the correlation of the South Mountain rocks with the copper- 
bearing rocks of Lake Superior (Keweenawan series), although at an 
earlier date he notes a resemblance. He says : ^ 

I may also note that I have observed bedded petrosilex rocks like those just 
noticed [South Mountain porphyries] to the north of Lake Superior, both in an 
island south of St. Ignace and on the adjacent mainland. The conglomerates or 
breccias, which, in the rocks of the Keweenawan series on the south shore of the 
lake, include the native copper of the Calumet and Hecla and the Boston and Albany 
mines, are also made up of the ruins of a precisely similar petrosilex porphyry. 

In October, 1879, Mr. J. F. Blandy made a brief reconnoissance of 
the lower portion of the South Mountain with not unfruitful results. 
He makes a suggestive correlation of the copper-bearing rocks of 
southern Pennsylvania with the Lake Superior copper »rmation,^ 
thus recognizing the volcanic nature of the greenstone at least, 
which he called "amygdaloidal trap." The acid rocks sdll remain 
<^ slates." 

In the final report of the Pennsylvania survey,'' Professor Lesley 

1 Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 339. 

« Proc. Am. Ass. Adv. Sci., 1876, pp. 211-212. 

» Second Geol. Surv. Pa., Vol. E, p. 193. 

4 Trans. Am. Inst. Min. Eng., Vol. VII, p. 331. 

sProc. Am. Ass. Adv. Sci., 1876, p. 211. 

8 Trans. Am. Inst. Min. Eng., Vol. VII, pp. 331-333. 

' Final Report of the Pa. Geol. Surv,, Lesley, Summary, Vol. 1, 189?, 

Bull, 136 8 



18 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull.u«. 

gives in substance the views of Dr. Frazer, which have already been 
quoted. He refers to Dr. Frazor's '^section 8" as representative of 
the South Mountain rocks and structure. In this section, as in the 
others in which the orl hofelsite appears, notably section 9, the bedded 
orthofelsite is represented as overlying the quartzose coiigloinerate. 
Professor Frazer concludes his summary with the statement that "it 
is hard to avoid the inference that our South Mountain rocks repre- 
sent the lluronian section of Murray and Logan." 

The geological map of Adams, Cumberland, and Franklin coanties 
made by this final Pennsylvania survey refers all the rocks of the 
South Mountain region by the use of a single color to the "Azoic 
system." 

The Second Geological Survey of Pennsylvania, while recognizing 
the extent and crystalline character of the Scmth Mountain rocks and 
emphasizing the absence of the Primal of Kogers, still failed to solve 
the problems of the structure of the region and the age and origin of 
its rocks. 

Professor Lesley has been very ready to acknowledge the incomplete- 
ness of the survey in the South Mountain, and since Mr. Walcott's 
determination of the age of its sedimentary rocks has appeared, he has 
expressed himself as desiring a new investigation of that region. 
While he claims that the survey of Pennsylvania *^ has been so minute 
and complete that comparatively little remains to be desired in the 
future," he adds: "The geology of the South Mountain is therefore 
[because of the new aspect given to it by the investigation of Mr. 
Walcott] in a very unsatisfactory condition and requires, in fact, a 
special and protracted investigation in the field. It is the most unsat- 
isfactory part of the work of the Geological Survey of the State." ^ 

Subsequent workers in the South Mountain are indebted to the 
Survey for the superior topographical maps of the mountain made on 
a scale of 1 inch to 1,600 feet (about 3 inches to the mile) by A. E. 
Lehman. These maps are invaluable to the field geologist and render 
possible accurate areal mapping. Owing to a confusion of cleavage 
planes and bedding the dips recorded on these maps are not reliable. 

While the geological explorations of the South Mountain have been 
careful and minute and conducted by able geologists, the petrography 
of its rocks has never been thoroughly investigated. The microscope 
has not been used to assist in determining the nature and origin of the 
rocks, and to correct impressions colored by preconceived ideas or by 
an experience more or less limited to sedimentary structures. Under 
microscopic scrutiny and the comparative study of recent lavas, an 
increasing number of the so-called sedimentary rocks are proving to 
be igneous in origin. That such has proved to be the case in the South 
Mountain, while not surprising, is a fact of considerable significance, 
both because of characteristics which the rocks themselves possess and 



* Report of the Board of Comm., pp. 5-6, 1893. 



BAscoM.] HISTORICAL REVIEW. 19 

because tliey form an important part of a belt of similar rocks extend- 
ing northward and southward along the Atlantic Coast. 

Following is a list of publications on the rocks of South Mountain, 
Pennsylvania : 

BIBLIOGRAPHY. 

1755. Lewis Evans. Analysis of a map of the British colonies. 

1787. Schopf. Bey triige zur mineralogischeii Kentniss des ostlichen Theils von Nord- 

amerika und seiner Gebiirge, chap. 30, pp. 66-101. 
1858. Rogers. Geology of Pennsylvania, Vol. I, pp. 203-209. 
1860. Tyson. Report on the Geology of Maryland, pp. 34-35. 
1875-1877. Frazer. Second Geological Survey of Pennsylvania ; Report of progress 

in the counties of York, Adams, and Franklin, Vol. CC, p. 285. 

1876. Hunt. Proc. Am. Ass. Adv. Sci., pp. 211-212. 

1877. Frazer. Copper ores of Pennsylvania; Polytechnic Review, Vol. Ill, p. 170. 

1878. Hunt. Second Geological Survey of Pennsylvania, Vol. E, p. 193. 

1879. Frazer and Hunt. Trans. Am. Inst. Min. Eng., Vol. VII, pp, 331, 332, 338. 

1879. Blandy. The Lake Superior copper rocks in Pennsylvania; Trans. Am. Inst. 

Min. Eng., Vol. VII, p. 331. 

1880. Frazer. Second Geological Survey of Pennsylvania, Vol. CCC." Appendix. 
1883. Frazer. Iron ores of the middle James River; Trans. Am. Jour. Min. Eng., 

Vol. II, p. 203. 
1883-1884. Frazer. An hypothesis of the structure of the copper belt of South 

Mountain ; Trans. Am. Inst. Min. Eng., Vol. XII, p. 82. 
1883-1884. Henderson. The copper deposits of South Mountain; Trans. Am. Inst. 

Min. Eng., Vol. XII, p. 90. 

1891. H. R. Geiger and A. Keith. The structure of the Blue Ridge near Harpers 

Ferry ; Bull. Geol. Soc. Am., Vol. II, pp. 155-164. 

1892. Lesley. A Summary Description of the Geology of Pennsylvania, Vol. I, p. 146. 
1892. G. H. Williams. The volcanic rocks of South Mountain, in Pennsylvania 

and Maryland; Am. Jour. Sci., Vol. XLIV, pp. 482^96, PI. X. Reprinted 
in the Scientific American, Jan. 14, 1893. 
1892. C. D. Walcott. Notes on the Cambrian rocks of Pennsylvania and Maryland, 
from the Susquehanna to the Potomac; Am. Jour. Sci., Vol. XLIV, pp. 
469-481. 

1892. A. Keith. The geologic structure of the Blue Ridge in Maryland and Vir- 

ginia; The American Geologist, Vol. X, No. 6, pp. 362-369. 

1893. G. H. Williams. Piedmontite and scheelito from the ancient rhyolite of 

South Mountain, Pennsylvania; Am. Jour. Sci., Vol. XLVI, pp. 50-57. 
1896. C. D. Walcott. The Cambrian rocks of Pennsylvania; Bull. U. S. Geol. Sur- 
vey No. 134. 



\ 



i 
i 

3 

i 



4 

i 



i 

4 

i 



BAscoM] THE SEDIMENTAEIES. 21 

The slates of tlie region thus prove to be both sedimentary and 
igneous. The former are argillaceous. The latter are either acid or 
basic, and are far more abundant than the former. 

SEDIMENTARY ROCKS. 

Areal distribution, — The first -mentioned of the rock types occupies 
the high altitudes only, Green Ridge, Monterey Peak, Pine Mountain, 
and Jacks Mountain being largely formed of it, while it caps Haycock 
Mountain and the foothill to the east of Jacks Mountain. 

Structural features. — ^The strike of the sediments is uniformly north- 
west and southeast, with a mean dip of about 45 degrees southeast. 
There are some exceptions to this southeasterly dip, notably at Monterey 
Peak, where the dip is northwest, forming, with the Green Ridge sand- 
stone, a gentle syncline. (See PI. V.) 

In the Jacks Mountain quartzite the bedding is exceedingly obscure 
and the cleavage very marked. There is an opportunity for error in 
determining the dip by the confusion of the two. 

At the northeast end of the tunnel which is cut through a spur of 
Jacks Mountain, on the Gettysburg Railroad, stratification is dis- 
tinctly visible, and shows that minor crumpling of the sediments as 
well as folding on a large scale resulted from their uplift. The rock 
is here folded in a series of small anticlines and synclines. (See PI. IV.) 

Thickness, — Professor Rogers estimated the thickness of the sand- 
stone at 1,000 feet. Professor Lesley, on the other hand, considers it 
^'immensely thick," and states that Frazer's "section 11" shows 32,000 
feet of quartzite and 64,000 feet of schistose conglomerate.^ 

Walcott^ and Keith,^ in their examination of a section of the same 
quartzite, sandstone, and conglomerate, displayed to the west of the 
Monterey district, agree essentially with Professor Rogers's estimate 
(1,000-1,200 feet). The writer did not find sufficient data within the 
Monterey district for a reasonably correct estimate of the thickness of 
the quartzite. There are undoubtedly a very great number of minor 
crumplings and foldings resulting in compressed synclines, anticlines, 
and thrust planes in Jacks Mountain. Since there are no means of 
determining the number of these folds, no estimate can be made of the 
thickness of the formation. If the minor folds are ignored, the dip and 
liorizontal extent of the sediments indicate a thickness which is enor- 
mous, an indication unsustained by sections only 2 miles to the west. 
Probably the estimate made by Mr. Walcott (1,200 feet) in that more 
favorable locality will be approximately correct for the Jacks Mountain 
quartzite. 

Age and superposition, — These siliceous elastics are the "Primal white 



J Summary, Final Report Geol. Pa., Vol. 1, 1892, pp. 145-146. 

* AValcott, Notes on the Cambrian rocks of Pennsylvania and Maryland from the Susquehanna to the 
Potomac : Am. Jour. Sci., Vol. XLIV, Dec, 1892, p. 481. • 

'Koith, The geologic strifiiture of the Blue Ridge in Maryland and Virginia: Am. Geologist, Deo.^ 
1892, Vol. X, No. 6, p. 365. 



22 VOLCANin EOCKS OF SOUTH MOUNTAIN, Ian 

aandstone" of Rogers, and iire represented by him as iuteibedded witt 
slates and occurring in a nerics of narrow anticlines. Tliey forintte 
"Mountain Creel; rock" of Fraxer and Lesley, mapped by them 
Azoic and represented both in sections and text ns niiderlyiiig (be 
"chlorite 8<'histH" sind "bedded orthofelaites." Messrs. Gcigjer and 
Keith,' in their earlier investigation of the simthward extoiisioBot 
this sandstone in the lihie Kidge, phured it above the limeKtono of ^ 
Cumberland Valley, nnd, upon strnctnral grounds, deU^nninod itftiKl 
as Upper Silurian (Medina). The true age of these sediments \|a 
recently been determined beyond ipiestion, through liie discoveiyof 
fossils, by Mr. Walcott." No fossils were found in the <|uartzo8e COB' 
glomerates and quartzit«s of the Monterey district {No. li of Mr. Wal' 
cott's section), but the interbedded argillaceous shales, of wbieb i 
tiou has been made, are foundtothe westof the Monterey district j 
ing beneath quartsiit* and other shales in which were discovered the 
remains of Scolithiis linearis, Oleneltua, and Camerella minor. These 
di.scoveries undoubtedly refer the conformably underlying Montenf 
quartz ite to a Lower Cambrian age. 

In a recent section made in the light of Mr. Walcott's discoveriea 
of his own subsequent discovery of fossils in the Blue Eidge, Mr. K« 
gives the Monterey sediments a similar position (No. 4),describiiigfch«!i 
as a " massive white sandstone with bluish black bands, feldspathie, ibi 
in places conglomeratic." Mr. Walcott calls them "a coarse-grained 
qnartzite, sometimes conglomeratic." 

Tlie sections (PI. V) through the Monterey district show the relative 
distribution and position of the three type rocks. It will be observed 
that the sandstone lies in a gentle syncline, or almost Hat, and whol^ 
above the eruptives. Tliese observations made by the writer in the 
Monterey district and elsewhere in the South Mountain, accoid witli the 
observations of Mr. Keith^ in Maryland, Owing to the entire abaeoce 
of exposure, save at the tunnel, the minor folding, which is inidoubtodly 
present at many other jjoints, is not indicated in the sections. 

Mr, Keith = finds that in Maryland, as a result of faulting, the igneous 
rocks occasionally overlie the sandstone. This is nowhere the caaa 
in the Monterey district. The facts that no inclusions of sandatone 
were found in the volcanic breccia, and that fragments of the aeidorap- 
tive occur in the quartzose conglomerate, suggest the auperflcial char 
acter of the latter. 

Further indications of the same nature will be discussed in consider- 
ing the comparative age of the sedimentary and igneous roclis. 



Sflc. Am., Vol. II, p. 163, pU. 4 Had 5. 

'C. D. Walcott. NotiiS on Cumbrian rocks 
XLIV, 18»2,p.481. 

>A. Keilh, Goologio Btructure of Bloe Eiaen 
p. 365. 

' Geiger and KBitb, loc. oil., p. 15S. pis. 4 sod S 


roof the Blue Ridge nB« 
of PcnnBylvaui» and Ma 
in MBrjUnU and VirRinia 


Har,™Forrj';Bnn.Oe«L 
j-land: Am-Jour. Sci..Vd. 
Ain.Goologiat,Vol.X.lg(B, 








E GETTVSBUHQ 



BA8COM.] ERUPTIVE ACID ROCKS. 23 

ACID ERUPTIVES. 

Areal distribution. — The flanks of the mountains, the hills, and the 
vfilleys are all forine<l of volcanic material ("Primal upper slate"), 
which, as Professor Rogers sugft ests, furnishes less resistance to erosion 
than the hard sandstone. Of lese rocks the acid lavas are much less 
abundant and tend to occupy the lower altitudes in this portion of the 
South Mountain. Farther north the reverse is true. 

With the exception of a large district northwest of Jacks Mountain, 
the acid rock 3 occur in insignificant areas, such as might be exposed 
by the ei .lOn of the surrounding greenstone. There are nineteen such 
limited and isolated occurrences. 

Character, — The acid volcanics are always readily distinguished from 
the basic rocks by their bright colors, which range from a brick-red, 
through pink, purple, blue, or blue gray, to a grayish-green, light-green, 
or buflf. Opaque white or red phenocrysts may be conspicuous or 
almost absent. Bands of spherulites, simulating bedding lines, are a 
prominent feature, while spherutaxites composed of spherulites ranging 
in size from a pea to a butternut are by no means uncommon. Eutax- 
ites with a marked yet delicate flow structure are abundant. 

Only faint suggestions of the lithophysal structure were found in the 
Monterey district. To the northwest, at Raccoon Creek, it was found 
in great perfection. 

An amygdaloidal character is less general than with the basic rocks. 
In a few localities, however, notably the Bigham copper mine region, 
it is very pronounced. Both flow breccias and tuff breccias occur. 
The rocks split readily into slabs, and in general are cleaved parallel 
to the structure planes of the sedimentary rocks. In some instances 
the porphyries have been sheared into spotted slates, still preserving 
the crystalline outlines of the feldspar phenocrysts, or into fissile seri- 
cite-schists. Such metamorphism is displayed in an exposure of a 
few feet, or even in a single hand specimen. 

There are not sufficient data for an estimate of the thickness of these 
rocks. In the x)orphyry area north of the Clermont House, Monterey 
Springs, several borings have been made for water. One well was 
bored to the depth of %^ feet in 1878, and carried 48 feet lower in 1886, 
when it was reported that a '^ soft rock" was reached. A few rods west 
of this other attempts were made, in 1892, to bore through the por- 
phyry. The greatest depth reached was 62 feet, when, the character of 
the rock not altering, the undertaking was abandoned because of the 
refractory nature of the porphyry. 

Previous descriptions. — These are the rocks characterized by Pro- 
fessor Rogers as ^^ a gray siliceous altered rock," " a reddish gray rock 
of the same series" (Primal upx)er slate), ^^ a highly metamorpbic Primal 
slate." They are the ^'orthofelsites" of Frazer, "showing no character 
of igneous action," and the "bedded petrosilex" of Hunt. Blandy 
covers them by the term " slates." 




24 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [b 

Economic value. — The susceptibility of these porphyries to 
polish and their suitability for ornamental purposes have been rema 
by many of those who have studied them. ^^ 

The earliest mention of this sort was made in 1822 by Dr. 13.^y^^^ 
who notes *' handsome porphyry, Nicholson's Gap, Blue Ridge, P< 
sylvan ia, crystals red and distinct." 

Speaking of Catoctin Mountain, the southern extremity of 
Mountain in Maryland, Tyson says:^ 

Its porphyries and amygdaloids are deserving of the attention that I propose 1m 
after to bestow npon them. Some of them will receive a beautiful polish, but 
hardness renders the process expensive. This can, however, be overcome by ap] 
priate machinery. 

Dr. Frazer^ has, as already quoted, described the porphyries 
*' suitable for an ornamental building stone." 

Dr. Hunt* has also called attention to the fact that "these pecal 
rocks, which make such a conspicuous figure in the South Mountain 
Pennsylvania south of the Susquehanna, are of interest economic 
from the fact that they are in other regions the repositories of 
iron ores, and also because they afford ornamental porphyries of 
beauty, similar to those wrought in Elfdalen, in Sweden.'' 

The Tenth Census '^ reports that "it [the South Mountain porph^ 
is well adapted to ornamental work, as it is rich in color, durable, 
susceptible of a good polish, and in many cases could be obtained 
abundant quantities. It has not yet been quarried for purposes 
construction." 

BASIC EBUPTIVES. 

Areal distribution, — The basic eruptives occupy, in this district, 
area fully twice as large as that covered by the acid eruptives constj* f' 
tuting the major part of the valleys, foothills, and mountain flanks. 

Character, — The rocks are massive, schistose, or slaty. They aie 
usually conspicuously amygdaloidal, and associated with these amygda- 
loids are banded fine-grained schists, which have been considered altered 
accumulations of volcanic ash. In a section exposed on the Crettysburg 
Railroad there are alteruating bands, from 2 to 3 feet wide, of a compact, 
fine-grained, epidotic rock which may also represent a basic volcanic 
ash. Bombs were found embedded in this epidotic rock. As with the 
acid rocks, there are accompanying basic breccias, though the latter are 
not abundant. The cementing material in every case was epidotic. 
An agglomerate formed of rounded fragments from an inch to 6 inches 
in diameter was also fouud. There was no opportunity for estimating 
the thickness of the accumulated basic flows. Wells have been bored 
in the basic rock to the depths of 55, 85, and 110 feet. 



> H. H. Uayden, Mineralogical notes : Am. Jour. Sci. and Arts, Vol. V. 1822, p. 255. 

* Tyson, First Annual Report, 1860, Appendix, p. 3. 
»Frazer, Vol. CC, p. 285. 

* Hunt, Proc. Am. Ass. Adv. Sci., 1876, p. 212. 

* Tenth Census Keport on the Building Stones of the United States, p. 168. 



BULLETIN NO. 136 PL.V. 



urg R Friends Creek 



Friends Creek 



*!^!?!^ 



V V 



V V 



I 

I 

« 



anch- 



[ONTEREY DISTRICT. 




Triassic Formation. 



Basic Volcanic s (Pre- Cambrian.) 



\ 



i: 






Cl; 




ficoM.] ORES. 25 

The basic rock, by reasou of its softer character, is more subject to 
deration under dynamic action than are the hard acid rocks. The 
feet of this is seen in the ahnost universal schistosity of the basic 
ok. This metamorphism is accompanied by a corresiJondingly greater 

emical alteration than is shown by the acid rocks. The alteration 
iisists largely in the abundant development of epidote and chlorite, 
:iich gives to the rock its uniform green color and its popular name of 
a^reen stone." 

JPrevious descriptions, — Dr. Hayden^ notes the remarkable develop- 
ent of epidote as follows: 

Most beautiful epidote, withgreeu and other shades of copper scattered in qnartz. 
le blue is prevalent. Abundant in Blue Ridge. * » * 

Quartz and epidote. with green carbonate and red oxid of copper and native 
»pper. Blue Ridge ; abundant. 

The greenstones are locally known as the "copper rock," because of 
ae copper ore which they carry. They are Eogers's "lower Primal 

ates," which "are highly indurated, and even decidedly crystalline, 
jntaining in some of their layers segregated specks, and even half- 
►rmed geodes [amygdules] of epidote and other minerals." "A highly 
Itered, greenish slate." "Dark-green slate, with its epidote and 
Lite, intrusive quartz." Tyson describes them as " slates." Dr. Frazer 
i.lls them "chlorite schists," and Dr. Hunt terms them epidotic or 
iloritic rocks. Blandy approaches most nearly the present idea of 
ieir nature in describing them as "amygdaloidal trap." 

ORE DEPOSITS OF THE MONTEREY DISTRICT. 

The workable deposits of ore in the South Mountain fall into two 
lasses : The limonite ores, which are deposited in Paleozoic sediment- 
ry rocks, and the copper ores, which are associated with pre-Paleozoic 
gneous rocks. 

Profitable limonite iron mines are being worked at various localities 
a the South Mountain, in the Cambrian sandstone, and along the con- 
acts between it and the Silurian limestone. They are fully discussed 
n the Final Report of the Second Geological Survey of Pennsylvania, 
^ol. I. 

One such ore bank occurs within the Monterey district, on its north- 
astern boundary, at the contact between Triassic limestone and the 
Janibrian sandstone, near Old Maria furnace. This bank is exhausted 
nd has not been worked for many years. 

An interesting occurrence of copper ore and native copper ^ is to be 
)und within the South Mountain chain, in a belt extending from a 
oint some 6 miles north of the State line, in a southwesterly direction. 



» Loc. cit., 1822, p. 255. 

2Frazer, Copper ores of Pennsylvania : Polytechnic Review, Vol. Ill, 1887, No. 16, p. 159; Au hypoth- 
iis of the stracture of the copper belt of the South Mountain, pp. 82-85. Henderson, The copper 
aposits ot the South Mountain : Trans. Am. Inst. Min., Eng., Vol XII, pp. 85-90. 



26 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [BUii.136. 

to and beyond the Stat<5 line into Mjiryland. This belt lies along the 
contact between the acid and basic erni)tivesJ 

The i)rincipal localities where shafts have been snnk within this belt 
are the Snively copper mines, the Kussel farm (the Bechtel shaft), the 
Reed copper pits, the Bigham coi)per mine, and the Headlig^ht mine. 
(See map, PI. III.) To the east of this belt, on the road between Fair- 
field and Fonntaindale, a fifth old copper shaft is located. 

The Snively copper mine, which is a mile north of the Monterey dis- 
trict, was not visited by the writer. 

Frazer reports that the copper is associated with quartz, epidotK 
rock, and azurite.^ 

At the Russel copper mine, which is located on the Eassel farm, at the 
fork of Copper linn, native copper occurs in quartz veins, associated 
with calcite, in a very siliceous epidotic rock, an epidosite. The mate- 
rial thrown out at the lower shaft was amygdaloidal greenstone; at the 
upper or Bechtel shaft, an acid slate, which indicates that the copper 
belt is along the contact between the two igneous rocks. 

The Keed copper pits lie on the north side of Toms Creek, at the 
forking of the highroad near Spring Kun. It has not been worked for 
forty years, and there is nothing visible to indicate the character of the 
copper occurrence. The dump pile shows scoriaceous greenstones 
with malachite stains. Dr. Snively reports that native copper occurs 
here in quartz veins. 

The excavation for copper ore on the Bigham property, which lies 
just northwest of the forking of Gladhills road, furnishes interesting 
material for the petrographer. It is here that acid amygdaloids have 
been exposed in great perfection. They are abundantly stained witii 
malachite, azurite, or cuprite. Metallic copper occurs in quartz veins 
traversing the amygdaloids, and in submacroscopic quantities in the 
amygdules. In the latter case the copper is frequently surrounded by 
zones of the oxide and the carbonate. An analysis of these rocks gives 
4 per cent of copper. The Headlight mine is tunneled beneath the turn- 
pike half a mile (eastward) below the Clermont House. Here the dump 
pile shows only greenstones, more or less stained with copper carbonates. 
This is also the case at the old copper shaft on the Fairfield road. 
Asbestos in quartz is quite generally found at these copper mines. 

Throughout this belt the copper is evidently of secondary origin, 
occurring in seams and amygdules, where it has been deposited from 
solution. This occurrence of metallic copper in the igneous rocks of 
South Mountain is interesting because of its similarity to the Lake 
Superior copper-ore deposits. Its associations are quite analogous. 
In the Lake Superior region it occurs in veins, quartzose and epidotic, 
in the open-textured amygdaloids (basic), and in interbedded con- 

----- ■■__- — - 

> Frazer says: "The ore belt lies in the orthofelsite which forms this portion of the chain." 
»P. Frazer, Copper ores of Pennsylvania: Polytechnic Review, VoL III, 1877, No. Ift, p. 170; alM 
Appendix, Vol. CCC, Second GeoL Surv. Pa., p. 310. 



BAscoM.] AGE OF THE ROCKS. 27 

gloraerates. In the South Mountain, in the absence of contemporaneouH 
interbedded elastics, the occurrence is limited to the igneous rocks. 

The replacement of the feldspar by chlorite and epidote, and tlieir 
replacement in turn by copper, characterizes the amygdaloids of the 
South Mountain also. 

The explanation of the precipitation of the copper is doubtless the 
same in both regions.^ 

A correlation of the South Mountain formations and the Keweena- 
wan copper bearing rocks was suggested by Mr. Blandy in the article 
before quoted. There is great petrographical similarity between the 
porphyrites and felsites and diabase porphyrites of the Keweenawan 
series and their equivalents in the South Mountain, but it seems to the 
writer unwise to j)arallelize on petrographical evidence the South 
Mountain igneous rocks and the Keweenawan of Lake Superior. All 
correlation upon such grounds, when applied to rocks widely separated, 
is well known to be untrustworthy. There has not as yet been found 
sufficient evidence to show to what horizon in the Algonkian series the 
igneous rocks of South Mountain should be referred. 

COMPARATIVE AGE OF SEDIMENTARY AND IGNEOUS ROCKS. 

CONTACTS DESCRIBED. 

That the Cambrian rocks do not underlie the "slates" and «*ortho- 
felsites," as stated in the reports of the Pennsylvania surveys, is quite 
plain, but whether the sediments are entirely subsequent to the 
igneous rocks or are in part, at least, contemporaneous, it is not so 
easy to decide. 

Contacts between the sedimentary and igneous rocks are finely 
exposed at two localities. About halfway through the tunnel on the 
Gettysburg Kailroad the basic igneous rocks and the Cambrian rocks 
are in contact. Both formations dip gently to the southeast (±20^). 
The sandstone has become an indurated quartzite. Close to the green- 
stone it has acquired a green color, due to the abundant development 
of chlorite. It has also become very schistose, and in fact might 
readily be confused with the greenstone itself. Granulation, undu- 
latory extinction, and the obliteration of all direct evidence of clastic 
origin, as revealed by the microscope, indicate the action of dynamic 
force on the quartzite. The greenstone is slaty and too decayed at the 
contact for microscopic study. 

Southwest of Old Maria Furnace, in an abandoned cut on the Tape- 
worm Kailroad, a great surface of red felsite'^ is partially exposed and 
partially overlain by the clastic formation. (PI. VT.) Here again 
there is a gentle dip of both rocks to the southeast (±450). The sand- 



^Pnmpelly, Geology of Michigan, Vol. I, Part III, p. 43. 

^Folsite is used here as a fiehl term for a compact, stony, nonporphyritic or inconspioaously por- 
phyritic volcanic rock. See pp. 37-38. 



28 VOLCANIC ROCKS OF SOUTH MOUNTAIN. Ie«i« 

Stone has become a vitreoua i|uartzito. The hard fe e d w not ahoir 
alteration. That tlio igneous itnd setlimeiitary rocks 'e been sub^ 
jectoi to tlie aaiue forcea of folding is bLowu by the un ity of thw 

cleavage dips at both these locabties. 

This conformity of structure plauee, the abseuoe of contact nielA' 
morpUisin, and the evidences of dynamic action indicate ptanea fil 
thrust rather tlian planes of original deiiositiou or of subtseqiunl 
igneous iutruHiou. 

These two contacts do not decisively prove the younger ageoflte 
overlying sediments, although they very strongly indicate it. If fte 
sandstone has been thrust over the lava from beneath, it would be BBC 
essary to suppose an enormous amount of erosiou to account for tlw 
entire absence of vohauic material above the sandstone. On the otiin 
hand, it is very easy to suppose that overlying sedimentB, on bflflg 
subjected to pressure, were thrust up over the igneous rocks frotttittit 
east. This explanation of the facts coincides with other evidence vSlk 
tive to the comparative age of the sedimentary mid volcanic rocka. 

(1) Nowhere in the South Mountaia has there beeu found adifcatf 
the volcanic material in tlie sedimentary rock. Were the lavas mm 
recent than the sediments, the former could hardly fail of tilling enMiki' 
in the latter, 

(2) Ko sedimentary beds have been fouTid intercalated with the lam 
flows. 

(3) There ia no evidence of the alteration of the sedinieutary root 
by igneous contact. 

(4) The igneous rocks, in their fluxion and amygdaloidal stnietimir 
and in their accompanying pyroclastica and ash deposits, besir eVW 
evidence of being subacrial lava flows. 

(5) The presence of igneous fragments in the basal conglomeratt ot 
the sedimentary formation shows that erosion of the igneous rocksin' 
taking place while the sediments were ai'cumulating along their edgB. 

While it is true that some erosion of the eruptives must have t^n 
place before and during the deposition of the C'ambriun sedinfientt- 
there is no evidence of extended erosion ; indeed, the character of sur- 
face flows, which the lavas retain so conspicuously, would incline oaelo 
suppose that no great load of material has been removed. Mr. Keitl^^ 
conception that the granit© which in Maryland is intruded into the 
"diabase" (greenstone) is younger than the volcanics uecessitntw 
an immense original thickness of the diabase flows and an extended 
subsequent erosion to expose tlie plutonic eruptive. In this respw' 
his ai'gumeut for the comparative age of the deep-seated and surfais* 
igneous rocks seems at fault'. Such unhmlted erosiou is unsupported 
by fleld observations in the South Mountain. 

■Gsolaeifliittuctiira of Bine Ridge in UiLrylund and Vireiaia; Am. Geulogiat, ToL X, ICM, p.W». 



BA8COM.] AGE OF THE ROCKS. 29 

COMPARATIVE AGE OF ACID AND BASIC ERUPTIVES. 

FIELD RELATIONS. 

The relative age of the acid and basic volcanics is an interesting 
question, a question to which it is not possible with our present knowl- 
edge to give an entirely satisfactory answer. 

On the one hand we find the acid rocks occupying, as a rule, the 
lower altitudes, and even overlain by the basic rock. Occurrences of 
the latter nature are located as follows : 

(1) At the junction of Gladhill's road and the Fountaindale turn- 
pike, in the northeast angle of the roads, a slight excavation exposes 
an outcrop of gray, slaty felsite overlain by greenstone. 

(2) Not far from the source of Minie Branch there is exposed by the 
cutting of the stream a gray felsite, overlain by greenstone. 

(3) On the road from Fountaindale to Fairfield, southeast of Jacks 
Mountain, a much-altered felsite is overlain by greenstone. 

(4) Lastly, on the Fountaindale turnpike, about half a mile above the 
Emmetsburg toUgate, the same superposition is exposed by the roadside. 

The only published statement of opinion as to the relative age of the 
volcanics has been made by Mr. Keith. ^ From his observations of the 
relative positions of the acid and basic rocks in Maryland, Mr. Keith 
expresses himself as confident that the " quartz-porphyry underlies the 
diabase." 

On the other hand, over against this somewhat meager proof must 
be considered a few facts of another sort. Professor Williams reports 
that a position of the acid and basic rocks the reverse of that which has 
been described in the Monterey district occurs north of that district. 
Throughout the northern portion of tlie range the acid eruptives, which 
abound almost to the exclusion of the basic eruptives, form the moun- 
tains, while the latter occupy the valleys. 

We must, moreover, bear in mind that the principles of stratigraphy 
employed in determining the age of sedimentary rocks may prove very 
misleading if applied to igneous rocks. Tlie younger lava may be 
zfound overlain by the materials of an earlier eruption where the 
former, in coming to the surface, breaks through the older formation. 
The latest eruption may fill the depressions. 

Where two different lavas occur on the same level, as is the case 
south of the Clermont House, the lava (in this case the acid lava) occur- 
ring as a narrow strip inclosed by the other might be an intrusive 
eruptive, and thus younger than the inclosing basic lava. 

PROBABLE EXPLANATION. 

With these principles in mind, with conflicting field evidence, and in 
the absence of genuine dikes of either acid or basic character, the data 
tor the determination of the comparative age of the acid and basic 



» hoc, cit., p. 367, 



30 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull.1M. 

volcanics are not suflicient. In all probability there were several 
sources of lava flow. 

The southern vents furnished the great areas of basic lava (green- 
stones) in Maryland and southern l^ennsylvania, while the northern 
vents poured out the enormous acid tiows. 

In the Monterey district the two lavas are mingled, and apparently 
the basic flow was preceded by the a<*id flow. 

Thus the facts observed by Professor W^illiams in the north, Mr. 
Keith in the south, and the writer in the Monterey district, maybe 
brought into accord. 

SUMMAUY. 

Three types of rocks are i)resent in the South Mountain : (1) Siliceoas 
sedimentary rocks of Lower Cambrian age, overlying with structural 
conformity and strati graphical unconformity igneous rocks. These 
igneous rocks are surface flows of (2) an acid and (3) a basic constita- 
tion. They are probably pre-Cambrian, and lithologically resemble the 
Keweenawan copper-bearing rocks of Lake Superior. 

There is not sufficient evidence to decide their comparative age, 
though in the Monterey district field observations, on the whole, indi- 
cate that the acid rocks are the older. 

The subaerial flows were subjected to a limited erosion before the 
sediments were deposited and while they were being deposited, so that 
the region must have possessed some elevation at that time. 

The intense dynamic action shown by the igneous rocks occurred 
after the deposition of the sediments. Since the sediments were laid 
down the whole region has been subjected to lateral pressure (at the 
time of the Appalachian uplift), whereby the igneous rocks were cleaved 
and sheared and the sedimentary formation was thrust up over them 
from the east — where it has been largely eroded, occurring now only 
sporadically — and the whole region was elevated. That erosion has 
removed a great thickness of material since tlds elevation is indicated 
by the cleavage dips of the igneous rocks. 



CHAPTER ITT. 

PETROGRAPHICAL DESCRIPTION OF THE CAMBRIAN ROCKS. 

MACEOSOOPICAL DESCRIPTION. 

The sedimentary formation exhibits two marked phases, the con- 
glomeratic and the quartzose. Its lowest member is conglomeratic. 

These conglomerates are frequently slaty, through the development 
of more or less sericite. They contain quartz pebbles from an inch to 
an inch and a half in length, and sometimes show included fragments 
of quartz-porphyry and of a green slate. 

From a conglomeratic character the sediments pass through a coarse 
sandstone into a compact quartzite exhibiting under the microscope the 
characteristics of a recrystallized clastic. 

The quartzose sandstone is exposed in great masses on the flanks and 
summit of Monterey Peak, on Haycock Summit, at the GladhiUs Switch 
on the Gettysburg Eailroad and at many points along the railroad 
as it skirts the east side of Jacks Mountain, and in massive pinnacles 
along the crest of that mountain. That the sandstone has been greatly 
fissured and broken is indicated by the conspicuous and frequent quartz 
veining, but metamorphism, save in a few cases, has been limited to 
the formation of a vitreous quartzite by the deposition of a siliceous 
cement. The original stratification planes are usually preserved, and 
cross bedding is frequently conspicuous. A secondary cleavage is also 
very marked, so much so as, in the absence of well-defined stratifica- 
tion planes, to be mistaken for the bedding. 

MICROSCOPICAL DESCRIPTION. 
QUARTZITE. 

Four specimens of typical quartzite were studied in the thin section. 
They were obtained from widely separated localities, and range in color 
from gray to a light green. Two of them illustrate the enlargement of 
quartz fragments and the genesis of a quartzite as perfectly as do any 
of the Lake Superior sandstones.^ (PI. XV, a.) 

* Quartz enlargements were first described by Torncbohm, subsequently by Sorby, Young, Irving 
and Van His^, Bonney, Phillips, and Iddings : 

A. E. Tornebohm, Ein Beitrag zur Frage der Quartzbildung: Geol. Foren. Stockb. 1876, Vol. Ill, 
p. 35. Ref. Neues Jahrbuch fiir Mineral., 1877, p. 210. 

H. Clifton Sorby: Anniversary Address, Quart. Jour. Gool. Soc. London, Vol. XXXVI, 1880, p. 62. 

A. A. Young: Am. Jour. Sci., Vol. XXn, July, 1881. 

R. D. Irving: Am. Jour. Sci., Vol. XXV, June, 1883. 

R. D. Irving and C. R. Van Hise: Bull. TJ. S. Geol. Survey No. 8, 1884. 

T. G. Bonney and J. A. Phillips: Quart. Jour. Geol. Soc. London, Vol. XXXIX, 1883, p. 19. 

R. D. Irving and C. R. Van Hise: The Penokee iron-bearing series of Michigan and Wisconsin: 
Tenth Ann. Rept. U. S. Geol. Survey (1888-89), p. 375, PI. XXVII. fig. 2, 

J. P. Iddings: Mon. U. S. GeoJ. Survey, Vol. XX, 1892, pp. 346-347, 

31 



32 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull, 136. 

The original grains are conspicuously outlined by a rim of iron oxide 
and surrounded by interlocking areas which are optically continuous 
with the inclosed grain. Rarely the grain itself is composed of two 
differently oriented areas, in whicb case the enlargement is also com- 
posed of two different areas oriented with the two portions of the grain. 

These same specimens show feldspar grains, usually with the *' grid- 
iron" structure of microcline. Occasionally a hornblende fragment 
is also present. The rock is evidently derived from granitic debris 
and might more accurately be called arkose. The fresh character of 
the fragments is in general noteworthy and suggests a source not far 
distant. 

The best examples of the vitreous quartzite, the extreme phase of 
metamorphism in the sediments, are to be found in the tunnel at and 
in the neighborhood of the contact with the greenstone, and on the 
unfinished Tapeworm Railroad southwest of the Old Maria Furnace at 
the similar contact of the sandstone and felsite. At both of these 
localities the rock shows macroscopically and microscopically the effect 
of dynamic action. Of two specimens from the first locality, which 
has already been described in sufficient detail on page 27, one is a pure 
quartzite with prominent cleavage, on the surface of which iron rust is 
deposited. The thin section shows a quartzite from which every ves- 
tige of the original waterworn grains has been obliterated. The rock 
consists of interlocking areas of quartz and some interstitial material. 
This is a fine aggregate of quartz grains, and may be a remnant of the 
granulation which must have occurred as a result of the movement of 
the grains against one another. 

A similar phenomenon in the sandstone of Sugar Loaf Mountain has 
been figured by Dr. Keyes.^ 

Another specimen from the same locality, but nearer the greenstone, 
shows a rock which, because of the shearing and chloritization that it 
has undergone, very closely resembles a chlorite schist. Its clastic 
character, however, becomes quite evident under microscopic examina- 
tion. This shows it to be a siliceous rock, greatly sheared and altered 
in character by the formation of sericite and chlorite. It is the only 
section of quartzite in which chlorite has been observed. Its close 
proximity to the greenstone, in which that mineral is so abundantly 
developed, suggests a probable source of the magnesia, iron, and alu- 
mina. Tourmaline, zircon, magnetite, and feldspar are also present in 
this rock. 

The quartzite at the felsite contact is similar, both in the hand speci- 
men and in the thin section, to the first of the two specimens just de- 
scribed. 1^0 trace of the original grain remains. Undulatory extinction 
is pronounced and recrystallization complete. Specimens obtained close 
to the acid igneous rock differ only in the presence of a large amount of 

> C. R. Keyea, A geological seotiopacrgss the Piedmont Plateau i^ Maryla}^4 « Bull. Gepl. 3oo. America, 
Vol. II, 1891, p. 321, "^'^" '-'' ''•'"" 



BAS0OII.1 SLATES. 33 

red iron oxide. The qnartz crystals are fn'(|iiently cracked, testifying 
to more intense dynamic action. The extreme induration at l>oth these 
contacts is confined to a selvage of the sandstone. 

SLATES. 

In a cut on the Gettysburg Railroad, southwest of the Old ^^aria Fur- 
nace, the sedimentary rock is locally of a difllerent character from that 
which has been described. It resembles the second si)ecimen described 
from the contact in the tunnel more closely than any other of the elas- 
tics. Shearing has been accompanied by the development of sericite 
and chlorite, producing a soft, green, slaty ro(*,k. The chloritic areas 
have usually clearly defined boundaries, and possibly represent former 
hornblende grains. Zircon is present. The major part of the rock con- 
sists of quartz grains, with undulatory extinction. 

Another local alteration of the clastic rock is to be found on the hill, 
tops southeast of Jacks Mountain. Uere it api)ears as a yellowish 
schistose rock. The thin section is characterized by the development of 
a large amount of sericite and granular e])idote. The sericitic scales 
show a parallel arrangement, which is the product of shearing. The 
color of the rock is due largely to an iron hydroxide. 

Among the sedimentary rocks within the Monterey district there 
rarely occurs, associated with the sandstone, an argillaceous slate. 
This is exposed just north of Jacks Mountain Station. It is the only 
representative in the Monterey district of the interbcdded slates occur- 
ring west of that district. It is silky, pearl-gray, crinkled, and cleaves 
readily into slabs. 

The thin section shows that the rock has been somewhat recrystal- 
lized, though its clastic origin is still apparent in the angular shape of 
its quartz fragments. Quartz and sericite are the principal constitu- 
ents of the rock. The other constituents are magnetite, hematite, and 
a gray cloudy aggregate which, under the highest i)ower, obscurely 
suggests leucoxene and granular epidote. 

OHEMIOAL ANALYSIS. 

The analysis of the quartzite given below shows proportions which 
would be possible only in a derivative rock where the exact chemical 
comx>ositi6n is a matter of accident and subject to no fixed laws. 

The silica percentage is disproportionately high for {*»iy but a clastic 
rock. The alkalies indicate the presence of a considerable amount of 
either orthoclase feldspar or sericite, probably the latter. The alumina 
and lime percentages denote the presence of epidote. 

BuU. 136 3 



34 



VOLCANIC ROCKS OF SOUTH MOUNTAIN. 



[bull. 136. 



Analysis of quartziie.^ 



SiOj (qiinrtz) . . . 
SiO.^(Coiubiiie(l) 
TiOj 

AiA:---------- 

FejOj 



Per o^nt. \ 

84.130 !■ 

7.870 !l 

.146 j, 

.205 •! 

4.210 I; 

1.800 ,1 





Per cent. 


CaO 


.040 

1. 11.0 

.160 

.960 


K./) 


Na/) 


Ignition , 


Total 


100.080 



SIMMAKY. 

The Cambrian sediments of the Monterey district may be classified 
as conglomerate, sandstone, ([uartzite, and slate. 

The first two of these phases represent original sedimentary deposits, 
of which the latter has been locally metamorphosed by induration into 
a quartzite. The slates are in some cases from deposits of a more 
finely divided material, and in other cases they are the product of shear- 
ing. 

There has been a limited and local introduction of new material, 
which shows itself in the i)roduction of chlorite and epidote. In every 
case the clastic character of the rock remains indubitable. 



' Specimen No. 1157, IJ miles S. of W. of tlie burned Hawmill on the Conococheague. This analysis 
was made by Dr. F. A. Genth, formerly of Pennsylvania University, for the Second Geolofi^cal Sur- 
vey of Pennsylvania, Vol. CCC, Second Geol. Surv. Pa. 



CHAPTER IV. 

PETROQRAPHICAL DESCRIPTION OF THE ACID ERUPTIVES. 

NOMENCLATURE. 

The question of the nomenclature of the acid volcanics has proved 
to be one of considerable interest and im])ortance. In the discussion 
of suitable terms for the description of these rocks, it will be necessary 
to anticipate, in some dejp^ee, the results of their microscopic study. 
A variety of names have been applied by petrographers to the acid 
type of the older volcanic rocks. Under the general group of quartz- 
porphyries, Rosenbusch classifies them as mierogranites, with a micro- 
granitic groundmass; granophijres, with a micropegmatic groundmass; 
felsophyresj with a microfelsitic base; and vitrophyres (including pitch- 
stones and pitchstoneporphyries), with a vitreous base. Fouque and 
L6vy employ microgranitite^ micropegmatite^ and porphyre petrosUwetix 
as corresponding terms. British j>etrographers have described these 
acid rocks under the terms horttsfouesy claystones and elaynione-por- 
phyrieSj felsites, quartz-felsitesj and feJsiteporyhyries, agreeing in this 
respect with the older German usage, when they have not followed 
Rosenbusch. In America both German and English usages have been 
followed, with more or less confusing results. In the nomenclature of 
the South Mountain rocks an effort has been made to avoid such con- 
fusion and to use such a term or terms as shall accurately characterize 
them and all similar rocks. 

Among the acid eruptives of South Mountain are typical represen. 
tations of Rosenbusch's quartz-porphyries. Closely associated with 
them and impossible of separation by any sharp line of demarcation are 
acid rocks ^vith every structural characteristic of modern lavas. 

Although possessing some characteristics in common with thefeho- 
phyresy these ancient prototypes of the rhyolite can not be included 
under that term, since they have a holocrystalline groundmass. Inso- 
much as many of the English feUitcH have been shown by Ilutley, 
Allport, Cole, and Bonuey to be devitrified obsidians and pitch- 
stones, and thus, like the American rocks, representatives of the glassy 
lavas of pre-Tertiary times, these South Mountain lavas might, with 
some propriety, be termed felsites. Strict consistency would then com- 
pel the replacement of the term quartz-porphyry by the more limited 
and less well-established term quartz-felsite, which was felt to be a dis- 
advantage. Moreover, the distinction between the rhyolitic lavas of 

35 



36 



VOLCANIC ROCKS OF SOUTH MOUNTAIN. 



[BULL. 136. 



South Mountain and the typical (juartz pophyries is not one of the 
absence or presence of pheiiocrysts. Thus the term felsite would be 
forced to cover not only nonp()ri)hyritic acid rocks, but also conspicu- 
ously porphyritic rocks. Finally, felsite, though useful as a field name, 
may well be objected to as an inaccurate petrograi)hical term. 

A brief hi»story of the word felsite and its synonyms in different coun- 
tries (petrosilex, eurite, and hiilletiinta,) will serve to illustrate the 
unfitness of it and these allied terms for exact petrographical usage. 

"Felsit" was introduced into (ierman petrographical nomenclature 
by Gerhard^ in 1814, who api)lied the term to a matrix which he claimed 
was common to all feldspar, claystone, and hornstone porphyries. This 
matrix was a compact homogeneous aggregate of feldspar and quartz, 
supposed by Gerhard to be compact feldspar. 

In 1858 Naumann^ adopted Gerhard's word for the feldspar-quartz 
groundmass, and called all porphyries with such a base felsite-porphy- 
ries. This term has since been applied by Tschermak to porphyries 
without quartz phenocrysts (orthofelsites, orthophyres). 

The more accurate German usage of the present day discards felsite 
save as a macroscopic term for an unresolvable porphyry base,^ while 
"felsitfels" is used if, in the absence of phenocrysts, the rock is com- 
posed of this base only.'* 

This same quartz-feldspar mosaic, confused with compact feldsimr, 
was distinguished as petrosilex by Wallerius^ as early as 1747, and 
subsequently by Dolomieu. This term was also early used by Bron- 
gniart. In 1819 the same groundmass was called eurite by Daubisson^ 
because of its fusibility. Michel- Levy^ uses the term petrosilex, but 
with a more limited meaning. He defines it as essentially synonymoiis 
with Eosenbusch's microfclsite, making '' porphyre petrosiliceux" equiY- 
alent to felsophyre. Petrosilex is, however, generally used by L6vy and 
other French petrographers in a more or less loose and vague way, to 
cover a crystalline or cryptocrystalline quartz-feldspar aggregate or a 
partially amorphous siliceous feldspathic magma. 

In Sweden, the fine-grained, apparently homogeneous acid rocks 
consisting essentially of quartz and feldspar and rarely i>orpliyritic, 
are called htilleflinta.^ They may be of aqueous or of igneous origiii. 

The early meaning of felsite in England was quite similar to that 
given it on the Continent. According to Pinkerton^ the term was 



'Boitriige, zur Gcschicbto dea WeiHsteins doH Felsit and auderer verwandten Arten: Abhandl- 
K. Akad. Wias. zu Berlin, 1814, 1815, pp. 18-26. 

'Lehrbuch dor Goognosie, 2d ed., Vol. I, 1858, p. 597. 

*RoH0ubu8ch, Mikro. Pbys., etc., 2d ed.. Vol. II, p. 373. 

4Rosonbu8ch, loc. cit., p. 354. 

*Sy8toinati<;a Mineraloginuii, 1847, French translation, 2 vols., Paris, 1853. 

« Traits do gi'ognosie, 1st ed., Vol. 1, 1819, p. 112. 

^structures et classification des roches 6ruptive8, p. 17. 

"Justus Roth, Allgemeiue und cheniische Geologie, Vol. II, p. 494. F. Zirkel, Lekrlmch d«r Tt^ 
rographie. Vol. I, p. 564. 

» T. Pinkorton, Petralogy, A Treatise on Rocks, Vol. I, 1811, p. 161. 



BA8COM.] • NOMENCLATURE. 37 

introduced in 1794 by Kirwan * for compact feldspar. Current usage as 
defined by TealP applies the term to '^ compact stony rocks, the min- 
eral composition of which can not be ascertained by examination with 
the naked eye or with the lens. ♦ ♦ ♦ These rocks are anhydrous 
(or nearly so), and except in this respect agree in composition with the 
acid glassy lavas." 

Dana ^ (as an exponent of American usage) defines ^^felsyte,'' arock, 
as ^< compact orthoclase with often some quartz intimately mixed, fine 
granular to flint-like in fracture, * ♦ ♦ both metamorphic and 
eruptive." The mineral he defines as follows: "Felsite is compact, 
uncleavable orthoclase, having the texture of jasi)er or flint, which it 
much resembles. It often contains some disseminated silica. It is 
distinguished from flint or jasper by its fusibility. * ♦ ♦ It is the 
base of much red i)orphyry.'' This is substantially the " felsitfels " and 
" felsit" of Eosenbusch. 

In short, "felsite" has been used to describe an acid base, unresolvable 
by the naked eye, and once supposed to be a single mineral. With the 
introduction of the microscope this macro" felsitic " base was resolved 
into the microgranitic, micropegmatitic, and microfelsitic groundmass, 
the point of ignorance having been shifted from the felsitic base, 
macroscopically unresolvable, to the microfelsitic base, which is micro- 
scopically unresolvable.'* 

On the Continent "felsite" has practically been replaced by these 
terms. British and American petrographers retain it as a useful field 
name for rocks formed of this macroscopically unresolvable base with- 
out phenocrysts or with inconspicuous phenocrysts. In this sense the 
word will be used, when used at all, in this paper. 

It is very generally recognized that structural features are not con- 
ditioned by the geological age of rocks, but are a function of the condi- 
tions of consolidation. That these conditions, while very varied and 
complex in any geological period, have not essentially altered since 
Paleozoic times, has been shown to be the case by some of the most 
able observers.^ 

With this recognition has come the growing conviction among petro- 
graphers that mere age should be eliminated as a factor in geolog- 
ical nomenclature. While this is true, it is felt on the other hand 
that the rock name should show some recognition of the alteration 

* Richard Kirwan, Elements of Mineralogy . 

* J. J. Harris Teall, British Petrography, p. 291. 

sj. D.Dana, Manual of Mineralogy and Lithology, 3d ed.. 1883, pp. 280, 442. Manual of Geology, 
3d ed., 1879, pp. 73, 77. 

^Rosenbusch does not assent to this interpretation of microfelsite, but regards it as a definite 
chemical compound allied to feldspar, just as felsite was once regarded. 

* Teall, Address of the Pres. of the Gool. Soc. of the British Ass. Adv. Sci., 1893. 

Judd, On the gabbros, dolerites, and basalts of Tertiary ago m Scotland and Ireland : Quart. Jour. 
Geol. Soc, London, Vol. XLII, 1886, pp. 49-97. 

Allport, Tertiary and Paleozoic trap rocks : Geol. Mag., London, 1873, p. 196. British Carboniferous 
Dolerites : Quart. Jour. (Jeol. Soc, London, Vol. XXX, 1874, pp. 529-567. 

Iddings and Hague, On the developmeDt of crystaiization in the igneous rocks of Washoe, Nov., 
with notes on the geology of the district : Bull. U. IS. Geol. iSurvoy No. 17, 188& 



38 VOtCANIC ROCKS OP SOUTH MOUNTAIN. * [bull. 136. 

which the rock has undergone subsequent to its solidification. If at 
the time of its solidification the rock presented the features of a 
rhyolite, as is believed to have been the case with much of the South 
Mountain acid lava, but since that time has become holocrystalline, 
both these facts — its original character and its present character — 
should be recognized in the name. It is believed that this result may 
be secured by the retention of such well-established names as rhyolite, 
obsidian, trachyte, etc., preceded by a prefix which shall have such a 
signification as will indicate the altered character of the rock. The 
prepositions meta, epi., and apo all indicate, as prefixes, some sort of an 
alteration. Their exact force has been thus defined by Professor Gil- 
dersleeve. Meta indicates change of any sort, the nature of the 
change not specified. This accords with the use of the prefix by Dana 
in such terms as ^'metadiorite" and ^^metadiabase.'' These terms have 
been recently revived to designate ^^ rocks now similar in mineralogical 
composition and structure to certain igneous rocks, but derived by 
metamorphism from something else."^ 

Upi signifies the x)roduction of one mineral out of and upon another. 
This prefix has not been much used. We find it in such terms as epi- 
diorite, epigenetic hornblende, and epistilbite. Apo may properly be 
used to indicate the derivation of one rock from another by some spe- 
cific alteration. 

If, therefore, we decide to employ this prefix to indicate the specific 
alteration known as devitrification (Entglasung) we may obtain, by com- 
pounding it with the names of the corresponding glassy rocks, a set of 
useful and thoroughly descriptive terms like aporhyolite, apoperlite, 
apobsidian, etc., as to the exact meaning of which there can be no doubt. 

In accordance with this usage, it is proposed in this paper to call all 
the acid volcanic rocks the structures of which prove them to have once 
beeu glassy, aporhyolites, while such as were originally holocrystalline, 
or whose original character is in doubt, will be termed quartz-porphyries. 

The writer feels that the introduction of a new name into petrograph- 
ical literature is to be deplored unless it can be shown that the name is 
formulated in accordance with certain well-defined principles. A good 
rock name should express composition, original structure, and as far as 
possible the process of alteration, if alteration has occurred. It is 
thought that aporhyolite and the suggested series of similarly formed 
terms meet these requirements. They are therefore adopted as prefer- 
able to any in present use.^ 



» Whitman Cross, "Ou a series of peculiar schists near Salida, Colo.: ' ProQ. Colo. Soi. Soc.Jan., 
1893, p. 6. 

•Since the above was written the terin eorhj'olite has been proposed by Dr. Nordenskjold to cover 
ancient acid volcanics identical with those of the South Mountain. In a review of Dr. Nordenskjold's 
able paper, "Ueber archieische Ergussgesteine aus Sm^land " (Bull. Geol. Instit. Upsala No. 2, vol.1, 
1893), the writer has discussed the disadvantages that attend the umc of any term or series of tenna 
which carry with them the idea of age. The reader is referred to this review, which appeared in the 
American Geologist for March, 1896, pp. 179-184, for a fuller statement of the writer's idea of the rrfa- 
tion existing between devitrification and age of volcanics. 



BAscoM.] QUARTZ-PORPHYRIES. 39 

It is a question whether it is always possible to distinguish between 
a primary and a secondary crystalline ^roundnuiss, and no attempt has 
been made to draw a sharp line between the quartz iK)rphyries and the 
aporhyolites. In the absence of sonu^ of the more marked structnres 
of a glass the presence of a secondary (uystsilhne structure has not 
been considered sufficient evidence for the completely swondary char- 
acter of the crystallization. It is very ]>robable that whik'. a large 
portion of the lava flow consolidated as a ghiss, much of thehiva solidi- 
tied at a sufficient depth to have secured a holocrystallin«' groundnmss. 

QUARTZ-PORPHYRIES. 

DISTRIBrTION. 

A deep-red porphyry outcrops along the turn])ike leading from the 
west to Monterey Station, and covers a small area north of the high- 
road.* One mile north of this, in the neighborhood of "Gum Spring," 
there are several limited areas of blue, purph», and brick-red porphyries. 
About 4 miles northeast, on the ^'Old Furnace Uoad,'' by tin* unfinished 
viaduct, is a large area of dark-blue porphyry, i)assing toward the 
southwest by insensible gradations into typical aporhyolites. 

These are the more important areas of i)ori)hyries. There are other 
porphyries so closely associated with the devi trifled glassy lava that 
they will not be separately located. 

MArHOSCOPICAL I>KS('RIPTI<)N. 

The beauty and variety of color of these porphyries have already 
been noted. The phenocrysts are not large (5 to 11 "'"' long), but are 
conspicuous against the dark or brilliantly colored matrix. The ortho- 
clase phenocrysts are opaque white, pink, or brick red. They usually 
possess a well-defined crystalline outline and show twinning in the hand 
specimen. The quartzes are colorless or a rich wine red. The fracture 
is conchoidal and there is a less-marked tendency to cleavage than in 
the aporhyolites. 

The porphyries show a reddish-brown color on the weathered surface, 
and on decomposing form a red earth. Tlu^ presence of manganese is 
abundantly exhibited in dendritic markings on the cleavage surfaces. 
This is especially true in the case of the slatcss developed from the por- 
phyries. (PI. XIV.) 

MICROSCOPKJAL DKSCiaPTlOX. 
PHENOCRYSTS. 

Feldspar. — The porphyritical feldsi)ars are more abundant than the 
quartz phenocrysts, and are remarkably fresh and unaltered. That 
they belong to the group of alkali feldspars, and that both inonoclinic 
and triclinic varieties are undoubtedly ])resent, are indicated by chemi- 
cal analysis (given on page (>1), by their specific gravity, and by their 



the geological raap of the Monterey diHtrict accoinpanj'ini; this bulletiu, PI. III. 



40 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

optical properties. Feldspar crystals were separated from specimens 
of the porphyry taken from different localities and their specific gravity 
was determined. The range was from 2.6 to 2.62. On acconnt of the 
presence m the heavier feldspars of minute inclusions of piedmontite, 
the lowest specific gravity was considered to represent the purest 
feldspar. 

Twinning is very common, according to either the Carlsbad or Mane- 
bacher (pericline) laws. PI. XV, h, shows a Manebacher penetration twin 
of feldspar. The section is nearly parallel to the clinopinacoid, as the 
following observations indicate: The axis of least elasticity makes an 
angle with the twinning line (rhombic section) of about 15 degrees on 
one side and 21 degrees on the other; the obtuse positive bisectrix 
emerges. There is a slight trace of a basal cleavage and a microper- 
thitic intergrowth parallel to c. The position of the plane of the optic 
axes, the distribution of the axes of elasticity, and the specific gravity 
all indicate that the crystal is the triclinic soda feldspar anorthoclase. 

The resemblance to this of the other feldspar phenocrysts makes it 
probable that this is the prevailing feldspar. Other sections show the 
albitic intergrowth (microperthite) developed in a pronounced maimer 
(PI. XVI, a). Every gradation of this structure is present, from the 
microperthitic to the cryptoperthitic. Sometimes the perthitic growth 
does not persist throughout the crystal, but is present in an incipient 
stage ^long its edge. 

The feldspars are often cracked and drawn apart in the direction of 
the schistosity of the rock, and the cracks cemented with sericite scales 
whose parallel arrangement conditions the schistosity (PI. XVI, 6). 
This is most striking in porphyry obtained 40 feet below the surface, 
which shows an abrupt passage into a sericite-schist. The phenocrysts 
are crushed and pulled apart. This action had been accompanied by 
an abundant development of sericite. Some of the phenocrysts jws- 
sess in the hand specimen a red color, which is due to a fine admixture 
of red iron oxide. These phenocrysts frequently prove, from » test of 
their optical properties, to be orthoclase. 

The alteration of the feldspars to epidote may indicate the pres- 
ence of lime in the feldspar. Undoubtedly, however, much of the lime 
is of secondary origin. This alteration of feldspar to epidote seems 
to be a direct one. There is no intermediate kaolin stage, such as 
observed byEutley.^ Accompanying the epidote is granular quartz. 
These two alteration products usually occupy the center of the crystal 
and are surrounded by a rim of unaltered feldspar. 

Brilliantly colored piedmontite frequently fills cavities in the feldspar 
crystals. This mineral is often surrounded by a rim of epidote. 

Quartz. — ^The bipyramidal or rounded quartz phenocrysts are remark- 
able only for their undulatory extinction and the cracks by which strain 



■Kutley, On some perlitic felsites and on the possible origin of some epidosites: Quart. Joor. 
G60l. Soc. London, Vol. XLIV, 1888, pji. 741-742. 



BASCOM.1 QUARTZ-PORPHYRIES. 41 

lias been relieved. They are fresh and show chanu^teristic embayineiits 
and inclusions. Like the orthoclase, they are sometimes reddened hy 
inclusions of hematite, or are more rarely given a rich wine-red color 
by inclusions of piedmontite. 

OROUNDMASfl. 

The holocrystalline groundmass of the ])orphyrie8 consists essentially 
of quartz and feldspar. These minerals form either a finely micro- 
granitic mosaic or a structure which becomes of considerable interest 
in connection with the question of the original (;harai*.ter of the crys- 
tallization. 

In the first case there is no reason per so for supimsing the crystal- 
hzation to be other than primary. It is only as the crystallization 
is associated with secondary structures, or structures testifying to a 
primitive glassy condition, that the presumption favors its secondary 
character. When the crystallization is microgranitic and presumably 
primary^ the rock is a genuine quartz-jwrphyry. 

The structure alluded to, which occasionally repla(;es the quartz-feld- 
spar mosaic in the porphyries, is the micropoikilitic,' where all the <[uartz 
of the groundmass has crystallized in irregular areas inclosing the feld- 
spar as lath-shaped microliths. These feldspar microliths are oriented 
quite independently of one another. This structure characterizes the 
aporhyoliteSy and the question of its primary or secondary character 
will be fully discussed in connection with these rocks. 

The alteration of the groundmass to sericite and the formation of a 
sericite schist will be discussed in connection with the iu^id slates. 

ACCE880BY rC)N8TlTUENT8. 

Piedmontite (manganese epidote) is the most remarkable of the acces- 
sory constituents. It is not only disseminated m microscopic quan- 
tities through the deep-red porphyry, but it also occurs in macroscopic 
masses, as a radiating aggregate, filling veins and cavities in the red 
felsite or replacing spherulitic crystallization. 

Two localities in the Monterey district furnish a great abundance of 
the piedmontite: the southwest flank of Pine Mountain, 1 mile north- 
east of Monterey station, and the hillside south of the Clermont House, 
between the turnpike and Minie Branch. Microscopic sections of the 
aggregate show brilliantly colored needles of piedmontite intergrown 
with clear quartz. (PI. XXV, b.) This intergrowth with quartz is also 
marked in the hand specimen. The radiating needles have the appear- 
ance of being broken, stretched apart, and the spaces filled with sec- 
ondary quartz. The only other locality where i)iedmontite was found 
in macroscopic quantities is in the Buchanan Valley, 2 miles north of 



*G. H. Williams, On tho use of the terms poikilitic and luicroiioikilitic in petrography: Jour, of 
GeoL, Vol. I (No. 2, Feb.-March, 1893), pp. 176-179. 



42 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull 136. 

the Charabersburg turnpike, and one-ei{;;lith mile west of Musser's store. 
Piedmontite occurs here in cavities associated with scheeliteJ 

p]pi(lote is also abundantly present in the porphyries, passing by 
gradations of rose colored epidote to the deep carmine of the pied 
montite. Both of these silicates are undoubtedly secondary products. 

Innumerable black globulitesof manganese oxide and black and red 
iron oxide crowd the groundmass. In their arrangement they show 
fluxion structuie, or form meshes inclosing the quartz areas which 
constitute a part of the micropoikilitic structure. This arrangement is 
sometimes so marked as to be conspicuous in the hand specimen, giving 
the rock a mottled appearance. Crystals of zircon are not infrequently 
present. 

A study of the (piartz- porphyries of the South Mountain shows us 
that they are present there in characteristic development, and offer no 
marked variations from the normal type described from other localities. 
For this reason a brief space only has been devoted to them. 

* APORHYOLITES. 

DISTKIBITTION. 

There remains to be described a large and important iK)rtion of tlie 
acid rocks of the Monterey district. These will be termed aporhj'olites, 
for reasons already suggested and to be more fully discussed in the 
sequel. 

The localities colored red upon the geological map of the Monterey 
district (PI. Ill) are aporhyolitic areas associated, in the cases already 
mentioned, with quartz-porphyries. The largest area begins just south 
of the Bigham copper mine, and, widening toward the north, extends 
far beyond the Monterey district. This area furnishes most of the 
spherulitic structures. A small detached area south of the Clermont 
House furnishes slaty and spherulitic aporhyolites. These aporhyolites 
and those of the four detached areas near the Maryland State line, 
separated from eacli other by a distance, of a mile to a mile and a half, 
show some marked i)oints of resemblance, to which attention will be 
called later. 

The areas covering the foothills southeast of Jacks Mountain show 
an altered aporhyolite. There are several small areas of red aporhyolite 
on the Gladhill road leading to the Bigham property. At the forking 
of this road with the Fountaindale turnpike occurs the ajwrhyolite slate 
mentioned on page 29. 

MACROSCOPIC^AI. DESCRIPTION. 

The aporhyolites have about the same range in color as the porphy- 
ries, varying from light bluish-gray, or buff, to many shades of red 
and purj)le. While UvSually compact and always fine-grained, they are 

1 G. II. Williams Piedmontite and scheclito from the ancient rhyolite of Soath Monntiiin, Pennsyl* 
vania. Am. Jour. Sci., Vol. XLVI, 3d series, July, 1893, pp. 50-57. 




FLOW STRUCTURE IN APORHrOLITE. NIONTEREV DISTRICT. 



BA8COM.] APORHYOLITES. 43 

also sometimes very vesicular. In the latter case the amygdulea of 
dark-green epidote and clear quartz, elongated by Huxion and con- 
spicuous against a pale-pink background, render the rock strikingly 
handsome. Phenocrysts are usually ))resent, but generally incon- 
spicuous. 

Delicate lines of flow structure, which arc brought out in great 
detail by weathering or are painted in rich colors on the material 
washed by the mountain brooks, are another marked feature of the 
aporhyolites. These flow lines are frequently very sinuous, showing con- 
tortion and crumpling of the lava. (Pis. VII and VIII.) The flowage 
is often emphasized by the mingling of two contrasting magmas, form- 
ing taxites of either a eutaxitic or an ataxitic character. A fuller 
descrlj)tion of these taxites is given on page 57. 

Another characteristic which these aporhyolites possess in common 
with their modern analogues is the spherulitic structure. Spherulites 
are rarely, if ever, altogether absent, and in some localities they are 
crowded so close together as to constitute the major part of the rock 
mass. They range in size from microscopic dimensions to those of a 
butternut. Where there is no regularity of arrangement and they are 
brought out in rehef by weathering, the rock has a superficial resem- 
blance to a conglomerate composed of rounded pebbles of remarkably 
uniform size (BB shot) and shape. (PI. IX.) The rich grays and blues 
and purples of the spherulites and matrix render this a conspicuous 
rock. Other specimens from the same locality (the south flank of the 
mountain northeast of the junction of Copper Run and Toms Creek) 
show spherulites elongated by flow. They are thus drawn out into 
solid cylinders with a diameter of 1'""' and a length of some 2^"'. 

Specimens of aporhyolites composed of spherulites about the size 
and shape of almonds (18"*'" by 9"'"') were also found. The rock had 
been greatly sheared and the spherulites flattened. 

Spherulites become a still more striking feature of the aporhyolites 
when arranged in layers which traverse the face of the rock in long, 
parallel, dotted bands. This arrangement has been described by 
Iddings in the obsidian of the Yellowstone National Park.^ 

While sometimes these bands are 4'""^ wide, at a nearly uniform dis- 
tance apart, and of an indefinite length, in other cases they are very 
narrow, dwindling into mere lines and dying out, to be succeeded by 
other lenticular bauds. (PI. X.) The planes of these spherulites have 
become planes of weakness and solution. The rock cleaves readily 
parallel to them and shows a coating of secondary silica on the cleav- 
age surfaces. This deposition of silica causes these planes to become 
the hardest part of the rock, and hence they often stand out as 
parallel ridges on the weathered surfaces. 

At the locality mentioned on pages 41-42, piedmontite, in radiating 
needles, associated rarely with scheelite, has, together with quartz, been 



lObeidian Cliff; Seventh Ann. Kept. U. S. Geol. Survey, 1888, p. 276, PI. XVIII. 



44 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [buix.136. 

deposited along these planes of former spherulitic crystallization. The 
silica occupies the center of the deposit and gives rise to ridged surfaces. 
These layers of spherulites do not always lie in a single x)lane. Cross 
sections of the layers show irregnlar and minute sinuosities, which were 
doubtless caused by movement in the hiva during consolidation. 

The bands consist always, •when not composed of piedmontite, of a 
central dark line (impurities) with a lighter band (opaque quartz) on 
either side. These in turn are bordered by dark lines (iron oxide) 
(PI. X). This parallel banding, conspicuous even at a considerable dis- 
tance, though recognizable as a true igneous structure by one familiar 
with the acid lavas of the Yellowstone National Park or of tbe Lipari 
Islands, counterfeits a sedimentary structure so closely that it is not 
surprising that the rocks should be described as "bedded orthofel- 
sites." 

Still other specimens show spherulites with a tendency to an arrange- 
ment in rows and chains which are in turn collected in bands some 
2 inches in width traversing the rock and alternating with bands 
approximately free from spherulites. This arrangement differs from the 
layer spherulites in the fact that the outline of individual spherulites is 
quite distinct and the grouping irregular. 

Some fine examples of the lithophysal structure were found in the 
Raccoon Creek region (PI. XI). Nothing that could be definitely recog- 
nized as lithophysse was observed in the Monterey district. 

In the areas southeast of Jacks Mountain the aporhyolites have 
been more or less silicifled or epidotized, or both, and are sometimes 
very difficult to distinguish, in the hand specimen, from the neighbor- 
ing sandstone. A good example of this rock is seen in the exposures 
on the turnpike about one-fourth of a mile above the Emmitsburg toll- 
gate, where silicification has produced a close resemblance to a quartz- 
ite. The fresh surface is white. Feldspar phenocrysts are sparsely 
distributed and scarcely discernable. Quartz blebs are more numer- 
ous. Pyrite is finely disseminated, and the exposed surfaces of the 
rock are tinged with yellow and red iron oxide. The rock cleaves in 
two directions oblique to each other. In this respect it resembles 
many of the felsites. 

The microscopic evidence of igneous character is supported by the 
field evidence, which shows a continuity between this altered rock and 
that which would be at once recognized as a felsite. 

MICROSCOPICAL DESCRIPTION. 

PHENOCRYSTS. 

Feldspar and quartz, — The feldspar phenocrysts are very like the 
feldspars of the quartz-porphyries, and little needs to be added to the 
description of them already given. Tbey are fresh, contain inclusions 
of a once glassy magma, show perthitic iiitergrowth, and are twinned in 
accordance with the Carlsbad (albite) and Manebacher (peridine) laws. 




I, RACCOON CREEK 



BAscoM ) AP0RHY0L1TE8. 45 

In the former case the twinning is sometimes repeated, and fiirnishes 
mother indication of the triclinic charac^ter of the feldspar. 

The feldspars are frequently bent or broken, causing the twinning 
striations to show curvature and faulting. Occasionally a phenocryst 
bas been completely broken, the fragments pulled apart, and the space 
filled with a qaartz-albite mosaic of much coarser grain than the ground- 
mass, and evidently of subsequent formation. (PI. XVII, a.) A com- 
plete replacement of the feldspar crystal by a quartz mosaic sometimes 
3ccurs.* 

The quartz phenocrysts have rounded outlines and characteristic 
embayments and inclusions; undulatory extinction is almost universal; 
frequently the quartzes are cracked, and sometimes completely granu- 
lated. Both the porphyritical constituents of the aporhyolites give 
rise, in connection with curving flow lines, to the " augen '' structure 
which has often been described.^ 

These flow lines leave clear, triangular spaces on one side of the 
phenocrysts, such as Futterer describes.^ 

Other porphyritic constituents. — The only ferromagnesian silicate 
mmistakably present in the aporhyolites is biotite. This was found 
n sections representing three dift'erent areas of acid volcanics which lie 
iiong the southern limit of the Monterey district. The first area is a 
small one at the head of Minie Branch. The second area is larger and 
3aps the mountain southeast of Eaven Rock Mountain. The third 
irea is directly to the east of this, on Friends Creek. The lavas of 
these three localities show a marked similarity, both in the hand speci- 
mens and in the thin sections. They are compact, dirty-gray rocks, 
weathering a yellowish-gray. Feldspar phenocrysts are somewhat 
sparsely and inconspicuously present. On a fresh surface biotite can 
be detected with the naked eye. Magnetite is present, and in one of 
two specimens from the mountain locality pyrite is abundant in minute 
3rystals. 

All of the thin sections of these rocks are distinguished by finely 
striated feldspars. The carlsbad and pericliiie twinning are sometimes 
3oth present in a single crystal. Quartz phenocrysts are not numerous. 

The presence of titanic iron oxide in abundance is attested by the 
brmation of leucoxene. The groundmass is holocrystalline and finely 
nicrogranitic, with a tendency toward the micropoikilitic structure, 
rhis tendency is fully developed in one of the sections. The hand speci- 
nen, from which this section was cut, shows a rock more completely 
Jilicified than the other specimens indicate, and also contains pyrite, 
ivhich is indicative of secondary crystallization. 



> Similar replacements have been described by Dr. Milch, Beitrage zur Kenninis des Verrucano, 
1892, p. 126, 

»J. Lehmann, Untersuchnngen iiher die Entstehung deraltkrystalliniacheu Schiefergesteine, Bonn, 
1884. G. H. Williams, Ball IT. S. Geol. Survey No. 62, pp. 85, 118, 207, PI. XV, fig. 1. 

3Karl Futterer, Der " Ganggranit " von Grossaobseu und der Quartz-porphyr von Thai Uu Thiiringer 
VVald, Iiiaug. disser., 1890, p. 32. 



46 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

There is some epidotization of the groundmass, resulting in the pro- 
duction of finely granular epidote concentrated at certain centers of 
development. 

The mica by which these lavas are characterized has a fibrous char- 
acter, aud is the andesitic type described by liosenbusch.' It is in 
idiomorphic clusters of reddish-yellow fibers. The absorption is cliar 
acteristically strong and the pleochroism marked. Parallel to the 
cleavage the fibers show a deep reddish-brown color, and at right 
angles a light yellowish-red or straw-yellow color. The marked resem 
blance of the lava from these localities suggests a continuity in the 
lava flow — a continuity probably maintained underneath the greenstone. 

If this is the case these areas are exposed by erosion of the overly- 
ing basic lava, if it ever completely overlay the aporhyolite. That th€ 
continuity was maintained above the greenstone is a scarcely tenable 
hypothesis, because of the extended erosion which such a superposition 
would necessitate. 

If any other ferromagnesian constituents were present in the apor- 
hyolites — and there are indications that there were— they have been 
totally decomposed and removed. 

Harker ^ found it to be true of the preCambrian acid volcanics of 
Wales, that clusters of biotite flakes were preserved in a compara- 
tively fresh condition, while only slight trace of augite or hornblende 
remained. 

In the South Mountain aporhyolites these minerals must have been 
rare originally, and are now completely replaced by epidote. Occa- 
sionally the outline of a perfect crystal section parallel to 010 is pre- 
served, while the substance of the crystal is entirely replaced by epi- 
dote individuals. In the absence of basal sections there is no clue to 
the specific character of the original crystal. 

GROUNDMASS. 

The groundmass is always a quartz-feldspar aggregate, of varying 
structure and grain (about -^ of a millimeter in diameter), more often 
finely than coarvsely niicrogranitic. The structures of the groundmass 
are of the highest interest and importance in their disclosure of the 
original cbjiracter of the rocks which possess them. 

The fluidal, micropoikilitic, spherulitic, axiolitic, lithophysal, rhyo- 
litic, micropegmatitic, perlitic, taxitic, amygdaloidal, and trichitic struc- 
tures are characteristically developed and merit detailed description. 

Fluidal structure. — The fluidal structure, which is so familiar to all 
students of rhyolitic lavas, is a conspicuous feature of the aporhyolites, 
both macroscopically, as has been described, and microscopically. 
Globulites of magnetite and hematite and indefinable opaque crystal- 
lites follow sinuous lines of flow, twisting around the phenocrysts and 

'Rosenbusch, Massige Gesteine, 2d ed., VoL II, p. 658. 
'Bala Volcanic Seriea of Kocks, p. 18. 



■f^]^-:- 






"Ml 






^;^?rr^'*v;>?.^ 




IC APORHVOLITE, MOMTEREY DISTRICT. 



BAscoM] AP0RHY0LITE8. 47 

imparting to them the appearance of eyes, previously mentioned. PI. 
XVIII, a, is the reproduction of a photomicrograph of a section show- 
ing this structure. 

Micropoikilitic structure, — The micropoikilitic structure has been 
defined in connection with the quartz-porphyries, where it was not 
infrequently a significant feature of the groundmass. It is still more 
characteristic of the aporhyolites, and is occasionally present in the 
basic eniptives. 

These irregular quartz patches, inclosing niicrolites of lath-shaped 
feldspars or other minerals of independent orientation, give a pro- 
nounced mottled or "patchy" appearance to the groundmass, an 
appearance which has been noted in volcanics of all ages. It has been 
observed and variously described, usually without being named, in 
quartz-porphyries, felsites, x>orphyrites, rhyolites, and peridotites, by 
numerous writers — Irving,' Williams,^ Haworth,*^ Cross,^ Iddings,^ 
Diller,^ Lindgren,'' Teall,^ Harker,^ Brogger,'^ and Nordenskjold.' ' Fel- 
site from the Archean rocks of Georgia shows the same structure.'^ This 
structure is likewise present in the felsites from the neighborhood of 
Boston," as is the case also with the quartz-porphyries and felsites of 
Marblehead Neck, Massachusetts. 

An acid lava of the Keweeuawan series from Minnesota, recently 
examined by the writer, shows with a high power and polarized light 
the micropoikilitic structure in perfection.'^ 

While the term micropoikilitic is not restricted to a quartz -feldspar 



I R. D. IrviDg, Copper beariog rocks of Lake Superior: Mon. U. S. Geol. Survey, Vol. V, 1883, pp. 
99-100, PI. XIII, figs. 13 and 14. 

« G H. Williams, Neues Jabrbacb fur Min. u Pet., Supp. Vol. II, 1882, p. 607, PI. XII, fig. 3. The 
peridotites of the Cortlandt series: Am. Jour. Sci., Vol. XXX, p. 30; Vol. XXXIII, p. 139. 

*E. Haworth, A contribution to the Archaean geology of MiHSOuri: Am. Geologist, 1888, Vol. I, j). 
368, PI. I, figs. 1 and 2; also. Crystalline rocks of Missouri: Ann. Kept. Mo. Geol. Survey, Vol. VIII, 
1894, p. 195, PL XXII. 

* Whitman Cross, On some eruptive rocks from Custer County, Colo. : Proc. Colo. Sci. Soc, Vol. II, 
1888, pp. 232, 242. On a series of peculiar schists near Salida, Colo. : Proc. Colo. Sci. Soc, Jan., 1893, p. 8. 

8 J. P. Iddingg, The eruptive rocks of Electric Peak and Sepulchre Mountain, Yellowstone National 
Park: Twelfth Ann. Kept. U. S. Geol. Survey, pp. 589, 646. 

*J. S. Diller, Mica-peridotite from Kentucky: Am. Jour. Sci., 3d series. Vol. XLIV, Oct., 1892, p. 
287. 

' Waldemar Lindgren, On sodalite syenite and other rocks from Montana: Am. Jour. Sci., 3d series, 
Tol. XLV, Apr , 1893, p. 287. 

8 J. J. Harris Teall, British Petrography, 1888, p. 337. 

9 Alfred Barker, Bala Volcanic Series of Kocks, pp. 23, 53, 54. 

'* W. C. Brogger, Der Mineralien der Syenitpegmatitgange der sildnorwegischen Augit und Nephe- 
linsyenit: Groth's Zeitschrift fiir Krystallographie, Vol. XVI, p. 40. 

^'Otto Nordenskjold, ZurKenntniss der sogen. Ilalletiinta dew nordostlichen Sm&lands: Bull. Geol. 
Xnst. Upsala, Vol. I, No. 1, 1893. 

'*A section of this felsite was loaned by Prof. L. V. Pirsson. It is of especial interest in its 
S^^at similarity to the South Mountain felsite, thereby showing the southward persistence of this 
x*ock type. 

'^J. S. Diller, Felsites and aasoc. rocks north of Bost<m: Proc. Boston Soc. Nat. Hist., Vol. XX 
Jan. 21, 1880, pp. 355-368; also Bull. Mus. Corap. Zool. Harvard Coll., whole series, Vol. VII; geol., 
•Series, Vol. I. Thin sections of these felsites were kindly loaned by Mr. Diller to the Johns Hopkins 
l«^boratory. They have many microscopic features in common with the South Mountain rocks, and 
1 i ke them were first supposed to be of sedimentary origin. 

'*The writer's attention was called to the presence of this structure in the Minnesota acid volcanics 
^^ Dr. U. S. Grant, of the Minnesota State Geological Survey. 



48 VOLCANIC RO(!K8 OP SOUTH MOUNTAIN. [BOTx.l3d 

intcMgrowtli, in most of the occurn»iices descnbed these have been the 
(toniponent niinoials. In the South Moiiutaiii rocks the feldspathic 
inatorial is usually so abundant as not to admit of the determination 
of the mineral chanu^ter of the host, lu such cases a clue to the 
nature of the cenuMitin^ material is found in its optical continuity with 
the porphyi'itic quartz. These phenocrysts are severally included 
within a single micr()]K>ikilitic area, with which they are always simi 
larly oric^nted (PI. XVI I, h). The cementing material acts as a sort of 
secondary enlargement of the. quartz phenocrysts. The feldspar phen 
ocrysts, on the otlu^T hand, hav(». no effect ui>on the orientation of tbe 
cement. 

In the basic rocks, which are coarser-grained, the character of the 
host can be diieetly tested and proved to be quartz. The mottled 
appearance, previously alluded to, is usually emphasized by tiie 
arrangeniei)t of globulites, longulites, and trichites of iron oxide. This 
is such as both to accord with the How structure and to outline the 
quartz areas, which in these cases have a somewhat oval Bhape. 

When shearing has led to the production of sericite, this mineral is 
formed around the micropoikilitic areas, rarely traversing a ringle 
area, when it seems to be cementing material filling a crack (PL XVII, 
h). These areas are persistent, and slowly disappear in the develop- 
ment of a slate. 

While in some cases this structure is undoubtedly of primary char- 
acter, as Professor Tddings considers it to be in many porphyrites, in a 
large class of rocks its secondary origin seems equally certain. Irving, 
w^ho gives one of the best as well Jis the earliest (1881) descriptions of 
this structure, considered it of a secondary character. His statements 
as to its nature and origin are so applicable to the South Mountain 
aporhyolites that they are quoted here in full:^ 

Although wholly absent from some sections, a very highly characteristic feature of 
the sections of many of these rocks, and more particularly of the felsites withoat 
porphyritic quartz, is a network quartz which can only be regarded as of secondary 
origin. I find no mention of such a feature in any of the descriptions of the felsites 
of other regions which I have examined. Only occasionally * * * is this net- 
work quartz coarse enough to be n^adily seen with a low power in the ordinary 
light. Usually both a high power and the use of the polarized light are required 
for its detection, when it appears in its most characteristic development as a deli- 
cate aboresccnt tracery or frost-work saturating the ground mass in all directions. 

In the polarized light all of the quartz network within each of these namberless 
irregularly round areas, whoso existence would not be suspected in the ordinary 
light, is found to be similarly oriented. 

From these more prouounced developments the secondary quartz is found through 
man^*^ degrees of lessening amount and less plainly marked character until it disap- 
pears altogether. It is plainly of the same nature as the secondary quartz of the 
already described orthoclase-gabbro, diabase-porphyrite, and quartzless porphyry, 
and of the augite-syenite described below. It never, however, reaches in the rock* 
now under description the coarseness nor presents the graphic forms with which it 
appears in the augite-syenites, its characteristic development here bemg the deli' 



1 Loc. cit., pp. 99-100. 






BAscoM.] APORHYOLITES. 49 

cate arborescent clusters above mentioned. Whether tb is secondary qnat* tz may ever 
be rather a result of devitrification than a truly secondary or alteration product, I 
have no means of deciding, though it is certainly the latter often, and I should sup- 
pose always. It surely can have no connection with the original solidification of the 
rock. 

Observations made on the South Mountain aporhyolites lead to 
essentially the same conclusions as those reached by Irving. As the 
nature of the structure is of both interest and importance in its bear- 
ing upon the question of the primary or secondary character of the 
groundmass, attention will be called to some of the observations which 
prove suggestive. 

It has been stated that a few sections of the basic lavas of South 
Mountain exhibited this structure. In these sections the nature of the 
structure could be more readily detected. (PI. XIX, a and b.) 

The outline of the lath-shaped feldspars, forming an original ophitic 
structure, is completely preserved, though none of the original constit- 
uents of the rock remain, unless some of the titaniferous iron oxide is 
original. 

At present the rock consists entirely of quartz, epidote, magnetite 
(or ilmenite), and leucoxene. It is amygdaloidal, and the vesicles are 
filled with secondary quartz. Quartz is also a cement for the minerals 
of the groundmass, and forms irregular interlocking areas, which are 
quite similar to the micropoikilitic areas of the finer-grained acid rocks, 
and produce in polarized light the familiar patchy eflFect. Fine cracks 
traversing the sections are still preserved in outline by the ferrite, but 
are prior to the quartz areas, which have obliterated all trace of cement- 
ing malerial. The epidote, which replaces crystals of some former ferro- 
magnesian constituent, is often pierced by these cracks, which become 
invisible in the quartz areas save for the outlinmg ferrrte. There can 
be no question as to the secondary character of the micropoikilitic 
structure in these cases. 

Some structures described by Zirkel * in the rhyolites of the Washoe 
district are suggestive in this connection. He notes (slide 350, fig. 1, 
PI. VI) perlitic parting in certain rhyolites (southeast from Wadsworth), 
where the cracks, semicircular and oval, traverse a glassy groundmass 
and are bordered by a narrow zone of microfelsite, "giving [to the sec- 
tion] the appearance of a network." The same general effect is produced 
in other rhyolites of the district (sees. 351, 352, figs. 2, 3, PL VI) by 
faint granular lines "which, by their fluidal running, form a net with 
a multitude of meshes of oval shape.'' These lines seemed to be the 
vestiges of perlitic cracks, though they could not be certainly deter- 
mined as such. A widespread and characteristic type of rhyolite (sees. 
333, 407, fig. 1, PI. VIII) shows the same network of dark granular lines, 
but in this case the meshes are filled with a spherulitic crystallization. 

There are, then, two types of crystallization which may occupy the 



» Geol. Expl., 40tli parallel, Vol. VI, Microscopic Petrography. 

BuU. 136 4 



50 VOLCANIC ROCKS OF SOUTH MOUNTAIN. Iuull.136. 

oval spaces, the spherulitic and the microfelsitic. The former does so to 
the exclusion of ii glassy groiindmass, and must have been formed prior 
to the consolidation of the rock. The latter shares the spaee with the 
glass into which it gradually passes and of which it may be a molecular 
alteration. Turnilig to the South Mountain aporhyolites (particularly 
the spherutaxites), we find a similarity in the development of faint gran- 
ular lines forming irregular oval meshes and giving to the section the 
network appearance ligured by Zirkel. These lines may be traces of a 
former peril tic parting or of a microfluidal structure. The meshes are 
now filled either by the quartz areas which condition the micropoiki- 
litic structure or by the vestiges of a spherulitic crystallization. 

The latter represents a primary structure, as in the rhyolites; the 
former may represent the molecular rearrangement of a spherulitic 
crystallization or of a glassy groundmass. In the last case it is the 
direct result ot devitrification and infiltration, processes more readily 
initiated along the borders of the perlitic cracks, as in the rhyolites of 
the Washoe district. 

A comparative study of some sections ' of the rhyolites of the Eosita 
Hills, Colorado, and of Obsidian Clitt*, Yellowstone National Park, 
side by side with the aporhyolites under discussion, also suggests the 
secondary character of the micropoikilitic structure with reference to 
the spherulitic crystallization. In the trichitic spherulites of the mod- 
ern rhyolites there is an appearance analogous to the micropoikilitic 
mottling, caused by the breaking up of the radiating spherulitic fibers 
into irregular areas which extinguish differently. An altogether simi- 
lar phenomenon occurs in some of the spherulites of the ancient rhyo- 
lites. It is indicative of an intermediate stage between the spherulitic 
and a completely micropoikilitic crystallization. This change from the 
spherulitic to the micropoikilitic structure is carried still further in 
some sections, notably in the case of a specimen crowded with minute 
red spherulites. Each spherulito extinguishes as an individual filled 
with inclusions of feldspar and iron oxide (hematite). The host can be 
determined by optical tests to be quartz. The shape of the spherical 
bodies in the hand specimen and in thin section, their tendency to form 
bands and chains, and their uniform color put their original spherulitic 
character and the secondary nature of the micropoikilitic structure 
beyond doubt. It is not supposed that a prior spherulitic crystalliza- 
tion always existed where now the aporhyolites show a micropoilitic 
structure, but these evidences of the derivation of the structure from 
spherulites establish a presumption for its secondary origin in other 
aporhyolites, where it may be the direct result of devitrification or 
may be due to the subsequent alteration, by infiltration, of a granular 
crystallization. 

On the whole, the plainly secondary character of the micropoikilitic 

1 Soctions of material from these localities wore kindly loaned the writer by Dr. Cross and Professor 
Iddings. 



BA8COM.] APORHYOLITE8. 51 

structure in the basic volcanic, the evidence in the aporhyolites of its 
being subsequent to fluidal lines or to perlitic parting, the indications 
that in many cases it is subsequent to a spherulitic crystallization, all 
denote a secondary origin for this structure in the South Mountain rocks. 

Spherulitic structure, — There are two sorts of spherulitic crystalliza- 
tion in the aporhyolites. They differ in no essential respect, but are 
unlike in appearance. The most numerous spherulites are also the 
simplest and smallest. They are colorless, microscopic spheres, scarcely 
or not at all perceptible in ordinary light, but between crossed nicols 
showing a distinct dark cross. Spherulites in every respect similar 
have been described and figured by Professor Iddings from the rhyo- 
lites of the Yellowstone National Park.^ Similar spherulites also occur 
in the rhyolites of Hungary and in the felsites of the Boston Basin. 
These radial growths are grouped in bunches and along lines, and are 
comx)osed of positive fibers. Further optical determination of the fibers 
could not be made. Their iwsitive character indicates either a prismatic 
section of quartz elongated in the direction of the vertical axis or a 
clinopinocoidal (010) section of orthoclase elongated in the same direc- 
tion. In the latter case the extinction would be slightly inclined. 
This is impossible, however, of determination. 

While it is not impossible that some of these spherulites are second- 
ary, in some cases there is evidence of their i)rimary character. One 
such case of spherulites whose formation was coincident with the con- 
solidation of the rock occurs in an aporhyolite from a cut made by the 
Gettysburg Bailroad north of Toms Creek trestle. Minute colorless 
spherulites are embedded in a base which suggests in every way its 
former glassy condition. In ordinary light there is no evidence ot 
crystallization except the porphyritical. 

The groundmass is traversed by irregular, angular cracks, evidently 
the result of crushing. These cracks, which are cemented by epidote, 
pass through the spherulitic aggregates (seen with crossed nicols), some- 
times cutting directly across a spherulite, portions of which appear on 
either side of the crack. Between crossed nicols the field breaks up 
into a mosaic of quartz and feldspar. The granular crystallization 
disregards the cracks, filling the spaces left by the (cementing epidote. 
It seems fair to conclude from these observations that the spherulitic 
crystallization is prior to the cracking, that the granular crystallization 
is subsequent, and that the cracking occurred in an already solidified 
glass. 

The secondary character of the granular crystallization admitted, it 
is easy to suppose that it is due to devitrification continued through 
lapse of time. The spherulites, on the other hand, being prior to 
the crystallization of the groundmass, and prior to the cracking, are 
doubtless primary and contemporaneous with tlte consolidation of the 
glass. 



» ObelcUan Cliff: Seventh Auu, Kept. U. S. Oeol. Survey, VI XVII, p. 27(5. 



52 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [bull. 136. 

The other class of spherulites correspond to those figured by Professor 
Iddings.^ They are much hirger than those that have just been 
described. The smallest is easily discernible by the naked eye, while 
the largest is 12J cm. in diameter. They are spherical, hemispher- 
ical, cylindrical, fan-shaped, oval, or irregular in form. While they 
all possess a clear-cut, conspicuous outline in ordinary light, in many 
sections they completely disappear between crossed nicols. The fresh 
spherulites (PI. XXII, />), which still show in polarized light a radi- 
ating structure, are usually colored red by a finely dissemiuated iron 
oxide. The globulites of hematite are distributed homogeneously 
throughout the spherulite, or they are grouped in radial and concentric 
lines. These lines are most dense near the margin of the spheruhte, 
and may be separated from the central portion by a narrow clear zone, or 
it is the outer rim of the spherulite which is clear. In the central por- 
tion of the spherulite the coloring matter shows a tendency to collect 
in bunches that correspond with areas which extinguish as individuals. 
Between crossed nicols the field is broken up into the minute areas 
which were referred to on page 50 as forming a structure approacbiDg 
the micropoikilitic. Feldspar phenocrysts usually occupy the center of 
the radiating crystallization, and two or more spherulitic centers may 
be included within a single outer zone. In specimens from Eaccoou 
Creek ^ these radial growths were remarkably well preserved and occur 
in a groundmass which retains the characteristics of a glass in great 
perfection (PI. XX, a; PI. XXI, a). It bears a close resemblance to 
the groundmass of some of the Colorado rhyolites, and in ordinary light 
would certainly be mistaken for the base of a fresh glassy lava. Deli- 
cate perlitic parting, which because of its delicacy is easily obliterated, 
is here preserved in wonderful detail. It is evidently subsequent to 
the radial crystallization to which it accommodates itself. The pres- 
ence of innumerable globulites accentuates the perlitic and rhyolitic 
structures. With crossed nicols the groundmass at once betrays its 
holocrystalline character (PL XX, h). All glassy structures disappear, 
to be replaced by granular quartz and feldsi)ar, a crystallization which 
closes the perlitic parting and thereby completely obliterates it. 

The porphyritic feldspars still show inclusions of a glassy base. Iti& 
impossible by any description to produce that definiteness of conviction 
as to the original glassy nature of the groundmass which the character 
of the sections justifies. To one who has studied these sections. in both 
ordinary and polarized light there can be no question as to the secondary 
character of the holocrystalline groundmass. One can not escape the 
conviction that the rock originally consolidated as a spherulitic perlite 
and has become holocrystalline by a process of devitrification. 

The sequence of events, as revealed by microscopic study^ is as fol- 
ows : There was first the intratellurie development of the porphyritic 

» Op. cit., p. 277, PL XVII. 

' In Frt^QkUu County, Pa., oust of Bocky Kidge aud south of Graefltoburg. 



BAscoM.j AP0RHY0LITE8. 53 

crystals, followed by tlie emergence of the magma, the development of 
globulites and fluxion structure, the commencement of radial crystalli- 
zation, and finally the consolidation of the magma as a glass and the 
development of perlitic parting. Subsequent to all this, and extending 
over a much longer i)eriod of time, the process of devitrification took 
place. 

Associated with a holocrystalline groundmass, which bears less 
marked evidence of an original glassy character, are the more altered 
spherulites. Their circular outline in the thin section and their spher- 
ical shape in the hand specimen testify to their former presence. 

In both hand specimen and thin section a threefold zonal arrangement 
is often clearly defined by the distribution of red or black iron oxide. 
With crossed nicols these boundaries become inconspicuous, and the 
field of the microscope shows only a uniform quartz-feldspar mosaic, 
or the former radial growth is indicated by zones of a finer-grained 
crystallization than the groundmass (PI. XXIII, a), or a micropoikilitic 
structure is present within the spherulitic boundaries when absent in 
the groundmass. Occasionally, vestiges of a radiate structure still 
remain. 

In the specimen already referred to on page 50 and figured in PI. 
XXII, a, the spherulites are colored red by a uniform dissemination of 
hematite particles, and are not more than one-half mm. in diameter. 
They traverse the rock in rows and chains, which are in turn grouped in 
bands about 2 inches wide. The rock has been sheared, a ready cleav- 
age has been produced, and sericite has developed around each spher- 
Tilite. As has already been mentioned, the spherulitic individuals have 
become micropoikilitic individuals. Another specimen shows spheru- 
lites which have been rendered almond-shaped through shearing or, 
less probably, by the fluidal movement of the magma during this 
consolidation. That the former was probably the case is indicated by 
the gradual passage of the rock into a slate and the development of 
biotite. 

Spherulites which, like these, have been replaced by a fine-grained 
mosaic of quartz and feldspar have been described by Klockmanu.' 

Chain spherulites. — The arrangement of spherulites in layers and along 
planes so that a cross-section shows a chain of si)herulites has been 
described on page 43 and figured on PI. XYIII, h, as it appears in the 
hand specimen. In ordinary light the microscope discloses somewhat 
sinuous or straight dark bands, with scalloped borders sharply outlined, 
inclosing an irregular clear chain which also has scalloped edges. 

Frequently there are detached clear spots with circular outline. 
These bands sometimes spread out so as to include phenocrysts and 
sometimes curve around them. (PI. XVIII, h.) A comparative study of 
these spherulitic chains and the chains of fresh spherulite of the Yel- 



' Die Porphyre; Dergeologische Aufbau des sogen. Magdeburger Uferrandes niit besonderer Beriick- 
sichtiguiig der anftretenden Eruptivgesteine : Jahrbuch K. preuss. geol. Landesanstalt, Vol. XI, 
1890, p. 179. 



54 VOLCANIC ROCKS OF SOUTH MOUNTAIN. tBULL.136- 

lowstoiie obsidians discloses a striking similarity in ordinary light — 
the same irregularly scalloped outline, the same central chain of clear 
spherules. With crossed nicols the close similarity vanishes, for in the 
ancient rocks the radial growth has utterly disappeared. The central 
clear chain consists now of a fine quartz mosaic. The dark borders, 
except for the crowded magnetite globulites, can not be distinguished 
from the holocrystalline quartz feldspar groundmass. This clear cen- 
tral zone where the spherulites converged evidently furnished the plane 
of weakness and easy solution, along which silica was infiltrated and 
parallel to which the rock cleaves. 

The impurities which have entered the rock along this cleavage plane 
give rise to the central dark line mentioned in the macroscopic descrip- 
tion, while the silica forms the opaque white band on either side. The 
central zone is sometimes more than a millimeter wide. Where a feld- 
spar crystal lies across this plane of weakness with its longest axis at 
right angles to the latter, the strain has proved too great for the crystal, 
which has been broken apart and the break cemented by infiltrated 
silica (PI. XXIII, b). 

The chain spherulite structure is of more common occurrence in the 
aporhyolites of the Monterey district than any other form of spheru* 
litic growth. The acid rocks east of the Bigham copper mine show 
them in great perfection. Kutley^ has figured some similar chain 
spherulites in the felsitic lavas of England and Wales. In felsite of 
the Keweenaw series from the Minnesota shore of Lake Sax>erior the 
writer has recently observed fine bands of silica so similar to the altered 
chain spherulites as to suggest a like explanation for them. 

Axiolitic structure, — Closely related genetically to the chain spheru- 
lites, but unlike them in being radial linearly rather than centrally, is 
the axiolitic growth.^ 

Axiolites are not particularly characteristic of the South Mountain 
apori)hyolites. Curving, linearly radiating growths do occur, however, 
in specimens from more than one locality. PI. XXI, ft, shows this 
structure. 

RhyoUtic structure, — The rocks in which the axiolites were observed 
are holocrystalline, yet they exhibit most strikingly the characteristics 
of a glass. Flow and vesicular structures, stringers and shreds, and 
curved patches of a brownish-red color, forming what has been called 
the rhyolitic structure, abound. (PL XXIV, a and b; PI. XXV, a.) 
This latter structure has been figured and described by Eutley,^ Nor- 
denskjold,* and de la Vall^e-Poussin,* and on a macroscopic scale by 



iFelsitic lavas of England and Wales: Mem. Geol. Survey, Gt. Brit., 1885, PI. VII, flgs. 11 and 12. 

' Zirkel, Microscopic Petrography : Geol. Expl. 40th parallel, p. 167. 

'Rutley, On the microscopic structure of devitrified rocks from Beddgelert and Snowden: Quart 
Jour. Geol. Soc. London, Vol. XXXVII, p. 406, figs. 1 and 2. 

^Nordenskjold, Zur Kenntuiss des sogen. Halleflinta des nordostlichen Sm&lands: Bnll. Geol. Inst 
Upsala, No. 1, Vol. I, p. 5, 1893. 

^Dc la Vall6e-Pou8sin, Les ancienues rhyolites, dites eurites, de Grand-Manil : Bull. Aoad. Boy.Bd* 
gique, 3d series, Vol. X, 1885, p. 271. 



pif 






- 

i 


|^|t^'|: 


'-« i*. 


' '^ 


i 






LITHOPHYS(€, 



BAscoM.] AP0RHY0LITE8. 55 

IrvingJ A perlite from Deer Creek Meadows, 25 miles south of Lassen 
Peak, displays a similar rliyolitic structure. This structure is essen- 
tially a phase of the fluidal structure.^ 

lAthophysal structure. — Very often the macroscoi^ic features of the 
aporhyolites disclose their original character more convincingly than do 
the microscopic. Lithophysae are best revealed in the hand sx)ecimen? 
where they are brought out in delicate relief by weathering. In such 
specimens from the Eaccoon Creek locality, the rose-pink x>etals of the 
lithophysae in a pale-pink base produce quite as beautiful examples of 
this glassy structure as any obsidian or rhyolite offers. (PI. XI.) No 
undoubted lithophysse were found within the Monterey district. The 
microscope discloses some vesicular structures which bear slight trace 
of a lithophysal character, but the alteration has been too great to 
allow of their Identification as hollow spherulites. 

Micropegmatitw structure, — Themicropegmatitic structure shows itself 
in microscopic pegmatoid groups of phenocrysts, such as have been 
frequently described in porphyries and rhyolites.^ It does not play an 
important part in the aporhyolites. 

Ferlitic structure. — That this structure is i)resent in the South Moun- 
tain rocks, ^nd in great perfection, has already been noted. (PI. XX, a, 
PI. XXI, a.) While its presence is a most reliable proof of the former 
character of the rock, its absence furnishes no evidence against the pre- 
vious glassy condition of the rock, both because many recent rhyolites 
showed no trace of that structure and because it is most readily effaced 
by devitrification. 

Amygdaloidal structure, — At Raccoon Creek, at the Bigham copper 
mine and its near vicinity, are light colored (pink and yellow), extremely 
vesicular aporhyolites. Tlie vesicles arc oval or elongated by How move- 
ment. (PI. XXVI, a and h,) They are uniformly filled with opidote or 
quartz, with both, or with either to the exclusion of the other. When 
both are present the quartz forms a rim around the epidote. The epidote 
has often a radial arrangement, while crystal boundaries are absent. Its 
color varies from a deep yellowish green to light yellow, and pleochro- 
ism is marked. In some of the larger amygdules the radiating needles 
of epidote have been broken and pulled apart at right angles to their 
longer axis and the spaces filled with silica. Piedmontite and quartz 
show the same relation, as described on page 41. The groundwork of 
these amygdaloids is the usual holocrystalline quartz -feldspar aggre- 
gate. Incipient alteration to granular epidote is more frequent in these 
open-textured amygdaloids than in the compact aporhyolites. These 



•Irving, Copper-bearing rocks, etc.: Mon. U. S. Geo!. Survey, Vol. V, pp. 312-313, fijj. 22. 

2 This is the " Aschenstructur" of Miigge(Unterauchnngcn iibcrdio "Lonneporphyro" in Westfalcn 
unil den angrenzenden Gebieten: Neues Jahrb. fiir Min., Gcol. \\. Pal., B. B. VIII, 1893, pp. G48, C49, 
713), who considers it dne to the original fragnioutal character of the lava. Whether it i.s always to 
be so interpreted is a question for further investigation. 

> Iddings, op. cit., p. 275, Pi. XV, fig. 5. 



56 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

rocks are pierced with coarse qaartz veins which bear native copper. 
Copper in microscopic (luautities and copper oxide also occur m the 
amygdules. Hand specimens are frequently coated Avith the copper 
carbonates, malachite and azurite. There were picked up on the road- 
side some specimens of amygdaloidal aporhyolites that are quite 
diverse from the amygdaloids of the Bigham copper mine which have 
just been describtnl. They are similar to specimens found north of the 
Monterey district at Kaccoon Creek. The amygdules, which are black 
against a yellowish- white background, are finely attenuated and elon- 
gated in long parallel bair lines, lending to the rock the appearance of 
an eutaxite. The black color is due to magnetite, which either is finely 
disseminated in quartz (the other infiltrated mineral), or is present in 
masses, or simply forms a heavy rim around the amygdules. 

Some of the ainygdaloids from Baccoon Creek merit a detailed descrip- 
tion. In these rocks the vesicles are usually bordered by a broad rim 
like the groundmass, in its present crystallization, but separated from 
it by a narrow, clear zone of quartz, and characterized by a greater 
abundance of magnetite (or ilmenite). On the inner edge of this 
border are spherulitic growtlis, while the rest of the vesicle is Med 
with quartz (PI. XXVII, h) or with quartz and an opaqu& black oxide 
(PL XXVII, a). In the latter case the black oxide occupies the center 
of the vesicle, leaving a clear zone of silica around the sphemlites. 
Crossed nicols show that the sphemlites are optically continuoas with 
the quartz, and that the radial appearance which has been retained is 
due to the arrangement of the impurities. The appearance of these 
vesicles is very suggestive of those figured by Professor Cole.* Pro- 
fessor Cole explains this type of spherulite by a dual mode of growth— 
a radial growth outward from J;he groundmass, as well as inward, origi- 
nating in the glass and converging toward a spherulitic center! He 
does not give the mineral character of the spherulite. Whatever may 
be the facts with reference to the Kocche Kosse obsidian, it is not 
necessary to postulate an abnormal method of crystallization to explain 
the phenomena observed in the South Mountain aporhyolites. 

The spherulites projecting into the vesicle, with their bases sunk into 
its walls, were recognized by Professor Iddings, who kindly examined 
the section, as tridymite spherulites, such as form on the walls of vesic- 
ular cavities in all kinds of modem lavas. Tangential sections of such 
spherulites are represented by granular aggregations. The form of 
the tridymite has been preserved by impurities, while its molecular 
arrangement has been altered to that of quartz. The presence of a 
border between the groundmass and the cavity suggests that crystal- 
lization, starting from the walls of the cavity, took place within the 
magma, iuitated, perhaps, by the gaseous content of the vesicle. 



'Grcnvillo A. J. Colo and Gerard W. Bntler, On the lithophyscs in tho obsidian of the Socohe 
Rosse, Lipari : Quart. Jour. Gool. Soc. London, Vol. XLVIII, 1892, j). 438. 



BAscoM.J AP0RHY0LITE8. 57 

Somewhat similar radial growths within vesicles in ancient rhyolites 
have been described and figured by de hi Vallee-Poussin.* 

Taxitic structure. — Another structure which the South Mountain rocks 
possess in common with rhyolites is what has been called the taxitic.^ 
This consists in the intimate mingling of two portions of the magma 
which from some cause (liquation) are slightly differentiated. The 
iron constituent, which evidently separated out in the original glass, 
has been still further crowded into bands and curved lines by the 
secondary crystallization. The result is the production, in some cases, 
of an irregular mottling, when the rock is called an ataxife; and in 
other cases of a more or less complex network of interlacing bands 
following lines of flow and forming a eutaxite. This mottling and 
banding is made the more striking by a marked contrast in color. The 
body of the rock is light-gray or pink, and the lines are dark blue-gray 
or red, according to the varying degrees of oxidation of the iron. 
Where the dark lines outline oval and spherical spaces and contain 
porphyritical crystals in or near their centers, the crystallization is 
regarded as having once been spherulitic and the rock is termed a 
spherutdxite. These have been described on page 50. 

The eutaxites are frequently so sheared as to give a hair-like tenuity 
to the bands in cross section, while the microscopic slide shows the 
effect of pressure on the rock in the parallel arrangement of the glob- 
nlites of black oxide. The universal presence of globulites, trichites, 
and microlites of black and red iron oxide in flow bands, or indiffer- 
ently distributed, or in concentric zones around spherulites and vesi- 
cles, is worthy of mention as a further point of resemblance to the mod- 
ern rhyolite. Such a trichitic structure in similar rocks has been 
described by various petrographers.^ 

SUMMARY OF PROOF OF DEVITRIFICATION. 

It is not easy to present the evidence for the secondary nature 
of the holocrystalline groundmass so that it shall have the weight 
which properly belongs to it. Very much depends upon effects which 
it is impossible to convey by description, but which carry conviction 
to the student of these rocks. The contrasting appearance of many of 
the sections in ordinary and polarized light can not be adequately repro- 
duced. The disappearance under crossed iiicols of rhyolitic, perlitic, 

'Lies anciennes rhyolites, dites eurites, do Grand-Manil : Bull. Acad. roy. Belgique, 3d series, Vol. X, 
1885, p. 292. 

'Fritsch and Keiaa, Teneriffe, 1868, p. 414. 

Roaenbuachf Mic. Ph^s. der Massigen Gesteine, 2d ed., p. 625. 

F. Loewinson-Lessing, Zar Bildungsweise uiid Classificatiou der klastischen Gesteine, 1888, pp. 228- 
235. Note sur lea taxites et sur les roches clastiquo vulcaniquo: Bull. See. Belg. gcol., etc.. Vol. V., 
1891- LoewinsoD'Lesaiiig's division of the taxites into ataxites, eutaxites, and spherutaxites has 
been foUowed in this bulletin. 

*R.cD. Irving, Copiier bearing Rocks, etc. : Mon. U. S. Geol. Survey, Vol. V, p. 312. 

S. Allport, On certain ancient devitrified pitchstonos and perlites from the Lower Silurian district 
of Shropshire. Quart. Jour. (Jeol. Soc. London, Vol. XXXIII, i). 449. 

Nordenslgold, op. cit. 



58 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull.136. 

sphcrulitie, and fiiixion structures, so clearly indicated in ordinary 
light, in a homogiMicous liolocrystallinc mosaic is one of the strongest 
evidences for the secondary character of the crystallization. 

There are also instances where the nature of the crystallization is 
distinctly inoved. On page 51 it was shown to be subsequent to the 
cracking which must have occurred in a solid rock. Pages 52-53 
describe the replacement of the radiating crystallization of the spheru- 
lites and chain si)herulites by a granular crystallization which is homo- 
geneous with a granular groundinass. Finally, on pages 49-50, the 
secondary chartuiter of the niicropoi kill tic crystallization has been 
indicated. One or another of these indications of secondary crystalli- 
zation is almost invariably present in the rocks which have been 
included under the name aporhyolite. 

The exceptional occurrences, where these structures are absent, sliow 
genetic relationship in the field to typical aporhyolites. The deter- 
mination of the character of the groundmass in the cases described 
thus practically determines it for all the aporhyolites. 

The secondary character of the holocrystalline groundmass once 
admitted, and the indications of an original glassy base recogni;jed as 
such, one is forced to conclude that the former was developed from the 
latter by a i)rocess of devitrification. 

That the processes of crystallization do not necessarily cease with 
the solidification of a magma is well known, for experiment has proved 
that crystallizing forces are active in a glass as well as in a molten 
magma.' This action is exceedingly sluggish, and requires, unless 
accelerated by heat and moisture, an immense amount of time. Devit- 
rification has been considered the result of dynamic action only;* but 
while dynamic action undoubtedly accelerates the process, if it does not 
initiate it, devitrification may also take i)lace indei)endently of dynamic 
action, as was the case in the famous example of the old cathedral win- 
dow glass ^ and the ancient devitrified glass from Nineveh investigated 
by Sir David Brewster.'* The nature of the process is in no way differ- 
ent from the process of crystallization in a fluid magma save in the 
rapidity of the action, and is of both a physical and a chemical character. 

The devitrification wliich has occurred in the South Mountain a|)0- 
rhyolites is not attributed to dynamic action, of which there are many 
evidences of another nature in the South Mountain, but to staticjil 
metamoi-phism. The former would, by shearing, obliterate the original 
structures of a glassy rock and produ(».e a slate, while the latter might 
be an imi)ortant initiatory and accelerating factor in the process of 
devitrification of the glassy rocks. 



'l)aul)r<''e, G^'-olo^io ox])<'Tiineiitale, 1879, p. 158. 

"Do la Vall6o Pouasin, Lea euritos quartzeuaea (rhyolit^^a ancieinies) <le NivelleH et den environs: 
Bull. Acad. roy. sci. lott. ot doa beaux arta do ]5el«;iqno, 57 aim^'O, 3d aories, Vol. XIII, N"o. 5, 1887. 
1)1». 521-522. T. G. Bornemann, l)er Quart zporphyr von Heiligeuatoiu und seine FluidElstmctur: 
Zeitaclir. Dciitach. gool. Geaell., Vol. XXXIX, 1887, p. 793. 

3Rritish Aas. Kept., 1840. 

4 Trans. Koyal Soc. Edinburgh, Vols. XXXII, XXXIII. 



BAscoM.] APORHYOLITES. 59 

Opinions of petrographers, — Paleozoic and pre- Paleozoic acid vol- 
caDics have long been studied on the European continent. Although 
their variation from the modern type of acid volcanics, rather than their 
resemblance to that type, has, for the most part, been emphasized by 
German and French petrographers, there have not been wanting able 
advocates of devitrification and of an original glassy base for the 
ancient lavas. 

R. Ludwig* (1861) and Vogelsang^ (1867) inclined to the opinion that 
the groundmass of certain quartz-pori^hyries is the result of the devit- 
rification of a glassy lava. 

The late Dr. K. Si Lossen^ (1869), on comparing the spherulitic por- 
phyries of the Harz Mountains with the obsidians of Lipari, Mexico, and 
Java, found the resemblance sufficiently striking to lead him to declare 
that the porphyry groundmass was originally crystallized as glass, and 
became cry ptocry stall ine through molecular rearrangement. Later, 
Kalkowsky* (1874) suggested that devitrification through the chem- 
ical iictivity of water was the process by which the microfelsitic base of 
certain pitchstones and felsites was developed; and still later, H. Otto 
Lang^ (1877) described a macroscopically un individualized base which 
is similar macroscopically to the devitrified base described by Kalkow- 
sky. Sauer ^ (1889) considered the Dobritz porphyries as the final altera- 
tion product of a pitchstone. More recently Klockmann^ (1890) has 
described the replacement of the spherulitic crystallization in quartz 
porphyries, through secondary processes, by a fine-grained aggregate 
of quartz and feldspar. 

Osanti^,(1891) described incipient devitrification in perlite and other 
glassy rocks from Cabo de Gata. Finally, Link (1892) considered that 
the fine-grained groundmass of some American rocks closely related 
to mica-syenite- porphyries was once glassy, or at least partially glassy, 
and C. Vogel^ (1892) reached the same conclusion as to the Umstadt 
porphyries in Hesseu. Many no less capable observers still hold to 
an original difference between ancient and recent acid volcanics, and 
the possibility of devitrification and an original similarity is yet an 
open question in Germany. 

In France, La Croix *° describes andesites from Martinique in which 
the glass has altered into quartz spherulites and a granular quartz 
aggregate. 

It is interesting to note that many of the hiilleflinta of Sweden, 



»Erl. zur geol. Karte Hessens, Bl. Dieburg, 1861, p. 56. 
2 Phil, de g6oIogie, pp. 144, 153, 194. 

^Beitrage zur Petrographie (ler plutonischen Gesteine: Abhaiidl. <ler Berliner Aca<l., 1869, p. 85. 
^Tschemiaks mineral. Mittheil, pp. 31, 58. 
^Grnndriss der GesteiDskundo, p. 43. 

«Erl. zur geol. Specialkarte Sachsens, Bl. Meissen, pp. 81-91. 

^Dio Pprphyrej Der geol. Aufbau sogen. Magde burger Uferrandes niit besonderer Beriicksichti- 
gung der auftretenden Emptivgesteine : Jahrbucb K., preusa. geol. Landesanstalt,Vol. XI. 
» Zeitsclir. Deutach. geol. Gesell., Berlin, pp. 691, 716. 
9 Abhandl. geol. Landeaanstalt von Hessen, Vol. II, p. 38. 
>o Comptes-rendus, CXI, p. 71. 



60 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [bull. 136. 

whicli, like the Soiitli Mountain volcanics, were OQce described as sedi- 
mentary, are proving to be acid volcanics, preserving the features of 
their modern equivalents. Quite recently glassy and rhyolitic struc- 
tures in these rocks have been observed and described by Otto Nor- 
denskjold.' 

In Belgium, de la Vallee-Poussin seems to be the only writer who has 
brought out the resemblance between the eurites of that country and 
modern rhyolites. lie describes at some length structures similar to 
those possessed by the aporhyolites of South Mountain. A vacillating 
state of mind as to the matter of nomenclature is indicated in the 
titles of his successive papers.^ 

In England the rhyolitic character of ancient acid volcanics has been 
recognized and emphasized, and the idea of devitrification is widely 
accepted. Allport, Cole, Bonney, Kutley, Judd, and Harker have 
accomplished most valuable work along this line. 

Dr. Wadsworth ^ was the first American petrographer to advocate 
the abandonment of age as a factor in rock classification, while at the 
same time he recognized devitrification as the process which was form- 
ing felsites out of rhyolites. What he says is of interest in its anticipa- 
tion of ideas now more generally accepted: 

This devitrification gives rise in the older and more altered rhyolites. to the feld- 
spar, quartz, and uiicrofelsitic (so called) base that has so puzzled lithologists in the 
study of the felsites. The rhyolites of all volcanic rocks preeminently show lamina- 
tion produced by ilowing, a fact which is doubtless due to their being bo siliceous. 
This structure and their devitrification enable us to trace a dlTect connection 
between the rhyolites and felsites, which are simply the older and more altered rhy- 
olites. * ^ * One of the best illustrations of this is to be found on Marblehead 
Neck, Massachusetts, where at least two distinct fiows of felsite ocouTy one cutting 
the other. They show the fluidal structure so characteristic of rhyolites — a char- 
acter that has been mistaken for lines of sedimentation by geologists, while the 
inclosed crystals of orthoclase have been taken for pebbles. * * ♦ While to the 
naked eye and under the microscope this rock shows the fluidal structure of a rhy- 
olite, in polarized light it is seen that the base has been completely devitrified, a 
process that is carried to a great extent in many known modern rhyolites. 

No other American petrographer has so distinctly advocated the 
identity of felsites and ancient rhyolites, in spite of the fact that many 
of our felsites illustrate it as unmistakably as do the English felsites. 

Dr. Irving/ in his description of the Beaver Bay group of the Kewee- 
naw series, repeatedly calls attention to the resemblance between the 
ancient felsites and quartz-porphyries and the modern rhyolites, though 
he does not express an opinion as to their equivalence. 



■Op. cit. Also Ueber arclueische Er^ussgeHteine aus Smalaml: Bull. Geol. Inst. Upaala, No. 3, 
Vol 1, 1893, pp. 1-127. 

' Los ancicnnes rhyolites, ditcs earites, du Grand-Manil : Ball. Acad. roy. Belgiqne, 3d Mries. VoI.X, 
1885, p]). 253-315. Les eurites quartzeuses (rhyolites anciennes) de Nivelles et dee environB: Bull. 
Acad. roy. des sci, et des beaux-arts de Belgique, 57 ann^c, 3d series, Vol. XIII, No. 5, 1887. 

*M. E. Wadsworth, Notes on the miueralogy and petrography of Boston and vicinil^ : Proc. Boston 
Soc. Nat. Hist., Vol. XIX (May. 1877), p. 236. On the claAsification of rocks: BuXL Hmb. Comp. ZooL 
Harvard Coll., Vol. V, No. 13, June, 1879, p. 277. 

•Op. cit., pp. 312, 313, note 5, p. 436. 



BAscoM.] CHEMICAL COMPOSITION OF THE ACID EKUPTIVES. 



61 



Messrs. Hagae and Iddings ^ make the statement <Hhat the degree of 
crystalhzation developed in igneous rocks is mainly dependent upon 
the conditions of heat and pressure under whicli tlie mass has cooled, 
and is Independent of geological time." 

On the question of devitrification the writer finds no more direct 
expression of opinion, but the fact of devitrification is recognized by 
Iddings, Williams, Cross, and Diller. In none of the felsites elsewhere 
described have the varied structures of the modern rhyolites been more 
perfectly and conspicuously preserved than in the aporhyolites of the 
South Mountain. 



CHEMICAL COMPOSITION OF THE ACID EBUPTIVES. 

The chemical study of the South Mountain rocks has been rendered 
easy by a number of analyses of these rocks made by the late Dr. Genth, 
under the auspices of the Second Geological Survey of Pennsylvania. 
Analyses of the sedimentary, acid, and basic igneous rocks of South 
Mountain are distributed through pages 252 to 282 of Report CCC. 

The analyses of the acid volcanics have here been brought together 
and tabulated. 

Table of analyses. 



SiO, .... 
PsOs.... 
A1,0,... 
FejO,... 

FeO 

TiOj.... 

CaO.... 
K2O .... 
Na,0 ... 
H2O .... 
Ignition 



Total 



I. 

79.970 
13.866* 
2.' 766 



ir. 



79. 920 

.190 

12.400 

2. 950 



1.230 
.220 



95. 038 



Trace. 
6.490 
2. 320 



.440 



100. 71 



iir. 



76.76 

.11 

12.64 

2. 63 



IV 



VI. 



68 



.11 
6.15 



1.53 



100. 77 



76.62 



12.26 
2.08 
4.03 



75. 570 

.190 

11.940 

3.830 



.26 

.34 

2.57 



.40 



99.09 



5.250 
3.010 



75. 31 

.05 

12.06 

3.05 



41 



3.15 
5.11 



.270 



.61 



100.06 



100.65 



I. Fissile green schist. Between Pino Grove Fnrnjicc and Laurel forge.^ 

II. *' Ortbofelsite." ** One-fourth of a mile north of Lerew's store. ''"^ 

III. **0rthofel8ite." '*Cut on turnpike 5 miles northwest of Petersburg, Cumber- 
land County.'' 3 

IV. "Laminated felsite.'^ ''East of Bighaui Copper Mine."^ 

V. '^ Laminated orthofelsite." "One-fourth of a mile southeast of Caledcmia Fur- 
nace." ^ 

VI. ''Finely laminated orthofelsite." '' One-fourth of a mile west of Cole's saw- 
mill, on the Shippenburgroad." » 

Their general uniformity and their close agreement with the analyses 
of typical rhyolitic lavas is the most striking feature of these analy- 



'On the development of crystallization in tho igneous rocks of Waahoc, Nov., with notes on the 
geology of the district: Bull. U. S. Geol. Survey No. 17, 1885, p. 40. 

.'Analysis made by A. S. McCreath for commercial purposes; alkalies undetermined. 

'Analyses made by Dr. Genth and Henry Trimble. Second Geol. Surv. Pa.. Vol. CCC. pp. 263-260. 

^Analysis made by C. Hanford Henderson, of Philadelphia, The copper deposits of the South 
Mountain: Trans. Am, Jnet, Mm. Eng., Vol. XII, 1884, p. 90. 



62 



VOLCANIC KOCKS OF SOUTH MOUNTAIN. 



[BULL. 136. 



ses, and is ii convincing proof of the igneous origin of the rocks which 
they represent. In the absence of samples of the rocks analyzed or of 
exact descri])tions of their diarac^ter, special points can be brought out 
only inferentially. 

The high percentage of tlie alkalies and the slight trace of lime 
plainl}'^ denote the character of the feldspathic constituents. This 
indication of their chemical character coincides with the optical and 
physical i)roperties enumerated on page 40. The rock from which 
Analysis IV was made is rei)orted to be from the same locality from 
which many of the aporhyolites were obtained, the optical character of 
whose feldspars were tested. 

Microscopic study of these rocks leads us to expect a percentage of 
titanium oxide.* In two instances the analyses show it. It is not 
unlikely that in the other cases the titanium oxide was not determined. 
The absence of manganese oxide from these analyses is surprising, as 
the Monterey porphyries and aporhyolites show a high percentage of 
it. The lime and magnesia present are doubtless due to the presence 
of epidote in the rock. The silica, alumina, and iron percentages are 
exactly normal and call for no remark. 

Analysis I, of a fissile schist, illustrates the slight change of chemical 
constitution which accompanies dynamic action in the acid rockSo 

Among the analyses made by Dr. Genth for the Second Geological 
Survey of Pennsylvania are the following, which, because of their 
anomalous character, have not been tabulated with the others : 



I. 



11. 



SiO, .... 

PoO..... 

AUOa-.. 
FejOa-.. 
FeO .... 
TiOj.... 
MnO.... 
MgO-... 
CaO . . . . 
KjO .... 
Na,0 ... 
Ignitiou 



53.86 



22.18 
7.11 
1.46 



2.79 
1.09 
4.56 
4.12 
2.55 



52.82 

.21 

20.73 



10.72 
1.92 

.52 
2.03 

.23 
G.68 



4.28 



Total 



100.42 i 99.62 



I. 'SSlaty rock.'' *' Nine miles southwest of Dillersburg." 

II. '*Puri>lish slaty ortliofelsite." "One aii<l oue-lialf miles sontbeast of Mount 
Alto.'' 

These rocks plainly do not represent the normal type of the South 
Mountain acid rock. In the absence of specimens or means of deter- 
mining the character of Ihe rocks from which these analyses were made, 
it is impossible to explain altogether satisfactorily the abnormally low 
percentage of silica and the high percentages of alumina and iron. 

If these slates were once normal aporhyolites, the shearing which 
])roduced the subsequent slaty character must have been accompanied 
by an abundant development of sericite from the feldspar. If the silica 



.jM 



BAscoM.i ACID VOLCANIC BRECCIA. 63 

thus set free was carried off by percolating water, a low silica percent- 
age and a correspondingly high alumina i)ercentage would result, while 
the alkalies would remain about the same. The increase in iron may be 
due to infiltration. 

ACID VOLCANIC BRECCIA. 

DISTRIBUTION. 

The presence of acid pyroclastics in the Monterey district has already 
been mentioned. Although a conspicuous feature of a portion of South 
Mountain, notably of the Buchanan Valley north of the Chambersburg 
tui^pike, where they cover about 2 square miles, they play an insig- 
nificant role among the rocks of the Monterey district. Their charac- 
ter, however, is unmistakable. They may be classified as tuffs, flow 
breccia, and true breccia or tuffaceous breccia. 

TUFF. 

TJiis is a dark-purple banded rock, the clastic character of which is 
hardly evident in the hand specimen. Microscopic examination dis- 
closes its tuffaceous nature. Minute angular fragments, not exceeding 
a millimeter in length, are thickly distributed through a crystalline 
groundmass. 

The fragments are usually spherulitic, with the replacement of the 
spheruli tic crystallization, as in the massive aporhyolites described on 
page 52, by a fine mosaic, so that inordinary light the spherulites are 
traceable only from their outline. In the same way, under crossed nicols 
a uniform granular crystallization obscures the fragmental character 
of the rock — a character which, m ordinary light, is sharply brought 
out by an outlining pigment of red iron oxide. Fragments of quartz 
and feldspar are among the inclusions. The groundmass, which is of 
-^the same chemical and mineralogical constitution as the included frag- 
ments, doubtless represents an ash recrystallized. 

FLOW BUECCIAS. 

These occur at widely separated localities — northwest of Old Maria 
ftirnace, near the source of Toms Creek, on the Gladhills road, and on 
tlio brow of the hill east of the Clermont House. These breccias are 
coinposed of fragments of considerable size, which plainly were caught 
imx a viscous acid magma, as is evidenced by their linear arrangement 
in ±3ow lines and by the way in which different fragments fit together, 
ft^rrning what was once a larger fragment. On the weathered surface 
t>"f the rock its brecciated character is rendered very manifest in the 
'^^^J'ing tints of pink, red, purple, and blue (PI. XII). The fragments 
ran^^ in size from the submacroscopic to those that are 2 J inches in 
f^^tHeter. Their spherulitic character is discernible by the naked eye. 
^ '^<iei the microscope, in ordinary light, the fragments frequently show 
^^x^ perlitic parting, a spherulitic character, or a regular arrangement 



64 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

of tlio coloring matter parallel to the boundaries of the fragments, due 
to water deposition. Crossed nicols again show^ uniform crystallization 
or a micropoikilitic structure. In one specimen of breccia this was not 
the case, however. A coarsely crystalline siliceous cement sis quite 
distinct in grain from the uniformly finely crystalline fragments. This 
maybe a lava How crushed and receniented. 

TrFFACKOrS BKBCCIA. 

In some instances the groundmass doubtless represents an altered 
ash, when the rock becomes a true breccia. A breccia of this sort from 
the Monterey district has epidote largely developed in the matrix. 
The granulated quartzes and the perthitic feldspars of the inclnded 
fragments show in a marked way the effect of dynamic action. 

At Raccoon Creek a fine specimen of breccia was found (PI. XIII), 
and in the Buchanan Valley breccia is extensively exposed. Someof the 
fragments contain chain spherulites. At Coles Corner (Buchanan Val- 
ley), some C ndles northeast of Graeffenburg, the breccia is sheared, 
and with the development of sericite the rock has become more or Jess 
slaty, while still conspicuously retaining its brecciated character. 

The presence of these tuffs, flow breccias, and breccias proper, which 
are the natural accompaniments of surface lava flows, is inexplicable 
under any other hypothesis of the origin of the acid rocks. 

METAMORPHOSED ACID ERUPTIVES : SEBIOITE-gCHISTS AND SLATES. 

The alteration of the feldspathic constituent of quartz-porphyries to 
sericite or some other micaceous mineral, under the action of dynamic 
forces, has been frequently described.^ 

The production, in tliis way, from massive acid emptives, of schists 
and slates resembling true pori)hyroids,^ is finely illnstrated in the 
South Mountain. In a single exposure (west end of Long Mountain, 
west of Gettysburg) felsites with distinct phenocrysts grade insensibly 
into a crinkled sericite slate. The shear zone is of limited width (10 
feet), and bounded on each side by massive felsites. The phenocrysts 
are only slowly obliterated and can be distinguished until the last stage 
in the alteration has been reached. 

Thin sections of five successive stages were studied. They show a 
development of sericite first around the feldspar phenocrysts and in a 
plane of dislocation. It is only sparingly developed in the ground- 

' J. Leliinann, I'nttirsucbuDgeii ilbcr die EutHtohniifi; der altkrystallinischen Sdiiefergesteine, Bonn, 
J 884, Cap. IX, Druckscliiefeniug und Gliniiiiorbildimg, p. 136. 

A. von Groddeck, Zur Kenntniss eiiiigor Seric-itgeateine, welche neben und in Erzlangerstfitten 
auftreteu : Noucs Jahrbuch fiir Mineral., etc. Supp. Vol. IV, 1886, p. 428. 

G.II. Williams, Bull. V. S. Geol. Survey No. 62, pp. 61, 121, 212. 

Bonn(;y, On some nodular felsites in tbe Bala group of North Wales: Qoart. Joar. Greol. See. Lon- 
don, Vol. XXXVIII, p. 289. 

Callaway, On the genesis of the crystalline schists of the Malvern HiUs: Quart. Joor. Geol. Soc. 
London, Vol. XLIII, pp. 530, 531. 

P. L. ;Miich, Beitrage zur Kcnutnis des Vermcauo, 1892, pp. 128, 129. 

'Kosenbusch, Pet. Mossigen Geateine, Vol. II, 2d ed., p. 411. 




D BRECCIA, RACCOON CREEK 



BAficoM.] BERIC1TE-8CHI8TS AND SLATES. 65 

mass. In the next stage the groundmass shows a decided tendency 
to a parallel arrangement, and serieite is more abundantly developed. 
Eventually the phenocrysts are obliterated and there is much serieite 
in the groundmass. It is not, however, developed to the exclusion of 
the feldspar, while the quartz remains unaltered. 

In general, the development of serieite stands in direct relation to the 
shearing, and increases up to an almost complete, if not quite com- 
plete, replacement of the feldspathic constituents of the groundmass. 
Silica remains as a constituent of the groundmass. 

Material obtained from an artesian well at a point 40 feet below the 
surface furnished a similar shear zone, which displayed an even more 
abrupt transition from a porphyry to a sericite-schist. A single micro- 
scopic section included both i)orphyry and schist in typical develop- 
ment. The former showed an early stage of the alteration which was 
complete in the latter, where only serieite and a very schistose siliceous 
microgranitic groundmass remained. In this case, and in the extreme 
stage of the transition previously described, it would be impossible, 
with the microscope alone, to decide whether the schists were of clastic 
or nonclastic origin. This is one of the instances where field evidence 
is quite essential to the authoritative determination of the origin of the 
rock. 

At the Bechtel shaft there has been thrown out a mottled red and 
T^hite schist which has been produced by the shearing of a massive 
felsite. Here the phenocrysts have been replaced by a quartz mosaic 
and some serieite, which is also largely developed in the groundmass. 
The red mottling is due to a more or less parallel arrangement of red 
iron oxide globulites. 

A light green sericite-schist found on the railroad near Blue Ridge 
Summit station, and closely resembling some schists in situ exposed 
on the Gettysburg Eailroad below the Clermont House, shows under 
the microscope phenocrysts of feldspar containing inclusions of a 
former glassy magma, still well preserved and showing twinning stria- 
tions. These phenocrysts occur in a groundmass of quartz, a little 
feldspar presumably, much serieite, epidote, ilmenite, or magnetite, and 
leucoxene. 

The color of the schist is due largely to the epidote. 

At the exposure just now mentioned on the Gettysburg Railroad, 
east of the Clermont House, there occurs a handsome, light, silvery- 
green, crinkled sericite-schist. Several rods to the north of this 
exposure the railroad cuts tlirough quartz-porphyry, but the contact 
between the porphyry and the schist is not exposed. The schist is sim- 
ilar to those already described, whose gradual passage into a massive 
porphyry could be followed in the field, and shows traces of pheno- 
crysts under the microscope, and in the hand specimen on the surface 
at right angles to the cleavage. The cleavage surfaces often display 
exquisitely delicate and manifold dendritic tracery. (PI. XIV.) 
BuU. 136 5 



06 VOLCANIC ROCKS OP SOUTH MOUNTAIN. [bcll.136. 

On the bijLchroad from Fount aindale to Fairfield, not far from tbe 
"Old Copper Shaft," occuirs a dark purple-gray spotted slate. Tbe 
light-green spots are sometimes irregular, but more frequently possess 
crystalline outlines and prove under the microscope to be a sericitic 
alteration of feldspar phenocrysts. Much of the feldspathic material 
still remains. The grouudmass consists largely of iron oxide, which by 
its prevalenc<^ obscures tbe other constituents — ^leucoxene and quartz. 

Microscopic evidence is sufficient in this instance to determine tbe 
origin of tbe rock. It is plainly a sheared eruptive, and probably a por- 
phyritic aporhyolite, although the irregular outline of some of tbe 
sericitic areas suggests a brecciated aporhyolite. 

The occurrence of these slates is an interesting feature of the geology 
of the South Mountain. In the hand specimen they might readily be 
confused with a porphyroid, that is, a metamorphosed clastic rock, and 
have been so confused by geologists. They did not escape the atten- 
tion of Professor Kogers, who alludes to them as 'Hhe fissile talcose 
rock" near the "reddish gray rock, containing specks of reddish feld- 
spar," and includes them among the primal slates whose highly altered 
condition he repeatedly contrasts with the other slightly altered sedi- 
ments (sandstone). If these slates were of clastic origin, a high degree 
of metamorphism was necessary to produce their present crystalline 
condition, and Professor Kogers was quite right in drawing a contrast 
between their extreme metamorphism and the comparatively unaltered 
condition of all the other sediments. The very fact that their develop- 
ment from a sediment calls for such a high degree of metamorphism con- 
fined to limited and isolated zones, and for which no adequate cause ean 
be assigned, renders such an origin as improbable as it is unnecessary. 
Such a "selective metamorphism^' is not demanded by the facts. 

As a matter of fact, these slates are scarcely more altered thaii the 
sandstone. Dynamic action in the latter has develojied a qnartzite. 
Dynamic action in the less resistant porphyry and aporhyolite has 
produced a sericite slate. That the chemical character of the acid 
rock remains essentially unaltered is evinced by analysis I, given on 
page 01. This shows exactly tlie composition of a rhyolite, and is 
totally unlike that of the sediments of the region. The sedimentary 
argillaceous slates of South Mountain are very little altered, and 
exhibit no tendency toward the development of porphyroids. 

All evidences — field relationship, successive stages shown in tbe 
hand specimen and under tbe microscope, chemical character, inherent 
improbability of clastic origin — combine to reveal the igneous character 
of these acid slates. 

SUMMARY. 

The acid igneous rocks of the South Mountain have proved to be 
quartz-i)orphyries, devitrified rhyolites (aporhyolites) with accompany- 
ing pyroclastics, and sericite-S(jhists. 



J 



BA8COM.] SUMMARY. • 67 

The first are typical holocrystalline porphyries, characterized by a 
soda-feldspar aod by the presence in many cases of accessory piedmont- 
ite. The second group are the prototypes of the modern rhyolites, 
differing from them only in the loss of a vitreous base through devitri- 
fication. They are without phenocrysts, with inconspicuous pheno- 
crysts, and with abundant and conspicuous phenocrysts. Like the 
porphyries, they are characterized by a sodafeUlspar — that is, they are 
of the pantellerite type. The evidence for devitrification lies in the 
abundant presence of structures peculiar to glassy lavas, in the present 
holocrystalline character of the rocks, and in the empirical knowledge 
that glass may become crystalline through lapse of time. 

The sericite schists are a metamorphic product of the first two 
classes by means of dynamic action. 

The alteration which the original types have undergone subsequent 
to consolidation is, in the case of the aporhyolites, devitrification 
(statical metamorphism); in the case of the schists, sericitization 
(dynamical metamorphism); and in the case of all three groups, includ- 
ing the quartz-porphyries, an epidotization (weathering). 



(3HAPTEK V. 

PETROGRAPHICAL DESCRIPTION OF THE BASIC ERUPTIVES. 

NOMENCLATURE. 

Many of the p(itrographi(jal data upon which uniformitarian argu- 
ments have been based have been drawn from the comparative study 
of basic eruptives, and there is a marked disposition to disregard age 
in the nomenclature of these rocks. Among German petrographers, 
Reyer,' Tietze,^ Reiser,^ Reusch (H. H.)/ and Suess* have supported the 
view that age is not a just ground of distinction between eruptive 
rocks, and Rosenbusch'^ predicts that in no very distant foture the 
separation of effusive rocks into an older and a younger series "will 
prove untenable." English and American petrographers are practi- 
cally disregarding age in their nomenclature of the basic igneous 
rocks. Among the former, Judd,^ Teall,^ Allport,^ Bonney,*® Phillips," 
and Hobson^^ are notable. Among American petrographers, the 
Danas^^ and Iddings'* have disregarded age in their usage of basic 
rock names. 

In plagioclaseaugite rocks, the distinction between the gabbro and 
the diabase groui)s has been jfinally recognized as structural and 
not mineralogical, and the distinction between the diabase and the 



'E.Reyer, BeitragozurFisikderEnip., 1877, pp. 142-171; ref. Hnssak: Neues Jahrbnch fBrMineial., 
etc., 1892, Vol. II, p. 147. Beitrago zurFisik der Erup. und der Eraptivgesteine. 1887, p. 135. 

*E. Tietze, Das Altersprincip bei der Nomenclatur der Emptivgesteine : Verhandl. k. k. gwl. 
Reichsanstalt, Wien, 1888, p 166; ref. F. Bocko: Neues Jalirbuch fiir Mineral.. Vol. II, 1884. p. 303- 

'Karl A. Keiser, Ueber die EniptivgesteiDO des Algaii: Tschermaks minenU. MittheiL, Vol. Xi 
1889, pp. 500-550. 

*H. H. Reuscb, Ueber Vulkanismiis, Berlin, 1883. 

«Sue88, Das Antlitz dor Erdo, Vol. I, i)p. 204-206, 1883. 

*H. Ilosenbnsch, Ueber die cbemische Beziebungen der Emptivgesteine: Tschermaks minenl' 
Mittbeil., Vol. XI. 1890, p. 146. 

'Judd, On tlic gabbro.s, dnlorito.s, and basalts of Tertiary age in Scotland and Irdand: Qnart Joor. 
Geol. Soc. London, Vol. XLII, 1H86, pp. 49-97. Tbo secondary rocks of Scotland: Qnart. Jour. Geol' 
Soc. Loudon, Vol. XXX, 1874, pp. 220-303. 

^Teall, Address of the president Geol. Sec. (e) of the British Assn. Adv. Sci., 1893. 

sAllport, On the basaltic rocks of the Midland coal fields: Geol. Mag., Vol. VII, Ko. 70, ISTDiPP* 
159-162 Tertiary and Palicozoic trap rocks : Geol. Mag. Vol. X, 1873, p. 196. 

'"Bonuey, Quart. Jour. Geol. Soc. London, Vol. XXX, p. 529. 

"Phillips, On the so-called greenstones of central and eastern Cornwall: Quart. Jonr. G«<d. So&i 
London, Vol. XXXIV, p. 471. 

)'nobson,On the basalts and andesites of Devonshire : Quart. Jour. Geol. Soo.,LoiidoB,yoLXLTin« 
1892, pp. 496-507. 

"J. D. Dana, On some points in lithology: Am. Jour. Sci., 3d series, VoL XXXVITT, 1878, fP> 
336, 438. E. S. Dana, Trap rocks of the Connecticut Valley: Proc. Am. Assn. Adv. Soi., 1884. 

'biddings. The columnar structure in the igneous rock on Orange MoQDtl^i ^91f ^VfW* ^ 
Jour. Sci., 3d series, Vol. XXXI, Mav, 1886, p. 331. 

68 



J 



BA8COM.] BASIC ERUPTIVE8. 69 

basalt (dolerite) aa one of the degree and granosity of the crystal- 
lization. It la easy to see that these features are determined by the 
geological conditions of consolidation. It follows from this that the 
essential characteristics of the rock groups are independent of age. 

A history of the classification of the gabbro and its allied groups 
and an exact statement of the final definition of these groups have been 
concisely given by Dr. Bayley.^ He suggests that the group of mela- 
phyres and augite-porphy rites will eventually be dispensed with, when 
the olivine diabases and the diabase ^'will take the position thus left 
vacant, and the plagioclase augite rocks will be found to occupy these 
places with respect to each other; the gabbros, the position of a deep- 
seated rock; the diabases, that of the corresponding holocrystalline 
effusive; and the basalt, that of the hypocrystalline equivalent." 
With this understanding of the use of the terms diabase and basalt, 
the South Mountain basic rocks fall into the diabase group. 

They are holocrystalline, effusive, plagioclase augite rocks, with or 
without olivine. They thus possess the characteristics of th^ augite- 
porphyrites and melaphyres (diabase group). They are so fine-grained 
as to appear homogeneous in the hand specimen, yet show no evidence 
in the thin section of an originally hypocrystalline character. There is 
no proof for or against devitrification. In the absence of such proof 
their present holocrystalline character will be recognized in their nomen- 
clature as primary. 

MELAPHYRES AND AUGITE-PORPHYRITES. 

DISTRIBUTION. 

The quartz-porphyries and aporhyolites in the Monterey district are 
limited to numerous small detached areas. The melaphyres and augite- 
porphyrites, on the other hand, occupy a large, irregular area, covering 
the valleys, the foothills, and the mountain flanks. Besides this area, 
which constitutes about one-half of the entire district, there are two 
small areas north of the old Maria Furnace, which are surrounded by 
the acid rocks, thus reversing the usual relation of the basic and acid 
eruptives. Along the State line and to the south of that line the dia- 
bases are massive or schistose, and inconspicuously amygdaloidal. In 
the Monterey district the amygdaloidal character of the diabases is their 
most marked feature. In the exposures on the Gettysburg Railroad 
narrow zones of inconspicuously amygdaloidal or nonamygdaloidal mel- 
aphyres grade above and below into conspicuously scoriaceous rocks. 

Basic igneous slates occur at the west end of the Gettysburg tunnel, 
where they grade into massive diabases. They also occur on Colonel 
BenchofPs place, at a locality just north of Gum Spring, on a line 
northeast of the Blue Ridge Summit station, and form a knoll south of 
the Fountaindale post-office. (See map, PI. I.) 



> The basic DMWMive rocks of the Lake Superior region: Jour, of Geology, July- August, 1893, Vol. 
I, No. 5, pp. 433-46«. 



70 VOLCANIC ItOCKft O^ SOUtrt MOUNTAm. tBtJLL.l36. 

There are a few other localities where slates occur. Where the slates 
were not studied in thin section their igneous origin has not been 
considered as proved. 

Ash beds, diabases crushed and recemented with epidote and quartz, 
are exposed along the Gettysburg Eailroad in the cuts west of the 
tunnel. 

A tuffaceous breccia, composed of fragments so rounded as to appear 
waterworn, was found at the head of Minie Branch. At the Eussel cop- 
per mine and a few other localities epidosites are abundant. At the 
former place they are evidently vein material, and carry the native 
copper. In other localities they undoubtedly represent the last stages 
of decomposition and alteration of the massive or more often of the 
tuffaceous diabases. 

MACR08C0PICAL DESCRIPTION. 

The augite porphyrites and melaphyres vary in color from a slate- 
blue or purple to all shades and tones of greei). Where epidote is the 
predominating alteration mineral the prevailing color is light yellowish- 
green; with chlorite or actinolite as the alteration products the color is 
a dark green. 

The most persistent and striking feature of the augite-porphyrites 
and melaphyres is their amygdaloidal character, to which allusion has 
already been made. 

Bowlders on the roadside and in the fields show a curiously rough 
and pitted surface, due to epidote or quartz amygdules brought out in 
relief by weathering. Sometimes the bowlders closely resemble con- 
glomerates composed of green or white oval pebbles; or the quartz 
amygdules, when perfectly spherical, mimic the spherulites of the 
acid rocks. 

The diabases (augite-porphyrites and melaphyres) are rarely mas- 
sive, usually schistose, sometimes slaty, and almost universally amyg- 
daloidal. 

As the amygdules quickly respond to pressure, they fdmishi a deli- 
cate test of the degree of schistosity present in the rock. Macroscopic- 
ally the schistosity is otherwise more or less obliterated by subsequent 
epidotization or chloritization. 

Genuinely massive diabases are exceptional in occurrence and lim- 
ited in extent. Occasionally a mass of rock has moved as a whole, 
under pressure, and thus close to schistose or even slaty diabase the 
rock may retain its massive character. In these cases, since there has 
been no shearing, the vesicles are often perfect spheres, showing uo 
elongation from magmatic movement. This is notably the case just 
north of Gum Spring, on the Old Furnace Eoad. Quartz amygdules 
show conspicuously as round white spots on the fresh surface of the 
rock, which is a dark blue-gray, or are brought out in relief on the 
weathered surface, giving the rock the appearance of being riddled 



BAscoM.] MELAPHYKES AND AUGITE-PORPHY RITES. 71 

with shot. In other localities there is ji j^^reater diversity both in the 
shape and in the composition of the amygdules. 

There is frequently a zonal arrangement of three minerals — epidote, 
chlorite, and quartz — the light-green epidote being developed on the 
edge of the vesicle and surrounding the chlorite and (jnartz, which are 
successively developed in the interior Where the vesicles are large, 
epidote sometimes occurs in beautiful radiating crystals, not com- 
pletely filling the vesicle, and occasionally associated with crystallized 
malachite. 

It is doubtless to these amygdules that Professor Kogers refers when 
he describes the " decidedly crystalline" primal slates as containing 
'^segregated specks and even half-formed geodes of epidote and other 
minerals," or "as gray slate spotted with epidote." Wherever there 
has been sufficient shearing to form a slate a micaceous mineral is formed 
in the vesicles. This micaceous mineral is either sericite, when the 
slate is conspicuously ornamented with greenish-white oval spots, or it 
is chlorite, when the slate is marked with brilliant dark-green oval 
spots. 

The nonamygdaloidal diabase is always more or less schistose and 
frequently slaty. At the second railroad cut beyond GladhiU's switch 
it hasa banded appearance, due to an alternation in color, purplish green, 
dark and light green rapidly succeeding one another. The rock is fine- 
grained. The form of the banding and the structure of the rocks as 
disclosed by the microscope suggest that the bands represent ash beds. 

At the west end of the tunnel there is a curious differentiation of 
the diabase in color and sensitivity to pressure. This difi'erentiation is 
limited to an irregular band which suggests in many ways an intrusive 
dike. The apparent dike traverses the nonamygdaloidal diabase in a 
direction oblique to the schistosity of the latter. At one end it diverges 
and sends out a branch which pursues a course vertically downward 
and disappears beneath the surface. 

Fragments of the schistose diabase are included within the dike. 
Its upper surface is somewhat amygdaloidal, the interior compact, and 
the lower surface bordered by a band of light-yellow epidote. This 
band is more irregular in outline than the upper surface. 

The dike is intersected by two systems of fine parallel, or approxi- 
mately parallel, quartz veins. Parallel to one of these systems is an 
easy cleavage. 

The diabase just above and below the dike is finely schistose. This 
schistosity is parallel to the course of the dike, and is particularly 
remarkable above the dike, where it follows every curve of the latter. 
The color of the dike is purplish, and contrasts with the surrounding 
dark-green diabase. 

The obliquity of this seeming dike to the general schistosity of the 
diabase, its inclusions of fragments of the surrounding schist, its 
divergent branches, and the foliation of the diabase i)arallel to the 



72 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull.136. 

band are all more readily explained on the supposition that we are deal- 
ing with a genuine igneous dike than in any other way. The structure 
of the rock under the microscope and its analysis (see Analysis III, 
p. 78) show that there is no essential diflference in these characters 
between the band and the schistose diabase which it traverses. Its 
chemical composition diiters only in the high percentage of iron which 
it carries. It is possible that for some cause there has been a local con- 
centration of iron (to which the color is due) within the limits of this 
band, which renders it harder than the surrounding diabase and enables 
it to resist pressure more successfully. Hence, while not yielding itself 
to the pressure which produced the schistosity of the diabase, it has 
also been the means of producing a foliation in the diabase parallel to 
itself. The only other tenable hypothesis is that it represents a later 
intrusive lava flow of the same general composition as that of the rock 
into which it was intruded. The manner in which it grades into a 
finely vesicular rock on its upper surface, and the inclusions of frag- 
ments of a green diabase, would be explained by this hypothesis. Its 
resistance to pressure would be due to the same cause in either case. 

In only a few instances do the diabases show a jwrphyxitical struc- 
ture apparent to the naked eye. The diabases have not infreqently 
suffered crushing, and are recemented with quartz, epidote^ and hema- 
tite, the former minerals predominating. Veins of asbestos with quartz 
occur in the more epidotic diabase. 

MICROSCOPICAL DESCRIPTION. 

Original structures. — There is a marked uniformity of structure and 
of mineral constituents in the South Mountain diabases. 

Unlike the aporhyolites, the porphyrites and melaphyres do not show 
the effects of magmatic movement. Their structure is universally the 
ophitic, which is produced only in a magma in a state of equilibriam. 
The vesicles also, as has already been noted, do not betray any fluidal 
movement. (PI. XXYIII, a.) 

Crystallization is fine-grained (see p. 69), corresponding to what has 
been called the '' microophitic." That originally this microophitic struc- 
ture was associated with and i)assed insensibly into the hyalopilitic is 
not impossible, although subsequent i)rocesses of alteration^ chief among 
which is silicification, have destroyed all trace of an unindividualized 
base. 

Shearing has obscured and sometimes obliterated the delicate ophitic 
structure through i)rocesses detailed later. Where dynamic action 
tbund relief in the crushing of the rock rather than in the production 
of a schist, the ophitic structure remains perfectly preserved in the rock 
fragments. The porphyritic structure is inconspicuoos. Among the 
nonolivinitic pori)hyrites intratelluric crystallization is nearly absonti 
Feldspar and augite phenocrysts are rare. This characteristic, togetbcr 
with the widespread development of the amygdaloidal ^tmcfcoreyallitf 
these rocks to Kosenburch's spilite type. 



BAsooM] MELAPHYRE8 AND AUGITE-PORPHYRITES. 73 

The olivinitic porphyrites, or true melaphyreSj contain olivine as a 
constituent of the groundmass as well as in the very plentiful porphy- 
ritical crystals. 

The distribution of the melaphyres is quite similar to that of the 
spilites. The history of the two types since consolidation has been the 
same, and they will be discussed together. 

The vesicular structure is a conspicuous feature of the melaphyres 
and spilites. They range from rocks almost as vesicular as a sponge to 
a compact rock containing only a few scattered vesicles. These vesicles 
are filled with material furnished by percolating waters, and a solid 
amygdaloid is formed. 

The mineral nature of the amygdules will be described under the 
secondary constituents. The vesicles vary in size from microscopic 
dimensions to 5 centimeters in length and 3 in breadth. They are very 
significant, both of the amount of shearing and of alteration present 
in the rocks which they characterize, and they have undoubtedly been 
a factor in determining the character of both processes (pp. 74-75). 

Secondary structures, — The micropoikilitic structure, as has been noted 
on page 49, is occasionally present. It is found in those melaphyres 
and spilites which have been thoroughly silicified by infiltration. The 
secondary nature of the structure is very plain. The original structure 
(the ophitic) is so well preserved, in spite of the replacement of the 
mineral constituents, that in ordinary light the altered character of 
the rock is scarcely apparent (PI. XIX, a and h). Polarized light at 
once betrays the extent of the alteration. 

Where the schistose character of the rock is pronounced in the hand 
specimen, it is also a marked feature of the thin section. The con- 
stituents, which in these cases are for the most part secondary, are 
arranged with their longest axes at right angles to the pressure. 

There has been so complete a recrystallization of the rock as to 
obscure its original character. It has been repeatedly pointed out that 
under conditions of pressure igneous rocks acquire a degree of schis- 
tosity which renders it almost impossible to determine their true char- 
acter and to distinguish authoritatively between foliated traps and 
metamorphosed slates (elastics). In the schists under discussion their 
relation to undoubted porphyrites leaves no room for doubt as to their 
origin, nor is their alteration so extended as has been described in 
other localities. The structure of the South Mountain porphyrites is 
usually far less altered than is the case with the greenstone schists of 
the Menominee and Marquette regions. 

Original constituents, — It is to be expected that these ancient rocks, 
comparatively soft and extremely vesicular, exposed as they have been 
to pressure, percolating waters, and weathering, should exhibit altera- 
tion. It is surprising that the alteration has not been so complete as 
to obscure altogether the original structures and constituents. The 
original constituents of the rock— plagioclase feldspar, augite, olivine, 
titaniferous magnetite — are either present in a comparatively fresh con- 



74 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

dilion or are represented by characteristic alteration products, which are 
often paramorphs of the original mineral. The former is only rarely 
the case, while the latter is the rule. 

The feldsi)ar occurs both as porphyritic crystals and as a constituent 
of the groundmass. The crystals of the first generation are fromO.C to 
0.8 millimeters in length to 0.2 millimeters in breadth. Those of the 
second generation are lath-shaped, 0.4 millimeters in length to 0.4 
millimeters in breadth, and condition the microophitic structure. They 
are both striated, and show undulatory extinction when any of the 
original substance remains. Usually the feldspars are altered to epi- 
dote and quartz, or they have been completely replaced by quartz, while 
their crystal outline is preserved by the iron constituent. 

It is doubtful whether augite is present otherwise than as a constitu- 
ent of the groundmass where it is allotriomorphic. It is universally 
replaced by the more stable amphibole minerals or by epidote or 
chlorite, and some porphyritic crystals of the latter minerals may 
represent augite phenocrysts. The chief alteration product of augite 
in the schistose porphyrites is actinolite. This mineral is not limited 
in its development to the augite outlines, and it thus obscures the 
ophitic structure. Olivine crystals of two generations are readily 
recognized by means of their characteristic form, their irregular frac- 
turing, and sometimes by their alteration products. Usually the olivine 
is altered to epidote. In a few cases (PI. XXVIII, h) the crystals are 
still sufficiently unaltered to resj)ond to the optical tests for olivine. 

There is a large amount of an opaque black oxide in the porphyrites. 
Where this was tested the powder was found to be magnetic. This 
fact, the crystal form of some of the oxide, and the analyses of these 
rocks point to the conclusion that much of it is magnetite, thougb 
undoubtedly very titaniferous. Occasional rhombohedral forms and 
cleavages indicate that ilmenite is also present. Both these minerals 
have given rise to an abundant development of leucoxene (PL 
XXVIII, Z^). 

In order of abundance the original constituents rank as follows: 
feldspar, augite, magnetite and ilmenite, olivine. 

Secondary constituents. — The processes of alteration have been greatly 
assisted by the open -textured, vesicular nature of the rocks^ and the 
mineral character of the amygdules is indicative of the character of 
the alteration of the rock mass. The vesicles are almost universally 
filled with one or two or all of three minerals: epidote, quartz, and 
chlorite. Whatever is the amygdaloidal filling is also the prevailing 
alteration mineral. If quartz fills the amygdules the rock mass is 
more or less completely silicified. The ophitic structure, while pre- 
served in outline, is replaced by the micropoikilitic. Quartz, titaniferous 
magnetite, leucoxene, and some epidote constitute the rock, which is 
distinguished in the hand specimen by its blue-gray color and white 
amygdules. 

One of the most common amygdaloi(ial fillings is epidote with a 



BAscoM.] MiELAPHYUES ANt) AlJGITK-PORPHYRITES. 75 

little quartz. In this case epidote is the predominating alteration 
product These rocks are recognized in the hand specimen by a light- 
green color and green araygdules. Feldspar, augite, and olivine have 
all been rephiced by epidote. Tlic material for this mineral has ^ 
undoubtedly been furnished by the interaction of feldspar and augite, 
and also has been bro.ught to the rock by percolating water from over- 
lying rocks. In the extreme phase of this alteration these rocks are 
true epidosites. 

In the presence of a larger amount of iron, actinolite is abundantly 
developed. There is also much free iron oxide, and the rock becomes 
dark green in color. Where there has been shearing movement, as in 
the case of the acid rocks, a micaceous mineral is developed. In the 
aporhy elites the mineral is sericite; in tlie porphy rites it is chlorite. 
In the incipient stages of the schistose structure chlorite occupies the 
center of the amygdules, with quartz and epidote filling the rest of the 
space. When the rock is so schistose as to be fairly called a slate the 
amygdules are represented by brilliant dark-green spots and consist of 
chlorite only. 

Chlorite in turn becomes the prevaling alteration i)roduct. Often 
none of the original constituents remain. Actinolite, chlorite, epidote, 
and secondary silica are the invariable constituents of the ^'spotted 
greenstone schists." The tictinolite and chlorite both blur the outlines 
of the original constituents and oblit(a\ate the original structure. These 
rocks are a medium green in color. 

The important secondary constituents of the porphy rites are quartz, 
epidote, actinolite, chlorite, and leucoxene. The prevalence of one or 
the other of these alteration products can be determined in the hand 
specimen by means of the color of the rock and the character of the 
amygdules. With reference to the character of the alteration which 
they have undergone, the melaphyres and spilites thus fall into the fol- 
lowing groups: 

1. A blue-gray rock with quartz amygdules which do not show 
shearing. Under the microscope it shows an ophitic structure well 
preserved, and a silicified groundinass. Localities: Near Gum Spring, 
on the Old Furnace road, on Minie Branch, and along the Gettysburg 
Bailroad. 

2. A light yellowish-green rock with epidote-quartz amygdules, and 
epidote as a prevailing constituent. The original structure is obscured. 
Localities: South of the Clermont House on the Gettysburg Railroad 
and at the Russel copper mine. 

3. A medium-green spotted schist. Chlorite is the prevailing min- 
eral. The original structure is more or less completely obscured. 
Localities: Along the State line at the west end of the tunnel and at 
many other places. This is a prevailing type of the porphyrite. 

4. A dark-green rock, more or less schistose. Epidote, quartz, and 
chlorite form the amygdules. Actinolite is abundant as an alteration 



76 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

product. Feldspar is often fresh and unaltered and the structure 
preserved. 

In the first three types actinolite may also be present, but not so 
abundantly as in the last. The first type passes into the second by 
increase in epidote, and the second type grades readily into the third 
by increase in chlorite. As this increase accompanies the development 
of schistosity, most of the schists belong in the third group. 

Types 3 and 4 are not sharply separated. Chlorite and actinolite are 
present in both. In the former chlorite predominates, and in the latter 
actinolite. 

Olivine may be present in any of the four groups. The crystal out- 
lines are best preserved in the first group; hence the ophitic structure 
is here best preserved; and it is most obscured in group 3, where orig- 
inal crystal outlines are lost. 

Group 1 contains the rocks which have been the least sheared, a fact 
which is perhaps accounted for by the silicified character of the rock. 

The peculiar dike-like band which traverses the basic eruptives at 
the west end of the tunnel is not unlike type 1 in color and compactness 
of texture. The color is several shades darker, and the specific gravity 
of the rock is greater. Under the microscope a further remsemblance 
is seen. The compact rock mass consists largely of titaniferous mag- 
netite (arranged in layers, or outlining obscurely an ophitic structure); 
chlorite, epidote, and quartz. 

The amygdaloidal selvage of the band shows the same constituents 
in an inverse proportion, and the ophitic structure is strongly marked. 
This was true in all the numerous thin sections made of the band. In 
the vesicular portion of the band the ophitic structure is well pre- 
served, and even olivine crystals with unaltered outline are present 
The amygdules are filled with quartz, granular and crystalline epidote, 
and chlorite. It is probable that the yielding of the vesicles, which 
would offer the least resistance to pressure, saved the rock. The 
tendency to a parallel arrangement of the feldspars on either side of 
the amygdules accords with this supposition. The passage of this 
band, which is sometimes only from an inch to two inches wide, into a 
green chlorite-schist or slate is very abrupt. The difference seems to be 
due to the presence in the band of a large amount of iron. Its chem- 
ical analysis, given on page 78, coincides with this view. 

Epidote and quartz are by far the most abundant and widely dis- 
tributed of the secondary minerals. This is true not only of the basic 
eruptives, but also of the acid eruptives when they are the prevalent 
alteration products and amygdaloidal filling, and where piedmontite, 
a member of the epidote grou}), occurs in macroscopic quantities. 
There seem to have been conditions favoring an extensive epidotization 
and silicification. Undoubtedly there has been ^vithin the rocks them- 
selves a mutual reaction between the decomposition prodacts of feld- 
spar and augite resulting in the production of epidote. The factthrt 



BAscoM.] MELAPHYRES AND AUGITE-P0RPHYRITE8. 77 

rarely some fresh feldspar still remains, while the augite is always 
decomposed, indicates that the decomposition products of augite have 
acted upon fresh feldspar, thus also develoi)in|? epidote. But the feld- 
spars and augite of the porphyrites under discussion will not account 
for all the epidote and quartz present in them, composing, as they do, 
the amygdules, and in many cases the entire rock. The dip of the 
foliation planes of the basic eruptives indicates a thickness formerly 
much greater. A large amount of erosion of the igneous rocks has 
occurred since they were elevated to their present i)osition. The water 
which percolated through this great thickness of igneous material 
brought with it the lime and alumina. 

It is plain that these processes of epidotizatiou and silicilication took 
place not only while the porphyrites were being elevated, but have con- 
tinued since the cessation of all dynamiti action. The filling of vents 
and cracks by these materials, the fresh, unschistose character of the 
epidote and quartz in vesicles which themselves show the effect of 
squeezing, the presence of granular epidote in the schists and slates, 
all lead to this conclusion. While this is true, there are, on the other 
hand, amygdules of epidote where the fan-shaped radiating crystals 
of epidote have been broken and i^ulled apart in consonance with the 
alteration in the shape of the vesicle, and the spaces thus formed filled 
with silica. 

The nonvesicular character of the acid eruptives has saved them 
from so extended an epidotizatiou as characterizes the basic eruptives. 
In the case of the amygdaloidal aporhyolites of the Bigham copper 
mine, the same conditions which obtained with the i)orphyrites have 
operated to effect with them an extended development of epidote. 
While there is so complete an alteration in mineral constituents, there 
is surprisingly little change in structure. 

In this respect the South Mountain basic eruptives are a contrast to 
similar greenstones of the Menominee and Marquette regions. A com- 
parative study of the greenstones of the two regions shows that the 
Lake Superior rocks, while more altered in structure, i^ossess feld- 
spars less altered than do the South Mountain greenstones. Calcite is 
much more abundant in the former rocks, and epidote in the latter, as a 
secondary product. 

Acessory minerals, — Copper occmrs in microscopic quantities in the 
amygdules of the basic eruptives, Just as it did in the amygdaloidal 
aporhyolites. This is true only of the aniygdaloids from the various 
copper- mine localities described on pages 25-27. At these localities the 
carbonates of copper, malachite and azurite, occur as thin stains. The 
former sometimes forms crystals of considerable size in the vesicular 
cavities (one-fourth inch). A silvery-green asbestos occurs in some 
abundance in quartz veins ^penetrating the basic eruptives. It is 
plainly a secondary product. A finely divided red hematite is some- 
times quite QORspicuous in the iuuygdules as a cementing material for 



78 



VOLCANIC ROCKS OF SOUTH MOUNTAIN. 



[BULL. 136. 



the crushed porpliy rites. It also occurs in crystalline form, and at a 
single locality (south of the Fouutaiudale turnpike, on the north flank 
of Haycock Mountain) micaceous hematite occurs in veins. Calcite is 
rare. It is occasionally present in the amygdules or as vein material; 
and in the case of a single specimen, broken from a roadside bowlder, it 
almost completely replaces the substance of the rock. 

The range of minerals, original, secondary, and accessory, found in 
the South Mountain rocks is a very limited one. 

DISCUSSION OF (CHEMICAL ANALYSES. 

The analyses tabulated below, with the exception of Ko. IV, are col- 
lected from analyses scattered through the publications of the Second 
Geological Survey of Pennsylvania. For Analysis IV the writer is 
indebted to the courtesy of Professor Daniells, of Wisconsin University; 

Table of analyses. 



SiOj. 
PjOs- 
AI2O3 

Fe,03 
FeO . 
TiO,. 



48.93 



15.42 
14.71 



CaO 

MgO..-. 
K2O .... 
Na.,0 ... 
Ignition 



18. IC 
2.78 



Total 



100.54 



II. 



48.02 

1.45 

17.84 

11.61 

.98 



18.25 



1.50 



III. 



41. 280 



18. 480 
9.440 
8.200 



7 040 
7.486 
2.208 
3.523 
2.740 



99.65 



100. 397 



ly. 



37.225 



i 44. 



82 



15.79 
1.14 



(•) 



98. 975 



{ 



37.03 

0.51 

24.13 



19.03 
4.05 
0.21 
1.44 
8.93 
Trace. 
3.54 



99.67 



1 Not determined. 

I. ^^Orthofelsite, containing epidote, llj miles west of Gettysburg."* 

II. ^'Epidotic rock, 2| miles from Mount Alto furnace." * 

III. '' Chloritic schist from Bechtel shaft." ^ 

IV. Differentiated band at the west end of the tunnel. ^ 

V. '* Variegated chlorite-schist Avitli chlorite (?), one-half mile northeast of Pine 
Grove." 1 

The characterization of these rocks by the Second Geological Survey 
is somewhat vague, and in the discussion of the analyses the writer 
is again hampered by the lack of hand specimens of the rock analyzed. 

The percentages are about those of the normal augite-porphjnites 
and melaphyres as given in Eoth's tables. They scarcely show as 
much variation as the altered augite plagioclase rocks of his tables. 

Although the analyses show some phosphorous pentoxide, no apatite 
was noted iu the microscopic study. The iron percentage is high in all 
of the analyses, though not abnormally so — not higher than the micro- 
scopic study w^ould lead us to expect. 

» Second Geological Survey of Pennsylvania, Vol. CCC, pp. 255-275. Analyses made by the late Pr- 
F. A. Gcnth. 

'Frazer: Ilypotbesia of the strncture of the copper belt of the South Mountain, p. 82: Tf'ans. Am. 
Inst. Mm. Eng.. VqI. XII, pp. 85-90. Analysis made by C. Hanford Hende^so^. 

» Analysis W^tlo by Prot W. W, ^)a^iell8, of Wisponsi^ University, 



BAscoM] BASIC SLATES. 79 

The *^epid.otic rock" (I and II) shows, as would be expected, an 
abnormally high lime percentage. 

The chief variation from the normal type lies in the addition of lime 
and iron oxide and the abstraction of the alkalies and magnesia. 

BASIC SLATES. 

DISTRIBUTION AM> DESCRIPTION. 

The localities where the basic slates occur in any considerable extent 
are colored a light yellow-green on the map of the Monterey district. 
The slight rounded eminence opposite the Fountaindale post-office is 
composed of a dark-gray crinkled and finely laminated slate. It is 
darker colored than the clastic slate of the region and is surrounded 
by the basic eruptives. The thin section shows very distinct traces of an 
ophitic structure. Iron oxide, chlorite, and sericite are the only con- 
stituents that can be determined. About three-fimrths of a mile north 
of Monterey station, on the road leading from the turnpike to the Old 
Furnace road, there is an exposure of a lighter-colored slate. The same 
constituents are found in this slate, with a larger proportion of sericite 
and the addition of leucoxene. The ophitic structure is barely discern • 
ible, and it is with considerable hesitancy that the slate is referred to 
the group of igneous rocks. Half a mile farther northeast, on the Old 
Furnace road, just beyond Gum Spring, occurs a light-gray spotted 
slate of undoubted igneous origin. It is not so finely foliated as the 
slates that have just been described, and much of the original structure 
remains. The constituents are the same as those of the last-mentioned 
slate. Leucoxene is more abundant and the ophitic structure is pro- 
nounced. 

The amygdules, which give the slates a spotted appearance, are com- 
posed of quartz, sericite, and some chlorite. These slates are related 
to the porphy rites of Grou]) I, some typical examples of which occur 
near by. 

At the west end of the Gettysburg tunnel, just above the iron-bear- 
ing band previously described, another spotted slate has been devel- 
oped from the basic igneous rock. In this case the slate is green, and 
the spots are a brilliant dark shade of the same color. Iron oxide, 
chlorite, epidote, and some silica are the constituents. The original 
structure is entirely obliterated. Some of the epidote grains faintly 
suggest olivinitic forms. Chlorite is the prevailing mineral. The spots, 
which as in the other slate are sheared amygdules, are formed of chlo- 
rite only. This slaty zone is only a few inches wide and passes some- 
what abruptly into a slightly schistose porphyrite. 

The first tunnel on the old Tapeworm Railroad, which was abandoned 
before an excavation was made, exposes a green slate which differs from 
the one just described only in its nonamygdaloidal character and in the 
greater abundance of epidote. The knoll northeast of Blue Eidge 



80 VOLCA.NI0 ROCKS OF SOUTH MOUNTAIN. [bull. 138. 

Sunmiit station, the roadway crossiDg the Gettysburg Eailroad in front 
of the Clermont House, and the hill to the east of the Fairfield-Fountain- 
dale road and north of the Fountaindale turnpike, are the other locali- 
ties where slates occur. While their association and their appearance in 
the hand specimen, which resembles that of the first two slates discussed, 
indicate an igneous origin, in the absence of thin sections that origin 
can not be considered beyond question. 

BASIC PYROCLASTICS. 

DISTRIBUTION AND DK8CRIPTION. 

The basic breccias of South Mountain may be classified as follows 
(1) Crushed i)orphyrites which have been recemented with epidote an^ 
quartz and sometimes brilliantly colored with red hematite; (2) TuflFa 
ceous breccia; (3) Ash. 

(1) Crushed porphy rites. — The crushed and broken porphyrites, whO 
perhaps not in a strict sense breccias, present a strikingly brecciate^ 
appearance. Fragments of all sizes, of a blue or purple-gray rock, an 
embedded in a bright-green and white or rose-colored matrix. The 
fragments have undergone either silicification or epidotization withoot 
affecting materially their structure (microophitic), which is outlined 
by iron oxide. 

(2) Tuffaceous breccia, — Some large bowlders found near the scarce 
of Minie Branch furnish the only unmistakable tuffaceous breccia. 
The fragments show a considerable range in size (see page 24) and are 
thickly crowded in a basic cement. As there has been no shearing, 
the structure of the fragments is i)erfectly preserved. (PI. XXVIII, «.) 
Epidote, quartz, and iron oxide are their present constituents. 

(3) Ash, — Above the Headlight copper mine on the Fountaindale 
turnpike and in the fourth cut beyond Monterey station (northeast), on 
the Gettysburg Eailroad, are intercalated bands of a light-green rock 
which, for the following reasons, have been considered altered ash: 
At the west end of this cut a fine-grained homogeneous rock is striped 
with alternating bands of light green and reddish green. Under the 
microscope these bands show no trace of any structure save a slight 
schistosity. They are composed almost wholly of angular grains of , 
epidote, magnetite and leucoxene, actinolite, chlorite, a^nd quartz. The ! 
difference in color is due to the presence in the reddish bands of rod 
iron oxide. Toward the eastern end of the same cut the whole face of 
the rock is banded with light-green epidotic layers from 1 foot to 2 j 
feet wide, running approximately parallel to one another. 

A microscopic slide of one of these bands consists wholly of granular 
epidote and quartz, with a little iron oxide, usually the red oxide. 

These rocks overlie and are in close proximity to scoriaceous basic 
lava. This fact, together with their variation in color and their struc- 
tureless and fragmental character, is very suggestive of an altered ash. 
At the first locality mentioned, the Headlight copper mine, the 9/h 



1 



MBOOM] SUMMARY. 81 



I 

I 

ii 
11 

i 



called ash occars as a light-green schist. It is not banded, but pre- 
sents under the microscope the same structureless character as the 
rocks abovft described. It consists of actinolite blades and needles, 
epidote granules, and some chlorite, magnetite, and leucoxenc. 

SUMMARY. 

The basic igneous rocks display but little variety of structure or 
mineral constitution. The former is that common to porphyrites and 
•melaphyres — the microophitic — and in spite of great alteration in the 
'mineral constituents of the rocks it still remains a marked structure. 
^^Bhearing obscures it, but in the extreme form of the sheared porphy- 
■rite — ^the slate — ^it is still discernible. 

The formation of chlorite and actinolite tends to confuse outlines; 
hence some of the slates in which there is not much of those minerals 
preserve their original structure better than the chlorite-schists. The 

I original mineral constituents, plagioclase, feldspar, augite, and olivine, 
have almost completely disappeared. Xo augite remains; olivine crys- 
tals are well preserved in outline, and sometimes a core of the original 
mineral remains. There is considerable feldspar still unaltered. It is 
always striated, but the crystals are too small to allow of an accurate 
determination of their character. That they belong to the basic end 
of the series is shown by their extended alteration to epidote, and by 
^ the chemical analyses of the rocks. 

The vesicular character of these rocks has aided in the extended 
replacement of their original minerals, and the amygdules are an index 
of the character of that replacement. The presence of vesicles has also, 
I doubtless, been a factor in preserving the internal structure of the rock 
in spite of dynamic action. Silicification, epidotization, and chloritiza- 
tion are the processes of alteration which have been most active. 

The source of material is twofold — from the rocks in which these pro- 
cesses have been described, and from the overlying rocks which have 
been removed by erosion. 
Bull. 136 6 






I 
1 1 



CHAPTER VI. 

SUMMARY OF CONCLUSIONS. 
EVIDENCE OF THE ERUPTIVE CHARACTER OF THE TWO ROCKS. 

The preceding chapters have treated in detail structural and chemical 
features possessed by the South Mountain rocks and characteristic of 
igneous rocks o)ili/. These are regarded as sufficient evidence of tlie 
igneous origin of the rocks which they characterize, without further 
proof on that point. Jt only remains to show on what grounds a sedi- 
mentary origin has been attributed to them, and to sum up the evidence 
against such an origin. 

FIELD KVIDKNCK. 

Schistosiiy, — The conformity of the foliation planes of the porphyries, 
the aporhyolites, and the porphyrites, with the foliation planes of the 
Cambrian sediments is a prominent and persistent feature, and one 
which, among others, has undoubtedly led to the ascription of a sedi- 
mentary origin to the former rocks. The confusion of foliation planes 
and bedding planes in the quartzite, owing to the obscurity of the 
latter, added force to this argument. 

This conformable schistosity is not, of course, inconsistent with the 
igneous origin of the underlying rocks. The schistosity is a secondary 
feature, x)roduced by forces which aii'ected the igneous and aqueous 
rocks alike. 

The cleavage is quite as plainly secondary in the clastic rocks as in 
the nonclastic, and sometimes conforms to the bedding and sometimes 
does not. 

Lamination. — An original lamination, conspicuous in the aporhyo- 
lites, the nature of which was described on pages 43-44, characterized 
as bedding by Hunt and others, doubtless furnished another reason for 
attributing stratification and an aqueous origin to the rocks possessing 
it. The real nature of the lamination has proved to be such that it 
becomes an evidence of the igneous character of the rocks in which it 
occurs. 

That the lamination is due to bands of spherulites has been pointed 
out. True spherulitic crystallization, such as has been described in 
these aporhyolites, has thus far been known only as the product of crys- 
tallization from a molten magma. 

The slates. — The slaty character of the rocks has been another reason 

for assigning a sedimentary origin to them. 

The slates, both acid and basic, have been considered clastic slates 
by Professors Rogers and Lesley, by Frazer, Tyson, Blandy, and Dr. 

82 



BAscoM.] SUMMARY OF CONCLUSIONS. 83 

Hunt, as the quotations from these writers, given iu Chapter 1, show. 
While the igneous slates of South Mountain do not resemble any of 
the Cambrian sediments of that region, their resemblance to i)orphy- 
roids from regions where there has been extended metamorphism is 
very great. This resemblance is internal as well as external, and fur- 
nishes an instance of tlie production of essentially similar results by 
either of two difterent methods. 

The true nature of these slates and the manner of their production 
are conclusively revealed throngli held evidence. Where a single expo- 
sure shows a shear zone of not more than liO feet in which every gra- 
dation from a porphyry to a fissile slate is displayed, or where a single 
hand specimen shows such a metamorphism, the evidence of such a 
genetic relationship is irrefutable. 

Absence of gradation betivven ufneouH and clastic rocks, — There is, on 
the other hand, no such gradation between the igneous rocks and 
undoubted elastics as might be expected if the former were metamor- 
phosed elastics. We have holocrystalline rocks sharply separated 
from uoncrystalline elastics, with an entire absence of intermediate 
stages. 

Professor Eogers was impressed with the high degree of metamor- 
phism which these rocks must have undergone in order to attain their 
present holocrystalline character. ''A gray siliceous altered rock," "a 
compact siliceous altered slate," are the terms he uses to describe the 
porphyries and aporhyobtes, while he speaks of the porphyrites as 
"primal slate in a highly metamorphic condition" and "highly altered 
greenish slate." The sediments and the igneous rocks have been sub- 
jected to the same dynamic forces, and, as a matter of fact, we find 
one no more highly metamorphosed than the other relatively to their 
respective powers of resisting alteration. 

Surface-flow features, — Positive field evidence for the nonsedimen- 
tary origin of these rocks is found in the features which they possess in 
common with surface flows. Their vesicular, scoriaceous, and pumi- 
ceous character, the accompanying xjyroclastics, their tiow structures, 
even grain, conchoidal fracture, and other characteristics of a glassy 
lava all testify to an eruptive origin. 

PETROGUAPIIICAL EVIDENCE. 

Structural, — The petrographical evidence of the origin of the porphy- 
ries, aporhyolites, and i)orphyrites is of an even more unmistakable 
character. 

Their porphyritic structure is indicative of their origin. Olivine and 
feldspar phenocrysts with crystalline outlines, idiomorphic quartz with 
embayments and edges rounded by magmatic corrosion, are possible 
only in rocks which have once been molten. 

The ophitic structure, preserved in great perfection in the porphy- 
rites, is peculiar to rocks which have consolidated from a molten magma. 



84 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

Other structures wliicli furnish additional and convincing proof of an 
igneous origin need only be mentioned. A detailed description of their 
appearance in the aporliyolites and porphyrites has been given in the 
previous chapters. Such structures are the spherulitic, axiolitic, lithe- 
physal, perlitic, rhyolitlc, lluidal, and amygdaloidal. 

MiheralogicaL — Metamorphosed sedimentary rocks are always accom- 
panied by certain characteristic minerals. The absence of such minerals 
in these South Mountain rocks is conspicuous. Epidote and sericite 
are the only prominent alteration minerals. Of these, the former is the 
product of weathering rather than a true metamorphic mineral; the 
latter is more or less limited to shear zones, where its development is 
directly related to the dynamic force acting upon massive porphyries 
and aporhyolites. The absence of all evidence of contact action indi- 
cates their eftusive character. 

Chemical, — The close conformity of the composition of these rocks in 
the one case with that of the rhyolites and in the other with that of the 
diabases and melaphyres from all parts of the world, as tabulated by 
Both, indicates an igneous origin. Their uniform composition is a con- 
trast to the composition of a series of clastic rocks, where the chemical 
proportions are largely a matter of accident. A similar test has been 
applied by Rosenbusch ^ to the determination of the origin of Archean 
gneisses. The association of these types of acid and basic lava accord 
with the laws of petrograi)hical consanguinity. 

ORIGINAL ROCK TYPES. 
ACID IGNEOUS ROCKS. 

The acid-lava flows in South Mountain are regarded by the writer as 
quite comparable, at the time of their consolidation, to similar flows in 
post-Tertiary time, such, for instance, as those which have been recently 
studied in the Yellowstone National Park. Certain portions of the flow, 
as in the case of the Obsidian Clift*, were completely vitreous save for 
spherulitic and lithophysal crystallization. In other localities the lava 
was lithoidal, and in the central portion of thick flows holocrystalline. 
In this way three types of acid volcanics would be developed— 
rhyolites, lithoidal rhyolites, and quartz-porphyries. Every gradatiou 
between these tyi)es would accompany these. 

Thus, while there are certain areas in the South Mountain, notably 
the Bigham Copi)er Mine and Kaccoon Greek localities, which exhibit 
typical ancient rhyolites, other regions display genuine qnartz-por- 
phyries. While in the latter rocks, which constitute a not inconsider- 
able portion of the acid flows, the groundmass may have been, and prob- 
ably was, originally holocrystalline, as in some modern lavas, in the case 
of the former rocks it is supposed that the groundmass was, at the time 
of consolidation, wholly or partly glassy. 



'Zur Auffassung der cUomiscUeu Katur des Gruud^eblrgos ; Tscbermaks miaenl. MittheiL, Vo^ 
XU,1891,pp.49-ei. 



BAscoM.] SUMMARY OF CONCLUSIONS. 85 

Chief amoDg the processes of alteration which have been going on 
since that time is devitrification. Ont of glassy and lithoidal rhyolitea 
devitrification has been developing aporhyolites. This process consists 
in the replacement of whatever glassy base was present in the original 
rock by a uniform quartz-feldspar mosaic. Sometimes the alteration is 
carried still further, and the original spherulitic crystallization is also 
replaced by this secondary granular crystallization. Earely, if ever, 
does the secondary crystallization completely obscure the former char- 
acter of the rock, while often all of the structures peculiar to a fresh 
glassy lava are retained. 

The other processes of alteration, which all of the original rock 
types have undergone in some slight degree, and some of them in an 
extreme degree, are the processes of sericitization and epidotization. 
The former process has been a chief factor in the development of slates 
from massive porphyrites and aporhyolites. 

These three processes of alteration — devitrification, sericitization, 
and epidotization — represent statical metamorphism, dyna;mic meta- 
morphism, and weathering, respectively. 

BASIC IGNEOUS KOCK8. 

In contrast with the acid rocks, the basic rocks have their original 
constituents so completely replaced that it is not easy to determine the 
original type or types. The original constituents were plagioclase, pyr- 
oxene, olivine, ilmenite, and magnetite. The original structures were 
the microophitic, the i)orphyritic (inconspicuous and mostly confined to 
the olivine-bearing type), and the amygdaloidal, a universal structure. 

In view of these constituents and structures, these rocks have been 
regarded as members of the diabase or augite-porphyrite group and of 
the olivine-diabase or melaphyre group. The augite-porpbyrites resem- 
ble the spilites in their scanty pori^hyritic crystals, their ever-present 
inclination to the amygdaloidal structure, and their susceptibility to 
weathering. 

The character of the alteration which has taken place in these mela- 
phyres and spilites varies with the amount of shearing which has 
occurred. Where shearing has been a factor in the alteration, chlorite, 
actinolite, and quartz replace th.e pyroxene and plagioclase. 

In the absence of shearing, epidote has resulted from the interaction 
of plagioclase and pyroxene. Olivine has altered to epidote, serpen- 
tine, and iron oxide; ilmenite has altered to leucoxene, and magnetite to 
hematite, when altered at all. In the absence of shearing the ophitic 
structure is preserved in outline, although sometimes the micropoiki- 
litic is added to it through the infiltration of silica; with the presence 
of shearing the development of chlorite and actinolite has obliterated 
the original structure and produce^l the schistosity characteristic of 
chlorite-actinolite rocks. 



86 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bcll.136. 

SIMILAR ROCKS IN OTHER REGIONS. 

These Soutli Mountain volcanics form a part of a belt of similar 
rocks which liave been recently recopiized along the Eastern Coast of 
the United States and Canada. 

Such vohmnics have been described in New Brunswick by the Cana- 
dian geologists — Bailey/ Matthew,^ and Ells.^ 

More recently, in the Sudbury district, similar rocks have been ob- 
served by Bell.^ They have been described in Maine by Shaler,*** and are 
recognizable in Canada, Maine, and New Hampshire through the 
writings of Hunt, Jackson^ and Hitchcock, although they were other- 
wise interi)reted by these observers. 

Spherulitic volcanics have recently been definitely recognized by W. 
S. Bayley^ at Vinal Haven, Maine, and have been studied in detail by 
Mr. G. O. Smith, of Johns Hopkins University. 

They have been identified in the Boston Basin by Wadsworth" and by 
Diller,^ who has studied them in some detail. The thin sections loaned 
by the latter for comparative study have already been mentioned as 
showing a marked similarity to the South Mountain acid volcanics. 

The continuation of the South Mountain volcanics in Maryland and 
Virginia has been studied by Keith.^ Similar volcanics have been found 
in North Carolina by Professor Williams,^" in South Carolina by Lieber,^^ 
and in Georgia by Professor Pirsson.^^ 

In Canada, Maine, in the neighborhood of Boston, and in Missouri" 
the felsites were, like the South Mountain rocks, first regarded as sedi- 
mentary in origin, and have only recently been identified as volcanic. 

With continued petrographic investigation of the pre-Cambrian rocks 
of North America volcanics may yet be recognized at other points where 
the rocks have been interpreted as sedimentary. 

In the Lake Superior region they have long been known through the 
writings of Irving and others, and their extent in that region has 
recently been still further enlarged.^^ 



'Bailey, Report on the pre-Silurian rocks of South Now Brnnswick : Kept. Can. Geol. Survey, 1877-78 
D. D. 

^Bailey, Matthew, and Ells, Report on Southern New Brunswick : Rept. Can. Geol. Survey, 1878-79. 
•Ells, Volcanic rocks of Northern New Brunswick: Rept. Can. Geol. Survey, 1870-80, D. 
<Ibid., 1889-00, F, 1891. 

^Shaler, Cobscook Bay, Maine: Am. Jour. Sci. (3), I'^l. XXXII, 1886, p. 40. Mount Desert: Eighth 
Ann. Rept. U. S. Geol. Survey, 1886-87, pp. 1043, 1054. 
«Bull. Geol. Soc. Am., vol. 6, 1894, pp. 474-476. 

'Wadsworth : Bull. Mus. Comp. Zool. Harvard Coll., Vol. V, No. 13, p. 282. 
'Diller, op. cit. 
•Keith, op. cit. 

"For full statement of distribution of volcanic rocks on Atlantic Coast, see paper by Professor Wil- 
liams in Jour, of Geology, Vol. II, No. 1, pp. 1-31. 
"Lieber, Report on the survey of North Carolin.a, 1856, 2d ed., 1858, p. 31. 
^*A section of a Georgian fol.site, loaned by Professor Pirsson, has already been alluded to. 
"Haworth: Am. Geologist, Vol. I, 1888, p. 280; Bull. Missouri Geol. Survey, No. 5. 
J4U. S. Grant, Volcanic rocks in the Keewatin of Minnesota: Science, Vol. XXIII, Jan. 12, 18M, 
p. 17. 



BAscoM.] LITERATURE. 87 

The reported rarity of volcanic action' in America in ]>reCambrian 
times is perhaps more apparent than real, ami is dnc ratlier to the failure 
to recognize the results of such action than to the actual absence of vol- 
canic action. 

LITERATURK. 

A list of papers in which points of resemblance between ancient and 
modern acid volcanics have been emphasized, or in which devitrilica- 
tion has been described, and a list of articles on the nature and origin 
of spherulites, are appended. 

PAxERS ON ACID VOLCANICS AND DKVITKIFICATION. 

AUport, S. On the microscopic study of the pitchst(»nes and felrtites of Arran. 
Geol. Mag., Vol. IX, 1872, pp. 536-545. 

. On ancient devitrilied pitchstones and perlites from the Lower Silurian dis- 
trict of Shropshire. Quart. Jour. Geol. Soc, Londou, Vol. XXXIII, 1877, p. 449. 

Bayley, "W. S. Spherulitic volcanics at North Haven, Maine. Bull. G«ol. Soc, 
Am., Vol. VI, 1894, pp. 474-476. 

Blake, J. F. On the felsites !ind conglomerates hetween Uethesda and Llanllyini, 
North Wales. Quart. Jour. Geol. Soc, London, Vol. XLIX, 1893, pp. 441-465. 

Bonney, T. G. On certain rock structures as illustrated by pitchstones and fel- 
sites in Arran. Geol. Mag., Vol. IV, 1877, pp. 499-511. 

. Note on the felsite of Brittadon, North Devonshire. Geol. Mag., Vol. V, 1878, 

• pp. 207-209. 

. On some nodular felsites in the Bala group of North Wales. Quart. Jour. 

Geol. Soc, London, Vol. XXXVIII, 1882, p. 289. 

Bomemann, T. G. Die Quart zporphy re von Heiligcnstoin uud seine Fluidalstruc- 
tur. Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XXXIX, 1887, p. 793. 

Chapman, P. On a method of producing perlitic and pumaceous Htructures in 
Canada balsam. Geol. Mag., Vol. VII, 1880, p. 79. 

Clements, J. Morgan. The volcanics of the Michigamme District of Michigan. 
Jour. Geol., Vol. Ill, No. 7, Oct.-Nov., 1895, p]). 801-822. 

Cole, Grenville A. J. On the artificial i)roduction of the perlitic structure. 
Geol. Mag., Vol. VIII, 1880, p. 115. 

. On hollow spherulites and their occurrence in ancient British lavas. Quart. 

Jour. Geol. Soc, London, Vol. XLI, 1885, p. 162. 

. On the alteration of coarsely spherulitic rocks. Quart. Jour. Geol. Soc, 

London, Vol. XLII, 1886, p. 186. 

. On the devitrification of cracked and brecciated obsidian. Min. Mag., 

Vol. IX, No. 44, 1891, pp. 272-274. 

Cole, Grenville A. J., and Butler, G. "W. On lithophysio in the obsidian of the 
Rocche Rosse, Lipari. Geol. Mag., Vol. IX, 1892, p. 488. 

. On the lithophysa*. in the obsidian of the Rocche Rosse, Lipari. Quart. 

Jour. Geol. Soc, London, Vol. XLVIII, 1892, p. 438. 

Davies, Thos. Preliminary note oil old rhyolites from Bouley Bay, Jersey. Min. 
Mag., Vol. Ill, 1880, pp. 118-119. 

Diller, J. S. Felsites and their associated rocks north of Boston. Proc Boston 
Soc. Nat. Hist., Vol. XX, 1880, pp. 355-368; Bull. Mus. Comp. Zool. Harvard Coll., 
Vol. VII, 1881, pp. 165-178. 

Futterer, Karl. Der Ganggranit von Gros-Sachsen und der Quartzporphyr von 
Thai im Thiiringer Wald. Inang. Dissor., 1890. 



> Dana, J. D., Manual of Geology, 4th ed., 1895, p. 938. 



88 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 

Hague, Arnold, and Iddings, J. P. The development of crystallization in the 
igneous rocks of Washoe, Nev., with notes on the geology of the district. Bull. 
IT. S. Geol. Survey, No. 17, 1885. 

Harker. Bala volcanic series of Caernarvonshire and associated rocks. Sedg- 
wick prize essay for 1888. Cambridge, 1889. 

Hatch, F. H On the Lower Silurian felsites of the southeast of Ireland. Geol. 
Mag., Vol. VI., 1889, pp. 545-549. 

Irving, R. D. The copper-bearing rocks of Lake Superior. Men. U. S. Geol. Sur- 
vey, Vol. V, 1883. 

Judd, J. "W. On composite dikes in Arran. Quart. Jour. Geol. Soo. London, Vol. 
XLIX, 1893, pp. 536-565, pp. 546, 551, 560. 

Kalkcwsky, Ernst. Mikroskopische Untersuchungen von Felsiten und Pech- 
steinen Sachsens. Tschermaks mineral. Mittheil., 1874, pp. 31, 58. 

Klockmann, F. Der Geologische Aufbau des sogen. Magdeburger Uferrandes 
mit besonderer BerUcksichtigung der auftretenden Eruptivgesteine. Jahrbuch K. 
preuss. geol. Landesanstalt, Vol. XI, 1890, pp. 171-203. 

La Croix, Alf. Comptes-rendus, CXI, p. 71. 

Lang, Heinrich Otto. Grundriss der Gesteinekunde^ 1877, p. 43. 

L6vy, A. Michel. Caract^res microscopiques des roches anciennes acides, con- 
sid^r^s dans leurs relations avec TAge des ^ruptives. Bull. Gdol. Soc, France, Feb., 
1875. 

Lossen, K. A. Beitriige zur Petrographie der plutonlschen Gesteine. Abhandl. 
K. Akad. Wiss. zu Berlin, 1869, p. 85. 

McMahon, Lieut. Gen. C. A. Notes on some trachytes, metamorphosed tuffs and 
other rocks of igneous origin on the western flank of Dartmoor. Qaart. Jour. Geol. 
Soc, London, Vol. L, 1894, p. 338. 

Milch, L. Beitriige zur Kenntnis des Verrucano, Leipzig, 1892; Part 2, 1896. 

Miigge, O. Untersuchungen Uber die " Lenneporphyre " in Westfalen und den 
angrenzenden Gebieten. Neues Jahrb. fiir Min. Geol. u. Pal., Vol. VIII, 1893, pp. 
534-719. 

Nordens^old, Otto. Zur Kenntniss der sogen. Halleflinta des nordostlichen 
SmSlands. Bull. Geol. Inst., Upsala, No. 1, Vol. I,- 1893. 

. Ueber archa^ische Ergussgesteine aus Sm^land. Bull. Geol. Instit. Upsala, 

No. 2, Vol. 1, 1893, pp. 1-127. 

08ann,'A. Zeitschr. Deutsch. geol. GeselL, Berlin, Vol. XLIII, 1891, pp. 691, 716. 

Rosi'wal, A. Petrogr.iphische Notizen iiber Eruptivgesteine aus dem Tejrovieer 
Cambrium. Verhaudl. K. k. geol. Keichsanstalt, 1894, p. 210. 

Rutley, P. Perl i tic and sperulitic structures in the lavas of Glyder Fawn. Quart, 
Jour. Geol. Soc, Loudon, Vol. XXXV, 1879, p. 508. 

. On the microscopic structure of devitrified rocks from Beddgelert and Snow- 
don. Quart. Jour. Geol. Soc, London, Vol. XXXVII, 1881, p. 403. 

. The microscopic character of the vitreous rocks of Montana. Quart. Jour. 

Geol. Soc, London, Vol. XXXVII, 1881, pp. 391-402. 

. On strain in connection with crystallization and perlitic structare. Quart. 

Jour. Geol. Soc, London, Vol. XL, 1884, pp. 340-346. 

. Felsitic lavas of England and Wales. Geol. Survey of England and Wales, 

1885. 

. Notes on alteration induced by heat in certain vitreous rocks. Proc. Royal 

Soc, London, No . 245, 1886. 

. On some eruptive rocks from the neighborhood of St. Minver, Cornwall. 

Quart. Jour. Geol. Soc, London, Aug., 1886, pp. 392-401. 

. On the rocks of the Malvern Hills. Quart. Jour. Geol. Soc., London, Vol. 

XLIII, 1887, p. 499. 

. On perlitic felsites and on the origin of some epidosites. Quart. Jour. Geol. 

Soc, London, Vol. XLIV, 1888, pp. 740-744. 



1 

J 



BAscoM.] LITERATURE. 89 

Rutley, F. On taohylite from Victoria Park, Whiteinch, near Glasgow. Quart. 
Jour. Geol. Soc., London, Vol. XLV, 1889, pp. 623-632. 

. On composite spherulites in obsidian from Hot Springs, near Little Lake, 

Colorado. Quart. Jour. GooUSoc, London, Vol, XL VI, 1890, pp. 423-428. 

. On some of the melaphyres and felsites of Caradoo. Quart. Jour. Geol. Soc, 

London, Vol. XLVU, 1891, pp. 534-544. 

. On a spherulitic and perlitic obsidian from Pilas, Mexico. Quart. Jour. 

Geol. Soc, London, Vol. XXVII. 1891, pp. 530-533. 

. On the sequence of perlitic and spherulitic structures : A rejoinder to a criti- 
cism. Quart. Jour. Geol. Soc, London, Vol. L, 1894, p. 10. 

Rutley, F., and Herman, D. On the microscopic character of some specimeiis 
of devitrified glass, with notes on certain analogous structures in rocks. Proc. Royal 
Soc, London, No. 239, 1885, pp. 87-107. 

Sederholm, J. J. Studien fiber archseische Eruptivgesteine aus dem sUdwestlichen 
Finland. Tsch. Min. Mith., Vol. XII, 1891, pp. 98-141. 

Smith, G. O. The volcanic series of the Fox Islands, Maine. Johns Hopkins 
University Circulars No. 121, Oct., 1895. 

De la Vall^e-Pousain, Chas. Lea anciennes rhyolites, dites eurites, de Grand- 
Manil. Bull. Acad. roy. Belgique, Vol. X, 1885, pp. 253-315. 

. Les eurites quartzeuses (rhyolites auciennes) de Nivelles et des environs. 

Bull. Acad. roy. des Soi. et des Lettr. et des Beaux- Arts de Belgique, Vol. XIII, 1887, 
pp. 498-535. 

Watts, "W. "W. Note on the occurrences of perlitic cracks in quartz. Quart. Jour. 
Geol. See, London, Vol. L., 1894, p. 367. 

. On perlitic structure. Geol. Mag.. Dec. IV, Vol. Ill, 1896, pp. 15-20. 

Woods, Hemy. The igneous rocks of the neighborhood of Biulth. Quart. Jour. 
Geol. Soc, London, Vol. L, 1894, p. 566. 

Vogel, Christoph. Die Quartzporphyre der Umgegend Von Gross-Umstadt. 
Abhandl. der grossherzlich hessichen geol. Landesanstalt zu Darmstadt, Vol.' II, 
Part I, 1891, pp. 1-52. 

Vogelsang, H. Philos. d. Geologie, 1867, pp. 144, 153, 194. 

Wadsworth, M. E. Notes on the mineralogy and petrography of Boston and 
vicinity. Proc. Boston. Soc. Nat. Hist., Vol. XIX, 1877, p. 236. 

. On the classification of rocks. Bull. Mus. Comp. Zool. Havard Coll., Vol. 

V, No. 13, 1879, p. 282. 

Zirkel, Ferdinand. Mikroskopische Uutersuchungen der glasigen und halb- 
glasigen Gesteiue. Zeitschr. Deutsch. geol. Gesell., Berlin, Vol. XIX, 1867, p. 784. 

PAPERS RELATING TO SPHERULITES. 

Brogger, "W. C. Die Mineralien der Syenitpegmatitgange, etc. Zeitschr. fiir 
Kryst. u. Min., Vol. XVI, 1890, pp. 552-553. 

Cohen, IS. Gottingsche gelehrten Anzeigeu, 1886, p. 915. 

Cole, Grenville A. J. On hollow spherulites and their occurrence in ancient 
British lavas. Quart. Jour. Geol. Soc, London, Vol. XLI, 1885, pp. 162-169. 

Cross, "Whitman. On the occurrence of topaz and garnet in the lithophysse of 
rhyolite. Am. Jour. Sci. (3), Vol. XXXI, 1886. p. 432. 

. The constitution and origin of spherulites in acid eruptive rocks. Bull. 

Philos. Soc, Washington, Vol. XI, 1891, pp. 411-444. 

Dana, B. S. Contributions to the petrography of the Sandwich Islands. Am. Jour. 
Sci. (3), Vol. XXXVII, 1889, p. 441-467. 

. Characteristics of volcanoes, 1891, p. 318. 

Delesse, IS. Recherches sur les roches globuleuses. M^m. de la Soc g^ol., France, 
Vol. IV, 1852, pp. 301-364. 

HolBt, N: O., and Bichstadt, P Klot Diorit Slattmassa. Geol. Foreningens 
Stockholm Forhandl, Vol. VII, 1884, p. 134. 



90 VOLCANIC RO€KS OF SOUTH MOUNTAIN. [bull. 136. 

Iddings, J. P. On the occurrence of fayalite in the lithophysas of obsidian mid 
rhyolite in the Yellowstone National Park. Am. Jonr. Sci. (3); Vol. XXX, 1885, 
pp. 58-60. 

. Obsidian Cliff, Yellowstone National Park. Seventh Ann. Rept. U. S. Geol. 

Survey, 1885-86, pp. 249-295. ^ 

. The nature and origin of lithophysau, and the lamination of acid rock. Am. 

Jour. Sci. (3), Vol. XXXIII, 1887, pp. 36-45. 

. Spherulitic crystallization. Bull. Philos. Soc, Washington, Vol. XI, 1891, 

pp. 445-464. 

Idding;s, J. P., and Penfield, S. L. The occurrence of fayalite in the litho- 
physae of the obsidian from the Lipari Islands. Am. Jour. Sci. (3), 1890, pp. 75-78. 

Johnston-LaviSy A. J. Note on lithophyssB in obsidian of the Rocche Rosse, 
Lipari. Geol. Mag., Vol. IX, 1892, p. 488. 

Judd, J. "W. Contributions to the study of volcanoes. Geol. Mag., Decade II, 
Vol. II, 1875, p. 65. 

Langorin, A. Ueber die Natur der Glasbasis, sowie der Krystallizationvorgiinge 
in Eruptiven Magma. Tschermaks mineral. Mittheil., Vol. VIII, 1827, p. 440. 

Lehmann, O. Molecular Physik, Vol. I, 1888, pp. 378-390. 

L6vy, Michel, A. Sur divers ^tats fabulaires de la silica. Boll. Soc. g^ol. 
France, Vol. Ill, 1875, p. 140. Comptes-rendus Acad. Sci., Paris, Vol. XXII, 1876 

. Sur uue nouvelle 6tat fabulaire du quartz suivant une senle orientation 

crystallographique. Comptes-rendus Acad. Sci., Paris, Vol. XXVII, 1876. 

. M^moire sur les divers modes de .structure des roches eniptives ^tudi^es an 

microscope an moyen de plaque minces, Ann. Min., Vol. VIII, 1876, p. 337. Comptes- 
rendus Acad. Sci.. Paris, Vol. VIII, 1875. Ref. Science, No. 21, 1871, p. 502. 

. Des differentes forms de spherulites dans les roches ^ruptives. Bull. Soc. 

geol. France, Vol. V, 1877, p. 257. 

. Sur la nature des spherulites paisant partie integrale des roches ^ruptives. 

Comptes-rendus Acad. Sci., Paris, Vol. XCIV, 1882, p. 464. Ref. Neues Jahrbuch fiir 
Mineral., Vol. IV, 1882, p. 381. 

. Structure et classification des roches <^ruptives, 1889, p. 20. 

McMahon, Lieut. G«n. C. A., and Hutchings, "W. Maynard. Note on pseudo- 
spherulites. Geol. Mag., Dec. IV, Vol. II, 1895, pp. 257-259. 

Raisin, Miss C. A. On some nodular felstones of the Lleyn. Quart. Jour. Geol. 
Soc. London, Vol. XLV, 1889, pp. 247-369. 

. Variolite of the Lleyn and associated volcanic rocks. Quart. Jour. Geol. Soc, 

London, XLIX, 1893, p. 145. 

Reinhold. Proc. Phil. Acad. Sci., 1882, p. 59. 

Richthofen, Baron von. Studien aus dem ungarischsiebenbiirgischeu Trachyt- 
gebergen. Jahrbuch, k. k. geol. Reichsanstalt, Vol. II, 1860, p. 180. 

. Natural system of volcanic rocks. San Francisco, 1868, p. 14. 

Rose, Gustav. Ueber die sogenannte krystallisirten Obsidian. Poggendorff 
Annalen, Vol. X, 1827, p. 323. 

Rosenbusch, H. Mik. Phys. der Massigen Gesteine, 2d ed., 1887, pp. 392-393. 

Roth, Justus. Beitriige zur Petrographie der plutonischen Gesteine, 1869, p. 168. 

. Allgemeine und Chemische Geologie, 1883, Vol. II, p. 216. 

Scrope, G. P. Considerations on volcanoes. Trans. Geol. Soc, London, Vol. II, 
1825, p. 202. Repr. 1873, p. 118. 

Szab6, Dr. Joseph. Die Trachj^te und Rhyolite der Ungebung von Tokay. Jahr- 
buchs, k. k. geol. Reichsanstalt, Vol. XVI, 1866, p. 89. 

Sztireniji, Hugo. Kugele und spherulitische Trachyte von Schenmitz and dem 
Matia Gebirge. Ref. Neues Jahrbuch fiir Mineral, Vol. II, 1883, p. 222. 

Teall, J. J. Harris. British Petrography, 1880, p. 402. 

Tenne, L. A. Ueber Gesteine des Ceno de las Navajos (Wesserberg) Mexico. 
Zeitschr. Deutsch. geol. Gesell., Vol. XL, 1888, p. 610. 



BAscoM.] LITERATURE. 9 1 

Vogelsang, H. Spbernlitic strnctiire iu diorites. Niederrh. Geol. Natnr and 
Heilkande, 1862. 

. Philosopbie der Qeologie imd mikroskopische Oesteinstudien, 1867. 

. Die Krystalliten, 1875. 

Von Hauer, Karl Ritter. Die Gesteine init Lithophysenbiidungen von Telki Haii 
in Ungern. Verhandl. k. k. geol. Reichsaustalt, 1866, p, 98. 

'Weiss, Charles E. Qnartz Porpbyr aus Tburingen. Zeitscbr. Deutscb. geol. 
Gesell., Berlin, Vol. XXIX, 1877, p. 418. 

Zirkel, Ferdinand. Lehrbucb der Petrograpbie, Bonn, Vol. II, 1866, p. 253. 

. MtcroBCopical Petrograpby, Geol. Expl. Fortieth Parallel, Vol. VI, 1876, p. 212. 



PLATES. 



93 



PLATE XV. 






PLATE XY. 

Fig. a. — Quartzite. Specimen I. Slide 1 D. Between Waterloo and the Blue 
Mountain House. In polarized light; X 32. Illustrates the induration of a sand- 
stone by enlargment of the original grains. 

Fig. h, — ^Penetration Manebacher twin of anorthoclase in quartz-porphyry. Speci- 
men 23. Slide 23 D. From well, at depth of 40 feet near Clermont House. In 
polarized light, X 10 (about). 

Piedmontite fills the cavities in the crystal. 
96 



J 



I 






PLATE XVI. 



Bull. 136 7 07 



■ t 






PLATE XYI. 

Fi^. a. — Microporthic structure iu a Carlsbad twin of anorthoclase in quartz-po 
phyry. Specinieu 181. Slide 181 D. Near the old viaduct on the Tapeworm Eai 
road. In polarized light, X 85. 

Fig. 1). — Broken feldspar crystal in quartz-porphyry. Specimen 166. Slid© Iwl 
Near Gum Spring on the Old PMrnace road. In polarized light, X 28. The phenocryj 
has been broken and i)nlled apart and the cracks cemented with sericite scales. Tfl 
ground mass shows the niicropoikilitio structure and the micropoikilitic areas af 
surrounded Avitli sericite scales. 
98 



PLATE XVII. 



99 



PLATE XYII. 

Fig. a. — Broken feldspar crystal in qnartz-porphyry, Specimen 177, Slide Gb. 
Old Furnace road north of the junction with the Gladhill's road. In polarized 
light, X 115. 

Fig. h. — Quartz-porphyry. Specimen 25. Slide 64. Artesian well near Clermont 
House. In polarized light, X 28. 

It is attempted in this figure to show the continuity of orientation of a quartz 
phenocryst with the micropoikilitic area surrounding it, and to show (in a very 
crude way) the patchy effect given to the groundmass by the micropoikilitic struc- 
ture. The quartz areas are bordered by sericite scales. 
100 



PLATE XVIII. 



101 




PLATE XYIII. 

Fig. a. — Flow structure in an Jiporhyolite. Specimen and slide loaned by Prof.S. 
L. Powell, of Newbury, S. C. From tbe South Mountain. In ordinary light, X 24. 

Fig. h. — Chain spherulites in an aporhyolito. Specimen 34. Slide 34 D. From the 
neighborhood of the Bighani copper mine. In ordinary light, X 24. Quartz pheno- 
cryst (now granulated) inclosed by chain sphernlites. Cleiar spherules in center of 
dark bands are still preserved. 
102 






PLATE XIX. 



lo:^ 



. „--X«g'*''^„tW***- 1 attain*' rtjWe8*»*^ 




PLATE XX. 



105 



PLATE XX. 

Fig. a. — Perl i tic parting in an aporhyolite. Specimen 279. Slide G. H. W. (un- 
num beared). Raccoon Creek, Franklin County. In ordinary light, X 88. 

Fig. b, — The same. In polarized ligbt, X 225. A quartz-feldspar mosaic obscares 
all trace of glassy stractures. 
106 



PLATE XXI. 



107 



PLATE XXI. 

Fi^. a. — Pcrlitic ]mrting in an aporhyolite. Specimen G. H. W. Slide G. I. 
(uiinunibero(l). Raccoon Creek, Franklin County. In ordinary light, X 40. 

Fig. /;. — Axiolites in an aporhyolite. Specimen 77. Slide 77. South Moiiu 
In ordinary light, X 40. 

108 



PLATE XXII. 



109 



PLATE XXII. 

Fig. a. — Altered sphernlite in au aporhyolite. Specimen 280. Slide 280 D. Peach 
Orchard; southwest flank of Pine Mountain. In ordinary light, X 5. In pdliiized 
light the slide shows an even-grained quartz-feldspar mosaio, with the nlonipoiki- 
litic structu-^e and with no trace of sphernlitic crystallization. 

Fig. b. — An unaltered spherulite in an aporhyolite. Specimen 279. SUda Q. H. W. 
(unnumbered). Raccoon Creek, Franklin County. In ordinary light, X 80. Gnmnd- 
mass devitrified, but sphernlitic crystallization still in a large part preeemd. 

110 



U&StOLOOtCAL SURVEY. 



BULLETIN N«. ISe PL.W 




a- ALTERED SFHERUI.ITE IN AN APORHYOLITE, 
b - UNALTERED SPHERULITE IN APORHYOLITE. 



PLATE XXIII. 



Ill 



PLATE XXIII. 



Fig. a. — Splierulitic aporhyolite. Specimen 226. Slide 226 D. South flank of 
mountain northeast of the junction of Copper Run and Toms Creek. In polarized 
light, X 28. 

The left-hand side of the figure shows the spherulites in ordinary light; the 
right-hand shows the same spherulites in polarized light. 

Fig. h. — Aporhyolite. Specimen 121. Slide 121 D. One-half mile beyond the 
Bigham copper mine, Old Furnace road. In polarized light, X 80. A crystal of feld- 
spar broken by the cleavage along the plane of former spherulitic crystallization. 
The figure shows the crystalline silica, which has replaced the spherulitic crystal- 
lization and which is much coarser in gram than that of the gronndmass. 
112 



PLATE XXIV 



Bull. 136 8 113 



PLATE XXIV. 

• 

Fig. a. — Rhyolitic structure in an aporhyolite. Specimen 153. Slide 153. Sot 
Mountain. In ordinary light, X 120. 

Fig. 6. — The same. In ordinary light, X 40, The structure is not satisfactor 
shown in either figure. 
114 



^ 


^k 


/^^i 


^'^/l 


M^^mi' 


!^^'^^%C 




^^^^^^■jjK^*^ <:£/ 


^^ 






^SJP\ 




•^im^'" / 


^ 





PLATE XXV. 



115 



PLATE XXY. 

Fig. a. — Khyolitic structure in an aporhyolite. Specimen 153. Slide 158. South 
Mountain. In ordinary light, X 120. Aschen-structur of MUgge. 

Fig. h, — Piedmontite in an aporhyolite. Specimen 162. Slide 482. Southeast 
flank of Pine Mountain. In ordinary light, X 120. 
116 



J 



LS.0EOL0GIGAL SURVEY. 



BULLETIN NT. I3e F 




RHrOMTE STRUCTURE IX AN APORHYOLITE 



PLATE XXVT, 



117 



PLATE XXYI. 

Fig. a. — Amygdaloidal aporhyolite. Specimen 48. Slide 48 D. Bigham eopper 
mine. In ordinary light; X 30. Epidote fills the center of the amygdtile and quartz 
surrounds the epidote. 

Fig. 6.— Amygdaloidal aporhyolite. Specimen G. H.W. Slide G. H. W. (unnum- 
bered). Raccoon Creek^ Franklin County. In ordinary light, X 30. 

118 







0,6 - AMYODAI.OIDALAPORHYOUTE- 



PLATE XXVII. 



119 



PLATE XXYII. 

Fig. a. — Amygdaloidal aporhyolite with tridymite spheralites. Specimeu 276. 
Slide G. H. W. Raccoon Creek, Franklin County. In ordinary lights X 30. 

Fig. h. — The same. Specimen G. H. W. Slide G. H. W. (annambered). Raccoon 
Creek, Franklin County. In ordinary light, X 30. 

120 




'.*- TRIDTAllTE si-he 



'"^■■-''^'■«>>*A.N-APORHTOUTE. 



PLATE XXVin, 



121 



PLATE XXVIII. 

Fig. a. — Augite-porpliyrite. Specimen 237. Slide 237 D. Head of Minie BTUich. 
In ordinary ligbt, X 120. 

Fig. h. — Melapliyre. Specimen 87. Slide 87 D. On Gettysburg Railroad, at fonrtli 
cut northeast of Monterey Station. In ordinary light, X 120. 

122 



JEOLOGICAL SURVEY. 



BULLETIN Nl. 136 F 




a- AL-On-K-POrtlMIYRITK, 
b - Mt;i.APFrYBE. 



I N I) E X . 



Page. 

t,S., cited 37,57,68 

laloidal structure in aporhyolites, 

characters of 55 

ses, chemical 33-34, 78 

yolites, character and distribution of. 42-61 
)-porphyrites, character and distribu- 
tion of 69-78 

tic structure in aporhyolites, charac- 
ters of 54 

,L.W., cited 86 

', W. S., cited 69,86 

obert, cited 86 

r, J. F., cited 17,27,82 

7, T.G.,cited 31,64,68 

iiann, T. G., cited 58 

ter, David, cited 58 

)r, W. C, cited 47 

liart, cited 36 

,G.W., cited 56 

ay, cited 64 

ian rocks, description of 31-34 

L'al analyses 33-34, 78 

John M., acknowledgments to 14 

•. A. J., cited 56 

: ore, occurrence of 25-27 

Whitman, cited 38,47 

knowledgments to 60 

J. D., cited 37,68 

E.H., cited 68 

Is, analysis by 78 

6e, A., cited 58 

ification, proofs of 57-58 

J.S., cited 47,86 

ieu, cited 36 

.W., cited 86 

, Lewis, cited 14 

larin quartz-porphyries, characters of 39-40 

lar in aporhj elites, characters of 44-45 

•, Persifor, cited 16-17,24,25,26,78,82 

h and Reiss, cited 57 

•er, Karl, cited 45 

', H. R., cited 22 

F. A., chemical analyses by . . . 34, 61, 62, 78 

rd, cited 36 

sleeve, cited 38 

U. S., acknowledgments to 47 

ed 86 

3ck, A. von, cited 64 

, Arnold, cited 37,61 

r, Alfred, cited 46,47 

th,E., cited 47,86 

n,H.H., cited 24,25 



Henderson, C. H., cited . 

analyses by 

Hitchcock, cited 

Hobson, cited 



Page. 

25 

61 

80 

68 

Hunt, T. Sterry, cited 17, 24, 25, 82, 83, 86 

Iddings, J. P., cited 31, 

37, 43, 47, 48, 61, 52, 55, 56, 61, 68 

acknowledgments to 50 

Irving, R. D., cited 31, 47, 55, 57, 60, 86 

Jackson, cited , 86 

Jacks Mountain, geology of 21, 42 

Judd,J.W.,cited 37,68 

Kalkowsky , E. , cited 59 

Keith, Arthur, cited 21, 22, 28, 29, 86 

Keyes,C.R., cited 32 

Kirwan, Richard, cited 37 

Klockmann, F., cited 53,59 

LaCroix, A., cited 59 

Lang, H. Otto, cited 59 

La Yall^e-Poussin, Ch. de, cited , 55, 57, 60 

Lehman, A. E., superior topographic maps 

of South Mountain prepared by 18 

Lehman, J ., cited 46, 64 

Lesley, J. P., cited 17-18,21,82 

L6vy, Michel, cited 36 

Lieber, cited 86 

Lindgren, Waldemar, cited 47 

Link, cited 59 

Lithophysal structure in aporhyolites, char- 
acters of 55 

Loewinson-Lessing, F., cited 67 

Lessen, K. S., cited :. ... 59 

Lud wig, R., cited 69 

Matthew, cited 86 

McCreath, A. S., analyses by 61 

Melaphyres, charapter and distribution of. . 09-78 

Mica of aporhyolites, characters of 46 

Micropegmatitic structure in aporhyolites, 

characters of 65 

Micropoikilitic structure in aporhyolites, 

description of 47-51 

Milch, P. L., cited 46,64 

Monterey district named and defined 20 

ore deposits of 25-27 

Naumaiin, cited 36 

NordenskjOld, Otto, cited 38, 47, 67, 60 

Osaun, A., cited : 59 

Perlitic structure in aporhyolites, charac- 
ters of 65 

PhilUps, J. A., cited 31,68 

Piedmontiie in quartz-porphyries, character 

and distribution of 41-42 

123 



124 



INDEX. 



Pinkerton, T., cited 36 

IMrHHon, L. V..citeil 47.80 

Pumpt'lly, li., ritt'd 27 

Qiiiirtx in <niartz-iM»ridiyrioH, clianu'ter 

of 4(U4l,4ri I 

Quart z-p<>rp1iyrieA,<'hiira<'ters anil dirttrihu- 

t i«)ii of :J0-42 

description of :«M2 

Kaccoon Creek, dencript ion of ainy^dnloidH 

fi-oni ri6 

ReiH*' r, cit ed r.8 

KiMisch, H. II., <-itt»d 08 

Reyer, K., cited 68 

KhyoJitic struct iir«' in aporliyolitcs, cliarac- 

tiTrt of 54-55 

Ko|,'erH,jr,I).,citcd.... 14-16,21.23.25,06.71,82,83 : 
Kosenburtch, U., cited .... 35, 36, 37, 46, 57, 64, 68, 84 

Koth, Justus, cited 36 

Kutley, F., cited 40,54 ! 

Sauer, cited 59 

Sohopf, J. I)., citeil 14 

Sedimentary rocks, destTiption of 21-22 j 

Sericite schist, localities of 64-66 ■ 

Slialer, N. S., cited 86 

Slates, localities of 43, 64-06, 7»-80 ■ 



i 



Page. 
Smith, G. O.. volcanic rocks in Maine 

Hiudietl by 

Sorby, H. C, cited 3* 

South Mountain, area of 1" 

account of surveys in 14- 

Spherulitic structure in aporhyolites, char- 
acter of 43-44, 5U 

Sue.Ms, PMuard, cited 

Taxitic structure of rooks, description of. . . 

Teall. J. J. Harris, cited 37.47^^^ 

Tietze, cited ^ 

Tomebohm, A.E.,cited gj^ 

Trimble, Henrj*, analyses by ^ 

Tyson, P. T., cited..., 16,2^.55 

Van Hise, C. R., cited 31 

Vogel.C, cited s$ 

Vogelsang H., cited 59 

Wadsworth,M.E., cited 90.99 

Walcott, C. D.. cited 21,92 

Wallerius, cited 36 

Williams, G. H., letter of transmittal by. ■ . 11 

cited 13,29.41,42,47,64 81 

Young. A. A., cited 81 

Zirkel, F., cited 3e,4»,M