LIBRARY
FACULTY OF FORESTRY
UNIVERSITY OF TORONTO
EOCKS, EOCK-WEATHEBING, AND SOILS
A TREATISE
ON
ROCKS, ROCK-WEATHERING
AND SOILS
BY
GEORGE P. MERRILL
CURATOR OF GEOLOGY IN THE UNITED STATES NATIONAL MUSEUM, AND PROFESSOR
OF GEOLOGY IN THE CORCORAN SCIENTIFIC SCHOOL AND GRADUATB
SCHOOL OF COLUMBIAN UNIVERSITY, WASHINGTON, D.C.
AUTHOR OF "STONES FOR BUILDING AND DECORATION," ETC.
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.
All rights rexerred
COPYRIGHT, 1897,
BY THE MACMILLAN COMPANY.
ae
i
• NortoooB
J. S. Gushing & Co. — Berwick & Smith
Norwood Mass. U.S.A.
- ELECTRONIC VERSION
AVAILABLE
"THE ruins of an older world are visible in the present structure
of our planet ; and the strata which now compose our continents
have been once beneath the sea, and were formed out of the waste
of pre-existing continents. The same forces are still destroying,
by chemical decomposition or mechanical violence, even the hard-
est rocks, and transporting the materials to the sea, where they
are spread out, and form strata analogous to those of more ancient
date." — BUTTON.
PREFATORY NOTE
IN the work here presented the writer has endeavored to
bring together in systematic form the results of several years'
study of the phenomena attendant upon rock degeneration
and soil formation. Although beginning with a discussion
of rocks and rock-forming minerals, the work must be con-
sidered in no sense a petrology as this word is commonly
used. What is here given relative to the origin, structure,
and composition of rock masses is regarded as an essential
introduction to the chapters on rock-weathering. The por-
tion dealing with the structure and composition of the result-
ant materials is an essential corollary to these same chapters.
It is believed that no apology is necessary even in this <lay
of many books for bringing out the present work. The origin,
structure, and mineral composition of rocks, particularly tilt-
eruptive varieties, are matters which have of late received much
attention. In fact, it is to these rocks that the petrologists
have devoted their best efforts. Since the introduction of the
microscope into petrographic work, there has, however, been
very little time devoted to the study of rocks in a weathered
condition. The chemists have made analyses, but have disre-
garded the physical and mineralogical nature of the material
analyzed. Other workers have studied the physical properties
of rocks decayed, — in the form of soils, — but have in their
turn disregarded their mineral and chemical nature. The
writer has aimed to bring together here such results obtained
by these workers in divers fields as it is believed will be for
the mutual benefit of all concerned. The state of comminu-
tion reached by j-ocks during the processes of long-continued,
viii PREFATORY NOTE
secular decay, and the amount of leaching such have under-
gone, are certainly of as much practical interest to the agri-
culturist as of theoretical interest to the geologist.
To the one, these residues are essential to the life and well-
being of man through furnishing the soils from whence is
derived directly and indirectly the food for life's sustenance ;
to the other they are but transitory phases in the earth's his-
tory, representing the materials from which, through a process
of fractional separation by running waters, have been made
up the thousands of feet of secondary rocks which to-day
occupy so large a portion of its surface.
The very general scheme of classification adopted in the
treatment of the unconsolidated clastic materials may at first
seem disappointing. It has, however, been the writer's special
aim to introduce into this preliminary volume as few new terms
as possible, using only those which through years of service
have become a part of our language. It is of course possible
that in his desire to avoid any possible confusion such as might
arise through putting forward a purely tentative classification
he has been overcautious.
It is possible, further, that in numerous instances it may
appear that too much reliance is placed upon single analyses,
particularly in the discussions relating to the character of
decomposed material. Regarding this it can only be said that
in those instances upon which most reliance is placed, the
materials were not merely collected by the author himself, but
that he made his own chemical analyses and microscopic deter-
minations as well. It is believed that the fresh and residual
materials examined are in each instance as truly representative
of the same rock mass, as would be samples of fresh rock col-
lected equal distances apart. In all cases special effort was
made to obtain material concerning the lithological identity
of which there could be no doubt, and in the majority of cases
the residuary matter was collected from positions immediately
overlying the still unaltered rock. Where such a procedure
PREFATORY NOTE ix
was impossible, especial care was exercised to obtain only such
as was originally of the same lithological nature as the fresh
rock, and which had suffered no contamination from extrane-
ous sources. The fact that stratified rocks are likely to vary
so greatly within short distances, and hence that a residual clay
cannot be relied upon to represent the residue from rocks of
the same nature immediately underlying, will serve to explain
in part the author's limiting himself so largely to a discussion
of massive eruptive materials. That so little use has been
made of other analyses, made in greater detail or by those more
skilled in analytical methods, is due to a lack of satisfactory
information relative to the mutual association of the fresh and
decomposed materials and the mineralogical and physical nature
of the residual product.
As will be readily perceived by those at all acquainted with
the general literature, the publications of the U. S. Geological
Survey, the U. S. National Museum, and the Bulletins of the
Geological Society of America 1m ve been drawn upon to furnish
materials for illustration. The writer is under special obliga-
tion to Dr. Milton Whitney of the U. S. Department of Agri-
culture for many of the mechanical analyses given, and to Mr.
L. H. Merrill of the Maine Experiment Station for numerous
criticisms and suggestions.
To the late Dr. G. Brown Goode he is indebted for permis-
sion to utilize photographs and specimens forming a part of
the collections of the National Museum and also for electro-
types of sundry plates and figures in its publications.
GEORGE P. MERRILL.
U. S. NATIONAL MUSEUM, January, 1897.
CONTENTS
PART I
THE CONSTITUENTS, PHYSICAL AND CHEMICAL
PROPERTIES, AND MODE OF OCCURRENCE
OF ROCKS
PAG*
I. INTRODUCTORY : ROCKS DEFINED 1
II. THE CHEMICAL ELEMENTS CONSTITUTING ROCKS ... 4
III. THE MINERALS CONSTITUTING ROCKS 9
IV. THE PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS . . 33
1. The Structure of Rocks, macroscopic and microscopic . 33
2. The Specific Gravity of Rocks 43
3. The Chemical Composition of Rocks 44
4. The Color of Rocks 45
V. THE MODE OF OCCURRENCE OF ROCKS 49
•PART II
THE KINDS OF ROCKS
GENERALITIES, AND CLASSIFICATION 56
I. IGNEOUS ROCKS: ORIGIN OF, AND CLASSIFICATION; RELATION-
SHIP EXISTING BETWEEN PLUTONIC AND EFFUSIVE ROCKS 59
1. The Granite-Liparite Group 65
2. The Syenite-Trachyte Group 73
3. The Foyaite-Phonolite Group 77
4. The Diorite-Andesite Group 81
5. The Gabbro-Basalt Group 85
6. The Theralite-Basanite Group 93
xii CONTENTS
PAGE
7. The Peridotite-Limburgite Group ..... 95
8. The Pyroxenite-Augitite Group 99
9. The Leucite-Nepheline Rocks 102
II. AQUEOUS ROCKS . . . 105
1. Rocks formed through Chemical Agencies .... 105
(1) Oxides 106
(2) Carbonates Ill
(3) Silicates 114
(4) Sulphates 117
(5) Phosphates 119
(6) Chlorides . . 119
(7) Hydrocarbon Compounds 120
2. Rocks formed as Sedimentary Deposits .... 129
(1) Rocks composed mainly of Inorganic Material . . 131
(1) The Arenaceous Group : Psammites . . 131
(2) The Argillaceous Group : Pelites . . .135
(3) The Calcareous Group : Calcareous Conglom-
erate and Breccia ...... 139
(4) The Volcanic Group : Tuffs . . . .139
(2) Rocks composed mainly of debris from Plant and
Animal Life. Organagenous .... 141
(1) The Siliceous Group : Infusorial Earth . 141
(2) The Calcareous Group : Limestone, Marl, etc. 143
(3) The Carbonaceous Group : Peat, Lignite,
and Coal 148
(4) The Phosphatic Group 151
III. 2EoLiAN ROCKS . 153
Volcanic Dust ; Dune Sands, etc 153
IV. METAMORPHIC ROCKS 155
Agencies and Results of Metamorphism and Metasomatosis . 155
*1. Stratified or Bedded 162
(1) The Crystalline Limestones and Dolomites . 162
2. Foliated or Schistose 164
(1) The Gneisses 164
(2) The Crystalline Schists 168
CONTENTS xiii
PART III
THE WEATHERING OF ROCKS
PAGE
I. THE PRINCIPLES INVOLVED IN ROCK-WEATHERING : Statement
of General Problem ; Weathering defined ; Reference to Au-
thorities and Opinions held 173
1. Action of the Atmosphere 176
(1) Nitrogen, Nitric Acid, and Ammonia of the
Atmosphere 176
(2) Carbonic Acid of the Atmosphere . . . 178
(3) Oxygen of the Atmosphere . . . .180
(4) Effects of Heat and Cold . . . .180
(5) Effects of Wind 184
2. Chemical Action of Water 186
(1) Oxidation 187
(2) Deoxidation 187
(3) Hydration 187
(4) Solution 189
3. Mechanical Action of Water and of Ice ; Erosion by
Water ; Daubree's Experiments ; Action of Freez-
ing Water and of Ice 195
4. Action of Plants and Animals ; Effect of Lichens,
Mosses, Root Action, Organic Acids, etc. ; Solvent
Power of Citric Acid ; Action of Bacteria ; Action
of Ants and Termites ; Action of Marine Inver-
tebrates ; Production of Carbonates . . . 201
II. CONSIDERATION OF SPECIAL CASES 206
(1) Weathering of Granite, District of Columbia . . . 206
(2) Weathering of Gneiss, Albemarle County, Virginia . . 214
(3) Weathering of Elaeolite Syenite, Little Rock, Arkansas . 216
(4) Weathering of Phonolites, Marienfels, Bohemia * . . 217
(5) Weathering of Diabase, Medford, Massachusetts . . 218
(6) Weathering of Diabase, Venezuela 222
(7) Weathering of Basalt, Kammar Bull, Bohemia . . . 223
(8) Weathering of Basalt, Haute Loire, France . . . 223
xiv CONTENTS
PAGB
(9) Weathering of Diorite, Albemarle County, Virginia . . 224
(10) Weathering of Peridotites and Pyroxenites . . . 225
(a) Serpentine of Harford County, Maryland . . . 226
(6) Soapstones of Albemarle and Fairfax Counties, Vir-
ginia 226
(11) Weathering of Clastic Rocks 228
(a) Arglllites of Harford County, Maryland . . . 229
(b) Cherts of Missouri and Arkansas .... 230
(12) Weathering of Limestones, Arkansas .... 232
(13) Resume: Importance of Hydration ; Loss of Constituents ;
Relative Durability of Various Minerals ; Discussion of
Processes involved in Feldspathic Decomposition . . 234
III. THE PHYSICAL MANIFESTATIONS OF WEATHERING . . . 241
(1) Disintegration without Decomposition .... 241
(2) Weathering influenced by Crystalline Structure . . 243
(3) Weathering influenced by Structure of Rock Masses . . 244
(4) Weathering influenced by Mineral Composition . . 248
(5) Results due to Position . . . . . . . 252
(6) Induration on Exposure 254
(7) Changes in Color incidental to Weathering . . . 257
(8) Relative Amount of Material removed in Solution . . 258
(9) Incidental Surface Contours 259
(10) Effacement of Original Characteristics . . . . 262
(11) Simplification of Chemical Compounds incidental to
Weathering 265
(12) Other Results incidental to Decomposition and Erosion . 266
IV. TIME CONSIDERATIONS 268
(1) Rate of Weathering influenced by Texture . , . 268
(2) Rate of Weathering influenced by Composition . . 269
(3) Rate of Weathering influenced by Humidity . . . 270
(4) Rate of Weathering influenced by Position . . . 270
(5) Relative Rapidity of Weathering among Eruptive and Sedi-
mentary Rocks 271
(6) Time Limit of Decay : Post-Cretaceous Weathering of
Granite ; Weathered Implements of Human Workman-
ship ; Post-Glacial Weathering of Diabase ; Post-Jurassic
CONTENTS XV
PAGE
and Post-Pliocene Decay of Rocks of the Sierras ; Pre-
Palseozoic Weathering of Archaean Rocks . . . 272
(7) Extent of Weathering : In the District of Columbia,
Georgia, Missouri, Nicaragua, Brazil, and South Africa . 276
(8) Relative Rapidity of Weathering in Warm and Cold Cli-
mates : Opinions hitherto held ; Supposed Protective
Action of Frost Effects of Forests 278
(9) Difference in Kind of Weathering in Cold and Warm Cli-
mates 283
(10) Relative Amounts of Materials lost through Weathering
in Hilly and Plains Regions 284
PART IV
TRANSPORTATION AND REDEPOSITION OF ROCK
DEBRIS
1. ACTION OF GRAVITY 286
2. ACTION OF WATER AND ICE 287
3. ACTION OF WIND 292
PART V
THE REGOLITH
I. CLASSIFICATION AND GENERAL DESCRIPTION .... 299
1. Sedentary Materials 300
(1) Residuary Deposits : Residual Sands and Clays ;
Terra Rossa ; Laterite, etc 301
(2) Cumulose Deposits : Peat ; Muck and Swamp Soils
in part ; Infusorial Earths 313
2. Transported Materials 318
(1) Colluvial Deposits : Talus, Cliff Debris and Material
of Avalanches 319
(2) Alluvial Deposits : Modern Alluvium ; Sea-coast
Swamps; Loess ; Adobe in part; Champlain Clays;
Beach Sands and Gravel 320
XVi CONTENTS
PAGE
(3 ) JEolian Deposits : Wind-blown Sand ; Sand Dunes ;
Volcanic Dust 344
(4) Glacial Deposits : Moraine Material ; Eskers ; Drum-
lins, etc. 350
3. The Soil 358
(1) The Chemical Nature of Soils 358
(2) The Mineral Composition of Soils . . . . 374
(3) The Physical Condition of Soils .... 379
(4) The Weight of Soils 382
(5) The Kinds and Classification of Soils . . .382
(6) The Color of Soils 385
(7) The Age of Soils 387
(8) Soils as Affected by Plant and Animal Life . . 390
ILLUSTRATIONS
FULL-PAGE PLATES
FACING PACK
PLATE 1 Frontispiece
Stone Mountain, Georgia. A Residual Boss of Granite. From
a photograph by J. K. Killers.
PLATE 2
Porphyritic and Flow Structures.
PLATE 3 35
Slaggy and Vesicular Structures.
PLATE 4 38
Brecciated Structures.
PLATE 5 .41
Microscopic Structures of Rocks.
PLATE 6 65
Fig. 1. Lithophysjc in Liparite.
Fig. 2. Cross-section of Stalagmite.
Fig. 3. Concretionary Aragonite.
Fig. 4. Pegmatite.
PLATE 7 70
Fig. 1. Liparite, Nevadite Form.
Fig. 2. Liparite, Rhyolite Form.
Fig. 3. Liparite, Ol»i<li;m Form.
Fig. 4. Liparite, Pumiceous Form.
PLATE 8 82
Fig. 1. Orbicular Diorite.
Fig. 2. Granite Spheroid.
PLATE 9 107
Fig. 1. Botryoidal Hematite.
Fig. 2. Septarian Nodule.
PLATE 10 113
View in Limestone Cavern.
xvii
XVlii ILLUSTRATIONS
FACING PAGE
PLATE 11 . . . . 130
Fig. 1. Shell Limestone.
Fig. 2. Coquina.
Fig. 3. Crinoidal Limestone.
PLATE 12 143
Fig. 1. Pisolitic Limestone.
Fig. 2. Oolitic Limestone.
PLATE 13 164
Banded and Foliated Gneisses.
PLATE 14 172
Weathered Granite, District of Columbia. From a photograph
by George P. Merrill.
PLATE 15 193
Corroded Limestones.
PLATE 16 199
Fig. 1. Diorite Boulder split along Joint Planes by Frost.
Fig. 2. Corroded Surface of Pyroxenic Limestone.
Fig. 3. Corroded Limestone.
PLATE 17 219
Weathered Diabase Dike, Medford, Mass. From a photograph by
G. H. Barton.
PLATE 18 241
Fig. 1. Exfoliated Granite in the Sierras. From a photograph
by H. W. Turner.
Fig. 2. Talus Slopes on Pike's Peak. From a photograph by W.
H. Jackson.
Fig. 3. Disintegrated Granite, Ute Pass, Colorado. From a
photograph by W. H. Jackson.
PLATE 19 248
Fig. 1. Weathered Schists, Coast of Cape Elizabeth, Maine.
Fig. 2. Sandstone bored by Bees.
Fig. 3. Slab of Glaciated Limestone.
PLATE 20 258
Fig. 1. Weathered Boulder of Oriskany Sandstone.
Fig. 2. Concentric Weathering in Diabase.
Fig. 3. Zonal Structure in Weathered Argillit.e.
Fig. 4. Weathered Sandstone showing Induration along Joint
Planes.
ILLUSTRATIONS xix
FACING PAGE
PLATE 21 . 267
Fig. 1. Sink-hole near Knoxville, Tennessee. From a photograph
by George P. Merrill.
Fig. 2. Beds of Marble corroded by Meteoric Waters, Pickens
County, Georgia.
PLATK 22 285
Fig. 1. Forest destroyed by Wind-blown Sand. From a photo-
graph by I. C. Russell.
Fig. 2. Calcareous Conglomerate carved and polished by Wind-
blown Sand.
Fig. 3. Rock being undermined by Wind-blown Sand. After G.
K. Gilbert.
PLATE 23 , 319
Rock Disintegration and Formation of Talus, Mount Sueffels,
Colorado. From a photograph by Whitman Cross.
PLATE 24 . 345
Fig. 1. Section of Beds of Leda Clay, Lewiston, Maine. From a
photograph by L. H. Merrill.
Fig. 2. Beds of Volcanic Dust, Reese Creek, Gallatin County,
Montana. From a photograph by George P. Merrill.
PLATE 23 357
Fig. 1. Section of Glacial Till. From a photograph by G. F.
Wright.
Fig. 2. Glaciated Landscape. From a photograph by L. H.
Merrill.
Plates 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 15, 19, and 20, and Fig. 3 Plate 16,
ami Fig. 2 Plate 22, from specimens in the Geological Department of
the United States National Museum.
FIGURES IN TEXT
». PACK
1. Augite partially altered into Hornblende 40
2. Mounted Thin Section of Rock 43
3. Microscopic Structure of Muscovite-Biotite Granite, Hallowell,
Maine 67
4. Microscopic Structure of Diabase, Weehawken, New Jersey . . 88
5. Microscopic Structure of Peridotite (Porphyritic Lherzolite) . 96
6. Microscopic Structure of Pyroxenite 100
XX ILLUSTRATIONS
7. Microscopic Structure of Oolitic Limestone 112
8. Pyroxene partially altered into Serpentine 115
9. Microstructure of Sandstone 131
10. Section through Lake Basin, showing Bed of Infusorial Earth . 142
11. Microstructure of Oolitic Limestone ...... 144
12. Microstructure of Fossiliferous Limestone ..... 145
13. Microstructure of Quartzite . 158
14. Microstructure of Crystalline Limestone ..... 163
15. Microstructure of Gneiss 165
16. Microstructure of Quartzite 169
17. Influence of Joints in the Production of Boulders .... 244
18. Exfoliation of Granite, Stone Mountain, Georgia .... 245
19. Concentric Exfoliation of Granite, Canada 246
20. Microstructure of Sandstone, with Large Absorptive Power . . 269
21. Microstructure of Diabase, with relatively Little Absorptive Power 269
22. Flint Implement showing Weathered Surface .... 274
2J5. Sketch showing Pre-Palseozoic Decay of Rocks .... 276
24. Diagram showing Direction and Rate of Motion of Soil . . 287
25. Diagram showing Flood Plain of River 289
26. Angular Outlines of Particles in Residual Soil from Gneiss . . 301
27. Section across Central Kentucky, showing Inherited Characteris-
tics of Soils 303
28. Angular Quartz Particles from Decomposed Gneiss . . . 304
29. Outlines of Kaolinite Crystals and Kaolin Particles . . . 309
30. Section across Small Lake 314
31. Talus Slopes 319
32. Alluvial Plains .323
33. Outlines of Particles in Chinese Loess , 329
34. Particles washed from Leda Clays 335
35. Cross-section of Marine Marsh 338
36. Quartz Granules in Beach Sand 343
37. Outlines of Particles of Glass in Volcanic Dust .... 349
38. Section through Carboniferous Soil 386
39. Section showing Varying Character of Residual Soil . . . 387
40. Section through Ant Nest 390
41 and 42. Sections showing the Effect of Tree Roots in Soil . . 395
Fig. 1, after G. W. Hawes ; 5 and 6, after G. H. Williams ; 18 and 22,
after Robert Bell ; 10, 23, 24, 26, 29, 30, 31, 34, 37, 38, 39, 40, and 41, after
Shaler, Twelfth Annual Report United States Geological Survey, 1890-1891.
ROCKS, ROCK-WEATHERING,
AND SOILS
PART I
THE CONSTITUENTS, PHYSICAL AND CHEMICAL
PROPERTIES, AND MODE OP OCCURRENCE OP
ROCKS *
I. INTRODUCTORY
A ROCK is a mineral aggregate ; more than this, it is an
essential portion of the earth's crust, a geological body occu-
pying a more or less well-defined position in the structure of
the earth, either in the form of stratified beds, eruptive masses,
sheets or dikes, or in that of veins and other chemical deposits
of comparatively little importance • as regards size and extent.
In giving this definition, origin, chemical composition, and state
of aggregation of the individual particles are for the time
ignored. From a strictly geological standpoint, the beds of
loose sand, and even the water of the ocean itself, may be
considered as rocks, and either, under favorable circumstances,
may undergo a process of induration such as shall be produc-
tive of the condition of solidity commonly ascribed to rocks
by the popular mind.
In ever-varying conditions as regards compactness, color,
texture, and structure, rocks form the entire mass of the globe
so far as it is as yet made known to us, with the exception of a
scarcely appreciable proportion of organic matter. It is rock
which forms the substance of mountain ranges and the vast
stretches of valley and plain. It is from the rocks that we
gain our food, our fuel, and the supplies of metal which are
seemingly so essential to our well-being ; we cannot ignore
2 INTRODUCTORY
them, even if we would. We borrow from the rocks that
which is essential to our life to-day, but when that brief day
is ended return it once more, with neither loss nor gain, to its
original source.
Those portions of the earth's crust which are available for
study comprise at best but a few thousand vertical feet, though
from the fact that the stratified rocks have been so extensively
thrown out of their original, horizontal position, and again
eroded, we are enabled to measure their thickness, and may
hence claim to know with a reasonable degree of accuracy the
character of the material forming this crust down to a depth
of perhaps twenty miles.1 Throughout all this vast thickness,
comprising millions upon millions of cubic feet, in weight far
beyond all comprehension, we find a constant recurrence of
materials alike in composition and similarity in origin to those
upon the immediate surface. There is at times, as noted later,
a difference in structure due to metamorphism, between the
older, deeper lying portions and those more recent, but the
ultimate composition is essentially the same, and all the know-
ledge thus far gained points to a wonderful unity in nature's
methods, and shows with seeming conclusiveness that the geo-
logical agencies of the past, the methods by which rocks were
made and again destroyed, differed in no essential particular
from those in progress to-day. What these processes were,
how they operated, and with what results, it shall be our aim
to here set forth.
Among the many interesting, and at first thought seemingly
unaccountable, things we shall encounter in the progress * of
our work, not the least is the fact that so large a proportion
of natural objects are more or less out of harmony with their
surroundings. Throughout life every organic being is in a
constant struggle with the elements to preserve that life, fulfil
all its functions, and gratify its natural desires. No sooner
does life depart than decomposition and disintegration ensue.
As with organic beings, so with inorganic substances. Every
mass of rock pushed up by the faulting and folding of the
earth's crust, exposed by denudation, or erupted as molten
matter from the earth's interior, finds almost at once that its
various elements, in their existing combinations, are not in har-
1 The total mean depth of the fossiliferous formations of Europe as stated by
Geikie (Text-book of Geology, p. 675) has been set down as 75,000 feet.
INTRODUCTORY 3
mony with their environment. The summer's heat and winter's
cold, the chemical action of atmospheres and acidulated rains,
combine their forces ; a breaking up ensues, to be succeeded
by new combinations and perhaps reconsolidations more in
keeping with the then existing circumstances. An intermedi-
ate product in all this endless cycle of change, of disintegration
and recombination, is a comparatively thin, superficial mantle
of loose debris, which, mixed with more or less organic matter,
nearly everywhere covers the laud, and by its combined chemi-
cal and mechanical properties furnishes food and foothold for
myriads of plants, and hence, indirectly, sustenance for man and
beast as well. In brief, what is commonly known as soil is but
disintegrated and more or less decomposed rock material, inter-
mingled, perhaps, with organic matter from plant decay. Such
being the case, a study of the processes of rock weathering and
the transportation, deposition, and physical properties of the
resultant debris, is but a study of the origin of soils on the
broadest and most comprehensive basis, and soils themselves
may justly be regarded as secondary rocks in a state of in-
complete consolidation. Their study belongs, therefore, as
legitimately to the realm of geology as does that of any sub-
ject relating to rock formation or other phases of the earth's
history.
Accepting the above, we will begin our studies by a consid-
eration of (1) the elements which in their single or combined
state make up the minerals ; (2) the minerals whicli make up
the rocks ; (3) the rocks themselves, with particular reference
to their mineralogical and chemical natures ; (4) the breaking
down or degeneration of rocks through processes in part chemi-
cal and in part mechanical ; and (5) the result of this clasmatic
process as manifested in the production of clay, sand, gravel,
and incidental soil. There are other points which will be
touched upon more briefly, in order to make our work system-
atic, as the action of wind and water in assorting and redeposit-
ing rock debris and tending to reduce the land surface to one
general level.
II. THE CHEMICAL ELEMENTS CONSTITUTING
ROCKS
Although there are 69 elements now known, but 16 occur in
any abundance or form more than an extremely small proportion
of the material of the earth's crust. Indeed, of this number
probably fully one-half, taken collectively, will not constitute
more than 4 or 5% of the earth's crust so far as known. These
16, arranged according to their chemical properties and order
of their abundance, are as follows : oxygen, silicon, carbon,
sulphur, hydrogen, chlorine, phosphorus, fluorine, aluminum,
calcium, magnesium, potassium, sodium, iron, manganese, and
barium. The eight more important, with their approximate
percentage amounts as given by Roscoe and Schorlemmer,1 are
as below : —
Oxygen 44.0 to 48.7%
Silicon 22.8 to 36.2
Aluminum 9.9 to 6.1
Iron 9.9 to 2.4
Calcium 6.6 to 0.9
Magnesium 2.7 to 0.1
Sodium 2.4 to 2.5
Potassium 1.7 to 3.1
It must not for a moment be imagined, however, that these
elements exist for the most part in a free or uncombined state :
on the contrary, in the majority of cases so great is their affinity
for one another that it is only momentarily, or under abnormal
conditions, that they are met with at all in this elementary
form. Those elements which are most common in the free
state, though even these occur more commonly combined with
others, are, (1) the gas oxygen, and (2) the solids, carbon, sul-
phur, and, more rarely, iron. Still more rarely, and under such
abnormal conditions, as exist during volcanic eruptions, are found
the free gases, hydrogen, chlorine, and fluorine. The gas nitro-
gen, although so abundant a constituent of the atmosphere,
1 Treatise on Chemistry, Vol. I, p. 55, 1878.
4
OXYGEN 5
is, as a primary constituent of the earth's crust, almost wholly
unknown, and needs no consideration at this stage of our
work.
Oxygen, as is well known, is the active, even the aggressive,
principle of the atmosphere, of which it constitutes about one-
fifth by bulk. Combined with other elements, it is, however,
of vastly greater geological importance, being estimated, as
noted above, to constitute from 44 to 48.7% of the entire mass
of the earth's crust ; that is to say, could the earth's crust be
once more resolved into its original elements, the oxygen thus
liberated would be found very nearly equal to all the other
elements taken together. The simpler forms of oxygen com-
pounds are known as oxides, and of these the oxide of hydrogen,
water (H2O), is by far the most common, and, anomalous as it
may at first seem, is a true mineral and to be classed as an
anhydrous oxide at that. Aside from being so essential to
human life, oxygen, as will be noted later, is a very potent
factor in the manifold changes which are constantly taking
place in the more superficial portions of the earth's crust.
Silicon. — Next to oxygen silicon is the most abundant of
the earth's constituents, though it exists only in combination,
either as an oxide (SiO2), or with other elements to form
silicates. In these two forms it is the predominating con-
stituent in all but the calcareous rocks. As silica (SiO2), or
quartz, it forms one of the most indestructible of natural com-
pounds, and hence is to be found as the prevailing constituent
in nearly all sands and soils.
Aluminum is next to oxygen and silicon probably the most
important element when regarded from our present standpoint.
It occurs mainly in combination with silicon and oxygen, form-
ing an important series of minerals known as aluminous sili-
cates. As a sesquioxide it is well known in the minerals
corundum and beauxite.
Iron, although less abundant than either oxygen or silica,
occupies a very important place as a rock constituent, owing to
the variety of compounds of which it forms a part, as well as
to the decided colors which are characteristic of its oxides and
of the iron-bearing silicates. The most conspicuous forms of iron
on the immediate surface of the earth are the oxides, but which
at greater depths, or where the atmosphere has as yet exercised
less influence, give way to carbonates, sulphides, and silicates.
6 CHEMICAL ELEMENTS CONSTITUTING THE ROCKS
Iron, although so common in combination with other ele-
ments, occurs but rarely free, owing to its affinity for oxygen.
It is possible that far below the surface, beyond the reach
of meteoric waters and atmospheric air, it is to be found in
a metallic state much more abundantly, but of this we have
no other proof than that the specific gravity of the globe, in
its entirety, is much greater than that of the most dense minerals
which constitute its outer portion. The inference seems un-
avoidable that at great depths some of these elements exist
uncombined, and in a state of greater molecular density than
at the surface.
Calcium is a very important element of the earth's crust,
although, as we have seen, it has been estimated to compose
only about one-sixteenth of its mass. Its most conspicuous
form of occurrence is in combination with carbon dioxide,
forming the mineral calcite (CaCO3), or the rock limestone.
In this form it is slightly soluble 'in water containing carbonic
acid, and hence has become an almost universal ingredient of
all natural waters, whence it furnishes the lime necessary for
the formation of shells and skeletons of the various tribes of
mollusca and corals. In combination with sulphuric acid,
calcium forms the rock gypsum. It is also an important con-
stituent of many silicates.
Magnesium is found in combination with carbonic acid as
carbonate, forming thus an essential part of the rock dolomite.
The bitter taste of sea-water and some mineral waters is due to
the presence of salts of magnesia. In combination with silica
as a silicate it forms an essential part of such rocks as serpen-
tine, soapstone, and talc.
Potassium combined with silica is also an important element
in many mineral silicates, as orthoclase, leucite, and nepheline.
In smaller amounts it is found in silicates of the mica, amphi-
bole, and pyroxene groups. ^The following table will serve to
show the varying amounts of potash (K2O) in rocks of various
kinds : —
Granite . 2.6 to 6.50%
Diorite 0.1 to 2.42%
Basalt . . . ' 0.058 to 0.50%
Gabbro 0.00 to 0.93%
Limestone , 0.19 to 1.22%
Sandstone 0.00 to 3.30%
Slate (fissile argillite) 0.00 to 3.83%
SODIUM 7
As a chloride, potassium is invariably present in sea-water,
and as a nitrate it forms the rare, but valuable mineral nitre, or
saltpetre.
• Sodium. — The most common and wide-spread form of the
element sodium is the compound with chlorine known as
sodium chloride (NaCl) or common salt. In this form it is
the most abundant of the salts occurring in sea-water, and
constitutes also rock masses of no inconsiderable dimensions
interstratified with other rocks of the earth's crust. Combined
with silica, lime, and alumina, sodium is an important constitu-
ent of the soda-lime feldspars, and of numerous other silicate
minerals. In the form of carbonate and sulphate it occurs as
an incrustation on the surface, or disseminated throughout the
soils in poorly drained portions of arid countries, giving rise to
the so-called " alkali soils," for which such regions are frequently
noted. As a nitrate, sodium occurs in the desert regions of
Chili, forming the soda nitre so valuable for fertilizing purposes.
Manganese is, next to iron, the most abundant of the heavy
metals, occurring as oxide, carbonate, or in combination with
two or more other elements as a silicate.
Barium is found mainly combined with sulphuric acid, to
form the mineral barite or heavy spar. It sometimes occurs
as a carbonate, and more rarely as a silicate.
Phosphorus, although existing in comparatively insignificant
proportions, is nevertheless an important element, though in
nature it occurs only in combination with various bases, prin-
cipally lime, to form phosphates. In this form it is found in
the bones of animals, the seeds of plants, and constitutes the
essential portions of the minerals apatite and phosphorite.
Though small in proportion, phosphorus is a very important
constituent of any fertile soils. Its chief source, in the older,
crystalline rocks, is the mineral apatite, as noted later. As
found in the secondary rocks, as limestones and marls, it is
evidently derived from animal remains. (Seep. 151.) Analy-
ses have shown that the amount of phosphorus, in the form of
phosphoric anhydride (P2O6), i*1 rocks rarely exceeds 1%, and
usually falls much lower, being most abundant in the basic
eruptive rocks, as diorites and gabbros, and most lacking in
the siliceous fragmentals, as sandstones and slates. The fol-
lowing table will serve to show the small percentages of this
constituent in rocks of various kinds : —
8 CHEMICAL ELEMENTS CONSTITUTING THE ROCKS
Granite 0.07 to 0.25%
Diorite 0.18 to 1.06%
Basalt 0.03 to 1.18%
Limestone 0.06 to 10.00%
Shale 0.02 to 0.25%
Sandstone 0.00 to 0.1 %
Of the solid elements occurring free, or uncombined, carbon
is by far the more abundant, being found in the forms known
as diamond and graphite, or when quite impure as coal. In
combination as a dioxide (CO2), it forms the well-known car-
bonic acid gas, which, like oxygen, is a powerful agent in
bringing about important changes in the rocks with which it
comes in contact. Free sulphur occurs more rarely, being as a
rule a product of volcanic activity, or due to the reduction of
the sulphides and sulphates of the metal with which it more
commonly exists in combination.
III. THE MINERALS CONSTITUTING ROCKS
A rock, as previously stated, is a mineral aggregate. As a
rule, the number of mineral species constituting any essential
portion of a rock is very small, seldom exceeding three or four.
In common crystalline limestones, the only essential constitu-
ent is the mineral calcite ; granite, on the other hand, is,
as a rule, composed of minerals of three or four independent
species. As has been elsewhere stated, the mineral composition
of rocks in general is greatly simplified by the wide range of
conditions, under which the commonest minerals can be formed,
thus allowing their presence in rocks of all classes and of what-
ever origin. Thus quartz, feldspar, mica, the minerals of the
hornblende or pyroxene group, can be formed in a mass cooling
from a state of fusion ; they may be crystallized from solution,
or be formed from volatilized products. They are therefore
the commonest of minerals and rarely excluded from rocks of
any class, since there is no process of rock formation which
determines their absence. Moreover, most of the common
minerals, like the feldspars, micas, hornblendes, pyroxenes, and
the alkaline carbonates, possess the capacity of adapting them-
selves to a very considerable range of compositions. In the
feldspars, for example, the alkalies, lime, soda, or potash may
replace each other almost indefinitely, and it is now commonly
assumed that true species do not exist, all being but isomorphous
admixtures passing into one another by all gradations, and the
names albite, oligloclase, anorthite, etc., are to be used only as
indicating convenient stopping and starting points in the series.
Hornblende or pyroxene, further, may be pure silicates of lime
and magnesia, or iron and manganese may partially replace these
substances. Lime carbonate may be pure, or magnesia may
replace the lime in any proportion. These illustrations are
sufficient to indicate the reason of the great simplicity of rock
masses as regards their chief constituents, and that whatever
may be the composition of a mass within nature's limits, and
9
10 THE MINERALS CONSTITUTING EOCKS
whatever may be the conditions of its origin, the probabilities
are that it will be formed essentially of one or more of a half
a dozen minerals in some of their varieties.
But however great the adaptability of these few minerals may
be, they are, nevertheless, subject to very definite laws of chemi-
cal equivalence. There are elements which they cannot take
into their composition, and there are circumstances which retard
their formation while other minerals may be crystallizing. In
a mass of more or less accidental composition it may, there-
fore, be expected that other minerals will form in consider-
able numbers, but minute quantities. It is customary to speak
of those minerals which form the chief ingredients of any
rock, and which may be regarded as characteristic of any
particular variety, as the essential constituents, while those
which .occur in but small quantities, and whose presence or
absence does not fundamentally affect its character, are called
accessory constituents. The accessory mineral which predomi-
nates, and which is, as a rule, present in such quantities as to
be recognizable by the unaided eye, is the characterizing acces-
sory. Thus a biotite granite is a stone composed of the essential
minerals quartz and potash feldspar, but in which the accessory
mineral biotite occurs in such quantities as to give a definite
character to the rock. The minerals of rocks may also be con-
veniently divided into two groups, according as they are prod-
ucts of the first consolidation of the mass or of subsequent
changes. This is the system here adopted. We thus have: —
(1) The original or primary constituents, those which formed
•upon its first consolidation. All the essential constituents are
original, but, on the other hand, all the original constituents
are not essential. Thus, in granite, quartz and orthoclase are
both original and essential, while beryl and zircon or apatite,
though original, are not essential.
(2) The secondary constituents are those which result from
changes in a rock subsequent to its first consolidation, changes
which are due in great part to the chemical action of percolat-
ing water. Such are the calcite, chalcedony, quartz, and zeo-
lite deposits which form in the druses and amygdaloidal cavities
of traps and other rocks.
Below is given a list of the more important rock-forming
minerals, arranged as above indicated. Although these are
sufficiently described as regards their chemical and crystallo-
ROCK-FORMING MINERALS
11
graphic properties in any of the mineralogies, it has seemed
advisable to devote some space here to a reconsideration of
those most prominent as rock constituents, in order that the
individual characteristics of the rocks of which they form a
part may be better understood. In passing them in review
we will also note briefly the characteristic alteration and de-
composition products to which they give rise, though the cause
of such changes must be left for another chapter.
A, ORIGINAL MINERALS.
1. Quartz.
2. The Feldspars.
2 a. Orthoclase.
2 6. Microcline.
2 c. Albite.
2 d. Oligoclase.
2 e. Andesite.
2/. Labradorite.
_ (i. Bytownite.
2 h. Anorthite.
3. The Amphiboles.
3 a. Hornblende.
3 6. Tremolite.
3 c. Actinolite.
3 d. Arvedsonite.
3 e. Glaucophane.
3/. Smaragdite.
4. The Monoclinic Pyroxenes.
4 a. Malacolite.
4 b. Diallage.
4 c. Augite.
4 d. Acmite.
4 e. ^Egerite.
.""». The Rhombic Pyroxenes.
r>«r. Enstatite (Bronzite).
5 &. Hypersthene.
0. The Micas.
6 a. Muscovite.
66. Biotite.
6 c. Phlogopite.
7. Calcite (and Aragonite).
8. Dolomite.
9. Gypsum.
10. Olivine.
11. Garnet.
12. Epidote.
13. Zoisite.
14. Andalusite.
15. Staurolite.
16. Scapolite.
17. Elaeolite and Nepheline.
18. Leucite.
19. Sodalite.
20. Hauyn (nosean).
21. Apatite.
22. Menaccanite.
23. Magnetite.
24. Hematite.
25. Chromite.
26. Halite (common salt).
27. Fluorite.
28. Graphite.
29. Carbon.
30. Pyrite.
B. SECONDARY MINERALS.
1. Quartz.
1 a. Chalcedony.
16.
1 c.
Opal.
Tridymite.
12
THE MINERALS CONSTITUTING ROCKS
2. Albite.
3. The Amphiboles.
3 a. Hornblende.
3 b. Tremolite.
3 c. Actinolite.
3d. Uralite.
4. Muscovite (Sericite).
5. The Chlorites.
5 a. Jefferisite.
5 b. Ripidolite.
5 c. Penninite.
5 d. Prochlorite.
6. Calcite (and aragonite).
7. Wollastonite.
8. Scapolite.
9. Garnet.
10. Epidote.
11. Zoisite.
12. Serpentine.
13. Talc.
14. Glauconite.
15. Kaolin.
16. The Zeolites.
16 a. Pectolite.
16 b. Laumontite.
16 c. Phrenite.
16 d. Thomsonite.
16 e. Natrolite.-
16/. Analcite.
16 g. Datolite.
16 li. Chabazite.
16 i. Stilbite.
16 k. Heulandite.
16 1 Phillipsite.
16m. Ptilolite.
16 n. Mordenite.
16 o. Harmotome.
17. Hematite.
18. Limonite.
19. Gothite.
20. Turgite.
21. Pyrite.
22. Marcasite.
Quartz. — Composition: Pure silica, SiO2; specific gravity 2.6;
hardness, 7.1
This is one of the commonest and most widely distributed
minerals of the earth's crust, and forms an essential constituent
in a variety of eruptive and sedimentary rocks, such as granite,
1 For convenience in determining minerals, the "scale of hardness" given
below has been adopted by mineralogists. By means of it one is enabled to
designate the comparative hardness of minerals with ease and definiteness.
Thus, in saying that serpentine has a hardness equal to 4, is meant that it is of
the same hardness as the mineral fluorite, and can therefore be cut with a knife,
but less readily than ealcite or marble.
1. Talc: Easily scratched with the thumbnail.
2. Gypsum : Can be scratched by the thumbnail.
3. Calcite : Not scratched by the thumbnail, but easily cut with a knife.
4. Fluorite : Can be cut with a knife, but less easily than ealcite.
5. Apatite : Can be cut with a knife, but only with difficulty.
6. Orthoclqse feldspar : Can be cut with a knife only with great difficulty and
on thin edges.
7. Quartz : Cannot be cut with a knife ; scratches glass.
8. Topaz : Will scratch quartz.
9. Corundum : Will scratch topaz.
10. Diamond: Will scratch corundum.
QUARTZ 13
quartz porphyry, liparite, gneiss, mica schist, quartzite, and
sandstones. In the granites, gneisses, and schists it occurs in
the form of irregular granules destitute of crystal outlines.
In the quartz porphyries and liparites i\ is found as a porphy-
ritic constituent, usually with well-defined crystal outlines,
which may however have become more or less obliterated
through the corrosive action of a molten rnagma. (See Fig. 3,
PL 5.) In the secondary rocks, quartzite and sandstone, the
quartz occurs as more or less rounded or irregularly angular
grains without crystal outlines, except it may be through a
secondary deposition of silica, as explained on p. 158. Quartz
is the hardest and most indestructible of the common constitu-
ents, and hence when rocks containing it decompose and their
debris becomes exposed to combined chemical and mechanical
agencies, it remains unaltered to the very last, forming the
chief constituent of beds of sand and gravel, which in turn
may become transformed into sandstones, quartzites, or con-
glomerates.
Quartz is usually easily recognized, either under the micro-
scope or by the unaided eye, by its clear, colorless appear-
ance, irregular, glass-like fracture, — having no true cleavage,
— hardness, and insolubility in any acids but hydrofluoric.
Under the microscope it appears in clear, pellucid grains, often
highly charged with minute cavities filled with liquid and
gaseous carbonic acid, the latter like the bubble in a spirit
level, dancing about from side to side of its minute chamber as
though endowed with life. In other cases the cavity may be
filled with a saline solution from which has separated out a
minute cube of common salt.
As a secondary constituent quartz occurs, filling veins and
cracks in other rocks, and in the impure crypto-crystalline and
amorphous forms known as chalcedony, chert, flint, opal, hya-
lite, and agate is found as an infiltration product in the cavities
of many trappean rocks, in lenticular and oval concretionary
masses in limestones, and replacing the organic matter of wood
and other organisms. The name tridymite is given to a quartz
occurring in minute, usually microscopic, tablets in cavities in
volcanic rocks, particularly the more acid varieties. (See fur-
ther on p. 71.)
The Feldspars. — Hardness, 5 to 7; specific gravity, 2.5 to
2.8. The feldspars are essentially anhydrous silicates of alu-
14 THE MINERALS CONSTITUTING ROCKS
minum, with varying amounts of lime, potash, or soda, and
rarely barium. They have in common the characteristics of
two easy cleavages inclined to one another at an angle of
90°, or nearly 90°; close relationship in optical properties;
similarity in colors, which vary from clear and transparent
through white, yellowish pink, and red; more rarely greenish,
and often opaque through impurities or decomposition; and
lastly, a constant intergradation in composition, as already
noted on p. 9.
Nine varieties of feldspar are commonly recognized, which
on crystallographic grounds are divided into two groups: the
first, crystallizing in the monoclinic system, including ortho-
clase and hyalophane; and the second, crystallizing in the
triclinic system, including microcline, anorthoclase, and the
albite-anorthite series albite, oligoclase, andesine, labradorite,
and anorthite.
The Monoclinic Feldspars : Orthoclase (Sanidiri), Potash Feld-
spars. — Composition : K2Al2Si6O16 = silicia, 64.7 % ; alumina,
18.4%; potash, 16.9%.
This is one of the commonest and most abundant of feldspars,
and forms an essential constituent of the acid rocks, such as gran-
ite, gneiss, syenite, and the orthoclase and quartzose porphyries;
more rarely it occurs as an accessory in the more basic erup-
tives. Under the name sanidin is included the clear glassy
variety of orthoclase occurring in tertiary and modern lavas,
such as trachyte, phonolite, and the liparites.
Among the older rocks orthoclase not infrequently occurs in
very coarse pegmatitic crystallizations with quartz and mica,
and is quarried for utilization in pottery manufacture. As a
rock constituent the potash feldspars are of primary impor-
tance, imparting by their preponderance, not merely color
and important structural features, but on their decomposition
yielding up the alkali potash, valuable for plant food, and the
mineral kaolin so essential for porcelain ware, or in its impure
state, as clay for pottery and brick making. In the thin sec-
tions, under the microscope, the orthoclase of the older rocks
is, as a rule, found to be quite opaque, or at least muddy,
through impurities or incipient kaolinization. In many erup-
tives it has been one of the first minerals to separate out from
the molten magma, and shows, therefore, more or less well-
defined crystallographic boundaries — is idiomorphic, to use a
THE TRICLINIC FELDSPARS
15
more technical term. A well-defined zonal structure is fre-
quently observed, which is due to interrupted periods of
growth, and not infrequently to a gradual change in the char-
acter of the magma, whereby the outer zones are more or less
translucent or opaque from impurities. Twin structure is very
common after what is known as the Carlsbad law, and when
the crystals are of sufficient size is easily recognized by the
unequal reflection of the light from the two sides of a crystal
011 a cleavage surface.
The Triclinic Feldspars. — The chemical relationship exist-
ing between the triclinic feldspars is shown in the following
table : —
SiO±
Al,0,
KjO
N:ul>
CaO
Microcline
65.00%
18.00%
17.00%
Albite
68.00
20.00
12.00%
gjQ flrp.
Oligoclase
62.00
24.00
9.00
5.00
Labradorite
53.00
30.00
4.00
13.00
Anorthite
43.00
37.00
20.00
Considering only the last four of these, as arranged, it will
be noted that they become gradually poorer in the acid element
silicia, and richer in alumina and other bases; that is, they
become more basic. Also that albite carries some 12 % of soda
and no lime; that oligoclase carries 9 % of soda and 5% of lime;
labradorite but 4 % of soda and 13 % of lime, while anorthite,
the most basic of all, has no soda, and carries 20 % of lime.
They have hence come to be known, respectively, as soda feld-
spar, soda-lime feldspar, lime-soda feldspar, and lime feldspar.
As a matter of fact, however, these varieties all grade into one
another, through the replacing power of the various elements,
and are regarded, not as true species, but rather as isomorphous
admixtures, forming what is known as the albite-anorthite
series.
Their distinction, either in hand specimens by the unaided
eye, or in thin sections by the microscope, is a matter of con-
siderable difficulty, and as in addition to other characteris-
;ics they have in common two eminent cleavages occurring at
)blique angles, it has become customary to group all under
;he general term of playioclase, a name derived from two
16 THE MINERALS CONSTITUTING ROCKS
Greek words signifying oblique and fracture. We can then
treat of the subject under the heads of (1) microline and
(2) plagioclase.
(-1) Microcline (Triclinic Potash Feldspar). — As a rock con-
stituent, this feldspar is in- every way nearly, if not quite,
identical with orthoclase, from which it can be distinguished
only in thin sections under the microscope. Its composition,
manner of occurrence, and associations are those of orthoclase,
and need not be repeated here. Anorthoclase is a triclinic
soda-potash feldspar of a form closely resembling that of ortho-
clase and which for all present purposes may be regarded as
orthoclase in which soda replaces a considerable proportion of
the potash.
(2) The Plagioclases. — With the exception of albite the
plagioclases are all prominent and essential constituents of
the basic eruptives. As a rule they are recognizable only as
feldspars by the unaided eye, and recourse must be had to
the microscope or to chemical tests for their final determina-
tion. Examined in thin sections and by polarized light, they
almost invariably show a beautiful parallel banding in light
and dark colors, which is due to multiple twinning, the alter-
nate bands becoming light and dark in turn as the stage of the
microscope is revolved. When the crystals are of sufficient
size, this twinning is sometimes evident in the form of fine
straight, parallel bands, or striae, but in rock masses, as already
noted, recourse must be made to microscopic methods. In form
the plagioclase of effusive rocks is most frequently slender and
elongated, lath-shaped, as commonly described, and often with
very perfect crystal outlines. In the norites and gabbros, they
are often short and stout, imparting a granular character to the
rock. They occur frequently in crystals of two or more gen-
erations, of which the earlier formed are usually the largest
and best developed. The common forms are described in de-
tail below : —
(1) Albite, or soda feldspar, occurs as an original constituent
in many granites in company with orthoclase; it is also found
in gneiss, the crystalline schists, and not infrequently in diorite,
phonolite, trachyte, and other eruptives. (2) Oligoclase, a soda-
lime feldspar, occurs like albite in the acid eruptives like gran-
ite and quartz porphyry, but is also a common constituent of
diorite, and the younger eruptives such as trachyte, the aride-
THE TRICLINIC FELDSPARS 17
sites, and more rarely of the diabases. It is also a constituent
of many gneisses. (3) Labradorite, or lime-soda feldspar, is a
prominent constituent of the basic eruptives of all geologi-
cal ages, such as the norites, diabases, diorites, and basalts.
Andesine and bytownite are closely allied varieties of similar
habit, the first being a trifle more acid, and the second more
basic than labradorite. (4) Anorthite, or lime feldspar, is
also a prominent and important constituent of the basic
eruptives, and has been found in meteorites and terrestrial
peridotites.
On account of their abundance and wide distribution, as well
as on account of the character of their decomposition products,
the feldspars are to be considered as among the most important
of rock constituents. As it is from the debris of the older
feldspathic rocks that have been made up a large proportion
of all the sedimentaries of more recent date, so too it may be
claimed that from the decomposition of this feldspathic con-
stituent has been derived a large share of the salts of potash,
lime, and soda, as well as aluminous silicates which form so
essential a portion of the soils. The method of feldspathic
decomposition as commonly understood is given on p. 237.
This decomposition usually manifests itself by a whitening
of the mass, accompanied by opacity and a general softening,
whereby it falls away to loose powder unless confined. As seen
in thin sections under the microscope, the decomposition goes
on most rapidly along lines of cleavage, naturally attacking the
outer portions first, so that the crystals show fresh unaltered
cores surrounded by opaque and u muddy " borders. In cases
where the feldspars carry iron this usually makes its presence
known by a reddening or browning of the mass, due to oxida-
tion. In presence of abundant carbonic acid, the liberated iron
may enter into combination as a carbonate and the color remain
unchanged.
Daubree, who submitted feldspathic fragments to trituration
in revolving cylinders of stone and iron, found that in all such
cases not merely were the particles worn down to the condi-
tion of fine silt, but that there was an actual decomposition,
whereby a certain proportion of the alkalies in the form of
soluble silicates were formed in the water with which the cyl-
inders were partially filled. When the trituration was carried
on in iron cylinders, a certain amount of iron oxides were
18 THE MINERALS CONSTITUTING ROCKS
formed which combined with the silica of the alkaline silicate,
leaving the alkali itself free. As in nearly all decomposing
rocks there exists more or less of iron oxides from decomposing
ferruginous minerals, it is not impossible that a similar reaction
is there going on.
The production of kaolin through feldspathic decomposition
has become so well recognized that it is customary to speak
of this form of decomposition as kaolinization, a term which we
shall have frequent cause to use as we proceed.
It should be noted that orthoclase, though so frequently
found muddied and impure, apparently in an advanced stage
of decomposition, does not in reality decompose so readily as
the plagioclase (soda-lime) varieties. This fact has been noted
by Lemberg,1 who states that the apparent decomposition may
be due to physical causes, as disintegration, inclusions of some
easily decomposable silicate, or to originally water-filled cavities
whose contents have been absorbed through the formation of
secondary hydrous silicates.
Leucite. — Composition: Silica, 55.0 % ; alumina, 23.5 % ; pot-
ash, 21.5%.
Leucite occurs as an original and essential constituent of
many volcanic rocks, such as leucitophyre, leucotephrite, and
leucitite. More rarely it occurs in trachyte. It is a common
associate of nepheline in recent lavas, and has been found asso-
ciated with elseolite in the elseolite syenites of Hot Springs,
Arkansas. When well developed it shows polyhedral, garnet-
like outlines.
Leucite as a rock constituent is not an abundant mineral
except in rare instances. Its chief interest, from our present
standpoint, lies in its high percentage of potash which must
become available as plant food on decomposition. Leucite is
a common constituent of certain Vesuvian lavas, and it is not
improbable that this fact may account in part for the well-
known fertility of the soils of this region, though naturally
climatic influence has much to do.
Nepheline (Elaeolite). — These names are given to what are
varietal forms of one and the same mineral. In composition
they are silicates of alumina, soda, and potash of the formula
(NaK)2Al2Si2O8 = silica, 41.24; alumina, 35.26; potash, 6.46;
soda, 17.04.
1 Zeit. Deut. Geol. Gesellschaft, 35, 1883.
THE AMPHIBOLES 19
Nepheline occurs in Tertiary and post-Tertiary eruptive rocks,
and is an essential constituent of phonolite, tephrite, and nephe-
linite. Secondary nepheline has been found in the ejected vol-
canic blocks found in the lava of Mount Somnaa. The variety
eheolite occurs only in older rocks, and is an essential constitu-
ent of elteolite syenite. Cancrinite is a yellowish granular
mineral, in some cases apparently resulting from the alteration
of ekeolite, with which it occurs.
Both nepheline and elaeolite gelatinize readily with hydro-
chloric acid, and the powdered rock when treated on a glass slide
with this acid yields abundant microscopic cubes of sodium
chloride. This is one of the easiest of microchemical tests for
the determination of the mineral. Nepheline occurs as a rule
in well-defined short and stout hexagonal prisms, which in
longitudinal sections show up as short, colorless rectangular
areas extinguishing parallel with the sides of the prism. ElaBO-
lite differs in being more opaque and occurring in less well-
defined, more granular forms. When occurring in sufficient
abundance in a rock mass it is readily recognized by its char-
acteristic greasy appearance. The mineral undergoes a ready
alteration, giving rise to zeolitic minerals and on ultimate
decomposition through weathering, yielding a rich and fertile
soil.
The Amphiboles. — Composition : Two principal varieties are
recognized. (1) Non-aluminous, consisting mainly of the
meta-silicates of magnesium and calcium, with 55 to 50% of
silica, 21 to 27 % of magnesia, 11 to 15 % of lime, and small pro-
portions of protoxides of iron and manganese. Under this head
are included the white, graj% and pale green, often fibrous forms,
as tremolite, actinolite, and asbestos. (2) Aluminous, contain-
ing silica, 40 to 51 % ; magnesia, 10 to 23 % ; alumina, 6 to 14 % ;
lime, 10 to 13%; ferrous and ferric oxides, 12 to 20%. Here
are included the dark green, brown, and black varieties.
The aluminous variety, common hornblende, is an original
and essential constituent of diorite, and of many varieties of
granite, gneiss, syenite, schist, andesite, and trachyte, and is
also present as a secondary constituent in many rocks, result-
ing from the molecular alteration of the augite. The non-
aluminous varieties occur in gneiss, crystalline limestone, and
other metamorphic rocks.
By the unaided eye, or by means of blowpipe tests, it is often
20
THE MINEEALS CONSTITUTING ROCKS
impossible to distinguish the minerals of this group from
the pyroxenes. In the thin sections this distinction is, however,
a matter of comparative ease, basal sections showing not merely
a greater development of prismatic faces, but also cleavages
cutting at angles of 66° and 124° instead of nearly at right
angles, as in the latter. Green fibrous hornblendes frequently
result from the molecular alteration of augite, and all varieties
are susceptible of alteration into chloritic and ferruginous
products with the separation of calcite. In the recent lavas it
is a common occurrence to find the hornblendes surrounded by
a black border, or wholly changed by corrosion of the molten
magma into an aggregate of small black opaque granules, which
in certain instances have been proven to be augites.
On decomposing, the amphiboles give rise to ferruginous and
aluminous or magnesian products, as do the pyroxenes, next to
be described. With the darker colored varieties, the decompo-
sition begins with hydration and the peroxidation of the iron
along lines of cleavage and fracture, whereby the crystal
becomes riddled with corroded areas filled with the liberated
iron in the form of hydrated sesquioxide.
When the disintegration is complete, the whole mass is con-
verted into an ochre-brown, earthy substance. These chemical
changes are indicated in the following analysis of I. fresh, and
II. decomposed hornblende from Haavi on Fillef jeld, Norway: 1 —
I
n
Silica
45.37
40.32
Alumina
1481
17.49
Iron protoxide ....
Manganese
8.74
1.50
Iron peroxide . . .
18.26
2.14
Lime
14.91
5.37
Magnesia
14.33
9.23
Water
8.00
99.66
100.81
The most striking features of the above analyses are (1)
the complete conversion of the protoxides into sesquioxides,
(2) the loss in lime and magnesia which have presumably
1 Bischofs Chemical Geology, Vol. II, p. 354.
THE MONOCLINIC PYROXINES 21
been carried away in the form of carbonates, and (3) the
assumption of 8% of water. As the dark aluminous and
ferruginous hornblendes are among the commonest and most
wide-spread of minerals, it is apparent from the above that
they may have an important bearing upon the color and physi-
cal qualities of the residual clays ; to which they thus give
rise. The peroxidation of the iron gives yellow, brown, or red
colors, while the hydrated aluminous silicate (clay) imparts
tenacity. The final product of such decomposition is, then, a
ferruginous clay.
The Pyroxenes. — The rock-forming pyroxenes are divided
upon crystallographic grounds into two groups, the one ortho-
rhombic in crystallization, and the other monoclinic. All varie-
ties, when in good crystalline form, show in basal sections an
octagonal outline bounded by prismatic and pinacoidal faces
and with a well-defined cleavage parallel with the prism faces.
Chemically they are silicates of magnesia and iron with lime
and alumina in varying proportions. They are hard, tough
minerals and have an important bearing upon the physical
properties of the rocks of which they form a part. Their dis-
tribution, in some of their varieties, is almost universal, being
found in metamorphic and eruptive rocks of almost every class
and every age.
The Monoclinic Pyroxenes. — Two principal varieties are recog-
nized. (1) Pyroxenes containing little or no alumina, and com-
posed of silica, 45.95 to 55.6 % ; lime, 21.06 to 25.9 % ; magnesia,
13.08 to 18.5 %, with sometimes varying quantities of iron oxides
and water. Under this head are included the lighter colored
varieties, malacolite, sahlite, and diallage. (2) Pyroxenes con-
taining alumina, and composed of silica, 49.40 to 51.50 %; alu-
mina, 6.15 to 6.70$,; magnesia, 13.06 to 17.69%; lime, 21.88
to 23.80%; iron oxides, 0.35 to 7.83%, with sometimes small
quantities of soda and water.. Under this head are included
the darker varieties, augite and leucaugite.
The lighter colored, non-aluminous varieties, malacolite and
sahlite, are common in mica and hornblendic schists, gneiss,
and granite, though not always in sufficient abundance to be
noticeable to the naked eye. The foliated variety, diallage,
is an essential constituent of the rock gabbro, and is also
common in peridotites. The darker colored, aluminous vari-
ety, augite, is an essential constituent of diabase and basalt,
22 THE MINERALS CONSTITUTING ROCKS
and also occurs in many syenites, andesites, and other eruptive
rocks.
In the thin sections the monoclinic pyroxenes are usually
readily recognized by their nearly rectangular cleavages on
basal sections (see Fig. 1), lack of pleochroism, and high
extinction angles on sections parallel to the clinopinacoids.
The aluminous varieties undergo alteration into chloritic and
ferruginous products, while the non-aluminous give rise to ser-
pentine, either process being attended by the separation of
free calcite.
JEgerine and acmite are soda-bearing pyroxenes corre-
sponding to the formula Na2OFe2O34SiO2. They are less
abundant than the above-mentioned varieties, and so far as
yet described seem to be confined mainly to the elseolite
syenites.
The Orthorhombic Pyroxenes. — These are essentially silicates
of magnesia and iron, the latter replacing the former in varying
proportions up to as high as 25%. Two principal varieties are
recognized, the distinction being founded mainly upon their
optical properties which seem to be affected very largely by the
percentages of iron. Enstatite is the theoretically pure mag-
nesian silicate of the formula MgSiO3, but which, as a matter
of fact, usually contains from 2 to 10 % or more of iron. The
highly ferruginous varieties are known as bronzite, from their
bronze-like lustre. HyperstJiene differs from enstatite in being
strongly pleochroic in thin sections, and it contains from 10 to
25 °/0 of ferruginous .oxide.
Both enstatite and hypersthene are common constituents of
basic igneous rocks, such as the gabbros, norites, and perido-
tites. Enstatite is a common constituent of meteorites, occur-
ring not infrequently in peculiar fan-shaped, radiating masses
not greatly unlike certain organic forms for which they were
once mistaken. Both forms are liable to alteration, giving rise
to serpentinous pseudomorphs to which the name bastite has
been given, and to talcose and chloritic products. The general
character of the decomposition products of the pyroxenes, as
well as the methods by which the decomposition progresses,
are in every way similar to those of the amphiboles, and need
not be further dwelt upon here.
The Micas. — There are several species of mica which are
prominent as rock constituents, the more important being the
THE MICAS 23
white variety, muscovite, and the dark variety, biotite. Both
occur, as a rule, in thin, platy forms, splitting readily into thin,
elastic folia, which in crystalline form are hexagonal in outline.
The folia are often bent and distorted, and the mineral not
infrequently undergoes alteration into a chloritic or sericitic
product. The micas exercise an important influence upon the
rocks containing them, on both color and structural grounds.
Other things being equal, the muscovite-bearing rocks are
lighter in color than those carrying biotite. If the mica plates
are arranged in definite planes, the rock assumes a schistose
structure and splits more or less readily into sheets — an impor-
tant feature from an economic standpoint. Muscovite, or
potash mica, a silicate of alumina and potash, is a constituent
of many granites, gneisses, and schists, but is rarely met with
in other rocks, and is wholly wanting in the basic eruptives.
Another white or nearly colorless mica is sericite, a silvery
white, or greenish, hydrous, secondary constituent of metamor-
phic schists, or occurring as an alteration product from feldspar:
paragonite and margarite are other hydrous micas, confined
mainly to the schists and to veins. Lepidolite, a lithia mica of
a white or faint pink color, is frequently found in pegmatitic
veins in the older rocks.
Biotite, the black iron mica, is a silicate of alumina, iron, and
magnesia, and is much more general in its distribution than is
muscovite, occurring in both eruptive and metamorphic rocks
of all kinds and of all ages. It undergoes alteration into
chloritic and ferruginous products and is often an impor-
tant feature in hastening rock disintegration. Other black
micas, sometimes distinguishable from biotite only by chemi-
cal means, are lepidomelane and houghtonite. A pearl gray
potash mica phlogopite is an important constituent of many
limestones, as in northern New York and adjacent portions of
Canada.
All micas, owing to their eminently fissile structure, allow the
ready percolation of moisture, and hence, though in themselves
of difficult solubility, are elements of weakness in any stone
of which they may form a part. The characteristic form of
decomposition begins as in other silicate minerals, with hydra-
tion. This in the dark varieties is accompanied by a higher
oxidation of the iron. The folia? gradually lose their elasticity
and crumble away, the bases being removed in solution as
THE MINERALS CONSTITUTING ROCKS
before. The complete decomposition of the micas is, however,
brought about very slowly, and almost any granitic soil, how-
ever thoroughly decomposed, will, on washing, show small flakes
of the mineral still remaining. However rusty, too, these may
appear, a little hydrochloric acid cleans them up, showing rem-
nant shreds still fresh and readily recognizable. For some
unexplained reason those granitic rocks containing a consider-
able proportion of white mica are almost invariably more friable
and easily disintegrated than those containing biotite.
Olivine (Chrysolite, Peridote). — Composition: Silicate of iron
and magnesia, (MgFe)2 SiO4.
This is an essential constituent of basalt, dunite, limburgite,
Iherzolite, and pikrite, and a prominent ingredient of many
lavas, diabases, gabbros, and other igneous rocks. It also occurs
occasionally in metamorphic rocks and is a constituent of most
meteorites. Olivine is subject to extensive alteration, becom-
ing changed by hydration into serpentine or talcose and chloritic
products, with the separation of free iron oxides. Under the
microscope olivine is as a rule easily recognized by its lack of
cleavage and brilliant polarization colors. It occurs in well-
defined crystals and also in irregular grains, either singly or
grouped in peculiar clusters to which the name polysomatic has
been applied by Tschermak. The serpentinous alteration takes
place along the irregular curvilinear lines of fracture, and under
favorable .conditions continues until the transformation is com-
plete. The following analyses by Holland, as quoted in Teall's
British Petrography, illustrate the simplicity of the chemical
changes which here take place: —
I
II
III
Si02
41.32%
42.72 %
43.48 %
AloO^
028
006
Fe203.
239
225
CrO
005
Trace
M>0 .
5469
42 52
4348
H2O
020
1339
1304
98.93 %
100.94%
100.00%
I. Olivine, Snarum, Norway. II. Serpentine derived from the same.
III. The theoretical composition of serpentine.
EPIDOTE AND ALLANITE 25
Aside from the assumption of some 13 % of water, the princi-
pal change, as will be noted, is a loss in magnesia which as a
rule separates out as a carbonate. The iron, which existed as
protoxide, is further oxidized and crystallizes out along lines of
fracture as magnetite or hematite, or in the hydrous sesquioxide
form known as limonite. Through decomposition, a portion or
all of the silica may be set free as opal or chalcedony, the mag-
nesia going over to the condition of carbonate, and the iron
passing into various hydrated oxide forms such as are most
stable under the existing circumstances.
Epidote. — Composition: Silica, 37.83%; alumina, 22.63%;
iron oxides, 15.98 % ; lime 23.27 % ; water 2.05 %.
This mineral is a common constituent of many granites,
gneisses, and schists, especially the hornblendic varieties. It
is particularly abundant, however, as a secondary constituent
in basic eruptives, where it results from the alteration of the
original ferromagnesian constituents such as the augites, horn-
blendes, or micas. It is the presence of this mineral or a sec-
ondary chlorite that gives the characteristic color to many of
the so-called greenstones (altered basalts, diabases, diorites, etc.).
The name piedmontite is given to a red manganese epidote,
which has been found in certain Japanese schists and has also,
in sparing amounts, been observed by Professor Haworth,1 in the
quartz porphyries of Missouri, and a few foreign porphyrites.
Zoisite is a closely related mineral crystallizing in the ortho-
rhombic system and relatively poorer in iron and richer in
alumina than is epidote. It is chiefly characteristic of the crys-
talline schists, though sometimes found in granitic rocks, inter-
grown with common epidote as has been noted in Maryland, by
Keyes.2
Allanite, or orthite, as it is sometimes called, is closely allied
to common epidote, but contains cerium and other of the more
rare alkaline earths. In the form of brown acicular crystals it
is a common constituent of New England granites and has
recently been described in a granite porphyry near Ilchester,
Maryland, where it occurs enclosed as a nucleus in the ordinary
epidote.
Calcite (Calcium Carbonate). — Composition: CaCO3 = Car-
bon dioxide, 44 % ; lime, 56 %• Hardness, 3.
1 American Geologist, Vol. I, p. 365.
2 15th Ann. Rep. U. S. Geol. Survey, 1890-94.
26 THE MINERALS CONSTITUTING KOCKS
This is an original constituent of many secondary rocks,
such as limestone, ophiolite, and calcareous shales. It is the
essential constituent of most marbles, of stalactites, travertine,
and calc-sinter. The shells of foraminifera, brachiopods, crus-
taceans, and many lamellibranchs and gasteropods are also of
this material. As a secondary constituent, resulting from the
decomposition of other minerals, it occurs almost universally,
filling wholly or in part cavities in rocks of all ages, such as
granite, gneiss, syenite, diabase, diorite, liparite, trachyte,
andesite, and basalt.
The effervescence of the mineral when treated with a dilute
acid furnishes the most ready means for its detection. Under
the microscope it appears as colorless grains with faint irides-
cent polarization, and is best recognized by its cleavage and
characteristic twinning lines as shown in the figure on p. 163.
Being soluble in carbonated waters, it is liable to complete
removal, or leaves only its impurities behind as a mark of its
decay.
Aragonite. — Composition : CaCO3 = Carbon dioxide, 44 % ;
lirne, 56 %.
This mineral has the same chemical composition as calcite,
but differs in its crystalline form and specific gravity. It
occurs with beds of gypsum and veins of ore, and also in
stalactitic and stalagmitic forms. In small quantities it occurs
as a secondary product in many trap rocks and basalts, and is
the substance of which the shells of many gasteropod and
lamellibranch molluscs are composed.
The mineral occurs nearly always in clustered aggregates of
radiating, divergent needles, and is distinguished from calcite
by its crystallization and cleavage. As a rock constituent it is
comparatively unimportant, but frequently occurs as a decom-
position product in basic eruptives. This form of calcium
carbonate, as long ago pointed out by Sorby, is less stable than
calcite, and in many instances where the substance has first
crystallized in the orthorhombic form aragonite, it is found to
have undergone a molecular alteration into calcite.
Dolomite. — Composition: (CaMg)CaO3 = Calcium carbonate,
54.35%; magnesium carbonate, 45.65 %. Hardness, 3.5-4.
This mineral, like calcite, is a wide-spread constituent of
rocks, and not infrequently forms extensive masses which are
of value as sources of building material. It is distinguishable
APATITE AND THE IRON ORES 27
from calcite by its greater hardness, higher specific gravity,
and in being but slightly acted on by acetic or dilute hydro-
chloric acid. In itself the mineral is less susceptible to atmos-
pheric influence than calcite, yielding much less readily to
decomposing agencies of a purely chemical nature. Never-
theless, Roth1 has shown that in the weathering of dolomitic
limestones the magnesia is sometimes removed by leaching, in
greater proportional quantities than the more soluble lime
carbonate.
Apatite. — Composition: Phosphate of lime. Hardness, 5.
Apatite is an almost universal constituent of eruptive rocks,
both acid and basic, though as a rule present only in micro-
scopic proportions. In the granular limestones, schists, and
other metamorphic and vein rocks it sometimes occurs in large
crystals or massive forms in such abundance as to be of value
as a source of mineral phosphate for fertilizing purposes. In
the thin sections the apatites of eruptive rocks are as a rule
colorless, and without evident cleavage, though presenting
good crystallographic forms. Rarely the mineral is pleochroic
in red or brown or bluish colors. If a drop of an acid solution
of ammonium molybdate be placed upon an apatite crystal in
an uncovered slide, the mineral will be slowly dissolved and
minute crystals of phosphomolybdate of ammonium be contem-
poraneously deposited. The process is an easy one, readily
performed while the slide is still on the stage, and forms one of
the most interesting and accurate of the many microchemical
tests. Though present in but small amounts, apatite is an
important constituent, since it is the only common rock con-
stituent containing the valuable element phosphorus.
THE IRON ORES
Under this head we may conveniently treat the several
forms of iron oxides which commonly occur as rock constitu-
ents, and which from their opacity in even the thinnest sec-
tions, and similarly in crystallographic outline, are separable
with difficulty by optical tests alone.
Magnetite. — Composition : FeO + Fe2O3 = iron sesquioxide,
68.97 %; iron protoxide, 31.03 %.
This is a wide-spread and almost universal constituent of
1 Chemische u. Allgemeine Geologic.
28 THE MINERALS CONSTITUTING ROCKS
eruptive rocks, occurring as a rule in the form of scattering,
small, and rather inconspicuous granules, which under the
microscope are characterized by a complete opacity and bluish
lustre. When of sufficient size to be distinguished by the
unaided eye, magnetite is easily recognized by its brilliant
lustre, weight, and its property of being readily attracted by
the magnet. It is as a rule one of the first minerals to sepa-
rate out from the molten magma, and hence presents good
crystal outlines in which octahedral forms prevail. Skeleton
forms of great beauty are not infrequent. Magnetite also
occurs as a constituent of metamorphic rocks and is some-
times found in large beds, constituting a valuable ore of iron.
Under continual alternations of heat and cold, moisture and
dryness, it slowly decomposes, giving rise to hydrated sesqui-
oxides which impart color, but no valuable qualities, to the
resultant sands and clays.
Menaccanite (Ilmenite or Titanic Iron). — Composition :
(TiFe)2O3, a mixture in varying proportions of the oxides
of iron and titanium.
This, like magnetite, occurs in scattering granules as an
original constituent of many eruptive rocks, and also in mica-
ceous lamellar and vein-like masses in other rocks. Under the
microscope it shows, by incident light, a brownish rather than
bluish lustre, but is best recognized by its characteristic altera-
tion products, which are whitish, gummy, and opaque. The
name leucoxene was given by Gumbel to the final product of
this alteration. This form of iron ore is extremely refractory
to atmospheric agencies and is to be found scarcely, if any,
changed in the residuary materials resulting from the breaking
down of the rocks in which it originated.
Hematite (Specular Iron Ore.) — Composition: Anhydrous ses-
quioxide of iron, Fe2O3 = iron, 70.9 % ; oxygen, 30.20 %. H =
5.5-6.5.
This mineral occurs in varying proportions and under vary-
ing conditions in rocks of all ages. In the form of minute
scales of a blood-red color, it is found not infrequently in
granitic and other eruptive rocks. It occurs, also, in large
beds, forming a valuable ore 'of iron. In the amorphous
condition, it may form the cementing constituent of sand-
stones, and is the cause of the red color of many rocks, both
clastic and metamorphic, and of soils as well. The usual color-
LIMONITE AND PYRITE 29
ing constituent is, however, limonite or turgite, as noted below.
The specular and massive forms are best recognized by opacity,
brilliant, black, metallic lustre, and red streak.
Limonite (Brown Hematite). — Composition: Hydrous ses-
quioxide of iron, H6Fe2O6 + Fe2O3=iron sesquioxide, 85.6%;
water, 14.4 %. H = 5-5.5.
This is a common constituent of rocks of all ages, but is as
a rule wholly secondary, resulting from the decomposition of
ferruginous silicates, sulphides, and anhydrous oxides. As a
coloring constituent it is even more abundant than hematite,
and like it forms a valuable ore of iron. (See p. 107.) Turgite
(Fe4H2O7) in the form of a brilliant red ochreous material is
also a common constituent of soils and clays resulting from the
decomposition of siliceous rocks, and is presumably, like limo-
nite, a product of the spontaneous hydration of the iron salts
thus set free. (See further under Color of Soils, p. 385.)
Pyrite (Iron Pyrites). — Composition: Iron disulphide, FeS2
= iron, 46.7 %-, sulphur, 53.3 %. H = 6-6.5.
Two principal forms of iron disulphide occur in nature, alike
in chemical composition, but differing in forms of crystalliza-
tion and in density. The one is common pyrites which crys-
tallizes in the isometric system, and is easily recognized by its
strong brassy yellow color and hardness. Its usual form of
occurrence is that of cubes, the corners and edges of which may
be more or less modified by secondary planes, and in concre-
tionary masses. The second form marcasite, also called gray,
white, or cockscomb pyrites, is of lighter color, inferior hard-
ness and density, and crystallizes in the orthorhombic system.
Its most common form of occurrence is that of irregular con-
cretionary masses.
Both forms of pyrite are susceptible to oxidation when
exposed to atmospheric agencies, though of the two the pyrite
proper is much the more refractory.
Mr. A. P. Brown has shown1 that in this form of the com-
pound a large proportion of the iron exists in a, ferric condition
while in marcasite it is ferrous. In other words, marcasite is
an unsaturated compound, and hence unstable. This readily
explains the relatively more rapid decomposition of the latter
mineral. There is also a difference in the character of the
products arising from the decomposition of the two compounds,
* Proc. American Philos. Soc., Vol. XXXIII, 1894, p. 225.
30 THE MINERALS CONSTITUTING ROCKS
pyrite yielding, as a rule, limonite and free sulphur, while mar-
casite, under the same conditions, yields ferrous sulphate, though
when decomposing under water, it may also yield much limonite.
The sulphate of iron, resulting from pyritiferous decomposition,
is, if present in quantity, injurious to plant growth. This fact
was well illustrated some years ago on the west front of the
National Museum at Washington. Several large masses of iron
sulphide, too large for exhibition within the building, were
placed here upon a floor of cement bordered by a narrow strip
of lawn. Under the oxidizing influence of rain and air the
sulphide became slowly converted into sulphate which was
washed down upon the cement and thence into the soil, which
it so poisoned as to kill the roots and necessitate an entire
resodding.
The experiments of Prichard, 1 however, showed that the
presence of a small amount of sulphate of iron in a soil may,
under certain conditions, be beneficial, in that it serves to pre-
vent the loss of ammonia in rapidly decomposing materials.
In processes involving slow decomposition, its antiseptic quali-
ties render it of doubtful value.
Chlorite (Viridite). — Under the general name chlorite are
included several minerals occurring in fibres and folia, closely
resembling the micas, from which they differ in their large per-
centage of water, and in their folia being inelastic. The three
principal varieties recognized are, ripidolite, penninite, and pro-
chlorite, any one of which may occur as the essential constitu-
ent of a chlorite schist. Chlorite as a secondary product often
results from and entirely replaces the pyroxene, hornblende, or
mica in rocks of various kinds, and also occurs filling wholly or
in part the amygdaloidal cavities of trap rocks. In this form
it is frequently visible only with the microscope, and owing to
the difficulties in the way of an exact determination of its
mineral species is sometimes called viridite. It is this mineral
which gives the green color to a large share of the more or
less altered eruptives, like the diabases and diorites, the
" greenstones " of the older geologists.
Serpentine. — Composition : A hydrous silicate of magnesium
corresponding to the formula H4Mg3Si2O9= silica, 44.1 % '•> mag-
nesia, 43.0 % ; and water, 12.9 %.
The prevailing color is green, though often spotted and
1 Ann. de Chemie et Physique, 1892.
GLAUCONITE AND THE ZEOLITES 31
streaked ; hence the name from the Latin serpentinus, a ser-
pent. It has a somewhat greasy lustre and may be cut with a
knife, having a hardness of about 4 of the scale. The mineral
is always secondary, resulting mainly from the hydration 'of
pure magnesian or lime magnesian silicates. (See further on
p. 115.)
Glauconite. — This name is given to a somewhat variable
compound consisting essentially of silica, iron, alumina, and
water, with smaller amounts of potash, and incidentally lime,
magnesia, and soda. The prevailing color is green, and as it
occurs in single granules or granular aggregates, it is com-
monly known as greensand. It is always a secondary mineral,
and has been formed and is still forming on many shallow sea-
bottoms which receive fine sediments derived from the breaking
down of siliceous crystalline rocks. (See under Greensand
Marl, p. 133.)
The Zeolites. — Under this head are grouped a number of
minerals alike in being hydrous silicates of alumina with vary-
ing percentages of lime, potash, and soda. They are altogether
secondary minerals, resulting from chemical changes taking
place in pre-existing rocks, and indicate not infrequently the
first or deep-seated stages of rock decay. In a more or less
perfect condition they have been assumed to occur in soils,
having been derived from the rocks, or, as is contended by some
authorities, having formed during the process of rock decompo-
sition or in the soil itself. It is possible that those constituents
of a soil which on analysis are found to be " soluble " as the
term is ordinarily used, may, in part at least, have existed as
zeolites. Hence their consideration in this connection is of
importance.
Out of the 22 species of minerals classified as zeolites by
Dana in this " System of Mineralogy " there are but 11 which,
on account of their abundance or chemical composition, need
consideration here. The theoretical composition of these, as
indicated from a comparison of several to many analyses, is
shown in the accompanying table. In addition to the true
zeolites are included several other hydrous silicates closely
related, both as regards chemical composition and mode of
occurrence, and which, in our present discussion, cannot well
be excluded.
32
THE MINERALS CONSTITUTING ROCKS
SILICA
(Si02)
ALUMINA
(A 1,0.)
LIME
(CaO)
BARIUM
(BaO)
POTASH
(K20)
SODA
(Na20)
WATER
(H,0)
Ptilblite . . .
70.0
11.9
4.4
2.4
0.8
10.5
Mordenite . .
Heulandite .
67.2
59.2
11.4
16.8
2.1
9.2
3.5
2.3
13.5
14.8
Fhillipsite . .
Harmotome . .
48.8
47.1
20.7
16.0
7.6
20.6
6.4
2.1
....
16.5
14.1
Stilbite . . .
Laumontite .
57.4
51.1
16.3
21.7
7.7
11.9
....
1.4
47.2
15.3
Chabazite . .
Analcite .
47.2
54.5
20.0
23.2
5.5
6.1
14.1
21.2
8.2
Natrolite
47.4
26.8
16.3
9.5
Thomsonite .
Prehnite .
36.9
43.7
31.4
24.8
11.5
27.1
....
6.4
13.8
4.4
Apophyllite . .
53.7
25.0
....
5.2
16.1
PLATE 2
FIG. 1. Quartz porphyry showing porphyritic structure.
FIG. 2. Quartz porphyry showing flow structure.
IV. THE PHYSICAL AND CHEMICAL PROPER-
TIES OF ROCKS
1. STRUCTURE
In considering the structure of rocks it will facilitate mat-
ters to do so under two heads : (1) the macroscopic (or rnega^
scopic) structures, or structures visible to the unaided eye
(macros, from Greek word /ia^/oo?, signifying large); and
(2) microscope structures, or those visible only with the aid
of the microscope.
1. Macroscopic Structures. — From a structural standpoint all
rocks may be classified sufficiently close for present purposes,
under the heads of : (1) Crystalline,, (2) vitreous or glassy,
(3) colloidal, and (4) clastic or fragmental. Of the first of
these, ordinary granite or crystalline marbles are good types,
being made up wholly of crystal aggregates, without interstitial
amorphous or fragmental material. The term crystalline gran-
ular, or granular crystalline, is often applied to such as have a
distinctly granular structure, as do many of the granitic rocks.
Vitreous or glassy structures are found only among igneous
rocks, and are due always to a cooling of the molten magma
too rapidly for the production of crystals. Obviously, as the
rate of cooling in rock masses must be extremely variable, so
we find all intermediate stages between the completely glassy
and the crystalline forms. To these intermediate stages such
names as felsitic and microlitic are given, names the precise
meaning of which will be stated under the head of microscopic
structures. Rocks originating as chemical deposits, and which
have since undergone no structural changes, often present a
jelly or glue like structure known as colloidal. Such are exem-
plified in the flints from the English chalk cliffs, the siliceous
sinters from the Yellowstone National Park, and by various
other forms of silica, as opal, agate, etc., and occasionally by
serpentines.
n 33
34 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
A clastic or fragmented structure is found only in secondary
rocks, and is the result of a breaking down or disintegration of
pre-existing rocks, and a reconsolidatioii of their particles with-
out crystallization. There are many minor points of structure,
some of which are common to all of the primary groups above
mentioned, while others are limited to one or more. Rocks
which are made up of distinct grains, whether crystalline or
f ragmental, are spoken of as granular ; when the structure be-
comes too fine and dense for macroscopic determination it is
spoken of as compact, though there is no reason why the term
should not equally well be applied to the coarser grained rocks
in which the individual grains are closely cohering without
interstices. The term massive is applied to such igneous rocks
as show no signs of bedding or stratification, while limestones,
sandstones, and such other rocks as are arranged in more or
less parallel layers are described as stratified. (See Fig. 1,
PI. 13.) The name foliated or schistose is given to a rock in
which the arrangement of the constituent minerals in parallel
planes is sufficiently marked to cause it to split in this direction
more readily than in any other. Not infrequently the quartzes
or feldspars occur in lens-shaped forms about which curve the
hornblende or mica folia as shown in Fig. 2, PI. 13. As ex-
plained elsewhere, this structure may be due to original deposi-
tion or may be secondary. In eruptive rocks a fluidal or fluxion
structure is not uncommon, as shown in Fig. 2, PL 2, and is due
to the onward flowing of the mass while gradually cooling and
passing into a solid state. Eruptive magmas at the time of
their extrusion contain more or less moisture, which, being
highly heated, expands whenever sufficient force is developed
to overcome the pressure of the overlying mass. In this way
are formed innumerable cavities or bubbles, comparable to the
cavities caused by carbonic acid from the yeast in well-raised
bread. Such cavities are called vesicles, and the rocks contain-
ing them are vesicular (Fig. 2, PI. 3). By the subsequent
action of percolating waters these cavities may become filled
with a variety of secondary minerals, among which chalcedony,
epidote, calcite, and various zeolites are not uncommon. Such
refilled cavities are called amygdules, from the Greek word
ajjivySaXov, an almond, in allusion to their shape, and the rocks
containing them are therefore described as amygdaloidal. The
upper part of a lava flow not infrequently cools in peculiar ropy
PLATE 3
f, l
FIG. 1. Basalt showing slaggy structure. FIG. 2. Basalt showing vesicular structure.
MACROSCOPIC STRUCTURE 35
forms like the slag from a smelting furnace. Such forms are
known as slaggy. (See Fig. 1, PL 3.)
When a rock consists of a compact, glassy, or fine and evenly
crystalline ground-mass, throughout which are scattered larger
crystals, usually of feldspar, the structure is said to be porphy-
ritic (Fig. 1, PL 2). This structure is quite common in granite,
but is not particularly noticeable, owing to the slight contrast in
color between the larger crystals and the finer ground-mass. It
is most noticeable in such effusive eruptives as the quartz por-
phyries, in which, as is the case with some of those of eastern
Massachusetts, the ground-mass is exceedingly dense and com-
pact and of a black or red color, while the large feldspar
crystals are white and stand out in very marked contrasts.
This structure is so striking in appearance that rocks possess-
ing it in any marked degree are popularly called porphyries,
\\ luitever may be their mineral composition. The term por-
phyry is said to have been originally applied to certain kinds
of igneous rocks of a reddish or purple color, such as the
celebrated red porphyry or " roseo antico " of Egypt. The
word is now used by the best authorities almost wholly in its
adjective sense, since any rock may possess this structure
whatever its origin or composition may be.
Glassy rocks on cooling sometimes have developed in them
a series of concentric cracks whereby the rock on a broken sur-
face shows numerous rounded or globular bodies with an onion-
like shell. This structure, which may be visible only with a
microscope, is known as perlitic. It is not uncommon in glassy
forms of Hungarian trachytes.
Glassy and felsitic eruptives, particularly of the liparite and
quartz porphyry groups, frequently show spherulitic masses of
all sizes, from microscopic to several inches or even feet in
diameter, usually with a well-defined radiating structure and
which are due to incipient crystallization. Such are known as
spherulites, and hence rocks in which they occur are described
as spherulitic.*
A concretionary structure is not infrequently developed in
rocks either as a primary structure or as due to segregating
processes acting subsequent to the formation of the rocks in
1 The structure and origin of these forms has been worked out in detail by
Whitman Cross. Bull. Philosophical Society of Washington, Vol. XI, 1891,
pp. 411-462.
36 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
which they are found. Many of the forms thus developed are
peculiarly deceptive, and it may not be out of place to enter
into a discussion of their nature and origin with some detail.
On genetic grounds we may divide such forms, as intimated
above, into two groups: (A) Primary concretions, formed con-
temporaneously with the rocks in which they are found, and
(.5) secondary concretions, or those which are due to segregat-
ing influences acting subsequent to the formation of the rocks
of which they now form a part. All are due to that peculiar
and little understood tendency which atoms or molecules of
like nature so often manifest in concreting or gathering in
amorphous masses or concentric layers about some foreign body
which serves as a primary point of attachment. The extreme
development of this tendency is seen in crystallization, of which
we may perhaps regard this first form of concretionary structure
as incipient stages. Under primary concretions may be included
the flint and chalcedonic nodules found in chalk and the older
limestones, the material of which was in part without doubt
derived from the siliceous remains of diatoms and sponges.
Such sometimes occur in the form of lenticular nodules with
or without an appreciable concentric structure and lying in
parallel layers or beds, sometimes continuous for long distances.
Clay iron stone, an impure carbonate of iron, occurs character-
istically in this form. These latter often crack on drying
and consequent shrinkage, the cracks extending from within
outward. In these cracks calcite is subsequently deposited,
whereby the nodule is divided up into septa of a white or
yellowish color. On being cut and polished, these often form
beautiful and unique objects. To such the name septarian
nodule is commonly given. (See Fig. 2, PL 9.) The car-
bonate of lime in inland lakes and seas may not infrequently
become deposited in the form of thin pellicles about a minute,
perhaps microscopic nucleus, forming small, spherical bodies
which, when ultimately consolidated into beds, give rise to the
oolitic and pisolitic limestones. (See p. 143.) All primary
concretions are not, however, chemical deposits ; but, rather,
aggregates of mineral particles in a finely fragmental condition.
Such are the clay concretions which are found in the beds
of streams and lakes, and which may not so closely simu-
late animal forms as to be very misleading. The manner in
which concretions of this nature are formed was shown in a
MACROSCOPIC STRUCTURE 37
very interesting manner a few years ago during the process of
the work of filling in the so-called Potomac flats, on the river
front at Washington, District of Columbia. For the double
purpose of raising the flats and deepening the channel, gigantic
pumps were employed which raised the sediment from the river
bottom in the form of a very thin mud and forced it through
iron pipes to the flats, where it flowed out, spreading quietly
over the surface. The material of this mud was mainly fine
siliceous sand and clay intermingled with occasional fresh-
water shells and plant debris. As this mud flowed quietly
from the mouth of the pipe and spread out over the surface,
the clayey particles began immediately to separate from the
siliceous sand in the form of concretionary balls, and in the
course of a very short time these would grow to be several
inches in diameter. Such, owing to the rapidity of their
formation, contained a large amount of sand and shells, though
clayey matter predominated.
In crystalline rocks concretionary structure is rarely devel-
oped. Cases such as shown on Plate 8 are quite unique, and
in the case of the orbicular diorite of the greatest interest on
account of the beauty of the stone and its adaptability for
small ornamentation.
Concretionary structure of a secondary nature may be de-
veloped through the process of weathering. Thus, by the
oxidizing action of meteoric waters percolating through a
porous sand or sandstone, included nodules of iron disulphide
(pyrite) may be converted into an oxide which gradually
segregates in zones about the original nodule. This oxide,
by its cementing action, binds the grains together in the form
of a hard crust, leaving the central portion, formerly filled by
pyrite, either empty or occupied by loose sand.1 A zonal
banding or shelly structure closely simulating concretionary
structure is common in rocks more or less weathered and
decomposed, but which is due not to original deposition or
crystallization of mineral matter about a centre, but rather to
the weathering of jointed blocks, the various chemical agencies
acting from without inward.
A botryoidal structure is not infrequent among rocks and
minerals of chemical origin. It is, as a rule, confined to such
1 See On the Formation of Sandstone Concretions, Proceedings U. S. National
Museum, Vol. XVII, pp. 87, 88.
38 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
as are amorphous or radiating crystalline aggregates of a single
mineral, such as chalcedony or the hematite iron ores. (See
Fig. 1, PI. 9.)
A brecciated structure, produced by the presence of angular
fragments in a finer ground, is of common occurrence among
fragmental rocks (the breccias), but is more rare among the
crystallines. It is sometimes produced in volcanic rocks by the
imbedding in the still pasty magma of angular fragments of
previously consolidated material, as shown in Fig. 2, PL 4.
Columnar structure, though comparatively common as the
structure of a geological body, is rarely developed among the
constituents of the rock itself. The columnar structure of
many lavas and dike rocks has already been alluded to : oc-
casionally the mineral constituents of some secondary rocks
are arranged after this manner. A cavernous or cellular struct-
ure is not infrequently developed through the removal by
solution of some constituent or the weathering out of a fossil.
As an original structure it occurs in many rocks of chemical
origin as the stalagmitic deposits in caves, travertines, etc.
A laminated or banded structure, due to the arrangement of
the constituents in parallel layers or bands, is common in rocks
of sedimentary origin, particularly in sandstones and shales.
2. Microscopic Structures. — Many, if not indeed the majority,
of rocks are so fine grained and compact that little of their
mineral nature or structural features can be learned from exami-
nation by the unaided eye. This difficulty made itself apparent
very early in the history of geological science, and to it is per-
haps due, more than to any other single cause, the apparent
crudities and fallacies of the early workers. As long ago as
1663, the microscope had been to some extent utilized for the
examination of minerals ; but its application to the study of
rocks remained long unrecognized, though early in the present
century Cordier and others utilized it in the study of rocks in
a pulverized condition. It was not until about 1850, when the
subject was taken up by H. Clifton Sorby of England, that the
possibility of studying rocks in thin sections under the micro-
scope began to be appreciated. Even then the idea failed to
bear its legitimate fruits until transplanted to German soils,
where, under the fostering care of Professor Zirkel of Leipzig,
it soon began to yield an abundant harvest ; and to-day the
branch of the science of geology known as microscopical pe-
PLATE 4
FIG. 1. Chert breccia cemented by zinc blende.
FIG. 2. Felsite breccia formed of felsitic fragments embedded in a matrix of the same
composition.
MICROSCOPIC STRUCTURE 39
trography holds a prominent place in all the leading universi-
ties, both domestic and foreign. The efficiency of the method
is based upon the fact that every crystallized mineral has cer-
tain definite optical properties ; i.e. when cut in such a way as
to allow the light to pass through it, will act upon this light in
a manner sufficiently characteristic to enable one working with
an instrument combining the properties of a microscope and
stauroscope to ascertain at least to what crystalline system it
belongs, and in most cases by studying also the crystal outlines
and lines of cleavage the mineral species as well. To enter
upon a detailed description of the method by which this is done
would be out of place here, since it involves the polarization of
light and other subjects which must be studied elsewhere. The
reader is referred to any authoritative work on the subject of
light, and to Professor J. P. Idding's translation of Professor
Rosenbusch's work on optical mineralogy.1
This method of study is of value, not merely as an aid in
determining the mineralogical composition of a rock, but also,
and what is often of more importance, its structure and the
various changes which have taken place in it since its first
consolidation. Rocks are not the definite and unchangeable
mineral compounds they were once considered, but are rather
ever-varying aggregates of minerals, which, even in themselves
undergo structural and chemical changes almost without num-
ber. It is a common matter to find rock masses which may
have had originally the mineral composition and structure of
dhihase, but which now are mere aggregates of secondary prod-
ucts, such as chlorite, epidote, iron oxides, and kaolin, with
perhaps scarcely a trace of the unaltered original constituents ;
yet the rock mass retains its geological identity, and to the
naked eye shows little, if any, sign of the changes that have
gone on. These and other changes are in part chemical and in
part structural or molecular. A very common mineral trans-
formation in basic rocks is that from augite to hornblende.
This takes place merely through a molecular readjustment of
the particles, whereby the augite, with its gray or brown colors
and rectangular cleavages, passes by uralitic stages over into a
green hornblende, a mineral of the same chemical composition,
but of different crystallographic form. This transformation in
1 Microscopic Physiography of Rock-making Minerals, Wiley & Son, New
York. See also Professor A. Barkers' Petrology for Students.
40 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
its incompleted state is shown in the accompanying figure, in
which the central, nearly colorless portion with rectangular
cleavage represents the original augite, while the outer dotted
portion with cleavage lines cutting at
sharp and obtuse angles is the second-
ary hornblende. This change is due
to slow and gradual pressure exerted
through unknown periods of time upon
the rock masses, and the final result is
the production of a rock of entirely
different type and structure from that
which originally cooled from the molt-
en magma. The change such as above
FIG. i.— Augite partially described is further alluded to in the
chapter on metamorphism.
This science of microscopic petrography, as it is technically
called, has also been productive of equally important results in
other lines. As an instance of this may be mentioned the dis-
covery that the structural features of a rock are dependent, not
upon its chemical composition or geological age, but upon the
conditions under which it cooled from a molten magma, portions
of the same rock varying all the way from holocrystalline
granular through porphyritic to glassy forms. To this fact
allusion has already been made.
The general subject of the microscopic structure of rocks of
various kinds, will be discussed more fully in describing the
rocks themselves. Nevertheless, as in describing these struct-
ures it has become necessary to use sundry technical terms, it
will be well to refer to them briefly here.
When a rock is made up wholly of crystalline matter, it is
spoken of as holocrystalline ; when, however, it shows interstitial
glassy or felsitic matter, it is hypocrystalline. Rocks wholly
without crystalline secretions are amorphous. The glassy, or
felsitic matter' occupying the interstices of the other constitu-
ents is spoken of as the base. This base; together with the
microlites and smaller crystallizations of the second generation,
is called the ground-mass; such may be made up of microlites —
small needle-like crystals imperfectly developed — when it is
called microlitic, or of a dense aggregate of quartzose, felds-
pathic and other materials, when it is known as felsitic. The
larger crystals developed in a glassy, felsitic, microlitic, or finely
PLATE 5
FIG. 1. Microstructure of granite.
FIG. 2. Mlcrostructure of micropegmatite.
FIG. 3. Microstructure of quartz porphyry.
FIG. 4. Microstructure of porphyritic obsidian.
FIG. 5. Microstructure of trachyte.
FIG. 6. Microstructure of serpentine.
MICROSCOPIC STRUCTURE 41
granular microcrystalline ground-mass are called phenocrysts.
When a mineral in a rock shows good crystal outlines, having
been uninfluenced in its growth by the proximity of other
minerals, it is called idiomorphic : when, however, its outline is
due not to crystallographic forces, but to interference — to the
action of external forces — it is allotriomorphic. Many rocks
show indications of two or more periods of crystallization,
whereby minerals of the same species may be developed. Thus
in a molten magma the augites may begin to form under such
conditions that for some time their growth is unimpeded and
they take on large and well-developed forms. After a time,
owing to changed conditions, their growth is stopped, and the
rock solidifies with a new crop of smaller and less perfectly
developed forms. It is customary to speak of such a mineral
as occurring in crystals of two generations. In the case above
described, the first developed form the porphyritic constitu-
ents, the phenocrysts, while the latter formed are a part of
the ground-mass. Vitreous or glassy rocks not infrequently
show, under the microscope, minute, hair-like or rod-shaped
forms, representing the first stages of crystallization, but in
which the process was arrested before they were sufficiently
developed to render possible an accurate determination of
their mineral nature. Such are termed crystallites; those in
drop-shaped or globular forms being called globulites, the
rod-shaped ones belonites, and the twisted, hair-like forms
triehitet.
The wide variation in microstructure in rocks of essentially
the same chemical composition, but whicli have cooled under
the varying conditions indicated above, is shown in Figs. 1 to
4 of PI. 5, Fig. 1 being a holocrystalline type, and Fig. 4 one
almost completely glassy, the first being a deep-seated rock, and
the last a surface lava flow. Intermediate structures are often
produced through a beginning of crystallization at certain
depths below the surface, after whicli, and while a portion of
the magma was still fluid, it was pushed upward toward the
surface, or brought under such other conditions as resulted in
a more rapid cooling, the final result being a glassy, or micro-
crystalline rock with scattering porphyritic crystals, or pheno-
crysts. It has not infrequently happened that, subsequent to
the formation of these earliest products of crystallization, a
second elevation of temperatures has taken place whereby the
42 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
magma has eaten into or corroded them, as is the case with
the quartz crystal shown in the centre of Fig. 3 of PL 5.
Inasmuch as this study by the microscope involves the prepa-
ration of thin sections, a brief description of the methods pur-
sued may well be given here. The fact that a chip of rock,
however dense, can, without breaking, be ground so thin as
to be transparent, may at first seem strange, but in reality it
is readily accomplished. The work requires only patience and
the skill which comes from practice. A small chip of the rock,
about the size of a nickel five-cent piece, is broken off with a
hammer, care being taken to get it as thin as possible without
fracturing. One side of this is then ground flat and smooth by
rubbing it in water and emery on a smooth, cast-iron plate.
Toward the close of the process fine flour of emery should be
used, as the final surface must be very smooth and free from
scratches. This chip is then cemented smooth side down on
a piece of ordinary double-thick window glass, a convenient
size being about 2x1 inches, the cementing material being
Canada balsam which has been evaporated to the extent that,
when cold, it is sufficiently hard to hold firmly, is not at all
sticky, but yet is not so hard as to be brittle. The exact degree
can only be learned by experience ; a hardness such as to be
barely indented by the thumb nail will be found about right.
This operation of cementing will be best done by means of a
thin iron plate laid horizontally on a support and heated not too
hot by a lamp beneath. The glass with the balsam upon it is
heated to the right temperature, the balsam being fluid and free
from bubbles. The rock chip, heated sufficiently to expel all
moisture, is then pressed firmly into the balsam, in such a way
as to exclude air bubbles, and brought within as close contact
with the glass as possible. It is then removed from the iron
plate and allowed to cool, when the grinding process is resumed,
the glass plate serving merely as support for the film of stone
and something for the fingers to hold by. Being transparent,
the worker can see just how the grinding is progressing without
continually stopping to examine. When sufficiently thin, —
usually from ^-^ to ^^ of an inch, — the film is remounted as
follows : While on the thick glass on which it was ground, it
is thoroughly washed with a brush — an ordinary tooth-brush
serves well — to get rid of all particles of emery and other dirt
that may adhere. It is then washed in alcohol to get rid of the
THE SPECIFIC GRAVITY OF ROCKS
43
old hard balsam, which is usually quite dirty from mud pro-
duced in grinding. Fresh mounting slips and clean cover
glasses being ready, the first is laid upon the warm iron plate
with a couple of drops of fresh balsam in the centre, and allowed
to heat until it just begins to smoke. Care must here be exer-
cised, as, if heated too much, the balsam becomes hard and
brittle, and if too little, the mount is sticky from the balsam
which constantly oozes from under the cover. The thick glass,
with its film of stone still adhering, is likewise laid upon the
warm iron plate, and a drop of fresh balsam placed upon the film.
This is then gently heated, and the cover-glass, first warmed,
gently laid upon it — one edge placed in position and lowered
gradually in such a manner as to force out any accidental air
bubbles, being finally pressed flat down against the stone film.
The film itself, if sufficiently warmed, no longer adheres to the
thick glass, and may be removed to the
clean slip for its final mounting. This is
best accomplished by taking up the thick
glass by means of a pair of forceps and
pushing cover-glass and film together, with
a needle point set in a handle, off into the
balsam on the new slide. The cover-glass
here serves merely as a support for the thin
film during the process of transferring.
AVithout it there is danger of breakage.
AVhen fairly transferred, the new slide is
removed from the hot plate, the cover
pressed close down against the film, ad-
justed in proper position and allowed to cool. Fl°- 2.— Mounted thin
The superfluous balsam may be then re-
moved with a hot knife and the section finally washed in alcohol.
Thus completed, it forms the "thin section''1 of the petrologist.
2. THE SPECIFIC GRAVITY OF ROCKS
The term specific gravity is used to designate the weight of
any substance when compared with an equal volume of distilled
water at a temperature of 4° C. This property is therefore
dependent upon the specific gravity of its various constituents
and their relative proportions. The exact or true specific
gravity of a rock may be obscured by its structure. Thus an
44 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
obsidian pumice will float upon water, buoyed up by the air
contained in its innumerable vesicles, while a compact obsidian
of precisely the same chemical composition will sink almost
instantly. This property of any subject is spoken of as its
apparent specific gravity in distinction from the actual com-
parative weight, bulk for bulk, of its constituent parts, which
could in the case of a pumice be obtained only by finely pul-
verizing so as to admit the water into all its pores. Inasmuch
as the structural peculiarities of any igneous rock — as will be
noted later — are dependent ifpon the condition under which it
cooled, it is instructive to notice that a crystalline aggregate
has a higher specific gravity, i.e. a greater weight, bulk for
bulk, than does a glassy, non-crystalline rock of the same chemi-
cal composition. The property is therefore dependent upon
chemical (and consequently mineral) composition and struct-
ure, and as a very general rule it may be said that among the
siliceous rocks those which contain the largest amount of silica
are the lightest, while those with a comparatively small amount,
but which are correspondingly rich in iron, lime, and magnesian
constituents, are proportionately heavy.
3. THE CHEMICAL COMPOSITION OF ROCKS
This varies naturally with their mineral composition. It is
customary to speak of sedimentary rocks as calcareous, sili-
ceous, ferruginous, or argillaceous, accordingly as lime, silica,
iron oxides, or clayey matter are prominent constituents.
Among eruptive rocks it is customary to speak of those show-
ing, on analysis, upwards of 60 % silica as acidic, and those
showing less than 50 %, but rich in iron, lime, and magnesian
constituents, as basic. The extremes, as will be noted, are rep-
resented by the rocks of the granite and peridotite groups.
A series illustrating the above-mentioned properties may be
arranged as below. With the eruptive rocks only the silica
percentages are here given. The results of the complete chemi-
cal analysis of each variety are given further on, in the pages
devoted to their description.
THE CHEMICAL COMPOSITION OF ROCKS
45
(1) STRATIFIED ROCKS
KIND
SPECIFIC GRAVITY
COMPOSITION
Calcareous :
Compact limestone . . .
Crystalline limestone . .
Compact dolomite . . .
Crystalline dolomite . .
Siliceous :
Gneiss
| 2.6 to 2.8
1 2.8 to 2.95
2.6 to 2.7
Carbonate of lime.
Carbonate of lime and magnesia.
Same as granite.
Siliceous sandstone . . .
Schist
2.6
2.6 to 2.8
Mainly silica.
60 to 80 per cent silica.
Argillaceous :
Clay slate (argillite) . .
2.5
Mainly silicate of aluminum.
(2) ERUPTIVE ROCKS
KIND
SPECIFIC GRAVITY
PEB CENT SILICA
Acidic group :
Granite
2.58 to 2.73
77.65 to 62.90
2.53 to 2.70
76.06 to 67.61
Obsidian .
2.26 to 2.41
82.80 to 71.19
Obsidian pumice
Floats on water.
82.80 to 71.19
Intermediate group :
Syenite
2.73 to 2.86
72.30 to 54.65
Trachyte
2.70 to 2.80
64.00 to 60 00
Hyalotrachyte
2.40 to 2.50
64.00 to 60.00
Andesite
2.64 to 2.79
66.75 to 54.73
Basic group :
Diabase ....'.
2.66 to 2.88
60.00 to 48.00
Basalt
2.90 to 3.10
60.59 to 40.74
Peridotite
3.22 to 3.29
42.66 to 33.73
Peridotite (iron rich)
3 86
23 00
Peridotite (meteorite)
3.51
37.70
4. THE COLOR OF ROCKS
The color of a rock is dependent upon a variety of circum-
stances, but which may all be generalized under the heads of
mineral and chemical composition and physical condition. Iron
and carbon, in some of their forms, are the common coloring
46 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
substances and the only ones that need be considered here.
The yellow, brown, and red colors, common to fragmental rocks,
are due almost wholly to free oxides of iron. The gray, green,
dull brown, and even black colors of crystalline rocks are due
to the presence of free iron oxides or to the prevalence of sili-
cate minerals rich in iron, as augite, hornblende, or black mica.
Rarely copper, manganese, and other metallic oxides than those
of iron are present in sufficient abundance to impart their char-
acteristic hues. As a rule, a white or light gray color denotes
an absence of an appreciable amount of iron in any of its forms.
The amber, bluish and black colors of many rocks, particularly
the limestones and slates, are due to the prevalence of carbona-
ceous matter.
Among siliceous crystalline rocks the more basic, like those
of the diabase, diorite, or basalt groups, are as a rule of a darker
color than the acid varieties, the color being due to the fine
grain and predominance of dark iron-magnesian silicates, such
as hornblende, augite, or black mica, or their chloritic alteration
products. The red or pink color sometimes occurring in gran-
itic rocks is due to the predominance of red or pink feldspars,
which in their turn owe their color to the presence of iron.
Among feldspar-bearing rocks the color is not infrequently
due to the physical condition of this important constituent.
Thus in many rocks like the norite of Keeseville (New York),
and the Quincy (Massachusetts) granite, the dark color is
largely due to the fact that the feldspar is clear and glassy,
allowing the light rays to penetrate and become absorbed. The
beautiful chatoyant play of colors sometimes shown by labra-
dorite-bearing rocks like those of northern New York and of
Norway is apparently due to a separation of the individual
crystals along cleavage lines, into thin, transparent plates which
reflect and partially polarize the light which would otherwise
penetrate and become absorbed. Through weathering, such
feldspars undergo a further physical change, becoming soft
and porous, and no longer allowing the light to penetrate, but
wholly reflecting it and causing the stone to appear white.
These white feldspars, as has been very neatly expressed by the
late Dr. Hawes, bear the same relation to the glassy forms
as does the foam of the sea to the water itself, the difference
in color being in both cases due to the changed physical con-
dition. Indeed, the color of rocks, as may be imagined, is
THE COLOR OF ROCKS 47
not constant, but liable to change under varying conditions,
particularly those of exposure. Rocks black with carbonaceous
matter will fade to almost whiteness on prolonged exposure,
owing to the bleaching out of the coloring materials. Rocks
rich in magnetite or free iron oxides, protoxide carbonates, or
sulphides, or in highly ferruginous silicate minerals, are like-
wise liable to a change of color, becoming yellowish, red, or
brown, through oxidation of the ferruginous constituents. (See
p. 257.) Translucent, nearly colorless rocks or minerals, as
those made up of crystals of calcite or selenite, will on exposure
become nearly opaque and snow-white, owing to purely physi-
cal causes, as already noted in the case of the feldspars. (See
further in chapter on weathering.)
The cause of the color variations in certain rocks and min-
erals is, however, a matter concerning which it will not do, as
yet, to speak too decidedly. Analysis of a mineral may show
the presence of metallic oxides, but it does not necessarily fol-
low that whatever color the mineral may have is due or in any
way related to these oxides. Thus the writer has shown 1 that
the onyx marbles (travertines) of Arizona and Mexico may
vary from pure white to green, and from yellow through brown
to red, without appreciable change in the actual amounts of
iron, though there may be a change in the form of combination.
In the white and green varieties the iron exists as a carbonate ;
in the yellow, red, and brown varieties as a more or less hydrated
sesquioxide. Certain dark amber and bright rose-colored va-
rieties from California, and the Californian Peninsula, show,
however, no iron or other of the usual metallic coloring con-
stituents, but burn perfectly white when submitted to high
temperatures and yield volatile organic compounds. The fact
that serpentines so frequently contain small traces of chromium,
early gave rise to the opinion that it was to this element that
was due the characteristic green color of the mineral. The
writer has elsewhere2 described serpentines of a beautiful oil
yellow and deep green color which, however, contain not a
trace of chromium or manganese, but only iron, which in this
case is in combination as a silicate. (See p. 114.)
These color characteristics are of greater importance than
1 Annual Report U. S. National Museum, 1893, p. 558.
2 On the Serpentine of Montville, New Jersey, Proc. U. S. National Museum,
1888, p. 105.
48 PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS
may at first appear, particularly from an economic standpoint.
One of the first essentials in a rock designed for architectural
use should be permanency of color. Deleterious changes are
particularly liable to occur in stone taken from below the water
level, where, protected from oxidation, or from variations in
temperature. Certain of the Ohio sandstones are of a blue-
gray color below the water level, but buff above, where the
included iron sulphides and protoxide carbonates have been
acted upon by oxidation. The student should early make
himself acquainted with these characteristics, as in the field it
is as a rule only the more or less weathered surfaces that pre-
sent themselves for inspection. This subject is again referred
to in the chapter on rock weathering.
Lustre as a property of rocks does not, owing to their com-
plex nature, possess the same value as a determinative charac-
teristic as among minerals. Certain of the more compact and
homogeneous varieties possess lustres which may be described
as vitreous, greasy, pearly, metallic, or iridescent.
The meaning of such terms is sufficiently evident, and the
subject need not be further dwelt upon here. The fracture,
or manner of breaking of any rock, is dependent more upon
structure than upon chemical or mineralogical composition.
Many fine and evenly grained crystalline or fragmental rocks
break with smooth, even surfaces, and are described as having
a straight or even fracture. Others break with shell-like con-
cave and convex surfaces, and are said to have a conchoidal
fracture. Still others are splintery, hackly, or shaly, words the
meaning of which is sufficiently evident without their being
described in detail.
V. THE MODE OF OCCURRENCE OF ROCKS
It is ordinarily assumed that the earth owes its present form
to its having originated from a mass of incandescent vapor, and
to have passed, by gradual cooling and consequent condensa-
tion, from gaseous through pasty or fluidal, and all intermediate
stages, to its present condition. This, in brief, is the hypothesis
of Kant, and which seems most readily to account for the facts
as we now know them. As to the character of the rock masses
resulting from this primary cooling, we know but little. Rea-
soning from analogy, it seems safe to assume that they resem-
bled the slags from a smelting furnace, or some form of modern
lavas, more nearly than any other rock masses of which we
have knowledge. Whatever may have been their nature, they
have long since been obscured by rocks of secondary origin,
or become so altered through dynamic and incidental chemical
agencies as to be no longer recognizable.
The oldest rocks of which we now have knowledge belong
to the group of gneisses and crystalline schists. They are as
a rule highly siliceous rocks, though not infrequently includ-
ing considerable thicknesses of crystalline limestone. They
contain no traces of what can be referred beyond doubt to an
organic origin, though from their banded or foliated structure,
so closely simulating bedding, they have in the past, as a rule,
been considered metamorphic rocks ; that is to say, rocks laid
down as sediments and crystallized by the complex processes
comprehended under the term metamorphism. Rocks of this
type, according to Dana, first appeared in North America in
the wide V-shaped area extending from Labrador southwesterly
to the Great Lake, and thence northwesterly to the Arctic
regions. This area has since been added to by the folding and
crumpling processes incident to the formation of the Appa-
lachian and Rocky Mountain systems. Concerning the geo-
graphical distribution of these rocks, as they now appear
exposed, we have little to say here. They seem to form, as
E 49
50 THE MODE OF OCCURRENCE OF ROCKS
has been stated, the actual floor of the continents upon which
all later deposits have been laid down, and through which and
into which have been extruded and intruded the great variety
of igneous rocks which form so conspicuous a feature in many
a mountainous regiqn. In order to properly understand that
which is to follow, we may well devote a little space here to a
consideration of the manner in which these rock masses occur,
so far as exposed to investigation.
Several varieties of igneous rocks, and particularly the gra-
nitic types, occur not infrequently in the form of immense oval
or rounded masses, protruded into overlying materials which
dip away on all sides ; such forms are ordinarily designated
as bosses. (PL 1.) It is a form common to granite, gab-
bros, norites, etc. A laccolite1 is a somewhat similar form
due to the welling up of a magma through a comparatively
small vent, but which, instead of coming to the surface, spread
out laterally into dome-shaped masses between the sheets of the
overlying strata. When the intruded matter has been so
forced into or between overlying bedded rocks as to appear
like more or less regularly denned beds, they are known as
sheets or sills. Such, as a rule, may be distinguished from
superficial lava flows by their like condition of compactness
along both upper and lower contacts, surface streams being
more or less vesicular along the upper portions, owing to the
expansion of their included moisture. The name dike is given
to an eruptive mass of varying width included between well-
defined walls, and occupying a fissure or fault in previously
consolidated rocks. Such are inclined at all angles with the
horizon, and are usually of very moderate width, but may ex-
tend for miles. The dikes in any one region will frequently
be found to belong to one or more well-defined systems, each
system occupying fissures essentially parallel with one another.
Any one dike may remain comparatively uniform in width for
long distances, excepting when split up into smaller dikes.
At times, dikes may be traced to the parent mass — a boss or
laccolite — from which they radiate with more or less regu-
larity, being in such cases widest at the start, and gradually
1 It is to be regretted that this name in its present form has been so generally
adopted by geologists, since its termination, ite, should indicate a kind of rock,
whereas, in fact, it but denotes a form of occurrence. Laccolith would be
preferable.
IGNEOUS ROCKS 51
thinning out to, it may be, mere knife-like edges. The name
volcanic neck or plug is given to the cylindrical mass which
results from the congealing of that portion of the lava which
remains in the volcanic vent when eruption ceases. Through
the erosion of the matter composing the cone of a volcano,
such are sometimes left, owing to their superior hardness, form-
ing thus a very striking feature of the landscape. The gen-
eral name lava is applied to any igneous rock, regardless of
geological age or mineral composition, which has been poured
out on the surface of the earth in a molten condition. Such
are characterized, as a rule, by less perfect crystallization and a
more slaggy and vesicular structure than the deep-seated rocks.
A columnar jointing, due to cooling, is by no means uncom-
mon, particularly among basaltic lavas, although it is by no
means confined to them.
But a comparatively small proportion of the rocks composing
the superficial portions of the earth's crust — the portions with
which we are more or less familiar — are eruptive. They are
rather what are known as secondary rocks ; that is to say, they
are rocks made over from these so-called primary rocks, which
we have been just discussing, by processes which will be described
later.
Any rock mass, be it eruptive or otherwise, lying exposed at
or near the surface of the ground finds itself subjected to a
multitude of disintegrating and decomposing agencies, such as
will be described more in detail under the head of rock weather-
ing. Leached and decomposed by meteoric waters, disintegrated
by heat and frost, or the mechanical action of waves and cur-
rents, the rock masses slowly succumb, their materials being
gradually removed in solution, or as debris mechanically trans-
ported by every wind, rain, or running stream, down the slopes
into the valleys, and from the valleys into the seas. This
debris, in various stages of coarseness and fineness, and to
which we give the name of bowlders, gravel, sand, and silt,
undergoes by these transporting agencies a system of assorting
more or less complete, and is carried to distances dependent
upon its weight and the force of the transporting agent. It
requires no geological or other special training to enable one to
understand that the force being the same, the finer and lighter
materials will be carried the farthest, and that all must be de-
posited when the force shall be expended. Consider, then, for
52 THE MODE OF OCCURRENCE OF ROCKS
purpose of illustration, a stream flowing out from a mountainous
region and emptying itself into a lake. Materials falling by
gravity from the mountain slopes, or washed by spasmodic rains
into the stream, are transported certain distances, according to
the strength of the current. For our present purposes, it is
sufficient to consider only those portions which are transported
quite to the mouth of the stream and dumped into the lake.
But as the water leaves its narrow channel and spreads out into
the lake, there is an almost instant diminution of the force of
its current, and consequent carrying power. As a result, it
begins to deposit its load, the coarsest and heaviest first, and
the finer materials further out from the shore, the very finest,
an impalpable silt it may be, remaining suspended until the
very last. There will thus be formed on the bottom of the lake
or sea, whichever it may be, a bed, or series of beds of varying
thickness, of gravel, sand, and clay, the coarsest at the bottom
and nearest the shore, and the finest and last the most remote.
But the streams emptying into the lake vary from time to
time in their carrying capacity, and the action of the waves in
the sea itself, together with the salts dissolved therein, exert a
modifying action, whereby this process of sedimentation, as it is
called, may not be quite so simple as it first appears.1 Enough
has, however, been said to show that beds of detritus laid down
in this manner must occur in approximately horizontal layers,
and that the layers may vary greatly in the coarseness and
fineness of their materials, as well as in their mineral character.
But there are still other processes of sedimentation than the
purely mechanical methods described above. All natural waters
contain more or less mineral matter, of which lime is the more
abundant. Through the secreting power of marine animals, this
lime is taken up in the form of a carbonate to form shells and
calcareous skeletons of molluscs, corals, and other forms of
marine life. On the death of the secreting animal, the calca-
reous material is left to accumulate in a more or less fragmen-
tal condition, forming thus the material of the coral islands,
and to a considerable extent the beds of limestone the world
over. I have said to a considerable extent, for the reason that
it is doubtful if many of our limestones are of purely animal
origin ; in many a true chemical precipitation plays a not unim-
1 See Conditions of Sedimentary Deposition, by Bailey Willis, Journal of
Geology, 1893, p. 476.
BEDDED OR STRATIFIED ROCKS 53
portant part. This is especially true of the oolitic varieties,
and the fact is readily apparent when we come to study such
in detail. Consider a shallow sea-bottom on which are gradu-
ally accumulating in a finely divided condition the fragmented
remains of calcareous organisms of any kind. By the undu-
latory action of the waves these are kept in almost constant
motion, though it may be but gently rolling from side to side.
Owing to evaporation, or a too rapid accumulation of the lime
for it to be abstracted by the lime-secreting animals, the water
becomes supercharged with this constituent, which is then pre-
cipitated in the form of a thin pellicle around the most availa-
ble nucleus, in this case the grains of calcareous sand upon the
bottom. Thus are gradually built up beds of no inconsiderable
thickness, such as the well-known Carboniferous oolitic lime-
stones of Indiana and Kentucky. The microscopic structure of
stones of this class is shown in Fig. 7 on p. 112. Rocks which
are laid down in the manner we have just described, whether
composed of inorganic particles or fragmental materials from
marine and fresh water organisms, are designated as sediment-
ary. They occur in more or less well-defined beds or strata,
ami hence are spoken of as bedded or stratified. Owing to the
fact that they have in most cases been deposited in compara-
tively shallow water, they retain not infrequently the superficial
markings made upon them by waves and other agencies prior to
their final consolidation.
Deposits laid down as above described naturally lie approxi-
mately horizontally where not subsequently disturbed by earth
movements. The earth's crust, however, is by no means in a
state of stable equilibrium, but, being subjected to continuous
stress or compressive force, is often broken, crushed, or folded,
and crumpled to an extraordinary degree. The name fault is
applied to the profound fractures made by these movements,
and which, inclined at various angles to the horizon, may extend
for miles. Usually the rocks on one side of a fault will be
found to have sunk <lo\\n. while those of the other remain sta-
tionary or are raised, producing thus an inequality of surface
that may assume mountainous proportions. Most mountain
ranges, in fact, are due to a combination of faulting and fold-
ing processes. It not infrequently happens that the masses
of rock, sliding over one another along a line of fault, produce
smooth or striated and often highly polished surfaces, to which
54 THE MODE OF OCCURRENCE OF ROCKS
the name slickensides is given. Such are particularly noticeable
among serpeutinous rocks, being apparently due to motion gen-
erated in the mass by increase in bulk incident to its conver-
sion into serpentine.1 The name vein is given to rock masses
of chemical origin, deposited along previously existing fractures
which may or may not be true faults. By some authorities
the name is also made to include the smaller injections of igne-
ous rocks. Such are here classed under the head of dikes,
though it must be understood that it is not in all cases pos-
sible to state to which of the two classes an occurrence is
to be referred. It is customary to divide the veins into two
classes : (1) the mineral veins, in which the materials have
been deposited from aqueous solution or sublimation between
the walls of a fissure ; and (2) segregation veins, in which the
component materials have crystallized or segregated out of the
still unconsolidated, pasty, or colloidal rock. It is not in all
cases possible to decide to which of the two classes a vein may
belong, but as a rule the mineral (or fissure) veins are separated
by sharp and well-defined walls from the country rock, and
show a comb or banded structure. The segregation type is less
distinctly marked, the vein material being welded to the enclos-
ing rock, or seemingly passing into it by gentle gradations.
The unconsolidated materials, as sands and gravels, occur
not only in regularly bedded or stratified forms, but also in
hillocks and ridges to which special terms are applied. The
loose material washed down the mountain slopes by ephemeral
streams, and deposited at the mouth of gorges, not infrequently
assumes the form of "a conical mass of low slope descending
equally in all directions from the point of issue." To such
forms Gilbert has given the name of alluvial cones. The mate-
rial of these cones, as described, varies in size from the finest
powder to angular rocks weighing many tons. It exhibits no
regular bedding or stratification, but coarse and fine debris are
mingled in endless variety. There is a well-marked gradation,
however, to be seen as one travels from the apex of a cone
toward its periphery. At the apex it is composed mostly of
coarse, angular material, with fine silt-like clays filling the
interspaces, while toward the periphery the fine material pre-
dominates. The name talus is given to the accumulations of
1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, 1888, p. 105.
CLASTIC MATERIALS 55
debris at the foot of rocky cliffs, and which are composed of
angular fragments, large and small, which have fallen from the
cliffs above. The name dune is given to the rounded hills of
wind-blown sand common in arid regions and on windy shores.
Such are naturally of moderately fine and quite uniformly
assorted materials. In form and position they are ever chang-
ing, like drifts of snow, but are usually much steeper on the
leeward than on the windward sides. The character of the
material of which they are composed is most commonly sili-
ceous sand.
The names kame, esker, osar, or horseback are given to ridges
and mounds of sand and gravel deposited by the melting ice of
the glacial epoch. The materials are as a rule well rounded,
and as deposited usually show rude lines of stratification.
Such, as described, vary greatly in breadth and height, some
being 400 to 500 feet broad at the base and from 25 to 60 feet
in height. Drumlin is the name given to the peculiar low,
gently and smoothly sloping lenticular hills composed of un-
assorted glacial debris, and which are common in eastern Massa-
chusetts and other glacial regions. The general name moraine
includes the heterogeneous materials brought down by glaciers
and ultimately deposited in undulating hills and ridges on
their final disappearance. (See further under The Regolith,
p. 299.)
PAET II
THE KINDS OP ROCKS
" Some rin up hill and down dale knapping the chucky stones to pieces wi
hammers like sae many road-makers run daft. They say it is to see how the
warld was made." — St. Bonan's Well.
REFERENCE has already been made to the fact that but
sixteen out of the sixty-nine known elements enter into the
composition of the earth's crust in other than comparatively
minute quantities. Also to the equally important fact that the
combination of these elements as represented in not above a
score of well-known mineral species go to make up the essential
portion of nearly all rock masses. Nevertheless, owing to the
variety of forms under which these rock masses occur, the vary-
ing forces or conditions under which they originated, or the
proportional quantities of the various minerals which they may
contain, we find numerous and widely varying types of rocks,
a satisfactory consideration of which necessitates first some
attempt at systematic classification. We may say at the outset,
however, that rock species, in the sense in which the word is
used in mineralogy and zoology, scarcely exist. It is true we
may have, and particularly among igneous rocks, certain forms
which on casual inspection, or indeed on close inspection, with
regard only to limited geographical areas, seem to possess an
individuality of their own sufficient to entitle them to being
considered as true species. Yet, when we come to compare
these with others, to take into account their physical and chem-
ical composition, their structure and mode of occurrence, and
above all to consider how any rock varies within its own mass,
and the still greater variation which may have been produced
through alteration, we shall see that one form grades into an-
other almost without limit, that, indeed, no two are exactly
alike, and that, were we to attempt any hard and sharp lines of
discrimination, our species-making would practically resolve it-
56
THE KINDS OF ROCKS 57
self into an enumeration of individual occurrences, or specimens.
This fact will become apparent as we proceed, and further
remarks on the subject may well be deferred until we come to
a discussion of individual groups. Indeed, in the present, tran-
sitional state of knowledge regarding the chemical and minera-
logical composition of rocks, their structural features, and
methods of origin, no scheme of classification can be advanced
that will prove satisfactory in all its details. The older sys-
tems, which were made to answer before the introduction of the
microscope into geological science, are now known to be founded
upon what were in part false, and what have proven to be
wholly inadequate, data. This is especially true in regard to
eruptive rocks. The time that has elapsed since this intro-
duction has been too short for the evolution of a perfectly satis-
factory system ; many have been proposed, but all have been
found lacking in some essential particulars. To enter upon a
discussion of the merits and demerits of the various schemes
would obviously be out of place here, and the student is re-
ferred to the published writings of Naurnann, Senft, Von Cotta,
Richtofen, Vogelsang, Zirkel, Rosenbusch, Michel-Levy, Cred-
ner, Jukes Brown, and Geikie, as well as those of the American
geologists, Dana,1 Wadsworth,2 and Iddings.8 In the scheme
here presented the writer has aimed to simplify matters so far
as is consistent with observed facts, and has not hesitated to
adopt or reject any such portions of systems proposed by others
as have seemed desirable.
All the rocks forming any essential part of the earth's crust
are here grouped under four main heads, the distinctions being
based upon their origin and structure. Each of the main
divisions is again divided into groups or families, the distinc-
tions being based mainly upon mineral and chemical composi-
tion, structure, and mode of occurrence. We thus have : —
I. Igneous Rocks : Eruptive. — Rocks which have been
brought up from below in a molten condition, and which
owe their present structural peculiarities to variations in con-
ditions of solidification and composition. Having as a rule two
1 On Some Points in Lithology, Am. Jour, of Science, Vol. XVI, 1878, pp. 335
and 431.
2 On the Classification of Rocks, Bull. Mus. Comp. Zool. Howard College,
No. 13, Vol. V ; also Lithological Studies.
8 The Origin of Igneous Rocks, Bull. Philosophical Society of Washington,
1892.
58 THE KINDS OF ROCKS
or more essential constituents. In structure massive, crystal-
line, or glassy, or in certain altered forms, colloidal.
II. Aqueous Rocks. — Rocks formed mainly through the
agency of water, as (.A) chemical precipitates or as (.6) sedi-
mentary beds. Having one or many essential constituents. In
structure laminated or bedded ; crystalline, colloidal, or f rag-
mental ; never glassy.
III. JEolisin Rocks. — Rocks formed from wind-drifted ma-
terials. In structure irregularly bedded ; fragmental.
IV. Metamorphic Rocks. — Rocks changed from their orig-
inal condition through dynamic or chemical agencies and
which may have been in part of aqueous, seolian, or of igne-
ous origin. Having one or many essential constituents. In
structure bedded, schistose or foliated, and crystalline.
I. ROCKS FORMED THROUGH IGNEOUS
AGENCIES. ERUPTIVE
This group includes all those rocks which having once been
in a state of igneous fusion have been forced upward and in-
truded into the overlying rocks in the form of bosses, laccolites,
dikes, and sheets, or poured out upon the surface as lavas.
Concerning the source of eruptive rocks we are yet in igno-
rance. By many they have been supposed to represent portions
of the still unconsolidated interior of the earth. The great
variety of igneous rocks, the wide variation in chemical compo-
sition as well as the apparent independence of closely adjacent
volcanoes, both in the matters of time of eruption and character
of erupted material, seem, however, to show that they come not
from a common reservoir, but from isolated and comparatively
small areas where, for reasons not now well understood, pre-
viously solidified rock masses have been so highly heated as to
become pasty or liquid ; and then, through their own expan-
sion, or that of included vapors, or by compressive forces
generated in the earth's crust, forced upward into the positions
they now occupy. The origin of igneous rocks belongs as yet
largely to the realm of speculation. We must here confine
ourselves more to their mineral and chemical nature, general
physical properties, and the conditions under which they occur.
Consider, then, a mass of molten rock material, — to which
the term magma may be conveniently applied, — and which by
the processes of eruption is forced upward toward the surface,
and let us first dwell briefly upon the forms assumed by this
magma on cooling under the various conditions in which it
finds itself. It is obvious at the start that we can have actu-
ally to do with but a comparatively limited portion of the
products of any eruption. If the molten material is poured
out upon the surface and there remains for our inspection
to-day, it is a necessary consequence that the deeper-lying
portions are obscured. If, on the other hand, the superficial
59
60 ROCKS FORMED THROUGH IGNEOUS AGENCIES
portions have been removed by erosion so as to expose the
deeply lying parts, we have only these for study and observa-
tion. It is rare indeed that erosion has so acted on any one
rock mass as to expose superficial and deep-seated portions
alike. In the older regions, — those of greatest geological an-
tiquity,— erosion, either glacial or otherwise, has not infre-
quently removed more or less completely the superficial parts
and left for our inspection those portions of a magma that at
the time of eruption never reached the surface, but cooled, it
may be, under thousands of feet of superincumbent matter.
Such rocks are as a rule more highly crystalline* than those
which in the newer, less eroded portions, flowed out upon the
surface like our modern lavas. Hence it is that from a very
early period it has been found convenient, for purposes of dis-
cussion, to divide the eruptive rocks into two general groups :
first, the intrusive or plutonic rocks ; and second, the effusive,
or volcanic rocks.
Although this classification has not been strictly adhered to
in the present work, a few words descriptive of the essential
distinctions between plutonic and effusive rocks will not be out
of place, since such distinctions, particularly in eroded regions,
afford the only criteria for discrimination as to the original
conditions under which a rock mass has been formed, and hence
are of value in the field.
As a general rule, it may be said that the structural features
of an eruptive rock depend upon the conditions under which
a magma has cooled, although undoubtedly the amount of
included vapor of water may exert a powerful influence. As
Professor J. P. Iddings has well expressed it, " the chemical
differences of igneous rocks are the result of a chemical differ-
entiation of a general magma, and the structure of a rock is
dependent upon the physical conditions attending its eruption
and solidification." Now it is at once apparent that the greater
the depth below the surface at which a magma undergoes
solidification, or the greater its mass, the slower, more gradual,
will be that solidification, and hence the more complete and
coarser will be the crystallization. Hence the strictly plutonic
rocks are always holocrystalline. And, inasmuch as the weight
of the superincumbent matter has been such as to prevent the
expansion of included vapors to form steam cavities, so these
rocks are never vesicular or pumiceous, but compact and gran-
STRUCTURAL FEATURES OF IGNEOUS ROCKS 61
ular throughout. In cases where a plutonic rock has been
voided upward to fill a pre-existing rift in the form of a dike,
those portions of the magma coming in contact with the cold
walls on either hand will cool most quickly. Hence a dike is
as a rule most coarsely crystalline near the centre, becoming
finer grained and perhaps microcrystalline or even glassy at
the immediate contact. These two phenomena often afford
the only means of determining whether a rock mass occurring
in the form of a sheet parallel with the stratiliration, between
sedimentary beds, is an intrusive or a contemporaneous lava
flow ; whether it was injected as we now find it between
two previously existing beds ; or whether, as a lava flow, it
was poured out over the lower, first formed, after which the
second was laid down upon its surface. If formed as an intru-
sive sheet, we may expect to find the rock more dense along
both contacts, in addition to which there may, very probably,
be more or less contact metamorphism on the sedimentary beds
from the action of the hot intruded material. If poured out
as a la.vu, on the other hand, contact metamorphism and the
dense, fine-grained portions will be limited to the lower con-
tarts. while, provided there had been no great amount of erosion
between the time of the pouring out of the molten mass as &
surface How and the deposition of the newer sediments, the
upper portions will be less dense, perhaps even vesicular, sco-
riaccdiis, ami glassy, while the sediments themselves, having
been laid down on cold consolidated material, remain wholly
unchanged. Such means of discrimination have been of the
greatest value in ascertaining the relative ages of portions
of the Triassic sandstones and associated traps in the eastern
United States.
'£he lava flows, cooling so much more rapidly than the plu-
tonic rocks, owing to their exposed position and relief from
pressure, often show but incipient forms of crystalli/.ation, or
are quite glasslike, as is the case with the obsidians of the
Yellowstone Park and elsewhere. Chemically these are iden-
tical with granite, but they have cooled too quickly for the
forces of crystallization to act. Owing, further, to the expan-
sive force of the included vapor of water, — a constituent of
all lavas, — these surface flows are not infrequently so filled
with cavities as to be quite pumiceous. The pumice pur-
chased at the drug-stores is but the froth from a lava which,
62 ROCKS FORMED THROUGH IGNEOUS AGENCIES
had it cooled under greater pressure, might have given us a
granite.
A common feature of the effusive or volcanic rocks is a flow
structure, sometimes visible only with the microscope, and which
is due to a flowing movement of the magma while undergoing
consolidation. (See Fig. 2, PI. 2.) The characteristic structure
of effusive rocks is porphyritic, instead of granular, and repre-
sents two distinct phases of cooling and crystallization : (1) an
intratellurial period, marked l>y the crystallization of certain
constituents while the magma, still buried in the depths of the
earth, was cooling very gradually, and (2) an effusive period,
marked by the final consolidation of the material on or near
the surface. As this final cooling was much the more rapid,
the ultimate product is a glassy, felsitic, or sometimes holo-
crystalline ground-mass, enclosing the porphyritic minerals, or
phenocrysts, formed during the first or intratellurial stage.1
Naturally the deeper-lying portions of an effusive mass, .those
forming the under or lower portions of deep lava streams, will
be under conditions essentially similar to plutonic magmas, and
may cool so slowly as to become holocrystalline. It is, more-
over, obvious that, could we trace any superficial mass of
erupted material back to its original deep-seated source, we
would pass gradually from the volcanic to the plutonic type
without at any one point being able to indicate the line of
separation. Hence it is that in the laboratory it is not always
possible, from the examination of the hand specimen or thin
section only, to determine to which of the two classes it may
belong. We can easily discriminate between the extremes,
but there is a wide intermediate zone where any such attempts
are impracticable, as indeed they are unnecessary.2
1 Whitman Cross has shown that there are exceptions to this rule. See. The
Laccolitic Mountain Groups of Colorado, 14th Ann. Rep. U. S. Geol. Survey,
pp. 231-235.
2 Intermediate between these plutonic and effusive types is still a third phase
of prevailing holocrystalline porphyritic structure, and which, owing to the fact
that such have thus far been found only in dikes, it has been proposed to group
under the head of dike rocks (gangesteine). Since such are but local phases of
plutonic magmas, which have been left to cool and crystallize between narrow
walls, instead of poured out upon the surface, such a subdivision seems scarcely
called for and as tending to still further confuse that which is already sadly
confounded. The same may be said with reference to the now prevailing ten-
dency to give varietal names to every phase of magmatic differentiation, and
which has resulted already in such monstrosities of nomenclature as ouachitite,
monchiquite, yogoite, and absarokite.
RELATIONSHIP OF PLUTONIC AND IGNEOUS ROCKS
63
Owing to a false impression which formerly prevailed relative
to the nature of the Palaeozoic effusives and those of Mesozoic,
Tertiary, and more recent times, dissimilar names have, in very
many instances, been applied to rocks which in other respects
than that of geological age are essentially one and the same.
Thus the name andesite is given to a rock in every respect
similar to porphyrite, with the possible exception of a slight
amount of devitrification the latter may have undergone owing
to its greater geological antiquity.
The name rhyolite likewise includes rocks with the structure
and composition of the older quartz porphyries, and though
intended by Richthofen to include only certain comparatively
modern acid lavas, has been shown by the late Dr. Williams l
to be equally applicable to the pre-Cambrian lavas of the South
Mountain region of Pennsylvania. These and other names have,
however, become too firmly engrafted upon the literature to be
too hastily set aside, and may well be retained here.
The following table will serve to show the relationship, so
INTRUSIVE OB PLUTONIC
EFFUSIVE OB VOLCANIC
Pala'ovolcanic
Neovolcanic
Acid ]
65% -75% ^Granites ....
Quartz porphyries . .
Liparites(rhyolites)
SiO2 J
Intermediate
55% to 65% H
L- : / i
• Syenites ....
Nepheline syenites |
(Foyaites) )
Quartz-free porphyries
Phonolites
Trachytes
Phonolites
OlUo
• Diorites ....
Porphyrites ....
Andesites
f Gabbros, norites, /
Melaphyrs and augitei
III C1 Itfl
and diabases \
porphyrites j
I * 1 1 > « I 1 tO
Basic
Theralites . . .
(Not known) ....
I Thephrites and
/ basauites
40% to 55%
Peridotites .
Picrite porphyrites
Limburgites
SiO2
Pyroxenites .
(Not known) . . .
Augitites
(Not known)
(Not known) . . .
Leucite rocks
(Not known)
(Not known) . . .
Nepheline rocks
. (Not known)
(Not known) . . .
Melilite rocks jf
far as known, which exists between the plutonic rocks and
their effusive equivalents of whatever age. Thus the palaeo-
1 Am. Jour, of Science, Vol. XLIV, p. 482, 1892.
64 KOCKS FORMED THROUGH IGNEOUS AGENCIES
volcanic equivalents of the syenites are the quartz-free por-
phyries, and the neovolcanic equivalents, the trachytes. The
terms acid, intermediate, and basic, as used, have reference to
the percentage amounts of silica, both free and combined,
contained by the representatives of the several groups. Rocks
which, like some of the peridotites, carry even less than 40 %
of silica are sometimes spoken of as ultra basic.
The researches of the past few years have made it apparently
evident that eruptive rocks are to be satisfactorily studied only
when considered in their geographical as well as geological
relationships ; that is to say, the eruptives of any particular
region must be considered with reference to their genetic rela-
tion to others of the same region ; such a relationship as is
suggested by regarding them all as but varying phases of a
process of differentiation from a common magma.
That such a relationship in many cases exists has apparently
been conclusively demonstrated by the work of Iddings l in the
Yellowstone Park, J. F. Williams2 in Arkansas, Pirsson3 in
Montana, and Brogger4 in Norway. The attempt at correla-
tion of local types with those of a somewhat similar nature at
a distance is interesting and instructive, as showing on the
whole a remarkable unity in nature's methods ; • but we must
never lose sight of the fact that each eruptive centre, through-
out periods of activity interrupted it may be by thousands of
years, works out its own results according to local conditions
which may or may not harmonize with those at distant points.
It is possible to conceive that, could all the rocks of any suc-
cessive periods of eruption from a single centre be once more
relegated to a common magma, such might, in its entirety, be
an exact equivalent of others in remote portions of the globe.
The consolidated results from the cooling of extruded portions
of this magma may, however, show ever-varying differences
due to local conditions. In short, eruptive rocks must be
considered by geographic groups and with reference to magmas.
Attempts at a satisfactory classification on other grounds
must prove invariably futile and tend only to retard, rather
than to promote, the science.
1 Bull. Philos. Soc. of Washington, XII, 1892. .
2 Ann. Rep. Geol. Survey of Arkansas, Vol. II, 1890.
8 Bull. Geol. Soc. of America, Vol. VI, 1895.
4 Die Eruptivgesteine des Kristianiagebiete, Christiana, Norway, 1894.
PLATE 0
FIG. 1. Lithophysae in liparite.
FIG. 2. Cross-section of stalagmite.
FIG. 3. Concretionary aragonite.
FIG. 4. Pegmatite.
THE GRANITE-LIPARITE GROUP
65
In the following pages the rocks are discussed in groups,
each group comprising all those rocks having essentially the
same chemical composition,, but differing (1) in degree of
crystallization, (2) in mode of occurrence, and (3) in geological
age. In all, there is, within certain limits, a considerable ran^v
in mineral composition, or at least in the relative proportion of
the various essential constituents.
1. THE GRANITE-LIPARITE GROUP
This group includes the most acid of all eruptive rocks ; that
is, those which on analysis are found to yield the highest per-
centages of silica. Their chief essential constituents aiv quart/,
and potash feldspars, while the more basic ferruginous minerals
are in quantities proportionatel}' small. The group includes a
deep-seated or plutonic type, granite, and two effusive or vol-
canic types, quartz porphyry, and liparite or rhyolite. They
may be described in detail as below : —
(1) THE GRANITES
Granite, from the Latin " granum," a grain, in allusion to the
granular structure.
Mineral Composition. — The essential constituents of granite
are quart/, and a potash feldspar (either orthoclase or micro-
cline), and plagioclase. Nearly always one or more minerals of
the mica, hornblende, or pyroxene group are present, and in
small, usually microscopic forms, the accessories magnetite,
apatite, and zircon ; more rarely occur sphene, beryl, topaz,
tourmaline, garnet, epidote, allanite, fluorite, and pyritr. !)»•-
lesse1 has made the following determination of the relative
proportion of the various constituents in two well-known gran-
ites : —
K'ivrriAN UKI> (IUAMTF.
PARTS
POBPIIYKITIC GRANITE, VOSOF.S
PARTS
Red orthoclase ....
43
White orthoclase .....
28
White albite
9
Reddish oli^oclase
7
Gray quartz
44
Gray quartz
69
Black mica
4
Mica
6
Total
100
Total .
100
Prestwich, Chemical and Physical Geology, Vol. I, p. 42.
66
ROCKS FORMED THROUGH IGNEOUS AGENCIES
Chemical Composition. — A general idea of the varying char-
acter of these rocks may be gained from the following analy-
ses :
KINDS AND LOCALITIES
Si02
A120S
FeO
Fe,03
CaO
MgO
K20
Na,0
Biotite granite, near Dublin,
73.0
13.64
2.44
1.84
2.11
4.21
3.53
Biotite granite, Silesia . .
Biotite granite, Raleigh,
North Carolina ....
Hornblende granite, Salt
Lake Utah
73.13
69.28
71.78
12.49
17.44
14.75
2.58
2.30
1.941
2.40
2.30
2.36
0.27
0.27
0.71
4.13
2.76
4.89
2.61
3.64
3.12
Hornblende granite, Sauk
Rapids, Minnesota . ._ .
Gneissoid biotite granite,
District of Columbia . .
Hornblende mica granite,
Syene Egypt ....
64.13
69.33
68.18
21
14.33
16.20
01
3.60
4.10
6.90
3.21
1.75
1.26
2.44
0.48
1.22
2.67
6.48 !
3.31
2.70
2.88
Although the mineral apatite is so universally a constituent
of granitic rocks, yet it occurs in such small quantities as to
be quite overlooked in the ordinary methods of analysis. Such
tests as have been made show that the amount of phosphoric
acid (P2^5) contained by rocks of this class rarely exceeds
0.2 % and may fall as low as 0.05 %. Small as is the amount,
it is nevertheless probable that it was from just such minute
quantities in granites and the more basic eruptives, that was
derived the main supply of phosphates existing in soils.
Structure. — The granites are holocrystalline granular rocks.
As a rule none of the essential constituents show perfect crystal
outlines, though the f eldspathic minerals are often quite perfectly
formed. The quartz has always been the last mineral to so-
lidify, and hence occurs only as irregular granules occupying the
interspaces. It is remarkable from its carrying innumerable
cavities filled with liquid and gaseous carbonic acid or with
saline matter. So minute are these cavities that it has been esti-
mated by Sorby that from one to ten thousand millions could
be contained in a single cubic inch of space. The microscopic
structure of a mica granite from Maine is shown in Fig. 3 and
in Fig. 1, PL 5.
1 Yielded also 1.09% manganese oxide.
THE GRANITE-LIPARITE GROUP
67
FIG. 3. •
• Microstructure of muscovite-biotite
grauite, Hallowell, Maine.
The rocks vary in texture almost indefinitely, presenting all
gradations from fine evenly granular rocks to coarsely porphy-
ritic forms in which the
feldspars, which are the
only constituents porphy-
ritically developed, are
several inches or feet in
length.
Concretionary forms
are rare. A variety from
Craftsburg, Vermont, is
unique on account of the
numerous concretionary
masses of black mica it
carries.
Colors. — The prevail-
ing color is some shade
of gray, though greenish.
yellowish, pink, to deep
red, are not uncommon.
The various hues are due to the color of the prevailing feldspar
and the abundance and kind of the accessory minerals. Granites
in which muscovite is the prevailing mica, are nearly always very
light gray in color. The dark gray varieties are due largely to
abundant black mica or hornblende, the greenish and pink or
ml colors to the prevailing greenish, pink, or red feldspars.
Classification and Nomenclature. — Several varieties are com-
monly recognized and designated by names dependent upon tin;
predominating accessory mineral. We thus have (1) musco-
vite granite, (2) biotite granite or granitite, (3) biotite-muscovite
granite, (4) hornblende granite, (5) hornblende-biotite </rii>iit<\ and
more rarely (6) pyroxene (7) tourmaline and (8) epidote granite.
The name protogine has been given to a granite in which the
mica is in part or wholly replaced by talc. The name is not
very generally used.
Graphic granite, or pegmatite, is a granitic rock consisting
essentially of quartz and orthoclase so crystallized together in
long parallel columns or shells that a cross-section bears a
crude resemblance to Hebrew writing. (See Fig. 4, PL 6.)
Aplit is a name used by the Germans for a granite very poor in
mica aud consisting essentially of quartz and feldspar only.
68 ROCKS FORMED THROUGH IGNEOUS AGENCIES
The names granitell and binary granite have also been used
to designate rocks of this class. Grreisen is a name applied to
a quartz-mica rock, with accessory topaz, occurring associated
with the tin ores of Saxony and regarded as a granite meta-
morphosed by exhalations of fluoric acid. Luxullianite and
Trowlesworthite are local names given to tourmaline or tour-
maline-fluorite granitic rocks occurring at Luxullian and
Trowlesworth, in Cornwall, England. The name Unakite has
been given to an epidotic granite with pink feldspars occurring
in the Unaka Mountains in western North Carolina and eastern
Tennessee.
The name granite porphyry is made to include a class of rocks
placed by Professor Rosenbusch under the head of "gange-
steine," or dike rocks, and differing from the true granites
mainly in structural features. They consist in their typical
forms of orthoclase feldspars and quartzes porphyritically de-
veloped in a finer holocrystalline aggregate of the minerals
common to the granite group.
The granites are among the most wide-spread and commonest
of rocks, and are of great economic importance for structural
and monumental work. In the United States they are to be
found mainly in the Appalachian region and from the front
range of the Rocky Mountains westward to the Pacific coast.
Geological Age and Mode of Occurrence. — The granites are
massive rocks, occurring most frequently associated with the
older and lower rocks of the earth's crust, sometimes inter-
stratified with metamorphic rocks or forming the central por-
tions of mountain chains. They are not, as once supposed, the
oldest of rocks, but occur frequently in eruptive masses or
bosses invading rocks of all ages up to late Mesozoic or Ter-
tiary times. Thus Professor Whitney considered the eruptive
granites of the Sierra Nevada to be Jurassic. Zirkel divides
the granites described in the reports of the 40th Parallel Sur-
vey into three groups : (1) Those of Jurassic age ; (2) those of
Paleeozoic age, and (3) those of Arcluean age. The granites
of the eastern United States, on the other hand, have, in times
past, been regarded as mainly Archtean, though Dr. Wadsworth
has shown that the Quincy, Massachusetts, stone is an eruptive
rock of late Primordial or more recent age, while Professor
Hitchcock regards the eruptive granites of Vermont as having
been protruded during Silurian or perhaps Devonian times.
THE QUARTZ PORPHYRIES 69
(2) THE QUARTZ PORPHYRIES
Composition. — The mineral and chemical composition of the
quartz porphyries is essentially the same as that of the gran-
ites, from which they differ mainly in structure. Their essen-
tial constituents are quartz and feldspar, with accessory black
mica or hornblende in very small quantities ; other acces-
sories present, as a rule only in microscopic quantities, are
magnetite, pyrite, hematite, and epidote.
Structure. — The prevailing structure is porphyritic. (Fin . 1 •
PI. 2.) To the unaided eye they present a very dense and com-
pact ground-mass of uniform reddish, brown, black, gray, or jrel-
lowish color, through which are scattered clear glassy crystals
of quartz alone, or of quartz and feldspar together. The quart/,
differs from the quartz of granites in that here it was the first
mineral to separate out on cooling, and hence has taken on a
more perfect crystalline form ; the crystal outlines of the feld-
spar are also well denned. "Tinier the microscope the ground-
mass in the typical porphyry is found to consist of a dense
felsitic, almost irresolvable substance, which chemical analysis
shows to be also a mixture of quartzose and feldspathie ma-
terial. The porphyritic (jnart/cs show frequently a marked
corrosive action from the molten magma, the mineral having
again been partially dissolved after its first crystallization.
( Ki--. -•'>. PI. ">.) This difference in structure in rocks of the
same chemical composition is believed to be due wholly to the
different circumstances under which the two rocks have solidi-
fied from a molten magma. The structure of the ground-mass
is not always felsitic, but may vary from a glass, as in the
pitchstones of Meissen, Isle of Arran, and the Lake Lugano
region, through spherulitic, micropegmatitic, and porphyritic
to perfectly microcrystalline forms as in the microgranites.
This difference in structure may be best understood by refer-
ence to Plate 5, which shows the microscopic structure of (1)
granite from Sullivan, Hancock County, Maine, (2) micropeg-
matite from Mount Desert, Maine, and (3) a quartz porphyry
from Fairfield, Pennsylvania. Marked fluidal structure is
common. (See PI. 2, Fig. 2.)
Colors. — The colors of the ground-mass, as above noted, vary
through reddish, brownish gray to black and sometimes yellowish
or green. The porphyritic feldspars vary from red, pink, and
70 ROCKS FORMED THROUGH IGNEOUS AGENCIES
yellow to snow-white, and often present a beautiful contrast with
the ground-mass, forming a desirable stone for ornamental pur-
poses.
Classification and Nomenclature. — Owing to the very slight
development of the accessory minerals, mica, hornblende, etc.,
it has been found impossible to adopt the system of classifica-
tion and nomenclature used with the granites and other rocks.
Vogelsang's classification as modified by Rosenbusch is based
upon the structure of the ground-mass as revealed by the micro-
scope. It is as follows: —
Ground-mass holocrystalline granular Micro-granite.
Ground-mass holocrystalline, but formed of quartz and feld-
spar aggregates, rather than district crystals Granophyr.
Ground-mass felsitic Felsophyr.
Ground-mass glassy Vitrophyr.
Intermediate forms are designated by a combination of the
names, as granofelsophyr, felsovitrophyr, etc. The name felsite
is often given to rocks of this group in which the porphyritic
constituents are wholly lacking. The names felstone and
petrosilex are also common, though gradually going out of use.
Elvanite is a Cornish miner's term and too indefinite to be of
great value. Eurite, now little used, applies to felsitic forms.
The name felsite pitchstone or retinite has been given to a glassy
form with pitch-like lustre, such as occurs in dikes cutting the
old red sandstone on the Isle of Arran. Kugel porphyry is a
name given by German writers to varieties showing spheroids
with a radiating or concentric structure. Micropegmatite is the
term not infrequently applied to such as show under the micro-
scope a pegmatitic structure. (Fig. 2, PL 5.) Various popular
names, as leopardite and toadstone, are sometimes applied to such
as show a spotted or spherulitic structure.
(3) THE LIPARITES
Mineral Composition. — These rocks may be regarded as the
younger equivalents of the quartz porphyries, or the volcanic
equivalents of the granites, having essentially the same mineral
and chemical composition. The prevailing feldspar is the clear
glassy variety of orthoclase known as sanidin ; quartz occurs in
quite perfect crystal forms often more or less corroded by the
molten magmas, as in the porphyries, and in the minute, six-
PLATE 7
#
FIG. 1. Liparite, nevadite form.
Fio. 2. Liparite, rhyolite form.
FIG. 3. Liparite, obsidian form.
FIG. 4. Liparite, pumiceous form.
THE LIPARITES
71
sided, thin platy forms known as tridymite. The accessory
minerals are the same as those of the granites and quartz
porphyries.
Chemical Composition. — Below is given the composition of :
(I) Nevadite, from the northeastern part of Chalk Mountain,
Colorado, as given by Cross.1 (II) That of a rhyolitic form,
from the Montezuma Range, Nevada, as given by King.- and
(III) that of a black obsidian from the Yellowstone National
Park, Wyoming, as given by Iddings.3
CONSTITUENTS
I
II
III
Silica (Si02)
74.50%
74.62%
74.70%
Alumina (AljOs)
14.72
11.96
18.78
Ferric oxide (FeoOs)
None
1.20
l.ni
Ferrous oxide (FeO)
0.66
0.10
0.62
Ferric sulphide (FeSj)
0.40
Manganese (MnO)
0.28
Trace
Lime (CaO)
0.83
0.36
0.78
Magnesia (MgO)
0.37
0.14
Sn.la i Na-,0)
3.97
2.26
3.90
l'«'t!i*li (K..O)
4.53
7.76
4.02
1'h'isphoric anhydride (PjOg)
0.01
Ignition
0.66
1.02
0.62
Specific gravity
100.38 %
99.28 %
2 2
99.91 %
2 3447
Colors. — These are fully as variable as in the quartz por-
phyries ; white, though all shades of gray, green, brown, yel-
low, pink, and red are common. Black is the more common
color for the glassy varieties of obsidian, though they are often
beautifully spotted and streaked with red or reddish-brown.
Structure. — The liparites present a great variety of structural
features, varying from holocrystalline, through porphyritic and
felsitic, to clear, glassy forms. These varieties, can be best
understood by reference to Plates 5 and 7, prepared from
photographs. Fig. 1, PI. 7, is that of the coarsely crystalline
variety nevadite from Chalk Mountain, Colorado ; Fig. 2 is
1 Geology and Mining Industry of Leadville, Monograph XII, U. S. Geol.
Survey, p. 349.
2 Geological Exploration 40th Parallel, Vol. I, p. 652.
8 Ann. Rep. U. S. Geol. Survey, 1885-86, p. 282.
72 ROCKS FORMED THROUGH IGNEOUS AGENCIES
that of a common felsitic and porphyritic type ; Fig. 3 is that
of the clear, glassy form, obsidian ; Fig. 4 shows also an obsid-
ian, but with a pumiceous structure ; Fig. 1 on PI. 6 shows the
hollow spherulites or lithophysce, which have been studied and
described by Professor J. P. Iddings, of the United States Geo-
logical Survey.1 Such forms are regarded by Mr. Iddings as
resulting " from the action of absorbed vapors upon the molten
glass from which they were liberated during the process of
crystallization consequent upon cooling." A pronounced flow
structure is quite characteristic of the rocks of this group, as
indicated by the name rhyolite. The microscopic structure of
an obsidian is shown in Fig. 4, PL 5. Transitions from com-
pact obsidian into pumiceous forms, due to expansion of included
moisture, are common.
Classification and Nomenclature. — The following varieties are
now generally recognized, the distinctions being based mainly
on structural features, as with the quartz porphyries. We thus
have the granitic-appearing variety nevadite, the less markedly
granular and porphyritic variety rhyolite, and the glassy forms
hyaloliparite, hyaline rhyolite, or obsidian as it is variously called.
Hydrous varieties of the glassy rock with a dull pitch-like lustre
are sometimes called rhyolite pitchstone.
The name rhyolite, from the Greek word pew, to flow, it may
be stated, was applied by Richtofen as early as 1860 to this
class of rocks as occurring on the southern slopes of the Carpa-
thians. Subsequently Roth applied the name Liparite to similar
rocks occurring on the Lipari Islands. The first name, owing
to its priority, is the more generally used for the group, though
Professor Rosenbusch in his latest work has adopted the latter.
The name Nevadite is from the state of Nevada, and was also
proposed by Richtofen. The name Obsidian as applied to the
glassy variety is stated to have been given in honor of Obsid-
ius, its discoverer, who brought fragments of the rock from
Ethiopia to Rome. The name pantellerite has been given by
Rosenbusch to a liparite in which the porphyritic constituent
is an orthoclase.
Rocks of these types occur, in the United States, only in
the regions west of the front range of the Rocky Mountains.
Apo-rhyolite is the name proposed by Dr. Williams for the
i Obsidian Cliff, Yellowstone National Park, Ann. Rep. U. S. Geol. Survey,
1885-86.
THE SYENITE-TRACHYTE GROUP 73
devitrified and otherwise altered pre-Cambrian rhyolite found
at South Mountain in Pennsylvania.
2. THE SYENITE-TRACHYTE GROUP
This group stands next to that of the granites in point of
acidity, from which it differs mainly in the lack of free silica
(quartz) as an essential constituent. On chemical grounds this
and the next group to be described belong to the intermediate
series, standing midway between the acid granites and the basic
basalts. As with the last, we have plutonic and effusive forms.
These may be described as below : —
(1) THE SYENITES
The name Syenite, from Syene, a town of Egypt. The word
was first used by Pliny to designate the coarse red granite from
quarries at Syene, and used by the Egyptians in their obelisks
and pyramids. Afterwards (in 1787) Werner introduced the
word into geological nomenclature to designate a class of gran-
ular rocks consisting of feldspar and hornblende, either with or
without quart/. Later, when a more precise classification
became necessary, the German geologists reserved the name
syenite to designate only the quart/less varieties of tin-c-
rocks, while the quartz-bearing varieties were referred to the
Imrnblendic granites. This is the classification now followed
by all the leading petrologists and is therefore adopted here.
Much confusion has arisen from the fact that the French geolo-
gist Roziere insisted upon designating the quartz-bearing rock
as syenite, a practice which has been followed to a considerable
extent both in this country and Kngland.
Mineral Composition. — The syenites differ from the granites
only in the absence of the mineral quartz, consisting essentially
of orthoclase feldspar in company with luotite, or one or more
minerals of the amphibole or pyroxene group. A soda-lime
feldspar is nearly always present and frequently microcline ;
other common accessories are apatite, zircon, and the iron ores :
more rarely sodalite.
Chemical Composition. — In column I below is given the com-
position of a hornblende syenite from near Dresden, Saxony,
in II that of a mica syenite (minette) from the Odenwald, and
in III and IV that of augite-sodalite syenites from .Montana.
ROCKS FORMED THROUGH IGNEOUS AGENCIES
CONSTITUENTS
I
II
III
IV
Silica (SiOg)
60.02%
57.37%
54.15%
56.45%
16.66
13.84
18.92
20.08
1
f 2.44
1
f 1.31
| 7.21
I 3.44
| 6.79
I 4.39
Magnesia (MgO)
2,51
6.05
1.90
0.63
Lime (CaO)
3.59
5.53
3.72
2.14
Soda (NaoO)
2.CL
1.53
5.47
6.61
Potash (K20)
6.50
4.47
8.44
7.13
Ignition (H20)
1.10
3.17
1.77
Chlorine (Cl )
0.42
0.43
Phosphoric acid (PgOg)
0.13
100.00%
97.84%
99.81
100.07 %
Structure. — The structure of the syenites is wholly analo-
gous to that of the granites, and need not be further described
here. In process of crystallization the apatite, zircon, and iron
ores were the first to separate out from the molten magma, and
hence are found in more or less perfect forms enclosed by the
feldspars and later-formed minerals. These were followed in
order by the mica, hornblende, or augite, and lastly the feld-
spars, the soda-lime feldspars, when such occur, forming subse-
quent to the orthoclase.
Color. — The prevailing colors are various shades of gray,
through pink to reddish.
Classification and Nomenclature. — According as one or the
other of the accessory minerals of the bisilicate group predomi-
nates we have (1) hornblende syenite, (2) mica syenite, or minette,
and (3) augite syenite.
Other varietal names have from time to time been given
by various authors. The name minette, first introduced into
geological nomenclature by Voltz in 1828 (Teall), is applied
to a fine-grained mica orthoclase rock, occurring only in the
form of dikes and further differing from the typical syenites in
having a porphyritic rather than granitic structure. Vogesite
is the name applied to a similar rock in which hornblende or
augite prevails in place of mica. These rocks are placed by
Professor Rosenbusch in his latest work in the group of
syenitic lamprophyrs. Monzonite is a varietal name for the
augite syenite of Monzoni in the Tyrol.
THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES 75
The mode of occurrence of the syenites is similar to that
of the granites, though they are much more limited in their
distribution. In the United States they have thus far been
described but sparingly. Marblehead Neck, Massachusetts;
Jackson, New Hampshire, are well-known localities; a beauti-
ful hornblende syenite is found among the glacial drift boulders
about Portland, Maine, but its exact source is not known. The
hornblende syenite described by Hawes as occurring at Red
Hill, Moultonborough, New Hampshire, has been shown by
Professor W. S. Bayley1 to carry elceolite, and to belong to the
group of elseolite syenites. Hornblende syenites occur in the
Vosges Mountains of Germany and in Saxony ; mica syenites
or minettes in the Odenwald, Germany, Baden, Saxony, and in
the Fichtelgebirge. A mica-augite syenite carrying sodalite
occurs as a Cretaceous eruptive in Jefferson County. Moinana.-
and a similar rock has been described by Lindgren from the
Highwood Mountains in the same state.3
(2) THE ORTHOCLASE OR QUARTZ-FREE PORPHYRIES
Mineral Composition. -- The essential constituents are the
same as those of syenite. They consist therefore of a compact
porphyry ground-mass with porphyritie feldspar (orthoclase)
and accessory plagioclase, quart/, mica, hornblende, or minerals
of the pyroxene group. More rarely occur zircon, apatite,
magnetite, etc.. as in the syenites.
Chemical Composition. — lleing poor in quartz, these rocks are
a trifle more basic than the quartz porphyries which they other-
wise resemble. The following is the composition of an ortho-
clase porphyry from 1'ivdazzo as given by Kalkowski:4 Silica,
(54.45% ; alumina, 10.31% ; ferrous oxide, 6.49% ; magnesia,
0.30%; lime, 1.10%; soda, 5.00%; potash, 5.45%; watrr.
0.85%.
Structure. — Excepting that orthoclase is the porphyritic con-
stituent, they are structurally identical with the quartz porphy-
ries, and need not be further described here.
Colors. — These are the same as the quartz porphyries already
described.
1 Bull. Geol. Soc. of America, Vol. Ill, 1802.
3Proc. U. S. Nat. Museum, Vol. XVII, 1K<>4.
* Proc. Cali. Acad. of Sciences. Vol. Ill, 2<1 series, p. 47.
* Elements der Lithologie, p. 80.
76 ROCKS FORMED THROUGH IGNEOUS AGENCIES
Classification and Nomenclature. — The orthoclase or quartz-
free porphyries bear the same relation to the syenites as do the
quartz porphyries to granite, and the rocks are frequently
designated as syenite porphyries. Like the quartz porphyries,
they occur in intrusive sheets, dikes, and lava flows associated
with the Palaeozoic formations. Owing to the frequent absence
of accessory minerals of the ferro-magnesia group, the rocks can-
not in all cases be classified as are the syenites, and distinctive
names based upon other features are often applied. The term
orthophyr is applied to the normal orthoclase porphyries, and
these are subdivided when possible into biotite, hornblende, or
augite orthophyr according as either one of these minerals is the
predominating accessory. The term rhombporphyry has been
used to designate an orthoclase porphyry found in southern
Norway, and in which the porphyritic constituent appears in
characteristic rhombic outlines, and which is further distin-
guished by a complete absence of quartz and rarity of horn-'
blende. The name keratopliyr has been given by Gumbel to a
quartzose or quartz-free porphyry containing a sodium-rich alka-
line feldspar. So far as can be at present judged, rocks of this
type are much more restricted in their occurrence than are the
quartz porphyries already described.
(3) THE TRACHYTES
Trachyte, from the Greek word rpa^v^ rough, in allusion to
the characteristic roughness of the rock. The term was first
used by Haiiy to designate the well-known volcanic rocks of the
Drachenfels on the Rhine.
Mineral Composition. — Under the name of trachyte are com-
prehended those massive Tertiary and post-Tertiary lavas, con-
sisting essentially of sanidin with hornblende augite or black
mica, and which may be regarded as the younger equivalents of
of the quartz-free porphyries. The common accessory minerals
are plagioclase, tridymite, apatite, spherie, and magnetite, more
rarely olivine, sodalite, humite, hauyne, and melilite.
Chemical Composition. — The following analyses show the
range in chemical composition of these rocks, I being that of
the trachyte of Game Ridge, Colorado, and II that of a La
Guardia stone.
THE FOYAITE-PHONOLITE GROUP
77
CON8TITTENT8
I
II
Silica (SiOo)
66.03%
56.09%
Alumina (A^Oa) '
18.49
26.09
Ferric oxide (FegOs)
2.18
Manganese oxide (MnO)
Trace
Trace
Lime (CaO)
0.96
3.41
Ma°nesia (MgO)
0.39
2.70
Potash (K2O)
6.80
6.49
Soda (NaaO)
5.22
3 38
Ignition (H2O)
0.85
1.05
Phosphoric acid (PgOg)
0.04
*'
Total
100.24 %
100.74 %
Structure. — In structure the trachytes are rarely granular,
but possess a fine, scaly or microfelsitic ground-mass, rendered
porphyritic through the development of scattering crystals of
sanidin, hornblende, augite of black mica. The texture is
porous, and the rock possesses a characteristic roughness to
the touch; hence the derivation of the name as given above.
Perlitic structure is common in the glassy forms. The micro-
scopic structure of the trachyte of Monte Vetta is shown in
Fig. 5, PL 5.
Colors. — The prevailing colors are grayish, yellowish, or
reddish.
Classification and Nomenclature. — They are divided into horn-
blende^ auyite, or mica trachytes, according as any one of these
minerals predominates. The name sanidin-oligoclase trachyte is
sometimes given to trachytes in which both these feldspars ap-
pear as prominent constituents. The presence of quartz gives
rise to the variety quartz trachyte*. (See under rhyolite.) The
glassy form of trachyte is commonly known under the name of
trachyte pitchstone, or if with a perlitic structure simply as per-
lite. In his most recent work Professor Rosenbusch has included
the glassy forms under the name of hyalotrachyte.
3. THE FOYAITE-PHONOLITE GROUP
This group differs from the last mainly in the partial replace-
ment of the potash feldspars by the closely related mineral
eheolite or nepheline. It includes therefore those plutonic and
effusive rocks commonly known under the name of elceolite or
78
KOCKS FORMED THROUGH IGNEOUS AGENCIES
nepJieline syenites and the phonolites. In their silica and potash
percentages it will be observed they differ not greatly from the
syenites proper, but are much more rich in soda and corre-
spondingly poor in lime. They may be described in detail as
follows : —
(1) THE NEPHELINE (EL^EOLITE) SYENITES: FOYAITS
Nepheline from the Greek ve^eA.^, a cloud, since the mineral
becomes cloudy on immersion in acid. Elseolite from eXaiW, oil,
in allusion to the greasy lustre. Syenite from Syene in Egypt.
Mineral Composition. — The essential constituents of this
group are nepheline (elseolite) and orthoclase, with nearly
always a pyroxenic or amphibolic mineral and a plagioclase
feldspar. The common accessory minerals are sphene, sodalite,
cancrinite, zircon, apatite, black mica, and the iron ores iline-
fiite and magnetite, with occasional leucite, eucolite, melino-
phane, and also tourmalines, perowskite, and oliviiie. Calcite,
epidote, chlorite, analcite, and sundry minerals of the zeolite
group occur as secondary products.
Professor W. S. Bayley has computed1 the relative propor-
tions of the various constituents in the elseolite syenite of Litch-
field, Maine, as follows : Ekeolite, 17 % ; potash feldspar, 27 % ;
albite, 47% ; cancrinite, 2 % ; and black mica (lepidomelane), 1%.
Chemical Composition. — The composition of the elseolite sye-
nite from several well-known localities is given below : —
CONSTITUENTS
ALGRAVE,
PORTUGAL
HOT SPRINGS,
ARKANSAS
LlTCHFIELD,
MAINE
BEEMERVILLE,
NEW JERSEY
Silica (Si02)
54.61 %
59.70%
60.39%
50.36%
Alumina (Al2Os) ....
Ferric oxide (Fe2Os) . . .
Ferrous oxide (FeO) . . .
Magnesia (MgO)
22.07
2.33
2.50
0.88
18.85
4.85
0.68
22.51
.42
2.26
0.13
19.84
J6.94
Manganese oxide (MnO) . .
Lime (CaO)
2 51
1.34
0.08
0.32
0.411
3.43
Soda (Na2O)
7.58
6.29
8.44
7.64
Potash (K2O)
5.46
5.97
4.77
7.17
Titanium oxide (Ti02) . .
Phosphoric anhydride (P20e)
Water (H2O)
0.09
0.15
1 13
1.88
.57
3.512 (loss)
1 Bull. Geol. Soc. of America, Vol. Ill, 1892, p. 231.
THE NEPHELINE SYENITES 79
The essential points to be noted are the larger percentages
of the alkalies over those yielded by syenites of the ordinary
type, or the granites.
Color. — The colors are light to dark gray, and sometimes
reddish.
Structure. — The syenites, like the granites, are massive holo-
crystalline granular rocks, and as a rule sufficiently coarse in
texture to allow a partial determination of the constituent
parts by the unaided eye. In the Litchfield (Maine) syenite
the elseolite often occurs in crystals upwards of 5 centimetres
in length, and zircons 2 centimetres in length are not rare.
Neither of the essential constituents occur in the form of per-
fect crystals, while the apatite, zircon, black mica, and pyrox-
enic constituents often present very perfect forms. The can-
crinite occurs both as secondary after the elseolite and as a
primary constituent in the form of long needle-like yello\fc
crystals with a hexagonal outline. This last form is especially
characteristic of the Litchfield rock. The sodalite occurs both
as crystals and in irregular massive forms, coating the walls of
crevices.
Classification and Nomenclature. — Several varietal names have
been given to the rocks of this group as described by various
authors. Miascite was the name given by G. Rose to the sye-
nite occurring at Miask in the Urals ; Ditroite to that occurring
at Ditro in Transylvania, and Foyaite, by Blum, to that from
Mount Foya, in the province of Algrave in Portugal. The
name zircon syenite, or Laurvikite, has been given to the vari-
ety from Laurvig in southern Norway, which is rich in zircons.
Tinguaite is the name proposed for a varietal form from Serra
de Tingua, province of Rio Janeiro, Brazil.
American petrographers have not been at all delinquent in
tjie matter of names, and have added to an already over-
burdened nomenclature such terms as Litchfieldite, Ouachitite,
Pulaskite, and Fourchite to varieties from Litchfield (Maine)
and the Hot Springs region of Arkansas. Liebnerite is the
name given to an elseolite syenite porphyry occurring in the
Tyrol.
Rocks of this group, although wide-spread in their distribu-
tion, are nevertheless not abundant. The more important
localities thus far described have already been noted ; there
remains to be mentioned only the locality at Red Hill, Moul-
80 ROCKS FOKMED THROUGH IGNEOUS AGENCIES
tonborough, New Hampshire, the rock of which was first de-
scribed as an ordinary syenite, and that of Hastings County,
Ontario.
(2) THE PHONOLITES
Phonolite, from the Greek word (fxovtj, sound, and 7u'0o9, stone,
in allusion to the clear ringing or clinking sound which slabs
of the stone emit when struck with a vhammer ; formerly called
clinkstone for the same reason.
Mineral Composition. — The phonolites consist essentially of
sanidin and nepheline or leucite, together with one or more
minerals of the augite-hornblende group, and generally hauyne
or nosean. The common accessories are plagioclase, apatite,
sphene, mica, and magnetite ; more rarely occur tridymite,
melanite, zircon, and olivine. The rock undergoes ready alter-
ation, and calcite, chlorite, limonite, and various minerals of
the zeolite group occur as secondary products.
Chemical Composition. — The average of six analyses given
by Zirkel1 is as follows: Silica, 58.02%; alumina, 20.03%;
iron oxides, 6.18 %; manganese oxide, 0.58%; lime, 1.89%;
magnesia, 0.80 %; potash, 6.18 %; soda, 6.35%; water, 1.88%;
specific gravity, 2.58.
Structure. — The phonolites present but little variety in
structure, being usually porphyritic, seldom evenly granular.
The porphyritic structure is due to the development of large
crystals of sanidin, nepheline, leucite, or hauyne, and more rarely
hornblende, augite, or sphene, in the fine-grained and compact
ground-mass, which is usually microcrystalline, rarely glassy or
amorphous.
Colors. — The prevailing colors are dark gray or greenish.
Classification and Nomenclature. — Three varieties are recog-
nized by Professor Rosenbusch, the distinction being founded
upon the variation in proportional amounts of the three miner-
als, sanidin, nepheline, or leucite. We thus have (1) nepheline
phonolite, consisting essentially of nepheline and sanidin, and
which may therefore be regarded as the volcanic equivalent
of the nepheline syenite ; (2) leucite phonolite, consisting
essentially of leucite and sanidin ; and (3) leucitophyr, which
consists essentially of both nepheline and leucite in connection
with sanidin, and nearly always melanite.
1 Lehrbuch der Petrographie, II, p. 193.
THE DIORITE-ANDESITE GROUP 81
So far as now known, these rocks are of comparatively rare
occurrence in the United States, having been described as
occurring only in the Black Hills of South Dakota and the
Cripple Creek district of Colorado.
4. THE DIORITE-ANDESITE GROUP
We come now to groups of rocks which show a still greater
falling off in their total amount of silica, as indicated by analy-
ses, and a like diminution in the amount of potash. The cause
of this falling off is due to the absence as an essential constituent
of quartz and potash feldspars, the latter being replaced by soda-
lime varieties, and which in their turn cause a corresponding in-
crease in the elements sodium and calcium. The group includes
the plutonic type diorite, and the effusive types hornblende por-
phyrite, and andeslte. These may be described as below: —
(1) THE DIORITES (GREENSTONES IN PART)
Diorite, from the Greek word Stopi&tv, to distinguish. Term
first used by the mineralogist Haiiy.
Mineral Composition. — The essential constituents of diorite
are plagioclase feldspar, either labradorite or oligoclase, and
hornblende or black mica. The common accessories are mag-
netite, titanic iron, orthoclase, apatite, epidote, quartz, augite,
black mica, and pyrite, more rarely garnets. Calcite and
chlorite occur as alteration products.
Structure. — Dioritesare holocrystalline granular rocks, and as
a rule, massive, though schistose forms occur. The individual
crystals composing the rock are sometimes grouped in globular
;p_ro-ivgates, thus forming the so-called orbicular diorite, kui/el
diorite, or napoleonite from Corsica. (Fig. 1, PL 8.) The
texture is, as a rule, fine, compact, and homogeneous, and its
true nature discernible only with the aid of a microscope ; more
rarely porphyritic forms occur as in the camptonites.
Colors. — The colors vary from green and dark gray to
almost black.
Chemical Composition. — The following table shows the wide
range in chemical composition found in rocks commonly grouped
under this head.
Classification. — Accordingly as they vary in mineral compo-
sition the diorites are classified as (1) diorite, in which horn-
82
ROCKS FORMED THROUGH IGNEOUS AGENCIES
CONSTITUENTS
I
II
ill
IV
V
VI
Silica (Si02)
67.54 %
61.75%
56.71%
50.47 %
43.50%
39.32%
Titanic oxide (TiO )
1.70
Alumina (Al20s) . . .
17.02
18.88
18.36
18.73
17.02
14.48
Ferric iron (Fe20a) . .
2.97
0.52
4.19
13.68
2.01
Ferrous iron (FeO) . .
0.04
3.52
6.45
4.92
8.73
Manganese oxide (MnO)
0.71
Lime (CaO)
2.94
3.54
6.11
8.82
8.15
8.30
Magnesia (MgO) . .
1.51
1.90
3.92
3.48
6.84
11.11
Potash (K20) ....
2.28
1.24
2.38
3.56
2.84
0.87
Soda (Na20)
4.62
3.67
3.52
4.62
2.84
3.76
Phosphoric acid (P205) .
)
0.61
Carbonic acid (CO2) . .
V0.55
4.46
0.58
5.25
Water (H20) ....
J
4.35
2.57
I. Quartz-mica diorite: Electric Peak, Yellowstone Park (J. P. Iddings).
II. Diorite: Penmaen-Mawr, Wales (J.A.Phillips). III. Diorite: Comstock
Lode, Nevada (40th Parallel Survey). IV. Augite diorite: Custer County,
Colorado (Whitman Cross). V. Porphyritic diorite (camptonite) : Fairhaven,
Vermont (J. F. Kemp). VI. Porphyritic diorite: Lewiston, Maine (G. P.
Merrill) .
blende alone is the predominating accessory; (2) mica diorite,
in which black mica replaces the hornblende, and (3) augite
diorite, in which the hornblende is partially replaced by augite.
The presence of quartz gives rise to the varieties, quartz, quartz
augite, and quartz-mica diorites. The name tonalite has been
given by Vom Rath to a quartz diorite containing the feldspar
andesine and very rich in black mica. Kersantite is a dioritic
rock occurring, so far as known, only in dikes, and consisting
essentially of black mica and plagioclase, with accessory apatite
and augite, or more rarely hornblende, quartz, and orthoclase.
It differs from the true mica diorite in being, as a rule, of a
porphyritic rather than granitic structure. Professor Rosen-
busch, in his latest work, has placed the kersantites, together
with the porphyritic diorites (camptonites), under the head
of dioritic lamprophyrs in the class of dike rocks or "gange-
steine." The name, it should be stated, is from Kersanton, a
small hamlet in the Brest Roads, department of Finistere,
France.
The diorites were formerly, before their exact mineralogical
nature was well understood, included with the diabases and
melaphyrs under the general name greenstone (Ger. Grrunstein').
PLATE 8
Fia. 1. Orbicular diorite.
FIG. 2. Granite spheroid.
THE PORPHYRITES AND ANDESITES 83
They are rocks of wide geographic distribution, but -apparently
less abundant in the United States than are the diabases. The
lamprophyr varieties are still less abundant, so far as now
known.
(2) THE PORPHYRITES
Mineral and Chemical Composition. — The essential constitu-
ents of the porphyrites are the same as of the diorites, from
which they differ mainly in structure.
Structure. — The porphyrites, as a rule, show a felsitic or
glassy ground-mass, as do the quartz porphyries, in which are
embedded quite perfectly developed porphyritic plagioclases,
with or without hornblende or black mica. At times, as in
the well-known " porfido rosso antico," or antique porphyries
of Egypt, the ground-mass is microcrystalline, forming thus
connecting links between the true diorites and diorite porphy-
rites. Indeed, the rocks of the group may be said to bear the
same relation to the diorites in the plagioclase series as do
the quartz porphyries to the granites in the orthoclase series,
or better yet, they may be compared with the hornblende an-
desites, of which they are apparently the Palaeozoic equivalents.
Colors. — The prevailing colors are dark brown, gray, or
greenish.
Classification. — According to the character of prevailing
accessory mineral, we have hornblende porphyrite, or diorite
porphyrite^ as it is sometimes called, and mica porpliyrite.
When, as is frequently the case, neither of the above minerals
are developed in recognizable quantities, the rock is designated
as simply porphyrite. The porphyrites are wide-spread rocks,
very characteristic of the later Palaeozoic formations, occurring
as contemporaneous lava flows, intrusive sheets, dikes, and
bosses.
(3) THE ANDESITES
The name Andesite was first used by L. V. Buch in 1835, to
designate a type of volcanic rocks found in the Andes Moun-
tains, South America.
Mineral Composition. — The essential constituents are soda-
lime feldspar, together with black mica, hornblende, augite, or
a rhombic pyroxene, and in smaller, usually microscopic pro-
portions, magnetite, ilmenite, hematite, and apatite. Common
ROCKS FORMED THROUGH IGNEOUS AGENCIES
accessories are olivine, sphene, garnets, quartz, tridymite, anor-
thite, sanidin, and pyrite.
Chemical Composition. — The composition of the andesites
varies very considerably, the quartz-bearing members naturally
showing much the higher percentage of silica. The following
table shows the composition of a few typical forms : -
CONSTITUENTS
I
II
III
IV
V
VI
Silica (Si02) ....
66.32 %
69.51 %
61.12 %
56.07 %
56.19%
58.33%
Alumina (Al20s) . .
Ferric oxide (Fe203) . .
Ferrous oxide (FeO) . .
Magnesia (MgO) . . .
Lime (CaO)
14.33
5.53
0.25
2.45
4.64
15.75
3.34
2.09
1.71
11.61
11.64
0.61
4.33
19.06
5.39
0.92
2.12
7.70
16.21
4.92
4.43
4.60
7.00
18.17
6.03
2.40
6.19
Soda (Na20)
3.90
3.89
3.85
4.52
2.96
3.20
Potash (K20) ....
Water (H20) ....
1.61
1.13
3.34
3.52
4.35
1.24
0.99
2.37
1.03
3.02
0.76
100.16%
99.63 %
101.03%
98.01 %
99.62
98.10%
I. Dacite from Kis Sebes, Transylvania. II. Dacite from Lassens Peak,
California. III. Hornblende andesite from hill north of Gold Peak, Nevada.
IV. Hornblende andesite from Bogoslof Island, Alaska. V. Hypersthene ande-
site, Buffalo Peaks, Colorado. VI. Augite andesite from north of American
Flat, Washoe, Nevada.
Structure. — To the unaided eye the andesites present as a
rule a compact, often rough and porous ground-mass carrying
porphyritic feldspars and small scales of mica, hornblende, or
whatever may be the prevailing accessory ; pumiceous forms
are not uncommon. Under the microscope the ground-mass is
found to vary from clear glassy through microlitic forms to
almost holocrystalline. The minerals of the ground-mass are
feldspars in elongated microlites, specks of iron ore, apatite in
very perfect forms, and one or more of the accessory ferro-mag-
nesian minerals.
Colors. — The prevailing colors are some shade of gray, green-
ish or reddish.
Classification and Nomenclature. — Specific names are given
dependent upon the character of the prevailing accessory. We
thus have : —
Andesites with quartz = Quartz andesites or dacites.
Andesites in which hornblende prevails = Hornblende andesites.
THE GABBRO-BASALT GROUP 85
Andesites in which augite prevails = Augite andesites.
Andesites in which hypersthene prevails = Hypersthene andesites.
Andesites in which mica prevails = Mica andesiti-s.
The glassy varieties are often known as hyaline andesites.
The name propylite was given by Richthofen to a group of
andesitic rocks prevalent in Hungary, Transylvania, and the
western United States, but these rocks have since been shown
by Dr. Wadsworth l and others to be but altered andesites, and
the name has fallen largely into disuse.
5. THE GABBRO-BASALT GROUP
We have here a large and variable group of rocks which on
structural and miueralogical grounds might well be subdivided.
Thus the gabbros, norites, and hypersthene andesites might
well be considered as a group by themselves, while the diabases,
augite porphyrites, inelaphyrs, and basalts could form a second.
( hving, however, to the similarity of the magmas from which
they have been derived, it is believed the wants of the student
will be best subserved by grouping them all together as above.
They may be described in detail as below : —
(1) THE GABBROS
Gabbro, an old Italian name originally applied to serpen-
tinous rocks containing diallage.
Mineral Composition. — The gabbros consist essentially of a
basic soda-lime feldspar, either labradorite, bytownite, or an-
orthite, and diallage or a closely related monoclinic pyroxene,
a rhombic pyroxene (enstatite or hypersthene), and more rarely
olivine. Apatite and the iron ores are almost universally pres-
ent, and often picotite, chromite, pyrrhotite, more rarely com-
mon pyrites, and a green spinel. Secondary brown mica and
hornblende are common. Quartz occurs but rarely.
Chemical Composition. — As with other groups, the percent-
age amounts of the various constituents obtained by analyses
is dependent upon the relative proportion of the constituent
minerals. In the tables given below, analyses like I and III,
showing very little iron and magnesia, but rich in lime and
soda and alumina, are of rocks in which the pyroxenic con-
1 Proc. Boston Society of Natural History, Vol. XXI, 1881, p. 260.
86
ROCKS FORMED THROUGH IGNEOUS AGENCIES
stituents are almost wholly lacking, and which consist essen-
tially of lime feldspars only.
CONSTITUENTS
I
II
III
IV
V
VI
Silica (SiOg)
59.55 %
54.72 %
53.43%
49.15%
46.85 %
45.66 %
Alumina (A^Og) . . .
Ferric iron (Fe203) . .
Ferrous iron (FeO) . .
Lime (CaO)
25.62
0.76
7.73
17.79
2.08
6.03
6.84
28.01
0.75
11.24
21.90
6.60
4.54
8.22
19.72
3.22
7.99
13.10
16.44
0.66
13.90
7.23
Magnesia (MgO) . . .
Potash (K20) ....
Soda (Na20) ....
Trace
0.96
5.09
5.85
3.01
3.02
0.63
0.96
4.85
3.03
1.61
3 83
7.75
0.09
1.56
11.57
0.41
2 13
Ignition and loss . . .
0.45
1.92
0.56
0.07
I. Anorthosite : Chateau Richer, Canada (T. S. Hunt). II. Gabbro : near
Cornell Dam, Croton River, New York (J. F. Kemp) . III. Anorthosite :
Labrador (A. Wickman). IV. Gabbro : near Duluth, Minnesota (Streng).
V. Gabbro: near Baltimore, Maryland (G. H. Williams). VI. Gabbro: North-
west Minnesota (W. S. Bay ley).
Structure. — The gabbro structure is quite variable. Like
the other plutonic rocks mentioned, they are crystalline granu-
lar, the essential constituents rarely showing perfect crystal
outlines. As a rule the pyroxenic constituent occurs in broad
and very irregularly outlined plates, filling the interstices of
the feldspars, which are themselves in short and stout forms
quite at variance with the elongated, lath-shaped forms seen in
diabases. This rule is, however, in some cases reversed, and
the feldspars occur in broad, irregular forms surrounding the
more perfectly formed pyroxenes. Transitions into diabase
structure are not uncommon. In rare instances the pyroxenic
constituents occur in concretionary aggregates as in the peculiar
kugel gabbro or potato rock from Smaalanene, in Norway.
Through a molecular change of the pyroxenic constituent, the
gabbros pass into diorites, as do also the diabases.
Colors. — The prevailing colors are gray to nearly black ;
sometimes greenish through decomposition.
Classification. — The rocks of this group are divided into
(1) the true gabbros — that is, plagioclase-diallage rocks — and
(2) norites, or plagioclase-bronzite and hypersthene rocks.
Both varieties are further subdivided according to the presence
or absence of olivine. We then have : —
THE DIABASES
87
True gabbro = Plagioclase + diallage.
Olivine gabbro = Plagioclase + diallage and olivine.
Norite = Plagioclase + hypersthene or bronzite.
Olivine norite = Plagioclase + hypersthene and olivine.
Nearly all gabbros contain more or less rhombic pyroxene, and
hence pass by gradual transitions into the norites. Through
a diminution in the proportion of feldspar they pass into the
peroditites, and a like diminution in the proportion of pyroxene
gives rise to the so-called forellenstein. Hyperite is the name
given, by Tornebohm, to a rock intermediate between normal
gabbro and norite. Anorthosite, as above indicated, is the name
given to the granular varieties poor or quite lacking in pyrox-
enes.
(2) THE DIABASES
Diabase, from the Greek word &a/3a<rt9, a passing over ; so
called by Brongniart because the rock passes by insensible
gradations into diorite.
Chemical Composition. — The table below shows the average
range in composition of (I and II) the plutonic diabase and
(III, IV, V, and VI) the effusive forms melaphyr and basalt.
CONSTITUENTS
I
II
Ill
IV
V
VI
Silica (SiO2) . .' . .
53.13%
45.46 %
66.52 %
51.02 %
67.25 %
46.90 %
Alumina (A12O3) . .
13.74
19.94
13.53
18.86
16.45
10.17
Ferric iron (Fe203) . .
Ferrous iron (FeO) . .
1.08
9.10
j 15.36
12.56
\6.57
)4.68
1.67
1.77
1.22
5.17
Lime (CaO) ....
9.47
8.32
5.31
7.M
7.65
6.20
Magnesia (MgO) . . .
8.58
2.95
2.79
6.57
6.74
20.98
Potash (K2O) ....
1.03
3.21
3.59
2.10
1.57
2.04
Soda (NaaO) ....
2.30
2.12
3.71
2.54
3.00
1.16
Iirnition .
0 90
0 30
081
2 86
0 45
5 42
Specific gravity . . .
2.96
•J. !»»."•
2.86*
I. Diabase: Jersey City, New Jersey (G. W. Hawes). II. Diabase: Palmer
Hill, Au Sable Forks, New York (J. F. Kemp). III. Melaphyr: Hockenberg,
Silesia. IV. Melaphyr, Falgendorf, Bohemia (quoted from Zirkel's Lehrbuch
der Petrographie). V. Quartz basalt: Snag Lake, California (J. S. Diller).
VI. Basalt (absarokite) : near Bozeman, Montana (G. P. Merrill).
Mineral Composition. — The essential constituents of diabase
are plagioclase feldspar and augite, with nearly always mag-
88
ROCKS FORMED THROUGH IGNEOUS AGENCIES
netite and apatite in microscopic proportions. The common
accessories are hornblende, black mica, olivine, enstatite, hyper-
sthene, orthoclase, quartz, and titanic iron. Calcite, chlorite,
hornblende, and serpentine are common as products of altera-
tion. Through a molecular change known as uralitization the
augite not infrequently becomes converted into hornblende,
as already described (p. 40), and the rock thus passes over
into diorite. The plagioclase may be labradorite, oligoclase,
or anorthite.
Structure. — In structure the diabases are holocrystalline.
Rarely do the constituents possess perfect crystal outlines, but
are more or less imper-
fect and distorted, owing
to mutual interference in
process of formation, the
granular hypidiomorphic
structure of Professor
Rosenbusch. The augite
in the typical forms oc-
FIG. 4. — Microstructure of diabase.
curs in broad and sharply
angular plates enclosing
the elongated or lath-
shaped crystal of plagio-
clase, giving rise to a
structure known as ophi-
tic. (See Fig. 4.) The
rocks are, as a rule, com-
pact, fine, and homoge-
neous, though sometimes porphyritic and rarely amygdaloidal.
Colors. — The colors are sombre, varying from greenish through
dark gray to nearly black, the green color being due to a dissemi-
nated chloritic or serpentinous product resulting from the alter-
ation of the augite or olivine.
Classification. — Two principal varieties are recognized, the
distinction being based upon the presence or absence of the
mineral olivine. We thus have: (1) diabase proper and (2) oli-
vine diabase.
Many varietal names have been given from time to time by
different authors. Gumbel gave the name of leucophyr to a
very chloritic, diabase-like rock consisting of pale green augite
and a saussu rite-like plagioclase. The same authority gave
THE DIABASES 89
the name epidiorite to an altered diabase rock occurring in
small dikes between the Cambrian and Silurian formations
in the Fichtelgebirge, and in which the augite had become
changed to hornblende. He also designated by the term pro-
terobase a Silurian diabase consisting of a green or brown,
somewhat fibrous, hornblende, reddish augite, two varieties of
plagioclase, chlorite, ilmenite, a little magnetite, and usually a
magnesian mica. The name ophite has been used by Pallarson
to designate an augite plagioclase eruptive rock, rich in horn-
blende and epidote, and occurring in the Pyrenees. The
researches of M. Levy Kuhn1 and others have, however, shown
that both hornblende and epidote are secondary, resulting from
the augitic alteration, and that the rock must be regarded as
belonging to the diabase.
The Swedish geologist, Torjiebohm, gave the name sahlite
diabase to a class of diabasic rocks containing the pyroxene
sahlite, and which occurred in dikes cutting the granite, gneiss,
and Cambrian sandstones in the province of Smaaland, and in
other localities. The name teschenite was for many years ap-
plied to a class of rocks occurring in Moravia, and which, until
the recent researches of Rohrbach, were supposed to contain
nepheline, but which are now regarded as merely varietal forms
of diabase. Variolite is a compact, often spherulitic, variety
occurring in some instances as marginal facies of ordinary
diabase. The name eukrite or eucrite was first used by G. Rose
to designate a rock consisting of white anorthite and grayish
green augite occurring in the form of a dike cutting the Car-
boniferous limestone of Carlingford district, Ireland. These
rocks were included by Professor Zirkel under the head of
"anorthitgesteine." The name is now little used, and rocks
of this type are here included with the diabases.
The diabases are among the most abundant and wide-spread
of our so-called trap rocks, occurring in the form of dikes,
intrusive sheets, and bosses. They are especially characteristic
of the Triassic formations of the eastern United States. It
should be noted, however, that many of these Triassic traps
have been shown to be true lava flows, and that on both litho-
logical and geological grounds such might with propriety be
classed with the basalts.
1 Untersuchungen iiber pyrenaeische Ophite, Inaugural Dissertation Univer-
sitat, Leipzig, 1881.
90 KOCKS FORMED THKOUGH IGNEOUS AGENCIES
(3) THE MELAPHYRS AND AUGITE PORPHYRITES
The term melaphyr is used to designate a volcanic rock
occurring in the form of intrusive sheets and lava flows, and
consisting essentially of a plagioclase feldspar, augite, and
olivine, with free iron oxides and an amorphous of porphyry
base. The augite porphyrites differ in containing no olivine.
The rocks of this group are therefore the porphyritic, effusive,
forms of the olivine-bearing and olivme-free diabases and
gabbros.
Structure. — As above noted, they are porphyritic rocks with,
in their typical forms, an amorphous base, are often amygda-
loidal, and with a marked flow structure.
Colors. — In colors they vary through gray or brown to nearly
black ; often greenish through chloritic and epidotic decompo-
sition.
Classification and Nomenclature. — According as olivine is
present or absent, they are divided primarily into melaphyrs
and augite porphyrites, the first bearing the same relation to
the olivine diabases as do the quartz porphyries to the granites,
or the hornblende porphyrites to the diorites, and the second
a similar relation to the olivine-free diabases. The augite
porphyrites are further divided upon structural grounds into
(1) diabase porphyrite, which includes the varieties with holo-
crystalline diabase granular ground-mass of augite, iron ores,
and feldspars, in which are embedded porphyritic lime-soda
feldspars, — mainly labradorite, — idiomorphic augites, and at
times accessory hornblende and black mica ; (2) spilite, which
includes the non-porphyritic compact, sometimes amygdaloidal
and decomposed forms such as are known to German petrog-
raphers as dichte diabase, diabase mandelstein (amygdaloid),
kalk-diabase, variolite, etc. ; (3) the true augite porphyrite, in-
cluding the normal porphyritic forms with the amorphous base,
and (4) the glassy variety augite vitrophyrite.
(4) THE BASALTS
Basalt, a very old term used by Pliny and Strabo to designate
certain blacks rocks from Egypt, and which were employed in
the arts in early times.1
1 Teall, British Petrography, p. 136.
THE BASALTS 91
Mineral Composition. — The essential minerals are augite and
plagioclase feldspar with oli vine in the normal forms ; accessory
iron ores (magnetite and ilmenite), together with apatite, are
always present, and more rarely a rhombic pyroxene, horn-
blende, black mica, quartz, perowskite, hauyne and uepheline,
and minerals of the spinel group. Metallic iron has been
found as a constituent of certain basaltic rocks on Disco Island,
Greenland.
Chemical Composition. — The composition is quite variable,
as shown by analyses in columns V and VI on p. 87. The fol-
lowing shows the common extremes of variation : Silica, 45 %
to 55 %; alumina, 10 % to 18 %; lime, 1 % to 14 %; magm-sia,
3 % to 10 % ; oxide of iron and manganese, 9 % to 16 % ;
potash, 0.058 % ; soda, 2 % to 5 % ; loss by ignition, 1% to 5 % ;
specific gravity, 2.85 to 3.10.
Structure. — Basalts vary all the way from clear glassy to
holocrystalline forms. The common type is a compact and,
to the unaided eye, homogeneous rock, with a splintery or
conchoidal fracture, and showing only porphyritic olivines in
such size as to be recognizable. Under the microscope they
show a ground-mass of small feldspar and augite microlites,
with perhaps a sprinkling of porphyritic forms of feldspar,
augite, and olivine, and a varying amount of interstitial brown-
ish glass; the glass may be wholly or in part replaced by devit-
rification products, as minute hairs, needles, and granules. A
marked flow structure is often developed, the feldspars of the
ground-mass having flowed around the olivine belonging to the
earlier period of consolidation, giving rise to an appearance
that may be compared to logs in a mill stream, the olivines
representing small islands. Pumiceous and amygdaloidal
forms are common.
Colors. — The prevailing colors are dark, some shade of gray
to perfectly black. Red and brown colors are also common.
Mineralogically it will be observed the basalts resemble the
olivine diabases and melaphyrs, of which they may be regarded
as the younger equivalents. Indeed, in very many cases it has
been found impossible to ascertain from the study of the speci-
men alone to which of the three groups it should be referred, so
closely at times do they resemble one another.
Classification and Nomenclature. — In classifying, the varia-
tions in crystalline structure are the controlling factors. As,
92 ROCKS FORMED THROUGH IGNEOUS AGENCIES
however, these characteristics are such as may vary almost
indefinitely in different portions of the same flow, the rule has
not been rigidly adhered to here. We thus have : —
(1) Dolerite, including the coarse-grained almost holocrys-
talline variety ; (2) anamesite, including the very compact
fine-grained variety, the various constituents of which are not
distinguishable by the unaided eye ; (8) basalt proper, which
includes the compact homogeneous, often porphyritic, variety,
carrying a larger proportion of interstitial glass or devitrifica-
tion products than either of the above varieties, and (4) tachy-
lite, hyalomelan, or hyalobasalt, which includes the vitreous or
glassy varieties, the mass having cooled too rapidly to allow it to
assume a crystalline structure. These varieties, therefore, bear
the same relation to normal basalt as do the obsidians to the
liparites. Other varieties, though less common, are recogniz-
able and characterized by the presence or absence of some
predominating accessory mineral. We have thus quartz, horn-
blende, and hypersthene basalt, etc. An olivine-free variety is
also recognized.
The basalts are among the most abundant and wide-spread of
the younger eruptive rocks. In the United States they are
found mainly in the regions west of the Mississippi River.
They are eminently volcanic rocks, and occur in the form of lava
streams and sheets, often of great extent, and sometimes show-
ing a characteristic columnar structure. According to Rich-
thofen, the basalts are the latest products of volcanic activity.
A quartz-bearing basalt has been described by Mr. J. S. Diller
as occurring at Snag Lake, near Lassens Peak, California, and
which is regarded by him as a product of the latest volcanic
eruption within the limits of the state. This lava field covers
an area of only some three square miles, and trunks of trees
killed at the time of the eruption are still standing.1
Under the name of melilite basalt is included a group of rocks
in which the mineral melilite is the characterizing constituent,
with accessory augite, olivine, nepheline, biotite, magnetite,
perowskite, and spinel. The normal structure is holocrystal-
line porphyritic, in which the olivine, augite, mica, or occasion-
ally the melilite, appear as porphyritic constituents. These are
rocks of very limited distribution, and at present known in
North America only near Montreal, Canada. Professor Rosen-
1 Bull. No. 79, U. S. Geol. Survey, 1891.
THE THERALITE-BASANITE GROUP 93
busch, in his latest work, separates this entirely from the basalts,
and considers it in a group by itself under the nam,e of Melilite
Rocks.
6. THE THERALITE-BASANITE GROUP
This is a small, and so far as now known, comparatively in-
significant group of rocks, representatives of which are confined
to limited and widely separated areas. They are described as
below : —
(1) THE THERALITES
This name, derived from the Greek word Qrjpav, to seek
eagerly, is given by Professor Rosenbusch to a class of intru-
sive rocks consisting essentially of plagioclase feldspar and
nepheline, and which are apparently the plutonic equivalents of
the tephrites and basanites.
The group is founded by Professor Rosenbusch upon certain
rocks occurring in dikes and laccolites in the Cretaceous sand-
stones of the Crazy Mountains of Montana, and described by
Professor J. E. Wolff,1 of Harvard University.
Mineral Composition. — The essential constituents as above
noted are nepheline and plagioclase with accessory augite,
olivine, sodalite, biotite, magnetite, apatite and secondary horn-
blende, and zeolitic minerals.
Chemical Composition. — The chemical composition of a sam-
ple from near Martinsdale, as given by Professor Wolff, is as
follows: Silica, 43.175%; alumina, 15.236%; ferrous oxide,
7.607 % ; ferric oxide, 2.668 % ; lime, 10.633 % ; magnesia,
5.810%; potash, 4.070%; soda, 5.68%; water, 3.571 %;
sulphuric anhydride, 0.94 %.
Structure. — The rocks are holocrystalline granular through-
out.
Colors. — These are dark gray to nearly black.
The theralites, so far as known, have an extremely limited
distribution, and in the United ' States have thus far been re-
ported only from Gordon's Butte and Upper Shields River basin
in the Crazy Mountains of Montana.
1 Notes on the Petrography of the Crazy Mountains and other localities in
Montana, by J. E. Wolff. Neues Jahrb. fur. Min., etc., 1885, I, p. 69.
94
KOCKS FORMED THROUGH IGNEOUS AGENCIES
(2) THE TEPHRITES AND BASANITES
«
Mineral Composition. — The essential constituent of the rocks
of this group as given by Rosenbusch are a lime-soda feldspar
arid nepheline or leucite, either alone or accompanied by augite.
Olivine is essential in basanite. Apatite, the iron ores, and
rarely zircon occur in both varieties. Common accessories are
sanidin, hornblende, biotite, hauyne, melanite, perowskite, and
a mineral of the spinel group.
Chemical Composition. — The following is the composition of
(I) a nepheline tephrite from Antao, Pico da Cruz, Azores, and
(II) a nepheline basanite from San Antonio, Cape Verde
Islands, as given by Roth.1
CONSTITUENTS
I
II
Silica (Si02)
47.44 %
43.09 %
Alumina (A^Og)
23.71
17.45
Iron sesquioxide (Fe203)
6.83
18.99
Iron protoxide (FeO)
3.53
Magnesia (MgO)
1.95
4.63
Lime (CaO)
6.47
9.76
Soda (Na20)
6.40
5.02
Potash (K20)
3.34
1.81
Water (H20)
1.73
0.33
Structure. — The rocks of this group are as a rule porphyritic
with a holocrystalline ground-mass, though sometimes there is
present a small amount of amorphous interstitial matter or
base; at times amygdaloidal.
Colors. — The colors are dark, some shade of gray or brownish.
Classification and Nomenclature. — According to their vary-
ing mineral composition Rosenbusch divides them into : —
Leucite tephrite = Leucite, augite, plagioclase rocks.
Leucite basanite = Leucite, augite, plagioclase and olivine rocks.
Nepheline tephrite = Nepheline, plagioclase rocks.
Nepheline basanite = Nepheline, plagioclase and olivine rocks.
The group, it will be observed, stands intermediate between
the true basalts and the nephelinites to be noted later. Their
distribution, so far as 'now known, is quite limited.
1 Abhandlungen der Konig. Akad. der Wissenschaften zu Berlin, 1884, p. 64.
THE PERIDOTITE-LLMBURGITE GROUP
95
7. THE PERIDOTITE-LIMBURGITE GROUP
This and the following groups include eruptive rocks in
which neither quartz nor feldspars of any kind longer appear
as essential constituents, and which are therefore very low in
silica, causing them to be classed as ultrabasic. Although in
most cases comparatively insignificant as rock masses, they are
peculiarly interesting as mineral aggregates, and even more on
account of the character of their alteration products. The
peridotites are further of interest in presenting the nearest
homologues to meteorites of any of our terrestrial rocks. The
group includes the plutonic peridotites (serpentine in part), and
effusive picrite porphyrites and limburgites. In detail these are
as below : —
(1) THE PERIDOTITES
Peridotite, so called because the mineral peridot (olivine) is
the chief constituent.
Mineral Composition. — The essential constituent is olivine
associated nearly always with chromite or picotite and the iron
ores. The common accessories are one or more of the ferro-
magnesian silicate minerals augite, hornblende, enstatite, and
black mica ; feldspar is also present in certain varieties and
more rarely apatite, garnet, sillimanite, perowskite, and pyrite.
CONSTITUENTS
I
II
III
IV
V
VI
Silica (SiO2) ....
41.58%
43.84 %
39.103%
42.94 %
38.01 %
45.68%
Alumina (Al2Os.) . . .
0.14
1.14
4.94
10.87
5.32
6.28
Magnesia (MgO) . . .
49.28
44.33
29.176
16.32
23.29
34.76
Lime (CaO) ....
0.11
1.71
3.951
9.07
4.11
2.15
Iron sesquioxide(Fe2O8)
....
8.76
4.315
3.47
6.70
9.12
Iron protoxide (FeO) .
7.49
....
11.441
10.14
4.92
Chrome oxide (Cr2Os) .
....
0.42
0.436
....
....
0.26
Manganese (MnO) . .
....
0.12
0.276
Trace
Potash (K2O) ....
....
....
Trace
0.15
0.22
Soda (NajjO) ....
0.90
4.15
Nickel oxide (NiO) . .
0.34
Water and ignition . .
1.72
1.06
5.669
6.09
10.60
1.21
Specific gravity . . .
....
3.287
2.93
2.88
2.83
3.269
I. Dunite : Macon County, North Carolina. II. Saxouite: St. Paul's Rocks,
Atlantic Ocean. III. Picrite : Nassau, Germany. IV. Hornblende picrite :
Ty Cross, Anglesia. V. Picrite : Little Deer Isle, Maine. VI. Lherzolite :
Monte Rossi, Piedmont.
96
ROCKS FORMED THROUGH IGNEOUS AGENCIES
Chemical Composition. — The chemical composition varies
somewhat with the character and abundance of the prevailing
accessory. The preceding table shows the composition of
several typical varieties.
Structure. — The structure as displayed in the different
varieties is somewhat variable. In the dunite it is as a rule
even crystalline granular, none of the olivines showing perfect
crystal outlines. In the picrites the augite or hornblende often
occurs in the form of
broad plates occupying
the interstices of the oli-
vines and wholly or par-
tially enclosing them, as
in the hornblende pic-
rite of Stony Point, New
York. The saxonites and
Iherzolites often show
a marked porphyritic
structure produced by
the development of large
pyroxene crystals in the
fine and evenly granular
ground-mass of olivines.
(See Fig. 5, as drawn by
Dr. G. H. Williams.)
The rocks belong to the
class designated as hypidiomorphic granular by Professor
Rosenbusch; that is, rocks composed only in part of minerals
showing crystal faces peculiar to their species.
Colors. — The prevailing colors are green, greenish gray, yel-
lowish green, dark green to black.
Nomenclature and Classification. — Mineralogically and geo-
logically it will be observed the peridotites bear a close resem-
blance to the olivine diabases and gabbros, from which they
differ only in the absence of feldspars. Indeed, Professor Judd
has shown that the gabbros and diabase both, in places, pass by
insensible gradations into peridotites through a gradual dimi-
nution in the amount of their feldspathic constituents. Dr.
Wadsworth would extend the term peridotite to include rocks
of the same composition, but of meteoric as well as terrestrial
origin, the condition of the included iron, whether metallic or
Fia. 5. — Microstructure of porphyritic Iherzo-
lite, partly altered into serpentine.
THE PERIDOTITES 97
as an oxide, being considered by him as non-essential, since
native iron is also found occasionally in terrestrial rocks, as
the Greenland basalts and some diabases.
In classifying the peridotites> the varietal distinctions are
based upon the prevailing accessory mineral. We thus have : —
Dunite, consisting essentially of olivine only.
Saxonite, consisting essentially of olivine and enstatite.
Picrite, consisting essentially of olivine and augite.
Hornblende picrite, consisting essentially of olivine and hornblende.
Wehrlite (or eulysite), consisting essentially of olivine and diallage.
Lherzolite, consisting essentially of olivine, enstatite, and augite.
The name Dunite was first used by Hochstetter and applied
to the olivine rock of Mount Dun, New Zealand. Saxonite
was given by Wadsworth, rocks of this type being prevalent in
Saxony. The same rock has since been named Harzburgite by
Rosenbusch. The name Lherzolite is from Lake Lherz in the
Pyrenees.
The peridotites are, as a rule, highly altered rocks, the older
forms showing a more or less complete transformation of their
original constituents into a variety of secondary minerals, the
olivine going over into serpentine or talc and the augite or
hornblende into chlorite. The most common result of this
alteration is the rock serpentine, the transformation taking
place through the hydratiou of the olivine and the liberation of
free iron oxides and chalcedony. (See Fig. 5.) Recent inves-
tigations have shown that a large share of the serpentinous
rocks were thus originated. The chemistry of the process has
been already discussed under the head of olivine, p. 24.
Since in this process of hydration the combined iron becomes
converted into the sesquioxide form, and the calcium of the
lime-magnesian silicates separates out in large part as free cal-
cite, or as mixed carbonates of lime and magnesia, so these ser-
pentinous rocks are rarely uniform in color or composition.
The prevailing color is some shade of green, though not infre-
quently brown, yellow, red, or nearly black. Through the
presence of still unaltered grains of pyroxene, many varieties
are porphyritic. The rock is almost universally badly jointed,
an evident necessary accompaniment to the alteration, and into
these joints have filtered the lime or magnesia carbonate solu-
tions, where, depositing their load, they have formed the numer-
98 ROCKS FORMED THROUGH IGNEOUS AGENCIES
ous white, yellow, and greenish veins with which the stone is
traversed. Many varieties indeed, like the rosso de Levante,
verde di Pegli, and verde di Grenora of Italy, are but breccias of
serpeiitinous fragments cemented by calcareous and ferruginous
cements.1
It is, perhaps, as yet too early to state definitely that all peri-
dotites are eruptive. In many instances their eruptive nature
is beyond dispute. Others are found in connection with the
crystalline schists, so situated as to suggest that they may them-
selves be metamorphic.
(2) THE PICRITE PORPHYRITES
Under this head is placed a small group of rocks so far as
now known very limited in their distribution, and which are
regarded as the effusive forms of the plutonic picrites, as bear-
ing the same relation to these rocks as do the melaphyrs to the
olivine diabases. The essential constituents are therefore oli-
vine and augite with accessory apatite, iron ores, and other
minerals mentioned as occurring in the true picrites. Struct-
urally they differ from these rocks in presenting an amorphous
base rather than being crystalline throughout. Rocks of this
type are supposed to have had an important bearing on the
origin of the diamond, the diamond-bearing rocks of South
Africa being picrite porphyrite (kimbeiiite) cutting highly
carbonaceous shales. An examination of the Kentucky peri-
dotite locality, where the same rock occurs under quite similar
conditions, failed to show that similar results have been there
produced, a fact which is supposed to be due in part to the
small amount of carbonaceous matter in the surrounding shales.
The group is very limited, and is represented in the United
States only in Elliott County, Kentucky ; Pike County, Arkan-
sas ; Syracuse, Onondaga County, New York.
(3) THE LIMBURGITES
This is a small group of lavas described by Rosenbusch in
1872 as occurring at Limburg, or the Kaiserstuhl in the Rhine.
The essential constituents are augite and olivine with the usual
iron ores. Structurally the rock is so far as known never holo-
crystalline, but glassy and porphyritic. The composition of the
1 See the Stones for Building and Decoration, Wiley & Sons, New York.
THE PYROXEXITE-AUGITITE GROUP 99
Prussian limburgite is given as below. So far as known, the
group has no representatives in the United States.
CONSTITUENTS
PER CENT
Silica (SiO2)
42.24
Alumina (AlaOs)
18.66
Iron sesquioxide (Fe2Os)
7.45
Magnesia (M^O)
12.27
Lime (CaO) :
11.76
Soda (Na<>0)
4.02
Potash (K20)
1.08
Water (H2O)
3.71
<i'.u;»
8. THE PYROXENITE-AUGITITE GROUP
Here are included a small group of eruptive rocks differing
from the last mainly in the absence of olivine as an essential
constituent. They are represented, so far as now known, only
by the plutonic pyroxenites and effusive augitites.
(1) THE PYROXENITES
Pyroxenite, a term applied by Dr. Hunt to certain rocks con-
sisting essentially of minerals of the pyroxene group, and which
occurred both as intrusive and as beds or nests intercalated with
stratified rocks. The author here follows the nomenclature and
classification adopted by Dr. G. H. Williams.1
Mineral Composition. — The essential constituents are one or
more minerals of the pyroxene group, either orthorhombic or
monoclinic. Accessory minerals are not abundant and limited
mainly to the iron ores and minerals of the hornblende or mica
groups.
Chemical Composition. — The following analyses serve to show
the variations which are due mainly to the varying character
of the pyroxenic constituents : —
i American Geologist, Vol. VI, July, 1890, pp. 35-49.
100
ROCKS FORMED THROUGH IGNEOUS AGENCIES
CONSTITUENTS
I
II
III
Silica (Si02)
50.80 %
53.98 %
55.14 %
Alumina (Al20a)
3.40
1.32
0.66
Chrome oxide (Cr20s)
0.32
053
025
Ferric oxide (Fe20s)
1.39
1.41
3.48
Ferrous oxide (FeO)
8.11
3.90
4.73
Manganese (MnO)
0.17
021
003
Lime (CaO)
12.31
15.47
8.39
Magnesia (MgO)
22.77
22.59
26.66
Soda (Na20)
Trace
030
Potash (K20)
Trace
Water (H20)
0.52
0.83
0.38
Chlorine (Cl)
024
023
100.03 %
100.24 %
100.25 %
I. Hypersthene-diallage rock : Johnny Cake Road, Baltimore County, Mary-
land. II. Hypersthene-diallage rock : Hebbville post-office, Baltimore County,
Maryland. III. Bronzite-diopside rock from near Webster, North Carolina.
Structure. — The pyroxenites are holocrystalline granular
rocks, at times evenly granular and saccharoidal, or again
porpliyritic, as in the
websterite
Carolina.
from North
The micro-
scopic structure of this
rock is shown in Fig. 6
from the original draw-
ing by Dr. Williams.
Colors. — The colors
are, as a rule, greenish or
bronze.
Classification and No-
menclature. — The pyrox-
enites, it will be observed,
differ from the peridotites
only in the lack of olivine.
Following Dr. Williams's
Fio. 6. — Microstructure of websterite, Webster,
North Carolina.
nomenclature, we have
the varieties diallagite,
bronzitite, and hypertthenite, according as the mineral diallage,
bronzite, or hypersthene forms the essential constituent. Web-
sterite is the name given to the enstatite-diopside variety, such
AUGITITE
101
as occurs near Webster, North Carolina, and hornblendite to
the hornblende-augite variety. The pyroxenites rank, in geo-
logical importance, next to the peridotites. Through processes
of hydration and other chemical changes, these rocks pass into
amphibolic and steatitic masses to which the name soapstone
or potstone is not infrequently applied. These are dark gray
or greenish rocks, soft enough to be readily cut with a knife
and with a pronounced soapy or greasy feeling ; hence the
name soapstone. The name potstone was given on account of
their having been utilized for making rude pots, for which their
softness and fireproof properties render them well qualified.
Although it is commonly stated in the text-books that soap-
stone is a compact form of steatite or talc, few are even ap-
proximately pure forms of this mineral, but all contain varying
proportions of chlorite, mica, and tremolite, together with per-
haps unaltered residuals of pyroxene, granules of iron ore, iron
pyrites, quartz, and, in seams and veins, calcite and magnesian
carbonates. The variation in chemical composition is shown in
the following analyses, I being that of a compact, homogeneous-
appearing, quite massive variety from Alberene, in Albemarle
County, Virginia, and II one from Francestown, New Hamp-
shire.
CONSTITUENTS
I
II
Silica (Si02)
39.06%
42.43%
Alumina (AloOa)
12.84
6.08
Ferric and ferrous iron (Fe208) and (FeO) . . : .
Lime (CaO)
12.90
5.98
13.07
3.27
Manrnesia (MgO)
22.76
25.71
Potash (K2O)
0.19
0.32
Soda (Na2O)
0.11
0.16
6.56
8.45
100.40%
99.49%
(2) AUGITITE
The effusive form, augitite, differs from the pyroxenite proper
mainly on structural grounds. In common with many lavas it
has a glassy base, in which are embedded the crystals of augite
and iron ores. The composition of an augitite from the Cape
Verde Islands, as given by Roth, is as below : —
102 KOCKS FORMED THROUGH IGNEOUS AGENCIES
CONSTITUENTS
PER CENT
Silica (Si02)
41.83
18.60
16.11
4.98
Lime (CaO)
11.83
Soda (Na20)
4.70
Potash (K20)
2.47
Water (H20)
0.91
101.43
9. THE LEUCITE-NEPHELINE ROCKS
Under this head are grouped two small but interesting groups
of effusive rocks, having, so far as known, no exact equivalent
among the plutonics, and characterized by the presence of leu-
cite or nepheline as essential constituents and which here seem
to play the role of feldspars. In detail they are as below: —
(1) THE LEUCITE ROCKS
Mineral Composition. — The essential constituent is leucite
and a basic augite. A variety of accessories occur, including
biotite, hornblende, iron ores, apatite, olivine, plagioclase, nephe-
line, melilite, and more rarely garnets, hauyne, sphene, chromite,
and perowskite. Feldspar as an essential fails entirely.
Chemical Composition. — The average chemical composition as
given by Blaas1 is as follows : Silica, 48.9 % '•> alumina, 19.5 % ;
iron oxides, 9.2%; lime, 8.9%; magnesia, 1.9%; potash,
6.5% ; soda, 4.4%.
Structure. — The rocks of this group are, as a rule, fine
grained and only slightly vesicular, presenting to the unaided
eye little to distinguish them from the finer-grained varieties
of ordinary basalt.
Colors. — The prevailing colors are some shades of gray,
though sometimes yellowish or brownish.
Classification and Nomenclature. — The varietal distinctions
are based upon the presence or absence of the mineral olivine
1 Katechismus der Petrographie, p. 117.
THE NEPHELINE ROCKS
103
and upon structural grounds and various minor characteristics.
We have the olivine-f ree variety leucitite and the olivine-holding
variety leucite basalt.
These rocks have also a very limited distribution, and, so far
as known, are found within the limits of the United States only
at the Leucite Hills, Wyoming.
(2) THE NEPHELINE ROCKS
Mineral Composition. — These rocks consist essentially of
nepheline with a basaltic augite and accessory sanidin, pla-
gioclase, mica, olivine, leucite, minerals of the sodalite group,
magnetite, apatite, perowskite, and melanite.
Chemical Composition. — Below is given the composition of
(I) a nephelinite from the Cape Verde Islands, and (II) a
nepheline basalt from the Vogelsberg, Prussia.1
CONSTITUENTS
I
II
Silica (SiO2)
46.95 %
42 37 %
Alumina (AljOs)
21.59
8 88
Iron sesquioxide (FejOs)
8 09
11 26
Iron protoxide (FeO)
7 80
Magnesia (MgO)
2.49
13 01
Lime (CaO)
7.97
10 93
Soda (NaaO)
8.93
4 51
Potash (K2O)
2.04
1.21
Water (H2O)
2.09
0.34
Specific gravity
3.103
Colors. — The prevailing colors are various shades of gray
to nearly black.
Structure. — Structurally they are porphyritic, with a holo-
crystalline or in part amorphous base, usually fine grained and
compact, at times amygdaloidal.
Classification and Nomenclature. — These rocks differ from the
basalts, which they otherwise greatly resemble, in that they bear
the mineral nepheline in place of feldspar. Based upon the
presence or absence of olivine, we have, first, nepheline basalt,
1 Roth, Abhandl. der Konig. Preus. Akad. der Wiss. zu Berlin, 1884.
104 ROCKS FORMED THROUGH IGNEOUS AGENCIES
and second, nepJielinite. The name nepheline dolerite has been
given in some cases to the coarser, holocrystalline, olivine-
bearing varieties.
Like the leucite rocks, the members of this group are some-
what limited in their distribution.
II. AQUEOUS ROCKS
1. ROCKS FORMED THROUGH CHEMICAL AGENCIES
This comparatively small, though by no means unimportant,
group of rocks comprises those substances which, having once
been in a condition of aqueous solution, have been deposited as
rock masses either by cooling, evaporation, by a diminution of
pressure, or by direct chemical precipitation. It also includes
the simpler forms of those produced by chemical changes in
pre-existing rocks. Water, when pure or charged with more
or less acid or alkaline material, and particularly when acting
under great pressure, is an almost universal solvent. Thus,
heated alkaline waters, permeating the rocks of the earth's
crust at great depths below the surface, are enabled to dis-
solve from them various mineral matters with which they come
in contact. On coming to the surface or flowing into crevices,
the pressure is diminished, or evaporation takes place, and the
water, no longer able to carry its load, deposits it wholly or in
part as vein material or a surface coating. In other cases alka-
line or acid water, bearing mineral matters, may, in course of
their percolations, be brought in contact with neutralizing solu-
tions, and these dissolved materials be thus deposited by direct
precipitation. In these various ways were formed the rocks
here described. It will be observed that the various members
of the group are composed mainly of minerals of a single species
only.
This group cannot, however, be separated by any sharp lines
from that which is to follow, inasmuch as many rocks are not
the product of a single agency, acting alone, but are rather the
result of two or more combined processes. This is especially the
case with the limestones. It is safe to assume that few of these
are due wholly to accumulations of calcareous, organic remains,
but are, in part at least, chemical precipitates, as is well illus-
trated by the oolitic varieties.
105
106 AQUEOUS KOCKS
According to their chemical nature, the group is divided
into (1) Oxides, (2) Carbonates, (3) Silicates, (4) Sulphates,
(5) Phosphates, (6) Chlorides, and (7) the Hydrocarbon Com-
pounds.
(1) OXIDES
Here are included those rocks consisting essentially of oxygen
combined with a base, though usually other constituents are
present as impurities.
Hematite. — Anhydrous sesquioxide of iron. Fe2O3 = oxy-
gen, 30 % ; iron, 70 %. In nature nearly always more or less im-
pure through the mechanical admixture of argillaceous silicates
or calcareous matter, manganese oxides, sulphur, phosphates,
etc. Several forms are recognized, the distinction being based
mainly upon physical properties. Specular hematite is a mica-
ceous or foliated variety with a black, metallic, often splendent
lustre ; this variety is mainly a metamorphic form, and prop-
erly should be classed with the metamorphic rocks. Compact,
columnar, fibrous, and earthy forms also occur, the latter often
known as ochre, as are similar forms of limonite. Although
classified here under the head of aqueous rocks, it does not
follow that the hematites have all originated in precisely the
same manner. To a limited extent the specular variety is found
about volcanic craters and fumaroles, where it was originally
deposited by a process of sublimation. Through a process of
oxidation, beds of magnetic iron become locally altered into
hematite, giving rise to pseudomorphous granular, octahedral,
and dodecahedral forms, to which the name martite is given.
Many extensive beds undoubtedly arise from the dehydration
by dynamic agencies — the folding and metamorphosing of the
enclosing rocks — of beds of limonite. Others, like the fossil
and oolitic ores of the Clinton formations, arise in part from a
process of chemical precipitation and subsequent segregation,
the ore being originally disseminated throughout a ferruginous
limestone, and having accumulated as an insoluble residue as
the lime carbonate was carried away through the action of car-
bonated waters. The extensive hematite deposits of the Lake
Superior region of Michigan are regarded as oxidation prod-
ucts from pre-existing carbonates (siderite), the oxide having
been precipitated from solution in synclinal troughs, and subse-
quently crystallized by metamorphism.1 The ores of the Mesabi
1 Van Hise Monograph XIX, U. S. Geol. Survey, 1892.
PLATE 9
FIG. 1. Botryoidal hematite.
FIG. 2. Clay-iron stone septarian uodule.
OXIDES 107
range, on the other hand, are regarded by at least one writer
as having originated through a somewhat complicated process
of oxidation and metasomatosis, whereby a pre-existing glauco-
iiitic rock (a ferruginous silicate) became converted into an
admixture of free iron oxide and silica, the one or the other,
according to the intermittent character of the permeating solu-
tions, being leached out and redeposited at no great distance in
a fair condition of purity.1 A discussion of this subject belongs
more properly to economic geology, and need not be dwelt
upon further here.
Limonite (Brown Hematite). — Iron sesquioxide plus water.
H6Fe2O6 + Fe2O3. An earthy or compact dark brown, black,
or ochreous-yellow rock, containing, when pure, about two-
thirds its weight of pure iron. It occurs in beds, veins, and
concretionary forms, associated with rocks of all ages, and
forms a valuable ore of iron. (See Fig. 1, PL 9.) On the bot-
toms of lakes, bogs, and marshes it often forms in extensive
deposits, where it is known as bog-iron ore. The formation of
these deposits is described as follows : Iron is widely diffused
in rocks of all ages, chiefly in the form of (1) the protoxide,
which is readily soluble in waters impregnated with carbonic
or other feeble acids, or (2) the peroxide, which is insoluble in
the same liquids. Water percolating through the soils becomes
impregnated with these acids from the decomposing organic
matter, and then dissolves the iron protoxide with which it
comes in contact. On coming to the surface and being exposed
i « i the air, as in a stagnant lake or marsh, this dissolved oxide
absorbs more oxygen, becoming converted into the insoluble
sesquioxide, which floats temporarily on the surface as an oil-
like, iridescent scum. Finally this sinks to the bottom, where
it gradually becomes aggregated as a massive iron ore. This
same ore may also form through the oxidation of pyrite, or
I teds of ferrous carbonate. At the Ktaadn Iron Works, in
Piscataquis County, Maine, the ferrous salt as it oxidizes is
brought to the surface by water and deposited as a coating
over the leaves and twigs scattered about, forming thus beauti-
fully perfect casts, or fossils.
Pyrolusite, Psilomelane, and Wad. — These are names given
to the anhydrous and more or less hydrated forms of manganese
1 J. E. Spun, Bull. No. 10, Geol. and Nat. Hist. Survey of Minnesota, 1894.
108 AQUEOUS ROCKS
oxides, and which, though wide in their distribution, are found
in such abundance as to constitute rock masses in comparative
rarity. The origin of such deposits is at times somewhat ob-
scure. In all cases they are doubtless secondary. The original
source of the material appears to have been the manganiferous
silicates of Archaean and more recent eruptive rocks, whence it
was derived by leaching, being transported in the form of
soluble salts and finally precipitated as oxide or carbonate, the
latter being subsequently converted into oxide. The deposits
which are of sufficient extent to be of commercial value occur
as a rule in residual clays, as interbedded strata in shales and
sandstones, or as occupying superficial seams and joints, and
in the form of pockets and nests. True fissure veins of man-
ganese oxide are not known. It is often associated with the
form of limonite known as bog-iron ore, and, apparently, has
been deposited contemporaneously.
Beauxite (so called from Beaux, near Aries, France) is the
name given to a somewhat indefinite mixture of alumina and
iron oxides, and occurring in the form of compact concretion-
ary grains of a dull red, brown, or nearly white color, and
also in compact and earthy forms. The mode of occurrence of
the mineral is somewhat variable. At Beaux and several other
localities it occurs in pockets in limestone, and also in beds
alternating with limestones, sandstones, and clays belonging
to the Cretaceous period. In the Puy-de-D6me the beds rest
directly upon gneiss, and are overlaid by basalt. At Oberhes-
sen, Germany, the mineral occurs in rounded masses embedded
in clay, as is also the case at Vogelsberg. In America, beaux-
ite has been found in Alabama, Georgia, and Arkansas. In
Alabama and Georgia it occurs in beds of irregular extent,
associated with limestones of Upper Cambrian age (the Knox
dolomite); in Arkansas the deposits are Tertiary.
The origin of the beauxite is somewhat obscure. It has been
argued that the beds at Beaux, and those of Var, are deposits
from mineral springs. Those of the Puy-de-D6me, the West-
erwald, Vogelsberg, and of Ireland, on the other hand, are
regarded as derived from basalt by a metasomatic process.
The Alabama and Georgia deposits, like those of Beaux, are
regarded as of chemical origin.1
1 See resume of the subject, by R. L. Packard, in Mineral Resources of the
United States for 1891.
OXIDES
109
According to C. Willard Hayes,1 the prevailing rocks of this
region are dolomites underlaid by aluminous shales. It is
assumed .that heated waters, in their passage upward from
greater depths, have oxidized the iron sulphides of the shale,
giving rise to sulphates of iron, of alumina, and the double
sulphates of alumina and potash. As the ascending water,
carrying these salts in solution, passes through the dolomite, it
becomes charged with calcium carbonate, which causes the pre-
cipitation of the aluminum salts in the concretionary, pisolitic
form so characteristic.
Beauxite has, of late, come to be of considerable economic
value as an ore of aluminum, and as a source of alum, in place
of clay.
The material from various sources varies greatly in chemical
composition, as shown by the following analyses : —
CONSTITUENTS
I
11
III
IV
V
Silica (SiO2)
2.8%
1.10%
21.08 %
2.80 %
10.38 %
Alumina (Al2Os) . . .
Iron sesquioxide (Fe20s) .
Water (H2O)
57.6
25.3
10.08
50.92
15.70
27.75
48.92
2.14
23.41
52.21
13.50
27.72
55.64
1.95
27.62
Titanium oxide (Ti02) .
3.1
3.20
2.62
3.52
3.50
I. Beaux, France. II. Vogelsberg, Germany. III. Jacksonville, Alabama.
IV. Floyd County, Georgia. V. Pulaski County, Arkansas.
Silica. — Silica, as has been already noted under the head of
rock-forming minerals, is one of the most abundant constituents
of the earth's crust. In its various forms, which are sufficiently
extensive to constitute rock masses, it is always of chemical
origin, that is, results by deposition from solution, by precipi-
tation, or evaporation, as noted above. Varietal names are
given to the deposits, dependent upon their structure, method
of formation, color, and degree of purity. Siliceous sinter,
geyserite, or fiorite is the name given to the nearly white,
often soft and friable, hydrated varieties formed on the evapo-
ration of the siliceous waters of hot springs and geysers, or
through the eliminating action of algous vegetation, as de-
scribed by W. H. Weed in the reports of the United States
1 Trans. Am. Inst. of Mining Engineers, February, 1894.
110 AQUEOUS ROCKS
Geological Survey.1 The material is, in reality, an impure
form of opal. Throughout the geyser regions of the Yellow-
stone Park, Iceland, and New Zealand, the sinter has been
deposited as a comparatively thin crust over the surface, or in
the form of cones about the throats of the geysers. The vari-
eties of silica known as opal are hydrous forms occurring in
veins and pockets, in a variety of rocks. Not infrequently it
forms the replacing material in silicified or " petrified " woods.
In the old lake beds of the Madison valley, Montana, may not
infrequently be found large logs composed wholly of this mate-
rial, no sign of organic matter remaining, but yet with the
woody structure beautifully preserved.
The origin of these silicified logs, so far as it has been traced,
appears to have been somewhat as follows : The water which
permeated the lake beds in which these logs lay, was more or
less alkaline, and carried small amounts of silica in solution.
As the logs slowly decayed, there were given off minute quan-
tities of organic acids which, neutralizing the alkaline water,
caused a gradual precipitation of the silica, building up thus an
exact cast of the decaying structure. Chalcedony is the trans-
lucent, massive, cryptocrystalline variety of silica occurring
mainly in cavities in older rocks, where it has been deposited
by infiltration. It is a common secondary product formed
during the decomposition of many rocks, and, like opal, not
infrequently forms the petrifying medium of fossil woods and
other organisms. Not infrequently, also, it occurs in continu-
ous layers of several inches in thickness, interstratified with
limestone, as may be seen in the walls of the Wyandotte caves
in southern Indiana, or, more rarely, in beds from 2 to 8 feet
thick, interstratified with coal and fire-clay, as at the well-
known " Flint Ridge " of Licking County, Ohio. Such depos-
its are considered to be due to accumulations of the siliceous
tests of diatoms. Flint is a variety of chalcedony formed by
segregation in chalky limestone, and is composed, in part, of
the broken and partially dissolved spicules of sponges, and
the siliceous casts of infusoria. The source of the silica is,
doubtless, the sponge spicules above noted and diatomaceous
remains. Chert is an impure flint containing not infrequently
fossil nummulitic remains, and with sometimes a pronounced
1 9th Ann. Rep. U. S. Geol. Survey, 1887-88. See also Bischof's Chemical
and Physical Geology, Vol. I, pp. 184-200.
CARBONATES 111
oolitic structure. It occurs in rounded, nodular, concretionary
masses interbedded with limestones, particularly Palaeozoic vari-
eties, and doubtless originated as did the flints in the chalky
limestones. Jasper is a dull or bright red, or yellow variety
of chalcedony containing alumina, and owing its color to iron
oxides. It is sometimes used in jewellery.
The name novaculite is frequently given to very fine-grained
and compact quartz rocks, such as are suitable for hones. As
commonly used, the name is made to include rocks of widely
different origin, some of which are evidently chemical precipi-
tates, while others are indurated clastic or schistose rocks. The
well-known novaculites of Arkansas are clear white masses of
chalcedonic silica, containing scattering quartz granules, minute
grains of garnet, and numerous small rhomboidal cavities which
seemingly were once occupied by crystals of calcite or dolomite.
Opinions differ as to the origin of this rock. Owen1 regarded
it as a sandstone metamorphosed by percolating hot water.
Branner2 looked upon it as a metamorphosed chert ; Griswold,3
as a chemical deposit in the form of a siliceous slime on a sea-
bottom, while Rutley4 argues that it is but a siliceous replace-
ment of beds of dolomite or dolomitic limestone. It seems
probable that the views of Branner or Rutley are the most
nearly correct.
Quartz is a massive form of crystalline silica occurring in
veins, disseminated granules, and pockets in rocks of all kinds
and all ages. It is one of the most wide-spread and commonest
of minerals, and is frequently quarried and crushed for abrasive
purposes or use in pottery manufacture. It is not infrequently
of a pink or rose color from metallic oxides. It is a common
gangue of ores of the precious metals, particularly of gold.
Lydian stone is an exceedingly hard impure quartz rock, of a
black color and splintery fracture. It was formerly much
used in testing the purity of precious metals.
(2) CARBONATES
Water carrying small amounts of carbonic acid readily dis-
solves the calcium carbonate of rocks with which it comes in
1 2d Rep. Geological Reconnaissance of Arkansas, 1860.
2 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1886, p. 49.
8 Ann. Rep. Geol. Survey of Arkansas, Vol. Ill, 1890.
4 Quarterly Journal Geological Society of London, August, 1894.
112
AQUEOUS ROCKS
contact ; on evaporation and through loss of a portion of the
carbonic acid, this is again deposited. In this way are formed
numerous and at times extensive deposits, to which are given
varietal names dependent upon their structure and the special
conditions under which they originated. Gale sinter or tufa is
a loose friable deposit made by springs and streams either by
evaporation or through intervention of algous vegetation. Such
are often beautifully arborescent and of a snow-white color, as
seen at the Mammoth Hot Springs of the Yellowstone National
Park. Somewhat similar deposits are formed by springs in
Virginia, California, Mexico, New Zealand. Others, like those
from Niagara Falls, New York, and Soda Springs, Idaho, were
formed by the deposition of the lime on leaves and twigs, form-
ing beautifully perfect casts of these objects.
Tufa deposits of peculiar imitative shapes have been described
by Mr. I. C. Russell of the United States Geological Survey,
as formed by the evaporation of the waters of Pyramid Lake,
Nevada. Oolitic and pi-
solitic limestones are so
called on account of their
rounded, fish - egg - like
structure, the word oolite
being from the Greek
word (oov, an egg. (See
PI. 12.) These are in
part chemical and in part
mechanical deposits. The
water in the lakes and
seas in which they were
formed became so satu-
rated that the lime was
deposited in concentric
coatings about the grains
of calcareous sand on the
bottom, and finally the little granules thus formed became
cemented into firm rock by the further deposition of lime in
the interstices. This structure will be best understood by
reference to Fig. 7. Rocks of this nature are now forming
along the beaches of Pyramid Lake. Concerning the occur-
rence of these Mr. Russell writes : —
" Among The Needles the rocky capes are connected by cres-
FIG. 7. — Microstructure of oolitic limestone.
CARBONATES 113
cent-shaped beaches of clean, creamy sands, over which the
summer surf breaks with soft murmurs. These sands are oolitic
in structure, and are formed of concentric layers of carbonate
of lime which is being deposited near where the warm springs
rise in the shallow margin of the lake. In places these grains
have increased by continual accretion until they are a quarter
of an inch or more in diameter, and form gravel, or pisolite, as
it would be termed by mineralogists. In a few localities this
material has been cemented into a solid rock, and forms an
oolitic limestone sufficiently compact to receive a polish. No
more attractive place can be found for the bather than these
secluded coves, with their beaches of pearl-like pebbles, or the
rocky capes, washed by pellucid waters, that offer tempting
leaps to the bold diver."
Such forms as these may or may not show a nucleus. It
seems safe to assume that such a nucleus, at first, in all cases
existed, though it may be in microscopic dimensions only.
Travertine is a compact and usually crystalline deposit formed,
like the tufas, by waters of springs and streams. The traver-
tines are%often beautifully veined and colored by metallic oxides
and form some of the finest marbles. Such are the so-called
" onyx marbles " of Mexico and Arizona.1
Stalactite and stalagmite are the names given to the deposits
formed from the roofs and on the floors of caves ; water, perco-
lating through the limestone roof, by virtue of the carbonic acid
it contains, dissolves out a small amount of the lime, which, on
evaporation, is again deposited either as pendent cones from
the ceiling, or as massive and pillar-like forms upon the floor.
The pendants are known as stalactites ; the corresponding
growths upon the floor as stalagmites. Stalactite and stalag-
mite sometimes meet, forming thus continuous pillars, or col-
umns extending from floor to ceiling. The lime "of these
deposits, it may be said, is as a rule in the form of calcite,
though sometimes, as in the old portions of the Wyandotte
caves in Indiana, it is aragonite. The so-called " oriental ala-
baster " of the ancients is a stalagmitic deposit derived in part
from crevices and pockets in the Eocene limestones of the Nile
valley.
Magnesite, a carbonate of magnesia, occurs frequently as a
1 The Onyx Marbles, Ann. Rep. U. S. National Museum for 1893. Also
Stones for Building and Decoration, Wiley & Sons, New York, 2d ed., p. 120.
114 AQUEOUS KOCKS
secondary mineral in the form of veins in serpentinous rocks,
but rarely itself forms rock masses of any importance. Rhodo-
chrosite, a carbonate of manganese, sometimes occurs in rock
masses, but is found most commonly in the form of veins asso-
ciated with ores of silver, lead, or copper.
Another carbonate, less common than that of lime, but which
sometimes occurs in such quantities as to constitute true rock
masses, is siderite, or carbonate of iron. A common form of
this is dull brownish or nearly black in color, very compact and
impure, containing varying amounts of calcareous, clayey, and
organic matter. In this condition it is found in stratified beds
and in the shape of rounded and oval nodules, or concretions,
which are called clay-ironstone nodules, septaria, and sphcero-.
siderite. (See Fig. 2, PL 9.) These septarian nodules are
often beautifully veined with calcite, and when cut and polished
form not undesirable objects of ornamentation. Other forms of
siderite are massive, coarsely crystalline, and of a nearly white
or yellowish color, becoming brownish on exposure. Pure sider-
ite yields about 48 % metallic iron, and is of value as an ore.
•
(3) SILICATES
Silica, combined with magnesia and water, gives rise to an
interesting group of serpentinous and talcose substances, which
are often sufficiently abundant to constitute rock masses. Pure
serpentine consists of about equal parts of silica and magnesia,
with from 12 to 13 % of water. It is a compact, amorphous, or
colloidal rock, soft enough to be cut with a knife, with a slight
greasy feeling and lustre, and of a color varying from dull
greenish and almost black, through all shades of yellow, brown-
ish, and red. It also occurs in fibrous and silky forms, filling
narrow veins in the massive rocks, and is known as amianthus,
or chrysotile. These fibres, when sufficiently long, are used for
the manufacture of fireproof material, and the mineral is com-
mercially confounded with asbestos, a fibrous variety of amphi-
bole. It is very doubtful if serpentine is ever an original
rock ; it is rather an alteration product of other and less stable
magnesian minerals. Here will be considered only those which
have originated by a series of chemical changes known as meta-
somatosis, a process of indefinite substitution and replacement,
in simple mineral aggregates occurring associated with the
SILICATES
115
older metamorphic rocks. Such are the serpentines derived
from non-aluminous pyroxenes, like those of Montville, New
Jersey, and Moriah, New York, and those from Easton, Penn-
sylvania, derived from a massive tremolite rock. The analyses
given below will serve to illustrate the chemical changes which
occur in this process of metasomatosis, I being that of a nearly
white pyroxene, and II that of the serpentine derived therefrom.
CON8TITCENT8
I
ll
Silica (Si02)
54.215%
42.38 %
Magnesia (MgO)
19.82
42.14
Lime (CaO)
24.71
0.00
Alumina (AljOs) .
0.59
0 07
Ferric oxide (Fe2O3)
0.20
0.97
Ferrous oride (FeO)
0.27
0 17
Ignition (HaO)
0.14
14.20
99.945 %
'.«».85%
The pyroxene, it should be observed, occurs in nodular
masses in a crystalline granular dolomite. Various stages of
the process are shown in Fig. 8, in
which the white and gray central por-
tions are nucleal masses of unchanged
pyroxenes, surrounded by the darker
crusts of secondary serpentine.1 Ser-
pentine as an alteration product of the
mineral chondrodite is also known to
occur, though this form is less common.
At Brewster, New York, are extensive
deposits of this nature. (See further
on, p. 158.)
Several varieties of serpentine are
popularly recognized. Precious or noble
serpentine is simply a very pure com-
pact variety of a deep oil-yellow or
green color. Amianthus, or chrysotile, as noted above, is the
name given to the fibrous variety. Williamsite is a deep bright
green, translucent, and somewhat scaly variety, occurring asso-
1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, Vol. XI, 1888, p. 105.
Fio. 8. — Pyroxene partially
altered to serpeutine.
116 AQUEOUS ROCKS
elated with the chrome iron deposits in Fulton township, Lan-
caster County, Pennsylvania. Deweylite is a hard, translucent
variety occurring in veins in altered dunite beds. Bowenite is
a pale green variety forming veins in limestone at Smithfield,
Rhode Island. Picrolite, marmolite, and retinolite are varieties of
minor importance. Serpentine alone, or associated with calcite
and dolomite, forms a beautiful marble, to which the names ver d
antique, ophite, and ophiolite are given. The so-called Eozoon
Canadense, a supposed fossil rhizopod, is a mixture of serpen-
tine and calcite or dolomite. The name serpentine is from the
Latin serpentinus, a serpent, in allusion to its green color and
often mottled appearance.
Those serpentines which were derived from basic eruptives,
or complex metamorphic rocks are described with those rocks
with which, in their unaltered state, they would naturally be
grouped.
The mineral steatite, or talc, when pure, differs from ser-
pentine in containing 63.5 % of silica, 31.7% of magnesia, and
4.8 °/o of water. Its common form is that of white or greenish
inelastic scales, forming an essential constituent of the talcose
schists. As is the case with serpentine, it sometimes results
from the alteration of eruptive magnesian rocks, such as the
pyroxenites, and rarely occurs as a direct result of precipita-
tion. It will be described more fully under the head of schists
and pyroxenites. Rensselaerite is a closely related rock of a
white or gray color, found in St. Lawrence County, New York.
Its composition is essentially that of talc.
Pyrophyllite, or agalmatolite, is a hydrous silicate of alumina,
somewhat harder than talc, which it otherwise resembles, and
which is used in making slate pencils and small images. It
occurs in a schistose form in the Deep River region of North
Carolina.
Kaolin, also a hydrous silicate of alumina, is a chemical
product in that it is a residue left by the chemical decomposi-
tion of the feldspars. These minerals, as explained elsewhere,
consist of silicates of alumina and lime, with more or less of
the alkalies potash and soda, and iron oxides. In the process
of decomposition new compounds are formed, the more soluble
of which are leached out, leaving the less soluble silicates,
including kaolin, behind in a condition of more or less purity.
The mineral is of great value for fictile purposes, and is de-
SULPHATES 117
scribed more fully under the head of argillaceous fragmental
rocks.
(4) SULPHATES
Gypsum. — The rock gypsum is chemically a hydrous sul-
phate of lime, that is to say, consists of sulphur, lime, and
water, and in the proportion of 32.6 parts of lime and 20.9
parts of water, combined with 46.5 parts of sulphur trioxide.
When crystallized, the mineral is nearly colorless and trans-
parent, and splits readily into thin, inelastic sheets. The com-
pact massive varieties are white, gray to black, and sometimes
pink from various impurities. The most characteristic feature
is its softness, which is such that it can be readily cut with a
knife or even by the thumbnail.
Four varieties of gypsum are recognized : (1) The common
massive form, dull in color and often more or less impure ;
(2) the pure white, fine-grained variety, alabaster; (3) the
fibrous variety, satin spar ; and (4) the broadly foliated, trans-
parent variety, selenite, so called from the Greek word o-eXei/e,
the moon, in allusion to its soft and pleasing lustre.
The following is an analysis of a commercial gypsum from
Ottawa County, Ohio, as given by Professor Orton : J —
Lime(CaO) 32.52%
Sulphuric acid (S08) 45.60
Water (H2O) 20.14
Magnesia (MgO) 0.56
Alumina (A12O8) 0.16
Insoluble residue 0.68
99.62 %
Gypsum occurs mainly associated with stratified rocks, and
is regarded as a chemical deposit resulting from the evapora-
tion of waters of inland seas and lakes ; it may also originate
through the decomposition of sulphides and the action of the
resultant sulphuric acid upon limestone ; through the mutual
decomposition of the carbonate of lime (limestone) and the sul-
phates of iron, copper, and other metals ; through the hydration
of anhydrite ; and through the action of sulphurous vapors and
solutions from volcanoes upon the rocks with which they come
in contact. According to Dana,2 the gypsum deposits in western
1 Geology of Ohio, 1888, Vol. VI, p. 700.
2 Manual of Geology, p. 234.
118 AQUEOUS KOCKS
New York do not form continuous layers in the strata, but lie
in embedded, sometimes nodular, masses in limestones. In all
such cases this authority says the gypsum is the product of the
action of sulphuric acid from springs upon the limestone. " The
sulphuric acid, acting on the carbonate of lime, drives off its
carbonic acid and makes sulphate of lime or gypsum ; and this
is the true theory of its formation in New York." W. C. Clarke,
however, regards it as a product of deposition from solution in
sea- water.1
The gypsum deposits of northern Ohio form apparently con-
tinuous beds over thousands of square miles, and are regarded
by Professors Newberry and Orton as deposits from the evapo-
ration of landlocked seas at the same time as was the rock-salt
which overlies it.
Geological Age and Mode of Occurrence. — As may be readily
inferred from what has gone before, beds of gypsum have
formed at many periods of the earth's history, and are still
forming wherever proper conditions exist.
In New York there are extensive deposits belonging to the
Salina period of the Upper Silurian. In Ohio, gypsum asso-
ciated with limestones and shales of Lower Helderbergage occur
over areas comprising thousands of square miles. The follow-
ing section of beds in Ottawa County, this state, will serve to
show the conditions under which the rock may occur : —
Drift clays ' 12 to 14 feet
Gray rock carrying impure gypsum 5 to 14 feet
Blue shale £ to 14 feet
Boulder bed carrying gypsum embedded in shaly limestone . . . 6 to 14 feet
Blue limestone 1 to 14 feet
Main gypsum bed 7 to 14 feet
Gray limestone 1 to 14 feet
Gypsum 3 to 5 feet
Anhydrite is an anhydrous variety of calcium sulphate some-
what less common than gypsum. Barite, or heavy spar, the
sulphate of barium, also occurs in nature, but less abundantly
than the calcium sulphates. It is found commonly in con-
nection with metallic ores (silver, lead, and zinc), or as a
secondary mineral associated with limestone, sometimes in
distinct veins, or, as in southwest Virginia, filling irregular
fractures in certain beds of the Cambrian limestones, or in
1 Bull. New York State Museum, Vol. Ill, No. 1, 1893.
PHOSPHATES 119
part replacing the limestone itself. It is easily distinguished
from coarsely crystalline calcite, for which it might possibly
be mistaken, by its weight, the specific gravity being about
4.5 as against 2.7 for the latter.
(6) PHOSPHATES
The mineral apatite, a phosphate of lime, as already noted, is
a common accessory, in the form of small crystals, in crystal-
line rocks of all ages, both metamorphic and eruptive. In
rare instances, as among certain Laurentian rocks of Canada, it
occurs in coarsely granular aggregates of a green or pinkish
color and of such dimensions as to constitute true rock masses.
Here we have to do, however, more with the amorphous, fibrous,
or concretionary forms to which the name phosphorite is com-
monly applied. These occur nearly if not quite altogether as
secondary products, due to the leaching out of phosphatic mate-
rial from older rocks, and its redeposition in clefts and cavities
at lower levels. It is thus that the phosphorites of Estre-
madura, Spain, are accounted for. From these very pure,
semi-crystalline masses, to the amorphous nodular and earthy
forms, such as are found in the eastern Carolinas and in Flor-
ida, there are no well-defined lines of demarcation. All have
resulted apparently either from the leaching out of the phos-
phate as above, or from the dissolving and carrying away of the
lime carbonate in a phosphatic limestone, leaving the phosphatic
material to accumulate as a residual product. Some of the latter
products, like the phosphatic sandstones of the Carolinas, might
with equal propriety be classed with the fragniental rocks, as
are the residual clays. (See p. 151.)
(6) CHLORIDES
Sodium chloride, or common salt, is one of the most wide-
spread constituents of the earth's crust, and from the standpoint
of human comfort a most important constituent as well. The
theoretically pure mineral consists of 66.6 parts of sodium and
39.4 parts of chlorine, though in nature it is almost univer-
sally contaminated with chlorides, sulphates, and carbonates
of potassium, calcium, and magnesium, together with oxides of
iron and aluminum. A large number of analyses of rock-salts
120 AQUEOUS ROCKS
from world- wide sources show them to range from 94 to 99 %
sodium chloride. The pure mineral is white in color, but
shows often yellow, red, or purplish hues due to iron oxides or
organic matter; When crystallizing freely from solution, it
ordinarily assumes the form of a cube, the faces being frequently
cavernous or hopper-shaped ; rarely it occurs in octahedrons,
and occasionally in fibrous forms. Sodium chloride in solution
is an almost universal constituent of carbonated waters, though
often in but the merest traces. Its prevailing solid form is that
of coarsely granular aggregates constituting the so-called rock-
salt, the beds of which are often of such thickness and extent
as to constitute true rock masses and entitle them to considera-
tion here. These rock masses are invariably products of depo-
sition from solution, a deposition brought about through the
evaporation of saline waters in enclosed lakes or seas. They
are not limited to any particular geological period, but are to be
found wherever suitable conditions have existed for their for-
mation and preservation. Some of the more important beds
now known belong to either the Upper Silurian, Carboniferous,
Triassic, or Tertiary ages, and vary in thickness from a mere
film to upwards of 1200 feet. In the United States, beds of
rock-salt are known to occur in the states of New York, Penn-
sylvania, Ohio, Virginia, West Virginia, Michigan, Kansas,
Kentucky, Texas, Wyoming, California, and Nevada. Canada,
England, the Carpathian Mountains, the Austrian and Bavarian
Alps, West Germany, the Vosges, the Jura, Spain, the Pyrenees
and Celtiberian mountains, all contain important beds. With
the rock-salt are not infrequently associated other salts, as above
noted. In the celebrated Stassfurth deposits, sixteen different
compounds in the shape of chlorides and sulphates of sodium,
potassium, magnesium, calcium, and iron have been determined,
many of them in sufficient quantity to be of commercial value.
(7) THE HYDROCARBON COMPOUNDS
Under this head are included a series of hydrocarbon com-
pounds varying in physical properties from solid to gaseous,
and in color from coal-black through brown, greenish, red, and
yellow to colorless. Unlike the other members of the hydro-
carbon series yet to be described, they are not the residual
products of plant decomposition in situ, but are rather distilla-
Bituminous
THE HYDROCARBON COMPOUNDS 121
tion products from deeply buried organic matter of both animal
and vegetable origin. The different members of the series
differ so widely in their properties and uses that each must be
discussed independently. The grouping of the various com-
pounds as given below is open to many objections from a strictly
scientific standpoint, but, all things considered, it seems best
suited for our present purposes.1
Gaseous Marsh gas (natural gas)
Fluidal Petroleum (naphtha)
, , / Pittasphalt (maltha)
Viscous and semi-solid ( Mineral tar
f Asphalt (bitumen)
Elastic -I Elaterite
( Wurtzilite
f Albertite
Solid •{ Grahamite
I Uintaite
f Succinite
Resinous •] Copalite
( Ambrite
. f Ozokerite
Cerous(waxy) 1 Hatchettite
Marsh Gas (Natural Gras). — This is a colorless and odor-
less gas arising from the decomposition of organic matter
protected from the oxidizing influence of atmospheric air. By
itself it burns quietly with a slightly luminous flame, but when
mixed with air forms a dangerous explosive. It is this gas
which forms the dreaded fire-damp of the miners.
Under this head may properly be considered the so-called
natural gas, which has of late years become of so much impor-
tance from an economic standpoint. This is, however, by no
means a simple compound, but an admixture of several gases,
samples from different wells showing considerable variation in
composition, as well as those from the same well collected at
different periods. This last is shown by the six analyses fol-
lowing, and which may serve well to illustrate the average
composition, though in some instances the percentage of marsh
gas has been found greater.
1 W. P. Blake, Trans. Am. Inst. of Mining Engineers, Vol. XVIII, 1890,
p. 582.
122
AQUEOUS ROCKS
CONSTITUENTS
I
II
III
IV
V
VI
Mash gas
57.85%
75.16 %
72.18%
65.25 %
60.70%
49.58%
9.64
14.45
20.02
26.16
29.03
35.92
Ethylic hydride ....
Olifiant gas ....
5.20
0.80
4.80
0.60
3.60
0.70
5.50
0.80
7.92
0.98
12.30
0.60
Oxvfpn
2.10
1.20
1.10
0.80
0.78
0.80
Carbonic oxide ....
Carbonic acid ....
Nitrogen
1.00
0.00
23.41
0.30
0.30
2.89
1.00
0.80
0.00
0.80
0.60
0.00
0.58
0.00
0.00
0.40
0.40
0.00
100.00 %
99.70%
99.40%
99.91 %
99.99 %
100.00%
Natural gas in quantities sufficient to be of economic impor-
tance is necessarily limited to rocks of no particular horizon.
The tendency of recent studies seems to be to show that it
results, as above stated, from the deeply buried organic matter,
of both plant and animal origin. It is not, however, indige-
nous to the rocks in which it is now found, but occurs in an
overlying, more or less porous, sand or lime rock into which
it has been forced by hydrostatic pressure. The first necessary
condition for the presence of gas in any locality may, indeed,
be said to depend upon the existence of such a porous rock as
will serve as a reservoir to hold it, and also the presence of an
impervious overlying stratum to prevent its escape. In Penn-
sylvania the reservoir rock is a sandstone of Carboniferous or
Devonian age ; in Ohio and Indiana, a cavernous dolornitic
limestone of Silurian (Trenton) age.
Natural gas, as may readily be understood, is still in process
of formation, though at a rate vastly slower than it is being
utilized, or wasted, in many regions. It is a necessary conse-
quence that the available supply must sooner or later become
exhausted. Indeed this contingency has already made itself
apparent in many fields, necessitating continuous activity in
prospecting, and in more than one instance all known sources
of supply are already exhausted. Few more marked illustra-
tions of man's unreasonable squandering of nature's resources
have ever been offered than that relating to the utilization of
natural gas.
Petroleum. — This is the name given to a complex hydro-
carbon compound, liquid at ordinary temperatures, though
varying greatly in viscosity, of a black, brown, greenish, or
THE HYDROCARBON COMPOUNDS
123
more rarely, red or yellow color, and of extremely disagreeable
odor. Its specific gravity varies from 0.6 to 0.9. Through
becoming more and more viscous, the material passes into the
solid and semi-solid forms, asphalt and maltha. Chemically it
is considered as a mixture of the various hydrocarbons included
in the marsh gas, ethyline, and paraffin series.
An ultimate analysis of several samples, as given by the
reports of the 10th Census of the United States (1880), shown 1
the following percentages of the three essential constituents : —
LOCALITIES
HYDROGEN
CARBON
NlTI:
West Virginia
13.359 %
85.200 %
0.540 %
Mecca, Ohio
13.071
86.316
ojao
California
11.819
86.934
1.100
As with marsh gas, petroleum is considered as a product of
organic decomposition, which has been for the most part forced
up from the rocks in which it originated into overlying strata.
It is therefore limited to no particular geological horizon, but
is found in rocks of all ages, from the Cambrian to the most
recent, its existence in quantities sufficient for economic pur-
poses being dependent upon local conditions for its generation
and subsequent preservation. Inasmuch as its accumulation
in liirge quantities necessitates a rock of porous nature to act as
a reservoir, the petroleum-bearing rocks are mostly sandstones,
though not uniformly so. Petroleums are found in California
and Texas, in Tertiary sands ; in Colorado, in the Cretaceous ;
in West Virginia, both above and below the Crinoidal (Car-
boniferous) limestones ; in Pennsylvania, in the Mountain sands
(Lower Carboniferous) and the Venango sands (Devonian); in
Canada, in the Corniferous (Lower Devonian) limestone ; in
Kentucky, in the Hudson River shales (Lower Silurian); and
in Ohio, in the Trenton limestone, also of Lower Silurian age.
In some instances petroleum oozes naturally from the ground,
forming at times a thin layer on the surface of pools of water,
whence in times past it has been gathered and used for chemical
and medicinal purposes. The so-called " Seneca oil " thus used
some fifty or sixty years ago was obtained from a spring in Cuba,
Alleghany County, in New York. The immense supply now
124 AQUEOUS ROCKS
demanded for commercial purposes is, however, obtained alto-
gether from artificial wells of varying depths, and which are
in some cases self-flowing, while in others the oil is raised by
means of pumps. Wells of from 500 to 1500 feet in depth are
of common occurrence, while those upwards of 2000 feet are not
rare. The principal sources of petroleum, in the United States,
are in New York, Pennsylvania, and Ohio, with smaller fields
in West Virginia, Kentucky, Tennessee, Indiana, Texas, Colo-
rado, and California. The chief foreign source is the Baku
region, on the Caspian Sea, and Galicia, in Austria.
The quantity of petroleum and semi-solid bituminous com-
pounds contained in "the rocks of certain areas is sometimes
enormous. Dr. Hunt estimated that the dolomite underlying
the city of Chicago and vicinity contains for each square mile
over 7,000,000 barrels. A like computation by Professor
Orton J led to the conclusions given in the following quotation
relative to the water-lime stratum of Ohio, which is almost
universally petroliferous : —
" Estimating its petroleum contents at one-tenth of one per
cent, and the thickness of the stratum at 500 feet, both of
which estimates are probably within the limits, we find the
petroleum contained in it to be more than 2,500,000 barrels to
the square mile. The total production of the great oil field
of Pennsylvania and New York to January, 1885, is 261,000,000
barrels. It would require only three ordinary townships, or a
little more than 100 square miles, to duplicate this enormous
stock from the water-lime alone. But if the rate of one-tenth
of one per cent should be maintained through a descent of
1500 feet at any point in the state, each square mile would, in
that case, yield 75,000,000 barrels, or nearly one-third of the
total product of the entire Pennsylvania and New York oil
fields. These figures pass at once beyond clear comprehension,
but they serve to give some idea of the vast stock of petroleum
contained in the earth's crust. If petroleum is generally dis-
tributed through a considerable series of rocks in any appre-
ciable percentage, it is easy to see that the aggregate amount
must be immense. Even one-thousandth of one per cent would
yield 750,000 barrels to the square mile in a series of rocks 1500
feet deep, but this amount is nearly equal to the greatest actual
production per square mile of any part of the leading Pennsyl-
i Ann. Rep. U. S. Geol. Survey, 1886-87, Part II, p. 507.
THE HYDROCARBON COMPOUNDS 125
vania fields. It is obvious that the total amount of petroleum
in the rocks underlying the surface of Ohio is large beyond
computation, but in its diffused and distributed state it is
entirely without value. It must be accumulated in rocks that
serve as reservoirs before it becomes of economic interest. In
respect to the importance of concentration, it agrees with most
other forms of mineral wealth."
Asphaltum (Bitumen, or Mineral Pitch'). — These are names
given to what are rather indefinite admixtures of various
hydrocarbons, in part oxygenated, and which, for the most part
solid or at least highly viscous at ordinary temperatures, pass
by insensible gradations into pittasphalts ^or mineral tar, ami
these in turn into the petroleums. They are characterized by
a black or brownish black color, pitchy lustre, and bituminous
odor. The solid forms melt ordinarily at a temperature of
from 90° to 100° F., and burn readily with a bright flame,
giving off dense fumes of a tarry odor. The fluidal varieties
become solid on exposure to the atmosphere, owing to evapora-
tion of the more volatile portions.
The crude asphalt of Trinidad has the following composition
and physical characteristics : 1 —
Specific gravity, 1.28 ; hardness at 70° F., 2.5 to 3, Dana's
scale ; color, chocolate-brown. Composition : —
Bitumen 39.83 %
Earthy matter 33.99
Vegetable matter .... 9.31
Water 16.87
100.00 %
The mode of occurrence of asphalt deposits varies greatly,
owing to the fact that, as with petroleum and natural gas, it
has come up through fissures and cracks in the earth's surface,
and as a rule no longer occupies its place of origin. On the
island of Trinidad is an immense superficial deposit having an
area of about 114 acres and a depth varying from 18 to 78 feet.
The surface is sufficiently solid over nearly every part for the
passage of teams, is of a brownish black color, and nearly level.
The deposit has in numerous publications been compared to a
lake, and stated to be fluidal and at a high temperature in the
centre. This statement is quite erroneous and misleading.
1 Trans. Am. Inst. Mining Engineers, Vol. XVII, 1889, p. 363.
126 AQUEOUS ROCKS
In Ventura County, California, the material occurs in a fissure
vein in siliceous clay of Miocene age, the vein being from 7 to
15 inches thick on the surface, but widening rapidly in descent
to a thickness of 5 feet at a depth of 65 feet below the surface.
The material of the vein is, however, far from pure asphalt ;
but rather an asphaltic sand. In western Kentucky the as-
phalt exudes from the ground in the form of "tar springs," and
occurs also disseminated through sandstones and limestones of
sub-Carboniferous age. Frequently, as in the dolomite under-
lying Chicago, Illinois, the bituminous matter is so diffused
throughout the rock as to give it, on exposure, a brownish
black appearance, and cause it to exhale an odor of petroleum
appreciable for some distance. In the Dead Sea, bituminous
masses of considerable size have in times past risen like islands
to the surface of the water, and furnished thus the material
used by the ancients in pitching the walls of buildings and
rendering vessels water-tight. The ancient name of this body
of water was Lake Asphaltites, and from it our word asphalt
is derived.
The above illustrations are sufficient to indicate the numerous
conditions under which the substance occurs. The material is
world-wide in its geographic distribution and equally cosmo-
politan in its geological range, being found in gneissic rocks of
presumably Archsean age in Sweden, and in rocks of all inter-
mediate horizons down to late Tertiary.
Elaterite (Mineral Caoutchouc). — This is the name given to a
soft and elastic variety of asphalt much resembling pure india-
rubber. It is easily compressible in the fingers, to which it
adheres slightly, of a brownish color, and of a specific gravity
varying from 0.905 to 1.00. It has been described from mines
in Derbyshire and elsewhere in England, but, so far as the
writer is aware, is of no commercial value. Its composition so
far as determined is, carbon, 85.47 %\ hydrogen, 13.28 %.
The name wurtzilite has been given by Professor W. P.
Blake to a hydrocarbon very similar in appearance to the
uintaite (described below), but differing in physical and chem-
ical properties. It is described as a firm black solid, amorphous
in structure, brittle when cold, breaking with a conchoidal
fracture, but when warm, tough and elastic, its elasticity being
best compared with that of mica. If bent too quickly, it snaps
like glass. It cuts like horn, has a hardness between 2 and 3,
THE HYDROCARBON COMPOUNDS 127
a specific gravity of 1.03, gives a brown streak, and in very
thin flakes shows a garnet-red color. It does not fuse or
rnelt in boiling water, but becomes softer and more elastic ; in
the flame of a candle it melts and takes fire, burning with a
bright, luminous flame, giving off gas and a strong bitumi-
nous odor. It is not soluble in alcohol, but sparingly so
in ether, in both of which respects it differs from elaterite
proper.
Albertite. — This is a brilliant jet-black compound, breaking
with a lustrous, conchoidal fracture, having a hardness of
between 1 and 2 of Dana's scale, a specific gravity of 1.097,
a black streak, and showing a brown color on very thin edges.
In the flame of a lamp it shows signs of incipient fusion, intu-
mesces somewhat, and emits jets of gas, giving off a bituminous
odor ; when rubbed it becomes electric. According to Dana,
it softens slightly in boiling water, is scarcely at all soluble in
alcohol, and only slightly so in ether and in turpentine. The
following is the composition as given by Witherill : Carbon,
86.04%; hydrogen, 8.96%; oxygen, 1.97%; nitrogen, 2.93%;
ash, 0.10%. The mineral occurs in fissures in rocks of sub-
Carboniferous age, at the Albert Mines, in Hillsborough County,
Nova Scotia ; hence the name. ««.
Formerly it was used for the distillation of oils for illumi-
nating purposes. Since the discovery of petroleum its use has
been discontinued.
G-rahamite. — This variety resembles the last in its general
appearance and its conduct toward solvents, and it is a question
if it is not identical therewith, jjjt was described by Dr. Wurtz
from Ritchie County, in West Virginia, where it occurred in a
vein some four feet in width in Carboniferous sandstones.
Uintaite (Gilsonite). — This is a black, brilliant, and lustrous
variety giving a dark-brown streak, breaking with a beautiful
conchoidal fracture, and having a hardness of 2 to 2.5 and a
specific gravity of 1.065 to 1.07. It fuses readily in the flame
of a candle, is plastic but not sticky while warm, and unless
highly heated will not adhere to cold paper. Its deportment
is stated to be much like that of sealing wax or shellac. Like
albertite and grahamite, it dissolves slightly in turpentine and
is not soluble in alcohol. It is a good non-conductor of elec-
tricity, but like albertite becomes electric by friction. Its
composition as given is, carbon, 80.88%; hydrogen, 9.76%;
128 AQUEOUS ROCKS
nitrogen, 3.30 %; oxygen, 6.05 % ; and ash, 0.01 %. The min-
eral as first described occurred in a vertical vein from 3 to 5
feet in thickness, cutting through nearly horizontal sandstones
some 3 miles east of Fort Duchesne, on the reservation of the
Uinta Indians.
/Succinite (Amber'). — The mineral commonly known as amber
is a fossil resin, consisting of some 78.94 parts of carbon, 10.53
parts of oxygen, and 10.53 parts of hydrogen, together with
usually from two to four-tenths of a per cent of sulphur. It
is not a simple resin, but a compound of four or more hydro-
carbons. According to Berzelius, as quoted by Dana, it " con-
sists mainly of (85% to 90%) two other resins in soluble
alcohol and ether, and an oil, and %^'% to 6 % of succinic
acid."
The mineral, as found, is of a yellow, brownish, or reddish
color, frequently clouded, translucent, or even transparent,
tasteless, becomes negatively electrified by friction, has a hard-
ness of 2 to 2.5, a specific gravity, when free from enclosures, of
1.096, a conchoidal fracture, and melts at 250° to 300° Fahr.
without previous swelling, but boils quietly, giving off dense
white fumes with an aromatic odor and very irritating effect
on the respiratory organs.
Amber, or closely related compounds, has been found in
varying amounts at numerous widely separated localities, but
always under conditions closely resembling one another. The
better known localities are the Prussian coast of the Baltic ; on
the coast of Norfolk, Essex, and Suffolk, England ; the coasts
of Sweden, Denmark, and the Russian Baltic provinces; in
Galicia, Westphalia ; Poland ; Moravia ; in Norway ; Switzer-
land ; France ; Upper Burma ; Sicily ; Mexico ; the United
States at Martha's Vineyard, and near Trenton and Camden,
New Jersey, and at Cedar Lake in Northwest Canada.
The amber of commerce comes now, as for the past 2000
years, mainly from the Baltic, where it occurs in a stratum of
blue earth of from 4 to 20 feet in thickness underlying the
brown coal formation.
Ozokerite (Mineral Wax; Native Paraffin). — This is a wax-
like hydrocarbon, usually with a foliated structure, soft and
easily indented with the thumb nail ; of a yellow, yellow
brown, or sometimes greenish color, translucent when pure,
with a greasy feeling, and fusing at 56° to 63° F.; specific
ROCKS FORMED AS SEDIMENTARY DEPOSITS 129
gravity, 0.955. It is essentially a natural paraffin. The name
is derived from two Greek words, signifying to smell, and wax,
Below is given the composition of (I) samples from Utah, and
(II) from Boryslaw, in Galicia.
CONSTITUENTS
I
II
Carbon .
85.47 %
85.78%
Hydrogen
14.57
14.29
100.04 %
100.07 %
The substance is completely soluble in boiling ether, carbon
disulphide, or benzine, and partially so in alcohol.
Ozokerite occurs in the United States, in Emery and Uinta
counties, Utah, where in the form of small veins in Tertiary
rocks it extends over a wide area. It is also found in Galicia,
Austria, in Miocene deposits, in Roumania, Hungary, Russia,
and other Asiatic and European sources. As a rule the de-
posits are in beds of Tertiary or Cretaceous age. The Galician
deposits are the most noted of the above. According to
Boverton Redwood,1 the material occurs here in the form of
veins from the thickness of a few millimetres to some feet, and
is accompanied by petroleum and gaseous hydrocarbons.
The names scheerite, hatchettite, fichtellite, and konlite are
applied to simple hydrocarbons allied to ozokerite found in
beds of peat and coal, but so far as the writer is aware never in
such abundance as to be of commercial value.
The name retinite includes a considerable series of fossil
resins allied to amber, differing mainly in containing no suc-
cinic acid. They occur in beds of brown coal of Tertiary and
Cretaceous age. The so-called copalite, a hard brittle, clear
yellow, or brownish variety used in making varnishes, belongs
here.
2. ROCKS FORMED AS SEDIMENTARY DEPOSITS AND FRAG-
MENTAL IN STRUCTURE: CLASTIC
The rocks of this group differ from those just described in
that they are composed mainly of fragmental materials derived
from the breaking down of older rocks, or are but the more or
1 Jour. Soc. of Chem. Industry, February, 1892.
130 AQUEOUS ROCKS
less consolidated accumulations of organic and inorganic debris
from plant and animal life. The group shows transitional
forms into the last, as will be illustrated by certain of the lime-
stones and the quarzites. They are water deposits, and, as a
rule, are eminently stratified or bedded, although this structure
is not always apparent in the hand specimen.
As will be readily comprehended when one considers from
what a multitude of materials the fragmental rocks have been
derived, the amount of assorting, admixture with other sub-
stances, solution, and transportation by streams these materials
have undergone, they cannot be classified by any hard and fast
lines, but one variety may grade into another, both in texture
and structure as well as in chemical composition, almost indefi-
nitely. Indeed, many of them can scarcely be considered as
more than indurated muds, and only very general names can
be given them.
Accordingly as these rocks consist of mechanically formed
inorganic particles of varying composition and texture, or of
the more or less fragmental debris from plant and animal life,
they are here divided into two main groups, each of which is
subdivided as below : —
I. Rocks formed by mechanical agencies, and mainly of in-
organic materials.
(1) The Arenaceous group — Psammites: Sand, gravel, sand-
stone, conglomerate, and breccia.
(2) The Argillaceous group — Pelites : Kaolin, clay, wacke,
shale, clayey marl, argillite.
(3) The Calcareous group : — Arenaceous and brecciated
limestones. The rocks of this group are often in part organic,
and in part chemical deposits. Only those are considered
here in which the fragmental nature is the most pronounced
characteristic.
(4) The Volcanic group : — Fragmental rocks composed
mainly of ejected volcanic material : Tuffs, lapilli, sand and
ashes, pumice-dust, trass, peperino, pozzuolano, etc.
II. Rocks formed largely or only in part by mechanical
agencies and composed mainly of the debris from plant and
animal life — Organagenous.
(1) The Siliceous group — Infusorial earth.
(2) The Calcareous group — Fossiliferous and oolitic lime-
stone, marl, shell-sand, shell-rock.
PLATE 11
FIGS. 1 and 2. Shell limestones. FIG. 3. Crinoidal limestone.
ARENACEOUS ROCKS: PSAMMITES
131
(3) The Carbonaceous group — Peat, lignite, coals, oil shale,
etc.
(4) The Phosphatic group — Phosphatic sandstone, guano,
coprolite nodules.
(1) ROCKS COMPOSED MAINLY OF INORGANIC MATERIAL
(1) The Arenaceous Group: Psammites. — Arenaceous, from
the Latin arenaceous, sandy or sand-like ; psainmite from the
Greek T/ra/z/uT?;?, sandy.
These rocks are composed mainly of the siliceous materials
derived from the disintegration of older crystalline rocks and
which have been rearranged in beds of varying thickness
through the mechanical agency of water. They are, in short,
more or less consolidated beds of sand and gravel. In composi-
tion and texture, they vary almost indefinitely. Many of them
having suffered little during the process of disintegration and
transportation, are com-
posed of essentially the
same materials as the
rocks from which they
were derived. Others,
in which the fragmental
materials suffered more
prior to their final con-
solidation, have had the
softer and more soluble
minerals removed, leav-
ing the sand composed
mainly of the hard, al-
most indestructible min-
eral quartz.
In structure, the sand-
stones also vary greatly,
in some the grains being rounded, while in others they are
sharply angular. Figure 9 shows the microscopic structure of
a brown Triassic sandstone from Portland, Connecticut.
The material by which the individual grains of a sandstone
are bound together is as a rule of a calcareous, ferruginous, or
siliceous nature ; sometimes argillaceous. The substance has
been deposited between the granules by percolating water or
FIQ. 9. — Microstructure of sandstone,
Portland, Connecticut.
132
AQUEOUS ROCKS
during the process of sedimentation, and forms a natural
cement. It sometimes happens that the siliceous cement is
deposited about the rounded grains of quartz in the form of a
new crystalline growth, converting the stone into quartzite ;
such are in this work classed with the crystalline rocks.
Upon the character of this cementing material and the close-
ness with which the grains are bound together, is very largely
dependent the power of the stone to resist disintegration under
the trying action of percolating carbonated waters and the
mechanical action of heat and frost. The calcareous, and to a
less extent the ferruginous cements are liable to removal in
solution, allowing the rock to fall away to sand, or at least
allowing it to absorb water, which, on freezing, brings about
the disintegration. The argillaceous cementing material, while
in itself inert, also permits a high degree of absorption, with
like results. Those sandstones cemented by silica, and which
therefore partake of the nature of quartzite (see p. 169), are
by far the more refractory.
The following analyses will serve to indicate the consid-
erable range in composition of rocks of this class : —
CONSTITUENTS
I
II
III
IV
Silica (Si02)
69.94%
84.40%
95.24%
90.86%
Alumina (AloOg)
13.15
7.49
0.56
4.76
Iron oxides (Fe203) and (FeO) . .
Manganese (MnO)
2.48
0.70
3.87
1.28
1.58
Lime (CaO)
3.09
0.74
1.40
0.15
Magnesia (MgO)
Trace
2.11
1.23
0.59
Potash (K20)
3.30
0.24
1.06
Soda (Na20)
5.43
0.56
0.45
Loss
1.01
0.56
Totals
99. 10 %
99.41%
99.27 %
99.45%
I. Brown Triassic sandstone : Portland, Connecticut. II. Gray sub-Carbo-
niferous sandstone : Berea, Ohio. III. Red Carboniferous sandstone : Anan,
Scotland. IV. Cambrian sandstone : Siskowit Bay, Wisconsin.
The table given on p. 166 will serve to show the close chemi-
cal relationship existing between many rocks of this group,
and their metamorphic equivalents.
The colors of sandstone are dependent upon a variety of
circumstances. The red, brown, and yellowish colors are due
ARENACEOUS ROCKS: PSAMMITES 133
to iron oxides in the cementing constituent. Some of the dark
colors are due to carbonaceous matter.
Many varieties of sandstone are popularly recognized. Cal-
careous, ferruginous, siliceous, or argillaceous sandstones are those
in which the cementing materials are of a calcareous, ferrugi-
nous, siliceous, or argillaceous nature. The name arkose is given
to a coarse feldspathic sandstone derived from granitic rocks,
with a minimum amount of loss of original material. Conglomer-
ate or pudding stone is merely a coarse sandstone ; it differs from
ordinary sandstone only as gravel differs from sand. Breccia
is a fragmental rock differing from conglomerate in that the
individual particles are sharply angular instead of rounded.
The term is made to include also certain volcanic rocks with a
brecciated structure. (See PL 4.)
G-reywacke or Grrauivacke is an old German name for brecci-
ated fragmental rocks made up of argillaceous particles. The
name is now little used. Other names, as flagstone, freestone, and
brownstone, are applied to such as are used for flagging or other
structural purposes. Itacolumite is a feldspathic sandstone, or
perhaps more properly quartzite, in which the feldspathic mate-
rial plays the role of a binding constituent to the quartz gran-
ules. The so-called flexible sandstone is an itacolumite from
which the feldspathic portions have been removed by decompo-
sition leaving the interlocking quartz grains with a small amount
of play between them. The rock is in no sense elastic, but
merely loose jointed.
The name greensand, greensand marl, and glauconitic sand are
given to a prevailing dull green, loosely coherent, clayey or
arenaceous deposit which owes its peculiarities to the presence
of the hydrous silicate of iron and potassium glauconite, but
which is variously contaminated with minute particles of quartz
and siliceous minerals such as feldspar, hornblende, augite,
garnet, epidote, tourmaline, zircon, and the iron ores, clay, rock
fragments, and particles of shells.
Beds of glauconitic sand are most abundant among terranes
of Cretaceous age, but are by no means limited to them, as has
been already intimated on p. 31. They are aqueous deposits,
formed during processes of slow sedimentation along coasts
receiving debris from the continental slopes and of a nature
such as is derived from the breaking down of granitic and other
feldspathic rocks. The depth at which such deposits form is
134
AQUEOUS ROCKS
naturally quite variable, but conditions most favorable to their
accumulation seem to lie just beyond the reach of wave agita-
tion and under a depth of 900 fathoms.
The following table of analyses of glauconitic marls is from
the Report of the Geological Survey of New Jersey, for 1893.
CONSTITUENTS
I
II
III
IV
VI
X
XI
XII
XIII
Phosphoric acid . .
Sulphuric acid . . .
Silica and sand . . .
Potash
01
lo
1.15
1.28
34.50
1.54
2.52
2.15
6.00
31.50
18.80
01
10
0.58
45.50
3.79
1.51
2.20
5.80
24.50
15.40
01
lo
1.61
2.40
55.69
5.27
0.65
0.79
6.61
21.63
8.85
01
10
1.14
0.14
38.70
3.65
9.07
1.50
10.20
18.63
10.00
6.14
01
lo
0.84
0.12
52.07
6.46
1.01
1.53
6.96
21.55
9.31
<y
lo
0.19
0.41
51.15
7.08
0.49
2.02
8.23
23.13
6.67
o/
10
0.50
0.34
47.50
5.29
0.56
2.70
8.60
20.52
13.57
01
lo
6.87
3.12
44.68
3.97
4.97
2.97
6.04
18.97
8.63
Of
10
3.73
2.44
49.68
4.98
4.14
0.47
?
28.71
6.54
Lime
Magnesia
Alumina
Oxide of iron ....
Water .
Carbonic acid . . .
Carbonate of lime .
99.43
99.18
102.40
99.16
99.85
99.37
99.58
99.32
99.69
I. Clay marl, from near Mattawan. II. Clay marl, from Matchaponix
Creek, three miles south of Spottswood. III. Lower marl, from Navesink
Highlands. IV. Lower marl, from north shore of Navesink River, at Red Bank.
VI. Lower marl, from northwest slope of Mount Pleasant Hills. X. Middle
marl, from near Eatontown. XI. Middle marl, from southeast of Freehold.
XII. Upper marl, from Poplar. XIII. Upper marl, from Shark River.
The most extensive and best known deposits in the United
States are included in what are known as the Upper, Middle,
and Lower marl beds of the Cretaceous formations in south-
eastern New Jersey, and which has been very thoroughly
described in the various reports of the State Survey.1 The
marl is somewhat variable in different localities, but may in a
general way be described as a dull green, arenaceous deposit of
such consistency as to be easily removed by the shovel alone,
or pick and shovel. The beds vary from 30 to 60 feet in thick-
ness, but the glauconitic layers are not uniformly distributed
through it. Through weathering, the ferruginous constituents
become more highly oxidized, and the color changed from dull
green to red and yellow.
1 The reader is especially referred to Professor W. B. Clarke's paper on " The
Cretaceous and Tertiary Formations of New Jersey," in the Ann. Rep. State
Geologist of New Jersey for 1892.
ARGILLACEOUS ROCKS: PELITES 135
Rocks belonging to the arenaceous group are world wide
in their distribution, covering not infrequently thousands of
square miles of territory to depths, it may be, of thousands of
feet. They are, in some of their varieties, among the most
common and wide-spread of materials. Being themselves the
products of disintegration and decomposition of pre-existing
rocks, and having become consolidated under conditions not
greatly different from those now existing at or near the surface
of the earth, the rocks of this group are as a whole in a state of
comparatively stable chemical equilibrium. Unless including
calcareous matter or readily oxidizing ferruginous compounds,
such are subject to disintegration more through physical than
chemical agencies, as will be noted later.
(2) The Argillaceous Group : Pelites. — The rocks of this
group are composed of more or less hydrated aluminous sili-
cates admixed in almost indefinite proportions with siliceous
sand, various silicate minerals in a more or less fragmental and
decomposed condition, and calcareous and carbonaceous matter.
In their least consolidated form they are best represented by
the common plastic clays used for brick and pottery manufac-
ture. Such, although alike in their general physical or even
ultimate chemical nature, have widely diverse origins. In fact,
the term clay, like silt, indicates physical condition rather than
chemical or mineralogical composition, and it may perhaps be
defined as an indefinite admixture of more or less hydrated
aluminous silicates, free silica, iron oxides, carbonates of lime,
and various silicate minerals which in a more or less decom-
posed and fragmental condition have survived the destructive
agencies to which they have been subjected. About the only
feature characteristic of all clays, is that of plasticity, when
wet, and this is dependent, apparently, wholly upon texture
and structure, i.e. upon the size and shape of the individual
particles. ' Pure quartz, chalcedony, flint, feldspar, or other
silicates, will, when reduced to an impalpable powder, possess
the plasticity and even odor usually ascribed to clay, and in the
pages following, the term is used only with reference to degree
of comminution, regardless of mineral nature or chemical com-
position. It includes residual products of any or all forms of
rock degeneration, and which may or may not have been re-
assorted through the agency of water. (See further under The
Regolith, Part V.) The oft-repeated statement that kaolin
136
AQUEOUS ROCKS
forms the basis of clays, or that clay is impure kaolin, is there-
fore to a certain extent misleading, and if accepted at all it
must be with the reservations made by Johnson and Blake,1
who limit the term kaolin itself to the impure material, quite
distinct from true kaolinite, which is a definite chemical com-
pound corresponding to the formula H4Al2Si2O9.
Throughout the glaciated region of the northeastern United
States the clays are mostly glacial or water deposits from the
floods of the Champlain epoch. The latter are often beauti-
fully and evenly stratified, as shown in the illustration on PL 24.
The plastic clays and siliceous sands about Woodbridge, New
Jersey, are regarded as derived from the Azoic rocks and
deposited by sea-water in enclosed basins. The exact source of
the material is not always apparent ; the porcelain clays of Law-
rence County, Indiana, on the other hand, are residual deposits
resulting from the decomposition of impure Carboniferous
(Archimides) limestones, the lime carbonate being removed in
solution, while the less soluble clay remains. Kaolin, as already
noted, is a residual deposit from the decay of feldspathic and
other aluminous rocks, while the ordinary brick and tile clays
of the Southern states, as well as the clayey soils, are residual
aluminous deposits resulting from the decay and leaching out
of soluble constituents from a variety of rocks, both sedimentary
and eruptive. (See chapter on rock weathering.)
As showing the comparative compositions of kaolins and
clays, the following table is given : —
CONSTITUENTS
I
II
III
IV
V
VI
Si02 (combined) . . .
Si02 (free)
A1208
46.4%
397
39.00%
3600
34.70%
12.20
31.34
28.30 %
27.80
27.42
42.71 %
0.70
39.24
J60.97%
26.38
H20 (combined) . . .
H20 at 212°
13.9
14.00
950
12.00
8 00
6.60
2 90
13.32
1.58
} 8.93
CaO and MgO ....
Alkalies
0.63
0 54
0.10
0 95
0.18
2 71
0.20
0 89
} 1.90
FegOs
0 16
2 68
0 46
146
99.00 %
99.67%
99.45 %
98.59%
99.10%
99.64%
I. Kaolin. II. Indianite, a white clay residual from St. Lawrence County,
Indiana. III. Potter's clay, from Pope County, Illinois. IV. Brick clay from
New Jersey. V. Fire clay from New Jersey. VI. Fire clay from Illinois.
1 Am. Jour, of Science, 1867, p. 351.
ARGILLITES AND SHALES
137
Amongst the older formations the clays have undergone
induration, giving rise to what are known as argillites, or if
fissile, dates or clay slates, such as are used for roofing and
similar purposes, the fissile property having been imparted by
pressure or shearing. Such forms pass by imperceptible gra-
dations into argillaceous schists which are classed with the met-
amorphic rocks. (See p. 170.) The argillites are, as a rule,
among the most indestructible of rocks, since they are them-
selves composed of the least destructible debris of pre-exist in^-
rocks. Their ultimate chemical composition is much like that
of the clays, and scarce any two samples will show similar
results when submitted to analysis. The table given below
shows the composition of some schistose argillites used for
roofing purposes from (I) Harford County, Maryland, (II)
Lancaster County, Pennsylvania, and (III) Llangynog, North
Wales.
CONSTITUENTS
I
II
III
Silica (SiOo)
68.37%
60.32%
60. 150 %
Sulphuric acid (H2S04)
0.22
Alumina (AlgOs)
21.985
23.10
24.20
Iron oxides (FeO) and (FeaOs) . .
Lime (CaO)
10.661
0.30
7.05
7.65
Magnesia (MgO)
1.203
0.87
Soda (NazO)
1.933
0.49
4.278
Potash (K2O)
3.83
Water (H20)
4.03
4.08
3.72
98.699 %
99.74%
99.998%
Shale is a somewhat loosely defined term, indicating struc-
tural rather than chemical or miueralogical composition. The
word is perhaps best used in its adjective sense, as a shaly
sandstone, or shaly limestone. By many authors it is used
with reference more particularly to thinly stratified or lami-
nated, clayey rocks. Many shales are but the finer, more fissile
portions of sandstone beds; such may represent the off-shore
or deep-water portions of arenaceous sediments, which, begin-
ning with gravels near the shore-line, become gradually finer
as the distance from the shore increases, passing through coarse
to finer sands and finally to sandy clays and silts as the water,
138
AQUEOUS ROCKS
through the lessening of its carrying power, lays down its load.
Or they may represent later stages in the cycle of sedimenta-
tion ; the finer silts brought down after erosion have so far
reduced the level of the land as to greatly diminish the currents
and consequent carrying power of the seaward-flowing streams.
Such beds, on consolidation, yield then what are commonly
known, in the order of their formation, as conglomerates, sand-
stones, shales and argillites, or clay slates, the shales occu-
pying, both in texture and composition, a position intermediate
between the argillites and sandstones.
The following table will serve to show the varying character
of the rocks included under this name. Those such as given
in columns I and II carry their sulphur in combination with
iron, as iron pyrites (FeS2). This, on decomposing, through
the action of meteoric waters, yields iron sesquioxides and sul-
phuric acid, the latter combining with a portion of the alumina
in the rock to form sulphate of aluminum, or common alum.
Hence they have been called alum shales.
CONSTITUENTS
I
II
ill
Silica (SiOo)
50. 13 %
72.40%
66.96 % .
Alumina (A^Os)
10.73
16.45
15.626
Iron sesquioxide (Fe2O3)
5.78
1.05
8.38
Lime (CaO)
0.40
0.17
0.493
Magnesia (MgO)
1.00
. 1.48
0.677
Potash (K2O)
5.08
3.295
Soda (Na20)
0.53
0.628
Sulphur (S)
4.02
1.21
Carbon (C)
22.83
Undet.
3.787
Water (H2O) 1
2.21
Undet.
Phosphoric acid (P20s)
0.154
I. An alum shale from Garnsdorf, near Saalsfeld. II. An alum shale from
Bornholm. III. A "marly shale " from Breckenridge County, Kentucky.
The name till or boulder clay is given to a sandy clay of
glacial origin and consisting of the usual indefinite mixture.
Professor W. O. Crosby, who has studied the composition of
the normal till of the Boston Basin, reports it as composed,
exclusive of the larger pebbles, of "about 25%, or one-fourth,
of coarse material which may be classed as gravel ; about 20 %•>
1 Ignition.
CALCAREOUS FRAGMENTAL ROCKS 139
or one-fifth, of sand; 40 to 45 % of extremely fine sand, or rock
flour, and less than 12 % of clay." 1
Laterite is a red, ferruginous residual clay found in tropic
and semitropic regions. (See p. 310.) Catlinite, or Indian
pipe-stone, is an indurated clay rock formerly used by the Da-
kota Indians for pipe material. The name porcellainite has
been given to a compact porcelain-like rock consisting of clay
indurated by igneous agencies. The name wacke is sometimes
used to designate an earthy or compact, dark-colored clayey
material resulting from the decomposition in situ of basaltic
rocks. Adobe is the name given to a calcareous clay of a
general gray-brown or yellowish color, very fine grained and
porous, and which is widely distributed throughout the more
arid regions of the West. It is described in greater detail
under the head of soils (p. 333). Loess is a somewhat similar
material forming the surface soil over wide areas in the Missis-
sippi valley, and at times sufficiently plastic for brick making.
(See also p. 327.)
(3) The Calcareous Group. — Here are brought together a
small series of fragmental rocks composed mainly of calcareous
material, but of which the organic nature, if such it had, is not
apparent. These rocks form at times beautifully brecciated
marbles. Their structure may be best comprehended by remem-
bering that the original beds, whether crystalline or amorphous,
whether fossiliferous or originating as chemical precipitates,
have by geological agencies been crushed and shattered into a
million fragments, and then, by infiltration of lime and iron-
bearing solutions, been slowly cemented once more into solid
rock. The composition is essentially the same as the ordinary
sedimentary limestones and need not be further dwelt upon
here. It may be stated, however, that owing to the softness
and ready solubility of their materials limestones do not, on
breaking down, except under rare instances, give rise to exten-
sive beds of arenaceous rocks, as do the siliceous varieties.
One of the best known rocks of this group is the breccia marble
near Point of Rocks in Maryland, which has been used in the
United States Capitol building at Washington.
(4) The Volcanic Group : Tuffs. — Under this head -are in-
cluded a great variety of fragmental rocks, composed of the
more or less finely comminuted materials ejected from vol-
1 Proc. Boston Society of Natural History, Vol. XXV, 1890, p. 123.
140 AQUEOUS ROCKS
canoes as ashes, dust, sand, and lapilli. Some of them are made
up of minute shreds of pumiceous glass. These occur, in many
instances, interbedded with lava flows of the same lithological
nature, and which are a product of the same periods of vol-
canic activity, the eruption of molten lava being accompanied
by intervals of explosive action, during which only fragmental
material was ejected. To such materials the name pyroclastic
(Greek irvpos, fire) is appropriately given.
The lithological character of the materials varies almost
indefinitely, and only very general names are given them in
the majority of cases. The name tuff or tuffa is given to the
entire group of volcanic materials formed as above, and also
by some authorities to fragmental rocks resulting from the
breaking down and reconsolidation of older volcanic lavas.
It would seem advisable to designate these last, as has F.
Lowinson-Lessing,1 as pseudotuffs or tuffoids.
The names volcanic ashes, sand, and dust are applied to
the finer of these volcanic materials, and lapilli or rapilli to the
coarser fragments.
The dusts and sands are not infrequently composed of
minute shreds of volcanic glass, which were blown from the
volcanic vents and carried unknown distances, to be ultimately
deposited as stratified beds in comparatively shallow water.
Such are described more in detail under the head of ^Eolian
rocks (p. 153). The term trass is used to designate a compact
or earthy fragmental rock composed of pumice dust, in which
are embedded fragments of trachytic and basaltic rocks, car-
bonized wood, etc., and which occupies some of the valleys of
the Eifel. Peperino is a tufaceous rock composed of fragments
of basalt, leucite, lava, and limestone, with abundant crystals
of augite, mica, leucite, and magnetite. It occurs among the
Alban Hills, near Rome, Italy. Palagonite tuff is composed of
dust and fragments of basaltic lava, with pieces of a pale yellow,
green, reddish, or brownish glass called palagonite. The general
name of volcanic mud is given to the finely comminuted volcanic
material which in a more or less pasty or liquid condition is thrown
from volcanic vents during the incipient stages of eruption.
The tuffs are as a rule more or less distinctly stratified, of
very uneven texture, and with rarely a pisolitic structure.
They are found associated with volcanic rocks of all ages, and
i Tschermaks Min. u. Petrog. Mittheilungen, Vol. IX, 1889, p. 530.
VOLCANIC TUFFS
141
at times so highly metamorphosed as to render the original
nature of some doubt. Certain English authorities have eon-
tended that a part of the so-called argillites and fire clays were
of finely comminuted volcanic materials.
The composition of the tuffs naturally varies with that of the
character of the lava from which they were derived. Being
in a more or less finely comminuted condition, often porous
and readily permeated by water or rootlets, they undergo de-
composition, forming soils the character of which is dependent
to some extent upon their lithological nature. The following
table shows the varying composition of rocks of this class : —
cf
a-«
i~
0
|
^
a"
l
KINDS AND LOCALITIES
!£•
= q.
°3>
2,
If
M
5,
^
5
8
= <
f^
a
1*
3
CS
•o
I
|«
3
9
~7.
<~
£~
a
— ^r
&
5
ii
?
01
10
%
%
%
%
%
%
%
Pozzuolana, Naples,
Italy ....
59.144
21.28
4.76
1.90
....
4.37
6.23
....
100.24
Tuff, Crater of
.M<>nte Nuova,
Chlorine
Italy ....
56.31
15.23
7.11
1.74
1.36
6.54
4.84
6.12
0.27
100.22
Trass, Andernach,
' . '
Prussia. . . .
54.00
16.50
6.10
4.00
0.70
10.00
7.00
....
99.00
Tuff, Lacher See,
I'russia . . .
60.49
19.95
9.37
3.12
1.43
3.40
1.33
99.09
(2) ROCKS COMPOSED MAINLY OF DEBRIS FROM PLANT AND
ANIMAL LIFE
(1) The Siliceous Group : Infusorial or Diatomaceous Earth.—
This is a fine white or pulverulent rock, composed mainly of
the minute shells, or tests, of diatoms, and often so soft and
friable as to crumble readily between the thumb and fingers.
It occurs in beds which, when compared with other rocks of
the earth's crust, are of comparatively insignificant proportions,
but which are nevertheless of considerable geological impor-
tance. Though deposits of this material are still forming, and
have been formed in times past at various periods of the earth's
history, they appear most abundantly associated with rocks
belonging to the Tertiary formations.
The beds are wide-spread, and some of them of economic
importance as a source of tripoli, absorbents for nitro-glycerine
142
AQUEOUS ROCKS
compounds, non-conducting materials, etc. A deposit in Biln,
Bohemia, is some 14 feet in thickness, and is estimated by
Ehrenberg to contain 40,000,000 shells to every cubic inch.
Beds occur in the United States at South Beddington, Maine ;
FIG. 10. — Section through lake basin showing the formation of infusorial earth,
a, bed rock; bb, floating peat; cc, decayed peat; d, infusorial earth.
Lake Umbagog, New Hampshire ; in Morris County, New Jer-
sey ; near Richmond, Virginia ; in Calvert and Charles coun-
ties, Maryland; in New Mexico; Graham County, Arizona; near
Reno, Nevada, and in various parts of California and Oregon.
The New Jersey deposit covers about 3 acres, and varies
from 1 to 3 feet in thickness ; the Richmond bed extends from
Herring Bay, on the Chesapeake, to Petersburgh, Virginia, and
is in some places 30 feet in thickness ; the New Mexico deposit
is some 6 feet in thickness and has been traced some 1500 feet.
Professor Leconte states that near Monterey, in California, is
a bed some 50 feet in thickness, while the geologists of the
Fortieth Parallel Survey report beds not less than 300 feet in
thickness, of a pure white, pale buff, or canary-yellow color,
as occurring near Hunter's Station, west of Reno, Nevada.
The earth is used mainly as a polishing powder, and is some-
times designated as tripolite. It has also been used to some
extent to mix with nitro-glycerine in the manufacture of dyna-
mite. Chemically the rock is impure opal, as will be seen from
the following analyses made on samples from (I) Lake Umba-
gog, New Hampshire, (II) Morris County, New Jersey, and
(III) Paper Creek, Maryland: —
CONSTITUENTS
I
II
ill
Silica (Si02)
80 53 %
80.60 %
81.53%
Iron oxides (Fe2O3 and FeO) . . .
Alumina (AlaOa)
1.03
5.89
3.84
3.33
3.43
Lime (CaO)
0.35
0.58
2.61
Water (H2O)
11 05
14.00
6.04
Organic matter
0 98
99.38%
99.02%
96.94%
Number III showed also small amounts of potash and soda.
PLATE 12
••••••• >.,v>.
FIG. 1. Pisolitic limestone.
FIG. 2. Oolitic limestone.
LIMESTONES . 143
(2) The Calcareous Group. — These rocks are made up of the
more or less fragmental remains of molluscs, corals, aud other
marine and fresh-water animals. Many of them are but con-
solidated beds of calcareous mud, full of more or less fragment-
ary shells or casts of shells, as shown in Fig. 1, PL 11. The
name coquina (Spanish for shell) is given to such as that
shown in Fig. 2, PL 11, from St. Augustine, Florida. The
rock, it will be observed, is composed almost wholly of very
perfect shells of a bivalve mollusc, loosely cemented by calcare-
ous materials in a finely divided condition. From such forms
as these we have all possible gradations to compact crystalline
limestone. Special names are often given these calcareous
rocks, designating the character of materials from which they
are derived. Coral and shell limestones, as the names denote,
are composed mainly of the debris from these organisms. In
like manner such names as crinoidal, fusulina, etc., are applied.
Lumachelle is the name given to a shell limestone from the
Tyrol, in which the shells still retain their pearly lining and
original beauty. Nummulitic limestone carries fossil nummulites.
Rocks of this type were used in the construction of the pyramids
of Cheops. Chalk is a fine-grained, white, pulverulent rock,
composed of finely broken shells of marine molluscs, among
which minute foraminifera are abundant. Shell sand is a
loose aggregate of shell fragments, formed on sea-beaches by
the action of the winds and waves. On certain Hawaiian
beaches, such sands give out a distinct note, or peculiar crunch-
ing sound when walked over, or even when shaken in a closed
vessel, and are popularly known as sounding, or singing, sands.
The property is manifested only when the sand is dry and is
assumed to be due to the minute air cavities enclosed by the
shells. Oolitic and pisolitic limestones, as previously noted, are
made up of rounded concretionary masses of calcium carbonate,
and are in part of mechanical origin, and in part chemical de-
posits (PL 12).
The microscopic structure of an oolitic limestone from Prince-
ton, in Caldwell County, Kentucky, is shown in the accompany-
ing figure (p. 144). It will be noticed that the first step in the
formation of this stone was the deposition of concentric coat-
ings of lime about a nucleus which is sometimes nearly round,
but more frequently quite angular and irregular. After the
concretions were completed there were formed in all cases about
AQUEOUS ROCKS
each one, narrow zones of minute radiating crystals of clear,
colorless calcite; then the larger crystals formed in the inter-
stices. The nuclei are composed in some cases of single frag-
ments or, again, of a group of fragments. Certain of the oolites
present no distinct concentric structure, but appear as mere
rounded masses merging gradually into the crystalline inter-
stitial portions. Recent microscopic studies have tended to
show that many of the oolitic limestones owe their structure to
the lime-secreting power of microscopic algae.1
Limestones vary almost indefinitely in structure and color.
From the soft tufaceous or highly fossiliferous varieties there
is a constant gradation
to dense compact rocks
breaking with a conchoi-
dal or splintery fracture
and the true nature of
which is sometimes to be
ascertained only by chem-
ical tests. There is a like
variation in color. White
through all shades of gray
to black is common, and
more rarely occur yellow,
brown, pink, or red vari-
eties, the colors depend-
ing on organic matter and
metallic oxides, mainly
ferruginous.
Owing to the readiness with which calcium carbonate
undergoes crystallization, even at ordinary temperatures, few
limestones are wholly amorphous, but grade insensibly into
holocrystalline varieties such as are classed with the metamor-
phic rocks. The name marble is given to such limestones as
are of sufficiently close texture to take a polish and of such
colors as to make them desirable for ornamental work. A large
proportion of the marbles belong, however, to the metamor-
phic group. (See p. 162.) Figure 12 shows the microscopic
structure of a dark gray, variegated, highly fossiliferous lime-
stone belonging to the Cincinnati group, near Hamilton, Ohio.
It is a natural result of their method of formation that few
1 American Geologist, Vol. X, No. 5, 1892.
FIG. 11. — Microstructure of oolitic limestone.
LIMESTONES
145
limestones are of pure calcium carbonate. A portion of the
calcium is not infrequently replaced by magnesium, giving rise
to magnesian limestones, or when the proportion of magnesia
rises to 45.65 % to dolomite. This last can as a rule be distin-
guished from limestone only by its increased hardness (3.5—4.5)
and specific gravity (2.8-2.95). Frequently chemical tests un-
necessary, limestone effervescing readily when treated with
dilute hydrochloric acid, while dolomite is unacted upon.
Mechanically included materials, as sand and clay, are com-
mon, giving rise to siliceous and argillaceous varieties. The so-
called hydraulic limestone
is one containing 10 %
and upwards of these
impurities, and which,
when burnt and ground,
forms a cement charac-
terized by its property
of setting under water.
Many limestones, like the
dolomitic varieties in
Cook County, Illinois,
contain so large a pro-
portion of bituminous
matter as to give off a
distinct odor of petro-
leum when struck with a
hammer, or even to be-
come blackened on the
surface by its exudation when exposed to the weather. Others
contain phosphatic matter, and pass by insensible gradations
through what are known as phosphatic limestones to true phos-
phates (phosphorites, etc.).
In chemical composition the limestones vary, like other sedi-
mentary rocks, almost indefinitely, as will naturally be inferred
from what is said above. As a general rule, those varieties,
which have been formed in deep waters and at a distance from
the shores, will be of greatest purity, since less likely to have
become contaminated through detrital materials washed in from
the land. Even these may, however, be intermingled to a very
considerable extent with the fine siliceous and ferruginous mat-
ter, such as deep-sea dredgings have shown to be common to
Fio. 12. — Microstructure of fossiliferous
limestone.
146
AQUEOUS ROCKS
our modern sea-bottoms, and which are assumed to be in part at
least of volcanic origin. (See under JEolian Rocks, p. 153.) The
following table will give some idea of the wide range in chemi-
cal composition to be found in rocks of this class : —
17,
s
Sii
i
C i
'«•?>'"
3 € .
K c c
c c
s .=-£~
a**
CONSTITUENTS
>,
5 --a
{•»«f g"i-
III*
l!--
o~ «*
'3hJ">.
C3 - /.
| .
111!
Sill
III!
U- ^
Ill
Carbonate of lime (CaC03) . . .
98.00 %
54.62 %
41.48 %
72.95 %
96.60 %
Carbonate of magnesium (MgCOs)
45.04
24.55
3.84
0.13
Oxides of iron (FeO and Fe203) .
Oxide of aluminum (A1203) . .
J0.23
J4.03
1.34
4.50
0.98
Silica (Si02) and insol. silicates .
0.57
29.93
14.79
0.50
Potash (K20)
1.22
0.31
Soda (Na2O)
1.12
0.40
Water (H20)
0.96
Sulphate of lime (CaS()3) . . .
1.75
Organic matter
1.46
Totals
98.57 %
99.89%
98.33 %
100.64 %
99.88%
Researches by the Kentucky Geological Survey have shown
that the older limestones are, as a general rule, richer in soda,
phosphoric acid, and, when non-magnesian, in lime carbonate,
than are the younger more recently formed, and correspondingly
poorer in silica and insoluble silicates. This inverse ratio is
shown in the table on the opposite page, in which the rocks are
arranged by geological horizons, the oldest at the bottom.
The name shell marl, or merely marl, is given to an illy defined,
often arenaceous, soft and earthy rock consisting essentially of
shell material in a more or less fragmented condition, and usu-
ally intermixed with more or less clayey matter or siliceous
sand and silt. Geikie l would limit the term to fresh-water
accumulations of remains of mollusca, entomostra , and fresh-
water algse, but unfortunately the word has not been so used
in much of the literature extant. These marls, being easily
decomposed, and on account of their occasional richness in
phosphoric acid, or, perhaps, merely on account of the lime
they contain, are of value as fertilizers. The following analy-
ses of North Carolina marls, consisting largely of comminuted
1 Text-book of Geology. 3d ed.
COMPOSITION OF LIMESTONES
147
-nig ONY Yomg
0° *
o o^
Ci O
c; co
o o
O t—
Ci O
%
0 O
o o
cs
co o o
r— 00 •**
o oo co
Ci ^
S2
:N
oo ss
1— 1
1— I
oo °
co
«> 1-1 •*
YOOg
— 2J
— t—
. o
CO CS
co
o co
:r =r
o
00 O .
0 X ^
HSYXOJ
QO CD
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aioy oiHiiH.ri.)s
3§o eo
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. o
K. CO
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1
:2 v — *
~ X ~ A
Co'd)
aiay . ii:n m. IM .11,1
<N CO T-H O
iQ 5H «— i CO
0
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s
co
o oo
O CO
os
00
p~" ^j —
0 d
§
eaaixo ssaNvo
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o00 °*
is
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co cs
co
2S
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-IIYQ
axYjjoa
vieaKovj^
0 CO
SI
II
§05
co
1-1 CS
I
-H cr
o o
o --
CO
cs
00
--(NO
0 O CS
OS CO
cs cs
1— 1
r- 1 CS
cs
^- 1-1
1-1
cs^cs
HXVN
-OHHY3 aw ii
00 •*
= ° S
§*
<^
co o
O CO
O3 t^-
Ci tC
CS I-H
1
o cs
00 rH
1-1 CO
(O
os
co O r-
CO 01 i-
s %
u-5 10
%s
•Tl Ci
§
-8
8
-3§§
AXI
-AYHQ •! 11 • i.!..
o : I
*0 CO
1
Si!
H H
ww
a
-^ —
XX
fc*
525
525525
<n
O
H
co
H
i
13
^% '
=
i
a .
31 03
o
u
Average composition of 0 Coal-measure limestone
Average composition of 10 upper sub-Carbonifen
limestones .
I
-
-
~i.
>H
9
_r
£'
E
limestones (hydraulic)
Average composition of (i Black Slate limestones
Average composition of 4 Carboniferous limeston
of which 2 are magnesian (hydraulic ?) .
and 2 are non-magnesian
Average composition of 14 Niagara group (or Up
Silurian) limestones
Average composition of 3 Clinton group limeston
Average composition of 3 upper Hudson group Hi
stones ,
Average composition of 7 middle Hudson grc
("siliceous mudstone")
Average composition of 9 lower Hudson limestom
Average composition of 7 Trenton limestones (n
magnesian)
Average composition of 1 1 Trenton limestones (m
nesian)
Average composition of 2 Bird's-eye limestones
Average composition of 3 Chazy limestones
H
148
AQUEOUS KOCKS
shells and sometimes coprolite nodules, will serve to show the
widely varying character of the materials grouped under this
name.1
CONSTITUENTS
I
II
III
IV
V
VI
VII
%
6.97
%
61.61
01
10
18.84
01
10
58.25
01
10
25.28
%
on o^>
oy.oo
01
In
5.65
Oxide of iron and alumina .
0.86
47.62
2.80
19.60
2.72
41.48
11.28
13.49
3.02
37.52
3.47
28.96
3.30
48.51
1.03
0.12-
0.16
1.96
Potash
0.37
0.56
0.22
0.75
0.23
Soda .
0.15
0.09
0.25
0.17
0.30
Phosphoric acid
0.19
0.18
0.40
0.11
trace
Sulphuric acid
0.41
0.06
0.64
0.40
0.18
0.31
Carbonic acid
38.15
15.37
32.07
10.59
29.02
22.73
39.80
Organic matter and water .
4.25
3.42
2.98
4.11
0.60
(3) The Carbonaceous Group : Peat, Lignite, and Coal. — Here
are included a variety of more or less oxygenated hydrocarbons
varying widely in physical and chemical properties, but alike
in originating from decomposing plant growth protected from
the oxidizing influences of the air. Plants, when decomposing
upon the surface of the ground, give off their carbon to the
atmosphere in the shape of carbonic acid gas (CO2), leaving
only the strictly inorganic or mineral matter behind. When,
however, protected from this oxidizing influence by water, or
other plant growth, decomposition is greatly retarded, varying
portions of the carbonaceous and volatile matters are retained,
and the material becomes slowly converted into coal. Accord-
ing to the amount of change that has taken place in the original
plant material, the amount of volatile matter still retained by it,
its hardness and burning qualities, several varieties are recog-
nized, which, however, pass into each other by insensible gra-
dations.
Peat is the plant matter in its least changed condition.
It results from the gradual accumulation in bogs and marshes
of growths consisting mainly of sphagnous mosses, a low order
of plants having the faculty of continuing in growth upwards
as they die off below. In this way the deposits often assume
a very considerable thickness. Where sufficiently thick, the
1 Geology of North Carolina, Vol. I, 1875, p. 105.
PEAT AND LIGNITE
149
lower portions have sometimes been converted into a dense
brownish black mass somewhat resembling true coal. The
deposits of peat are all comparatively recent and occur only
in humid climates. They are developed to an enormous
extent in Ireland — about one-seventh of the entire country
being covered by them — and average in some cases 25 feet
in thickness. They are also abundant in Europe and various
parts of North America. In Europe, and especially in Ireland,
the material is extensively utilized for fuel, and there would
seem no good reason for not so utilizing it in America. An
impure variety containing a considerable quantity of siliceous
sand, and locally known as " muck," is used as a fertilizer and
for " multching " throughout New England. Below are given
the results of analyses of (I) peat from the bog of Allan,
Ireland, (II) Maine (United States), and (III) Commander
Islands in Bering Sea.
CONSTITUENTS
I
II
ill
Carbon
61.04%
21.00%
60 48 %
Volatile matter
37.53
72.00
39.53
Ash
1.83
7.00
3.30
Liynite, or brown coal, is the name given to a brownish black
variety characterized by a brilliant lustre, conchoidal fracture,
and brown streak. Such contain from 55 % to 65 % of carbon,
and burn easily, with a smoky flame, but are inferior to the
true coals for heating purposes. They are also objectionable
on account of the soot they create, and their rapid disintegra-
tion and general deterioration when exposed to the air. They
occur in beds under conditions similar to the true coals, but
are of more recent origin. The lignitic coals of the regions of
the United States, west of the Mississippi River, are mainly of
Laramie (Upper Cretaceous) age, and, as a rule, show easily
recognizable traces of their organic origin, such as compressed
and flattened stem and trunks of trees with traces of woody
fibre.
Bituminous Coal. — Under this name are included a series of
compact and brittle products in which no traces of organic
remains are to be seen on casual inspection, but which, under
150
AQUEOUS ROCKS
the microscope, often show traces of woody fibre, spores of
lycopods, etc. These coals are usually of a brown to black
color, with a brown or gray brown streak, breaking with a
cubical or conchoidal fracture, and burning readily with a
yellow, smoky flame. They contain from 35 % to 70 % of fixed
carbon, 18 % to 60 % of volatile matter, and from 2 % to 20%
of water, and only too frequently show traces of sulphur due
to included iron pyrites. Several varieties of bituminous coals
are recognized, the distinctions being based upon their manner
of burning. Coking coals are so called from the facility with
which they may be made to yield coke ; such give a yellow
flame in burning, and make a hot fire. They are soft, and
break with a cubical fracture. Other varieties of apparently
the same composition and general physical properties, cannot,
for some unexplained reason, be made to yield coke, and are
known as non-coking coals. Oannel coal has a very compact
structure, breaks with a conchoidal fracture, has a dull lustre,
ignites easily, and burns with a yellow flame. It does not coke.
Its chief characteristic is the large amount of volatile matter
given off when heated, whereby it is rendered of particular
value for making gas. Before the discovery of petroleum it
was used for the distillation of oils. Below is given the com-
position of (I) a coking coal from the Connelsville Basin of
Pennsylvania, and (II) a cannel coal from Kanawha County,
West Virginia.1
CONSTITUENTS
I
II
Water
1.05%
Volatile matter
29.885
Fixed carbon
57.754
58.00%
Ash
9.895
23.50
Sulphur
1.339
18.50
100.00 %
100.00 %
Anthracite Coal. — This is a deep black, lustrous, hard and
brittle variety, and represents the most highly metamorphosed
variety of the coal series. Such have been generally regarded
as bituminous coals from which a very large proportion of the
1 F. P. Dewey, Bull. 42, U. S. National Museum, 1891.
THE PHOSPHATES 151
volatile constituents have been driven off by the agencies in-
volved in the production of mountain systems, or by the heat
incident to the injection of igneous rocks. Traces of organic
nature are almost entirely lacking in the matter of the anthra-
cite itself, though impressions of ferns, lycopods, sigillaria, and
other coal-forming plants are frequently associated with the
beds in such a manner as to leave little doubt as to their origin.
Anthracite is ignited with difficulty and burns with little flame,
but makes a hot fire. Below is given the average composition
of anthracite from the Kohinoor Colliery, Shenandoah, Penn-
sylvania.1
Water 3.163%
Volatile matter 3.717
Fixed carbon 81.143
Sulphur . . . ' 0.899
Ash 11.078
100.000 %
Like the other coals, anthracite occurs in true beds, but is
confined mostly to rocks of the Carboniferous age. Thin seams
of anthracite sometimes occur in Devonian and Silurian rocks,
but which are too small to be of economic value. Rarely
coals of more recent geological horizon have been found locally
altered into anthracite by the heat of igneous rocks. Through
a still further metamorphism, whereby it loses all its volatile
constituents, coal passes over into graphite.
The principal anthracite coal regions of the United States are
in eastern Pennsylvania. From here westward throughout the
interior states to the front range of the Rocky Mountains the
coals are all soft, or bituminous coals. Those of the Rocky
Mountain regions proper are largely lignitic, passing into the
bituminous varieties.
(4) Phosphatic Group : Phosphatic Sandstone ; Bone Breccia ;
Guano ; Coprolite Nodules. — This is a group of rocks limited in
extent, but nevertheless of considerable economic importance,
owing to the high values of certain varieties for fertilizing
purposes.
G-uano consists mainly of the excrements of sea fowls, and is
to be found in beds of any importance only in rainless regions
like those of the western coast of South America and southern
Africa. The most noted deposits are on small islands off the
1 Bull. 42, U. S. National Museum, 1891.
152 AQUEOUS ROCKS
coast of Peru. Immense flocks of sea fowls have, in the course
of centuries, covered the ground with an accumulation of their
droppings to a depth of sometimes 30 to 80 feet, or even more.
An analysis of American guano gave : Combustible organic
matter and acids, 11.3 % ; ammonia (carbonate, etc.), 81.7 % ',
fixed alkaline salts, sulphates, phosphates, chlorides, etc.,
8.1%; phosphates of lime and magnesia, 22.5%; oxalate of
lime, 2.6 % ; sand and earthy matter, 1.6 % ; water, 22.2 %
(Geikie).
Coprolite nodules are likewise the excrements of vertebrate
animals ; those among the Carboniferous shales of the basin of
the Firth of Forth are regarded as accumulated excretions of
ganoid fishes.
Phosphatic sandstones, as the name denotes, are arenaceous
rocks containing more or less phosphatic matter. Inasmuch
as the phosphatic material is derived largely by leaching and
segregation, these rocks have been already described under the
head of chemical deposits (p. 119). In the river beds of the
Carolinas are found rounded and nodular masses of this nature,
consisting of siliceous and calcareous sand, with embedded
bones, teeth of sharks, and other animal remains. Bone breccia
consists of fragmental bones of mammals cemented by argil-
laceous, earthy, or calcareous matter.
III. AEOLIAN ROCKS
This group comprises a small and comparatively insignificant
class of rocks formed from materials drifted by the winds, and
more or less compacted into rock masses. They are, as a rule,
of a loose and friable texture and of a fragmental nature.
Many of the volcanic fragmental rocks (tuffs) are grouped
here, their materials having been thrown from the volcanic
vents in small fragments and drifted long distances by wind
prior to falling upon the surface of the ground or into the
water for their final consolidation.
One of the most common results of wind action on the land
is the production' of sand-dunes — billowy masses of loose sand
which, like drifts of snow, though more slowly, gradually
change their outlines and creep onward under the restless goad-
ing of the wind.
Such, owing to their superficial nature, recent origin, and
loose state of consolidation, are considered more in detail in
the chapter on The Regolith, p. 299. On undergoing consoli-
dation, these dune sands may give rise to sandstones in many
instances indistinguishable from those of aqueous origin, though
less regularly bedded. The finely disintegrated shell and coral
material thrown up by the waves on the beaches of Bermuda
is caught up by the winds and drifted inland, forming hills
which, in some instances, are 250 feet in height. Being soluble
in the water from rainfalls, these become shortly reconsolidated
through the deposition of lime carbonate in the interstices of
the fragments, and form thus the drift rock which comprises a
large portion of the mass of the islands above tide level.
The finely comminuted materials ejected from volcanic vents
may be likewise transported by atmospheric currents and, far
from their source, again deposited in beds of no insignificant
proportions. These, on induration, give rise to fine-grained
tuffs, and, where the final deposition has taken place in water,
to distinctly laminated, fine white rocks the lithological nature
153
154
AEOLIAN ROCKS
of which can be made out only by means of the microscope.
Such are many of the Pliocene sandstones of Idaho and Mon-
tana.1 The following analyses of samples of tuffs from (I)
Marsh Creek Valley, Idaho ; and (II) Little Sage Creek, Mon-
tana, will serve to show their composition.
CONSTITUENTS
I
II
Ignition (H20)
7.60 %
7.62%
Oxide of iron and aluminium (FeoOg and A1203) .
Silica (Si02)
16.22
68.92
18.24
65.56
Lime (CaO)
1.62
2.58
Magnesia (MgO)
Trace
0.72
Soda (Na^O)
1.56
2.08
Potash (K2O)
4.00
3.94
99.92%
100.74%
1 On the Composition of Certain Pliocene Sandstones from Montana and
Idaho, Am. Jour, of Science, Vol. XXVII, 1886, p. 199.
IV. METAMORPHIC ROCKS
Before proceeding to describe in detail the metamorphic
rocks, it will be well to devote a brief space to a discussion
of the processes by which this metamorphism has been brought
about.
The word metamorphism as used in geology includes changes
in the structure of rocks induced through agencies in part
physical, and in part chemical, in their nature. It is, in fact,
a very general terra, and indicates any transformation taking-
place in the composition and structural features of rocks
of any kind, whether sedimentary or igneous, and from any
cause whatever. Rocks laid down in the form of sediments
may become so deeply buried as to be subject to intense lu-at
from the earth's interior, as well as to pressure from weight of
the overlying material. In this way, a partial or complete
fusion of the constituents takes place, which is followed by a
crystallization whereby the original fragnu'iital nature may be
wholly or in part obscured. This form of change is included
under the general name of regional metamorphism. In this
manner, it was once assumed, were formed the gneisses, a part
of the granites, and the vast series of crystalline schists and
calcareous rocks (marbles, etc.). It has, however, been shown
that the banded and foliated structure shown by gneisses and
schists is not in all cases necessarily an indication of an original
bedded structure, but may be due to pressure acting through-
out long periods of time, and accompanied by the heat thereby
generated. A common and readily understood illustration of
this principle of metamorphism by pressure is offered by the
roofing slates. • These, first laid down as fine silts, rarely show
their eminent cleavages whereby they are rendered so useful to
man, parallel to their original bedding, but inclined at any and
all angles thereto. In such cases the bedding is not infre-
quently indicated by the dark bands or " ribbons " which are
so evident on a split surface.
155
156 METAMORPHIC ROCKS
But it is not alone* the fragmental rocks which, thus become
schistose under pressure. Originally massive, igneous rocks,
in regions of profound disturbance have been found converted
into schistose aggregates, indistinguishable from rocks ordina-
rily assumed to be sedimentary. Thus the greenstone schists
of the Menominee and Marquette regions of Michigan have
been shown by Williams 1 to be highly altered eruptive rocks,
mainly gabbros, diabases, and diorites, originally massive, but
now foliated, schistose, and variously crumpled through the
squeezing- and shearing to which they have been subjected since
the period of their first extrusion. The changes in these and
similar cases, is rarely purely physical, though at times the
chemical alterations may be quite inconspicuous. The ulti-
mate composition of the rock may remain essentially the same,
while the method of combination of its various elements may
have undergone extensive alteration. Quartzes and feldspars
may be crushed and distorted, drawn out into lens-shaped and
variously elongated forms, while secondary minerals like feld-
spars, quartz, zoisite, garnet, hornblende, epidote, and the micas
may be abundantly generated.
One of the commonest results of pressure effects upon
igneous rocks is the conversion of augite or other minerals of
the pyroxene group into hornblendes. The coarse hypersthene
gabbro occurring about Baltimore is found locally altered into
a rock consisting essentially of a schistose aggregate of horn-
blende and plagioclase feldspars, or what, on mineralogical
grounds, might be classed as a diorite.2 The chemical compo-
sition in this case has undergone no appreciable change ; there
has been simply a molecular rearrangement of the particles.
In such cases proof of the character of the change that has
taken place is usually found in the fractured and otherwise
distorted condition of many of the constituent minerals, as well
as intermediate stages of alteration, whereby a residual augite
crystal is found enclosed in an envelope of secondary horn-
blende, as shown in Fig. 1, on p. 40. To the secondary
minerals formed in this way the technical name paramorphic
is applied. To such changes as are above described the name
dynamic metamorphism is given.
The protrusion of a mass of molten matter into the over-
1 Bull. 62, U. S. Geol. Survey, 1890.
2 Bull. 28, U. S. Geol. Survey, 1880.
CONTACT METAMORPHISM 157
lying strata may give rise to a series of changes differing from
the last in that they are due mainly to heat and to the chemical
action of accompanying vapors and solutions. Since these
changes are confined to limited areas along the line of the
contacts between the two bodies, they are defined as contact
metamorphisms. As illustrative of such changes, a few cases
may be described.
Near Gefrees, in Bavaria, an eruptive biotite granite has
been protruded into clay slates and phyllites. At the line
of contact both phyllites and slates are converted into a
hard, compact blue-black "hornfels" consisting of a crystal-
line granular aggregate of quartz, deep reddish brown inira
(biotite), a little muscovite and andalusite. This zone, some
120 paces in width, is succeeded by a second some 380 paces in
width in which the rocks are converted into andalusite mica
schists, and this by a third zone some 500 paces wide in which
the gradually failing energy was sufficient only to give rise to
a spotted mica schist (knoten schiefer), and lastly, a zone snmr
400 paces wide in which the clay slates has become converted
into a chiastolite schist, and the phyllites to a biotite-bearing
variety. In all these cases the chemical character of the rock
remains essentially the same. Through the metamorphosing
action of intruded basic rocks crystalline schists near Peekskill,
New York, have near the line of contact become puckered and
filled with lens-shaped eyes of quartz containing garnets and
other minerals, while crystals of staurolite, sillimanite, cyanite,
and garnet appear, the amount of change being directly propor-
tional to the nearness of the line of contact. At the contact
the schistose structure is almost completely obliterated and the
schists become hard and massive, appearing more or less fused
with the eruptive, and consist of a large number of minerals.
Briefly expressed, the progressive change, approaching the line
of contact, consists in a gradual decrease in the proportional
amount of silica and alkalies, with a corresponding increase in
iron and alumina, this being accompanied by a disappearance
of the quartz and muscovite and the development of biotite,
sillimanite, staurolite, cyanite, and garnet, as above mentioned.
Where limestones abound, they have become bleached and
rendered more closely crystalline, while a variety of meta-
morphic minerals, as lime-bearing pyroxenes, hornblendes,
zoisite, sphene, and scapolite have been developed.
158
METAMORPHIC ROCKS
A common form of metamorphism is manifested in the pro-
duction of a quartzite from siliceous sandstone. This, in its
simplest form, is brought about by a secondary deposit of silica
about the original rounded granules of sand, whereby the entire
mass is converted into an aggregate of quartz crystals, the out-
lines of which are more or less imperfect through mutual in-
terference in process of
growth. The microscopic
structure of a quartzite
of this nature is shown in
Fig. 13. In this case the
original rounded gran-
ules are readily recog-
nized from the fact that
not merely did they fre-
quently contain small
cavities and needle-like
enclosures, but exteriorly
they were covered with a
thin pellicle of iron ox-
ide, while the secondary
deposit, which now fills
all the interspaces, is free
from enclosures of all
kinds and quite pellucid.
In many quartzites a shearing force has acted a prominent
part, whereby the granules have become elongated and more or
less pulverized along their margins by the friction of rubbing
one over the other. In such cases mica and other secondary
minerals are often developed, and the rock passes over into a
mica schist.
Still another form of change, or metamorphism, is that
known by the name of metasomatosis, a process of indefinite
substitution and replacement. Through the chemical action
of percolating solutions certain constituents of a rock may be
leached out and replaced by others in indefinite proportions.
It is by such processes that have originated a large share of
the serpentinous rocks, dolomites, etc. The mineral olivine,
an anhydrous ferruginous silicate of magnesia, passes over into
serpentine by a simple process of hydration, and a more or less
complete change of its combined iron from the ferrous to the
FIG. 13. — Microstructure of quartzite, showing
secondary deposit of silica about the original
quartz grains.
METASOMATOSIS 159
ferric state ; this constituent not infrequently separating out
during the process of change, and crystallizing as magnetite,
or remaining as an amorphous hematite or limonite. Provided
there be no loss in silica, this change in the olivine, according
to T. Sterry Hunt, must be accompanied by an increase of
volume amounting to some 33%. Through the hydration of
eruptive olivine-bearing rocks, or rocks rich in other magnesian
silicate minerals, have originated a large proportion of the so-
called serpentines and verd-antique marbles. Many serpentines
and serpentinous limestones are derived from metamorphic
rocks rich in lime-magnesian pyroxenes or amphiboles, as mala-
colite and tremolite. To such an origin are to be referred
such serpentinous limestones as those of Essex County, New
York ; Easton, Pennsylvania, and Montville, New Jersey. In
the last-named instance the original rock was a coarsely crys-
talline dolomitic limestone containing numerous nodular masses
of white pyroxene (malacolite). Under this metasomatic pro-
cess they yielded up their calcium, which recrystallized as
calcium carbonate or calcite, while the silica and magnesia,
combined with some 13 % of water, remained as a beautiful
green and yellow serpentine. The transformation was accom-
panied by a considerable increase in bulk, whereby the exterior
of the nodules, pressed against the rough walls of the enclos-
ing rock, became scratched and polished like boulders from the
glacial drift, or the entire mass even took on a platy, schistose
structure. Figure 8, from a specimen in the National Museum,
illustrates a transitional phase of this change, the interior
rounded mass of a gray color being of still unaltered pyroxene,
while the dark material forming the exterior shell, or travers-
ing the gray in fine thread-like veins, is the secondary ser-
pentine. In a like manner in all probability originated the
peculiar structure imitative of animal organisms known as
Eozoon Canadense.1
The conversion of a limestone into a dolomite is believed to
have been brought about by a somewhat similar process. Indeed
it is doubtful if this last-named rock is ever a product of direct
sedimentation or precipitation. Although sea-water contains
1 See On the Serpentine of Montville, New Jersey, Proc. U. S. National
Museum, Vol. XI, 1888 ; Notes on the Serpentinous Rocks of Essex County,
New York, etc., ibid., Vol. XII, 1889; and On the Ophiolite of Thurman,
Warren County, New York, Am. Jour, of Science, Vol. XXXVII, 1889.
160 METAMORPHIC ROCKS
from three to four times as much magnesia as lime, evidence is
wanting to show that the material is ever secreted in appre-
ciable quantities by marine animals, and hence the sedimentary
deposits, resulting from the accumulation of the remains of these
animals, must be correspondingly lacking in this constituent. It
has been argued by Beaumont and others that through a process
of partial molecular replacement (metasomatosis) pre-existing
limestones were converted into dolomites, the process consisting
in the replacement of every other molecule of calcium carbonate
by one of the magnesium carbonate. As the dolomite molecule
is the more dense of the two, such replacement, in any given
limestone bed, must result in a contraction amounting to some
12^- o/o . Assuming that a dolomitic mass resulting in this way
is of the same bulk as the original limestone, this shrinkage
must manifest itself in the production of interstitial rifts and
cavities, such as do actually occur in many dolomitic lime-
stones, as those of the Ohio Trenton formations. The principal
objection to this theory lies in the difficulty of accounting for
the large amount of magnesia in solution ; whence its source,
etc. The same objections apparently apply to the explanation
given by M. C. Klement.1 This writer describes a series of
experiments in which solutions of sodium chloride and magne-
sium sulphate were made to act upon pulverized calcite and
aragonite. From the results obtained, he concludes that dolo-
mite is formed by the action of sea-water, concentrated in en-
closed basins and heated by the sun, on the aragonite deposited
by marine organisms, in such a way that a mixture of carbon-
ates of calcium and of magnesium is first produced, and which
is subsequently converted into dolomite.
Still another theory is that which regards the dolomite as a
residuary product formed by the leaching out of the lime car-
bonate from beds of impure, slightly magnesian limestone,
leaving behind the less soluble magnesian carbonate. The
amount of material lost, and the consequent contraction of the
original beds, must necessarily vary with their purity ; but in
any case where the residual mass has reached the condition of
a true dolomite, the proportional loss must have been enormous,
since in no cases are unaltered sediments known to contain
more than 4 or 5 % of magnesian carbonate. Although on
first thought this theory seems the more plausible of the two,
i Bull, de la Societe Geologique de Beige, Tome IX, 1895.
HYDRO-METAMORPHISM 161
it is apparently rendered invalid by the presence in these dolo-
mites of very perfect casts of fossils which have undergone no
crushing or distortion whatever, and which tend to show that
the beds as a whole, so far from having undergone a shrink-
age of 95 % and upwards, are of essentially the same bulk as
when laid down. The subject is too large for complete dis-
cussion here, and the reader is referred to standard works on
chemical geology, as well as the current literature. l
Still another form of change in the structure and mineral
composition of a rock is that brought about through the action
of water below the zone of oxidation and of true weathering.
It may be best described as a process of hydro-metamorphism,
since the influence of water is paramount. It is to this form
of metamorphism that is due the production, in part, of secondary
epidote, chlorite, sericite, leucoxene, kaolin (?) pyrite, and vari-
ous zeolitic compounds from pre-existing minerals, but without
in any way changing the character as a geological body of the
rock mass in which they occur. Such changes are in part meta-
sornatic, and in many instances are rendered more intense by
dynamic causes. This form of change has, unfortunately, been
too frequently confounded with weathering and decomposition.2
Under the head of metamorphic, then, is grouped a large
series of rocks which have been changed from their original
condition through the dynamical and chemical agencies above
described, and which may have been in part of aqueous and in
part of eruptive origin. Were it possible, it might have been
better to describe each class of these rocks together with the
corresponding igneous or aqueous form from which it was de-
rived by this process of change, or metamorphism. In only
too many cases, however, the change has been so complete as
to quite obliterate all such traces of the original character as
would lead to safe and satisfactory conclusions, and consistency
demands that all be grouped together.
1 See The Magnesian Series of the Northwestern States, by C. W. Hall and
F. W. Sardeson. Bull. Geol. Soc. of America, Vol. VI, 1895, p. 167.
2 While it is true that no new compound can be formed without first a break-
ing up, or decomposition, of those already existing, still, as this decomposition
affects only the individual minerals, and not the integrity of the rock mass as a
whole, it would seem preferable to include such changes under the name of
alteration and metamorphism. Weathering it certainly is not, though 'it is
essentially the form of change which Roth (Allegemeine u. Chemische Geologie,
Vol. I, pp. 159-412) has designated as complex weathering (Complicirte Vtr-
witteruny).
162 METAMORPHIC ROCKS
Accordingly as they vary in structure, we may divide these
metamorphic rocks into two general groups as below: 1. Strati-
fied or bedded; 2. foliated or schistose.
1. STRATIFIED OR BEDDED
(1) THE CRYSTALLINE LIMESTONES AND DOLOMITES
Here are included the metamorphosed form of the sedimen-
tary rocks described on p. 143.
Mineral Composition. — The essential constituent of the crys-
talline limestones is the mineral calcite. The common acces-
sories are minerals of the mica, amphibole, or proxene group,
and frequently sphene, tourmaline, garnets, vesuvianite, apatite,
pyrite, graphite, etc.
Chemical Composition. — As may be inferred from the mineral
composition, these rocks, when pure, consist only of calcium
carbonate. They are, however, rarely if ever found in a state
of absolute purity, but show more or less magnesia, alumina,
and other constituents of the accessory minerals. The analyses
given on pp. 146-47 will serve equally well here, and need not
be repeated.
Structure. — The limestones are eminently stratified rocks,
though this peculiarity is not always sufficiently marked to be
seen in the hand specimen. The purest and finest crystalline
varieties often show a granular texture like that of loaf sugar,
and hence are spoken of as saccharoidal limestones. Statuary
marble is a good illustration of this type. Under the micro-
scope the stone is shown to be made up of small grains, which,
having mutually interfered in process of growth, do not possess
perfect crystal outlines, but are rounded and irregular in out-
line, as shown in Fig. 14. All grades of textures are common,
the coarser forms sometimes showing individual crystals an inch
in length. Though in their unchanged conditions highly fossi-
liferous or tufaceous, these structural features may be wholly
or in part obliterated by crystallization.
Colors. — The color of pure limestone is snow-white, as seen
in statuary marble. Other common colors are pink or reddish,
greenish, blue-gray through all shades of gray to black. The
pink and red colors are dup to iron oxides, the greenish as a
STRATIFIED OR BEDDED
163
rule to micaceous minerals, the blue-gray and black to carbo-
naceous matter.
Geological Age and Mode of Occurrence. — The crystalline
limestone and dolomites are but the metamorphosed sedimentary
deposits such as have al-
ready been described on
p. 143. They occur asso-
ciated with rocks of all
ages, but only in regions
that have been subjected
to disturbances such as the
folding and faulting inci-
dent to mountain-making,
or the heat from intruded
igneous rocks. From an
economic standpoint, the
rocks of this group are
not infrequently of great
economic value for struct-
ural and decorative pur-
poses.
Classification and Nomenclature. — It is common to speak of
this entire group of rocks as simply limestones, though many
varietal names are often rather indefinitely applied. The name
marble is given to any calcareous or magnesian rock suffi-
ciently beautiful to be utilized in decorative work. Argilla-
ceous and siliceous limestones carry clayey matter and sand.
Dolomite (so named after the French geologist Dolomieu)
consists of 45.50% carbonate of magnesia and 54.40% car-
bonate of lime, as already noted. The names ophiolite and
ophicalcite are popularly applied to stones consisting of a
granular aggregate of calcite and serpentine, such as occur
in Essex County, New York, and are used as marbles. The
so-called Eozoon Canadenses, a supposed fossil rhizopod, belongs
here. The serpentinous matter in such cases originates from a
non-aluminous pyroxene by a process of hydration, as already
explained.
FIG. 14. — Microstructure of crystalline lime-
stone (marble).
164 METAMORPHIC BOCKS
2. FOLIATED OR SCHISTOSE
(1) THE GNEISSES
Gneiss, from the German G-neis, a term used by the miners
of Saxony to designate the country rock in which occur the
ore deposits of the Erzgebirge. The word is pronounced as
though spelled nice.
Mineral and Chemical Composition. — The composition of the
gneisses is essentially the same as that of the granites, from
which they differ only in structure and origin. They, how-
ever, present a greater variety and abundance of accessory
minerals, chief among which may be mentioned (besides those
of the mica, hornblende, or pyroxene group) garnet, tour-
maline, beryl, sphene, apatite, zircon, cordierite, pyrite, and
graphite.
Structure. — Structurally the gneisses are holocrystalline
granular rocks, as are the granites, but differ in that the
various constituents are arranged in approximately parallel
bands or layers, as shown in PL 13.
In width and texture these bands vary indefinitely. It is
common to find bands of coarsely crystalline quartz several
inches in width, alternating with others of feldspar, or feld-
spar, quartz, and mica, or hornblende. A lenticular structure
is common, produced by lens-shaped aggregates of quartz or
feldspar, about and around which are bent the hornblendes or
mica laminae. The rocks vary from finely and evenly fissile
through all grades of coarseness, and become at times so mas-
sive as to be indistinguishable in the hand specimens from
granites. The causes of the foliated structure are mentioned
below.
Colors. — Like the granites, they are all shades of gray,
greenish, pink, or red.
Geological Age and Mode of Occurrence. — The true gneisses are
among the oldest crystalline rocks, and have been considered by
many geologists as representing "portions of the primeval crust
of the globe, traces of the surface that first congealed upon the
molten nucleus." By others they are regarded as metamor-
phosed sedimentary deposits resulting from the breaking down
of still older rocks, and may not in themselves, therefore, be con-
fined to any particular geological horizon. They are in large
PLATE 13
THE GNEISSES 165
part, however, indisputably the oldest known rocks, lying be-
neath or being cut by all rocks of later formation or injection.
The origin of the gneisses, as above suggested, is in many
cases somewhat obscure, the banded or foliated structure being
considered by some as representing the original bedding of the
sediments, the different bands representing layers of varying
composition. This structure is now, however, considered to be
due to mechanical causes, and in no way dependent upon origi-
nal stratification. The name, as commonly used, is made to in-
clude rocks of widely different structure, and which are beyond
doubt in part sedimentary and in part eruptive, but in all cases
altered from their original conditions.
FIG. 15. — Microstructure of gneiss, showing at the points a broken feldspars.
This alteration, it should be stated, has been brought about
not by heat and crystallization alone, but in many cases by
processes of squeezing, crumpling, and folding so complex as
almost to warrant the application of the term kneading thereto.
It is even possible to conceive that some of them may be origi-
nal massive or foliated rocks into which eruptive materials have
since been injected along lines of foliation or of weakness due to
shearing, and the entire mass again submitted to such a knead-
ing as to render it practically impossible to now decide what
are portions of the original rock and what of the subsequently
injected.
166
METAMORPHIC ROCKS
The close chemical relationship which may exist between
clastic, metamorphic, and eruptive rocks is shown in the selected
series of analyses here given.
CONSTITUENTS
GRANITE
GNEISS
Q
K
o
SANDSTONE
a
•<
oi
•
H
•<
J
OS
DISINTE-
GRATED
GKANITE
I
II
in
IV
V
VI
VII
Silica (Si02)
<y
lo
68.18
01
lo
61.96
"/
lo
69.24
Of
In
69.94
o/
lo
61.91
°l
lo
60.32
01
lo
65.69
Titanium oxide (Ti02) .
1.66
Not det
0.31
Alumina (A1203) ....
Ferric oxide (Fe203) . . .
Ferrous oxide (FeO)
16.20
4.10
19.73
4.60
14.85
2.62
13.15
2.48
21.73
4.73
23.10
7.05
15.23
4.39
Ferrous sulphide (FeS2)
4.33
Manganese oxide (MnO) . .
Lime (CaO)
1.75
Trace
0.35
0.45
2.10
0.70
3.08
0.09
Not det.
2.63
Magnesia (MgO)
0.48
1:81
0.96
Trace
0.59
0.87
2.64
Soda (Na20)
2.88
0.79
4.30
5.43
0.25
0.49
2.12
Potash (K2O)
6.48
2.50
4.33
3.30
3.16
3.83
2.00
Ignition
1.82
0.70
1.01
7.43
4.08
4.70
100.07
99.55
99.56
99.10
99.89
99.74
99.71
I. Granite : Syene, Egypt. II. Gneiss : St. Jean de Matha, Province of Que-
bec, Canada. III. Gneiss : Trembling Mountain, Province of Quebec, Canada.
IV. Sandstone : Portland, Connecticut. V. Shale : England. VI. Slate : Lan-
caster County, Pennsylvania. VII. Disintegrated granite : District of Columbia.
Figures 1 and 2 on PI. 13 shows two rather extreme types of
these gneissoid rocks. Figure 1 is that of a banded gneiss from
Madison County, Montana, and which, so far as we know, may
be an altered sedimentary rock. In Fig. 2 of the same plate
is shown a foliation rather than a banded rock, and whatever
may have been its origin, it undoubtedly owes its foliated
structure to dynamic agencies. The effect of the shearing
force whereby the foliation was produced is evident in the
figure, even to the unaided eye, to the left and just above the
centre, where an elongated feldspar is seen broken transversely
in four pieces. The same features are brought out even more
plainly in Fig. 15, which shows the structure of this same gneiss
as seen under the microscope.
As in the present state of our knowledge it is in most cases
impossible to separate what may be true metamorphosed sedi-
GNEISS
167
mentaiy rocks from those in which the foliated or banded
structure is in no way connected with bedding and which may
or may not be altered eruptives, all are grouped together here.
Classification and Nomenclature. — The varietal distinctions
are based upon the character of the prevailing accessory min-
eral, as in the granites, forming a parallel series. We thus
have biotite gneiss, muscovite gneiss, biotite-muscovite gneiss, horn-
blende gneiss, etc. Rarely the mineral cordierite occurs in suffi-
cient abundance to become a characterizing accessory. Such
forms occur in Gilford County, Connecticut, and in Saxony.
The name granulite or leptynlte is applied to a banded quartz-
feldspar rock, the constituents of which occur in the form of
small grains and show under the microscope a mosaic structure.
The Saxon granulites are regarded by Lehman as eruptive
rocks altered by pressure. Halleflinta is a Swedish name for
a rock resembling in most respects the eruptive felsites or
quartz porphyries already described. Such, however, show a
banded structure and are, as a rule, regarded as metamorpliic
rocks. Porphyroid is also a felsitic rock with a more or less
schistose structure, and with porphyritic feldspar or quartzes.
Such have been described from the Ardennes, France.
GNEISS
ANALOGOUS MASSIVE TYPE
OF IGNEOUS ORIGIN
ORIGIN UNKNOWN
Granite :
Granite gneiss :
Granitic gneiss :
Biotite granite . . .
Biotite granite gneiss .
Biotite granitic gneiss.
Hornblende granite . .
Hornblende granite \
Hornblende granitic
gneiss . . . . /
gneiss.
Syenite :
Syenite gneiss :
Syenitic gneiss :
Hornblende syenite . .
Hornblende syenite \
Hornblende syenitic
gneiss . . . . /
gneiss.
Mica syenite ....
Mica syenite gneiss. .
Mica syenitic gneiss.
Pyroxene syenite . .
Pyroxene syenite gneiss
Augite syenitic gneiss.
Diorite :
Diorite gneiss :
Dioritic gneiss : '
Mica diorite ....
Mica diorite gneiss . .
Mica dioritic gneiss.
Gabbro
Gabbro gneiss . .
Gabbroic gneiss, or gab-
brie gneiss.
Pyroxenite
Pyroxenite gneiss . .
Pyroxenitic gneiss.
Inasmuch as the structure characteristic of gneisses is found
developed in rocks of diverse types, many petrologists now use
168 METAMORPHIC ROCKS
the term in an almost wholly structural sense, as in itself non-
committal as to composition or origin, but merely designating a
rock of foliated or schistose structure. C. H. Gordon has pro-
posed1 a scheme of classification of gneissoid rocks as above,
and which has much in its favor.
(2) THE CRYSTALLINE SCHISTS
Under this head are grouped a large and extremely variable
series of rocks, differing from the gneisses mainly in the lack of
feldspar as an essential constituent. They consist, therefore,
essentially of granular quartz, with one or more minerals of the
mica, chlorite, talc, amphibole, or pyroxene group. In acces-
sory minerals the schists are particularly rich. The more
common of these are feldspar, garnet, cyanite, staurolite,
tourmaline, epidote, rutile, magnetite, menaccanite, and pyrite.
Through an increase in the proportional amount of feldspar the
schists pass into the gneisses, and through a decrease in micar
hornblende, or whatever may be the characterizing mineral,
into the quartz schists, in which quartz alone is the essential
constituent. Occasional forms are met with quite lacking
in quartz and other accessory minerals and consisting only of
schistose aggregates of minerals of a single species, as is the
case with the pyrophyllite schists (or, more properly, schistose
pyrophyllites) from North Carolina, talcose schists, and with
the more massive "soapstones."
The rocks of this group are characterized as a whole by a
pronounced schistose structure, due to the parallel arrangement
of the various constituents, this structure being most pro-
nounced in those varieties in which mica is the predominating
accessory mineral. They are ordinarily considered as having
originated from the crystallization of sediments, and in many
cases the microscope still reveals existing " traces of the origi-
nal grains of quartz sand and other sedimentary particles of
which the rocks at first consisted." Like the gneisses, they
are in part, however, mechanically deformed massive rocks and
their schistosity in no way relates to true bedding, as has been
already noted (p. 156).
The varietal names given are dependent mainly upon the
character of the prevailing ferro-magnesian silicate. We thus
1 Bull. Geol. Soc. of America, Vol. VII, p. 122.
THE CRYSTALLINE SCHISTS
169
have mica schists, chlorite schists, talc schists, hornblende, actinol-
ite, glaucophane schists, etc. The term slate was originally
applied to these and other types of rocks of schistose or fis-
sile character. In the arrangement here adopted this term is
restricted to the argillaceous fragmental or semi-crystalline
and foliated rocks next to be described.
Of the above-mentioned varieties the mica schists are the
most common and widely distributed, the mica being in some
cases biotite, in others inuscovite, or perhaps a mixture of
the two. The principal accessories sufficiently developed to
be conspicuous are staurolites, chiastolites, garnets, and tour-
malines. In the sericite schists the hydrous mica sericite
prevails; paragonite schist carries the hydrous sodium-mica par-
agonite ; ottr elite schist carries the accessory mineral ottrelite.
The name phyllite is used by German petrographers to desig-
nate a micaceous semi-crystalline rock standing intermediate
between the true schists
and clay_slates. Quart-
zite is a more or less
schistose or banded rock
consisting essentially of
crystalline granules of
quartz. Such originate
from the induration of si-
liceous sandstones. This
induration is brought
about through a deposi-
tion of crystalline silica
in the form of a bind-
ing material or cement
around each of the sand
particles of which the
stone is composed. Each
of these granules then forms the nucleus of a more or less per-
fectly outlined quartz crystal. This structure is shown in Fig.
16, drawn from a thin section of a Potsdam quartzite from
St. Lawrence County, New York. The rounded, more or less
shaded, portions represent the original grains of quartz sand,
and the clear, colorless, interstitial portions the secondary silica.
The quartzites consist, as a rule, only of silica, or silica
colored brown and red by iron oxides. At times a greenish
FIG. 1C. — Microstructure of quartzite.
170 METAMORPHIC ROCKS
tinge is imparted through the development of chloritic minerals;
accessory minerals are not, as a rule, abundant.
Among the hornblende schists there are but few needing
especial attention. These are, as a rule, less finely schistose
than are the mica-bearing schists, owing to the fact that the
mineral hornblende itself has not a platy structure. The glau-
cophane schists are perhaps the least abundant of the hornblendic
varieties. Such have been described from the Isle of Syra, in
the Mediterranean Sea, Switzerland, Wales, and Italy ; a more
massive form, probably an altered eruptive, is found near the
mouth of Sulphur Creek, Sonoma County, California. Am-
phibolite is the name given to an extremely tough and often
massive rock of obscure origin, and consisting essentially of
the mineral amphibole or hornblende. In some instances the
varieties of amphibole, actinolite, and tremolite take the place
of the common hornblende. The tremolite rock may undergo
alteration into serpentine under proper conditions. Eclogite is
a tough, massive, or slightly schistose rock, consisting of the
grass-green variety of pyroxene, omphacite, and small red gar-
nets, with which are frequently associated bluish kyanite, green
hornblende (smaragdite), and white mica. Crarnet rock, or
garnetite, is a crystalline granular aggregate of garnets with
black mica, hornblende, quartz, and magnetite. Kinzigkite is
a somewhat similar, though fine-grained and compact, rock
consisting of garnets, plagioclase feldspar, and black mica,
and which is found in Kinzig and the Odenwald.
Many of the rocks of this group are but products of dynamic
or contact metamorphism, as is the case with many of the
chiastolite and argillaceous schists or roofing slates. Rocks of
the latter group pass by insensible gradations into clastic ar-
gillites. They owe their cleavable property to shearing, as
already explained. Under the microscope these rocks are
found to be quite variable. Hawes describes clay slate from
Littleton, New Hampshire, as consisting of a mixture of quartz
and feldspar, in particles as fine as dust. They contained also
amorphous carbonaceous matter and little needles of a mineral
assumed to be mica. A slate from Hanover, in the same state,
contained garnets and staurolites. Wichman found slates from
Lake Superior to consist of a colorless, isotropic ground-mass
carrying quartz and feldspar particles, scales of iron oxide, car-
bonaceous matter, minute tourmalines, and mica fragments.
THE CRYSTALLINE SCHISTS
171
The red slates of New York state are composed of an impal-
pable red, dust-like ground-mass, carrying grains of quartz
and feldspar, all arranged with their longer axes parallel to
the plane of schistosity. These can scarcely be considered as
other than clastic rocks, the dynamic action not having been
sufficient to produce crystallization in more than incipient
stages. In this case the plane of schistosity is very nearly
parallel with that of bedding, but in many cases, as in the
roofing slates of Pennsylvania, the schistose structure is devel-
oped at a very considerable, though ever-varying, angle with
the bedding. In such cases the true bedding plane is often
determinable only by the dark bands, or ribbons, by which the
split slates are traversed.
Chemical Composition.' — As may be readily imagined, the
schists vary almost indefinitely in composition, approximating
pure quartzite on the one hand and the gneisses on the other.
The table given below is intended to show the composition of
a few characteristic types only. All gradations, from the most
acid of quartzites to the most basic of the ampliibolites, may
readily be found.
CONSTITUENTS
I
II
III
IV
V
VI
Silica (SiO2)
82.38%
49.00 %
52.39%
49.18%
50.81 %
97.1 %
Alumina (A1203) . . .
Ferric oxide (Fe20a) . .
Ferrous oxide (FeO) . .
Lime (CaO)
11.84
2.28
23.65
8.07
0.03
16.33
1.64
1.44
8.76
15.09
12.90
10.59
4.53
3.52
4.26
1.30
1.25
0 18
Magnesia (MgO) . . .
Potash (K2O) ....
Soda (Na2O)
1.00
0.83
0.38
0.94
9.11
1.76
4.70
1.42
2 59
5.22
1.51
3 64
31.55
0.13
Ignition
0.77
3.41
0.17
1.87
4.42
I. Mica schist : Monte Rosa, Switzerland. II. Sericite schist : Wisconsin.
III. Hornblende schist : Graad Rapids, Wisconsin. IV. Chlorite schist : Klippe,
Sweden. V. Talc schist: Gastein, Austria. VI. Quartzite: Chickies Station,
Pennsylvania. All analyses quoted from J. F. Kemp's Lecture I?otes on Rocks.
PART III
THE WEATHERING OF ROCKS
" In the economy of the world, I can find no traces of a beginning, no prospect of
an end." — HUTTON.
THE stability of chemical compounds is governed by prevail-
ing conditions. A form of combination stable under conditions
existing to-day may, under those of to-morrow, become impos-
sible. As was suggested in the introductory chapter, the con-
ditions under which the more superficial portions of the earth's
crust exist are ever changing, and as a result old compounds
are broken up and new continually formed. All over the earth
rocks laid down as sediments on oceanic floors have been
folded, faulted, and pushed out of place until brought under
influences as different from those under which they were formed
as it is possible to conceive. Molten magmas cooling suddenly
on the immediate surface formed compounds in which mere loss
of heat was the controlling factor, but which time proves to be
unstable. Slow cooling, deep-seated magmas have been, and
are being, continually exposed by denudation, and thus brought
under new influences and environments. Hence a constant
readjustment is everywhere, going on, which, as we shall see, is
manifold in its physical manifestations. As where an entire
building is razed to the ground, and another of quite different
architectural features constructed from the old materials ; or
again, where, without change of general plan, old timbers are
here and there replaced by new, so here we have at work a
series of processes in part seemingly destructive and in part
constructive, but all tending toward one end.
The firm and everlasting hills we must learn to regard as
neither firm nor everlasting. Whole mountain chains of the
geological yesterday have disappeared from view, and as with
172
PLATE 14
Weathered grauite, District of Columbia.
THE WEATHERING OF ROCKS 173
the ancient cities of the East, we read their histories only in
their ruins. Yet, in all this seemingly destructive process of
breaking down, decomposition, and erosion, there is traceable
the one underlying principle of transformation from the un-
stable toward that which is to-day more stable. Nothing is
lost or wasted: It is a change which began with the beginning
of matter ; which will end only with the blotting out of matter
itself. There are no traces of a beginning, there is no prospect
of an end.
I. THE PRINCIPLES INVOLVED IN ROCK-
WEATHERING
The processes involved in this readjustment from unstable
to stable compounds, as above outlined, and of incidental
soil formation, are in part physical and in part chemical in
their nature ; they operate under ever-varying conditions, and
through processes at times simple, or again complex. What
these processes are, and how they operate, it must be our
purpose to now consider.
It may be said at the outset, that whatever the forces en-
gaged, they are, with a few isolated exceptions, superficial, —
they work from without downwards. However much they may
have accomplished since the first rock masses appeared above
the primeval ocean, in no case can the actual amount of debris
in situ have formed at one time more than a scarcely appreciable
film over the underlying and unchanged material. The decom-
posing forces early lose their active principles and become quite
inert at depths comparatively insignificant. It is only where
through erosion the results of the disintegration are gradually
removed, that the processes have gone on to such an extent as
to perhaps quite obliterate thousands of feet of strata or of
massive rock, and furnished the necessary debris for the vast
thicknesses of sandstone, slate, and shale which characterize the
more modern horizons. In certain isolated cases, it is true,
ascending steam and heated waters, arising from depths un-
known, have been instrumental in promoting decomposition, as
is well illustrated in the areas of decomposed rhyolites in the
Yellowstone National Park. Nevertheless, it is to the almost
incalculably slow process of superficial weathering that we owe
174 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
a very large share of the apparent rock decomposition and inci-
dental soil formation.1 r
This transformation, as already noted, takes place through
processes that may be simple, or again complex. It is but
rarely that one, alone, prevails for any length of time, and as a
rule several or many go merrily on together. Were it possi-
ble, it might be well to consider briefly each of these in its turn
and by itself. From the fact, however, as above stated, that
any one, either physical or chemical, rarely goes on alone, it is
thought best to treat the subject as below, and describe in more
or less detail the action, first, of the atmosphere, second, of water,
in both the solid and liquid form, and third, that of plant and
animal life, finally considering the combined action of all these
forces, as manifested on the various types of rock which go to
make up the earth's crust.
So striking a phenomenon as the breaking down, or degenera-
tion as we may call it, of a mass of firm rock, naturally did not
escape the observation of the earlier workers in this and allied
branches of science, and the older literature from the time of
Hutton contains numerous references to it, though the full
significance of atmospheric agencies in bringing about the
results, seems not at first to have been fully realized.
The exciting cause of the degeneration, particularly in warm
latitudes, where phenomena of this nature are, as a rule, more
apparent, has been a matter of some speculation, and at the out-
set it may be well to indicate in brief their tendencies.
1 The term weathering, as here used, is applied only to those superficial
changes in a rock mass brought about through atmospheric agencies, and result-
ing in a more or less complete destruction of the rock as a geological body, as
where granitic rocks are resolved into sand, and kaolinic material, with liberation
of carbonates of the alkalies and of lime, and oxides of iron. It does not include
those deeper-seated changes — changes taking place below the zone of oxidation
arid which result mainly in hydration and the production, it may be, of new
mineral species, as chlorite, sericite, zeolites, etc., but during which the rock
mass as a whole retains its individuality and geological identity. The distinction
is not one that has been sharply insisted upon, and indeed geologists and petrolo-
gists as a rule have been extremely careless in their use of such terms as altera-
tion, decomposition, and weathering. The distinction drawn here is essentially
that made by Roth (Allegemeine u. Chemische Geologie), between Verwitterung
and Complicirte Verwitterung. For reasons above stated and others given on
p. 161, it seems best to limit the terms weathering and decomposition to processes
involving the destruction of the rock mass as a geological body, and to designate
the purely mineralogical deeper-seated changes as alteration, which may or may
not be due wholly to hydrometamorphism.
OPINIONS OF EARLY WORKERS 175
Fournet, as quoted elsewhere, writing as early as 1833, in-
sisted upon the efficacy of water containing carbonic acid in
promoting the decomposition of igneous rocks, while Brogniart,
writing with particular reference to feldspathic decomposition
and the origin of kaolin, laid great stress on the acceleration
of the ordinary process of decay through the electric currents
resulting from the contact of heterogeneous rock masses. Dar-
win 1 believed the extensive decomposition observed by him in
Brazil, to have taken place under the sea, and before the present
valleys were excavated. Hartt 2 gave it as his opinion that the
decomposition was due to the action of warm rain water soaking
through the rock, and carrying with it carbonic acid derived
not only from the air, but from the vegetation decaying in the
soil as well, together with organic acids, nitrate of ammonium,
etc. Further, that the decomposition had gone on only in re-
gions once covered by forests. Heusser and Claraz 3 suggested
that the decomposition was brought about through the influence
of nitric acid. They say "it is without doubt determined by
the violence and frequency of the tropical rains, and by the dis-
solving action of water, which increases with the temperature.
It is necessary to observe, moreover, that this water contains
some nitric acid, on account of the thunder storms which follow
each other with great regularity during many months of the
year."
Belt,4 in discussing the extensive decomposition observed by
him in Nicaragua, says : u This decomposition of the rocks
near the surface prevails in many parts of tropical America,
and is principally, if not always, confined to the forest regions.
It has been ascribed, and probably with reason, to the percola-
tion through the rocks of rain water charged with a little acid
from the decomposing vegetation."
The elder Agassiz laid much stress on the decomposing effects
of the hot water from rainfall,5 while Mills and Branner,6 in
addition, attributed no insignificant amount of the decomposi-
tion to the action of decomposing organic matter carried into
1 Geological Observations, p. 417.
2 Phys. Geog. and Geol. of Brazil.
8 Ann. des Mines, 5th series, 17, 1860, p. 291.
4 The Naturalist in Nicaragua, 1874.
6 Journey in Brazil, p. 89.
3 Bull. Geol. Soc. of America, Vol. VII, 1896.
176 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
the ground by ants, and also to the acid secretions of the ants
themselves.
The chemical changes involved in the process of decompo-
sition received attention from several of the earlier workers,
among whom the names of Berthier, J. G. Forschammer,
Brogniart, Gustav Bischof, and Ebelmen stand out in greater
prominence. More recently the name of Sterry Hunt becomes
conspicuous, while the purely geological side of the question
has been ably set forth in numerous papers by L. Agassiz, R.
Pumpelly, N. S. Shaler, O. A. Derby, R. Irving, J. C. Branner,
and others, to whom reference is frequently made in these pages.
1. ACTION OF THE ATMOSPHERE
Atmospheric air, as is well known, consists in its normal state
of a mechanical admixture of free nitrogen and oxygen in the
proportion of four volumes of the former to one of the latter.
In addition are small and comparatively insignificant amounts
of various combined gases and salts, of which carbonic acid is
by far the most abundant, constant, and, from our standpoint,
important. Still smaller quantities of ammoniacal vapors exist,
and in volcanic regions there have been detected appreciable but
variable quantities of sulphuric and hydrochloric and nitric acids
as well. With rare exceptions these last exist in combination
as sulphates, chlorides, and nitrates and with the exception of
the last-named need little consideration.
(1) Nitrogen, Nitric Acid, and Ammonia. — Nitrogen, by it-
self, is believed to be wholly inoperative in promoting rock
decomposition. In works on agricultural chemistry, much has,
however, been written concerning the presence in the atmos-
phere of the compounds of nitrogen, nitric acid, and ammonia,
and it will be well to devote a little space to a consideration of
the facts as known, and their possible application to the subject
under consideration.
The well-known experiments of Cloez, Boussingault, De Luca,
Kletzinsky, and Way, as well as the recent ones of G. H.
Failyer,1 prove conclusively the existence of ammonia and nitric
acid in the air, from whence it is brought to the surface of the
earth in the water of rainfalls.
1 Ammonia and Nitric Acid in Atmospheric Waters, 2d Ann. Rep. Kansas
Experiment Station, 1889.
ACTION OF THE ATMOSPHEKE
177
In nearly every case, however, the percentage of ammonia, as
determined, equalled or exceeded the amount necessary to com-
bine with the acid, forming thus the salt ammonium nitrate.
Failyer's experiments in Kansas, carried on for a period of four
years, during which time water was collected from 266 rain-
falls, showed in but seven instances nitric acid equalling or
exceeding the ammonia. In all other cases the amount is less,
with the possible exception of the reported occurrence (at
Nismes, in 1845) of a fall of hail sufficiently acid to be sour
to the taste. As direct promoters of rock decomposition,
neither atmospheric nitrogen nor free nitric acid need, ilu-n,
serious attention. The following tables are, however, of inter-
est, the first being abridged from Johnson's How Crops Feed,
and the second from Professor Failyer's paper above quoted.
AMOUNTS OF RAIN AND OF AMMONIA, NITRIC ACID, AND TOTAL NITROGEN THEREIN,
COLLECTED AT ROTHAMSTEDU, ENGLAND, IN THE YEARS 1855-56, CALCULATED
PER ACRE, ACCORDING TO MESSRS. LAWE8, GILBERT, AND WAY.
Total . .
Quantity of rain in
Imperial pallons.
1 pal. = 10 Ib. water
Ammonia
(in pounds)
Nitric Acid
(in pounds)
Total Nitrogen
(in pounds)
1855
663.332
1856
616.051
1855
7.11
1850
9.53
1855
2.98
1856
2.80
1855
6.63
1856
8.31
AMOUNTS OF RAIN AND OF AMMONIA, NITRIC ACID, AND NITROGEN THEREIN, COL-
LECTED AT MANHATTAN, KANSAS, 1887-90, ACCORDING TO G. H. FAILYER.
Total Nitrogen.
Nitrogen in
ammonia.
Nitrogen in
nitric acid.
Means for 3
Means for 3
years
\i-:ii>
Parts per million of water ....
0.522
.388
0.156 .
Grammes per acre
1663.0
1196.0
480.0
Pounds per acre
344
2.63
1.06
It has been demonstrated, however, that nitrogen compounds
and nitrogenous matter in the soil may become subject to nitri-
fication through the action of bacteria, whereby ammonia,
nitrous or nitric acid, carbon dioxide, and water are formed,
178 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
though, as Wiley says, " The ammonia and nitrous acid may
not appear in the soils, as the nitric organism attacks the latter
at once and converts it into nitric acid."1 (See further under
influence of plant and animal life, p. 203.)
In considering the possible efficacy of these compounds, one
must not lose sight of the fact that the amount of nitrogen in
the soils is as a rule far too small to supply the demands of
growing plants, and it is probable that a very large proportion
of that which finds its way there is quickly taken up again by
these organisms. It is possible that other salts of ammonium
than the nitrate may be locally efficacious. Thus M. Beyer,
as quoted by Van Den Broeck,2 has shown that the feldspars
decompose very rapidly under the influence of water contain-
ing ammonium sulphate or even sodium chloride, either of
which substance may be found in vegetable soil. Daubree,
who experimented by means of revolving iron cylinders (see
p. 197), found, however, that the presence of sodium chloride
retarded decomposition.
(2) Carbonic Acid. — The amount of carbonic acid in the air
under natural conditions is not a widely variable quantity, ex-
cepting near volcanoes and the immediate vicinity of gaseous
springs. In the vicinity of large cities and manufactories
consuming great quantities of coal, the amount is naturally
increased. Although carbonic acid is the most abundant gas
given off by decomposing vegetable matter, it has apparently
been definitely ascertained that the amount of this gas in regions
of abundant vegetation is no greater than elsewhere. This has
been accounted for on the assumption that, as fast as liberated, it
is taken up by growing organisms or carried by rains into the soil.3
1 Wiley, Principles and Practice of Agricultural Analysis.
2 Mem. sur les phenomenes d'Alteration des Depots Superficial, p. 16.
3 The researches of Boussingault and Lewey (Mem. de Chemie Agricole,
etc.), as quoted by Johnson (How Crops Feed, p. 139), showed the following
proportions existing between the C02 of the air of the atmosphere and of various
soils : —
CO2 m 10,000 PARTS
BY WEIGHT
Ordinary atmosphere 6 parts
Air from sandy subsoil of forest 38 parts
Air from loamy subsoil of forest 124 parts
Air from surface soil of forest 130 parts
Air from surface soil of vineyard 146 parts
Air from pasture soil 270 parts
Air from soil rich in humus 543 parts
ACTION OF THE ATMOSPHERE 179
Twenty-one tests of the air in various parts of Boston, during
the spring, 1870, showed the presence of 385 parts of carbonic
acid in 1,000,000. Eleven tests of the winter air in Cambridge
yielded 337 parts in 1,000,000.! Dr. J. H. Kidder found the
out-door air of Washington to contain 387 to 448 parts in
1,000,000, while Dr. Angus Smith, after an elaborate series of
experiments, reported the atmosphere of Manchester (Eng-
land) as containing 442 parts in 1,000,000. 2
These amounts are considerably in excess of those reported
by Miintz and Aubin,3 who give the following figures relative
to the proportional amounts in 10,000 by volume, as determined
at the various widely separated stations. The amount, it will
be perceived, is slightly greater during the night than during
the day.
DAY Niiiiir
Hayti 2.704 2.920
Florida 2.897 iv.'IT
Martinique 2.735 2.850
Mexico 2.665 2.860
Santa Cruz, Patagonia 2.664 2.670
Chubut, Patagonia 2.790 3.120
Chili 2.665 2.820
The general mean is then 2.78 parts in 10,000, that for the
night alone being 2.82. For the north of France the mean is
given as 2.962, for the plain of Vincennes 2.84, and for the
summit of the Pic du Midi 2.86.
Fischer, as quoted by Branner,4 has shown that in rain and
snow water the amount of carbdnic acid varies between 0.22%
and 0.45 % by volume of water. Assuming that the mean of
these figures fairly represents the general average, it is easy,
knowing the rainfall of any region, to calculate the amount of
the gas thus annually brought to the surface. Professor Bran-
ner has thus calculated that from 3.21 to 11.80 millimetres of
carbonic acid (CO2) are annually brought to the surface in cer-
tain parts of Brazil. The same method of calculation applied
to the various parts of the United States, would give us for the
Atlantic coast states 3.75 mm.; for the upper Mississippi val-
ley, 2.50 mm.; for the lower Mississippi valley, 4.50 mm. ; and
1 2d Ann. Rep. Mass. State Board of Health, 1871.
2 Air and Rain, p. 52.
» Comptes Rendus, Vol. XCIII, 1881, p. 797 ; also XCVI, 1883, pp. 1793-97.
* Op. cit.
180 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
for the northern Pacific states, 6.25 mm. As it is mainly when
this carbonic acid is thus brought to the surface by the rain
and snows that its effects become of direct significance in our
present work, the matter may be dropped here, to be taken up
again when considering the chemical action of water.
(3) Oxygen. — Under ordinary conditions oxygen is the most
active principle in atmospheric air, and it is to this agent that
is due the process of oxidation which almost invariably char-
acterizes the decomposition of silicates and other minerals con-
taining iron in the protoxide state. Such oxidation is, however,
almost inactive unless aided by moisture, and a further discus-
sion of the subject may well be deferred, to be taken up again
when discussing the action of water.
(4) Heat and Cold. — The ordinarily feeble action of the air
is greatly augmented through natural temperature variations.
That heat expands and cold contracts is a fact too well known
to need elaboration. That, however, the constant expansion
and contraction due to diurnal temperature variations may be
productive of weakness and ultimate disintegration in so inert
a body as stone, seems not so generally understood, or is, at
least, less well appreciated, and hence a little space is devoted
to the subject here. Rocks, it must be remembered, as the
writer has noted elsewhere,1 are complex mineral aggregates of
low conducting power, each individual constituent of which
possesses its own ratio of expansion, or contraction, as the case
may be. In crystalline rocks these various constituents are
practically in contact. In clastic rocks they are, on the other
hand, frequently separated from one another by the interposi-
tion of a thin layer of calcareous, ferruginous, or siliceous matter
which serves as a cement. As temperatures rise, each and every
constituent expands and crowds with almost resistless force
against its neighbor ; as temperatures fall, a corresponding con-
traction takes place. Since in but few regions are surface tem-
peratures constant for any great period of time, it will be readily
perceived that almost the world over there must be continuous
movement within the superficial portions of the mass of a rock.
The actual amount of expansion and contraction of stone
under ordinary temperatures has been a matter of experiment.
W. H. Bartlett 2 has shown that the average rate of expansion
1 Stones for Building and Decoration, Wiley & Sons, New York.
2 Am. Jour, of Science, Vol. XXII, 1832, p. 136.
ACTION OF THE ATMOSPHERE 181
for granite amounts to .000004825 inch per foot for each de-
gree Fahrenheit ; for marble .000005668 inch, and for sandstone
.000009532 inch. Adie, in a series of similar experiments,
found the rate of expansion for granite to be .00000438 inch,
and for white marble .00000613 inch.1 Slight as these move-
ments may seem, they are sufficient to in time produce a decided
weakening and afford a starting-point for other physical and
chemical agencies, such as are ever lying in wait for an oppor-
tunity to get in their work. The writer well remembers the
peculiar impressions produced during one of his earlier trips
into the comparatively arid regions of Montana, at finding, at a
certain place, the slopes and valley bottoms strewn with small,
beautifully fresh, concave and convex chips of a dense, coal-
black, andesitic rock that occupied the crest of one of the higher
hills. So fresh were the fractures, so free were they from oxi-
dation or other signs of decomposition, it was at first felt that
they must be of human origin, that they were chips flaked off
by aboriginal workmen in making stone implements, and some
time was wasted in seeking for the more complete results of
their handiwork. It, however, did not take long to convince
him that the flakes were far too abundant and too widely spread
to have originated in any such way, while the finding, on the
top of the hill, of the coal-black rock, broken into larger colum-
nar blocks, each with its angles rendered more obtuse or even
fluted by the springing off of just such flakes, — this, coupled
with the knowledge that during the day, exposed under a
cloudless sky, the rocks became so highly heated as to be un-
comfortable to the touch, whilst at night the temperature sank
nearly to the freezing-point, sufficed to teach, as it must have
taught the most obtuse, that the ordinary daily temperature
variations were amply sufficient to account for the phenom-
enon.
Shaler states 2 that rock surfaces in the eastern United States
may be subjected to temperatures varying from 150° F. at
midday in summer to 0° and below in winter. This change of
150° in a sheet of granite 100 feet in diameter would produce a
lateral expansion of about one inch of surface. That this ex-
pansion must tend to lessen the cohesion and tear the upper
from the deeper lying layers, is self-evident. As exemplifying
1 Trans. Royal Soc. of Edinburgh, Vol. XIII, p. 366.
2 Proc. Boston Soc. of Nat. History, XII, 1869, p. 292.
182 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
this, Professor Shaler states that there are on Cape Ann (Massa-
chusetts) hundreds of acres of bare rock surface completely
covered with blocks of stone, which have been separated from
the mass beneath by just this process.1
The size of such flakes may vary from those of microscopic
proportions to masses of several tons' weight. The higher
slopes of Lone Mountain, east of the Madison, in Montana,
are covered above timber line with thousands upon thousands
of these loose flakes of all sizes up to ten or more feet in diam-
eter. Such, here, as in general, are characterized by a roughly
lenticular outline in cross-section, possessing a large superficial
area in proportion to their thickness, and are further distin-
guished from boulders of decomposition by the entire freshness
of their materials even to the very surface. In close-grained,
black andesitic and basaltic rocks the chip or flake not infre-
quently shows a beautiful concave and convex form and is
greatly elongated in proportion to its breadth, resembling the
long and slender chips of obsidian or flint found on the sites
of aboriginal workshops. The surface left by the springing off
of these flakes is of course fluted as though the work were
done with a carpenter's gouge.
It is natural that this form of disintegration should be most
pronounced in massive, close-grained rocks. In regions of great
extremes of daily temperature the rupturing of these masses
from the parent ledge is frequently attended by gun-like reports
sufficiently loud to be heard at a considerable distance. H. von
Streeruwitz states 2 that the rocks of the Trans Pecos (Texas)
region undergo a very rapid disintegration from diurnal tem-
perature variations, which here amount to from 60° to 75° F.
He says: "I frequently observed in summer, as well as in winter
time, on the heights of the Quitman Mountains a peculiar crack-
ling noise and occasionally loud reports, . . . and careful
1 The rifting action of heat upon granitic masses is said to have been made a
matter of quarry utility in India. It is stated (Nature, January 17, 1895) that a
wood fire built upon the surface of the granite ledge and pushed slowly forward
causes the stone to rift out in sheets six inches or so in thickness, and of almost
any desired superficial area. Slabs 60 x 40 feet in area, varying not more than
half an inch from a uniform thickness throughout, have been thus obtained. In
one instance mentioned, the surface passed over by the line of fire was 460 feet,
setting free an area of stone of 740 square feet of an average thickness of five
inches. This stone is undoubtedly one of remarkably easy rift, but the case
will, nevertheless, serve our present purposes of illustration.
2 4th Ann. Rep. Geol. Survey of Texas, 1892, p. 144.
ACTION OF THE ATMOSPHERE 183
research revealed the fact that the crackling was caused by the
gradual disintegration and separation of scales from the surface
of the rock, and the loud reports by crackling and splitting of
huge boulders." The scales thus split off, he says, vary in
thickness from one-half to four inches, and their superficial
area from a few square inches to many feet. This form of
disintegration is necessarily confined to slopes unprotected by
vegetation, and is the more pronounced the greater the diurnal
variations.
In Arabia Petrea, according to Marsh,1 " when a wind pow-
erful enough to scour down below the ordinary surface of the
desert and lay bare a fresh bed of stones is followed by a sudden
burst of sunshine, the dark agate pebbles are often cracked and
broken by the heat." According to Livingstone, the rock tem-
peratures in certain parts of Africa, on the immediate surface,
rise during the day as high as 137° F. and at night fall so rap-
idly as to throw off by their contraction sharp, angular masses
in sizes up to 200 pounds' weight. Stanley, in his reports,
is inclined to lay considerable stress on the effects of cold rains
upon the heated rock surfaces, though it is doubtful if this is
as powerful an agent as his descriptions would give us to under-
stand. (See further under action of water.) Throughout the
desert regions of lower California, as observed by the writer,
the granitic and basic eruptive rocks subject to very little
rainfall, and hence almost completely bare of vegetation, under
the blistering heat of the desert sun have weathered down into
dome-shaped masses, their debris in the form of angular bits of
gravel being strewn over the plain. Particles of this gravel,
when compared with those which are a product of chemical
agencies, are found to differ in that each, however friable, is a
complex molecule of quartz, feldspar and mica or other mineral
that may have composed the rock from which it was derived.
Aside from a whitening of the feldspathic constituent, due to
the reflection of the light from its parted cleavage planes,
scarcely any change has taken place, and indeed it more resem-
bles the finely comminuted material from a rock-crusher than
a product of natural agencies.
Owing, however, to the low conducting power of rocks, dis-
integration from this cause alone can go on to any extent only
at the immediate surface, and on flat and level plains, where
1 The Earth as modified by Human Action, p. 552.
184 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
the debris is allowed to accumulate, must in time completely
cease.1 It is only on hillsides and slopes, or where by the-
erosive action of running water, or by wind, the debris is
removed as fast as formed, that such can have any geological
significance, although the rate of such disintegration is suffi-
ciently rapid in exposed places to be of serious consequence in
stone used for architectural application. (See further on p. 198,
Action of Ice.)
(5) Wind. — But it is to the action of the air when in motion
— to the wind — that is due a very considerable part of atmos-
pheric work. Particles of sand drifting along before the wind
become themselves agents of abrasion, filing away on every hard
object with which they come in contact. As a matter of course,
this phenomenon is most strikingly active in the arid regions,
though the results, when looked for, are by no means wanting
in the humid east. It is thought by Professor Egleston that
many of the tombstones in the older churchyards of New York
City have become illegible by the wearing action of the dust
and sand blown against them from the street. There is among
1 Observations on soil temperatures made at the Orono, Maine, Experimental
Station showed that the mean daily range of temperatures from April to Octo-
ber, at a depth below the surface of 1 inch, was 5.62° ; at a depth of 3 inches,
5.26° ; at 6 inches, 1.9° ; and at 9 inches, 1.18° ; and at 12 inches very slight. At
the depth of 1 inch the temperature was lower than that of the air by 2.4° ; at
3 inches by 2.11° ; at 6 inches by 3.16° ; at 9 inches by 3.94° ; at 12 inches by
4.18° ; at 24 inches by 6.78° ; and at 36 inches by 7.10°.
The following table, compiled by Forbes (Trans. Royal Society of Edinburgh,
Vol. XVI, 1849), from observations made near Edinburgh, Scotland, during
1841-42, shows the range of earth temperatures at varying depths in soil, sand-
stone, and trap rock.
TRAP EOCK
SAND OF GARDEN
CRAIGLEITH SANDSTONE
Max.
Min.
Range
Max.
Min.
Eange
Max.
Min.
Ran<*e
3 feet . . .
52.85°
38.88°
13.97°
54.50°
37.85°
17.65°
53.15°
38.25°
14.90°
6 feet . . .
51.07
40.78
10.29
52.95
39.55
13.40
51.90
38.95
12.95
12 feet . .
49.00
44.20
4.80
50.40
43.50
6.90
50.30
41.60
8.70
24 feet . .
47.50
46.12
1.38
48.10
46.10
2.00
48.25
44.35
3.90
It has been shown that the thermal conductivity of rocks varies in direction
according to their structure, being greatest in the direction of their schistosity,
where such exists. In massive, homogeneous rocks the conductivity is the same
in all directions. In finely fissile rocks, on the other hand, it may be four times
as great in the direction of their fissility as at right angles thereto.
ACTION OF THE ATMOSPHERE 185
the heterogeneous collections of the National Museum at Wash-
ington a large sheet of plate glass, once a window in a light-
house on Cape Cod. During a severe storin, of not above forty-
eight hours' duration, this became on its exposed surface so
ground from the impact of grains of sand blown against it as to
be no longer transparent, and to necessitate its removal. Win-
dow panes in the dwelling-houses of the vicinity are, it is stated,
not infrequently drilled quite through by the same means.
Apply now this agency to a geological field in a dry region.
The wind, sweeping across a country bare of verdure and
parched by drought, catches up the loose particles of dust and
sand and drives them violently into the air in clouds, or sweeps
them along more quietly close to the surface, where they are
at first scarce noticeable. The impact of a single one of these
moving grains on any object with which it may come in con-
tact is far too small to be appreciable ; but the impact of
millions, acting through days, weeks, and years, produces re-
sults not merely noticeable, but strikingly conspicuous. We
have here, in fact, a natural sand blast, an illustration on a
grand scale of a principle in common use in glass-cutting, and
to a small extent in stone-cutting also. Constantly filing away
on every object with which they come in contact, the grains
go sweeping on, undermining cliffs, scouring down mountain
passes, wearing away the loose boulders, and smoothing out all
inequalities. Naturally the abrading action on exposed blocks
of stone is most rapid near the ground, as here the flying sand
grains are thickest. First the sharp angles and corners are
worn away, and the masses gradually become pear-shaped,
standing on their smaller ends. Finally the base becomes
too small for support, the stone topples over, and the process
begins anew without a moment's intercession, and continues
until the entire mass disappears, — becomes itself converted
into loose sand drifted by the wind and an agent for destruc-
tion. Professor W. P. Blake was the first, I believe, to call pub-
lic attention to this phenomenon, having observed it while in the
Pass of San Bernardino (California) in 1853. G. K. Gilbert
has also published some interesting facts as noted by himself
while geologist of the Wheeler Expedition west of the 100th
meridian, in 1878. l In acting on the hard rocks, the sand cuts
1 It should be noted that the "sand-blast carving" described by Gilbert in
this report is not due wholly to the action of wind-blown sand. The rock is fine
186 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
so slowly as at times to produce only grooved or fantastically
carved surfaces, often with a very high polish. The geologists
of the 40th Parallel Survey in 1878 described like interesting
phenomena as observed on the western faces of conglomerate
boulders exposed to the sand blast of the desert regions of Ne-
vada. The surface of the otherwise light-colored rock was
found to have assumed a dark lead-gray hue and a polish equal
to that of glass, while the sand had drilled irregular holes and
grooves, often three-fourths of an inch deep and not more than
an eighth of an inch in diameter, through pebbles and matrix
alike. Professor W. M. Davis,1 G. H. Stone,2 and J. B. Wood-
ward 3 have described pebbles occurring in the glacial deposits
of Cape Cod and of Maine, carved and facetted by the same
agencies.
2. CHEMICAL ACTION OF WATER
Pure water, although an almost universal solvent, neverthe-
less acts with such slowness upon the ordinary materials of the
earth's crust, that its results are scarcely appreciable to the
ordinary observer. But it by no means follows that its effects
are not worthy of our consideration here. This is particularly
true when we reflect that the results being discussed are not
merely those of days and weeks, but of years even when counted
by the tens of thousands and millions. Moreover, absolutely
pure water, as a constituent of our sphere, presumably does not
exist. We have to consider its action as well when contami-
nated with sundry salts and acids which it almost universally
holds, having taken them up in passing through the atmosphere,
and in filtering through the overlying layer of organic matter
and decomposition products which cover so large a portion of
the surface of the land. It is when thus contaminated that are
manifested the wonderful solvent and other chemical reactions
which have been instrumental in promoting rock destruction,
and it is here, then, that will be considered the complex chemical
/
calcareous shale. Through the solvent action of meteoric water the calcareous
cement is removed, the fine, argillaceous interstitial material mechanically
eroded, while the more resisting granules of quartz sand stand in relief, giving
rise to elevated points and ridges.
1 Proc. Boston Soc. of Natural History, Vol. XXVI, 1893, p. 166.
2 Am. Jour. Science, Vol. XXXI, 1886, p. 133.
a Ibid., Jan., 1894, p. 63.
CHEMICAL ACTION OF WATER 187
proc^ses commonly grouped under the he^.d of oxidation, deoxi-
dation, hydration, and solution.
('!) Oxidation. — Oxidation is perceptibly manifested only in
rocks carrying iron either as sulphide, protoxide carbonate, or
silicate. The sulphides, in presence of water and when not
fully protected from atmospheric influences, readily succumb,
producing sulphates which, being soluble, are removed in solu-
tion, or hydrated oxides, sulphuretted hydrogen, and perhaps
free sulphur, as already noted (p. 29). Such an oxidation is
attended by an increase in bulk, so that if nothing escapes by
solution, there may be brought to bear a physical agency to aid
in disintegration. Weathered rocks, containing iron sulphides.
may not infrequently be found with cubical cavities quite empty
or partially filled with the brownish, yellow, or red product of
its oxidation in a more or less powdery condition. Pyrites,
though a wide-spread constituent, is, nevertheless, a less con-
spicuous agent in promoting rock decomposition than the pro-
toxide carbonates and silicates. In these the iron passes also
over to the hydrated sesquioxide state, as is indicated by the
general discoloration, whereby the rock becomes first streaked
and stained, and finally uniformly ochreous. The more com-
mon minerals thus attacked are the ferruginous carbonates of
lime and magnesia, and silicates of the mica, amphibole, and
pyroxene groups. As the oxidation progresses, the minerals
become gradually decomposed and fall away into unrecogniz-
able forms. The red and yellow colors of soils are due invari-
ably to the iron oxides contained by them. In many cases, the
mineral magnetite, a mixture of proto- and sesqui-oxides, under-
goes further oxidation and also loses its individuality.
(2) Deoxidation is a less common feature than oxidation.
Water, carrying small quantities of organic acids, may take
away a portion of the combined oxygen of a sesquioxide,
converting it once more into the protoxide state. The local
bleaching of certain ferruginous sands and sandstones is due
to this action and a partial removal of the ferriferous salt
in solution. Through a similar process of deoxidation, ferrous
sulphates may be converted into sulphides, a process which
undoubtedly takes place in marine muds protected from atmos-
pheric action.
(3) Hydration — the assumption of water — more commonly
accompanies oxidation, and, indeed, is an almost constant accom-
188 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
paniment of rock decomposition, as may be observed in com-
paring the total percentages of water in fresh and decomposed
minerals and rocks, as given in the analyses.
This assumption, provided it be not accompanied by a loss of
constituents, either by solution or erosion, must be attended by
an increase in bulk, such as may be quite appreciable. The
Comte de la Hure, as quoted by Branner,1 has expressed the
opinion that some of the hills of Brazil have actually increased
in height through this means. The present writer has calcu-
lated that the transition of a granitic rock into arable soil, pro-
vided the same took place without loss of material, must be
attended by an increase in bulk amounting to 88 %.
Hydration as a factor in rock disintegration is, in the writer's
opinion, of more importance than is ordinarily supposed. Granitic
rocks in the District of Columbia have been shown 2 to have be-
come disintegrated for a depth of many feet with loss of but
comparatively small quantities of their chemical constituents
and with apparently but little change in their form of combina-
tion. Aside from its state of disintegration, the newly formed
soil differs from the massive rock, mainly in that a part of its
feldspathic and other silicate constituents have undergone a cer-
tain amount of hydration. Natural joint blocks of the rock
brought up from shafts were, on casual inspection, sound and
fresh. It was noted, however, that on exposure to the atmos-
phere such shortly fell away to the condition of sand. Closer
inspection revealed the fact that the blocks when brought to the
surface were in a hydrated condition, giving forth only a dull,
instead of clear, ringing sound, when struck with a hammer, and
showing a lustreless fracture, though otherwise unchanged.
That such had not previously fallen away to the condition of sand
was evidently due to the vice-like grasp of the surrounding rock
masses. These observations seem to have since received confir-
mation from Professor Derby,3 who states that the sedimentary
rocks of Sao Paulo, Brazil, as seen in the deep railway cuttings,
" are almost invariably soft even when they show no signs of
decay, and go to pieces by a kind of slaking process when
broken up and exposed to the air, though they may have
required blasting in the original opening of the cuttings."
1 Op. cit., p. 284.
2 Bull. Geol. Soc. of America, Vol. VI, p. 321.
» Decomposition of Rocks in Brazil, Jour, of Geol., Vol. IV, 1896, p. 205.
CHEMICAL ACTION OF WATER 189
Professor W. O. Crosby l gives it as his opinion that the dis-
integration of the Pike's Peak (Colorado) granite is due mainly
to hydration, the mica particularly being affected.
Professor Alexander Johnstone showed 2 by experimentation
that normal muscovites, when submitted to the action of pure
and carbonated waters for the space of a year, underwent very
little change other than hydration, and a diminution in lustre,
hardness, and elasticity. They appeared, in fact, to be converted
merely into hydromuscovites, the hydration in pure water hav-
ing gone on nearly as rapidly as in that which was carbonated.
Biotite, when similarly treated, showed a slight discoloration
or bleaching on the edges, accompanied also by hydration, and,
in the case of that in carbonated water, a distinct loss of iron
and magnesia through solution. Lepidolite, voigtite, vermicu-
lite, and pyrosclerite were similarly acted upon, the iron and
magnesia being removed in the form of carbonates. The fact
was noted " that whenever anhydrous micas, or lower hydrated
micas, become hydrated, they always at the same time increase
in bulk." This fact he regarded as accounting for the rapid
weathering of micaceous sandstones. £
(4) Solution. — The solvent action of water is perhaps the
most important of its immediate effects, though there are many
incidental chemical changes set in operation which, in the end,
are of equal or even greater significance. It is the solvent
action only that concerns us here.
Rain and nearly all superficial waters contain small quantities
of carbonic, humic, ulmic, crenic, and apocrenic acids, which
greatly increase their solvent capacities. The last-named forms
are complex, unstable, and little understood products of plant
decomposition,3 and might logically be considered under effects
1 Personal Memoranda to the Writer.
2 Quar. Jour. Geol. Soc. of London, Vol. XLV, 1889.
8 The following are the chemical formulas of these acids, as commonly
given : —
ULMIN AND ULMIC ACID
Carbon °7.1%1
Hydrogen 4.2 I Corresponding to C^^sOu + H20
Oxygen 8.7 j
HUMIN AND HUMIC ACID
Carbon 64.4 %-j
Hydrogen 4.3 [.Corresponding to C2iH24Oi2 + 3 H2O
Oxygen 31.3 j
190 THE PRINCIPLED INVOLVED IN ROCK-WEATHERING
of plant and animal life, but that they act only in presence of
moisture.
" There is reason to believe that in the decomposition effected
by meteoric waters and usually attributed mainly to carbonic
acid, the initial stages of attack are due to the powerful solvent
capacities of the humus acids. Owing, however, to the facility
with which these acids pass into higher stages of oxidation, it is
chiefly as carbonates that the results of their action are carried
down into deeper parts of the crust or brought up to the sur-
face. Although CO2 is no doubt the final condition into which
these unstable organic acids pass, yet during their existence
they attack not merely alkalies and alkaline earth, but even
dissolve silica."1 P. Thernard found that the solvent power
of these acids was largely controlled by the amount of nitrogen
they contained.2
CRENIC ACID
Carbon 44.0 %>
SoglT. '.'.'.'.'.'. '. '. 3.9 [Corresponding to C12H1208?
Oxygen 46.6 J
APOCRENIC ACID
Carbon 34.4%-,
Nitrogen". 3.0 \ Corresponding to C2iU2iOn ?
Oxygen 39.1 J
Berthelot and Andre (Comptes Rendus Academic de Paris, 114, 1892, pp. 41-
43) have shown that the brown substance of humus and analogous compounds
undergo direct oxidation under the influence of the air and .sunlight, forming
carbonic acid. These reactions are purely chemical, taking place without the
intervention of microbes, and are accompanied by a change in color of the orig-
inal humus. The oxidation is rendered more active through the division and
mellowing of the humus by cultivation. Through chemical union of the carbonic
acid with certain bases, as lime soda and potash, there are found soluble car-
bonates which may be leached out by meteoric waters.
1 Geikie, Text-book of Geology, 3d ed., p. 472.
The writer was shown not long since, by Professor Charles E. Munroe, a
very practical illustration of the remarkable corrosive power of organic acids.
A highly ornate French clock, with case of black marble, was packed for storage
in excelsior which was a trifle damp. The clock remained in storage from the
last of May until about the first of October of the same year. When the pack-
ing material was removed, the marble was found to be so corroded as to need
rehoning and polishing. The roughness could be easily felt by passing the
finger over the surface, and long lustreless lines indicating the contact of excel-
sior fibres traversed the surface in every direction.
2 Julien, The Geological Action of Humus Acids, Proc. Am. Assoc. Adv. of
Science, 1879, p. 324.
CHEMICAL ACTION OF WATER 191
It is stated by Storer1 that "on the tops of the higher hills
of New Hampshire, and on the coast of Maine also, a cold, sour
black earth will often be noticed at the surface of the ground,
immediately beneath which is sometimes a layer of remarkably
white earth. The whiteness is due to the solvent action of
acids that soak out from the black humus, and which leach out
from the underlying clay and sand the oxides of iron that for-
merly colored them." ,
As long ago as 1848 the Rogers brothers showed2 that pure
water partially decomposed nearly all the ordinary silicate
minerals which form any appreciable part of our rocks. The
action of carbonated water was recognizable in less than ten
minutes, but pure water required a much longer time before
its effect was sufficient for a qualitative determination. So pro-
nounced was the action of carbonated water that the presence of
the alkalies of lime and magnesia could be recognized in a single
(In ip of the filtrate from the liquid in which the powdered min-
t-nils were digested. By digestion for forty-eight hours they
< ilit ained from hornblende, actinolite, epidote, chlorite, serpen-
tine, feldspar, etc., a quantity of lime, magnesia, oxide of iron,
alumina, silica, and alkalies amounting to from 0.4 % to \% of
the whole mass. The lime, magnesia, and alkalies were ob-
tained in the form of carbonates ; the iron, in the case of horn-
blende, epidote, etc., passing from the state of carbonate to that
of peroxide during the evaporation of the solutions. Forty
grains of finely pulverized hornblende, digested for forty-
eight hours in carbonated water at a temperature of 60°, with
repeated agitation, yielded — silica, 0.08%; oxide of iron,
0.095%; lime, 0.13%, and magnesia, 0.095%, with traces of
manganese. Commenting on these results, Bischof remarks'5
that "by repeating this treatment 112 times with fresh carbon-
ated water, a perfect solution might be effected in 224 days.
If now," he says, "40 grains of hornblende, unpowdered, in
which, according to the above assumption, the surface is only
one millionth of the powdered, were treated in the same way,
and the water renewed every two days, the time required for
perfect solution would be somewhat more than six million
years." In considering these figures and their practical bear-
1 Chemistry as applied to Agriculture.
2 Am. Jour, of Science, Vol. V, 1848.
8 Chemical and Physical Geology, Vol. I, p. 61.
192 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
ing, it must be remembered that while in nature the quantity
of water coming in contact with a crystal embedded in a rock
during a given time is much less than that assumed above, the
mineral is undergoing a gradual splitting up, becoming more
and more porous, so that the process is gradually accelerated.
To quote Bischof again, it is probably admissible to assume
that the time in which water produces similar effects of decom-
position or solution on minerals, is inversely as the magnitude
of the surface of contact. If, therefore, a mineral were so far
subdivided that the surface was increased ten million-fold, the
quantity then dissolved during a certain time would be the same
as that dissolved during a period ten million times as long.
Richard Miiller has also shown1 that carbonic acid waters
will act even during so brief a period as seven weeks upon the
silicate mineral with such energy as to permit a quantitative
determination of the dissolved materials. The accompanying
table shows (1st) the percentages of the various constituents
thus taken out by the carbonated water, and (2d) the total per-
centages of the materials dissolved. That is to say, the figures
0.1552 given for adular under SiO2, indicate that 0.1552% of
the total 65.24% of the silica contained by the mineral have
been removed, and so on. The last column gives the total per
cent of all the constituents extracted.
MINERAL
Si02
A1208
K2O
NajO
MgO
CaO
P20S
FeO
Total
Adular . . .
°/
10
0.1552
01
10
0.1368
01
10
01
lo
o/
lo
01
10
o/
lo
trace
o/
10
0.328
Oligoclase . .
Hornblende .
0.237
0.419
9.1713
trace
2.367
3.213
8528
trace
4.829
0.533
1.536
Magnetite .
trace
0942
0.307
Apatite . . .
2 168
I 822
2.018
Olivine . . .
0.873
trace
1.291
trace
8.733
2.111
Serpentine
0.354
2649
1.527
1.211
• The summary of his investigations he gives as below : —
(1) All the minerals tested were acted upon by the carbonated
water.
(2) In this process there were formed carbonates of lime, iron,
manganese, cobalt, nickel, potash, and soda.
1 Untersuchen iiber die Einwirkung des kohlensaurehaltigen Wassers auf
einige Mineralien und Gesteine, Tschermaks Min. Mittheilungen, 1877, p. 25.
Corroded limestones.
CHEMICAL ACTION OF WATER 193
(3) In the action of the carbonated waters upon the alkaline
silicates, like the feldspars, a small amount of silica went
always into solution, presumably in the form of hydrate.
(4) Even alumina was dissolved in appreciable quantities.
(5) Adular proved more resisting to the action of the acid than
did the oligoclase.
(6) The first stage of decomposition in the feldspars is a redden-
ing process ; the second, kaoliuization.
(7) Hornblende was more easily decomposed than feldspar.
(8) Increase of pressure on the solution was productive of more
energetic action than prolonging the time.
(9) Of all the minerals tested, the magnetic iron was least
affected.
(10) Apatite was readily acted upon, as could be detected by its
appearance under the microscope.
(11) Olivine was the most readily attacked of all the silicates
tested, probably twice as easily decomposed as the ser-
pentine.
(12) Magnesian silicates were attacked by the carbonated waters.
Hence serpentine cannot be considered a final product of
decomposition.1
Of all the materials forming any essential part of the earth's
crust the limestqnes are most affected by the solvent power of
water. It is stated that pure water will dissolve lime carbon-
ate in the proportions of one part in 10800 when cold and one
part in 8875 when boiling.
Since rock-weathering is, as already stated, a superficial
phenomenon, we have to do only with waters of ordinary tem-
peratures and under ordinary conditions of pressure, though
this expression must not be taken as necessarily meaning cold
waters, since, if we accept the statements of Caldcleugh,2 rain
waters falling upon the heated rocks may have their tempera-
tures raised as high as 140° F. The enormously destructive
effect of carbonated waters on limestone is scarcely apparent
on casual inspection, owing to the fact that the material is
carried away in solution, leaving only the insoluble impurities
behind. In such cases it is possible to estimate the amount of
corrosion through a comparison of the proportional amounts of
various constituents in this residue with those in the fresh rock
1 Serpentine, however, cannot be properly considered a decomposition prod-
uct. It is rather a product of alteration.
2 Trans. Geol. Soc. of London, 1829.
194 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING
(see p. 209 et seq.^), and the time limit of corrosion through
determining the percentage amounts of the constituents in the
water which annually drains from any given area. By such
methods it has been estimated1 that some 275 tons of calcium
carbonate are annually removed from each square mile of Cal-
ciferous limestone exposed in the Appalachian region alone ;
while a well-known English authority2 has calculated that with
an annual rainfall of 32 inches, percolating only to a depth of
18.3 inches, there are annually removed by solution from the
superficial portions of England and Wales an average of all
constituents amounting to 143.5 tons per square mile of area.
He further calculates that the average amount of carbonate of
lime annually removed from each square mile of the entire
globe amounts to 50 tons.3 It is to this corrosive action of
meteoric waters that still another authority4 would attribute
the slight thickness and nodular condition of many beds of
Palseozoic limestone. He argues that originally thick-bedded
limestones have, during the ages subsequent to their formation
and uplifting, become so impoverished through the dissolving
out and carrying away in solution of the lime carbonate, as to
have been quite obliterated, or reduced to mere nodular bands,
and given rise to important palseontological breaks in the geo-
logical record. Other than organic acids may locally exert a
potent influence. Thus Robert Bell has described the dolomitic
limestones underlying the waters along Grand Manitou Island,
the Indian peninsula, and adjacent portions of Lake Huron and
the Georgian Bay, as pitted and honeycombed in a very pecu-
liar and striking manner. This corrosion, it is believed, is
produced through the solvent action of sulphuric acid in the
water, the acid itself arising from the decomposition of the sul-
phides of iron, pyrites and pyrrhotite, which exist in great
quantities in the Huronian rocks to the northward.5
1 A. L. Ewing, Am. Jour, of Science, 1885, p. 29.
2 T. Mellard Reade, Chemical Denudation in Relation to Geological Time.
3 The total dissolved constituents thus removed are divided up as follows :
Carbonate of lime, 50 tons ; sulphate of lime, 20 tons ; silica, 7 tons ; carbonate
of magnesia, 4 tons ; peroxide of iron, 1 ton ; chloride of sodium, 8 tons ; alka-
line carbonates and sulphates, 6 tons.
4 F. Rutley, The Dwindling and Disappearance of Limestones, Quar. Jour.
Geol. Soc. of London, August, 1893.
5 Bull. Geol. Soc. of America, Vol. VI, pp. 47-304.
Messrs. C. VV. Hayes and M. R. Campbell, of the United States Geological
MECHANICAL ACTION OF WATER AND OF ICE 195
3. MECHANICAL ACTION OF WATER AND OF ICE
Aside from its solvent capacity, water acts as a powerful ero-
sive agent, as well as an agent for the transportation of the
eroded materials. It is only its erosive power that need con-
cern us here, though, as will be seen, this is to a considerable
extent dependent upon its power of transportation. Every
raindrop beating down upon a surface already sorely tried by
heat and frost serves to detach the partially loosened granules,
and, catching them up in the temporary rivulets, carries them
to the more permanent rills, to be spread out over the valley
bottoms, or perhaps if the slopes be steep and the current ac-
Survey, have recently reported some remarkable examples of corroded quartz
pebbles which should be mentioned here, although a satisfactory explanation for
the phenomenon has not yet been given.
Dr. Hayes, in a personal memorandum to the writer, describes the occur-
rence as follows: —
"At three rather widely separated points in the South, conglomerates have
been observed in which the projecting portions of the pebbles have been etched
or partly dissolved.
"The first, observed by Mr. Campbell, is at Nuttall, West Virginia. The
conglomerate in question, which belongs to the coal measures, is composed of
rather coarse quartz sand with slightly yellowish cement, in which are embedded
well-worn pebbles of white vein quartz. The latter vary in size up to three-
quarters of an inch in diameter, and are somewhat irregularly distributed.
Ordinarily the pebbles, wholly unaltered, weather out by the chemical or
mechanical disintegration of the sandy matrix. In the case observed, however,
where the conglomerate received the drip from an overhanging cliff, the project-
ing portions of the pebbles are deeply pitted, evidently by solution. Mechanical
wear is precluded by the form of the resulting surface, which is not smooth like
the portions of the pebble still protected by the matrix, but is rough and irregu-
lar. The outer portion of the pebbles is evidently less easily affected by the
solvent than the interior, and forms a sharp rim about the irregular cavities
hollowed out within. In some cases a third of the pebble has thus been re-
moved. The surface of the sandstone matrix in which the pebbles are embedded
is also pitted, possibly by the same process of solution as that which has affected
the pebbles, but such a surface might also be produced by mechanical means in
case the cement were less indurated in some places than in others.
" The second case is on Clifty Creek, White County, Tennessee. The con-
glomerate, also a member of the coal measures, forms the bottom of a small
canon, and is covered by the creek at high water, but uncovered throughout
the greater part of the year. The matrix is a coarse white sandstone which
weathers yellow by the oxidation of the slightly ferruginous cement. Embedded
in this are rather abundant pebbles, varying in size up to two inches in diameter,
and composed chiefly of quartz, with a few of chert and possibly of quartzite.
The projecting portions of these pebbles have been in part removed, though they
still project somewhat above the enclosing matrix. As in case of the Nuttall
conglomerate, the exterior portions of the. pebbles are less easily affected than
196 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
cordingly strong, to the rivers and thence to the sea. The
amount of detrital matter thus mechanically removed from
the hills and spread out over valley and sea-bottoms quite ex-
ceeds our comprehension, but it is estimated that at the rate
the Mississippi River is now doing its work, the entire Ameri-
can continent might be reduced to sea-level within a period of
four and one-half million years. The Appalachian Mountain
system, whose uplifting began in early Cambrian times and
terminated at the close of the Carboniferous, has already
through this cause lost more material than the entire mass of
that which now remains. But the rivers, like the winds and
glaciers, in virtue of this load they bear, become themselves
converted into agents of erosion, filing away upon their rocky
beds, undermining their banks, and continually wearing away
the land by their ceaseless activity. The pot-holes in the bed
of a stream, formed by the constant swirl of sand and gravel
in an eddy, furnish on a small scale striking illustrations of this
cutting power, while the rocky canons of the Colorado of the
West, where thousands of feet of horizontal strata have been cut
through as with a file, show the same thing on a scale so gigan-
tic as to be at first scarce comprehensible.1 An item of no
insignificant importance to be considered here is the possibility,
the interiors, and when the pebble has been a third or half removed the outer
shell forms a rim within which is a depression with a slight elevation in the
centre. The chert pebbles show less evidence of corrosion by a solvent than
those composed of quartz. Their upper surfaces are somewhat worn down and
even slightly hollowed, but this might easily have been produced by mechanical
means, which is not the case with the quartz.
" The third case is a block of conglomerate from Starrs Mountain, Tennessee,
collected by Mr. Bailey Willis. This is of Lower Cambrian age. The matrix is
a coarse feldspathic sandstone containing layers of well-rounded pebbles, mostly
quartz, with a few probably of some feldspar. The former are between one-half
and one inch in diameter and the latter somewhat larger. The projecting por-
tions of the quartz pebbles on one side of the block are almost entirely removed,
and as in the other cases evidently by solution. A slight rim projects above the
matrix in which the pebbles are embedded ; within this is a depression, while a
slight elevation occupies the centre.
"The projecting portions of the feldspathic pebbles also are partly removed,
but this may be due to corrasion instead of corrosion, that is, to the action of
mechanical rather than chemical agents. The pebbles on the lower side of the
block have their original water-worn surfaces without any trace of etching."
1 Captain C. E. Button has estimated (Tertiary History of the Grand Canon
of the Colorado) that from over an area of 13,000 to 15,000 square miles drained
by the Colorado River, an average thickness of 10,000 feet of strata have been
removed.
MECHANICAL ACTION OF WATER AND OF ICE 197
indeed probability, of an incidental chemical decomposition
taking place during this abrasive action. Daubree showed l
that when feldspathic fragments were submitted to artificial
trituration in a revolving cylinder containing water, a decompo-
sition was effected whereby the alkalies were liberated in very
appreciable amounts. He found further that the principal
product of mutual attrition of feldspar fragments in water was
not sand, but an impalpable mud (limori). This mud was of
such tenuity as to remain for many days in suspension, and
on desiccation became so hard as to be broken only with
the aid of a hammer, resembling in many respects the argillites
of the coal measures, but differing in that it carried a high
percentage of alkalies. Granitic rocks thus treated yielded
angular fragments of quartz and very minute shreds of mica,
while the feldspars ultimately quite disappeared in the form
of the impalpable mud above mentioned. It was noted that
after the quartzose particles had reached a certain degree of
fineness further diminution in the size ceased, owing to the
buoyant action of the water, which in the form of a thin film
between adjacent particles acted as a cushion and prevented
actual contact to the extent necessary for mutual abrasion. It
is to a similar action on the part of sea- water that Shaler 2 would
attribute the lasting qualities of the sand grains upon our sea
beaches. Indeed the conditions of Daubree's experiments as
a whole were not so different from those existing in nature that
we need hesitate, as it seems to the writer, to conclude similar
action, both chemical and physical, may be going on wherever
abrasion takes place in the presence of continual moisture, as in
the bed of a river or glacier.
1 It will be remembered that this authority placed rock fragments in stone and
iron cylinders containing water and made to revolve horizontally at a measured
rate of speed, so that the actual distance travelled by any of the particles dur-
ing a given time could be readily calculated. The product of this disintegration,
even when carried to the condition of fine silt, was always sharply angular. His
experiments further showed that when feldspathic fragments were thus treated,
there was always a certain amount of decomposition, whereby salts of potash were
liberated ; in one instance, when 3 kilogrammes of feldspar were revolved for 192
hours in iron cylinders containing 5 litres of water, 2.72 kilogrammes of finely
comminuted mud were obtained, and in solution in the water, 12.6 grammes of
potash, or 2.52 grammes per litre. The presence of carbonic acid in the water
increased the amount of potash. When the feldspar was triturated dry and then
treated with water, no such solvent action could be detected. — Geologic Experi-
mental, p. 268.
2 Bull. Geol. Soc. of America, Vol. V, p. 208.
198 THE PRINCIPLES INVOLVED IN KOCK- WEATHERING
The hammering action of waves upon the sea-coast exerts a
powerful erosive action, particularly upon particles of rock of
such size as to be lifted or moved by wave action, but too heavy
to be protected from attrition by the thin film of water above
alluded to. Shaler's observations1 at Cape Ann were to the
effect that ordinary granitic paving blocks (weighing perhaps
twenty pounds) were, when exposed to surf action, worn in
the course of a year into spheroidal forms such as to indicate
an average loss of more than an inch from their peripheries.
Experiments made with fragments of hard burned brick showed
that in the course of a year they would be reduced fully one-half
their bulk. Even the crystallization of the salt thrown up by
wave action and absorbed into the pores of rocks 2 serves in its
way the purposes of disintegration.
The Action of Freezing Water and of Ice. — The action of
dry heat and cold in disintegrating rocks has already been
described. The effects of such temperature changes upon
stone of ordinary dryness are, however, slight in comparison
with the destructive agencies of freezing temperatures upon
stones saturated with moisture. The expansive force of water
passing from the liquid to the solid state has been graphically
described as equal to the weight of a column of ice a mile high
(about 150 tons to the square foot). Otherwise expressed, 100
volumes of water expand, on freezing, to form 109 volumes of
ice. Provided, then, sufficient water be contained within the
pores of a stone, it is easy to understand that the results of
freezing must be disastrous. That stones as they lie in the
ground do contain moisture, often in no inconsiderable amounts,
is a well-known and well-recognized fact by all those engaged
in quarrying operations, and indeed no mineral substance is
absolutely impervious to it. The amount contained, naturally
varies with the nature of the mineral constituents and their
state of aggregation. According to various authorities, granite
may contain some 0.37% by weight; chalk, 20%; ordinary
compact limestone, 0.5% to 5 % ; marble, about 0.80% ; and
sandstones, amounts varying up to 10% or 12%, while clay
1 Bull. Geol. Soc. of America, Vol. V, p. 208.
2 According to Dana (Wilkes' Exploring Expedition, Geology, p. 529), the
sandstones along the coast of Sydney, Australia, are subjected to a mechanical
disintegration through the crystallization of salt which is absorbed from the
saline spray of the ocean waves.
MECHANICAL ACTION OF WATER AND OF ICE 199
may contain nearly one-fourth its weight. This water is largely
interstitial — the quarry water, as it is sometimes called. In
addition to this, the quartz, particularly of granitic rocks,
almost universally contains innumerable minute cavities par-
tially filled with water, and which are, in extreme cases, so
abundant as to make up, according to Sorby, at least 5 % of the
whole volume of the mineral.
That the passage of this included moisture from the liquid
to the solid state, must be attended with results disastrous to
the stone is self-evident, though the rate of disintegration may
be so slow under favorable circumstances as to be scarce notice-
able. Freezing of the absorbed water is one of the most fruit-
ful sources of disintegration in stones confined in the walls of
a building, and even in the quarry bed it is by no means uncom-
mon to have stone so injured as to render it worthless. How-
ever slight may be the effects of a single freezing, constant
repetition of the process cannot fail to open up new rifts, and
still further widen those already in existence, allowing further
penetration of water to freeze in its turn and to exert a chemical
action as well. So year in and year out, through winter's cold
and summer's heat, the work goes on until the massive rock
becomes loose sand to be caught up by winds or temporary
rivulets and spread broadcast over the land. In some instances,
it may be, the rock is of sufficiently uniform texture to be af-
fected in all its mass alike. More commonly, however, it is
traversed by veins, joints, or other lines of weakness along
which the rifting power is first made manifest, as in our illus-
tration. Naturally disintegration of this kind is confined to
frigid and temperate latitudes. As bearing upon the extreme
rapidity with which such disintegration may take place, the
following is quoted from a letter of Dr. L. Stejneger, of the
United States National Museum, who passed several months
among the islands of Bering Sea.
"In September, 1882, I visited Tolstoi Mys, a precipitous
cliff near the southeastern extremity of Bering Island. At the
foot of it I found large masses of rock and stone which had
evidently fallen down during the year. Most of them were
considerably more than six feet in diameter, and showed no
trace of disintegration. The following spring, April, 1883,
when I revisited the place, I found that the rocks had split up
into innumerable fragments, cube-shaped, sharp-edged, and of
200 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
a very uniform size, — about two inches. They had not yet
fallen to pieces, the rocks still retaining their original shape.
I may remark, however, that the weather was still freezing
when I was there. The winter was not one of great severity,
and several thawing spells broke its continuity. These cubic
fragments did not seem to split up any further, for everywhere
on the islands where the rock consisted of the coarse sandstone,
as in this place, the talus consisted of these sharp-edged stones."
Ice acts as a disintegrating agent in still other ways than
that mentioned. The phenomenon of the- glacier is now so
well known that we need dwell upon it but briefly here. Long-
continued precipitation of snow upon regions of such elevation,
or in such latitudes as to preclude anything like an equally
rapid melting, gives rise to deep fields of snow, compacted in
the lower portions into the condition of ice. These, in virtue
of the weight of the overlying mass, and perhaps the steepness
of the slopes, aided by a certain amount of plasticity possessed
in some degree by even the most rigid of so-called solids, creep
slowly down the slopes in the form of glaciers or rivers of ice.
Advancing, it may be, but an inch or several feet a day, now scarce
moving at all, or even retreating temporarily through a diminu-
tion in the amount of their supplies, or an increase in the sun's
heat, these bring, either upon their surfaces as moraines, or
frozen into their mass, large quantities of f ragmental rock mate-
rial fallen upon them from above, or picked up from the surfaces
over which they flow. Those fragments which remain upon the
upper surface, or frozen into the upper portions, are but trans-
ported to the lower levels where, the temperature being suffi-
cient, the ice is melted and the load deposited in the form of a
moraine.
Beneath, and frozen into the lower portion of the ice sheet,
there is, however, a variable amount of rock material, which, as
the glacier moves along, is crowded with all the weight of the
overlying mass, and all the resistless energy of the ice behind,
over the surface of the underlying rock. In virtue of this
material, this sand, gravel, and boulder aggregate, the glaciers
become converted into what we may compare to extremely
coarse files, to tear away the rocks over which they pass, and
grind and crush them into detritus of varying degrees of
fineness. The small streams which originate from the melt-
ing of these glaciers become, hence, not infrequently charged
ACTION OF PLANTS AND ANIMALS 201
to the point of turbidity with the fine silt-like detritus ground
from the ledges and in part from the boulders themselves.
Figure 3 of plate 19 shows a slab of limestone still bear-
ing upon its surface the evidences of the severity of the
onslaught. A consideration of the amount of detritus thus
brought down either merely as transported or as abraded
material belongs properly to the chapter on transportation,
but a few illustrations are not without interest here. The
Aar in Switzerland is stated by Geikie to discharge every day
in August some 440,000,000 gallons of water, carrying some
280 tons of sand. A portion of this is in a state of such
minute subdivision as to remain a long time in suspension,
and give the water a milky appearance for several miles.
I. C. Russell has described l the Tuolumne River, issuing from
the foot of the Lyell Glacier in the Sierras of California, as
turbid with silt which has been ground by the moving ice.
At the foot of the Dana Glacier there is a small lakelet
whose waters are of a peculiar greenish yellow color from
the silt held in suspension, and which, when submitted to
microscopic examination, is found to be made up of fresh
angular fragments of various silicate minerals of all sizes from
0.35 mm. in diameter down to impalpable silt.
4. ACTION OF PLANTS AND ANIMALS
Both plants and animals aid to some extent in the work of
rock disintegration. Plants are also not infrequently an im-
portant factor in promoting sedimentation, while burrowing
insects and animals may exert an important influence upon
the texture of soils and in bringing about a more general
admixture by transferring to the surface that which is below.
The lowest forms of plant life, — the lichens and mosses, —
growing upon the hard, bare face of rocky ledges send their
minute rootlets into every crack and crevice, seeking not
merely foot-hold, but food as well.
Slight as is the action, it aids in disintegration. The plants
die, and others grow upon their ruins. There accumulates
thus, it may be with extreme slowness, a thin film of humus,
which serves not merely to retain the moisture of rains and
thus bring the rock under the influence of chemical action,
1 6th Ann. Rep. U. S. Geol. Survey, 1883-84.
202 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
but supplies at the same time small quantities of the humic
and other organic acids to which reference has already been
made.1 These act both as solvents and deoxidizing agents.
As time goes on, sufficient soil gathers for other, larger and
higher types of life, which exert still more potent influences.
It may be the rock is in a jointed condition. Into these joints
each herb, shrub, or sapling pushes down its roots, which, in
simple virtue of their gain in bulk, day by day, serve to enlarge
the rifts and furnish thereby more ready access for water, and
the wash of rains, to still further augment disintegration.
This phase of root action is often well shown in walls of
ancient masonry, either of brick or stone, whereby the usual
rate of destruction is greatly accelerated. The depth to which
such roots may penetrate has often been noted, varying, as is
to be expected, with the nature of the soil.2 In the limestone
caverns of the Southern states, the writer has often been im-
pressed by the number of long thread-like rootlets, so fine as
to be almost imperceptible, which have found their way through
rifts in the rocky roof.
H. Carrington Bolton has shown that very many minerals
are decomposed by the action of cold citric acid for a more or
less prolonged period, the zeolites and other hydrous silicates
being especially susceptible. Such tests have a peculiar sig-
nificance when we consider that the roots of growing plants
secrete an acid sap, which, by actual experiment, has been found
capable of etching marble. The exact nature of this acid is
not accurately known, but it is considered probable that in the
rootlets of each species of plant there exists a considerable
variety of organic acids.3
But the effects of plant growth are not necessarily always
destructive ; such may be conservative or even protective. In
glaciated regions, it is often the case that the striated and pol-
ished surfaces of the rocks have been preserved only where pro-
tected from the disintegrating action of the sun and atmosphere
1 It is stated by Storer (Chemistry as applied to Agriculture) that some
lichens have been found to contain half their weight of oxalate of lime.
2 Aughey has found roots of the buffalo berry (Sherperdia aryophylla) pene-
trating the loess soils of Nebraska to the depth of 50 feet.
3 See Application of Organic Acids to the Examination of Minerals, H. Car-
rington Bolton, Proc. Am. Assoc. for the Advancement of Science, XXXI, 1883,
and Available Mineral Plant Food in Soils, B. Dyer, Jour. Chem. Society, March,
1894.
ACTION OF PLANTS AND ANIMALS 203
by a thin layer of turf or moss. As a general rule, however,
the manifest action of plant growth is to accelerate chemical
decomposition, through keeping the surfaces continually moist,
and to retard erosion. (See further on p. 280.)
Action of Bacteria. — The researches of A. Miintz,1 Wido-
gnidsky, Schlosing, and others tend to show that bacteria may
i-xt-irise a very important influence in promoting rock disinte-
gration and decomposition. Their influence in promoting nitri-
rication has been already alluded to. It would appear that
while these organisms secrete and utilize for their sustenance
the carbon from the carbonic acid of the atmosphere, as do
plants of a higher order, they may also assimilate carbonate
of ammonium, forming from it organic matter and setting free
nitric acid. Being of microscopic proportions, the organisms
penetrate into every little cleft or crevice produced by atmos-
pheric agencies, and throughout long periods of time produce
results of no inconsiderable geological significance. The depth
below the surface at which such may thrive is presumably but
slight, and their period of activity limited to the summer months.
They have been found on rocks of widely different character —
granites, gneisses, schists, limestones, sandstones, and volcanic
rocks — and on high mountain peaks as well as on lower levels.
The Pic Pourri, or Rotten Peak, in the Lower Pyrenees of south-
western France, is composed of friable and superficially decom-
posed calcareous schists, throughout the whole mass of which
are found the nitrifying bacteria, which are believed to have
been instrumental in promoting its characteristic decomposition.
The organism acts even upon the most minute fragments, reduc-
ing them continually to smaller and smaller sizes. Each frag-
ment loosened from the parent mass is found coated with a film
of organic matter thus produced, and the accumulation begun
by these apparently insignificant forces is added to by residues
of plants of a higher order, which come in as soon as food and
foothold are provided.2
Mr. J. E. Mills,3 and after him J. C. Branner,4 lay con-
siderable stress on the decomposing effect of vegetable matter
1 Comptes Rendus de 1' Academic des Sciences, CX, 1890, p. 1370.
2 It is, perhaps, as yet, too early to say to what extent the presence of bacteria
may be incidental to decomposition, rather than causative.
8 American Geologist, June, 1889, p. 357.
* Bull. Am. Geol. Soc. of America, Vol. VII.
204 THE PRINCIPLES INVOLVED IN ROCK- WEATHERING
carried into the ground by ants in certain parts of Brazil, Mills
going so far as to describe the ants as continually pouring car-
bonic acid into the ground. Be this as it may, the evacuations
of the ants themselves are undoubtedly of such a nature as to
further the processes of decomposition. Certain species of ants,
locally known as saubas, or sauvas, live, according to Brainier,
in enormous colonies, burrowing in the earth, where they exca-
vate chambers with galleries that radiate and anastomose in
every direction, and into which they carry great quantities of
leaves. Certain species of termites, the white ants of Brazil, are
also active promoters in bringing about changes in the structure
of the soil, and incidentally accelerating decomposition. The
organic matter carried by these creatures into the ground, there
to decompose, furnishes organic acids to promote further decay
in the material close at hand, and by its downward percolation
to attack the still firm rocks at greater depths. Indeed, these
numerous channels, through affording easy access of air and
surface waters with all their absorbed gases or alkaline salts,
may serve indirectly a geological purpose scarcely inferior to
that of the joints in massive rocks. (See further under soil
modified by plant and animal life.)
The mechanical agency which has already been referred
to as instrumental in bringing about a certain amount of de-
composition in silicate minerals, is greatly augmented when
such trituration takes place in connection with organic matter.
J. Y. Buchanan has shown,1 that the mud of sea-bottoms is being
continually passed and repassed through the alimentary canals
of marine animals, and that in so doing the mineral matter not
merely undergoes a slight amount of comminution and conse-
quent decomposition, but a chemical reduction takes place
whereby existing sulphates are converted into sulphides. Such
sulphides and the metallic constituents of the silicates and other
compounds, particularly those of iron and manganese, would
on exposure to sea-water become converted into oxides. It is
through such agencies that he would account for the presence
of sulphur in marine muds, and the variations in color, from
shades of red or brown to blue and gray, in the former the iron
occurring as oxides, while in the latter it exists as a sulphide.
Of course either form may be more or less permanent according
1 On the Occurrence of Sulphur in Marine Muds, Proc. Royal Soc. of Edin-
burgh, 1890-91.
THE PRODUCTION OF CARBONATES 205
as the mud may be devoid of animal life, or protected from
oxidizing influences. These reactions, being subaqueous, are
somewhat beyond the scope of the present work, but are never-
theless not without interest in this connection.
One* of the most conspicuous results of rock-weathering
through the agencies of water and organic acids, as above enu-
merated, is manifested in the production of carbonates of lime
and more rarely of magnesia, iron, and the alkalies. Thus in
the decomposition of lime-bearing silicates, as the feldspars,
pyroxenes, and amphiboles, the lime almost invariably separates
out as calcite or aragonite, and often may be found filling cracks
and crevices, as veins of " spar " in the very rock masses from
which it was derived. The celebrated verde di Geneva and
verde di Prato marbles are but secondary rocks derived by
hydration from pre-existing pyroxenic masses and in which the
lime and magnesia have separated out as carbonates forming
the white veins by which the stone is traversed. The almost
universality of carbonate formation incident to rock-weathering
manifests itself in the ready effervescence of freshly decomposed
material when treated with an acid. It is indeed difficult to
find weathered rocks of any kind that will not show at least
traces of secondary carbonates, of which those of calcium are by
far the more abundant.
It is further to be noted that the solvent and general chemical
activity of water is often greatly augmented by the salts and
acids it acquires through the decomposition of various minerals
with which it comes in contact. Thus through the decomposi-
tion of iron pyrites there may be formed free sulphuric acid,
or through the decomposition of a feldspar, carbonates of the
alkalies, any of which, when in solution, are more energetic
factors in promoting decomposition than water alone. Hence
under certain conditions the process of decomposition once set
in operation augments itself, and goes on with increasing vigor
until such a depth is reached that the percolating solutions
become neutralized and further action, aside from hydration,
practically ceases.
THE "WEATHERING OF ROCKS (Continued)
II. CONSIDERATION OF SPECIAL CASES
Let us now enter into a consideration of the composition of
a few prominent rock types, and note the changes they have
undergone in this process of weathering, assuming, as we must
for the time being, that they have been all subjected to essen-
tially the same conditions. Inasmuch, as has been noted already,
there are divers types of rocks, differing not merely in chemical
and mineral composition, but in structure as well, it is an easy
assumption that the results of prolonged weathering may be
widely divergent. Yet, as will become apparent, the ultimate
products from all but the purely quartzose rocks, present strik-
ing similarities.
In the tables following are given the results of chemical and
mechanical analyses of rocks of various kinds and in varying
stages of degeneration. We will begin with a consideration
of the granitic rocks of the District of Columbia.1
The rock (see PL 14) in its fresh condition is a strongly
foliated gray micaceous granite showing to the unaided eye
a finely granular aggregate of quartz and feldspars arranged in
imperfect lenticular masses from 2 to 5 mm. in diameter, about
and through which are distributed abundant folia of black
mica. In the thin section the structure is seen to be cataclastic.
Quartz and black mica are the most prominent constituents,
though there are abundant feldspars of both potash and soda-
lime varieties, which, owing to their limpidity, can by the
unaided eye scarcely be distinguished from the quartz. The
potash feldspar has in part a microcliiie structure. Aside from
these minerals, a primary epidote, in small granules and at times
quite perfectly outlined crystals, is a strikingly abundant con-
stituent. Small apatites, a few flakes of white mica (sericite),
1 Disintegration of the Granitic Rocks of the District of Columbia, Bull. Geol.
Soc. of America, Vol. VI, 1895, pp. 321, 332.
206
WEATHERING OF GRANITE
207
and widely scattering black tourmalines and iron ores complete
the list of recognizable minerals.
The outcrops from which the samples for the analyses to
which attention is first called were selected are shown in the
plate. At the very bottom, the rock is hard, fresh, and com-
pact, without trace of the decomposition products other than
as indicated by minute infiltrations of calcite from above. Just
above the level of the small creek which flows at the foot of
the bluff, at the point indicated by the first series of right-and-
left joints near the centre of the view, the character of the rock
changes quite suddenly, becoming brown and friable, though
still retaining its form and easily recognizable granitic appear-
ance. A few feet above a third zone begins, in which the rock
is converted into sand and gravel and which becomes more and
more soil-like to the top of the bank, where it becomes admixed
with organic matter from the growing plants. The amount
of organic matter is quite small, however, and in making the
analyses care was taken to remove such as was recognizable in
the form of rootlets, leaves, and twigs.
Bulk analyses of these three types, (I) fresh gray granite,
(II) brown but still moderately firm and intact rock, and (III)
the residual sand, yielded the results given in the columns cor-
respondingly numbered below: —
CONSTITrENTO
I
II
III
I'rniti<>n
1.22%'
3.27 %
4.70%
Silira (SiO2)
69.33
M.89
65.69
Titanium (TiOo)
not det.
not det.
0.31
Alumina. (AUOg)
14.33
15.62
16.23
Iron protoxide (FeO)
3.601
1.69
Iron sesquioxide (FejOg)
1.88
4.88
Lime (CaO)
3.21
3.13
2.63
Magnesia (MgO)
2.44
2.76
2.64
Soda (NasO)
2.70
2.58
2.12
Potash (K2O)
2.67
2.04
2.00
Phosphoric acid (P205)
0.10
not det.
0.06
99.60 %
99.79%
99.77 %
In glancing over these figures it is at once apparent that
there is a surprisingly small difference in ultimate composition
1 4.00% when calculated as Fe2O3.
208 ROCK DISINTEGRATION AND DECOMPOSITION
between the sound rock and the residual sand, the more marked
differences being a slightly smaller amount of silica, more alu-
mina, and slightly diminished amounts of lime, magnesia, pot-
ash, and soda, with a considerable increase in the amount of
water. The ferrous salts have moreover been converted into
ferric forms. It does not necessarily follow, however, that no
more actual gain or loss of material or change in manner of
combination than is here indicated may not have taken place,
and at the very outset it may be well to enter into a discussion
of the manner in which the results of such analyses are to be
considered.
We must first of all remember that any indicated loss or
gain of a constituent may be only apparent, and that the true
relative proportions can be learned only by calculating results
of analyses of both fresh and decomposed materials on a com-
mon basis. Thus the first glance at analysis III, as given,
might lead one to surmise that the decomposed rock had actually
lost only some 3.3% of silica. This, however, is not strictly
the case, since this analysis shows 4.7% volatile constituents
against 1.22% in analysis I of the fresh material. Could we
assume that this difference of 3.48 % was due wholly to a
uniform absorption of moisture, as by a clay, the problem would
resolve itself into simply recalculating all analyses upon a
water-free basis.
The results obtained thus are not quite satisfactory, however,
and it is thought a more correct view of the changes taking
place may be obtained by assuming for one of the constituents
a fairly constant value and using this as a basis for comparison.
Of all the essential constituents occurring in appreciable
quantities in siliceous crystalline rocks the alumina and the iron
oxides are the most refractory and the least liable to be removed
by a leaching process, although they may undergo manifold
changes in mode of combination. Although not absolutely
correct, therefore, we will for our present purposes assume the
one or the other of these (in this case the iron as Fe2O3) as a
constant factor, and in order to show the proportional or actual
amount of loss of any constituent will recalculate the analyses
upon this basis, a proceeding for which, so far as alumina is
concerned, we have already good authority.1 This method will
be adopted, however, only with the siliceous crystalline rocks,
1 G. Roth, Allegemeine u. Chemische Geologie, 3d ed.
WEATHERING OF GRANITE
209
in which, for reasons noted later, the process of decomposition,
we have reason to suppose, is more complex than in calcareous
and magnesian rocks poor or lacking in the alkalies. The
entire discussion is one beset with great difficulties, since we
lack definite knowledge as to the exact processes which have
been going on and need constantly to guard against assump-
tions too hastily drawn or based upon insufficient data. Indeed,
any assumption based upon the results of chemical analyses
alone is likely to lead to grave error.
If, then, we consider the iron in the form of Fe2O3 as a constant
factor, we may, by proper calculation, obtain the results given
in column (IV) below, which represent the proportional gain
and loss of the various constituents of the rock in passing from
the condition indicated in column (I) above, to that indicated
in column (III). Such a comparison is instructive as showing
not merely the relative loss and gain, but also the total loss of
material, in this case 13.47 %, accompanied by a gain of 2.16%,
in volatile matter.
DISINTEGRATED AND DECOMPOSED GRANITE, DISTRICT OP COLUMBIA, SHOWING
PROPORTIONAL Loss OF CONSTITUENTS
IV
V
VI
CONSTITUENTS
PERCENTAGE
Loss FOR EN-
TIBE ROCK
PERCENTAGE
or EACH CON-
STITUENT SAVED
PERCENTAGE
OF EACH CON-
STITUENT LOST
Silica (Si02)
10.60 %
85.11%
14.89%
Alumina (Al2Oj)
0.46
96.77
3.23
Iron sesquioxide (Fe2O8)
Iron protoxide (FeO)
} 0.00
100.00
0.00
Lime (CaO)
0.81
74.79
26.21
Magnesia (MgO)
0.36
98.51
1.49
Soda (Na2O)
0.77
71.38
28.62
Potash (K2O)
0.85
68.02
31.98
Phosphoric anhydride (PaOs) . . .
Ignition
0.04
2.161
60.00
100.00
40.00
0.00
Total loss
13.47 %
Such results are still far from satisfactory, and it is believed
the tables will be more useful and instructive can we show the
1 Gain.
210 ROCK DISINTEGRATION AND DECOMPOSITION
percentage loss and gain of each constituent as compared with
the same constituent in the original rock. This can also readily
be accomplished by a process the formula for which is given
below,1 and by which are obtained the results given in columns
V and VI.
From a perusal of these figures, it appears that the residual
sand retains 85.11% of the original silica; 96.77% of the alumina;
all the ferric oxide; 74.79% of its lime; 98.51% of its mag-
nesia, together with 71.38 % of its soda and 68.02 of the potash,
while there has been an actual gain, as was to be expected, in
volatile matter.
Let us not, however, too hastily assume that we have ex-
hausted the subject.
We must remember, further, that while an analysis shows the
actual composition of a rock so far as the various elements are
concerned, it quite fails to show the manner in which those
elements are combined. While the ultimate composition of
the fresh and decomposed samples may be closely similar, it is
possible, indeed probable, that in some cases at least the manner
of combination of these elements is quite different. This is
well illustrated in the case of the figures showing the percent-
ages of alumina in analyses I and III and which differ only
nine-tenths of one per cent in total amount; yet in the first the
alumina exists mainly in the form of anhydrous silicates of
alumina, potash, iron, and magnesia (as in the feldspars and
mica), while in the last a very considerable proportion, or indeed
all in extreme cases of weathering, may exist as a hydrous sili-
cate of alumina only (kaolin). It is in instances of this kind
that the microscope may render efficient service, and much may
be learned by means of such mechanical analyses as can be made
by sifting and washing. Such separations made on this disin-
tegrated rock showed it to consist of particles as given in the
following table, the 4.25 % silt being obtained by washing the
A
1 The formula employed in these calculations is as follows : — — — = x : and
B x C
100 — x = y, in which A = the percentage of any constituent in the residual
material ; B = the percentage of the same constituent in the fresh rock, and
C = the quotient obtained by dividing the percentage amount of alumina (or
iron sesquioxide, whichever is taken as a constant factor) of the residual mate-
rial by that in the fresh rock, the final quotient being multiplied by 100. x then
equals the percentage of the original constituent saved, in the residue, and y the
percentage of the same constituent lost.
WEATHERING OF GRANITE 211
10.75% of material which passed through fine bolting-cloth of
120 meshes to the lineal inch, and which represents the impal-
pable mud remaining in suspension while the 6.5 % of fine sand
sank quickly to the bottom of the beaker in which the washing
was made. The residual sand yielded then: —
Silt 4.25% Largest grains 0.1 mm. in diameter
Very fine sand 6.50 " " 0.18
Fine sand 11.25 " " 0.25
Medium sand 3.80 " " 0.65
Sand) 11.00 « " 1.00
Sand I 23.50 " " 1.50
Coarse sand 29.50 " " 2.00
Gravel . 10.20 " " 8.00
Total 100.00%
The coarser of these particles, like the gravel and coarse
sand, are of a compound nature, being aggregates of quartz
and feldspar, with small amounts of mica and other minerals.
In the finer material, on the other hand, each particle repre-
sents but a single mineral, the process of disaggregation having
quite freed it from its associates, excepting, of course, the
microscopic inclusions which could be liberated only by a
complete disintegration of the host itself. These particles,
as seen under the microscope, are all sharply angular, and in
many cases surprisingly fresh, though the analyses, as given
above, had suggested only a slight change in chemical composi-
tion. The mica shows the greatest amount of alteration, the
change consisting mainly in an oxidation of its ferruginous
constituent, whereby the folia becomes stained and reduced
to yellowish brown shreds. The feldspars are, in some cases,
opaque through kaolinization, but in others are still fresh and
unchanged even in the smallest particles. The finest silt,
when treated with a diluted acid to remove the iron stains,
shows the remaining granules of quartz, feldspar, and epidote
beautifully fresh, and with sharp, angular borders, the mica
being, however, almost completely decolorized.
An analysis of the silt, which was found to constitute 4.25%
of the entire mass of disintegrated material, as noted above,
is given below, and also a partial separation and analysis of
the 39.7% soluble, and 60.3% insoluble portions.1
1 In all analyses made by or under the direction of the author, the matter
tabulated as soluble is that extracted by boiling for three hours in hydrochloric
212 EOCK DISINTEGRATION AND DECOMPOSITION
ANALYSES OF SILT FROM DISINTEGRATED GRANITE
CONSTITUENTS
I
II
III
BULK ANALYSIS
OF SILT
ANALYSIS OF
SOLUBLE PORTION
(39. 7%) SILT
ANALYSIS OF
INSOLUBLE PORTION
(60.3%) SILT
Ignition
8.12%
49.39
23.84
3.69
4.41 A
4.60 1
3.36 [
2.49 J
8.12%
InHCl 1.123
InNa2C0311.147
9.21
4.47
Not det.
0.97%
} 37.30
13.40
0.82
(2.90
Trace
2.75
1.07
Silica (Si02)
Alumina (Al20g) ....
Iron sesquioxide (FegOs) .
Lime (CaO)
Magnesia (MgO) ....
Soda (Na2O)
Potash (K2O) . . .
99.90%
34.07
59.21
93.28 %
From these analyses it would appear that of the 17 grammes
of silt, representing 4 % of the total disintegrated material,
only 39.7% is soluble ; and, further, that a very considerable
proportion of the insoluble residue, as indicated by the high
percentages of alkalies and lime, still consist of unaltered soda-
lime and potash feldspars, the iron and magnesia alone having
been largely removed.
These results are not quite what one would be led to expect
from a perusal of the literature bearing upon the subject of
rock decomposition. As long since noted by J. G. Forch-
hammer, G. Bischof, T. Sterry Hunt, and others, the ordinary
processes of decay in siliceous rocks containing ferruginous
protoxides and alkalies consists in the higher oxidation and
separation of the protoxides in the form of hydrous sesqui-
oxides and a general hydration of the alkaline silicates, accom-
panied by the formation of alkaline carbonates, which, being
readily soluble, are taken away nearly as fast as formed. More
or less silica is also removed, according to the amount of car-
bonic acid present, a portion of the alkalies forming soluble
acid of one-half normal strength, to which is added the silica set free in a gelati-
nous form by the acid and subsequently extracted by sodium carbonate solu-
tion. All analyses made on material first dried at 100° C.
WEATHERING OF GRANITE 213
alkaline silicates when the supply of the acid is insufficient to
take them all up in the form of carbonates. The apparent
anomaly here shown is partially explained by examination of
the various separations with the microscope. Thus the low
percentage of silica is found to be in large part due to the fact
that the residual quartz granules are, in many cases, too large
to pass the 120-mesh sieve, or, if passing, have been largely
separated in the process of washing. Further, it is found that
the sifting has served to concentrate the small epidotes in the
fine sand, and a portion of them have even come over with
the silt. The presence of this epidote also explains in part the
high percentage of lime shown, since the mineral itself carries
some 20 to 24 % of this material. The large percentages of
magnesia, soda, and potash cannot, however, be thus accounted
for, and we are led to infer that either these elements are there
combined in minute amorphous zeolitic compounds, unrecog-
nizable as such under the microscope, or, as seems more prob-
able, the feldspathic constituents, to which the alkalies are to be
originally referred, have undergone a mechanical splitting up
rather than a chemical decomposition. This view is, to a
certain extent, borne out by microscopic studies, but it is diffi-
cult to measure by the eye the relative abundance of these
constituents with sufficient accuracy to enable one to form any
satisfactory conclusion. "The magnesia must come from the
shreds of mica, many of whi'ch, from their small size and almost
flocculent nature when decomposed, would naturally be found
in the silt obtained as stated.
It is to be noted that the magnesia, together with the iron,
exists almost wholly in a soluble form.
It is evident at once that we have had to do here with
but the preliminary stages of granitic weathering, that the
process is more one of disintegration than decomposition, and
it will be well to consider now a case in which the decom-
position has gone on to the condition of a residual clay, as
found in many of the Southern states. For this purpose a
biotite gneiss or gneissoid granite found near North Garden,
in Albemarle County, Virginia, is selected. The rock is a
coarse gray feldspar-rich variety with abundant folia of black
mica. Under the microscope it shows the presence of both
potash and soda-lime feldspars, a sprinkling of apatite and
iron ores, sporadic occurrences of an undetermined zeolite, and
214 ROCK DISINTEGRATION AND DECOMPOSITION
an extraordinary number of minute zircons which are mostly
enclosed in the feldspars. There are also present occasional
small garnets and aggregates of decomposition products the
exact nature of which was not made out. The residual soil
resulting from the decomposition of this rock is highly plastic,
of a deep red-brown color, and has a distinct gritty feeling in
the hand, owing to the presence of quartz and undecomposed
silicate minerals. In columns I and III below are given the
results of analyses of fresh rock and residual soil, and in II, IV,
and V the analyses of the soluble and insoluble portions. In
columns VI, VII, and VIII are given the calculated percentage
amounts of the various constituents saved and lost, as before.
The particular features to which attention need here be
called, are (1) that 30.47 % of the fresh rock and 69.18 % of
the decomposed are soluble in hydrochloric acid and sodium
carbonate solutions, and that more than half the potash and
nearly the same proportion of the soda in the fresh rock is
found in the acid extract. (2) That the insoluble portion of
the residuary material is mainly in the form of free quartz.
(3) That 44.67 % of the original matter has been leached away,
and that (4) of the original silica 52.45 % is lost, while 85.61 %
of the iron and all the alumina remain. All the lime has dis-
appeared, 83.52 % of the potash, 95.03 % of the soda, and 74.70 %
of the magnesia. The total amount of water, as indicated by
the ignition, has increased very greatly, as was to be expected.
The small original amount of phosphoric acid prohibits our
placing too much reliance upon the indicated gain in this con-
stituent, since it may be due to errors in manipulation.
Passing from the acid group of granular crystalline rocks,
we will consider next a closely allied form differing mainly in
the absence of quartz as an essential constituent, and in the
presence of elseolite, the elseolite syenites of the Fourche Moun-
tain region of Arkansas. These are somewhat coarsely crystal-
line granitic-appearing rocks, in which an orthoclase feldspar
in broadly tabular forms is the prevailing constituent, though
always accompanied by nepheline, biotite, pyroxene, titanite,
and apatite, while fluorite, .analcite, and thomsonite, together
with calcite, occur as secondary products. The rock weathers
away to a coarse gray gravel which ultimately becomes a clay,
from which, by washing, may be obtained kaolin in a fail-
degree of purity.
WEATHERING OF GNEISS
215
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ANALYS
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216
ROCK DISINTEGRATION AND DECOMPOSITION
The following analyses from the work of Dr. J. F. Williams1
will serve to show the changes which have here taken place in
the transformation from (I) fresh syenite through (II and III)
intermediate stages of decomposition to (IV) a kaolin-like
residue.
ANALYSES OF FRESH AND DECOMPOSED SYENITE, ARKANSAS
CONSTITUENTS
I
II
ill
IV
Silica (Si02) ....
59.70%
58.50%
50.65 %
46.27 %
Alumina (Al20s) . .
18.85
25.71
26.71
38.57
Ferric oxide (FejOa) .
4.85
3.74
4.87
1.36
Lime (CaO) ....
1.34
0.44
0.62
0.34
Magnesia (MgO) . . .
0.68
Trace
0.21
0.25
Potash (K2O) ....
5.97
1.96
1.91
0.23
Soda (Na20) ....
6.29
1.37
0.62
0.37
Ignition (H2O) . . .
1.88
5.85
8.68
13.61
99.56%
97.57 %
94.27 %
101.00%
Recalculating the numbers given in columns I and IV upon
the basis of 100, we may obtain by further calculation, as already
described, the figures given in columns V and VI and VII below,
which represent the proportional loss of each constituent, as
before.
CALCULATED Loss OF MATERIAL
V
VI
VII
CONSTITUENTS
PERCENTAGE
Loss FOR ENTIRE
EOCK
PERCENTAGE
OP EACH CON-
STITUENT SAVED
PERCENTAGE
OF EACH CON-
STITUENT LOST
Silica (Si02)
37.28 % loss
37.82 %
62.18%
Alumina (Al2Os)
0.00
100.00
0.00
Ferric oxide (Fe20s)
4.19
13.83
86.17
Lime (CaO)
1.19
12.10
87.90
Magnesia (MgO)
0.57
17.90
82.10
Potash (K20)
5.90
18.15
81.85
Soda (Na2O)
6.15
2.89
97.11
Water (H2O)
0.00
100.00
0.00
Total loss of original material, 56.28%.
1 Ann. Rep., Vol. II, 1890, Arkansas Geol. Survey.
WEATHERING OF SYENITE AND PHONOLITE
217
Here, as with the granitic rocks, it will be noted we have a
gradual increase in the percentage of water as the decomposi-
tion advances, and a decrease in the amount of silica even more
pronounced. This last, as may be readily imagined, is due to
the absence of free quartz in the Fourche Mountain rocks.
The phonolites of Marienfels, near Assig, in Bohemia, have
been described by Lemberg l as weathering into a bright-
colored, porous, friable mass, the composition of which, as
compared with the fresh rock, is shown below. Each column,
it should be stated, represents an average of three analyses,
I being the fresh and II the weathered material, while in III,
IV, and V are given the percentage calculations of gain and
loss, as before.
ANALYSES OF FRESH AND DECOMPOSED PHONOLITE, BOHEMIA
I
II
III
IV
V
CONSTITUENTS
FRESH
PHONOLITE
DECOMPOSED
PHONOLITE
Loss or
CONSTITCKNTS
PERCENTAGE
OF EACH
CONSTITUENT
SAVED
PERCENTAGE
"i- K.\' 11
CONSTITUENT
LOST
Silica (SiO2)
55.67 %
56.72%
4.83 %
91.46%
8.54 %
Alumina (AlgOs)
20.64
22.19
0.37
98.40
1.60
Ferric oxide (Fe20g)
3.14
3.44
0.00
0.00
100.00
Lime (CaO)
1.40
1.28
0.25
83.66
16.34
Magnesia (MgO)
0.42
0.44
0.02
95.65
4.35
Potash (K2O)
6.56
6.26
O.OO1
100.00
0.00
Soda (NaaO)
7.12
2.65
4.79
34.01
65.99
Ignition
4.33
7.79
O.OO1
100.00
0.00
98.28 %
99.77%
10.26 %
....
....
This phonolite, it should be remarked, consisted essentially
of sanidin feldspars and a soda zeolite, together with accessory
augite, black mica, magnetic and titanic iron, and possibly
hauyne. The zeolite is assumed to have originated from the al-
teration of the nepheline. The process of decomposition would
seem to consist, then, in the breaking down of this zeolite, and
the conversion of the rock into an earthy mass, with little other
1 Zeit. der Deutschen Geol. Gesellschaft, Vol. 35, 1883, p. 559.
a Gain. The calculations for potash in column IV gives: 107.79% and for
ignition 164.77%.
218 ROCK DISINTEGRATION AND DECOMPOSITION
change, so far as ultimate composition is concerned, than a loss
of a considerable proportion of its soda, and an assumption of
nearly 3.5% of water. The decomposed rock yielded 55.44%
of material insoluble in hydrochloric acid, and with essentially
the composition of sanidin, showing that this mineral underwent
only a physical disintegration, the decomposition proper being
limited to the other constituents.1
Turning to still more basic rocks, we will next consider
a disintegrated diabase occurring in the form of a large dike
extending from Granite Street in Somerville, Massachusetts, to
Spot Pond in Stoneham, and beyond.2 The rock at the point
selected for study (Medford) is a coarsely granular admixture
of lath-shaped feldspar, black mica, augite, and brown basaltic
hornblende, with the usual sprinkling of apatite, magnetite, and
ilmenite. Secondary uralite, chlorite, biotite, leucoxene, kaolin,
calcite, pyrite, and quartz are common.3
The rock has undergone extensive disintegration, giving rise
to loose sand and gravel of a deep brown color, in which lie
rounded boulders of all sizes of the still undecomposed material.
These boulders, as is usually the case, show a more or less con-
centric structure, from without inward, until a solid core of
unaltered diabase is met with. (See PI. 17, and Fig. 2, PL 20.)
A mechanical separation of the disintegrated material yielded
results as below : —
1. Coarse gravel above 2 mm. in diameter 42.300%
2. Fine gravel
3. Coarse sand
4. Medium sand
5. Fine sand
6. Very fine sand
7. Silt
8. Fine silt
9. Clay
2-1 mm. in diameter 20.355
1-5 mm. in diameter 12.723
.5-.25 mm. in diameter 9.567
.25-.! mm. in diameter 4.907
.1-.05 mm. in diameter 4.181
.05-.01 mm. in diameter 1.128
.01-.005 mm. in diameter 0.370
.005-.0001 mm. in diameter . 1.670
10. Loss at 110° C 0.660
11. Loss on ignition 1.730
99.691 %
1 In calculating these analyses, it was found that the loss of alumina had
exceeded that of iron oxide, necessitating the assumption of the last-named as
a constant for comparison. The apparent gain in potash is presumably due to
errors in analysis, since, as will be noted, the analysis of the fresh material, given
in column I, foots up only 98.28 %.
2 See Disintegration and Decomposition of Diabase at Medford, Massachu-
setts, by G. P. Merrill, Bull. Geol. Soc. of America, Vol. VII, 1896, pp. 349-362.
3 On the Petrographic Characters of a Dike of Diabase in the Boston Basin,
by W. H. Hobbs, Bull. Mus. Comp. Zoology, Vol. XVI, No. 1, 1888.
WEATHERING OF DIABASE 219
Of the above, the first three sizes could be easily recognized
by the unaided eyes, as composed of particles of a compound
nature. In number 4 the separation had gone a trifle farther,
though even here inspection with a pocket lens revealed the
compound nature of many of the granules, somewhat obscured
by the prevailing discoloration from the oxides of iron. It
forms a gray-brown sand composed of feldspathic particles,
dirty brown augites, and lustrous scales of brown mica. Num-
bers 5 and 6 seemed composed almost wholly of beautifully
lustrous, dark mahogany -brown mica scales, while 7 would pass
for a finely micaceous umber. Numbers 8 and 9 were uni-
formly ochreous, the last being several shades lighter than
number 8, and without appreciable grit.
The chemical nature of the fresh and decomposed rock is
shown in the accompanying table, the results being in nearly
every case averages obtained from two or more analyses. The
" fresh" material, obtained from the interior of one of the boul-
ders, is firm in texture, has a bright clean fracture, and shows to
the unaided eye no signs of decomposition. When pulverized
and treated with acid, however, it effervesces distinctly, indi-
cating the presence of free carbonates, which are also observ-
able as secondary calcite when thin sections are examined under
the microscope. Some of this calcite is evidently a deposit from
infiltrated waters, being derived from the surrounding decom-
posed material, while a portion results from the decomposition
of the silicate minerals in place. Aside from a slight kaolini-
zation of the feldspars and development of chlorite from the
ferruginous silicates, there are no other observable signs of de-
composition, though the presence of a soda-bearing zeolite is indi-
rutrd by cubes of chloride of sodium, which separate out when
an uncovered slide is treated with a drop of hydrochloric acid.
A glance at this table is sufficient to show that the disinte-
gration is accompanied by decomposition and a leaching action
which has resulted in the removal of a portion of the more
soluble constituents. The fact that the fresh rock yields the
larger percentages of its constituents to the solvent action of
acid and alkaline solutions is readily explained on this ground,
though it may be doubted if the full significance of the fact, so
far as it relates to siliceous crystallines, is as yet appreciated.
It will be observed that 36.23% of the fresh rock and 32.28%
of the decomposed is thus extracted.
220 KOCK DISINTEGRATION AND DECOMPOSITION
ANALYSES OF FKESH AND DISINTEGRATED DIABASE FROM MEDFORD
SILT FROM DISINTEGRATED
FRESH DIABASE
DISINTEGRATED
DIABASE, Nos. VII, VIII,
DIABASE
AND IX OF TABLE, ON
P. 218
I
II
III
IV
V
VI
VII
CONSTITUENTS
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Silica (SiOa)
finHCl 1
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47.28
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0.47
22.63
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23.19
4.86
21.98
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3.66
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40.68
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12.83
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8.89
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7.09
3.09
6.03
1.50
3.32
0.12
3.44
Magnesia (MgO)
3.17
2.20
2.82
1.84
3.23
0.79
4.02
Manganese oxide
MnO. . . .
0.77
Not det.
0.52
Not det.
Not det.
Not det.
Not det.
Potash (K20) .
2.16
1.21
1.75
0.68
1.30
0.52
1.82
Soda (Na20) . .
3.94
0.50
3.93
0.17
0.90
1.24
2.14
Phosphoric acid
(P206) . . .
0.68
Not det.
0.70
Not det.
Not det.
Not det.
Ignition .
2.73
2.73
3.73
3.73
10.86
0.11
10.97
100.59
36.23
99.81
32.28
77.52
22.17
99.68
Of the material classed as silt in columns V, VI, and VII, or
as silt and clay, on p. 218, and which constitutes only some
3.17 % of the entire residual debris, 77.87 % is soluble in dilute
hydrochloric and sodium carbonate solutions. The insoluble
portion, constituting 22.13% of the silt, consists of unaltered
feldspar and iron, lime and magnesian silicates, which are easily
recognized under the microscope, in the form of minute, sharply
angular particles. Recalculating, as before, the matter in col-
umns I and II on the basis of 100 and considering the alumina
as a constant factor, we get the results given in columns VIII to
XII inclusive, representing, so far as it can be obtained by this
WEATHERING OF DIABASE
221
method, the actual percentage loss of materials attending the
breaking down.
CALCULATED Loss OF MATERIAL
CONSTITUENTS
VIII
IX
X
XI
XII
RECALCULATED ON
BASIS OF 100
Percentage Loss
I'm- Kmiiv Ko.-i,
Percentage of
Kach Constitu-
ent saved
Percentage of
Kin-li Constitu-
ent lost
Kn->h
Diabase
Decomposed
Diabase
Silica (Si02)
47.01 %
20.11
3.63
8.83
7.06
3.15
0.77
2.14
3.91
0.68
2.71
44.51%
23.24
! 12.71
6.04
2.85
0.52
1.75
3.94
0.70
3.74
8.48
0.00
2.42
1.83
0.68
0.32
0.62
0.50
0.08
0.00
81.97%
100.00
81.90
74.11
78.30
58.43
70.85
87.17
88.61
100.00
18.03%
0.00
18.10
25.89
21.70
41.57
29.15
12.83
11.39
0.00
Alumina (Al20s) . . .
Ferric oxide (FeaOa) . .
Ferrous oxide (FeO) . .
Lime (CaO) . . .
Magnesia (MgO) . . .
Manganese (MnO) .
Potash (K2O) ....
Soda (Na2O)
rimsphoric acid (P205) .
Ignition
100.00%
100.00%
14.93%
From the figures in column X it appears that there has
been a loss of some 14.93% of all constituents. The increase
in \\itter, as indicated by the ignition, is a natural consequence
of hydration and the presence of a small amount of organic
matter. This increase, it should be stated, is greater than may
be at first apparent, for the reason that the fresh rock contains
a considerable amount of secondary calcite, which is quite lack-
ing in the residual sand. A large part of the ignition in col-
umns I and VIII is therefore to be accredited to carbonic acid,
and not to water of hydration.
From columns XI and XII it appears that of all the essential
constituents, the lime and potash salts have suffered the most,
though the iron oxides have been carried away to the amount
of 18.10 %. Magnesia has also proven very susceptible to the
solvent action, disappearing to the amount of 21.70%; and
lastly, silica, to the amount of 18.03%. The small original
amounts of manganese and phosphoric acid render the results
222
KOCK DISINTEGRATION AND DECOMPOSITION
obtained by these calculations of doubtful value, since it is pos-
sible they may be due to errors of analysis.
In this case, as in that of the granite from the District
of Columbia, we have to do with only the earlier stages of de-
generation, with conditions which are as much in the nature
of mechanical disintegration as of chemical decomposition. As
before, then, it will be instructive to consider cases in which, in
rocks of similar nature, the decomposition has proceeded much
farther. For this purpose we will select a diabase from Spanish
Guiana,1 and basalts from Bohemia and the Haute Loire as
described by Ebelmen;2 in each instance the actual analysis
being recalculated to the basis of 100.
ANALYSES OF FRESH AND DECOMPOSED DIABASE FKOM SPANISH GUIANA,
VENEZUELA
I
II
III
IV
V
CONSTITUENTS
1
pi
E*
DECOMPOSED
PERCENTAGE
Loss FOR EN-
TIRE BOCK
PERCENTAGE OF
EACH CONSTIT-
UENT SAVED
PERCENTAGE OF
EACH CONSTIT-
UENT LOST
Silica (Si02)
49.35%
43 38 %
20 92 %
57 60 %
42 40 %
Alumina (A1203) . . .
Ferric iron (Fe2O3) . .
Ferrous iron (FeO) . .
Lime (CaO)
15.30
12.28
9.60
18.36
20.39
2.37
3.27
0.00
8.05
78.62
100.00
16.17
21.38
0.00
83.23
Magnesia (MgO) . . .
Potash (K20) ....
Soda (Na2O)
7.38
0.85
1.98
3.45
0.59
0.14
5.12
0.33
1.82
30.63
54.12
4.63
61.37
45.88
95.37
Ignition ......
3.25
11 34
0 00
O.OO3
0.00
100.00%
100.00 %
39.51 %
....
In the case of the diabase, it appears, from a comparison
of the figures in columns I and III, that the total loss of
material equals 39.51 %, there being the usual gain in volatile
matter. •
1 Quar. Jour. Geol. Soc. of London, Vol. XXXV, 1879, p. 586.
2 Ann. des Mines, Vol. VII, 1845.
8 Gain.
WEATHERING OF DIABASE AND BASALT
223
ANALYSES OF FRESH AND DECOMPOSED BASALT FROM KAMMAR BULL,
BOHEMIA
I
II
III
IV
V
VI
CONSTITUENTS
u
c
K ^
« 3
3
rt —
• 7 B
§ ,
E
i • P
o
J e
2, °,
< H
5 c >
X x x
OS
< X
s g
go ;
*a s
~
- o
j O
H M
a _
- ~
1
s *
< 5
- *
H O
£ « g
u 5 o
s - S
u < x
a ^ r.
• 4 P
5.
- :
33 0
C KM
^- - _
- - -
%
01
h
10
%
%
%
Silica (Si02) ....
43.61
43.00
43.27
15.04 loss
67.01
88.M
Alumina (Al2Os) . .
13.26
13.90
18.13
0.00 "
100.00
0.00
Ferric iron (Fe2O3) . .
Ferrous iron (FeO) . .
3.51
12.16
5. 40 \
8.30 (
11.70
9.10 "
49.83
60.17
Lime (CaO) ....
11.37
12.10
2.60
9.60 "
64.47
84.53
Magnesia (MgO) . . .
9.14
7.30
3.40
6.83 "
25.90
74.10
Soda (NagO) ....
2.72)
Potash (K2O) ....
••••l
0.81 /
0.50
0.20
3.39 "
38.31
61.69
Water (H2O) ....
4.42
9.50
20.70
0.00
100.00
....
100.00 %
100.00 %
100.00 %
43.96 loss
....
ANALYSES OF FRESH AND DECOMPOSED BASALT FROM CROUZET, IN THE
HAUTE LOIRE, FRANCE
I
II
III
IV
V
1
h
O ,
o ,
H
CONSTITUENTS
H
c
H a
t} at
5 i
5
o
H S
< Z "
b * »
C Z
M
g(
W Ji
H ^p •
£ O «J
§M
e
B 8 11
S u z
B S S
h
c.2
2 E«
£ w S
B ^ fl
CM E
Silica (SiO2) ....
48.29 %
37.09 %
30.34% loss
34.44 %
65.56%
Alumina (A12OS) . . .
13.25
30.75
0.00 "
100.00
0.00
Ferric iron (Fe«O8) .
Ferrous iron (FeO) .
0.00
16.66
4.31 )
0.00 j
16.64 "
11.16
88.84
Lime (CaO) ....
7.33
8.97
3.46 "
52.76
47.24
Mamiesia (MgO) . . .
7.03
0.61
6.77 "
3.62
96.38
Potash (K2O) ....
1.81
0.71
1.51 "
16.66
83.34
Soda (NaoO) ....
2.71
1.01
1.40 "
25.59
74.41
Ignition
4.92
16.55
0.00
100.00
0.00
100.00 %
100.00 %
60. 12% loss
224 ROCK DISINTEGRATION AND DECOMPOSITION
Of the individual constituents, 83.23% of the original lime,
61.37 % of the magnesia, 45.88% of the potash, 95.37 % of the
soda, 42.40 % of the silica, and 21.38 % of the alumina have dis-
appeared, the calculations being made on a Fe2O3 constant
basis.
In the case of the Bohemian basalt, the decomposition com-
menced with the formation of boulders, which, when the
process had not gone too far, still showed fresh, unchanged
basalt interiorly, but became more and more altered toward
their peripheries. The first stage of decomposition (column II),
it will be noted, consists, aside from hydration, in a slight appar-
ent loss of silica, a considerable oxidation of the iron magnesia
minerals, accompanied by a slight loss of both constituents, and
an almost complete loss of alkalies. In the second stage (column
III) lime and magnesia are both lost in considerable amounts,
the iron passing over wholly to the condition of sesquioxide, and
there is a further slight diminution in the proportional amount
of silica. It is evident that here the feldspars were the first of
the constituents to yield to the decomposing forces, the augite
and olivine proving most refractory. The total loss of material,
it will be noted, amounts to 43.96 %, the lime, magnesia, alka-
lies, iron oxides, and silica disappearing in the order here
mentioned.
In the case of the basalt from Crouzet, the analyses show a
total of 60.12 % loss, or over one-half of the original material.
This loss includes nearly two-thirds of the original silica,
88.84% of the iron, and 96.38% of the magnesia. The loss
of both iron and magnesia in such proportionally large quan-
tities is quite unusual, and indicates, so far as the iron is con-
cerned, that the decomposition took place under conditions
excluding a sufficient supply of oxygen to convert the same
into the insoluble sesquioxide, or where subjected to the de-
oxidizing and solvent action of organic acids. The removal of
the magnesia, which must have existed mainly in the mineral
olivine, indicates that the decomposition has gone on even to
the production of carbonate of magnesia and the separation
of free silica and iron oxides.
An analysis by the present writer of a closely related rock,
a diorite, and its residual soil, from North Garden, Albemarle
County, Virginia, yielded the results given in columns I and
II below. The rock here was fine-grained, of an almost coal-
WEATHERING OF DIORITE
225
black color finely speckled with whitish flecks due to the
presence of feldspars. The microscope showed it to be com-
posed mainly of hornblende with interstitial soda-lime feldspars
and scattering areas of titanic iron. The clay, or soil, to which
it gave rise was deep brownish red in color and highly plastic,
though distinctly gritty from the presence of undecomposed
minerals. In columns III, IV, and V are given the loss and
gain of the various constituents calculated on an alumina
constant basis, as before.
ANALYSES OF FRESH AND DECOMPOSED DIORITE FROM ALBEMARLE COUNTY,
VIRGINIA
I
II
III
IV
V
CONSTITUENTS
FRESH
DECOM-
POSED
CALCULATED
Loss FOR Ks-
•111:1: ROCK
PER CENT
OP EACII
CONSTITU-
ENT SAVED
PER CENT
or EACII
CONSTITU-
ENT LOST
Silica (SiO2)
46.75%
42.44 %
17. 43% loss
62.69%
37.31 °/
Alumina (A12O8) . . .
Iron sesquioxide (Fe2Os) 1
Lime (CaO)
17.61
16.79
9.46
26.51
19.20
0.37
0.00
3.53
9.20
100.00
78.97
2.70
0.00
21.03
97.30
Magnesia (MgO) . . .
Potash (K20) ....
Soda (Na-jO)
6.12
0.55
2.56
0.21
0.49
0.56
4.97
0.21
2.17
2.83
61.25
15.13
97.17
38.75
84.87
Phosphoric acid (P2O)g .
Ignition
0.25
0.92
0.29
10.92
0.00
0.00
80.11
100.00
19.87
0,00
100.01 %
99.99 %
37.51% loss
....
The ultra basic rocks, — peridotites and pyroxenites, — from
the very nature of their composition, must yield on decompo-
sition residues poor in the presence of alkalies and rich in
iron or aluminum and magnesian compounds. Owing, further,
to their poverty in alkali-bearing silicates, the process of decom-
position must be less complex, consisting essentially in hydra-
tion, oxidation, and a production of iron, lime, and magnesian
carbonates and a liberation of chalcedonic silica.
During the process these rocks as a rule become brownish,
and, on the surface, often irregularly checked with a fine net-
work of rifts which become filled with secondary calcite, mag-
nesite, and chalcedony. If the original rock is an olivine-rich
1 All iron calculated as Fe2O3.
226
ROCK DISINTEGRATION AND DECOMPOSITION
peridotite, these clefts may become filled with the silicates of
nickel, noumceite and garncerite, which may be of sufficient
abundance to form valuable ores. This, in brief, is the history
of the nickel ores of Riddles, Oregon, and of New Caledonia,
though the process is more properly a form of hydrometamor-
phism than weathering.
The deep green serpentines of Harford County, Maryland,
weather slowly down into a gray-brown soil, which consists of
60.17% silica, 10.40% of iron oxides, 14.81% of alumina, and
only 7.23% magnesia. The fresh rock, on the other hand, car-
ries nearly 40% of magnesia, 8.50% iron and other metallic
oxides, and less than one-half of one per cent of alumina.
Natural joint blocks occur in which the preliminary stages
of weathering are manifested by a brown, ferruginous, though
tough and hard, vesicular crust of from a millimetre to two or
more centimetres' thickness, enclosing the slightly hydrated but
otherwise unchanged material.
ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE (ALTERED PYROXENITE)
CONSTITUENTS
I
II
III
IV
V
0
a
i
&
RESIDUAL SOIL
PERCENTAGE OF
Loss FOR ENTIRE
ROCK
PERCENTAGE OF
EACH CONSTIT-
UENT SAVED
PERCENTAGE OF
EACH CONSTIT-
UENT LOST
Silica (SiO2)
38.85%
12.77
12.86
6.12
22.58
0.19
0.11
6.52
38.82%
22.61
13.33
6.13
9.52
0.18
0.20
9.21
16.92 %
0.00
5.33
2.66
17.20
9.03
0.00
1.32
56.42 %
100.00
58.52
55.55
23.81
52.94
100.00
79.74
43.58%
0.00
41.48
44.45
76.19
47.05
0.00
20.26
Alumina (Al20s) . . .
Iron sesquioxide (Fe203) l
Lime (CaO) . " . . . .
Magnesia (MgO) . . .
Potash (K2O) ....
Soda (Na20)
100.00 %
100.00%
52.46%
....
In columns I and II above are given (I) the composition of
an altered pyroxenite (soapstone) from Albemarle County,
Virginia, and (II) a residual soil derived from the same, the
1 All iron calculated as Fe203.
WEATHERING OF PYROXENITES
227
latter being of a dull, ochreous, brown-red color, somewhat
lumpy, but with no appreciable grit when rubbed between the
thumb and fingers.
The fresh rock is of a- blue-gray color, close texture, and
consists, as shown by the microscope, of elongated crystals of
colorless tremolite, with folia of talc and chlorite, and occasional
opaque granules of chromic iron. The general petrologic feat-
ures are those of an altered pyroxenite.
Recalculated as before, the analyses give the results shown
in columns III, IV, and V.
Total loss of material 52.46%, including water of hydration.
The most striking feature brought out is the fact that the mag-
nesia has been carried away in greater proportional quantity
than has the lime. A like result was noted by Ebelmen in his
analyses of the decomposed basalts of Crouzet, which are given
on p. 223.
ANALYSES OF FRESH AND DECOMPOSED SOAPSTONE, FAIRFAX COUNTY, VIRGINIA
I
II
III
IV
V
CONSTITUENTS
M
£
RESIDUAL SOIL
PERCENTAGE OF
Loos FOB ENTIRE
ROCK
PERCENTAGE OF
EACH CONSTIT-
UENT SAVED
PERCENTAGE OF
EACH CONSTIT-
UENT LOST
Silica (SiOo)
68.40%
64.84%
46.31 %
20.70%
79.30 %
Alumina (ALjOs) . . .
Iron oxides(FeOandFe2C>8)
Lime (CaO)
I 7.44
0.00
33.75
0.00
0.00
100.00
0.00
Magnesia (MgO) . . .
Alkalies (K2O and Na2O)
Ignition (H2O) ....
29.19
0.00
4.97
4.36
0.00
7.05
28.23
3.41
3.29
31.28
96.71
68.72
100.00%
100.00%
77.96%
A varietal form of this same rock occurring near Fostoria in
Fairfax County, this state, is thoroughly decomposed throughout
nearly the entire area to a depth of twenty or more feet. The
fresh rock is composed mainly of a light greenish, almost white
talc, with sporadic patches of chlorite some five or more millime-
tres in diameter, and scattering granules of iron ores. The
228 ROCK DISINTEGRATION AND DECOMPOSITION
decomposed material is dull brownish or gray, and when washed
and submitted to microscopic examinations is found to consist
almost wholly of brown and yellow-brown scales of talcose
material, intermingled with an impalpable silt, composed so far
as determinable of talcose and chloritic shreds. It is wholly
without grit, and with a decided soapy or greasy feeling.
Analyses of fresh and decomposed material, and calculations as
already given, yielded results as shown in table on p. 227.
The principles involved in the decomposition of fragmen-
tal and crystalline stratified rocks are not so different from
those we have been discussing as to call for detailed considera-
tion. It is well to note, however, that the materials composing
rocks of this type are themselves a product o£ these very dis-
integrating and decomposing agencies, but which have become
consolidated into rock masses and now, once more in the infinite
cycle of change, are undergoing a breaking up. It follows from
the very nature of the case that such rocks, with the exception
of the purely calcareous varieties, will undergo less chemical
change than do those we have been discussing. Their feld-
spathic and easily decomposable silicate constituents long ago
yielded to the decomposing processes, and were largely removed
before consolidation took place. Thus, most sandstones are
composed largely of quartzose sand, the least soluble and least
changeable product, it may be, of many a previous disintegra-
tion/ Hence, the processes involved in the degeneration of the
sandstones, shales, and argillites are largely mechanical, with
the exception of those which carry a feldspathic or calcareous
cement. In these last-named, the cementing material is gradu-
ally leached away, and the rock becomes susceptible to the action
of frost, or falls away to loose sand simply through loss of cohe-
sion. Heusser and Claraz 1 described the itacolumites of Brazil
as subject to this mechanical degeneration, the process being
characterized by fissuration, succeeded by complete disintegra-
tion. Among siliceous sandstone it is the binding constituent
that yields first, as is naturally to be expected, and as has been
shown by the experiments conducted by R. Schutze.2
The rocks grouped under the name of argillites, though com-
posed of detrital materials from pre-existing rocks, and of parti-
1 Ann. des Mines, 5th, Vol. XVII, 1860.
2 Ueber Verwitterungsvorgange bei Krystallinischen u. Sedimentargesteinen,
Inaug. Dissertation der Friedrich-Alexanders Universitat, Berlin, 1886.
WEATHERING OF ARGILLITE
229
cles reduced to an extreme degree of fineness, are, nevertheless,
quite variable in composition, as already noted. As a rule, they
are among the most indestructible of rocks, and on breaking
down yield only clays which differ from the original argillites
inainl}' in degree of hydration and condition of oxidation of the
iron and other metallic constituents. Those argillites which
carry appreciable quantities of still undecomposed silicates, par-
ticularly alkali-bearing varieties, are, of course, more susceptible,
other things being equal, as texture, fissility, etc.
The deep blue-black argillites of Harford County, Mary-
land, as shown in the analyses given below, do contain very
l
II
III
IV
V
CONSTITUENTS
FRESH ARGILLITE
RESIDUAL CLAY
PERCENTAGE op
Loss FOR Ks-
TIRE ROCK
PERCENTAGE OF
EACU CONSTIT-
UENT SATED
PERCENTAGE OP
EACIlCONBTIT-
I-P.NT LOST
Silica1 (SiOs) .
44. 15%
24.17%
25.34 %
42.43 %
57.57 %
Alumina (AljOs)
30.84
39.90
0.00
100.00
0.00
Iron oxide (FeO and FeaO8) . .
Lime (CaO)
14.87
0.48
17.61
None
1.23
0.48
91.22
0.00
8.78
100.00
Magnesia (MgO)
Q.27
0.26
0.08
71.84
28.10
Potash (K2O)
4.36
1.24
3.39
22.04
77.95
Soda (NaaO)
0.51
0.23
0.33
0.36
99.64
Ignition (C aud HjO)
4.49
16.62
0.00
287.37
None
99.97 %
100.02 %
40.83%
....
....
considerable quantities of these undecomposed silicates, and
though extremely tough and enduring from a human stand-
point, in time decompose in a very interesting manner. In the
field these rocks are found standing nearly, if not quite, verti-
cally, that is, with their evident cleavage vertical, and form-
ing steep, high ridges flanked by valleys carved from the softer
rocks on either hand. In the fresh cuts made during the work
1 With traces of TiO2.
phur; hence no pyrite.
Manganese in traces, but not determined. No sul-
230 EOCK DISINTEGRATION AND DECOMPOSITION
of stripping, to open new quarries, the sound rock is found over-
lain by a variable thickness of ferruginous residual clay. Joint
blocks and splinters of the slate scattered through this clay, in
all stages of decomposition leave no doubt as to its origin.
Blocks, deep velvety black on the interior, are surrounded by
a crust of ochreous brown-red decomposition product, the decay
penetrating irregularly like the processes of oxidation into a
piece of metal. The first physical indication of decay is shown
by a softening of the slate, so that it may be readily scratched
by the thumb nail, and an assumption of a soapy or greasy feel-
ing, the entire mass finally passing over to the deep red-brown
unctuous clay, sufficiently rich in iron to serve as a low-grade
ochre, for paints. The incidental chemical changes are surpris-
ingly large, as shown by the analyses given on p. 229, column I
being an average of two analyses of the black, little altered
material from the interior of one of these blocks, and II that of
the residual clay. In III, IV, and V are given the calculated
losses of constituents, as before.
This residual clay, when boiled with hydrochloric acid and
sodium carbonate solutions, yielded up nearly 10% of its matter
to these solvents, leaving a residue which, when examined under
the microscope, shows only faint yellow-brown scale-like par-
ticles, rarely over a tenth of a millimetre in diameter, acting
very faintly, if at all, on polarized light, and with borders often
serrate, through corrosion, though this latter feature may be
due, in part, to the action of the solvents used.
Among siliceous rocks poor in alkalies or iron-bearing
silicates the degeneration is mainly disintegration, though a
small amount of silica, existing in either crystalline or chalce-
donic forms, is usually lost through solution. Thus the cherts
of southwest Missouri break down into porous friable forms,
sometimes passing into the condition of loose powder, or again
retaining sufficient tenacity to be utilized for filter discs and
tubes, as at Seneca, in Newton County.
Analyses of fresh and altered forms of this material, as given
by Dr. E. O. Hovey,1 show no differences that are of sufficient
importance to warrant us in assuming any of them as the direct
cause of disintegration. The change is evidently mainly physi-
cal, though it is more than probable that a certain amount of
interstitial silica has been removed. It is, of course, possible
1 Appendix A, Vol. YII, Missouri Geological Survey, 1894, pp. 727-739.
WEATHERING OF CHERTS 231
that here, as in other forms of decomposition, extensive solution
may have taken place, leaving a residue which, so far as compo-
sition is concerned, gives no clew to the changes which have
occurred. Dr. Penrose, however, describes l a process of chert
decay, or more properly disintegration, as manifested in the
Batesville region of Arkansas, in which the cause of the break-
ing down is more apparent. There are two stages in the proc-
ess, as described: (1) A transition into a light, porous, opaque,
buff-colored rock of the consistency of ordinary pressed brick,
and (2) into an impalpable white or brown powder, locally
known as a polishing powder. This second stage is not so con-
spicuous a feature as. the first, since the finer materials thus
formed are carried off by surface waters. The white residual
powder often contains masses of the porous, semi-decomposed
rock, the latter in turn encircling kernels of hard, unaltered
chert. Throughout this region, the cherts (of Carboniferous
age) are generally decomposed into the condition of a more
or less porous mass to all depths up to ten or more feet.
In all cases the disintegration may be traced to the removal,
by leaching, of a small amount of interstitial carbonate of
lime.
When we come to a consideration of the Calcareous rocks,
we find, almost invariably, the chemical agencies of degenera-
tion preponderating over those that are purely physical. In
arid regions, and with granular crystalline types, physical
agencies may for a time prevail, but as a rule the process
is largely chemical, and notable for its simplicity. The de-
composition is due mainly to the action of meteoric waters
trickling over the surface, or filtering through cracks and crev-
ices, under ordinary conditions of atmospheric pressure and
atmospheric temperature. Hence the process is one of super-
ficial solution, and the incidental chemical processes set in
motion, as in the feldspar-bearing rocks, are almost entirely
lacking. It follows that only the lime carbonate is removed
in appreciable quantities, while the less soluble impurities are
left to accumulate in the form of ferruginous clays, admixed
with quartzose particles, chert nodules, etc. Since in many
limestones the amount of these constituents is reduced to a
minimum, even perhaps to the fraction of one per cent, so it
happens that hundreds, or even thousands of feet of strata may
1 Ann. Rep. Geol. Survey of Arkansas, Vol. I, 1890.
232
ROCK DISINTEGRATION AND DECOMPOSITION
disappear without leaving more than a very thin coating of soil
in their place.
An interesting illustration of the changes taking place in the
decomposition of an impure Carboniferous limestone is described
by Penrose in his treatise on the genesis of manganese deposits.1
The stone in its least changed condition is of a granular crys-
talline structure and dark chocolate-brown color. The residual
clay from its decomposition is a trifle darker, highly plastic,
and quite impervious. Below are given the analyses of (I) the
fresh rock and (II) the clay, both being taken from the same
pit, the latter being of about fifteen feet in thickness and over-
lain by a capping of chert, which reduced to a minimum the
possibility of any admixture of foreign matter. The materials
were dried at a temperature of 110° to 115° C. before analyzing.
ANALYSES OF FRESH LIMESTONE AND ITS RESIDUAL CLAY
I
II
III
IV
V
CONSTITUENTS
FRESH
LIMESTONE
KEBIDUAL CLAY
PERCENTAGE OF
Loss FOR ENTIRE
ROCK
PERCENTAGE OF
EACH CONSTIT-
UENT SAVED
PERCENTAGE OF
EACH CONSTIT-
UENT LOST
Silica (Si02)
4.13 %
33.69%
0.00 %
100 00 %
0 00 °/
Alumina (A1203) . . .
Ferric iron (Fe2O3) . .
Manganic oxide (MnO) .
Lime (CaO) ....
4.19
2.35
4.33
44 79
30.30
1.99
14.98
3 91
0.35
2.13
2.49
44 32
88.65
10.44
42.41
1 07
11.35
89.56
57.59
98 93
Magnesia (MgO) . . .
Potash (K20) ....
Soda (Na20)
0.30
0.35
0 16
0.26
0.96
0 61
6.25
0.23
0 085
10.62
33.63
46 74
89.38
66.37
63 26
Water (H20)
2 26
10 76
0 95
58 37
41 fi3
Carbonic acid (C02) . .
Phosphoric acid (P205) .
34.10
3.04
0.00
2.54
34.10
2.73
0.00
10.24
100.00
89.76
100.00 %
100.00 %
97.635%
These analyses have been recalculated in the same manner as
before, excepting that silica, instead of alumina, is taken as the
constant factor. This for the reason above suggested. It is
believed that one is safe in assuming little or no silica is lost
1 Ann. Rep. Geol. Survey of Arkansas, 1890, p. 179.
WEATHERING OF CALCAREOUS ROCKS 233
here through the action of alkaline carbonates, since the alka-
lies are almost wholly lacking in the fresh rock, and a large
portion of the silica doubtless exists as free quartz. Recalcu-
lating, then, in the same manner as before, but on a silica con-
stant basis, we obtain the matter in columns III, IV, and V.
These columns bring to light some unexpected features, not
the least interesting of which is the fact that the residual clay,
in spite of its highly hydrated condition, in reality contains
scarcely half the amount of water it would, had the small amount
(2.20%) in the original limestone been allowed to accumulate
without loss. A more important, though perhaps more to be ex-
pected, feature is the entire removal of that portion of the lime
which existed as carbonate, as indicated by the absence of car-
bonic acid in the clay. It will be noted that 97.635 % of the
t-ntire rock mass has disappeared through leaching, leaving only
2.365 % to accumulate as an insoluble residue in the form of soil.
This leaching out of the lime carbonates and the accumula-
tion of insoluble residues is a strikingly conspicuous feature in
regions abounding in limestone caverns, and to it is due the
tenaceous ferruginous clays which cover their floors. So rich
indeed are some of these residual deposits in iron oxide that
in some instances they are locally used for pigments, under the
name of ochre or mineral paint, or again, where occurring in
large quantities, as ores of iron. (See p. 267.)
It is possible that loosely consolidated beds of shell limestone
may undergo a process of change, perhaps more nearly akin to
alteration than decomposition, through agencies quite different
from those we have been considering.
Darwin, it will be remembered, found the shells in the raised
sea-beaches of San Lorenzo, South America, altered to the con-
dition of a white powder without trace of organic structure, and
consisting of carbonate, sulphate and chloride of lime with sul-
phate and chloride of sodium. This alteration he believed to
be due to a mutual reaction taking place between the original
sodium chloride derived from the sea-water and the lime car-
bonate of the shells, and he speaks of it as an interesting illus-
tration of the fact that the dry climate of the west coast of
South America is much less favorable to the preservation of
shell structures than would be a moist one where the salt would
be removed too rapidly for the double decomposition to be
brought about.
234 KOCK DISINTEGRATION AND DECOMPOSITION
Resume. — Making all due allowance for possible sources of
error in our methods, there are certain general deductions that
may be safely drawn. Not, it may be, from our own analyses
alone, but from numerous others as found in existing literature.1
Let us briefly review the subject and make the deductions
accordingly.
In glancing over the columns of our analyses, it is at once
apparent that hydration is an important factor, the amount of
water increasing rapidly as decomposition advances. In the
earlier stages of degeneration it is doubtless the most important
factor. There is, moreover, among the siliceous crystalline
rocks, in every case a loss in silica, a greater proportional loss in
lime, magnesia, and the alkalies, and a proportional increase in
the amounts of alumina and sometimes of iron oxides, though
the apparent gain may in some cases be due to the change in
condition from ferrous to ferric oxide. As a whole, however,
there is a very decided loss of materials. Among siliceous
crystalline rocks, this loss, so far as shown by available analyses
and calculations, rarely amounts to more than 50 % of the entire
rock mass. Among calcareous rocks, on the other hand, it may,
in extreme cases, amount to even 99 % .
Of all the ordinary essential mineral constituents the free
quartz is the most refractory toward purely chemical agen-
cies, and the amount of silica lost from this source must be
small, though Sorby2 thinks to have distinguished chemically
corroded quartz granules in some of the sands examined by him.
It is, however, safe to say that the mineral suffers chiefly from
mechanical disruption, — that silica in any rock which is re-
moved during the process of decomposition comes mainly from
the silicates, and not from the free quartz. According to Bis-
chof, and as shown by our own work, the silicates in any rock
that are most readily decomposed are, as a rule, those contain-
ing protoxides of iron and manganese, or lime, and the first
indication of decomposition is signalled by a ferruginous dis-
coloration and the appearance of calcite. The evidence bearing
upon the relative durability of the various minerals consti-
tuting rocks is, however, quite conflicting and unsatisfactory.
Doubtless much depends on local conditions.
1 See especially Roth's Allegemeine u. Chemische Geologie, Vol. Ill, and Ebel-
men's papers in Ann. des Mines, Vols. VII, 1845, and XII, 1847.
2 Proc. Geol. Soc. of London, 1879.
GENERAL DEDUCTIONS 235
Dana observed1 that in the decomposition of the granitic
rocks of the Chilean coast the feldspars yielded first, becoming
white and opaque and of a friable earthy appearance. But it
should be noted that there is nothing in Professor Dana's de-
scription to show that this change may not have been a purely
physical one, and due to the splitting up of the feldspars along
cleavage lines. Fournet, from a study of the processes of kao-
liuization, was led to state 2 that hornblende yields less readily
to decomposing forces than does feldspar, when the two are
associated in the same rock. Becker, however, in studying
deep-seated decomposition in the Comstock Lode of Nevada,
arrived at a precisely opposite conclusion, the feldspars as a
whole offering more resistance than the augite, hornblende, or
mica.
The present writer has described 8 thick sheets of augite por-
phyrite in Gallatin County, Montana, in which the feldspathic
disintegration has gone on so far that the mass falls away to a
coarse sand, from which still perfectly outlined crystals of coal-
black augites may be gleaned in profusion. This last is,
however, a semi-arid region, and the process thus far one of
disintegration more than decomposition. In a moist, or perhaps
in any climate, minerals consisting essentially of silicates of
alumina and magnesia are less liable to decomposition than
those containing considerable proportions of iron protoxides or
of lime. This for the reason that the first-named are scarcely
at all affected 'by the ordinary atmospheric agents of solution.
Bischof goes so far as to say that the silicate of alumina is not
at all affected by carbonic acid, but the researches of Miiller, to
which reference has been made, and our own calculations tend
to disprove this. Dana states4 that in the decomposition of
basalt, on the island of Tahiti, the olivine is the earliest to give
way, becoming first iridescent and finally falling away to a soft,
pulverulent, ochreous yellow or brown powder. The compact
base of the rock yielded next, the augites holding out until the
last. Those silicates which are least liable to atmospheric de-
composition are, as is to be expected, those which have resulted
from the alteration of less stable silicates, as serpentine from
1 Report, Wilkes's Exploring Expedition, Geology, p. 578.
8 Ann. de Chimie et de Physique, Vol. LV, 1833, p. 240.
8 Bull, U. S. Geol. Survey, No. 110, 1894.
* Op. cit., p. 298.
236
olivine, epidote from hornblende, or kaolin from feldspar, etc.
A few silicates like tourmaline and zircon, or garnet, or oxides
like rutile and magnetite, or the salts of rarer earths like mona-
zite, etc., are scarcely at all affected by any of the ordinary
agents of decomposition, but remain in the form of residual
sands in the beds of streams, from whence the lighter, more
decomposed material is removed by erosion.
In the weathering of potash-feldspar rocks carrying black
mica, the latter mineral is as a rule the first to give- way, and at
times almost wholly disappears. With basic rocks, on the
other hand, the dark mica is one of the most enduring of the
constituents, and in the residual sands may be found in surpris-
ingly large proportions.
In the kaolinized gneisses of northern Delaware, the biotite,
as a rule, is in an advanced stage of decomposition, while the
small amount of primary muscovite is still fresh and intact,
retaining all its original lustre and elasticity.
Among the feldspars the potash varieties are, as a rule, far
more refractory than the soda-lime, or plagioclase varieties.
This is shown not merely by our own investigations, but by
those of others as well. Roth shows1 from analyses of fresh
and weathered phonolites, nepheline basalts, and dolorites, that
the loss of soda is almost invariably greater than that of
potash.
In the coarse, pegmatitic dikes of Delaware County, Penn-
sylvania, the microcline masses, as mined for pottery purposes,
are beautifully fresh and translucent, while the associated oligo-
clase is snow-white through a splitting up along cleavage lines
and partial decomposition. Where thrown out upon the dumps,
this whitened mineral shortly falls away to fine sand, resembling,
at first glance, kaolin, but is distinctly gritty.
Max Geldmacher noted2 that in the weathering of quartz
porphyry oligoclase always gave way before the oligocla,se.
Indeed, as shown in our analyses, in certain phases of rock
degeneration, the potash feldspars may lose very little by
decomposition, but be converted into the condition of fine
silt merely through a mechanical splitting up. This fact will
in part explain the relative scarcity of free potassium salts
1 Op. cit., 3d ed., 2d Heft.
2 Beitrage zur Verwitterung der Porphyre, Inaug. Dissertation, Konigl.
Freidrich Alexander Universitat, Leipzig, 1889.
GENERAL DEDUCTIONS
237
(carbonates, sulphates, and nitrates) as compared with those of
soda.1
The chemical processes involved in this feldspathic decompo-
sition are of sufficient importance to warrant further discussion,
even though it may involve a certain amount of repetition of
what has gone before.
Berthier, Forschammer, Brogniart,2 Fournet,3 and others ex-
plained more than fifty years ago the process of fektapathio dis-
integration through the breaking up of its complex molecule
into alkaline silicates soluble in water, and aluminous silicates
which are insoluble. The loss in silica, as noted above, was
supposed to be due to the removal, by solution, of these alka-
line silicates. Ebelmen,4 however, subsequently showed that
silicate minerals poor or quite lacking in alkalies lost a portion
of their silica with equal facility, as is also shown in our analy-
ses of pyroxenites on pp. 226 and 'I'll. He accounted for this on
the supposition that the silica set free — in a nascent state — was
soluble either in pure water, or water containing carbonic arid.
Bischof states that when meteoric waters containing carbonic
acid filter through rocks containing alkaline silicates, the lirst
1 An oligoclase occurring in a tourmaline granite on the southern slope of
Monte Mulatto, near Predazzo, undergoes, according to Leinberg (Zeit, der Deut.
Geol. Gesellsuhaft, 28, 1876), a much more rapid decomposition than the ortho-
chisr with which it is associated, and gives rise to a green, lustreless, serpentine-
like product. The chemical changes incidental to the alteration are as shown in
the following tables, I being the fresh oligoclase, and II the decomposition
product.
CON8TITCWJT8
I
II
Silica (SiO2)
59.51 %
45.29 %
Alumina (AljOg)
25.10
26.68
Iron sesquioxide (FegOs)
1.08
12.49
Lime (CaO)
4.03
0.52
Magnesia (MgO)
Trace
2.88
Potash (K2O)
2.10
3.00
Soda (NaaO)
7.26
2.14
Water (HaO)
0.92
8.00
100.00 %
100.00 %
2 Arch du Museum, Vol. I, 1839 (cited by Ebelmen).
8 Ann. de Chimie et de Physique, Vol. LV, 1833.
4 Ann. des Mines, Vol. VII, 1845.
238 HOCK DISINTEGRATION AND DECOMPOSITION
action consists in the partial decomposition of these substances
by the carbonic acid and the formation of alkaline carbonates,
which are dissolved. If the water thus impregnated, on pene-
trating further below the surface, comes in contact with cal-
careous silicates, another change will take place consisting of a
decomposition and replacement of these calcareous silicates by
the alkaline silicates, and a removal of the lime set free, as a
carbonate, provided the water still contains a sufficient amount
of carbonic acid. This replacing process and the retention of
the alkaline silicates is accounted for on the supposition that,
in their nascent state, they form new combinations with the
other silicates present, while the lime remains as a carbonate to
be removed or not, as the case may be. He further states that
the alkaline carbonates originating in the manner described
are among the most soluble substances known ; the carbonate
of soda requires for solution. only six times its weight of water
at ordinary temperatures. Silica, on the other hand, even in
its most soluble form, requires ten thousand times its weight of
water for solution. If, therefore, the decomposition of feld-
spar by such carbonated water were ever so energetic, there
would be sufficient water for the solution of the carbonate of
soda formed. But if the silica separated meanwhile amounted
to more than Ytruro °^ ^ne water present, the excess could not
be dissolved, but would remain mixed with the kaolin.
The case is very different when the decomposition of feldspar
is affected by fresh water containing only the minute quantity
of carbonic acid derived from the atmosphere. By the action
of such water, only very small quantities of alkaline carbonates
are formed ; consequently it is possible that the silica separated
at the same time, also small in quantity, may find enough water
for solution. In such cases the whole of this silica would be
removed with the alkaline carbonates, and pure kaolin would
be left. Such an action as this does not, however, appear to
take place ; for the purest of kaolin nearly always contains an
admixture of quartz sand, or of free silica in some of its forms.
K. V. Murakozy has shown l that in the decomposition of
rhyolite from Nagy-Mihaly, the sanidin passes into kaolin and
opal, the latter separating out as hyalite in veins or impure
concretionary forms.
It follows from this consideration that in the decomposition
1 Abstract by F. Becke, Neues Jahrbuch, 1894, 1 Band, 2 Heft, p. 291.
GENERAL DEDUCTIONS 239
of feldspar into kaolin more of the silica separated remains
mixed with the kaolin formed, the greater the quantity of
carbonic acid in water, and that, perhaps, the amount of car-
bonic acid is never so small that the whole of the silica sep-
arated in the decomposition of feldspar can be removed.1 The
above, however, overlooks the possible presence of nitrates, such
as we now know from the researches noted on p. 203 may in
many cases exist, even though in extremely small proportions.
It is probable that the small amount of nitric arid formed
by the bacteria would, if not taken up by plant growth, com-
bine immediately with the alkalies, forming nitrates which,
owing to their ready solubility, would be carried away. The
larger the proportion of nitric acid, therefore, the greater
would be the amount of silica intermingled with the kaolin,
since whatever proportion of the alkalies failed to be carried
away as nitrates would pretty certainly disappear as carbo-
nate. There is also the possibility, especially in the rocks
rich in iron protoxides, that a portion of the silica may com-
bine with the iron, as already noted.
In cases where the decomposition takes place under the
influence of a sufficient supply of oxygen, all iron, and presum-
ably the manganese as well, would be converted into the in-
soluble hydrous sesqiiioxide form and remain with the residue.
Where, however, the supply of oxygen is insufficient, a por-
tion or all of these constituents may be removed in the form
of j ) rot oxide carbonates, or, in the case of iron, of a ferrous
sulphate. These facts well account for the variation in sta-
bility of the iron, as indicated in the preceding analyses.
Reference has already been made to the fact that the mag-
nesia from the decomposition of maguesian silicates was some-
times removed in greater relative portions than was the lime.
This seeming anomaly is also sometimes met with in cal-
careous stratified rocks. Roth2 showed that in the weather-
ing of dolomitic limestones, the magnesia is often removed in
greater proportional quantities than the more soluble lime
carbonate.
The researches of Hitterman 3 showed, however, that carbonic
1 Chemical and Physical Geology, by Gustav Bischof, Vol. II, pp. 182, 183.
-Op. cit., Vol. III.
8 Die Verwitterungeprodoctfl von Gesteinen der Triasformatkm Frankers,
Inaug. Dissertation, Freidrich-Alexanders Universitat, Munich, 1889.
240 KOCK DISINTEGRATION AND DECOMPOSITION
acid solutions may exert a scarcely appreciable effect upon mag-
nesian carbonate, which therefore accumulates in the residual
soils.
It is safe to say that while the general process of rock-
weathering may be quite simple, as outlined, there are many
minor reactions which it is not possible to describe in detail.
It has been shown that even in firm rocks a mutual chemi-
cal reaction is not uncommon among minerals lying in close
juxtaposition, giving rise to what are known as reaction rims
or zones composed of secondary minerals. This is a par-
ticularly conspicuous feature in many gabbros, where olivine
and feldspar are closely adjacent. In these cases, a mutual
interchange of elements may take place, giving rise to garnets,
free quartz, or other minerals as the case may be. This is,
to be sure, a deep-seated change, to be classed as alteration
rather than decomposition, and taking place presumably under
conditions of temperature and solution quite at variance with
those existing on the immediate surface. It is, nevertheless,
self-evident that when elements are set free through any
process, they must almost immediately recombine, taking those
forms which existing circumstances may dictate and that close
contact of particles would be favorable to the more rapid for-
mation of new compounds. In a mass of decomposing rock,
circumstances are almost continually changing, and the infer-
ence is fair that new combinations are continually being made
and unmade, the intricacies of which we are unable to follow.
PLATE 18
FIG. 1. Exfoliated granite in the Sierras.
FIG. 2. Talus slopes on Pike's Peak.
FIG. 3. Disintegrated granite, Ute Pass, Colorado.
THE WEATHERING OP ROCKS (Continued)
III. THE PHYSICAL MANIFESTATIONS
Rock-weathering manifests itself in a great variety of ways,
much depending upon climate, though naturally the controlling
factor is that of mineral composition. The manner of weather-
ing is often sufficiently characteristic to be of -great import aim-
in determining surface contours, as well as incidentally afford-
ing a means for the identification of rock masses when t la-
outcrops themselves are obscured by decomposition products.
Such a means is of only local importance, however, since under
varying conditions the resultant forms assumed, even by similar
rocks, are themselves quite variable. It is, nevertheless, not
without interest to note the varying phases of weathering in
different kinds of rocks, the incidental contours assumed, the
character of the resultant debris, and, at the same time, tin-
controlling forces that have been instrumental in bringing
about the final result.
(1) Disintegration without Decomposition. — That in weather-
ing, physical and chemical agencies may go on either singly or
conjointly has been noted in previous pages. In the case of
single minerals, the preliminary disintegration is beautifully
illustrated in the large oligoclase masses associated with micro-
cline in the feldspar mines of Delaware County, Pennsylvania.
In the dumps of waste about the mines these are found, in all
stages of disintegration, the mineral splitting up along cleavage
lines, becoming snow-white, and ultimately falling away to a
kaolin-like product, but which, when submitted to microscopic
examination, is found to be made up of sharply angular cleavage
particles, showing no sign of decomposition other than that in-
dicated by occasional opacity. In the analyses given below are
shown (I) the composition of a fresh oligoclase (as given by
Dana) from near Wilmington, Delaware, (II) the snow-white
R 241
242 THE PHYSICAL MANIFESTATIONS OF WEATHERING
cleaved, but still moderately firm mineral mentioned above, and
(III) the flour-like or kaolin-like product.
I
II
III
CONSTITUENTS
FRESH OLIGOCLASE
OPAQUE WHITE,
BUT STILL FlKM
OLIGOCLASE
FINE DUST FROM
DISINTEGRATED
OLIGOOLASE
Si02
64.75%
61.23 %
56.73%
A12O3
23.56
25.65
28.44
CaO
2.84
2.37
2.95
K20
1.11
0.72
1.12
Na20
9.04
7.66
5.81
Ignition
1.00
5.67
101.33%
99.63%
100.72%
The fact that granitic and gneissic rocks may undergo ex-
tensive disintegration with slight decomposition, even in a
moist climate, was noted by Nordenskiold l in Ceylon. He
says: "The boundary between the un weathered granite and
that which has been converted into sand is often so sharp that
a stroke of the hammer separates the crust of granitic sand
from the granite blocks. They have an almost fresh surface,
and a couple of millimetres within the boundary the rock is quite'
unaltered. No formation of clay takes place and the alteration
to which the rocks are subjected, therefore, consists in a crum-
bling or formation of sand, and not, or at least only to a very
small extent, in a chemical change. At every road section
between Galle, Colombo, and Ratnapoora the granite and gneiss
crumbled down to a coarse sand, which was again bound to-
gether by newly formed hydrated peroxide of iron to a peculiar
porous sandstone, called by the natives cabook.2 This sandstone
forms the layer lying next the rock in nearly all the hills on that
part of the island which we visited. It evidently belongs to
an earlier geological period than the Quaternary, for it is older
than the recent formation of valleys and rivers. The cabook
often contains large, rounded, unweathered granite blocks, quite
resembling the rolled stone blocks in Sweden. In this way
1 Voyage of the Vega, Vol. II, 1881, p. 420.
2 Laterite ? It seems so regarded by H. F. Alexander, Trans. Edinburgh
Geol. Society, Vol. II, 1869-74, p. 113.
WEATHERING INFLUENCED BY STRUCTUUH
tin- re arises at places where the cabook stratum htfs again
been broken up and washed away by currents of w^er, forma-
tions which are so bewildering, like the ridges (osars) and hills
with erratic blocks in Sweden and Finland, that I was aston-
ished when I saw them."
The same features are brought out in the previous descrip-
tions relative to the weathering of the granite of the District
of Columbia, the diabase of Medford, Massachusetts, and other
localities mentioned in these pages. (See pp. 206 and 218.)
This tendency toward disintegration \vithout decomposition is
exaggerated among coarsely crystalline rocks, as is abundantly
exemplified in the rocks of the Pike's Peak (Colorado) area. (See
PI. 18.) Among those of finer grain, particularly the quartz-
free varieties, as the Fourche Mountain (Arkansas) syenites,
decomposition may follow so closely on disintegration that little
or no sand is formed, sound fresh rock passing within the space
of a few millimetres into the condition of residual clay.1
(2) Weathering influenced by Crystalline Structure. — It is else-
where observed that, other things being equal, a coarsely gran-
ular rock will disintegrate more rapidly than one of finer grain.
Lone Mountain, one of the high eruptive peaks on the west
side of the Madison valley in Montana, presents in its upper
portions all the features of a volcanic crater broken down on
one side by the lava flow. The facts of the case are, however,
that the coarser grained central portion has been disintegrated,
and swept by wind and rain into the valleys, while the fine-
grained, more compact outer portions, those which solidified near
the line of contact with adjacent rocks, remain intact. Pro-
fessor Bell 2 describes an interesting case of this kind where the
coarsely crystalline central portion of a " greenstone " dike has
yielded more readily to erosion than at the sides and afforded
channel-way for the Mattagami River, north of Lake Huron, in
Canada. The gneiss adjoining the dike having been shattered,
yielded also to decomposing agencies and forms now a second
parallel channel on each side of the central one. " Between
them the finer grained, hard, and undecayed ' greenstone ' con-
1 Dr. Max Fesca has noted that the granitic rocks of Kai province, Japan,
yield on decomposing gravel, sand, and clayey loams, while those rocks poor in
quartz, such as the syenites, give rise only to clays (Abhandlungen und Erlau-
terungen zur Agronomischen Karte de Prov. Kai, Kaiserlich Japanischen Geo-
logischen Reichsanstalt, 1887).
2 Bull. Geol. Soc. of America, Vol. V, 1894, p. 364.
244 THE PHYSICAL MANIFESTATIONS OF WEATHERING
stituting the outer portions of the dike rises up in the shape of
ridges and chains of islands, so that the river flows as a main,
central channel, more or less separated from the smaller lateral
ones." The same writer describes several instances in which
long straight valleys in the Archaean regions of Canada, now
occupied by straight river stretches, long narrow lakes or inlets
of the larger lakes, are due to the decay and removal of the
wide " greenstone " dikes, or of parallel dikes with narrow belts
of rock between. Long Lake, north of Lake Superior, some 52
miles in length, is mentioned as typical of lakes of this class.
(3) Weathering influenced by Structure of Rock Masses. — In
any rock mass weathering is greatly augmented by lines of
weakness, such as joint and bedding planes, since these furnish
so many additional points of attack. In homogeneous massive
rocks the rate of disintegration is retarded by a lack of vulner-
able points, and the resultant form is that of rounded bosses
such as are shown in plate 1.
As a rule, however, the most massive of rocks are traversed
by one or more series\pf joints (see PI. 14) whereby they are
FIG. 17. — Showing the influence of joints in the production of boulders.
divided up into rhomboidal blocks of varying sizes. Even
when not sufficiently developed to be conspicuous, such joints
not infrequently exist as lines of weakness along which moisture
and the accompanying agents of disintegration make their way,
gradually rounding the corners until there is left an oval niass
of which the so-called " niggerheads " of the gabbro area about
Baltimore are typical examples. In nearly all such rocks the
exfoliation and decomposition take place in the form of con-
centric layers, like the coatings on an onion. This holds true
with the huge granitic bosses, as well as with the smaller joint
blocks, and has been argued by some of the earlier geologists
as indicative of an original concretionary structure. Such an
WEATHERING INFLUENCED BY STRUCTURE 245
assumption seems, however, wholly uncalled for. If the block or
mass is reasonably homogeneous, the agencies of decomposition
will penetrate nearly uniformly from all exposed surfaces, pro-
ducing an exfoliation nearly parallel to that surface, and the
concentric structure is inevitable, as was long ago pointed out
by Werner.
In some cases the tendency to assume the boss-like form is
accentuated through the presence of joints running approxi-
mately parallel to the exposed surface, such joints as give rise
to the step-like arrangement of the stone so frequently seen in
granite quarries. Stone Mountain, Georgia, an immense boss
of light gray granite some 2 miles long by l.\ wide and 650 feet
Fio. 18. — Exfoliation of granite.
high, owes its form, apparently, wholly to exfoliation parallel
to pre-existing lines of weakness. The entire mass, so far as
exposed by quarrying operations, is made up of imbricated sheets
of granite, which, of unknown thickness beneath the surface,
thin out to mere knife edges above, like shingles on a roof.
Through prolonged exposure the superficial layers have become
detached from the parent mass, and doubtless hundreds of feet
in vertical thickness completely disintegrated and swept away.
With many geologists these joints, in themselves, would be
accepted as due to atmospheric action. In the writer's present
opinion they are, however, the result of torsional strains and
once existing are lines of weakness which become more and
more pronounced as weathering progresses. The boss-like form
is therefore incidental and consequent. The process of exfo-
liation has, in the case mentioned, been productive of some
peculiar results which may be described in detail.
246 THE PHYSICAL MANIFESTATIONS OF WEATHERING
As above mentioned, the sheets of granite, varying from a few
inches to several feet in thickness, conform in a general way to
the present surface of the hill. Constant expansion and con-
traction from temperature changes have, in the manner already
described, so expanded these sheets that, bound at the sides,
they have found relief in an upward direction where resistance
was least, and risen in dome or roof shaped forms, as shown in
the sketch. The weight of the sheets higher up the slopes, im-
pinging upon the edges of those below, has in some cases
undoubtedly aided in the work, but the larger part is due to
simple expansion, such as was referred to on p. 180.
These ruptured sheets are rarely more than 10 inches thick,
but 10 or 20 feet in diameter. The material, though quite
fresh appearing, is loosely granular and friable, easily reduced
to sand. In a few instances small avalanches have been caused
by the giving way of the sheets below and the consequent slid-
ing down of those above through lack of support. (See Fig. 18.)
This same mass of granite sometimes shows upon its surface
peculiar circular depressions, one within another, separated by
intervening ridges of low
relief, such as have been
described in a much more
perfect stage of develop-
ment by Dr. Robert Bell 1
in the Huronian rocks of
Canada. These, as shown
in Fig. 19 from Bell's
paper, are some 3 or 4
feet in diameter and 3 or
4 inches high. The cause
of this form of weather-
ing at Stone Mountain
is not apparent, though
Bell'in *he case/^rf'
regards it as induced by
an original concretionary
structure.
The spheroidal structure so frequently seen in basaltic rocks,
and as typified in the sphaeroidische absonderung of German
writers, may perhaps be due to an original spheroidal ten-
Geol. Soc. of America, Vol. V, 1894, p. 362.
FIG. 19.
WEATHERING INFLUENCED BY STRUCTUK1 247
dency caused by cooling,1 but a very large proportion of the
spheroidal masses so typical of the decomposition of massive
rock is, as already suggested, due wholly to external causes.
W. P. Blake in 1855 called attention to this form of disintegra-
tion in the massive sandstones near San Francisco (California )
and pointed out the true explanation.2
This sandstone is described as occurring in the form of layers
from a few inches to 6 and 8 feet in thickness, alternating with
beds of slate and shale. Down to a depth of 10 or 20 feet, in-
to the limits of atmospheric action, all the beds have turned from
i,n -a Y to rusty brown or drab. " There are, however, parts of the
upper beds that have not yet been reached and changed by de-
composition ; these parts are found in the condition of spherical
or ellipsoidal masses, from which the weathered parts scale off
in successive crusts. These nuclei have the appearance of great
rounded boulders, and have accumulated in great numbers at
tin- base of the cliff." In this case the sandstone is composed
mainly of grains of quartz and a little feldspar cemented by
caleite, the disintegration being due mainly to the removal of
this cement by percolating water, while the change in color is
doubtless due to oxidizing pyrite or ferrous carbonate.
The effect of percolating waters is not, however, always im-
mediately destructive. Though in themselves carrying cement-
ing materials, or causing an oxidation of the iron carbonates or
sulphides, a local induration may be induced along the joint
lines such as becomes conspicuous only through the weathering
away of the non-indurated portions. Resultant forms may be
extremely regular or again irregular, according to the character
of the lines along which percolation takes place, and that of the
rock itself. An interesting illustration of this form of w*-at her-
inij is that given by Wyville Thompson8 as occurring on the
islands of Bermuda.
u This dissolving and hardening process," he writes, " takes
place irregularly, the water apparently following certain courses
in its percolations, which it keeps open, and the walls of which
it hardens ; and in consequence of this, the rock weathers most
unequally, leaving extraordinary rugged fissures and pinnacles,
1 T. G. Bbnney, Quar. Jour. Geol. Soc. of London, Vol. XXXII, 1876, p. 153.
2 Expl. and Survey for a Railroad from the Mississippi to the Pacific Ocean;
Report on the Geology of the Route, near the 32d Parallel, by W. P. Blake.
3 See The Atlantic, Vol. I. Also Bull. 25, U. S. National Museum.
and piled up boulders, the cores of masses which have been
eaten away, more like slags or cinders than blocks of limestone.
The ridges between Harrington Sound and Castle Harbor are a
good example of this. It is like a rockery of the most irregular
and fantastic style, and there seems to be something specially
productive in the soil ; for every crack and crevice is filled with
the most luxuriant vegetation, mossing over the stones and train-
ing up as tier upon tier of climbers, clinging to the trees and
rocks. Frequently the percolation of hardening matter, from
some cause or other, only affects certain parts of a mass of rock,
leaving spaces occupied by free sand. There seems to be little
doubt that it is by the clearing out of the sand from such
spaces, either by the action of running fresh water or by that
of the sea, that those remarkable caves are formed which add
so much to the interest of the Bermudas."
A form of weathering due to similar causes, but productive
of results much more regular in arrangement, is shown in
Fig. 4, PI. 20, from a block of weathered sandstone in the
National Museum. The original joints through which the
waters filtered are easily recognized in the sharp straight lines
running diagonally across the specimen. Blocks of fine shale
and argillite, in their incipient stages of weathering, often show
concentric bands of varying color, due to the oxidizing effect
of water percolating inward from all sides of the natural joints
as shown in Fig. 3, PL 20.
In stratified rocks there is, as a rule, a lack of homogeneity,
certain layers being more porous than others, or containing
mineral constituents more susceptible to the attacking forces.
Such rocks, therefore, weather unevenly, and give rise to ex-
ceedingly ragged contours. The finely fissile schists standing
nearly on edge along the coast of Casco Bay, in Maine, under
the combined influence of wave and atmospheric action, weather
into peculiarly fantastic forms resembling nothing more than
piles of old lumber in which the multitudinous channels formed
by boring coleopterous larvae have become irregularly enlarged
by decay. (See Fig. 1, PI. 19.) The numerous quartz veins by
which these schists are traversed stand out in bold relief until
no longer supported by the matrix, when they fall to the beach,
where, together with fragments of the schist, they are gradually
reduced to pebbles and fine sand.
(4) Weathering influenced by Mineral Composition. — Although
PLATE 19
WEATHERING INFLUENCED BY COMPOSITION 249
tin- soda-lime feldspars yield to the decomposing agencies more
readily than the potash varieties, basic eruptives do not in all
decompose more rapidly than the granitic rocks into which
they are intruded, as is well illustrated in some of the glaciated
areas about Boston, when- small, compact dikes form low ridges
a few inches above the surface of the enclosing granite. Miu-li
seems to depend upon the character of the secondary minerals
which have been generated in a rock at some period prior to
its decomposition proper. Thus those dikes containing so large
a proportion of secondary epidote as to be of a dull greenish
hue are almost invariably more enduring than the granites,
while those, on the other hand, in which the secondary minerals
are largely chlorite, calcite, and zeolitic compounds, yield to the
decomposing agencies more readily. Even when the dike as
a whole gives way, the presence of epidotic aggregates fre-
quently manifests itself in protruding knots and bunches above
the corroded surface. Knots caused by segregations of black
tourmalines stand out in the same way from the surface of the
granite boss called Stone Mountain, near Altanta, Georgia.
(iaruets, staurolites, quartz veins, and other of the less easily
decomposed minerals may stand out in like manner from the
surface of the rocks of which they form a part.
( i ranitic and other complex crystalline granular rocks will, on
exposure, sometimes take on a pitted surface, owing to the re-
moval of the more easily decomposed materials. The boulders
of nepheline syenite in the glacial drift about Portland, Maine,
are thus corroded to the depth of several millimetres through
the removal of the granular nepheline, while the feldspars and
hornblendes project irregularly.
Calcareous rocks containing silicates, like the amphiboles or
pyroxenes, show like roughened surfaces due to the dissolving
away of the calcareous matter, leaving the silicates projecting,
or, as is the case with some of the tremolite-bearing dolomites
used for building, may become pitted by the dropping out of
the tremclite as the calcareous cement gives way.1
Many sandstones become likewise roughened through the
removal of a portion of the cementing constituent, leaving the
siliceous granules projecting. In the coarsely crystalline lime-
stones and dolomites the solution and weathering effects are
often first manifested along cleavage lines and the contacts of
1 As in the U. S. Capitol Building at Washington.
250
the individual granules, as may be observed in many an old
tombstone or polished column.
Even where the decomposition, is almost purely chemical, the
corroded surfaces are peculiarly irregular, as shown in PL 17.
This feature is doubtless due to some imperceptible difference in
the texture of the stone, or to the presence of joints and flaws
which give direction to the solvent fluids. Calcareous rocks
consisting of an admixture of calcite and dolomite crystals may
undergo disintegration through a complete or partial removal of
the calcite granules by solution, the dolomite remaining almost
untouched. Certain dolomitic limestones near Stockton, Min-
nesota, have been described1 as peculiarly subject to this form
of disintegration. The mass of the rock consists of dolomitic
crystals and granules, but often interlaminated with narrow
bands of calcite. Through the removal of the latter, the
stone becomes porous and its degeneration so complete that
" shovelfuls of loose sand consisting of dolomitic rhombohedra
can be taken up."
Fine-grained, compact, and seemingly homogeneous rocks
may, on account of imperceptible differences in composition
and structure, weather out in strikingly irregular and peculiar
forms. Figure 2 on PI. 15 is that of a limestone fragment from
Harrisonburg, Virginia. The resemblance to cuneiform charac-
ters is so close that it is not surprising that such were at first
supposed to be of human origin.
Massive granitic rocks seemingly of quite uniform composi-
tion will sometimes weather very irregularly, giving rise to
oven-like cavities, in general shape resembling the pot-holes in
the beds of streams. Reusch has described2 such in exposed
faces of granite ledges on the island of Corsica, the holes
extending inward horizontally, or sometimes with a slight up-
ward tendency. The cause of this is not apparent from the
description given, but it is presumably due to slight textural
differences such as are not readily discernible in the decom-
posed rock.
In any rock consisting of a variety of minerals, disintegration
is likely to constitute a more prominent feature of weathering
than in one of less complexity of composition, owing to the
unequally refractory properties of its constituents. Thus a
* Hall and Sardeson, Bull. Geol. Soc. of America, Vol. VI, 1895, p. 184.
2 Forhandlinger i Videnskabs-Selskabet i Christiania, 1878, No. 7, pp. 24-27.
WEATHERING INFLUENCED BY COMPOSITION 251
granite must yield a sand, while a purely feldspathic, pyrox-
enic, or calcareous rock may yield only clays.
Beds of feldspathic quartzite, through the decomposition of
the feldspar, undergo disintegration, giving rise to beds of
friable siliceous sand interlaminated with kaolin, as described by
Dana.1 The same author also describes an interesting pseudo-
breccia formed by a quartzite divided up by a succession of
cracks into which limonite from decomposing pyrite has fil-
tered and acted as a colored cement. He says : •• Many of the
pieces lie in place barely separated from one another, and ap-
pear to be undergoing new divisions. But in the lower part,
large pieces look as if there had been wide displacements ; yet
the hardly disturbed condition of the upper half proves that
the apparent displacement is due to the extension of the color-
ing and penetrating limonite. The cracks are made in part
by the extremely slow, wedge-like action of the depositing
limonite."
Heusser and Claraz2 describe somewhat similar breccias
formed in Brazil through the weathering of crystalline schists
rich in iron. These breccias consist of angular fragments of
schist, more or less decomposed, firmly cemented by limonite.
The boulders of Oriskany quartzite in the Cretaceous gravel
about Washington, District of Columbia, are composed of
rounded and angular quart/ fragments tightly bound together
by a fine granular crystalline aggregate of quartz and feldspar.
Disintegration first manifests itself on the exterior of the
boulders in the form of an irregular network of grooves or
channels, which gradually become more and more conspicuous
until the boulder falls into bluntly pyramidal fragments and
finally into sand. The microscope shows that the disintegra-
tion is due wholly to the disaggregation and partial kaoliniza-
tion of the binding constituents whereby all cohesion is lost,
and disintegration follows from necessity. (Fig. 1, PI. 20.)
This form of disintegration seems to take place only in
boulders exposed at or near the surface, and is believed to be
due primarily to expansion and contraction from alternations
of temperature.
.Many rocks, owing to a lack of homogeneity, weather with
extreme irregularity and give rise to odd and sometimes fan-
i Am. Jour, of Science, Vol. XXVIII, 1884.
a Ann. des Mines, 6th Series, Vol. XVII, 1800, p. 290.
252 THE PHYSICAL MANIFESTATIONS OF WEATHERING
tastic forms. In the case of a friable sand or limestone, sub-
ject to wind or rain erosion or to solution, certain portions may
be protected by a capping of other rock while the intervening
material is carried away. There thus arise spindle-shaped
forms of varying proportions, each capped by the roof or hat-
like block to which it owes its origin. Such have been noted
in many regions, and have been described by Hayden as occur-
ring on a colossal scale in Colorado. In the case of strata
lying nearly horizontal, it rarely happens that all possess the
same power of resistance, the more friable weathering away
with the greatest rapidity, leaving the harder layers for a
time projecting in rib-like masses, to ultimately break down
in large angular blocks as the support below is gradually
removed. Friable beds of sedimentary rock are thus not infre-
quently protected by a capping of impervious lava. Continual
percolation of water through existing joints and fractures in
time, however, erode away, in part, the underlying material,
causing the landscape to assume the Table Mountain appear-
ance, where each flat-topped hill represents residual masses of
a once continuous plateau, now isolated in the manner described.
(5) Results due to Position. — In very many instances loose
blocks of stone lying exposed upon the ground, will undergo
a more rapid disintegration from the lower surface, a feature
evidently due to the fact that this portion of the rock is kept
in a state of continual moisture. This form of disintegration
results in the production of oval, flattened, scale-like masses,
quite independent of the original jointing. In other cases
decomposition going on from all exposed sides of a joint block
may in time produce the so-called rocking-stones or " logans "
and "tors" of English writers, though some of these are un-
doubtedly nicely balanced boulders from the glacial drift.
A mass of rock may be prevented from undergoing disinte-
gration, even though partially decomposed, by its surroundings.
Thus, in driving the tunnel for the waterworks extension, in
Washington, natural joint blocks of hard and apparently firm
rock brought to the surface would fall away to loose sand in
course of a few days, or months, as the case might be, much
depending on the conditions of the weather and the state of
decay. This characteristic was sufficiently pronounced to
attract even the attention of the workmen, who described the
rock as "slaking" and believed it to contain quicklime.
RESULTS DUE TO POSITION 253
The fact was that percolating waters had brought about a
partial kaolinization of the feldspar, and hydration, without
great oxidation of the iron-magnesian constituent. The origi-
nal pressure, coupled with that incidental to expansion from
hydration, had, however, been sufficient to hold the mass intact
until exposed briefly to atmospheric influences.
The protective action of water, as sometimes shown in the
beds of streams and in deep ravines, may be only apparent, and
due to the fact that erosion exceeds decomposition, the stream
having cut its way down to fresh bed-rock. Professor Dana.
to be sure, writing more than half a century ago,1 described the
basaltic rocks of Kiama, Australia, as in a condition of advanced
decomposition except where protected by sea-water. He says :
" It is a general and important fact that a rock which alters
rapidly when exposed to the united action of air and water, is
wholly unchanged when immersed in water, or exposed to a
constant wetting by the surf." While no exception can be
taken to the conclusion regarding those rocks wholly immersed,
the question naturally arises in one's mind, if the absence of
decomposition products in those rocks constantly wetted by
the surf and in many stream beds may not be due, in part at
least, to erosion. That rocks so situated are in a condition far
from fresh, is well known to any petrologist who has attempted
to gather specimens.
It is obvious that where a large series of sedimentary rocks
composed, it may be, of interbedded limestones, sandstones, and
argillites are turned up on edge and exposed alike to atmos-
pheric agencies, they will become eroded very unequally. If
chemical agencies alone prevail, the limestone will dwindle
away and perhaps give rise to long valleys or depressions
walled in by the more enduring sands and shales, and carry-
ing upon its bottom a fertile clayey soil representing not
merely the insoluble impurities contained by the original lime-
stone, but also the mechanically disintegrated particles washed
in from the hills on either hand. This indeed may be consid-
ered the history of the fertile Shenandoah valley of Virginia,
famous alike for soft contours, beautiful scenery, and the exu-
berant fertility of its soils.
When stratified rocks lie nearly or quite horizontally, much
must depend upon the character as regards permeability, etc.,
1 Reports of Wilkes's Exploring Expedition, Geology, p. 614.
254 THE PHYSICAL MANIFESTATIONS OF WEATHERING
of the upper layers, since these may so protect the lower lying
as to retard or quite stop further disintegration. Further than
this, an easy and rapidly disintegrating superficial layer may
yield a residual clay so impervious as to protect the underlying
rocks as securely as a mass of rock itself, or so hard and tough
as to put a stop to purely mechanical erosion, as in the case of
the laterite beds of central India.
In cases where thinly bedded rocks lie sharply inclined, it
nearly always happens that certain layers decompose more
readily than others. There may thus arise strikingly ragged
saw-tooth contours, the more enduring layers standing out in
sharply serrate or wall-like masses, while the softer give way
and become obscured by their own debris.
(6) Induration on Exposure. — Many rocks, instead of becom-
ing disintegrated on exposure, undergo a kind of induration
upon the exposed surfaces. This is particularly the case with
some siliceous sandstones. The water with which the stone is
permeated holds in solution certain constituents, as silica, car-
bonate of lime, or iron oxides. When the rock is so situated
that this " quarry water," as it is popularly called, is brought
to the surface and evaporated, it binds together the granules
composing the stone, forming thus a more or less superficial
coating of a more enduring nature. The induration sometimes
takes place so rapidly that even an exposure of but a few months
is sufficient to produce very marked results on freshly broken
surfaces. This peculiarity of certain classes of rocks has long
been known to quarrymen and stone workers, who recognize
the fact that a well-seasoned stone yields much less readily under
the chisel than one that is newly quarried.1
A somewhat similar induration, due to purely superficial
causes, has been described by Dr. M. E. Wads worth as taking
place on the surface of exposed blocks of siliceous sandstone in
Wisconsin. " The St. Peters Sandstone " he writes,2 " is com-
posed almost wholly of a pure quartz sand, and in the outliers
of it found on the hilltops south of the town, the parts covered
by the soil were more or less friable, and the grains distinct;
while the exposed portions of the same blocks and slabs were
greatly indurated, the grains almost obliterated, and the rock
possessed the conchoidal fracture and other characteristics of a
1 See Stones for Building and Decoration, p. 415.
2 Proc. Boston Soc. of Natural History, Vol. XXII, 1883, p. 202.
INDURATION ON EXPOSURE 255
quartzite." In this and other cases cited by Dr. Wadsworth,
the cementing matter is silica.
The explanation given (in letter to the present writer) is to
the effect that all water, including that of rains, as well as ter-
restrial, dissolves silica, which is again deposited under suitable
conditions. Part of the silica apparently comes from the solu-
tion of the quartz, chalcedony, and opal, and a part from the
alteration and destruction of the silicates. Both solution and
deposition seem at times to take place on the immediate surface,
the interior waters in such cases playing no part.
P. Choffat regards it as possible that silica set free through
feldspathic decomposition in granitic rocks may, on evaporation,
be redeposited in an insoluble form in the interstices of the fresh
rock in the immediate vicinity, thus retarding if not wholly
preventing further decay in that direction.1
Professor W. O. Crosby, in a personal memorandum to the
writer, calls attention to the fact that in the disintegrated
granites of the Pike's Peak, Colorado, area, the rock is almost
invariably exceptionally firm and impervious along the joints,
indicating a local induration due }>erhaps to infiltration of iron
oxides or silica. Where a joint face bounds a ledge of rock, it
often maintains its integrity, weathering out in relief like a
quartz vein, while the granite is in a condition of advanced
degeneration all around. A slight break in the face of a joint
plane, in such cases, may lead to extensive disintegration behind
it, until it finally falls away from the disintegrating mass, a slab
of relatively sound rock.
Andesitic rocks in regions of limited rainfall have been
noted by Professor G. Vom Rath as having become covered
on the upper surface with a thin layer of brown iron oxide,
which protected them from further disintegration. Such
crumbled away only from the under surfaces, where they ab-
sorbed moisture from the ground, and gave rise thus to peculiar
tent-like and mushroom-shaped forms.
The present writer has noted in the Madison valley, north
of the Yellowstone Park, rounded masses of a vesicular rhyolite
which have, through the same causes, been reduced to the con-
dition of mere shells with openings on the under sides and that
1 Sur quelques cas d'erosion atmospherique dans les garnites du Minho, Com-
mimiea^fies da Direcsao Dos Trabalhos Geologicos de Portugal, Tome 3, Fasc. 1,
1895-96, p. 17.
256 THE PHYSICAL MANIFESTATIONS OF WEATHERING
facing the direction of the prevailing winds. In these cases,
however, the wind seemed to have aided their formation, not
merely through transporting the disintegrated material, but by
catching up and whirling about the loosened granules within
the gradually enlarging cavity, where, by force of impact, as
already described, they became themselves agents of abrasion.
Some of the cavities observed were of sufficient size to afford
shelter for a human being and had in some instances served as
temporary dens for wild animals.
Roth mentions 1 an induration evidently somewhat similar to
that described by Vom Rath above, as having taken place, on
the surface of a reddish yellow sandstone in Fezzan, North
Africa. The crust thus formed was so dense and hard as to
break with a shell-like fracture resembling basalt. A similar
incrustation on sandstone from the Lydian desert was found to
consist of : manganese oxide, 30.57 % ; iron oxide, 36.86 % ;
alumina, 8.91% ; silica, 8.44% ; barium oxide, 4.89% ; sul-
phuric acid, 4.06 % ; phosphoric acid, 0.25 % ; and water, 5.90 %.
W. P. Blake has described boulders from the Colorado
desert colored exteriorly by what he regarded as organic matter
received from water during a period of submergence. Similarly
discolored quartzitic boulders brought by G. K. Gilbert from
the Sevier desert in Utah, and examined by the present writer,
show a thin dark varnish-like coating, not inaptly named by Mr.
Gilbert " desert varnish," and which consists largely of oxides
of iron and manganese, though a slight amount of organic
matter is present. In this case the rock is composed not wholly
of quartz granules, but carries interstitial calcite and feldspathic
granules. Near the discolored surface of the boulders these in-
terstitial calcites are found quite dissolved away, leaving cavities
stained by a dark deposit which reacts for iron and manganese.
Inasmuch as acid solutions obtained from fresh and uncolored
portions of the boulders give faint reactions of the same nature,
it seems very probable that the crust is due to a concentration
of these metals in a condition of higher oxidation on the surface,
whither they have been brought by capillarity, while the more
soluble lime carbonate was removed.2
1 Allegeineine u. Chemische Geologie, 2d ed., Vol. Ill, p. 215.
2 Although such discolorations seem to have been noted principally in desert
regions, they are by no means limited thereto. The quartzitic boulders in the
superficial deposits of the District of Columbia show at times a like discoloration,
due to a very thin coating of iron and manganese oxide.
INCIDENTAL COLOR CHANGES 257
The Potsdam quartzites of Minnesota have had, in many in-
stances, an almost glass-like polish imparted to their exposed
surfaces through no other apparent agency than that of wind-
blown sand. Unlike a polish produced by artificial met hods,
this wind polish extends to the bottoms of every little groove
and cavity, or over every protruding knob alike. In softer
rocks, or rocks of less homogeneous structure, the same agencies
carve out the softer portions, leaving the more resisting pro-
truding, as already described on p. 186. This polish is so per-
fect, even on rough surfaces, as to suggest a partial solution ot
the granules, and a redeposition of the dissolved matter in tin-
form of a glaze, but the microscope proves to the contrary.
The gloss is due wholly to superficial smoothing and has no
thickness whatever, nor has any new matter been deposited
either on the surface or between the granules.
(7) Changes in Color incidental to Weathering. — That in
nearly every rock a change in color, the assumption of a
brownish or reddish hue, is an early indication of decomposition
has been made sufficiently apparent in the chapter devoted to a
discussion of the chemical changes involved. This discolor-
ation is, however, merely incidental, and not essential, and is
found to diminish, if not wholly disappear, as the distance from
the surface increases, as was noted in the case of the granites of
the District of Columbia (p. 207) and the diorites of the Sierra
Nevadas (p. 274. See further under Color of Soils, p. 385).
Granitic and other highly feldspathic rocks carrying pro-
portionately small amounts of iron become almost invariably
bleached or whitened on the immediate surface, owing in part to
kaolini/.ation and in part to the splitting up of the feld.sj.ars
along cleavage lines.
1 n extreme cases rocks consisting of an admixture of feldspars
and iron-bearing silicates, but in which the first-named, owing
to its glassy nature, is in the fresh rock quite inconspicuous,
become almost snow-white in the earlier stages of weathering.
This, as in the case above mentioned, is due to the change in
the feldspars and the consequent obscuring of the darker sili-
cates by the white product of kaolinization. Continued decom-
position must, however, attack the ferruginous constituent and
the usual staining ensue, unless, as in some cases possible, suffi-
cient carbonic acid may exist to convert the iron immediately
into carbonate and permit of its removal in solution.
258 THE PHYSICAL MANIFESTATIONS OF WEATHERING
Allusion has been already made to the fact that oxidation
or other chemical action, with the possible exception of hydra-
tion, practically ceases below the permanent water level. Hunt
and Le Conte have both called attention to the fact that the
hornblendic and feldspathic rock fragments occurring in the
Pliocene auriferous gravels of California are firm and intact in
those portions below the drainage level (the blue gravel layer),
but more or less completely oxidized, kaolinized, and otherwise
altered in the red or upper gravel.
Van den Broeck has called attention 1 to the possibility that
the so-called red and gray diluvium of the Quaternary deposits
near Paris may be but portions of one and the same geological
body, the " diluvium rouge " being but an upper member of
the " diluvium gres" oxidized and impoverished in lime by the
action of meteoric waters.
The same feature is noticeable in many of our quarries for
building stone, as those in the Berea sandstones of Ohio.
These below the drainage level, are of a gray or blue-gray
color, while above, where they have been subjected to the
oxidizing influence of meteoric waters, they are buff. The
Jurassic oolites of England, are blue-gray at some depths below
the surface, but white above.
In cases where natural joint blocks are exposed to the perco-
lation of meteoric waters, the weathering may for a time mani-
fest itself only in differential oxidation and zonal segregation
of the iron whereby are produced concentric bands of varying
hues. Figure 3, PI. 20, is a slab from a natural joint block of
argillite in the collections of the National Museum, in which
the bands, due to this cause, vary from yellow-brown, drab, to
ochreous yellow and red, while the rock as a whole still retains
its compact structure and susceptibility to polish, forming an
ornamental stone of no mean order.2
(8) Relative Amount of Material removed in Solution. -
Among siliceous rocks, chemical action proceeds but slowly,
and the amount of material actually removed in solution is
rarely over 50 %, and may be so small that, as the writer has
shown,3 the residue in extreme cases occupies some 80 % more
space than the rock from whence it was derived. Carbonate
1 Bull. Soc. Geologique de France, 6, 1876-77, p. 298.
2 Stones for Building and Decoration, p. 169.
8 Bull. Geol. Soc. of America, Vol. VI, 1895, pp. 321-332.
PLATE 20
Fi<;. 1. Weathered boulder of Oriskany sandstone.
Fio. 2. Concentric weathering in diabase.
FIG. 3. Zonal structure in weathered argillite.
FKI. 4. Weathered sandstone, showing induration along joint planes.
INCIDENTAL SURFACE CONTOURS 259
of lime, the essential constituent of ordinary limestone, is,
however, as has been observed, soluble in the carbonated water
of rainfalls, and, in time, may undergo complete removal,
leaving but the insoluble impurities behind. This is, indeed.
tin- almost universal history of limestone soils. They are not
infrequently so siliceous or ferruginous as to be quite barren
and of a nature to be benefited by the applieation of lime as .1
manure.
Throughout the areas occupied by the Trenton limestones, in
Maryland, nearly every farm has, in years past, had its quarry
and lime-kiln where the stone was iitted for supplying lime
once more to soils from whieh it had been so thoroughly leached
as to render them lean and poor. It is almost wholly to this
solvent action that is due the formation of the multitudinous
caverns, large ami small, of the limestone regions. Even where
caverns are not apparent, the corrosive action is evident to the
practised eye. In the quarry regions of Tennessee surface
blocks of limestone are often grooved to a depth of an inch or
more with wonderful sharpness, simply from the water of rain-
falls with its acids absorbed from the atmosphere and surface
soils, while in the quarry bed the stone is found no longer in
continuous layers, but in disconnected boulder-like masses.
( Fi.^s. 3 and 2, Pis. 1»» and 21.) In such cases casual
examinations give very little clew to the rapidity of the de-
struction going steadily on, since all is removed in solution
excepting the comparatively small amount of insoluble matter
(usually clay or silica) existing as an impurity.
(9) Incidental Surface Contours. — In limestone regions the
solvent action of water has frequently gone on so extensively
as to leave its imprint upon the topographic features of the
landscape. The drainage is no longer wholly superficial, but
by subterranean streams sinking entirely into the ground to
reappear again at lower levels, it may be miles away, having
traversed the intervening distance in some of the numerous,
passages (fissures enlarged by solution) with which the rocks
abound. Entire landscapes are undulating through the abun-
dance of sink-holes — shallow depressions down through which
the water has. percolated and escaped into the underground
passages.
The writer recalls a beautiful illustration of this nature seen
in the limestone regions of southern Indiana, some years ago.
260 THE PHYSICAL MANIFESTATIONS OF .WEATHERING
The season was that of the wheat harvest. On every side, far as
the eye could reach, were undulating fields of waving grain, of
that charming golden hue of which poets sing, with intervening
patches of woodland. From every farm was heard the click of
the reaper, and from every fence the whistle of the " Bob
White." Owing to the fact that the ridges between these de-
pressions were drier than the bottoms, the wheat here ripened
earlier, and field after field showed long reaches of saucer-
shaped depressions green in the centre, with intervening ridges
of golden brown, making, with that charming hazy atmosphere,
a picture long to be remembered. Through accident or design,
the opening in the bottom of these sink-holes sometimes becomes
closed, giving rise thus to temporary pools, or ponds, as shown
in the accompanying plate. It is this same action that has
given rise to the so-called " sandpipes " of the English geolo-
gists. These are slender funnel- or tube-shaped cavities found
in chalk and calcareous sandstone, sometimes filled with drift
gravels, sands, brick-earths, or again with fragments! materials
fallen into them from the overlying beds as the support beneath
was gradually removed. In all these cases it is assumed that
direction was given the percolating water by pre-existing fissures
or lines of weakness.1 (Fig. 1, PL 21.)
In regions underlaid by massive siliceous crystalline rocks,
and where mechanical erosion is reduced to a minimum, land-
scapes are softly undulating, with few abrupt escarpments or
precipitous ledges, owing to the uniform rotting away of the
materials, and the gradual accumulation of the debris. It is to
this form of weathering that is due the beautiful rolling hills
of southwestern Maryland. The prevailing rock is granite or
gneiss. Decomposition follows out each line of weakness.
Streams erode through the softened material down to hard
bed-rock, while the relatively large proportion of insoluble
debris is left to accumulate on the gentle slopes which form
such an enchanting feature of these landscapes.
In regions of gneissic or granitoid rocks traversed by large
veins of quartz, as in the northwestern part of the District of
Columbia, the superior resisting power of the quartz causes it
to stand out in relief from the gradually dwindling rock masses
on either hand, giving rise thus to prominent knolls, or ridges,
. 1 See Prestwich's paper, Quarterly Journal Geological Society of London,
1855, p. 62.
INCIDENTAL SURFACE CONTOURS 261
the occasion for which is a mystery until we come to examine
their foundation materials. Belt, in describing the auriferous
quart/, lodes at San Domingo,1 states that the prevailing trend
of the main ranges is nearly east and west, and is probably due
to the direction of the outcrops of the lodes whieh have resisted
the action of the elements better than the soft dolerites.
So striking a feature of the landscape as the Devil's Tower
or Bear Lodge on Little Sun Dance River, Wyoming, is due to
the weathering away and erosion of sedimentary beds from
around a dense crystalline core or plug of eruptive rock in-
truded into them in some past period of volcanie aetivity.
Through its greater powers of resistance, this still stands.
towering over 1000 feet above the level of the river, though in
time this, too, must go. Quite similar forms have resulted,
within a comparatively brief geological period through the
erosion of tufaceous cones from around the compact, crystalline
plug of lava which solidified within the crater when volcanic
aetivity ceased. Beautiful examples of these are to be seen in
Arizona and New Mexico, where they are known as " volcanic
necks." The formation of bosses through the influence of
joint planes has been described elsewhere (p. 244).
In regions abounding in intrusive olivine or pyroxene rocks
which have undergone alteration into serpentine and talc or
"soapstone," one frequently finds these materials forming the
main mass of the hills, while the valleys are carved out of the
softer, more readily decomposed granite, or whatever the count ry
rocks may be. The same feature is prominently developed in
the slate regions of Harford County, Maryland, where the slate
is the more enduring rock, and forms steep ridges, flanked l>\
valleys, carved out from less resisting materials. Regions of
trappean dikes in siliceous schists or gneisses, particularly
along sea-shores where swept by incoming tides, are often
characterized by narrow, straight-walled chasms, or canons due
to the weathering out of the basic rocks, while the more refrac-
tory schists on either hand remain.
In cases where trappean dikes have cut through friable sand-
stones, they have in some instances so indurated these rocks
along either contact as to cause them to be more durable than
the original rock or than even the trappean rock itself. There
may thus arise long parallel ridges of indurated sandstone sepa-
1 The Naturalist in Nicaragua.
262 THE PHYSICAL MANIFESTATIONS OF WEATHERING
rated by an intervening depression due to the weathering out
of the dike material.
In regions where climatic conditions or the nature of the
rock are more favorable to mechanical disintegration than
chemical decomposition, contours may be ragged in the ex-
treme. Entire crests may be but successions of jagged peaks
and intervening narrow valleys which are gradually becoming
choked up by the debris fallen from the cliffs above.
(10) Effacement of Original Characteristics through Weather-
ing.— In cases of extreme decomposition, in place, the residual
products may so slightly resemble the parent rock as to give
rise to very conflicting opinions concerning their origin. This
was for a long time the case with the laterite of India, already
described, and the terra rossa of Europe.
Dana describes * an interesting case of basaltic decomposition
which, on account of the peculiar nature of the residual product,
is worthy of mention here. He writes: " The process of decom-
position is finely exhibited on the second cliff north of Kiama
(Australia) towards the north end. At first sight, a distinct
argillaceous deposit was supposed to overlie the columnar basalt;
for it was twenty feet thick, and of a whitish color, resembling
a soft crumbling marl, thus wholly unlike the basalt, and the
common results of basaltic decomposition. Still it had pro-
ceeded from the alteration of a regular columnar variety, having
a dull grayish blue color. The original rock is exceedingly
compact, showing no trace of crystallization, excepting an
occasional minute crystal of feldspar ; and within the reach
of the swell, it was still compact and solid.
" The rock has a concentric structure, and to this it owes in
part its rapid decomposition. The alteration commences be-
tween the concentric layers, rendering them apparent, although
not so before. At first a thin ochreous line appears, arising
from iron ; either magnetic iron disseminated in the rock, or
from that of the constituent mineral augite. This ochreous
color afterwards mostly disappears, and the concentric coats
become separated by thin clayey layers of a white color, more
or less striped with ochreous lines. In a more advanced stage
of the process large ovoidal masses of basalt (but little changed
in appearance excepting the development of a slaty concentric
structure) lie in the cliff separated by a considerable thickness
1 Reports Wilkes's Exploring Expedition, Geology.
EFFACEMENT OF ORIGINAL CHARACTERISTICS 263
of the whitish clayey layers, which are stained by irregular
ochreous lines. At last the centres of the spheroidal masses
yield, and finally the change is so complete that the concentric
arrangement is entirely lost, and a soft whitish or yellowish-
white argillaceous deposit, with few ochreous spots or lines,
takes the place of the compact basalt.
"In basalts of more compact structure these changes take
place more slowly. The grayish blue basalt in the Illawarra
range, near Broughton's Head, when long exposed, is discoluivil
exteriorly to a depth of an inch and a half. The colors, begin-
ning within, are dirt-brown, grayish yellow, oohre-yellow,
brownish red; and they are evidently dependent mostly mi
changes in the condition of the iron which the rock or its
minerals contain.
•• When the rock includes much chrysolite, the results of
decomposition in some instances give a fissile or micaceous
appearance to the rock. At Prospect Hill, five miles west of
Paramatta, this change is in progress. The rock is a black
ferruginous basalt of homogeneous aspect, breaking with a
smooth fracture and no appearance of crystallization. It con-
tains chrysolite ; but the grains are small and not apparent
except on very close examination. . . .
" Were we unable to trace the transitions, and distinguish
the columnar structure through the whole, we should scarcely
suspect its basaltic origin. Indeed, it was pointed out to me
as an instance of mica slate overlying basalt. Particles of
rusted mica, as they seemed, were distinct, and it much re-
sembled a decomposing variety of that rock. On close inspec-
tion and an examination of the rock in different stages of
change, it became evident that the pseudo-mica was nothing
but altered chrysolite, which had rusted from partial decompo-
sition, and split into thin cleavage scales.
" The crystals of chrysolite have evidently a parallel position
in the rock, and hence the plane of easiest cleavage lies in the
same direction, or, as the cleavage shows, parallel with the
upper surface, that is, at right angles with the vertical axis of
the columns. The passage from the compact to the decomposed
rock is, in this case, unusually abrupt. Alteration takes place
(through the elimination of oxide of iron as before suggested)
slowly at the surface, which therefore chips off as soon as de-
composed and exposes a new portion. This sudden transition
264 THE PHYSICAL MANIFESTATIONS OF WEATHERING
may, in part, proceed from the absence of any natural planes of
fracture (which are brought out when there is a concentric
structure), and perhaps in part also from the presence of
chrysolite. The layer of pseudo-mica schist is in some places
five feet thick and has a rusty brownish color. Above it passes
into three feet of earth of the same origin, having a brownish
black color, and this is covered again by four feet of brownish
red soil.'1
Such an effacement is not, however, an invariable accom-
paniment of decomposition, since where the amount of residuary
material is relatively large, and allowed to accumulate in place,
the mass may for a long period retain its original structural char-
acteristics. Indeed, the original features are sometimes so per-
fectly preserved that casual inspection alone quite fails to reveal
the havoc that has gone on. Every detail of bedding, jointing*
or foliation, or even of internal structure, as brought about by
the arrangement or size of the individual particles, may be re-
tained with perhaps only a slight change of color due to oxida-
tion. This feature is often strikingly conspicuous in the newer
railway cuts of the southern Appalachian regions, particularly
where the country rock is of the nature of gneisses or schists.
In the work of grading the streets, in the extensions of the city
of Washington, masses of strongly foliated granites, so soft as.
to be readily removed with pick and shovel, would be cut
through, and which yet showed every vein or other structural
detail as plainly marked as in the original rock, and it was
only when by thrusting one's cane or other implement into it
that its thoroughly decomposed condition became apparent.
Russell describes1 a similar condition of affairs prevailing in
the coarse Triassic conglomerate near Wadesborough, North
Carolina. This conglomerate is here composed of rounded and
angular pebbles of talcose schist and other crystalline rocks.
In the fresh cuts along the line of the North Carolina railroad,
every detail of the original rock is brought out almost as sharply
as in the so-called " Potomac marble " phase of the same forma-
tions as used in the Capitol building at Washington. "On
examining more closely, however, one is surprised to find that
it is completely decomposed, and that when moist it can be cut
with a pocket knife through pebbles and matrix alike, as easily
as so much potter's clay. The full depth of the alteration in this
1 Bull. 52, U. S. Geol. Survey, 1889.
INCIDENTAL SIMPLIFICATION OF COMPOUNDS 265
instance is not revealed, but it extends more than 30 feet below
the surface without change in character."
\V. B. Potter described1 the feldspar porphyry of Iron Moun-
tain, Missouri, as decomposed to the extent that it can be easily
whittled away with a penknife or scratched with the thumb nail.
" At the same time," he writes, *' the original porphyritic
structure of the individual crystals scattered through tin'' mass
is beautifully preserved, and is even frequently more distinctly
visible than in the original rock, owing to stronger contrasts of
color in the kaolinized material." In many dense massive rocks,
indeed, such features as flow structure and inequalities of u-xt-
ure are frequently rendered evident only on weathered surfaces.
The same is often true of fossiliferous limestones, a weathered
surface revealing the presence of organic forms wholly imper-
ceptible on one freshly broken.
The crude kaolin as removed from the pits near linmdywinr
Summit, Pennsylvania, and at Hockessin, Delaware, still retains
more or less distinctly the structure of the original gneiss or con-
glomerate from whence it was derived. The quartz granules
of the gneiss are, in these cases, almost invariably shattered,
as though crushed by dynamic agencies, and show distinctly
corroded surfaces, presumably caused by the alkaline carbo-
n;ites formed during the kaolinizing of the feldspars. The
black mica makes its former presence known by rust-colored
spots which, in those cases where the mineral was sufficiently
abundant, have ruined the material for the purposes of the
potter.
(11) Simplification of Chemical Compounds, incidental to
Weathering. — It has been noted on p. 172 that the process of
weathering is but an attempt on the part of the elements in
their various combinations to adjust themselves to existing con-
ditions. This adjustment consists in the formation of new com-
pounds which are characterized by a less complex structure than
those first formed.
Indeed, one of the most striking features of chemical geology
is the tendency toward simplification in composition as mani-
fested all over the superficial portions of the earth. During
the process of decomposition there is almost invariably a con-
stant breaking down of complex molecules of mixed silicates Of
alumina, iron, lime, magnesia, and the alkalies, and a recombi-
1 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 26.
266 THE PHYSICAL MANIFESTATIONS OF WEATHERING
nation of their various elements as simpler silicates, carbonates,
sulphates, and oxides.
(12) Other Results incidental to Decomposition and Erosion. -
That all the minerals of a rock mass are not equally acted upon
by atmospheric agencies has been sufficiently noted in previous
pages. The more refractory, freed by the breaking down of
their host, remain to gradually accumulate in vastly greater
proportions than they existed in the original rock. If, in
addition to their refractory qualities, such possess, as is usually
the case, greater density, decomposition and erosion may act but
as agents of concentration, and in such residues minerals like
xenotime and monazite have been found in abundance, although
occurring so sparingly in the fresh rock that their existence was
scarcely suspected.
It is in this manner that has originated the gem sand of
Ceylon. Precious stones have been found disseminated in lim-
ited numbers in the granite converted into the cabook described
on p. 242. In weathering, the difficultly decomposable precious
stones have not been attacked, or attacked only to a limited ex-
tent. They have therefore retained their original form and hard-
ness. When in the course of thousands of years streams of water
have flowed over the layers of cabook, their soft, already half-
weathered constituents have been for the most part changed into
a fine mud, and as such washed away, while the hard gems have
only been inconsiderably rounded and little diminished in size.
The current of water therefore has not been able to wash them
far away from the place where they were originally embedded
in the rock$ and we now find them collected in the gravel bed,
resting for the most part on the fundamental rock which the
stream has left behind, and which afterwards, when the water
has changed its course, has been again covered by new layers of
mud, clay, and sand. It is this gravel bed which the natives
call nellan, and from which they chiefly get their treasures of
precious stones.1 The same process in states bordering along
the Appalachian Mountain system in North America has given
rise to auriferous sands, as well as to sands bearing monazite,
zircons, and other valuable minerals, which become segregated
merely through their greater density and power to resist decom-
position. The stream tin ores of the Malayan Peninsula, the
1 Nordenskiold, Voyage of the Vega. See also Judd, On the Rubies of Burma,
etc., Philos. Trans. Royal Soc. of London, Vol. CLXXXVII, 1896, p. 151.
PLATE 21
FIG. 1. Sink-hole near Knoxville, Tennessee.
FIG. 2. Beds of marble corroded by meteoric waters, Pickens County, Georgia.
RESULTS INCIDENTAL TO DECOMPOSITION
267
diamond-bearing gravels of Brazil, and indeed placer deposits in
general are illustrative of this same principle. The very soil
itself, although so indispensable, to human existenee. is but an
incidental and transitory phase of rock-weathering, as has been
made siiiti. iciitly apparent in previous pages. The deposits of
kaolin in western Pennsylvania and northern Delaware, as else-
where noted, are but decomposed highly feldspathie gneisses
and conglomerates, while the phosphate deposits of middle 'Fen-
are insoluble residue left by the leaching out of tin- 0*1-
cium carlionate from phosphatie limestones.1
According to Russell,2 the Clinton iron ore of Alabama is
but the insoluble residue from ferruginous Silurian limestones.
On the immediate surface the ore is quite pure, containing, it
may be, but a trace of lime. When followed downward, tin-
amount of lime is found to gradually increase, until the ores
may become so poor in iron as to be valueless. The following
figures show this gradual increase in lime carbonate, and neces-
sary decrease in iron, from the surface downward.3
PERCENTAGE OF CALCIUM CARBONATE is Ci IM--N II;.-N OUE
DEPTH
PM CE.XT
DKPTU
I'l , < 1 N
Surface
Trace
70 feet below surface .
26.01
10 feet below surface . . .
Trace
80 feet below surface . .
Mow surface . . .
True
90 feet below surface . .
30 feet below surface . . .
Trace
100 feet below surface . .
40 feet below surface . . .
21.00
110 feet below surface . .
60 feet below surface . . .
23.00
120 feet below surface . .
11,89
60 feet below surface . . .
27.01
130 feet below surface . .
80.66
William Whitaker in 1864 4 noted the decomposition of the
English chalk beds, in Middlesex, and the gradual accumulation
of a stiff brown-red residual clay interspersed with many flint
nodules. It is by this same leaching action on aluminous lime-
stones that is formed the so-called " rottenstone" so commonly
used in polishing brasses and other metals.
1 J. M. Safford, American Geologist, October, 1890, p. 261.
2 Op. cit., p. 22.
8 Trans. Am. Ins. of Mining Engineers, Vol. XV, 1886, p. 189.
4 Mem. Geological Society of Great Britain, 1804, p. 64.
THE WEATHERING OP ROCKS (Continued-)
IV. TIME CONSIDERATIONS
Concerning the rate of decomposition of rocks of various
kinds, only very general rules can be laid down, since much
depends upon climatic conditions and the position of rock
masses relative to the action of frost, moisture, and the various
growing organisms.
(1) Rate of Weathering influenced by Texture. — From the
study of building materials it has become apparent that a
coarsely crystalline rock will, all other conditions being the
same, disintegrate more rapidly than one of finer grain. This
is doubtless owing in part to expansion and contraction from
ordinary temperature variations, which act the more energeti-
cally the larger the crystalline particles.1
It has already been remarked (ante, p. 44) that crystalline
rocks have a greater density than do glassy forms of the same
chemical composition. This indicates a contraction during the
processes of crystallization, which manifests itself, according to
at least one authority, in the development of minute interspaces
between the individual crystals. The coarser the crystalliza-
tion, then, the greater the amount of interstitial space, and
hence the greater the absorptive power.
These coarser rocks, owing to their tendency to undergo a
mechanical disintegration, or disaggregation, may also yield to
1 The coefficient of cubical expansion for several of the more common rock-
forming minerals has been determined as follows : —
Quartz 0.0000360 Tourmaline ...... 0.000022
Orthoclase 0.0000170 Garnet 0.000025
Hornblende 0.0000284 Calcite 0.000020
Beryl . 0.0000010 Dolomite 0.000035
The strain brought to bear upon a mass of rock through the unequal rate of
expansion of its various constituents is further complicated through the unequal
expansion of the individual minerals along the direction of their various axes.
Thus quartz gives a coefficient of 0.00000769 parallel to the major axis, and of
0.000001385 at right angles thereto. Adularia gives 0.0000156, 0.000000659, and
0.00000294 for its three axes, and hornblende 0.0000081, 0.00000084, and 0.0000095
(Stones for Building and Decoration, p. 419).
268
RATE OF WEATHERING
269
the decomposing agencies more readily than those of finer
grain, though from the fact that they first fall away to coarse
sand, whereby the rock-
like character is lost, one
might, on casual inspec-
tion, be led to the oppo-
site conclusion. It need
scarcely be said that.
among rocks having the
same composition, wheth-
er fragmental or crystal-
line, those of a granular
structure will undergo
disintegration more
quickly than will those
in which the individual
minerals are closely com-
pacted or interknit, as in
many quartzites or dia-
bases.
(•2) Rate of Weather-
ing influenced by Compo-
sition.— Among rocks of
the same structure as re-
gards crystallization and
size of particles, the basic
varieties, such as the dia-
bases and gabbros, as a
rule succumb more iva<l-
ily than do the more acid
varieties like the gran-
ites. This for the reason
that the iron-magnesian
as well as the soda-lime
minerals are more sus-
ceptible than are the pot-
ash silicates and other
essential constituents of
the rocks of the granitic group. It is possible also that these
dark colors cause them to become more highly heated, where
exposed to direct sunlight, and hence subject to mechanical dis-
FIG. 21.
Microstructure of sandstone (Fig. 20), showing
relatively large amount of interstitial space
and absorptive power, and (Fig. 21) of dia-
base, with relatively little.
270 TIME CONSIDERATIONS
integration. The fact that many of our trappean rocks, as seen
in dikes cutting other rocks, do not in all cases succumb with
greater comparative rapidity is due to their very compact struct-
ure, whereby percolating waters are so largely excluded.
(3) Rate of Weathering influenced by Humidity. — The ra-
pidity of rock weathering and soil formation is, even among
rocks of the same nature, widely variable, being dependent
upon climatic conditions of any particular locality. In the arid
regions north of Flagstaff, Arizona, are wide areas of country
covered with coal-black lapilli ejected from volcanoes whose
craters are now occupied by growing pines upwards of two
feet in diameter. Yet these fields are, with the exception of the
pines, as bare of vegetation as though but yesterday scorched
by fire. The fine lapilli, resembling nothing more than crushed
coke, cover everywhere the undulating plains, greedily absorb-
ing the moisture from melting snows and scanty rainfalls, but
undergoing no appreciable decomposition and affording foot-
hold for only a few desert shrubs and grasses. Yet in a
moister clime, and one more adapted for luxuriant vegetation,
we might expect that these lapilli should long ago have suc-
cumbed and given fairly fertile soils.
(4) Rate of Weathering influenced by Position. — Among the
siliceous crystalline rocks superficial disintegration is undoubt-
edly greatly aided by temperature variations, which, by render-
ing the rocks porous, facilitate chemical decomposition. Such
action must, however, be merely superficial, and at considerable
depths below the surface the change must be purely chemical.
The chief conditions favoring chemical action are those of con-
tinual percolation by waters carrying the organic acids already
described. It naturally follows, therefore, that a purely chemi-
cal decay will progress more rapidly where the rock mass is
covered by such a layer of vegetable soil as shall give rise to
the decomposing solutions. Hence, that such an accumulation
having begun, decomposition will keep on at an ever-increasing
rate to a depth concerning which we have at present no data
for calculation. It must not be too hastily assumed from this
that rocks thus protected do in reality break down more rapidly
than those exposed on bare hillsides, since here, where physical
causes predominate, the loosened particles are removed as fast
as formed, and, besides leaving no measure of the destruction
going steadily on, new surfaces for attack are being continually
RELATIVE RAPIDITY OF WEATHERING 271
exposed. Moreover, in assuming that rocks decay rapidly where
covered by vegetation, we must not overlook the fact that the
character of the overlying soil may be such as to be protect i\r
rather than otherwise. Thus in glaciated regions it is a well-
known fact that the strue on rock surfaces are found best pre-
served where they have been protected from heat and frost
by a mantle of drift, or the compact turf so characteristic of
the Northern states. (See further under Influence of Forests.
p. 280.)
(5) Relative Rapidity of Weathering among Eruptive and
Sedimentary Rocks. — As to the relative rapidity of chemical
decomposition among eruptive and sedimentary rocks, there
can — with the exception of the calcareous varieties — be no
question, the eruptives being far the more susceptible. This
for reasons which will be at once apparent when we consider
their origin. The eruptive rocks result from the comparatively
sudden cooling of magmas originating far below the action of
atmospheric agencies, and which are pushed up and allowed to
solidify under conditions which are not at all conducive to chemi-
cal equilibrium. They are compounds of elements which have
combined according to the conditions under which they tempo-
rarily existed, but which undergo continual changes as they
become exposed by erosion and other causes. They become, in
short, out of harmony with their surroundings, and there are at
once set up a series of physical and chemical changes such as
shall result in products more in harmony with existing condi-
tions, and hence more stable. These changes, briefly put, are
those involved in the weathering processes we have described.
Indeed, we may well say that rock weathering and all the seem-
ingly endless processes of rock decay and rock consolidation
are but stages in the continual efforts being made by these inor-
ganic particles to adjust themselves to existing conditions. But
the sedimentary rocks (exclusive of the calcareous varieties) are
themselves the actual products of these adjustments. The con-
glomerates, sandstones, shales, and argillites are but the detri-
tal remains of eruptive rocks which under the various weathering
influences have become disintegrated and decomposed, their more
soluble constituents quite or in part removed, and the residues
laid down and consolidated under conditions such as to-day
exist upon or near the surface of the earth. They have, it is
true, been laid down under water ; they are subaqueous, but
272 TIME CONSIDERATIONS
their decomposition and disintegration was subaerial. Hence,
when elevated above the ocean's level to become a part of the
dry land, they are for the most part comparatively stable, sub-
ject to only such chemical changes as oxidation, and it may be
dehydration. All other things being equal, then, those siliceous
rocks which are the product of mechanical sedimentation will be
found far less susceptible to the chemical action of the atmos-
phere and meteoric waters than are the eruptives. While they
may undergo a transformation into soils, it is mainly through
the disintegrating effects of heat and frost. Sedentary soils
resulting from such disintegration resemble, therefore, their
parent rock more than those of any other class.
Turning now to calcareous rocks, we shall find a quite differ-
ent state of affairs prevailing, owing to the different chemical
nature of the material and its ready solubility. These rocks
represent, in fact, the soluble portions of the eruptive rocks
which have been leached out during the process of decomposi-
tion. They are themselves solution products, although their
immediate deposition may have been brought about through
mechanical agencies, as in the laying down of beds of shell
marl upon a sea-bottom. The lime leached out of terrestrial
rocks is carried in solution into the sea, where, taken up by
molluscs and corals as a carbonate, it becomes precipitated to
the bottom on their death, and may reappear as a limestone, or,
if mixed with sufficient quantities of other constituents, as a
marl, calcareous sandstone, or shale. Such on their re-elevation
are still subject to chemical change, owing to the ready solu-
bility of lime carbonate in terrestrial waters, and so the endless
round begins once more. Reference has already been made to
the amounts of lime carbonate that may thus be annually re-
moved from the earth's surface, but one may add here, that,
according to J. G. Goodchild, certain English limestones waste
away, superficially, at the rate of one inch in 300 years.1
(6) Time Limit of Decay. — We are sometimes enabled to
put a time limit on the beginnings of decomposition such as
shall enable us to gain at least a geological measure of the
rapidity of the process. This is the case with the disintegrated
granite of the District of Columbia described on p. 206. The
residual material is here now overlaid by clastic deposits of such
a nature as to force the conclusion that they were laid down by
1 Geological Magazine, 1890, p. 463.
TIME LIMIT OF DECAY ^73
water under such conditions as would have thoroughly eroded
away all underlying pre-existing' decomposed material. It is-
therefore inferred that this decomposition has taken place since
the clastic material was deposited, or, since these are of Creta-
ceous age, that it has taken place since the close of Cretaceous
times. In the same way, since glaciation must have carried
away the pre-existing disintegrated matter from the dike of
diabase at Medford, leaving the surface smooth and hard, so
here it is inferred that the decomposition is post-glacial. It is
but rarely that the rate of decomposition of any rock has been
sufficiently rapid since the beginning of human history, to be
of geological significance, though weathered surfaces in old
quarries, or the walls of old buildings, not infrequently offer
abundant illustration of what we might expect, could observa-
tion be extended over whole geological periods instead of at
most a few years. We must not forget, however, that, in the
latter case, the conditions are quite different from those exist-
ing in nature, and the rate of weathering may be accelerated or
retarded, as the case may be.
Stone implements, made by prehistoric man, as now found
in graves, or dug from the soil, sometimes show incipient signs
of decomposition, as indicated, when broken across, by a change
in color and texture from without inward. Flint arrow and
spear-heads from prehistoric caves or mounds in Europe,
England, or America, often present on the outer surface a thin
crust or patine of a gray or white color extending inward, it
may be, for the distance of two or more millimeters. A grooved
stone axe of diorite found in eastern Massachusetts and now in
the collections of the National Museum at Washington,1 shows
concentric exfoliation in every way comparable to that on the
diabase boulder figured on PL 20, extending inward to a depth
of from three to six millimetres. It is of course possible that
the axe was made from a boulder, itself not quite fresh, but this
seems scarcely probable, and the inference is fair that both the
patine and the exfoliation are due wholly to weathering sub-
sequent to the manufacture of the implements on which they
occur.
Mills2 regards the extreme condition of decomposition exist-
ing in the Archaean rocks of Brazil as having taken place prior
1 Specimen No. 172,794, Archaeological Series.
2 American Geologist, June, 1889, p. 345.
274
TIME CONSIDERATIONS
to the deposition of the loess, that is, in the long interval between
the elevation of the Archaean rocks and the beginning of Qua-
ternary times. Inasmuch, however, as the Quaternary gravels
and loess are all readily permeable by water and not of a nature
to be themselves readily affected, it would seem possible that
at least a portion of the decomposition might have been brought
about since their deposition and, indeed, to be still in progress.
The writer is informed by Mr. W. Lindgren that the granitic
diorites of the Sierra Nevadas of California, and which are of
FIG. 22. — Flint implement showing weathered surface.
late Jurassic or early Cretaceous age, are often decomposed and
disintegrated to a maximum depth of 200 feet, the extreme
upper, more superficial portions being reduced to the condition
of a red clay, while the lower are merely rendered soft and
friable, with little if any change in color. This disintegration
has gone on to such an extent that where the rock is traversed,
as is sometimes the case, by numerous gold-bearing quartz veins,
the entire mass of material is washed down by water — hydrau-
licked — as in the ordinary process of placer mining. The
Pliocene andesites are also in places decomposed to a depth of
TIME LIMIT OF DECAY 275
20 feet. The region is one of heavy annual precipitation, but
the rainfall is limited almost wholly to the winter season.
Rock disintegration and decomposition, after the manner
already described, has been by no means limited to the present
era, but has been going on since the first land appeared above
the surface of the primeval ocean. The results of the recent
decomposition are more apparent, since the derived materials are
still recognizable as rock debris, while that formed in past ages
may have been so changed by the solvent and assorting power
of water, the chemical action of the atmosphere, and the general
agents of metamorphism, as to have quite lost its identity.
Dr. 11. Bell, of the Canadian Geological Survey, has described l
an interesting illustration of pre-Palteozoic decay in the crystal-
line rocks north of Lake Huron. The red granite, where it has
been protected from glacial action, is found to be eaten into
hollows in the form of round and sack-like pits and small
caverns, the last-named generally occurring on steep slopes or
perpendicular faces of the rock. These pits are, in places, of
sufficient size to allow two men to crouch within. The sack-
like ovens, such as are shown in Fig. 23, are most usually on
sloping surfaces. The granite around these pits shows no in-
dications of decay. That they are of pre-Palseozoic origin is
demonstrated by the presence in them of residual patcnes, in
situ, of the fossiliferous Black River limestone and which Pro-
fessor Bell regards as veritable inliers of the Black River forma-
tion, which once filled all the inequalities and still overlies the
granite at lower levels, though elsewhere almost wholly removed
by erosion. Figure 23, after Bell, shows diagrammatically the
old granitic corroded floor up on which the calcareous sediments
were laid down, with pits still containing residual masses of the
limestone, and the intact beds passing under the waters of Lake
Huron at the lower right.
Pumpelly, too, has shown 2 that the diabase dike at Stamford,
Massachusetts, had undergone extensive decomposition prior
to the deposition of the Cambrian conglomerates. Of equal
interest and still greater economic importance was the sugges-
tion by this same authority, subsequently abundantly confirmed
by W. B. Potter,3 that beds of iron ore lying on the western
l. Geol. Soc. of America, Vol. V, 1894, pp. 35-37.
2 Ibid., Vol. II, 1891, p. 209.
8 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 23.
276 . TIME CONSIDERATIONS
flank of Iron Mountain, Missouri, and covered by Silurian lime-
stones, were true detrital deposits resulting from the pre-Silurian
breaking down of the ore-bearing porphyry forming the mass
of the mountain. These and other 1 illustrations that might be
given point unmistakably to the identity of geological processes
and correspondence in results since the earliest times, even did
not analogy and the thousands of feet of secondary rocks furnish
us safe criteria upon which to base our inferences.
FIG. 23.
(7) Extent of Weathering. — The depth to which weather-
ing has penetrated necessarily varies greatly. In cases where
the detrital material is removed nearly or quite as rapidly as
formed, it may go on indefinitely, until, it may be, thousands
of feet of material have melted away ; where, however, remain-
ing in place, decomposition must be gradually retarded until a
time comes when it practically ceases. In the region about
Washington, District of Columbia, the writer has observed the
granitic rock so disintegrated at a depth of 80 feet from the
present surface as to be readily removed by pick and shovel.
Even greater depths have been noted by writers on the geology
of our own Southern states and Central and South America.
aSee also T. Sterry Hunt, The Decay of Rocks Geologically Considered,
Am. Jour, of Science, Vol. XXVI, 1883, p. 190.
EXTENT OF WEATHERING 277
Spencer states1 that in the region about Atlanta, Georgia, the
rocks are "completely rotted" to a depth of 95 feet, while
"incipient decay" may reach to a depth of 300 feet. W. B.
Potter describes2 the feldspar porphyry of Iron Mountain in
Missouri as decomposed to a visible extent as- far into the hill
as mining operations had been carried, while to depths varying
from 10 to 80 feet the kaolinization is complete.
The coarse granite of Pike's Peak, Colorado, is reported as
disintegrated to a depth of from 20 to 30 feet. Belt3 describes
dolerites in Nicaragua as decomposed, as shown by deep cut-
tings in mines, to a depth of 200 feet. " Next the surface," he
says, " they were often as soft as alluvial clay, and might be
cut with a spade."
Derby describes* certain shales in Rio Grande do Sul, Brazil,
as decomposed into the condition of reddish, drab, greenish,
black, and umber-colored clays to the depth of 120 metres
(394 feet).
W. H. Furlonge has described6 the granite of the Dekaap
gold fields, in the Transvaal, South Africa, as decomposed to a
depth of 200 feet. Rain erosion has carved out from this
decomposed mass deep "dongas," as they are locally called,
and which sometimes present most striking and picturesque
appearances.
The apparent depth to which weathering has gone on is
often greater among siliceous than calcareous rocks. This is,
however, due merely to the facts that (1) the siliceous rocks
are composed largely of insoluble materials, and hence leave a
proportionately large amount of debris, and (2) that among
calcareous rocks the change is mainly chemical and takes place
(inly from the immediate surface. As a result of this, residuary
nodules of limestone may be found perfectly fresh and unal-
tered at a depth of but a few millimetres below the surface,
while granites and allied rocks may show signs of disintegra-
tion and incipient decay for many inches, or even feet.
Pumpelly states6 that in the Ozark Mountains of Missouri
the secular dissolving away of limestones containing from 2 to
1 Geol. Survey of Georgia, 1893.
2 Jour. U. S. Assoc. Charcoal Iron Workers, Vol. VI, p. 25.
8 The Naturalist in Nicaragua, p. 86.
4 Am. Jour, of Science, February, 1884, p. 138.
6 Trans. Am. Inst. of Mining Engineers, Vol. XVIII, 1890, p. 337.
8 Am. Jour, of Science, 1879, p. 136.
278
TIME CONSIDERATIONS
9 a/o of insoluble matter has left residual clays from 20 to 120
feet in thickness, indicating a removal of not less than 1200
vertical feet by solution. According to Whitney, the dark,
reddish brown, residual clays of southern Wisconsin, of an
average depth of perhaps 10 feet over the entire area, repre-
sent the insoluble accumulations from the decomposition of
from 350 to 400 vertical feet of dolomite, limestone and cal-
careous shale.
(8) Relative Rapidity of Weathering in Warm and Cold Cli-
mates. — For many years an impression has prevailed to the
effect that rocks decomposed more rapidly in warm and moist
than in cold climates. While, owing to abundance of vegeta-
tion and other supposed favorable conditions, a more rapid
decomposition might be expected, such has not as yet been
proven to actually take place, and indeed many facts tend to
prove the impression quite erroneous. Lack of decomposition
products in high latitudes is not infrequently due to glaciation
or erosion by other means. Whitney,1 Irving,2 Chamberlain,
and Salisbury3 have shown the presence of residual clays of
all thicknesses up to 25 feet in the driftless area of Wiscon-
sin, and Chamberlain has described 4 limited areas of strongly
decomposed gneiss in the non-glacial areas of Greenland.
Moreover, we have no actual proof that the action of frost
is, on the whole, protective, as is stated by Branner.5 It must
be remembered that frost, excepting in the extreme north,
penetrates to but a slight depth, and while it undoubtedly puts
a temporary stop to chemical action on 'the immediate surface,
it remains yet to be shown that the mechanical disruption that
ensues, and as described in previous pages, is not as efficacious
as would have been the chemical agencies alone, had they been
permitted to continue their work. Through bringing about a
finely fissile or pulverulent structure, whereby a vastly greater
amount of surface becomes exposed, frost prepares the way for
chemical action at a thousand-fold more rapid rate than could
otherwise have been possible. If, further, as the writer has
elsewhere at least suggested,6 hydration is the most potent
1 Rep. Geol. Survey of Wisconsin, 1861.
2 Trans. Wisconsin Acad. of Science, Vol. Ill, 1875.
a Ann. Rep. U. S. Geol. Survey, 1884-85, p. 254.
4 Bull. Geol. Soc. of America, Vol. VI, 1895, p. 218.
6 Bull. Geol. Soc. of America, Vol. VII, 1896, p. 282.
6 Bull. Geol. Soc. of America, Vol. VI, p. 331.
RELATIVE RAPIDITY OF WEATHERING 279
factor in rock disintegration, the process can go on uninter-
ruptedly below the level of freezing.
Professor H. P. Gushing has described1 the argillites in the
vicinity of Glacial Bay, Alaska, as in a condition of great dis-
integration, wholly through the action of frost. " Disintegra-
tion," he says, "takes place with amazing rapidity, as shown
by the enormous piles of morainic matter furnished to the tribu-
taries of Muir Glacier, whose valleys are adjoined by mountains
of argillite, and by the massive talus heaps that are rapidly
accumulating at the bases of other mountains made up of the
same material." In a private communication to the present
writer, he further states that the diabases of the region are
fully as much decomposed as are those in the Adirondacks of
New York, and that the blocks of eruptive rocks occurring in
the moraines of Muir Glacier are far gone in decomposition.
Mr. C. W. Purrington has made similar observations, and
states 2 that on the south side of Silver Bow Basin, some three
miles west of Juneau, at an elevation of 2000 feet above sea-
level, he found schistose diorites disintegrated over a consider-
able area to a depth of 20 feet. The particular locality cited
was on a mountain slope, where landslides were frequent, and
other conditions prevailed such as would prevent the accumula-
tion of the debris throughout a prolonged geological period or
to a very great depth. There could be, however, no doubt as
to the residuary character of the material observed, and the
inference drawn was to the effect that the disintegration had
taken place within a comparatively brief space of time. G. E.
Culver has also described 3 a diabase dike in Minnehaha County,
South Dakota, an arid region lying within the glacial area, as
decomposed throughout the whole exposures from its upper
surface down to a depth of 20 or 25 feet, the limit of disinte-
gration being the drainage level of the region as marked by
the bed of a stream cutting through it.
On the other hand, Professor I. C. Russell, who has devoted
much attention to the subject of rock- weathering in both high
and low latitudes, is of the opinion that rock decay is a direct
result of existing climatic conditions. He states further that
decay goes on most rapidly in warm regions where there is an
1 Trans. N. Y. Academy of Science, Vol. XV, 1895.
a Personal Memoranda to the writer.
8 Wisconsin Academy of Sciences, Art, and Literature, 1886-91, p. 206.
280 TIME CONSIDERATIONS
abundant rainfall, and is scarcely at all manifest in arid and
frigid regions.1 Professor Russell's observations are of more
than ordinary value, since he has discriminated between decay
and disintegration, which most writers have failed to do.
Relative to the subject of rock degeneration in temperate
regions, we have further to consider the possible increased
amounts of atmospheric gases brought down by snowfalls, over
those brought by rain. The snowflakes, in 'falling, so com-
pletely fill the air as to rob it of a larger proportion of its
impurities than would a corresponding amount of precipitation
in the form of rain. Further, the snow in melting slowly away
affords the water better facilities for soaking into the ground
than though it was poured down during- the comparatively brief
period of a shower. How far these agencies may go toward
counterbalancing the effects of the continued higher tempera-
tures of the tropics, we have no means of judging.2
It is even questionable if decomposition has actually gone on
to greater depths in regions covered by forests, as contended
by Hartt3 and Belt4 than elsewhere. The accumulation of a
large amount of organic matter is undoubtedly favorable to
decomposition, but the growing vegetation constantly robs the
soil beneath of moisture and other elements necessary for its
growth, storing it away in the form of woody fibre or sending
it off into the atmosphere once more. The amount of moisture
that a full-grown tree evaporates daily through its leaves is
simply enormous, and is often made conspicuously apparent
by the dry knolls that may be seen surrounding isolated trees
or groups of trees in swampy areas. Indeed, Mr. R. L. Fulton,
in discussing ? the influence of forests in the mountain regions
of the West, states it as his belief that the local springs and
streams are " more diminished by the water used by the trees
than by evaporation in their absence."
It has been shown 6 that the total amount of moisture returned
1 Surface Geology of Alaska, Bull. Geol. Soc. of America, Vol. I, 1890.
2 There is an old saying among Eastern farmers to the effect that a late
spring snowstorm is as good as a dressing of manure. It undoubtedly arose
from an appreciation by the farmers of the fact that the snow was more benefi-
cial than rain for the reasons above mentioned.
8 Physical Geography and Geology of Brazil,
* The Naturalist in Nicaragua, p. 86.
5 Science, April 10, 189G.
e See Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893.
INFLUENCE OF FORESTS
281
into the atmosphere from a forest by transpiration and evapora-
tion from the trees and underlying soil, is about 75 % of the
total precipitation. For other forms of vegetation it varies
between 70 % and 90 %, the forest as a rule being surpassed by
the cereals, while the evaporation from a bare soil is but 30 %
of the precipitation. To this should be added the fact that
the activity of evaporation from forested areas is continued
throughout a longer period of each year, as a rule, than in
non-forested, for the simple reason that the grasses and cereals
early ripen, and practically cease to exhale altogether. This
is particularly the case in cultivated areas and prairie regions.
Hence, while the daily evaporation from given areas might for
a time be nearly equal, the animal amount is likely to be great-
est for that which is forested.
Further, it has been shown that only 70 % as much rainfall
reaches the soil in the woods as in the open fields, the rest
being caught in the leaves, branches, and trunks, whence it is
returned directly to the atmosphere by evaporation. These
percentages naturally vary with the character of the forest
growth. In this connection the following table, showing the
measured amounts of water at varying depths in a loamy soil
under forests of spruce, twenty-five, sixty, and one hundred
and twenty years old, and one base of all vegetation, is instruc-
tive. It will be observed that the average amount is apprecia-
bly greater in the bare soil, and that the least amount is found
under forests 60 years old, when we may assume the trees are
in their prime.
WATER CONTENTS OF A LOAMY SAND ; RESULTS BY SEASONS EXPRESSED ix
PERCENTAGES OF THE WEIGHT OF THE SOIL
SPRUCE
SEASON
25 TEAKS OLD
60 TEARS OLD
16 inch
82 inch
Average
16 inch
82 inch
Average
%
%
01
to
01
10
01
10
01
10
Winter (January and February) .
20.23
17.00
18.61
18.06
17.76
17.01
Spring (March to May) ....
18.62
18.02
18.32
15.29
16.28
15.78
Summer (June to August) . . .
15.10
16.22
15.96
14.42
17.03
15.72
Fall (September to November) . .
16.57
17.57
17.07
13.49
16.52
15.00
282
TIME CONSIDERATIONS
SPRUCE
NA n COIT
SEASON
120 YEARS OLD
16 inch
32 inch
Average
16 inch
32 inch
Average
01
fa
o/
10
o/
fa
01
fa
01
fa
01
fa
Winter (January and February) .
19.75
22.44
21.09
19.96
24.73
22.35
Spring (March to May) ....
17.47
20.83
19.15
20.66
20.51
20.58
Summer (June to August) . . .
17.78
20.90
19.97
19.77
19.98
19.97
Fall (September to November) . .
14.88
19.46
17.17
20.04
20.20
20.12
Other experiments have shown a marked difference in the
distribution of the water in the forest-covered and naked soils,
in the first-named a much larger proportion being held in the
extreme upper portion than in that which was unprotected.
This is a natural consequence of the absorptive properties of
the accumulated humus. The following table, as compiled by
Fernow l from the work of Ebermayer, illustrates this point.
AVERAGE OF WATER CAPACITY, EXPRESSED IN PERCENTAGES OF THE WEIGHT
OF THE SOIL
SPRUCE
UNSHADED
DEPTH
25 Years
Old
60 Years
Old
120 Years
Old
SOIL
30.93%
29.48 %
40.32 %
22.33 %
6 to 8 inches
19.19
18.99
19.30
20.62
12 to 14 inches . ....
19.10
16.07
18.28
20.54
19 to 20 inches
18.40
16.26
20.16
20.14
30 to 32 inches
17.91
17.88
21.11
20.54
It is obvious that it is only that portion of the water which
passes through this superficial blanket of mould that can be
instrumental in promoting rock decomposition. Hence the
presence of such a blanket may exert a protective, or at least
conservative, rather than destructive action. Further than this,
we have to remember that plant growth tends to reduce the
extremes of temperature and, even more, to diminish evapora-
1 Bull. No. 7, Forestry Division, U. S. Dept. of Agriculture, 1893.
WEATHERING IN COLD AND WARM CLIMATES 283
tion from the immediate surface.. The constant action of grav-
ity and capillarity in pumping the water down and up through
the soil is therefore largely diminished. Since it is by temper-
ature changes and water action that decomposition is so largely
I in night about, it is apparent that we must not be too hasty in
assuming that forest action is actually destructive ; it may be
largely conservative. It is possible that the apparent amount
of decomposition in wooded areas is due to protection from ero-
sion, and the consequent accumulation of the residuary material.
Mtiiv facts are necessary before this question can he decided.
(9) Difference in Kind of Weathering in Cold and Warm
Climates. — That, however, there may be a difference in khuf
in the degeneration in warm and cold climates, or at least in
moist and dry climates, is possible and even probable.1 In cold
and in dry climates subject to extremes of temperature, as in
the arctic regions or in the arid regions of lower latitudes, the
weathering is at first almost wholly in the nature of disintegra-
tion, a process of disaggregation whereby the rock is resolved
into, tirst, a gravel and ultimately a sand composed of the
isolated mineral particles which have suffered scarcely at all
from decomposition. The writer has elsewhere referred to this
form of degeneration as manifested in the desert regions of the
Lower California!! peninsula.2 In a warm, moist climate chem-
ical decomposition may or may not keep pace with the disin-
tegration, according to local conditions, so that the resultant
material may be in the form of an arkose sand, as in the District
of Columbia, or a residual clay, as in the more superficial portions
of the residual deposits to the southward. In certain cases, or
among certain classes of rocks, the decomposition proceeds at so
rapid a rate that there is scarcely any apparent preliminary dis-
integration. Local circumstances and character of rock masses
being the same, we are, however, apparently safe in assuming
that in warm and moist climates decomposition follows so closely
upon disintegration as to form the more conspicuous feature of
the phenomenon, while in dry regions, or those subject to ener-
getic frost action, mechanical processes prevail and disintegra-
tion exceeds decomposition.
1 The majority of writers have failed to discriminate between decomposition
and disintegration. That there may be a very marked difference, due mainly to
climatic conditions, is the point I wish to emphasize here.
2 Bull. Geol. Soc. of America, Vol. V, 1894, p. 499.
284 TIME CONSIDERATIONS
Accepting these facts, there is at once suggested the idea
that the lithological nature of sedimentary rocks, as well as
their fossil contents, may be regarded as indicative of prevalent
climatic conditions.
The possibility of estimating these conditions by the char-
acter of the debris resulting from the degeneration of feld-
spathic rocks was first suggested by the geologists of the Indian
Survey,1 the undecomposed feldspars in the Panchet (Mesozoic)
sandstones being regarded as indicating a recurrence of a cold
period during which mechanical forces preponderated over those
purely chemical. The same idea was subsequently put forth,
quite independently, by the present writer.2 That rocks in arid
regions do actually undergo less decomposition during the
weathering processes is shown not only by the fresh character
of the residuary material. Judd has shown3 that rivers like
the Nile, draining regions of great aridity, though in a con-
dition of high concentration from prolonged evaporation, carry,
in solution, smaller proportional amounts of derived salts than
do those of humid regions.
Russell has noted that in the Yukon River region of Alaska
disintegration so far exceeds decomposition that the talus from
the mountains, composed of loose, angular masses of rock quite
free from vegetation, forms what he calls dSbris streams, which
actually creep slowly down the slopes, the movement taking
place principally in the winter time and being due apparently
to the slow settling, or creep, of deep snows. He states it as
his opinion that the mountains of the region have suffered more
through this form of disintegration than have those of Colorado
or the southern Appalachians, but less than those of the Great
Basin area. The range of limestone mountains along the Yukon
is pictured as presenting a crest of sharp, blade-like crags, flanked
by vast slopes of loose, angular stones on either side, the rock
being everywhere fresh and undecomposed, but badly shattered
and fissured.
(10) Relative Amount of Material lost. — Other things being
equal, it is also safe to infer that more material has actually
been lost through disintegration and decomposition in moun-
1 Geol. of India, 2d ed., Vol. I, p. 201.
2 Bull. Geol. Soc. of America, Vol. VII, p. 362.
8 Report on Deposits of the Nile Delta, Proc. Royal Society of London, Vol.
XXXIX, 1885.
PLATE 22
FIG. 1. Forest destroyed by wind-blown sand.
FIG. 2. Calcareous conglomerate carved and polished by wind-blown sand.
FIG. 3. Rock being undermined by wind-blown sand.
RELATIVE AMOUNT OF MATERIAL LOST 285
tainous and hilly countries than from the level plains. This
for the reasons that (1) through the upturning of the beds there
were exposed, it may be, friable and soluble strata that might
otherwise have been protected, and (2) that through the shat-
tering incident to this upturning the rocks were rendered more
susceptible to the weathering forces. Further, (3) the steeper
slopes in mountain regions promote more rapid removal of the
resultant debris, whereby fresh surfaces are continually exposed,
such as might otherwise shortly become protected through its
accumulation, as above noted.
PART IV
THE TRANSPORTATION AND RBDBPOSITION OP
ROCK DEBRIS
IT rarely happens that more than a comparatively small pro-
portion of the products of disintegration and decomposition are
left to accumulate on the site of the parent rock. In most
instances a very considerable proportion, in some instances all,
the debris is removed immediately, or soon after its formation,
and deposited elsewhere. A portion of this material is removed
in solution, as has already been described (ante, p. 194). A
still larger portion is transported mechanically, and it is to a
discussion of the method of this transportation that a few pages
may now be devoted with profit.
The chief agencies involved in this transportation are grav-
ity, water, in either a solid or liquid form, and the wind. Un-
doubtedly the major part of the work is done by water, but as
the wind's action is so frequently overlooked, and as, moreover,
the results thus produced are of more than ordinary interest
from our present standpoint, it may perhaps be well to dwell
upon this branch of the subject with considerable detail.
(1) Action of Gravity. — Gravity, especially when aided by
the lifting power of frost, may locally exert no insignificant
influence. The tremendous power of landslides, or avalanches,
have, owing to their devastating effects, been impressed upon
us from the beginnings of written history. There are, how-
ever, other results, due to similar causes, but which, going on
on an almost microscopic scale, are wholly overlooked by the
ordinary observer, and the full meaning of which can be dis-
covered only when the results of years are taken into account.
Professor W. C. Kerr, in 1881, described1 the manner in which
the superficial cap of soil from the decomposition of micaceous
1 Am. Jour, of Science, 3d Series, Vol. XXI, p. 345.
286
ACTION OF WATER AND ICE 287
and hornblendic gneisses near Philadelphia had crept down
the inclined surface on which it rested, and the gradual attenu-
ation of the bands of variously colored debris of which it was
composed. This creeping process he ascribed wholly to the
expansive action of included water passing into the condition
of ice, the expansion taking place laterally and the material
being pushed down the slope along the line of least resistance.
Mr. C. Davidson has since taken up the subject experimentally
FIG. 24. — Showing direction and rate of motion of soil: the arrows showing. l>y
their relative lengths, the rate of movement at various i»>ints. a, soil; 6, bed-
rock.
and shown that the amount of the creeping could be accounted
for by the ordinary laws of gravity, the frost, by its expansion,
raising the individual particles a slight distance, and, on thaw-
ing, allowing them to drop back again a greater or less distance
down the slope, according to the angle of inclination. Dr.
Milton Whitney has, however, shown * that there is an almost
continual movement among soil particles, dependent upon
meteorological conditions quite aside from those involved in
freezing and thawing. The creeping appears therefore to be
but the manifestation, in mass, of the inclination of each indi-
vidual particle to slide down the slope.
The accumulations of talus at the foot of every cliff and on
the slopes of hills and mountains arc matters of such every-day
observation as to need no mention in detail.
(2) The Action of Water and Ice.2 — The power of a stream
to transport rock debris depends naturally upon its volume
and the rapidity of its current. This, on the supposition that
the character of the sediment to be transported remains the
1 Some Physical Properties of Soils, Bull. No. 4, U. S. Weather Bureau, 1892.
2 Students are referred to Professor R. I). Salisbury's article on Agencies
which transport Material on the Earth's Surface, Journal of Geology, Vol. Ill,
1895, p. 70.
288 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS
same. According to the calculations of Hopkins, as quoted by
Geikie,1 the capacity of transport increases as the sixth power
of the velocity of the current ; that is to say, the motor power
is increased sixty-four times, by doubling the velocity. The
following table is taken from the work quoted as showing the
power of transport of river currents of varying velocities : —
INCHES MILES
PER SECOND PER HOUR
3 0.170 : will just move fine clay.
6 0.240 : will lift fine sand.
8 0.4545 : will lift sand as coarse as linseed.
12 0.6819 : will sweep along fine gravel.
24 1.3638 : will roll along rounded pebbles 1 inch in diameter.
36 2.045 : will sweep along slippery, angular stones of the size of an
egg.
There are, of course, other factors that should be taken into
consideration, such as the character of a river bed, the density
of the water, etc., but which lack of space prevents our touch-
ing upon here, and which are, moreover, sufficiently enlarged
upon in other works.
The writer has stood at the head waters of the Missouri, and
seen the Jefferson, Madison, and Gallatin rivers uniting their
floods to form one grand rushing stream of clear green water,
full of trout and grayling. He has seen it again at Mandan,
Dakota, a sluggish stream actually yellow with suspended silt.
At St. Louis, one beholds it a mighty torrent, whirling along
trunks and stumps of trees, twigs, and all manner of organic
debris and inorganic detritus picked up from its banks, or
washed in by rains and tributary streams, till, one vast sea of
liquid mud, it pours every year into the Gulf of Mexico a mass
of sediment equal to 812,500,000,000,000 pounds (7,468,694,400
cubic feet), or enough to cover a square mile of territory to
a depth of 268 feet. But only a portion of the detritus car-
ried by running streams reaches the ocean ; otherwise we need
devote little time here to its consideration. Nearly all streams,
in some part of their courses, flow through level plains with low
banks which are subject to inundation during seasons of high
water. Picture a muddy stream such as is shown in cross-sec-
tion in Fig. 25, and which at ordinary periods is confined
within the narrow channel near the centre. In time of freshet,
however, the volume of water is so greatly augmented as to
1 Text-book of Geology, 3d ed.
ACTION OF WATER AND ICE
289
cause it to overflow its banks and spread out over the plains on
either hand. But no sooner does the water leave the channel
than the force of its currents becomes checked, its carrying
power lessened, and it therefore begins to deposit its load of
silt upon this flood plain, as it is called, where it remains to
permanently enrich the land when the waters subside. It is to
such processes of formation that we owe some of the most fer-
tile lands in existence, as the valley of the Mississippi, that of
the Red River of the North, the Nile, and scores of others that
be mentioned readily attest.1
Fio. 25.
To the same process, coupled with the accumulation of
organic matter, we owe the filling in and gradual extinction of
thousands of glacial lakes throughout New England and the
North, and the formation of rich, flat-bottomed valleys known
locally as meadows, swales, and bogs.
Ice in the form of glaciers is an efficient agent for transpor-
tation as well as for erosion, as already noted. While the work
being done by existing glaciers may seem comparatively insig-
nificant, that done by the ice sheet of the glacial epoch was by
no means so, and deserves a more than passing notice. The
manner in which the ice carries and deposits its load has already
received attention in speaking of its erosive power, and but
little more need be said on the subject. That material which
existed in a loose, unconsolidated condition, on the surfaces on
1 The Arkansas River is stated by Owen (Geol. of Arkansas, 2d Rep., 1860,
p. 52) to be at certain seasons of the year almost blood-red from the quantity of
suspended fine ferruginous clay and saliferous silt brought down from the regions
of ferruginous shales, which prevail in the Cherokee County, through which the
river flows. This material, deposited along the banks and in the eddies of still
water, produces the celebrated red buckshot land. Material washed from the
bluffs of argillaceous shell marl, near the confines of Jefferson and Pulaski
counties, is deposited again farther down the stream as a fine silt, imparting,
like the red silt, extraordinary fertilizing properties to the soil.
290 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS
which the glacier formed, was pushed and dragged along by
the onward movement of the ice, which in extreme cases may
have exerted a pressure of 200,000 pounds to the square foot.
On the final retreat of the glacier, this was left in the form of a
compact structureless mass of almost stony hardness, commonly
known as till or ground moraine. Materials falling upon the
surface from greater heights were likewise transported, so long
as the ice sheet continued to advance, and finally deposited in
the form of terminal or frontal moraines.
Inasmuch as the ice sheet was almost continually melting
upon its surface, it is practically impossible to consider its
action wholly independent of that of water also. Thus,
streams resulting from such melting would gradually wear
channels in the ice, as on the land. In these channels would
accumulate sand and boulders of such size and weight as to
resist the current, and such accumulations would, on the final
melting of the sheet, be deposited on the surface of the ground
in the form of ridges known as eskers, or osars. Other forms
of water action on the materials of the ice sheet, are hillocks
of stratified sand and gravel deposited near the terminal mo-
raines, and known as kames. Since during the advancing of
this ice sheet existing rivers flowing eastward must have been
dammed, we can safely imagine the formation of large tempo-
rary lakes, on the bottom of which would be deposited the
glacial silt, like the so-called loess of the Mississippi valley.
Lake Agassiz, a glacial lake of this type, is supposed to have
occupied an area of more than 100,000 square miles in north-
western Minnesota, northeastern Dakota, and a considerable
portion of Manitoba. On the bottom of this lake there was
deposited during the comparatively brief time of its existence,
silt to a depth as yet undetermined, but known to be at least
100 feet.1
Waters issuing from the melting ice sheet tend to reassert the
material of the terminal moraine, redepositing it in approxi-
mately concentric zones beyond its margin. These deposits
are naturally thicker and coarser near the moraine and thinner
and finer at increasing distances. Their form and mode of
occurrence is such as to have suggested for them the name of
glacio-fluvial aprons, or frontal aprons. Their materials are
nearly always loose sands and gravels, the lithological nature
1 Ice Age in North America, by G. F. Wright, p. 355.
ACTION OF WATER AND ICE 291
of the individual particles being of course dependent upon that
of the moraines from which they are derived.
The effects upon the landscapes of this ice sheet have been
lasting and peculiar. We may safely imagine that, before the
ice invasion, the surface was covered with decayed and softened
materials like the residual soils of our Southern states, and
which had been cut up into valleys and intervening ridges by
the stream of that time. The ice sheet stripped from these
surfaces their mantle of decomposed materials, and in addi-
tion cut, in many cases, into the fresh rock, actually planing
the entire country so deeply that in most cases the preglucial
surface is no longer recognizable. The hills were thus lowered
and the valleys in some cases deepened or again tilled by sand
and gravel. Since a protruding rock mass would, from neces-
sity, be most eroded on the side from whence the ice sheet
approached, and since, moreover, such would serve to catch
and hold back a part of the loose earth and stony matter
brought from the north, a peculiar feature in the topography
of glaciated hills has been brought about as shown in Fig. 2,
PI. 25.
The direction taken by this drift material was quite variable.
It was, as a rule, from the north toward the south, with many
minor deflections. Boulders of Laurentian rocks north of Lake
Huron are abundant in the drift about Oberlin, Ohio, and even
further south. Boulders of native copper from the Lake Supe-
rior region are found even as far south as Kankakee, Illinois,
and a large boulder of a peculiar conglomerate known in place
only near Ontario, has been found a few miles south of the
Ohio River in Kentucky. Dawson states " that boulders from
the Laurentian axis of the continent, which stretches from
Lake Superior northward to the west of Hudson Bay, have
been transported westward a distance of 700 miles, and left
upon the flanks of the Rocky Mountains at an elevation of
something over 4000 feet." l
All over the states once occupied by this ice sheet the ma-
terial originating from the decomposition of rocks in situ, or
deposited on alluvial plains, was, with a few minor exceptions,
carried away to the southward and in part dumped into the
Atlantic, while its place was supplied by mongrel hordes from
the north. In process of digging for the foundations of the
1 Ice Age in North America, p. 171.
292 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS
Maine Experiment Station at Orono, the fresh and highly
polished slaty rock was found but a few feet below the sur-
face, proving incontestably that, with the exception of the
small amount of organic matter that had since been added,
not an ounce of the soil was truly native, but all of foreign
birth, and a mongrel creature of compulsory migration. We
shall dwell more fully upon the character and distribution of
these soils later. The single illustration above given will
answer present purposes.
In a less degree the ice along the shores of lakes and rivers
may exert a transporting influence. Thus the ice first formed
along the shores encloses sundry pebbles, boulders, and sand.
Through the expansion force of the freezing water as the entire
surface becomes frozen over, this shore ice, together with its
enclosures, may be pushed up some distance beyond the water
line, where the included debris is deposited on melting. Or,
on the breaking up of the ice in the spring, the shore ice may
be drifted to other parts of the lake, or down the stream, per-
haps for miles before melting sufficiently to cause it to deposit
its load.
(3) Action of Wind.1 — While abrasion by the wind is im-
possible without transportation, the converse is by no means
true ; indeed it is as an agent of transportation for rock detri-
tus, without appreciable abrasion, that the wind accomplishes
its greatest work, though in like manner this phase is most
manifest in arid regions.
It is stated by Darwin that for several months of the year
large quantities of dust are blown from the northwestern shores
of Africa into the Atlantic over a space some 1600 miles in
width and for a distance of from 300 to 600 and even 1000
miles from the coast. During a stay of three weeks at St. Jago
in the Cape Verde Archipelago, this authority found the atmos-
phere almost always hazy from the extremely fine dust coming
from Africa and falling upon the land and water for miles
around. So abundant was this dust that a distance of between
300 and 400 miles from the coast the water was distinctly
colored by it. In the arid lands of Central Asia the air is also
reported as often laden with fine detritus which drifts like snow
around conspicuous objects and tends to bury them in a dust
1 See article on Erosion performed by the Wind, by Professor J. A. Udden,
Journal of Geology, Vol. II, 1894, p. 318.
ACTION OF WIND 293
drift. Even when there is no apparent wind, the air is described
as often thick with fine dust, and a yellow sediment covers
everything. In Khotan this dust sometimes so obscures the
sun that even at midday one cannot see to read fine print with-
out the aid of a lamp. The tales of the overwhelming of trav-
ellers and entire caravans by sand storms in the Great Desert
of Sahara are familiar to every schoolboy. Greatly exagger-
ated though these may be, the accounts of Layard and of
Loftus show us that the sand storms which are of frequent
occurrence during the early part of summer throughout Meso-
potamia, Babylonia, and Susiana are by no means of insignifi-
cant proportions. Layard states that during the progress of
the excavations at Nimrud, whirlwinds of short duration but
almost inconceivable violence would suddenly arise and sweep
across the face of the country, carrying along with them clouds
of dust and sand. Almost utter darkness prevailed during
their passage, and nothing could resist their force ; the Arabs
would cease their work and crouch in the trenches almost suf-
focated and blinded by the dense cloud of fine dust and sand
which nothing could exclude.
The accounts of Loftus are equally impressive. Describing
their departure from Warka to Sinkara, he says: "A furious
squall arose from the southeast and completely enveloped us
in a tornado of sand, rendering it impossible to see within a
few paces. Tellig and his camels were as invisible as though
they were miles distant. A continuous stream of the finest sand
drove directly in our faces, filling the eyes, ears, nose, and mouth
with its penetrating particles, drying up the moisture of the
tongue, and choking the action of the lungs." With such
descriptions before one it is not difficult to believe that these
ruined cities have in the course of centuries been completely
hidden and their sites obscured by mounds of wind-drifted
sand and dust.
We need not, however, confine ourselves wholly to the Old
World for illustrations. Not longer ago than May of 1889 a
dry southwesterly wind which for several days had prevailed
in various parts of the Northwest, particularly in Dakota, cul-
minated in a storm peculiarly suggestive from a geological
standpoint. It is stated1 that during the prevalence of this
wind, on the 6th and 7th of the month mentioned, the air be-
1 American Geologist, June, 1889, p. 398.
294 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS
came filled with flying particles caught up from the ploughed
fields, fire-blackened prairies, public roads, and sandy plains.
The particles formed dense clouds and rendered it as impos-
sible to withstand the blast as it is to resist the blizzard
which carries snow in winter over the same region. The soil
to a depth of 4 or 5 inches in some places was torn up and
scattered in all directions. Drifts of sand were formed in
favorable places, several feet deep, packed precisely as snow-
drifts are packed by a blizzard. It seemed as if there were
great sheets of dust and dirt blown recklessly in mid air, and
when the wind died down for a few moments, the dirt, fine
and white, appeared to lie in layers in the atmosphere, clouding
the sun and hiding it entirely from sight for an hour or more
at a time. (See also on p. 184.)
Over the wide, dry, and bare flat-topped terraces of the upper
Madison valley the wind sweeps in a strong steady current
for days together, or during the heated portion of the year,
when the sun pours from a cloudless sky its hottest rays upon
the parched soil, starts up spasmodically here and there in the
form of small whirlwinds made visible by the dust they carry,
and which wander spectre-like across the plain to noiselessly
disappear in the distant mid air.
Dust columns of this nature are common in all arid regions,
and doubtless have been observed by the many who have
crossed the Humboldt desert in Nevada. Seated comfortably
in a Pullman car on the Union Pacific, one may not infrequently
see at a single view not less than a half dozen of these geologi-
cal spectres, each in the distance doing its apportioned task
and silently disappearing, laying down its load of sand as its
strength gives out and leaving it for its successor.1
Under proper conditions such of these wind-blown sands as
are too heavy to be carried into the air as dust accumulate
upon the surface in the form of drifts, or dunes, all lying with
their longer axes approximately at right angles with the pre-
vailing currents. Excepting during periods of calm, such are
in a state of almost constant, though it may be imperceptible,
motion, ever changing their shapes and moving onward like
long parallel drifts of snow. The rate of motion of a dune
1 Professor J. A. Udden estimates that the dust in a cubic mile of lower air
during a dry storm weighs not less than 225 tons, while in severe storms it may
reach 126,000 tons (Popular Science Monthly, September, 1886).
ACTION OF WIND 295
from necessity is governed by the strength and constancy of
the winds, and the fineness and dry ness of the sand. Urged
into temporary activity, each little grain goes scurrying up the
slope, across the crest, and tumbles to rest in the steeper
declivity upon the leeward side, to be slowly buried 1>\ those
which follow. This is the sum total of the movement taking-
place in the march of a dune, whatever its pace and however
great its bulk. Yet in this very faculty of moving itself for-
ward by but a ten billionth part of its bulk at a time lies the
whole secret of its power. Silently, imperceptibly it may be
except when measured by months and perhaps years of time,
retarded by no walls nor ordinary declivities, it relentlessly
performs its task.1
A writer in one of the recent popular magazines estimates
the dunes of Hatteras and Henlopen as in some cases upwards
of 70 feet in height and moving at least 50 feet a year. Swamps
have thus been filled, forests and houses buried, and it is stated
that but a few years can elapse before the entire island lying
north of Cape Hatteras will be rendered uninhabitable. The
sand dunes on the coast of Prussia commenced not over a
century ago, and already fields and villages have been buried
and valuable forests laid waste by them. In one instance a
tall pine forest covering many hundred acres was destroyed
during the brief period intervening between 1804 and 1827.
Loftus, writing of Niliyga, an old Arab town a few miles east
of the ruins of Babylon, says that in 1848 the sand began to
accumulate about it, and in six years the desert within a radius
of six miles was covered with little undulating domes, while
the ruins of the city were so buried that it is now impossible
to trace their original form and extent. A still more striking
illustration of the rapidity of these sand accumulations is offered
by the same authority in describing the burial customs of some
of these ancient people, it being stated that the earthen coffins
were merely stacked in layers one on top of another, and left
thus to be covered by the finer sand sifted over them by the
winds from the desert. Even Nineveh, founded some twenty
centuries before Christ and destroyed 1400 years later, became
so covered by drifted sands that at the time of the Greek
Xenophon (about 400 B.C.) the very site of the once famous
i The Wind as a Factor in Geology, Engineering Magazine, 1892, p. 596.
296 TRANSPORTATION AND REDEPOSITION OF ROCK DEBRIS
city was unknown. Marsh 1 gives the rate of movement of dunes
along the western coast of Jutland and Schleswig-Holstein as
averaging 13^ feet a year, while Anderson estimates the aver-
age depth of the sand over the entire area as about 30 feet,
equalling therefore about 1£ cubic miles for the total quantity.
It is not in all cases possible to trace the drifted sands to
their various sources. Dunes along the sea-coasts are in nearly
all cases composed of materials thrown up by the waves on
the beaches in the immediate vicinity. This is the case with
those of Hatteras, Cape Cod, Gascony, Algeria, and Schleswig-
Holstein. But the origin of the large inland dunes, like those
of Nevada, is not always so clear. It has been suggested that
these last are formed of beach sand driven in by the prevail-
ing westerly winds from the Pacific coast. This is, however,
a matter of very grave doubt, and it seems more probable, as
stated by geologist Russell,2 that they were derived from the
disintegrating granites of the Sierras. They certainly have
travelled far, and are not a product of disintegration of rocks
in the immediate vicinity.3
By wind action, accompanied by the carrying power of spas-
modic or perennial streams, were formed the wide stretches
of adobe in the western United States, and according to many
authorities the deposits of loess .which cover, as in Europe and
Asia, areas aggregating many square miles and which have .a
depth, in extreme cases, of 2000 feet.4
The tendency of the wind is not, however, in all cases toward
1 The Earth as modified by Human Action, p. 562.
2 Quaternary History of Lake Lahonton, Nevada, Monograph, U. S. Geol.
Survey, 1885.
3 The sands covering the Egyptian Sphinx and Pyramids are stated to have
come mainly from the sea on the north, and not from the desert, as is popularly
supposed. Sand showers having their origin in the desert of Sahara extend
across the Mediterranean, and as far as northern Italy (Nature, July 18, 1889,
p. 286).
4 The wind plays an important part in the transportation of soils in Wyoming,
owing to the incoherent state of the -soils, due to the lack of clay. The arid
regions of this state, which are chiefly Tertiary and Cretaceous plains and table-
lands, receive very little rain. Consequently the soils become loosened by great
earth cracks, and during the dry and windy winter weather are transported in
dense clouds, which almost suffocate travellers, to the broken country and dis-
tant hills and mountains. In a single season it is not an uncommon sight to see
banks of- earth, like huge banks of snow, behind a reef of rock, or in the lee of
large bunches of sage brushes (U. S. Dept. of Agriculture, Office of Experiment
Stations, Vol. V, No. 6, 1894, p. 567).
ACTION OF WIND 297
forming drifts and ridges, but at times rather to reduce the land
in one general level. Thus J. Flinders Petrie1 states that near
tin- ancient cemetery of Tell Nebesheh, on the Isthmus of Suez,
the surface of the country has been cut down at the rate of 4
inches a century until some 8 feet have been removed from
the dry areas and deposited in the intervening depressions,
slowly converting the existing lakes into marshes, and the
marshes into dry land. An even more rapid change of con-
tours is that described by Dwight2 as having taken place on
Cape Cod, Massachusetts. The entire country here is com-
posed of sand so susceptible to the drifting action of the wind
that it has for years been the custom of the people to sow pines
and coarse beach grass to hold it in place. In the instance
described by Dwight, however, reckless pasturage had so far
destroyed the grass as to lessen its protecting power, and
under the strong breezes from the open Atlantic it began to
drift rapidly. Over an area of about 1000 acres the sand was
Mown away to a depth, in many places, of 10 feet. "Nothing,"
says Dwight, " could exceed the dreariness of this scene. Not a
living creature was visible; not a house, nor even a green thing
except the whortleberries which tufted a few lonely hillocks
rising to the height of the original surface, and prevented by
this defence from being blown away also. The impression made
by this landscape cannot be realized without experience. It
was a compound of wildness, gloom, and solitude. I felt
myself transported to the borders of Nubia, and was well
prepared to meet 'the sand columns so forcibly described by
Bruce, and after him by Darwin. A troup of Bedouins would
have finished the picture, banished every thought of my own
country, and set us down in an African waste."
One more instance of contour changes of this sort must suffice.
It is stated3 that in Pipestone and Rock counties in Minnesota,
the bluffs facing to the westward are, as a rule, more precipi-
tous and more rocky than those facing in the opposite direc-
tion. This fact is regarded by Professor Winchell as due to
the action of the prevailing westerly winds, combined with
the drying effects of the southwestern sun in summer. Such
winds would uncover and keep bare the coarser materials of
. Royal Geographic Soc., November, 1889, p. 648.
2 Travels in New England and New York, Vol. Ill, p. 101.
8 Geol. of Minnesota, Vol. I, p. 575.
298 TRANSPORTATION AND REDEPOSITION OF ROCK DEBHIS
the western surface by blowing away the sand and clay, while
the bluffs on the east are not only protected from the winds,
but collect upon their slopes all the flying particles from the
prairies above.
The finely comminuted rock dust blown from volcanic vents
is often drifted for long distances by atmospheric currents, and
ultimately deposited in beds of no insignificant proportions.
Dense clouds of such dust were blown from Icelandic volcanoes
to the coast of Norway in 1875, and subsequent to the eruption
of Krakatoa (in 1883) the ship Beaconsfield of Philadelphia,
while at a distance of 831 miles from the source, sailed for three
days through clouds of dust which fell upon her decks at the
rate of an inch an hour. That such are not or have not in
the past been unusual instances is shown by results obtained
by the Challenger Expedition, volcanic ashes and sand being
repeatedly dredged up from almost abysmal depths at points
in the central Pacific far remote from land areas. The day
following the explosive eruption of St. Vincent, in 1812, the
Barbadoes Island, 80 miles to the windward, was completely
shrouded in darkness for many hours, the light of the sun being
almost wholly obscured by the cloud of impalpable dust which
in the form of a slow, silent rain fell over the whole island.
"The trade wind had fallen dead; the everlasting roar of the
surf was gone; and the only noise was the crushing of the
branches snapped by the weight of the clammy dust. About
one o'clock the veil began to lift, a lurid sunlight stared in
from the horizon, but all was black overhead. Gradually the
dust cloud drifted away; the island saw the sun once more,
and saw itself inches deep in black, and in this case fertiliz-
ing, dust." l
1 Kingsley, as quoted by Belt, in The Naturalist in Nicaragua, p. 354.
PAKT V
THE REGOLITH
THROUGHOUT the millions of years which have elapsed since
the earth assumed its present form and essentially solid con-
dition, the rocks composing its more superficial portions have
been constantly undergoing degeneration in the manner de-
scribed, and, in so doing, have given rise to the immense masses
of materials which constitute the thousands of feet of secon-
dary rocks, and the still unconsolidated sands, gravels, and other
products which will be .considered in detail later. With those
products which have undergone lithification, which are now in
the state of consolidation commonly ascribed to rocks by the
popular mind, we shall have little more to say. These have
already been sufficiently described as rocks in Part II of this
work. It is to the most superficial and unconsolidated portion
of the earth's crust that we will now devote our attention.
Let tfie reader for a moment picture to himself the present
condition of this crust, with particular reference to the land
areas. Everywhere, with the exception of the comparatively
limited portions laid bare by ice or stream erosion, or on the
steepest mountain slopes, the underlying rocks are covered by
an incoherent mass of varying thickness composed of materials
essentially the same as those which make up the rocks them-
selves, but in greatly varying conditions of mechanical aggrega-
tion and chemical combination.
In places this covering is made up of material originating
through rock-weathering or plant growth in situ. In other
instances it is of fragmental and more or less decomposed mat-
ter drifted by wind, water, or ice from other sources. This
entire mantle of unconsolidated material, whatever its nature
or origin, it is proposed to call the regolith, from the Greek
words ^7/705, meaning a blanket, and Xt0o<?, a stone. Within
299
300
THE REGOLITH
certain limits it varies widely in composition and structure, and
many names have, on one ground and another, been applied to
its local phases, the more important of which are given in tabu-
lar form below, and described in detail in the pages following.
According to its origin, whether the product of transporting
agencies as noted above, or derived from the degeneration of
rocks in situ, the regolith is found lying upon a rocky floor of
little changed material, or becomes less and less decomposed
from the surface downward until it passes by imperceptible
gradations into solid rock.
The
regolith
Sedentary
Residual deposits
/ Residuary gravels, sands and clays,
I wacke, laterite, terra rossa, etc.
f Peat, muck, and swamp soils, in
[Cumulose deposits | ' t
f Talus and cliff dfibris, material of
Colluvialdeposus { avalanches.
Alluvial deposits f Modern alluvium, marsh and swamp
(including aqueo- -I (paludal) deposits, the Champlain
Transported^ glacial) ( clays, loess, and adobe, in part.
f Wind-blown material, sand dunes,
I adobe and loess, in part,
f Morainal material, drumlins, es-
1 kers, osars, etc. •
jEolian deposits
Glacial deposits
The extreme upper, most superficial portion of this regolith,
that which affords food and foothold for plant life, is commonly
designated as soil; that immediately underlying the soil, and
passing into it by insensible gradations, is known as the sub-soil.
This last differs from the soil proper only in degree of compact-
ness and in such chemical changes as may have been induced
in the soil through growing organisms and more extensive
weathering. Indeed, the soil is but derived from the sub-soil,
and were it entirely removed, would shortly be replaced through
the same agencies as first gave it birth.
The characteristics of individual soils can be best discussed
when speaking of those loc.al phases of the regolith of which
they form a part, and with this understanding we will proceed.
1. SEDENTARY MATERIALS
Here are to be considered those deposits which, resulting
from chemical decomposition or disintegration, from any or all
of the processes involved in rock-weathering, or from organic
SEDENTARY MATERIALS
301
accumulation, are found to-day occupying their original sites.
They are, in fact, the primeval types of nearly all soils and sec-
ondary rocks, since those of drift origin are but derived from
sedentary materials through the transporting agencies of air
and water. They may be
conveniently divided into
two classes, (1) residual 1
and (2) cumulose.
(1) Residuary Deposits.
Under this name, then, are
included all those prod-
ucts of rock degeneration
which are to-day found oc-
cupying the sites of the
rock masses from which
they were derived, and im-
mediately overlying such
portions as have as yet
escaped destruction. The
name is peculiarly appro- Fio. 26. — Showing angular outlines of residuary
priate, since thev are actu- Particles from decomposed gneiss. 1, mica;
i r*. u u- i 2» 'eldspar; 3, quartz.
ally residues, left behind
while the more soluble portions have been leached away by
meteoric waters.
The residual deposits of North America reach their maximum
development in the portion of the United States east of the
Mississippi and south of the southern margin of the ice sheet
of the Glacial epoch. Their mode of accumulation and gen-
eral characteristics have been very thoroughly discussed by
Professors I. C. Russell, Chamberlain, and Salisbury,2 on whose
papers \ve shall draw for some of the facts given here.
1 Various .names have from time to time been proposed for deposits of this
nature, but obviously it is impossible to include under a single lithological term
materials so widely variable. The term saprolite (from the Greek <ra.irpos, rotten,
recently suggested by G. F. Becker, 16th Ann. Rep. U. S. Geol. Survey, Part III,
p. 289) is objectionable as conveying the idea of putridity. The old provincial
term tjeest adopted by De Luc, and recently endorsed by McGee (llth Ann.
Rep. U. S. Geol. Survey, 1889-90, p. 279), has lost whatever precise meaning it
may have had, being defined in both the Standard and Century dictionaries as
(1) a bed derived from rock decay in situ, (2) high gravelly land, and (3) gravel
or drift. The term gruss, although advocated by some American authorities, is
of old German origin and open to the same objection.
2 Bull. 52, U. S. Geol. Survey and Ann. Rep. U. S. Geol. Survey, 1884-85.
302 THE REGOLITH
The prevailing characteristic of an old residual deposit, from
whatever rock it may be derived, is a ferruginous clay. Exam-
ined by a microscope, its mineral particles, when not too thor-
oughly decomposed, are found to be sharply angular in outline.
With the exception of the quartz, the various mineral constitu-
ents are often in an advanced stage of decay, and the more
soluble constituents are wholly or partially lacking, having been
leached out, in the manner already described.
Owing to the prevalence of the aluminous constituents, these
deposits, when thoroughly decomposed, as on the immediate sur-
face, are very tenacious, and may well be termed clays. Their
colors are dull, or some shade of brown or red, owing to the
higher oxidation and perhaps dehydration of the ferruginous
matter set free by the decomposition of the iron-bearing sili-
cate constituents. Such in general are the residual soils of the
southern Appalachian regions of the United States and which
are apparently in every way comparable with the terra rossa of
Europe, but only in a slight degree with the later ite of India,
to which they have often unfortunately been referred.1 From
a chemical standpoint the soils forming the upper portion of the
residuary deposits, though of a prevailing aluminous character,
vary widely from the rock masses from whence they were de-
rived, much depending upon their age and the amount of actual
decomposition and leaching that has taken place. On p. 306
are given a few typical but widely varying analyses which will
serve to illustrate this point.
Deposits of this nature are never truly stratified, excepting
where, through having remained wholly undisturbed, they re-
tain the original structure of the parent rock. (See under
Effacement of Original Characteristics, p. 262.)
The residuary differ from the drift deposits in that they con-
tain no materials foreign to their vicinity, but only s«6h more
enduring matter as has been handed down to them from the
LThe term terra rossa, according to Neumayer (Erdgeschichte, Vol. I, p.
405) was first applied to the red residual deposits in the Karst maritime lands
of the Adriatic Sea. The material is described as a highly ferruginous clay
resulting from the leaching out, by meteoric waters, of the soluble portions of
the prevailing limestones. Its distribution is by no means limited to the mari-
time provinces of the Karst, but it is found also on the Grecian coasts and in
the Schwabia-Frankonia Jura Plateaus of Bavaria. In fact it is to be found any-
where in these regions where the prevailing country rock is a marine limestone
and erosion not sufficiently active to remove the residuary material.
RESIDUARY DEPOSITS 303
parent rock. In the case of limestones such matter consists
mainly of aluminous and ferruginous matter, grains of sand,
and nodular masses of chert which existed as mechanically
admixed impurities.
The inherited characteristics of deposits of this nature may
be illustrated by the accompanying exaggerated section across
central Kentucky where, it is easy to see, the regolithic mate-
rial overlying the Lower Silurian and Cambrian limestones may
FIG. 27.
contain a portion of all the insoluble residues from the hundreds
of feet of Upper Silurian, Devonian, Lower and Upper Carbonif-
erous beds which formerly stretched above them. Upon the
imtiire of this inheritance must depend the adaptability of the
regolith to soil purposes and its consequent fertility.
The transition from a regolith of this type to fresh rock is
usually quite sharp, owing to the fact that limestones decompose
mainly through solution from the immediate surface. Never-
theless there is a gradual change in the character of such a
deposit from above downwards, owing to the oxidizing influence
of the air and percolating waters. (See p. 307.)
As above noted, the mineral particles in the older residuary
deposits are, with the exception of the quartz, found to be as a
rule in a state of advanced decomposition. Nevertheless the
ultimate individual constituents of even the darkest clays of the
driftless regions of Wisconsin, as examined by Messrs. Chamber-
lain and Salisbury, are transparent, although stained by iron
oxides.
Concerning the physical properties of limestone residues as
occurring in this driftless area, the following statements are
made by Messrs. Chamberlain and Salisbury. "Above, the
clay graduates into soil which, outside the valleys, is uniformly
shallow. Beneath the soil, the clay loses the dark color of the
latter, due to the presence of organic matter, but is for a certain
distance downward not unlike the superior portion in texture.
The deeper lying clay, where limestone is the subjacent rock,
is the most characteristic member of the residuary earth series.
304
THE REGOLITH
It is not like that above, structureless, although, like that, it is
without trace of stratification. It generally shows a tendency to
cleave, breaking up into little pieces which are roughly cubical.
This is often conspicuous, and especially so on the faces of
sections which are thor-
oughly dry. In such sit-
uations large quantities
of the clay in small angu-
lar blocks may be removed
by slight friction. The
size of the cuboids varies,
within somewhat narrow
limits, from a small frac-
tion of an inch to one or
two inches in diameter.
This cleavage is probably
a phenomenon of shrink-
age due to drying, as it
partially disappears when
FIG. 28. — Showing angular character of quartz ^}ie clav becomes wet.
particle8 in decomposed gneiss. ^^ structure hag giyen
rise to the local name of ' joint ' clay, an appellation not alto-
gether inappropriate.
" Upon drying, this variety becomes very hard and rock-like.
It only becomes adapted to serve as soil by surface amelioration,
as is shown by the fact that, from the thousands of mineral holes
scattered over the southern part of the mining district, the
material ejected still lies beside the excavations as heaps of clay,
without covering of vegetation, although it has been exposed in
most cases for many years. Notwithstanding this fact, the
clay, even in its deepest parts, wherever examined, is found to
abound in minute perforations. These, in many cases at least,
indicate the penetration of rootlets, for the rootlets themselves
may sometimes be found. In some cases, too, the perforations
have been seen to undergo a gradual variation in size, and to
branch now and then, much as rootlets do. On the other hand,
it is probable that some of the perforations have had a different
origin, for in one case a small insect was found in one of the
little canal-ways. The clay is exceedingly tenacious, and hence
the perforations, once formed, would endure for long periods of
time.
RESIDUARY DEPOSITS 305
" Another characteristic of certain portions of the clay is its
power of retaining moisture. It can rarely be found, even in
the driest season, unless exposed to the direct rays of the sun,
without visible moisture a few inches from the surface. The
regions where it is present are conspicuously less affected by
drought than adjacent localities where it is wanting. For this
reason it is a valuable sub-soil.
u Fragments of residuary rock are not uncommon in the deeper
portions of this earth. Of these, ^kert fragments are most
abundant, and occur scattered sparingly throughout the clay or
sometimes arranged in more or less distinct layers in it. Even
where they appear to be entirely wanting, the microscope often
reveals minute flakes scattered sparsely throughout the clay.
The larger pieces are more numerous near the basal portion of
the clay than higher up.
" It is natural to suppose that the residuary earths derived
from the decomposition of limestone would differ very notably
from those which take their origin from sandstones or from
shales or mixed crystalline rocks. Yet the difference is far
less than might be anticipated. There usually overlies the
sandstone strata a loamy earth not very far removed in char-
acter from that which mantles limestones. It is somewhat
more sandy, and consequently less cohesive, and presents the
opposite variations in vertical sections, becoming less cohesive
below, instead of more so. In the limestone region the tough-
est clay lies next to the rock. In the sandstone regions the
soil graduates below into sand. The difference is most con-
spicuous where the mantle has been washed and redeposited
and mingled with mechanically derived sand and secondary
products, as occurs in some of the valleys. " l
The following analyses, in part from this same report, will
answer, in connection with those already given, to show the
prevailing type of the residuary deposits throughout widely
separated areas. It will be noted that silica exceeds as a rule
all other constituents, while alumina, iron oxides, and moisture
make up the main bulk of the residue. This generalization
holds good of nearly all sedentary soils, whatever the character
of the rocks from which they were derived, and is the more
pronounced the more advanced the decomposition.
i 6th Ann. Rep. U. S. Geol. Survey, 1884-85, pp. 240-242.
306
THE REGOLITH
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W
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RESIDUARY DEPOSITS 307
Columns I, II, III, and IV of this table (see opposite page)
are limestone residuals from southern Wisconsin. Columns I
and II are from the same vertical section, I being 4£ feet from
the surface, and II 8£, and in contact with the underlying lime-
stone. Columns III and IV are similarly related, III being 3
feet from the surface, and IV 4^ feet, the lower sample lying on
the unchanged rock. The larger percentages of silica in tluj
samples from nearest the surface indicate a higher state of
decomposition, the soluble portions having been more largely
removed. The presence of larger percentages of alkalies in
these same samples indicates that these salts existed in the form
of silicates which have resisted the decomposing influences, and
remain mechanically included in the residues. Column V is a
clay from the decomposition of the Knox dolomite at Morris-
ville, Alabama ; VI the characteristic red earth from the decom-
position of coralline limestone on the islands of Bermuda; VII
a product of the decay of a diabase dike at Wadesboro, North
Carolina; VIII a gabbro sub-soil from Maryland; IX a sub-soil
from the decomposition of Trenton limestone near Hagerstown,
Maryland ; and X a residual soil from the decomposition of a
Triassic sandstone, Maryland.
A microscopic examination of the material represented by
analyses I and IV, as given by the authorities quoted, showed
it to consist of particles in an extreme condition of comminu-
tion. An actual measurement of over 700,000 of these particles
yielded results as below : —
Particles less than .0025 mm. in diameter 721.866%
Particles between .0025 mm. and .005 mm. in diameter 9.812
Particles over .005 mm. in diameter 0.634
732.312 %
Of those over .005 millimetre in diameter, particles reaching
0.06 millimetre were not rare. Nearly all those above 0.1 milli-
metre were found to be of flints and cherts which graded up
into chips and flakes of notable sizes. Particles much coarser
than those above enumerated do indeed occur, but their actual
number is comparatively small, though their comparative bulk
may be considerable.
Work of a like nature, but done under somewhat different
conditions, by Dr. Milton Whitney, showed the residues from
the Trenton limestones near Hagerstown, Maryland, to contain
308
THE REGOLITH
011 an average some 45% of finely comminuted material, the
individual particles of which vary in size between .005 and
.0001 millimetre in diameter, and which may appropriately
be termed day. As Dr. Whitney has calculated, there are
approximately 22,000,000,000 grains of sand and clay in each
gramme of such a sub-soil, presenting in every cubic foot not
less than 158,000 square feet of surface to the action of water
and air, as well as to the roots of growing plants.
The results of mechanical analyses of (I and II) resi-
dues from the Trenton limestone, (III) Triassic sandstone,
(IV) gabbro, and (V) gneiss are presented in tabular form
below.1
DIAMETER
OF
PARTICLES
MM.
CONVENTIONAL NAMES
I
II
III
IV
V
2-1
Fine gravel .......
%
0.54
°/
10
0.17
<>/
lo
0.00
01
10
0.00
01
10
0.19
1-.5
Coarse sand
0.32
0.00
0.23
0.26
1.80
.S-.25
Medium sand
0.72
0.16
1.29
0.18
3.12
.25-.!
Fine sand *
0.62
0.25
4.03
0.66
6.96
.1-.05
Very fine sand
4.03
2.34
11.57
6.73
8.76
.05-.01
Silt , .
36.02
19.04
38.97
47.32
34.92
.01-.005
Fine silt
14.99
20.88
8 84
10 04
12 14
.005- 0001
Clay
41 24
51 77
32 70
34.90
28 82
Total mineral matter ....
Organic matter, water, and loss
88.48
1.52
94.60
5.40
97.63
2.37
94.44
5.66
96.71
3.29
100.00
100.00
100.00
100.00
100.00
Many of the products of weathering of siliceous crystalline
and calcareous rocks are of economic importance as soils, clays,
and iron ores, as elsewhere noted. The kaolin beds of northern
Delaware and southwestern Pennsylvania are mainly decom-
posed, highly feldspathic, gneissic rocks, and which as dug
from the pits still retain their gneissic structure, but which
are now plastic clays full of angular quartz fragments, mica
scales and feldspar particles in various stages of decomposi-
tion. The change that has taken place consists in a kaoliniza-
tion of the feldspars, whereby the alkalies are largely removed,
and a residue consisting essentially of a hydrous silicate of
1 Bull. No. 21, Maryland Agricultural Exp. Station, by Milton Whitney, 1893.
RESIDUARY DEPOSITS
309
alumina left in their place. The quartz granules are disaggre-
gated, and their surfaces sometimes slightly etched by the
action of the alkaline car-
bonates ; the black mica,
where such existed, de-
composed, giving rise to
rust-colored spots. The
material is dug from the
pits and washed with wa-
ter to separate the im-
purities, the " kaolin " or
clay remaining in sus-
pension, and being ulti-
mately saved by filtration
through canvas. This
finest material, as seen
under the microscope, still
contains particles of un-
decom posed feldspars and
slnvils of white mica, to-
gether with other extremely irregularly outlined, sometimes
almost amoeba-shaped forms, as shown in Fig. 29. An average
of two mechanical analyses of this clay, made under Dr. Whit-
ney's direction, yielded the results given below : —
FIG. 29. — Showing, on the left, the mineral
kaolinite as seen under the microscope, and
on the right, washed kaolin.
MoiSTt'RB IN A IK-
DRY MATERIAL AT
100° C.
M • • I- 1 i l: F ON
IGNITION
SILT
.05-.01 MM.
FINE SILT
.01-.005 MM.
CLAV
.005-.0001 MM.
0.41 %
11.41%
31.79%
7.31 %
47.78%
Chemical analyses of the same material, made in the labora-
tories of the United States Geological Survey, yielded : — ^
Silica (SiO2) 48.73%
Titanic oxide (Ti02) 0. 17
Alumina (A12O8) 37.02
Ferric iron (Fe2O8) 0.79
Lime (CaO) 0.16
Magnesia (MgO) 0.11
Potash (K2O) 0.41
Soda (NaoO) 0.04
Water at 100° 0.52
Ignition 12.83
Phosphoric acid (P2O6) 0.03
100.81 %
310 THE KEGOLITH
Among the special names that have from time to time been
given to local phases of residuary accumulations, there remain
two, the laterite and wacke, which are sufficiently common to
merit some attention. The first mentioned of these, laterite,
like loess and several other terms that might be mentioned,
has to a considerable extent lost its true lithological signifi-
cance through careless usage. Originally the name was applied
to a vesicular highly ferruginous clay, soft in the mass, but hard-
ening on exposure to the weather, and which has a wide distribu-
tion throughout India and Ceylon. Two forms are commonly
recognized, — the one capping the summits of hills and plateaux
on the highlands of central and western India, and underlain
by the Deccan traps; and the second occurring on the lowlands,
in part overlying gneisses and granites. The prevailing colors
of the laterite, when freshly broken, are various tints of brown,
red and yellow mottled, or whitish ; after exposure it is usually
t covered "with a brown or blackish brown coating of limonite.
When first dug out, the material is sufficiently soft to be cut
with a pick or shovel, but becomes greatly indurated on expos-
ure. In some instances the material is of so compact a texture
and so hard as to resemble jasper. In many forms of laterite
the material is traversed by "small irregular tortuous tubes
from a quarter of an inch to upwards of an inch in diameter."
These penetrate the mass in all directions, though most com-
monly nearly vertical, and are often lined with a coating of
limonite. On weathering, these give rise to extremely irregu-
larly pitted or scoriaceous surfaces, which, together with the
dense, often botryoidal structure, cause it to resemble certain
types of igneous rocks, for which it has more than once been
mistaken. The more massive forms show usually a horizontal
banding. Some forms of laterite show a brecciated structure,
due to its detrital fragments becoming recemented into masses
closely resembling the original rock. The high level form, that
which occurs capping the hills and plateaux on the highlands
of central and western India, is fine grained and compact and
of a fairly homogeneous structure, although the iron oxide may
be somewhat irregularly distributed and sometimes segregated
in pisolitic nodules sufficiently abundant to form an ore. The
lower level form, that which covers large areas of both east and
west coasts, frequently contains grains of sand and pebbles
embedded in a ferruginous matrix. It is, as a rule, less homo-
LATERITE AND WACKE
311
geneous than the high level form, but nevertheless passes into it
by insensible gradations.
The origin of both high and low level forms of the laterite
has been the subject of much speculation. It is probable that
all of it is of a residual nature, i.e. represents the less soluble
portions of pre-existing rock masses. That which is found on
the high levels occurs overlying the Deccan trap sheets, into
which it can in many instances be traced, proving conclusively
its origin from this rock by the ordinary processes of weather-
ing. The low-lying variety can, in many instances, in like man-
ner be traced back to its origin from more siliceous, gneissic, and
granitic rocks. A part of the material, however, has the ap-
pearance and structure of a clastic rock of sedimentary origin,
and so it is considered by the best authorities to be.
The chemical composition of a very ferruginous laterite from
Rangoon is as below : —
CONSTITUENTS
INSOLUBLE
SOLUBLE
BULK
Silica (SiOj)
30.728 %
I 2.728 j
I 6.802
6.848%
5.783
46.279
0.742
. 0.090
37.676%
I 62.802
6.892
Alumina (A1»O8)
Iron sesquioxide (FuoOs)
Lime (CaO)
Alkulit's
40.258
69.742
100.00 %
100.00%
"The surface of the country composed of the more solid
forms of laterite is usually very barren, the trees and shrubs
growing upon it being thinly scattered and of small size. This
infertility is due, in great part, to the rock being so porous that
all the water sinks into it, and sufficient moisture is not retained
to support vegetation. The result is that laterite plateaux are
usually bare of soil, and frequently almost bare of vegetation."1
Wacke is an old German name now but little used, designat-
ing the gray, brown to black earthy residue or clay resulting
from the decomposition in place of basic eruptive rocks, as
1 Manual of the Geology of India, by R. D. Oldham, 2d ed., 1893, pp. 369-390.
312 THE REGOLITH
basalt, melaphyr, etc. In composition the material naturally
varies with the character of the rock from which it was derived,
and the amount of decomposition and leaching it may have
undergone.
It seems advisable to call attention here, a little more emphati-
cally, to the fact that the same processes which in ages past
have been instrumental in the formation of sandstones, shales,
slates, or marls are to-day, and have in late Tertiary and in
Quaternary times, given us soils; in other words, many of our
soils are but secondary rocks in a state of loose consolidation,
and many of the accumulations classed as residual were de-
rived by disintegration, in situ, of alluvial materials ; materials
brought down years ago and deposited in shallow seas. The
amount of consolidation undergone by the more recent of these
sediments has in many instances been so slight that on elevation
above the water level they are ready almost at once to assume
the role of soil with little if any preparatory disintegration.
Nevertheless consistency demands that such be here grouped
as residuary.
Over what is known as the coastal plain of the middle Atlan-
tic slope, a narrow belt bordering on the Atlantic and extending
from the Hudson River on the north to the Roanoke on the
south, have been deposited in late Mesozoic and Tertiary times
a series of gravels, sands, and clays which constitute the well-
known Potomac, Appomattox, and Columbian formations of
Darton, McGee, and others. These are all detrital deposits
from the eastern Appalachian regions, brought down by streams
and deposited in the shallow estuaries and deltas of these
periods, but which have remained in a condition of slight con-
solidation, and through subsequent elevation and weathering
form the soils. Such vary widely and abruptly. In the region
northeast of Washington^ the Potomac formation consists of
feldspathic sands, gravels, and clays irregularly bedded and
often enclosing notable accumulations of rounded pebbles of
quartzite brought down from the Appalachian and Piedmont
regions. The Appomattox formation, from which was derived
surface soil in the vicinity of the Rappahannock and Appomat-
tox in Virginia, is a yellowish or orange-colored clay and sand
with sometimes interbedded gravel. The Columbian formation,
which yields the surface soil of the main portion of Washington
City and the immediate valley of the Potomac and contributary
CUMULOSE DEPOSITS
313
streams southward, is a delta and littoral deposit made up of
materials worked over from the older Potomac and Lafayette
formations and also of granitic sands and clays from the decom-
posed rocks of the Piedmont plateau.
The clays of the Potomac formation above mentioned are not
infrequently sufficiently homogeneous and plastic to be utilized
in the manufacture of brick, tiles, and pottery. The following
table shows the finely comminuted condition of the materials
which go to make up these clays in Maryland, as determined
by Whitney.1
DIAMETER
MM.
CONVENTIONAL NAMES
KED CLAY,
TILE
RED CLAY,
PUDDLING
BLUE CLAY,
STONEWARE
2-1
Fine gravel
0.00 %
0.31 %
OOO0/
1-.5
Coarse sand .
0.00
082
000
.S-.26
Medium sand
0.50
2.69
029
.25-.!
Fine sand
2.63
323
1 27
.1-.05
Very fine sand
9.02
889
893
.06-.01
Silt
25.13
26.17
20 16
.01-.005
Fine silt
13.44
11.18
1672
.005-.0001
Clay
42.34
42.36
50.02
3
Total
93.76 %
95.65 %
97 39 %
Organic matter, water loss . .
6.24
4.35
2.61
(2) Cumulose Deposits. — To be classed with the sedentary
deposits, in that they result from the gradual accumulation of
material in situ, but differing radically in both composition and
origin" from those just described, are those portions of the rego-
lith which result from the gradual accumulation of organic
matter with only small amounts of foreign detritus ; which are
made up almost wholly of the combined accumulations, organic
and inorganic, of growing plants. Such may not infrequently
be found in all stages of formation, in enclosed ponds or lakes,
without appreciable inlet or outlet, being merely due to stand-
ing water in low places. "Such pools, when not exposed to
periodical drying up, are invaded by a peculiar vegetation, first
mostly composed of confervae, simple thread-like plants of vari-
ous color and of prodigious activity of growth, mixed with a
mass of infusoria, animalcules, and microscopic plants, which,
1 Bull. 4, U. S. Dept. of Agriculture, 1892.
314 THE REGOLITH
partly decomposed, partly containing the floating vegetation,
soon fill the basins and cover the bottom with a coating of
clay-like mould. So rapid is the work of these minute beings,
that in some cases from 6 to 10 inches of this mud is deposited
in one year. Some artificial basins in the large ornamental
parks of Europe have to be cleaned of such muddy deposits
of floating plants, mixed with small shells, every three or four
years.
" When left undisturbed, this mud becomes gradually thick
and solid ; in some cases, of great thickness ; affording a kind of
soil for marsh plants, which root at the bottom of the basins or
swamps and send off their stems and leaves to the surface of
the water or above it ; where their substance becomes in the
sunshine hard and woody.
" As these plants periodically decay, their remains of course
drop to the bottom of the water ; and each year the process is
repeated, with a more or less marked variation in the species
of the plants. After a time the basins become filled by these
successive accumulations of years or even centuries, and the
top surface of the decayed matter, being exposed to atmospheric
action, is transformed into humus and is gradually covered by
other kinds of plants, making meadows and forests. In other
cases when basins of stagnant water are too deep for vegetation
FIG. 30. — Section across a small lake, a, bed rock ; 66, drift ; cc, growing peat ;
dd, decaying peat ; ee, climbing bog.
of aquatic plants, nature attains the same result by a different
special process ; namely, by the prolonged vegetation of certain
kinds of floating mosses, especially the species known as sphagna.
These grow with prodigious speed, and expanding their branches
in every direction over the surface of ponds or small lakes, soon
cover it entirely. They thus form a thin floating carpet, which
as it gradually increases in thickness serves as a solid soil for
another kind of vegetation, — that of the rushes, the sedges, and
some kinds of grasses, which grow abundantly mixed with the
mosses, and which by their water-absorbing structure furnish
CUMULOSE DEPOSITS
315
a persistent humidity sufficient for the preservation of their
remains against aerial decay. The floating carpet of moss be-
comes still more solid, and is then overspread by many species
of larger swamp plants, and small arborescent shrubs, especially
those of the heath family ; and so, in the lapse of years, by the
continual vegetation of the mosses, which is never interrupted,
and by the yearly deposits of plant remains, the carpet at last
becomes strong enough to support trees, and is changed into a
floating forest, until, becoming too heavy, it either breaks and
sinks suddenly to the bottom of the basin, or is slowly and grad-
ually lowered into it and covered with water."1
It is to such processes that are due, in large part, the inland
swamp soils of many localities. Beginning at and near the
shore and upon a soil of wet sand, the organic matter has accu-
mulated year by year till now several feet in thickness and in
some cases covering miles of territory. The proportion of or-
ganic matter in such a deposit naturally increases from the shore
outward until in the upper and central layers it may comprise
90 % of the total weight.
This feature is well brought out in the following analyses of
material from an open ground prairie swamp in Carteret County,
North Carolina.
CONSTITUENTS
I
II
Silica (insoluble) (SiO2)
80.84%
1.52 %
Silica (soluble) (SiO,)
3.70
0.00
Alumina (AloOs) ...
2.69
0.39
Oxide of iron (FegOg)
1.18
0.15
Lime (CaO)
0.44
0.36
0.22
0.14
Potash (K2O)
0.07
0.06
Soda (Na,O)
0.02
0.13
Phosphoric acid (PjOj)
0.08
0.06
Sulphuric acid (SOg)
0.06
0.00
Chlorine (Cl)
Trace
0.02
Organic matter (C)
7.70
87.25
Water (H2O)
2.50
9.60
Column I of the above is from the margin — the oak fringe —
of this great swamp, near North River, about 8 miles north of
1 Geol. Survey of Pennsylvania, 1885, p. 106.
316 THE REGOLITH
Beaufort ; it is light gray to ash-colored with a growth of white
oak, gum, maple, pine, and palmetto trees ; the situation is low
and flat. " This margin belt of semi-swamp is from a half mile
or less in width to above a mile. The surface rises towards the
interior and is covered by a soil, if it may be called such, repre-
sented by column II, which is 2 to 3 feet deep and upwards, and
lies on a bed of white sea-sand. It consists of a loose open mass
of half-decayed woody matter, of a brown color, and is in fact
a superficial, uncompressed lignite ; for it will be observed that
the analysis includes nearly 10 % of water, so that the dry sub-
stance would give but 3^ % of inorganic matter, not more than
would be accounted for by the ash of the woody matter. The
growth is a dense thicket of spindling shrubs with small scat-
tered maples and bays."1
Wiley has described 2 deposits of a somewhat similar nature
as covering 1,000,000 acres in the Kissimmee valley of Florida.
These, which are of a dark brown to deep black color, contain
in some cases as much as 96.16% of volatile matter, and vary
from 3 to 20 feet in depth. Such, when properly drained, may
be made extremely fertile, though in periods of drought endan-
gered by fire which, once started, may burn for months, doing
immense damage. The partially reclaimed areas of the Great
Dismal Swamp of Virginia are fair representative types of
swamp soils.
The formation of cumulose deposits is not, however, limited
to lakes, stagnant ponds, or even to swamps as the word is ordi-
narily used, excepting as the swamp itself may be incidental
and consequent. Regions of poor drainage, particularly in
moist and cool climates, may give rise to growths of sphagnous
mosses and subsequently to plants of a higher type, which in
course of years assume no insignificant proportions.
In accounting for such accumulations, we have but to remem-
ber that ordinarily when a plant dies, its organic constituents
are returned to the atmosphere once more in a comparatively
brief period of time through the usual processes of decay. It
needs only such conditions of moisture as shall prevent the
complete decay and hence favor the accumulation of the organic
matter, to give us beds of peat and ultimately of coal. Plants
of the type of sphagnous mosses, growing continuously above
1 Geology of North Carolina, Vol. I, 1875.
2 Agricultural Science, Vol. VII, No. 3, 1893, pp. 106-120.
SWAMP DEPOSITS 317
and dying beneath, hold in their mass sufficient moisture to
exclude atmospheric air, and thus themselves bring about the
proper conditions for bog making. In virtue of this property
such may gradually rise above the level of the surrounding
country, as is the case with the Great Dismal Swamp of Vir-
ginia and numerous others that need not be mentioned here.
Instances are on record where bogs of this nature have grown
so far above the natural level, that during seasons of unusual
rainfall they have burst, and flooded adjacent regions, with dis-
astrous results. The rate of growth of such accumulations is
naturally quite variable. H. S. Gesner, as quoted by T. Rupert
Jones,1 states that in Bavarian moors the observed increase in
peat, in forty-five years, amounted to from 2 to 3 feet in thick-
ness ; in Oldenberg, in one hundred years, to 4 feet ; in Ham-
melsmoor, Denmark, to 2£ feet; and in Alpine districts to 4 and
5 feet in from thirty to fifty years.
The peat bogs, so characteristic of Ireland, Scotland, and
other northern latitudes, are of this type. A section of the
well-known Bog of Allen, made in county Kildare, is given
below.2
THICKNESS
(1) Dark reddish brown; mass compact; no fibres of moss visible ;
surface decomposed by atmosphere 2 feet
(2) Light reddish brown ; fibres of moss very perfect 3 "
(3) Pale yellowish brown ; fibres of moss very perceptible 6 "
(4) Deep reddish brown ; fibres of moss perceptible 8J "
(6) Blackish brown ; fibres of moss scarcely perceptible, contains
numerous twigs and small branches of birch, elder, and fir . 3 "
(6) Dull yellow-brown ; fibres not visible ; contains much empyreu-
matic oil ; mass compact 3 "
(7) Blackish brown ; mass compact; fibres not visible ; contains much
empyreumatic oil 10 "
(8) Black mass, very compact ; has a strong resemblance to pitch or
coal ; fracture conchoidal in all directions ; lustre shining . . . 4 "
Total depth of bog 38£ feet
Underlaid by 3 feet of marl containing 64 % carbonate of lime, 4 feet of blue
clay, and this in its turn by clay mixed with limestone gravel of an unknown
thickness.
1 Proc. Geologists' Association, Vol. VI, No. 5, January, 1880.
3 T. Rupert Jones, Proc. of the Geologists' Association, London, Vol. VI, No.
5, January, 1880. This authority classifies the peat bogs, swamps, and marshes,
as follows : —
I. Peat bogs and turf moors on such plateaux as flat mountain tops and wide
hill moors.
318 THE REGOLITH
Deposits of the cumulose type pass by all gradations into
the paludal, swamp, or marsh type and these in turn into ordi-
nary alluvium. Or it would perhaps be better to reverse this
order, since, as in the gradual silting up of an enclosed lake,
we may have, in the first stages, stratified alluvium, then when
the waters become sufficiently shallowed, swamp and muck
deposits, and lastly the deposits of pure organic, or cumulose
material.
2. TRANSPORTED MATERIALS
Because of the constant action of gravity, the well-known
transporting power of water, the wind or moving ice, few re-
sidual products retain for any length of time their virgin purity,
but become more or less contaminated with materials from near
or distant sources. The avalanches of mountain regions afford
an illustration of the bodily transfer of, it may be, millions of
tons of matter from the mountain slopes to be debouched into
the valley below ; the slow-creeping glacier brings down its
load and deposits its moraine when, succumbing to the blan-
dishments of warmer climes, it is no longer able to bear it fur-
ther : spasmodic winds catch up the smaller particles as clouds
of dust to be transported, assorted, and redeposited as their
II. Peat bogs of valleys : (1) At the heads of valleys ; (2) at the salient angles
within river curves ; (3) in deserted beds of rivers ; (4) in plains and lakes of
expanded valleys ; (5) special peat bogs of Denmark and the black earth of Rus-
sia ; (6) river deltas ; (7) maritime peat marshes, where certain valleys and plains
open to the sea.
Regarding the black earth of Russia, it should be stated that this is now
regarded by at least one authority (Hume, Geol. Mag., Vol. I, No. 2, 1894) as
being but a local phase of the loess, the color being due to the prevalence of
organic matter.
Shaler (Ann. Rep. U. S. Geol. Survey, 1888-89), on a 'basis of physical char-
acters, classifies the inundated lands of the United States as below : —
r A , ( Grass marshes.
Above mean tide . . - 1 Mangrove marshes.
Marine marshes J r Mud banks.
[Below mean tide . . . { Eel.grass areas.
<• . < Terrace.
River swamps . . . .tEstuarine.
/ Lake margins.
Fresh-water swamps
Lake swamps . . . . j Quaking bogs.
f Wet woods.
Upland swamps . . . { Climbing bogs.
.Ablation swamps.
COLLUVIAL DEPOSITS
319
force is spent. It is, however, through the continual transpor-
tation of running streams, both in the past and present, and
through the action of moving ice in ages gone, that have been
brought about the great amount of transportation and admixt-
ure characteristic of that part of the regolith comprised under
the general name of drift. According to which of the agencies
enumerated prevailed, we may subdivide our subject as follows :
(1) Colluvial deposits, (2) alluvial deposits, (3) seolian de-
posits, and (4) glacial deposits, though as we proceed we shall
in id that the lines of separation are not in all cases sharply
drawn, and in many an area the regolith bears impress of com-
pounded agencies.
(1) Colluvial Deposits.1 — Under this head it is proposed to
include those heterogeneous aggregates of rock detritus com-
monly designated as talus and cliff debris. The material of
avalanches may also be classed here. Such result
"~\ wholly from the transporting action of gravity. The
"X..
&
deposits in themselves are comparatively limited
in extent, ever varying in composition, and are
\ composed of an indiscriminate admixture of
X particles of all sizes, from those as fine as
dust to blocks it may be of hundreds of
tons' weight. Such are necessarily limited
to the immediate vicinity of the cliffs or
mountains from which they are derived.
As loosened by heat or frost from the
FIG. 31. — Diagram
showing the history
of a talus, a, bed
rock ; 66, talus ; c, de-
stroyed portion of a cliff,
the material being now in
the talus.
Sail bearing portion
-"
parent masses, the fragments
tumble down the slopes, gradually
accumulating in beds the slope of which is limited only by the
laws of gravity and the character of the debris. (See PL 23.)
Inclinations of 30° are common ; less commonly of 40°. From
1 From the Latin "colluvies," a mixture. The term as here used is more
restricted in its meaning than as used by Professor Hilgard.
320 THE REGOLITH
their mode of origin it is natural that the individual particles
should be mainly angular and comparatively fresh. In fact,
they represent rock- weathering through disintegration, and
not decomposition, which will come later. Below, i.e. further
down the slopes and in the edges of the valleys, these coarse,
illy assorted deposits pass gradually into soils ; above, they
consist simply of masses of loose rock wholly unfitted for the
support of vegetable life. (Fig. 31.) Through becoming sat-
urated with water, ice, or snow, such at times become loosened
from the steep slopes on which they lie and slide down in the
form of avalanches into the valleys. Although comparatively
limited in their extent, these latter, owing to the resistless
energy and suddenness of their advance, are sometimes appall-
ingly destructive, as has been repeatedly illustrated in the Swiss
Alps, and other mountain regions. The geographic distribution
of talus deposits as controlled by climatic conditions has been
already noted (p. 283).
(2) Alluvial Deposits. — The deposits included under this
head differ structurally from those thus far described in that
they are always more or less distinctly stratified, or bedded.
In writing of the formation of sedimentary rocks, and again
when treating of the action of running water, a few figures
were given relative to the amount of transported debris de-
posited yearly in the Gulf of Mexico. In a similar way the
amount of debris carried annually to the ocean by some of the
chief rivers of the world has been estimated as below : —
CUBIC FEET
Mississippi . . . . . 7,468,694,400
Upper Ganges . . . 6,368,077,440
Hoang-Ho 17,520,000,000
CUBIC FEET
Rhone 600,000,800
Danube 1,253,738,600
Po 1,510,147,000
. * •
The muddy condition of the water, caused by this sus-
pended matter, is so conspicuous a feature of certain rivers that
they have received special names on this account. Hoang-Ho
means simply yellow river ; Missouri is the Indian name
for Big Muddy ; while the famous Red River of the North is
so called merely because of the red mud it carries. Such silt-
bearing streams, flowing into lakes and tideless seas, begin
depositing their loads so soon as their currents are checked,
building up thus the so-called delta deposits for which the
Mississippi, the Po, Ganges, and the Nile are noted.
The character of the material in the delta deposits is vari-
ALLUVIAL DEPOSITS
321
able only within certain limits, consisting always of siliceous
sand and mud intermingled with organic matter.
Professor Judd, who examined samples from borings in the
alluvial deposits of the Nile delta, found the materials to vary
abruptly in texture from the surface downward, the variations
following no recognizable law. The percentage amounts of
constituents classed as sand and mud, as obtained from (I)
borings at Kasr-el-Nil, Cairo, (II) Kafr-ez-Zayat, and (III)
Tantah, are given in the table below.
i ii m
DEPTH
SAND
Mil'
SAND
MUD
SA.ND
MUD
01
10
%
01
h
%
%
o/
/o
3'0" . . .
2.35
97.65
4'0" . . .
30.42
69.58
1.71
98.29
6'0" . . .
6.77
94.33
....
8' 6" . . .
7.27
92.73
11' 0" . . .
60.99
49.01
10' 0" . . .
86.27
13.73
• • • •
17' 6" . . .
79.65
20.35
18' 0" . . .
....
....
8.78
91.22
19' 0" . . .
....
....
87.41
12.59
22' 6" . . .
....
....
31.16
68.44
26' 0" . . .
90.19
9.81
31' 0" . . .
....
39.43
60.57
35' 0" . . .
....
86.42
13.58
38' 6" . . .
65.05
34.95
....
....
....
40' 0' . . .
81.94
18.06
80.70
19.30
40' 0' ...
80.83
19.17
B g
45' 0' ...
68.72
31.28
....
....
46' 0' . . .
....
....
....
95.90
4.10
48' 0' . . .
....
87.23
12.77
....
65' 0' . . .
0.25
99.75
97.71
* . • .
66' 0' ...
....
....
99.53
2.29
68' 0' . . .
. .•. .
....
3S.09
0.47
60' 0' ...
....
12.60
87.40
....
40.91
66' 0" . . .
....
....
62.07
37.93
68' 0" ...
....
7.76
73' 0" . . .
69.95
92.24
75' 0" . . .
....
66.38
36.62
40.05
The material described as sand consists of rounded, angular,
and sub-angular grains. The well-rounded granules are mainly
of quartz and feldspar ; the angular and sub-angular of quartz,
feldspars, hornblende, and augite, with smaller quantities of
mica, tourmaline, sphene, iolite, zircon, fluor-spar, and magnetite
322 THE REGOLITH
i
all in a nearly unaltered condition. The feldspars are mainly
orthoclase and microcline — rarely a soda-lime variety — and
in a state of surprising freshness. The quartz is in part the
quartz of granitic rocks and the larger grains well rounded,
best described as microscopic pebbles. He says : " It is evi-
dent that these sand grains have been formed by the breaking
up of granitic and metamorphic rocks, or of older sandstones
derived directly from such rocks. The larger grains exhibit
the perfect rounding and polishing now recognized as charac-
teristic of aeolian action ; the smaller ones from their larger
surfaces in proportion to their weight, have undergone far less
attrition in their passage through the air ; but it is fair to con-
clude that they are really desert sand, derived from the vast
tracts which lie on either side of the Nile valley, and swept
into it by the action of the wind." The material described
as mud is composed of essentially the same materials as the
sands, but in a more finely divided state. There is an entire
absence of anything like kaolin, though there are present
particles of organic matter and frustules of diatoms. The
surprising freshness of the materials and lack of kaolin is
regarded as indicative of an origin through the action of heat
and frost ; i.e. through mechanical agencies rather than through
the processes of rock decomposition.1
But, as has been already noted, only a part of the sediment
carried by any stream reaches its mouth. A comparatively
small, but, from our present standpoint, very important portion
is carried during seasons of high water beyond the usual chan-
nels and spread out over the flood plains, as described on p. 287.
Such deposits are, as a rule, plainly stratified, and consist of
mineral matter in a finely comminuted condition derived, it may
be, from the breaking down erf a great variety of rocks. Their
physical and chemical properties, as well as the periodic char-
acter of their deposition, are favorable to the formation of soils
possessing great strength and fertility. Both fertility and rate
of deposition in such cases are augmented through plant growth,
which takes place with great rapidity wherever climatic condi-
tions are favorable. So soon as the water leaves the flood plain,
a host of moisture-loving plants, as reeds and rushes, spring up
in countless numbers to die down again in the fall, and yield
the carbon and nitrogeneous constituents to serve as fertilizers,
i Proc. Royal Soc. of London, Vol. XXXIX, 1885, p. 213.
ALLUVIAL DEPOSITS 323
and augment the crop of the following year. Moreover, the
remaining stems and fallen leaves of the plants serve to retard
the running waters of each succeeding flood, catching in their
meshes the floating sediments which might otherwise be carried
seaward. The Anacostia, which empties into the Potomac River
east of Washington, serves as a good illustration of the working
of these agencies. A century ago the stream was navigable by
coasting crafts as far as Bladensburg. Now, owing to shallow
waters, nothing but rowboats can navigate beyond the Navy
Yard at Washington. Each season the stream, murky with
suspended silt from cultivated fields along its shores, comes
down, till, ponded back by tides, it begins to deposit its load.
As year by year its bed was thus raised, water plants, encroach-
ing more and more from shallow shores, still further dammed
FIG. 32.
its sluggish current till now, during summer months, it is little
more than a stagnant pond full of rank vegetation, and a source
of odors foul and atmospheres enervating. The so-called
" Potomac Flats " south of the city of Washington owed their
origin and unhealthy conditions to similar processes.
The method of alluvial deposition iii^the flood plain, or
delta, of the lower Mississippi has been worked out by McGee,1
from whom we cannot do better than quote in considerable
detail.
In length this flood plain reaches from the mouth of the Ohio
1100 miles measured along the river, or half as far measured
in an air line, to the Gulf, and is bounded on the east by the
bluff rampart separating it from the contiguous district ; it is
bounded on the west by a less continuous and less conspicuous
rampart crossing the Arkansas River at Little Rock and grad-
ually failing southward until this district and its more westerly
1 The Lafayette Formation, Ann. Rep. U. S. Geol. Survey, 1890-91.
324 THE REGOLITH
neighbor nearly blend. The surface of this otherwise monoto-
nous district is relieved by a few small tracts of higher land.
Most conspicuous of these is Crowley Ridge in eastern Arkansas,
a long belt of upland stretching from the southeastern Missouri
southward between the White and St. Francis rivers to the
Mississippi at Helena. This belt of upland rises 100 or 200 feet
above the insulating flood plain, and in its steepness of slope
and rugosity of outline . fairly simulates the eastern rampart
overlooking the " delta " in corresponding latitudes.
The vast lowland tract comprised in and constituting most of
this district is at once the most extensive and most complete
example of a land surface lying at base-level or a trifle below
that the continent affords.
It is trenched longitudinally by the Mississippi, and trans-
versely by the White, Arkansas, Red, and other large rivers ;
between these greater waterways it is cut into a labyrinth of
peninsulas and islands by a network of lesser tributaries and
distributaries, the former gathering the waters from its own
surface and from adjacent country, and the latter aiding the
main river to discharge its vast volume of water and its immense
load of detritus into the Gulf. The whole surface lies so low
that it is flooded by periodic overflows of the Mississippi and
its larger tributaries, and with each flood receives a fresh coat-
ing of river sediment ; and much of the flood plain, fertilized by
freshet deposits, is clothed with luxuriant forests and dense
tangles of undergrowth, or with brakes of cane, or with sub-
tropical shrubbery, only a few of the broader inter-stream tracts
being grassed. Partly by reason of this mantle of vegetation,
the current of each overflow is checked as the river rises above
its banks, and most of the sediment is dropped near by; and so
the Mississippi, the White, the Arkansas, and the Red, as well
as each lesser tributary and each distributary from the great
Atchafalaya down, are flanked by natural levees of height and
breadth proportionate to the depth and breadth of the stream.
The network of waterways is thus a network of double ridges
with channels between; and each inter-stream area is virtually
a shallow, dish-like pond in which the waters of the floods lie
long, to be drained finally, perhaps, through fresh-made breaks
in the natural dikes, weeks after the stream flood subsides. In
the southern part of the district the inter-stream basins approach
tide level and drain still more slowly ; in the sub-coastal zone
ALLUVIAL DEPOSITS
325
many of the basins are permanent tidal marshes. In the western
part of the district is an area in which the inter-stream basins
lie so high that they are invaded only by the highest floods and
veneered with only the finest sediments ; in some cases these
sediments are so fine and so compactly aggregated and the
surface is so ill drained and watered that trees may hardly
take root, and these are either drowned by the floods or with-
ered by the sun in the drought. Such portions of the sur-
face are but scantily covered with coarse grass and form
the "black prairies" of southern Arkansas and northwestern
Louisiana.
It is to just such processes as those described that the Nile
valley owes its remarkable fertility. The sediments depos-
ited over these plains during the season of freshets consist of
fine sand brought down by the Blue Nile and the Atbara from
the decomposing siliceous rocks of mountainous Abyssinia. The
gneisses and granites yield their detritus to the lixiviating in-
fluence of the mountain torrents and majestic Nile, the clayey
p.trticles being borne seaward, while the fresh quartzose, feld-
sjiathic and other siliceous particles, and smaller traces of apa-
tite and alkaline carbonates remain in just the right stage of
subdivision to yield a soil, which has brought forth for a period
of over 4000 years crop after crop without artificial fertilization.
The following table will serve to show the physical character-
istics of alluvial deposits, a portion of which are but reasserted
materials from the glacial drift.
APPROXIMATE NUMBER OF GRAINS OF SAND, SILT, AND CLAY IN ONE GRAMME
OF ALLUVIAL SUB-SOIL FROM ILLINOIS
DIAMETER
CONVENTIONAL
(a)
<«
(c)
MM.
NAMES
CniLLICOTRE
KOCKFOBD
AMEBICAN BOTTOMS
2-1
Fine gravel .
0
1
0
1-.5
Coarse sand .
83
48
0
.5-.25
Medium . . .
6,755
3,428
5
.25-.!
Fine saud . .
18,660
29,300
194
.1-.05
Very fine sand
53,470
212,400
151,400
.05-.01
Silt ....
4,670,000
6,888,000
12,230,000
.01-.005
Fine silt . .
86,860,000
115,100,000
195,600,000
.005-.0001
Clay ....
2,637,000,000
3,842,000,000
14,680,000,000
Total. . . .
2,628,608,968
3,963,232,177
14,887,981,599
(a) Terrace of Glacial age.
(bottom land of Mississippi) .
(6) Flood deposits, (c) Post-glacial terrace
326 THE REGOLITH
The same processes active in delta formation are manifested
on a smaller scale in the gradual silting up of many inland
lakes, particularly those of glacial origin, the rapidity of the
filling being augmented by aquatic plants.
These lakes lie not infrequently between high hills, being
fed by one or more streams flowing through narrow valleys,
and having outlets at the opposite extremity. Soon after the
close of the Glacial epoch, we may imagine one of these to
have existed as a lake of clear blue water of varying depths,
filled with abundant fish and wild fowl. But the little streams
which fed it brought down continually sand and silt to be de-
posited at varying distances so soon as the currents fall to sleep
within the bosom of the lake. Hence each year it shallows,
and the pure white water-lily, reeds, and the rotting trunks of
trees and shrubs encroach upon its shores until in course of
time there remains but a flat plain, for a time subject to annual
inundations, but ultimately permanently above the level of but
the most severe floods, and through which flow in a meander-
ing course the sluggish streams that first gave it birth and then
wrought its extinction. This is the story of thousands of the
so-called meadows, swales, swamps, and intervals throughout
the northern portion of the United States, and the process in
some easily recognizable stage may be found in almost any lake
or pond now remaining.
It is a striking thought that all our lakes are but transient
enlargements of pre-existing streams, and will in time, perhaps
even before our own species is extinct, become converted into
broad expanses of meadow lands ; and that our children's chil-
dren may yet sow and reap from rich and fertile areas which
now echo only to the cry of water-fowls, and whose blue ex-
panse is broken but by wind-born waves and leaping fish.
The lithological character of the deposits thus formed vary
within certain limits almost indefinitely, since everything de-
pends on the character and quantity of the silt brought down
by the streams. Rarely, if ever, are they clayey, since the finer
particles are carried beyond. In nearly all instances they are
found to consist of very fine sand, largely siliceous, permeated,
often quite blackened, through the presence of organic matter.
Such are the mucks or mucky soils of New England.
So abundant is this organic matter that, when dried, such are
not infrequently used locally for mulching purposes, though
ALLUVIAL DEPOSITS 327
in their fresh condition they are sour and almost worthless
except for growing sedges and the ranker kinds of forage
grass. During the later stages of the process of filling up,
deposition of sediments may almost entirely cease, since the
water no longer rises above the level of past accumulations.
In such cases the final stages consist simply in the accumu-
lation of organic matter and the deposits come to closely
resemble, or are even superficially identical with, the cumulose
deposits already described. This same statement holds good
also for the closely related salt-water marsh or paludal deposits,
to be noted later.
Loess and Adobe. — Under the head of transported deposits,
we must also consider the so-called loess of the Mississippi val-
ley in our own country ; of the Rhine valley, and other parts
of Europe ; of northern China and the Russian steppes, though,
as we shall see, the name includes deposits which, while having
many physical properties in common, may vary widely in com-
position as well as in method of deposition. It is more than
doubtful, indeed, if the name, through misapprehension, has not
been so loosely applied as to rob it of its proper geological
significance.
The loess of China, made famous through the researches of
Richtofen, is now regarded by some authorities l as of the
same nature as our adobe. Richtofen himself, it will be re-
membered, regarded the Chinese loess as largely an aeolian
deposit, as due to the action of wind in transporting for long
distances the fine detritus swept by rain and wind from moun-
tain slopes into enclosed basins, to ultimately become entangled
and deposited among the growing vegetation. This foreign
material, intermingled with the collective residue of herba-
ceous plants, with the inorganic residuum from the decay of
prairie vegetation for countless generations, makes up its mass
over many hundreds of square miles of territory, and to depths
in places of thousands of feet. The characteristics of the loess,
as found in China, are those of a fine calcareous silt or clay, of
a yellowish or buff color, so slightly coherent that it may be
readily reduced to powder between the thumb and fingers, and
yet possessing such tenacity as to resist the ordinary weather-
ing action of the atmosphere, and, wherever cut by stream
erosion or other means, to stand with vertical walls, even
1 See I. C. Russell, Subaerial Deposits of North America, Geol. Mag., August, 1889.
328 THE REGOLITH
though they may be hundreds of feet in height. The loess
country is described as thus cut up by an almost impassable
system of gorges, so that to cross it in any fixed direction is
almost an impossibility. "Wide chasms are surrounded by
castles, towers, peaks, and needles, all made up of yellow earth,
between which gorges and chasms radiate labyrinthically up-
wards into the walls of solid ground around. High upon a
rock of earth — steeper than any rock of stone — stands the
temple of the village, or a small fortress which affords the
villagers a safe retreat in times of danger. The only access
to such a place is by a spiral stairway dug out within the mass
of the bluff itself. In this yellow defile there are innumerable
nooks and recesses, often enlivened by thousands of people,
who dwell in caves dug in the loess."1
One of the striking features of the loess, both in China and
elsewhere, is the abundance of minute tubes or canals — lined
with carbonate of lime — which traverse it from above down-
ward, and which are assumed by some to be due to root fibres.
It is the presence of these presumably that causes the vertical
cleavage, and at the same time the remarkable absorptive quali-
ties for which the loess is noted. Such is the material which
for more than three thousand years has brought forth crops
continuously, and without exhaustion, over many square miles
of the Chinese Empire. Its distribution in Europe is given as
extending from the French coast at Sangatte, eastward across
the north of France and Belgium, filling up the depressions of
the Ardennes, passing far up the valleys of the Rhine and its
tributaries, the Neckar, Main, and Lahr ; likewise those of the
Elke above Meissen, the Weser, Mulde, and Saale, the upper
Oder and Vistula. Spreading across upper Silesia, it sweeps
eastward over the plains of Poland and southern Russia, where
it forms the substratum of the tschernoseun, or black earth.
It extends into Bohemia, Moravia, Hungaria, Galicia, Transyl-
vania, and Roumania far up into the Carpathians, where it
reaches heights of from 2000 to 5000 feet above sea-level. In
northern China it spreads over a large portion of the region
drained by the Hoang-Ho. For nearly a thousand miles
from the borders of the great alluvial plain of Pechele, through
the provinces of Shan si, Shensi, and Kansu, everywhere to the
1 The Chinese Loess Puzzle, by J. D. Whitney, American Naturalist,
December, 1877.
LOESS AND ADOBE
329
northern base of the range of the Tsing-ling-shan, the loess
may be followed to the very divide which separates the basin
of the Hoang-Ho from the region destitute of drainage, into
the sea. Toward the north it reaches almost to the edge of
the Mongolian plateau. The entire area covered continuously
is stated to be as large as the whole of Germany, while it is
found in more or less detached portions over an area in addi-
tion, nearly half as large. In the United States the loess
covers thousands of square miles throughout the drainage
basin of the Mississippi River. It is found in Ohio, Indiana,
Michigan, Iowa, Kansas, Nebraska, Illinois, Tennessee, Ala-
bama. Mississippi, Louisiana, Arkansas, Missouri, Kentucky,
and the Indian Territory. According to Professor Aughey it
prevails over at least three-fourths of Nebraska, to a depth
ranging from 5 to 150 feet, and furnishes a soil of extraor-
dinary strength and fertility. As here found, however, the
aeolian hypothesis fails to satisfactorily explain all the exist-
ing conditions, arid there is little doubt but that it represents
in large part the fine silt, the glacial flour brought down by
the ice of the Glacial epoch, borne southward by streams, and
deposited in water just sufficiently in motion to carry the fine
clay farther away. The
loess, in fact, illustrates
in a remarkable manner
the wonderful assorting
power of water.
.Microscopic and chemi-
cal examinations of loess
sustain this hypothesis.
The particles are as a rule
quite fresh and sharply
angular. Out of 150,000
particles examined under
the microscope only about
3 % measures above .0025
of a millimetre and 1 %
over .005 of a millimetre.
Quartz is the prepon-
derating material, with
lesser amounts of orthoclase and plagioclase feldspars, white
and dark micas, hornblende, augite, magnetite, dolomite, and cal-
FIG. 33. — Showing outlines of particles in
Chinese loess.
330
THE UEGOLITH
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LOESS AND ADOBE
331
cite. The loess of the Rhine valley and of China offers no differ-
ences that can be readily described, though, as will be noticed
by reference to the analyses, there may be a wide difference in
chemical composition. Indeed, the essential characteristic of
the loess is a physical rather than a chemical one, and it is
doubtless to this that is due its uniform fertility. On p. 330
are given analyses of loess from the United States, the Rhine
valley, and from Switzerland.
The following table will serve to show the fine state of sub-
division in which the particles exist in loess as well as in the
dust brought down by snow, which will be described on p. 344.
I
II
ill
IV
CONSTITUENTS
UPLAND LOESS :
VIRGINIA Cmr,
ILLINOIS
RIVER LOESS :
VIRGINIA CITY,
ILLINOIS
LOESS :
NEBRASKA
DUST FROM SNOW :
ROCKY ILLE,
INDIANA
Moisture
5.40%
3.17%
Organic matter '
4.96
11.98
Gravel
0.00 %
0.00 %
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.01
0.10
0.00
0.00
7.68
24.84
23.14
0.00
Silt
61.85
60.98
54.81
69.37
9.60
2.80
2.46
5.80
Clay
15.15
6.16
9.45
9.68
Aughey l gives the following section of the loess and soil in
Nebraska.
(1) Loess 4 feet
(2) Black soil 2
(3) Loess 4
(4) Black soil
(6) Loess 6
(6) Black soil
(7) Stratified loess 15
This alternation is accounted for on the assumption of fre-
quent changes of level during the loess-forming period. It
would seem that the loess was deposited in shallow water and
that as the lake became filled plant life came in as in modern
i Physical Geology and Geography of Nebraska, p. 276.
332 THE REGOLITH
bogs and marshes, and throve until sufficient organic matter was
formed to make the black soil layer. A period of subsidence
followed, more loess was deposited and the previous condition
repeated, this process going on till all the layers were formed.
The material of the loess, in this case, would seem most likely
to have been of seolian origin.
The name adobe is given to a calcareous clay of a gray-brown
or yellowish color, very fine-grained and porous, which is suffi-
ciently friable to crumble readily in the fingers, and yet, like
loess, has sufficient coherency to stand for many years in the
form of vertical escarpments, without appreciable talus slopes.
The material of the adobe is derived from the waste of the
surrounding mountain slopes, the disintegration being largely
mechanical. According to Prof. I. C. Russell,1 from whose de-
scriptions is drawn a portion of what is given here, it is assorted
and spread out over the valley bottom by the action of ephemeral
streams, where it becomes mixed with dust blown by the winds
from the neighboring mountains, and rendered more or less
coherent by the cementing action of interstitial carbonate of lime.
Hilgard 2 limits the name adobe to the distinctly clayey soils of
the arid regions, and divides them into two classes, — the upland
and the valley adobes, the first being derived mainly from the
disintegration, in place, of clay shales, while the second are
mostly paludal or swamp formations, and represent either the
finest materials that remain suspended in slack water, from any
source, or sometimes the direct washings of the clayey soils of
the hills. Whichever authority we follow, it is evident the
name includes materials alike not in mode of origin or com-
position, but only in physical characteristics.
Adobe forms the soil of a large portion of the rainless region
of the United States. It is found therefore in Colorado, Utah,
Nevada, southern California, Arizona, New Mexico, and west-
ern Texas, as well as in the southern portion of Idaho, Wyoming,
and Oregon. It has also a wide distribution in Mexico. In
the United States it occurs from near the sea-level in Arizona,
and even below it in southern California up to an elevation of
at least 6000 or 8000 feet along the eastern border of the Rocky
Mountains, and in the elevated valleys of New Mexico, Colorado,
and Wyoming.
1 Subaerial Deposits of North America, Geol.. Mag., August, 1889.
2 Bull. 3, U. S. Weather Bureau, Dept. of Agriculture, 1892.
LOESS AND ADOBE 333
The maximum thickness of the various deposits grouped
under this name is not in all cases readily determined, for the
reason that it is still accumulating and has not been sufficiently
dissected by erosion to expose sections to any considerable
depth. Many of the valleys of the arid region have been filled
by it to a depth of 2000 or 3000 feet. In the larger valleys
there are rocky crests, called " lost mountains," which project
above the broad level desert surface, and which are in reality
the summits of precipitous mountains that have been almost
completely buried beneath these recent accumulations. The
prevailing color of adobe is light buff to gray, excepting when
contaminated with organic matter. In its typical form it is so
line as to be quite without grit when rubbed between the fingers.
When examined under the microscope, it is seen to be com-
posed of irregular unassorted flakes and grains, principally
quartz, but fragments of other minerals are also present. The
adobe of Salt Lake shows flocculent masses of amorphous matter,
which, when thoroughly disintegrated, are found to consist of
mi 11 ute sharply angular fragments of quartz and feldspar with
much calcareous matter, and only rarely a shred of micaceous
or hornblendic material. In size the particles vary from those
too small for measurement up to .08 millimetre in diameter.
The valuable characteristics of the adobe are its extreme
fineness, great depth, and wonderful fertility.
Although comprising the soil of almost the entire region that
was but recently known as the Great American Desert, it needs
but water to make it laugh with harvests. While its physical
properties undoubtedly have much to do with its fertility, this
quality must also be in part due to the fresh and undecomposed
condition of its constituent parts. Originating doubtless by
purely mechanical agencies, it has been swept by winds and
spasmodic rains into closely adjacent basins occupied by but
temporary lakes, where, spread out over a floor sometimes almost
absolutely level, it has been subjected to a minimum amount of
leaching and retains until to-day its youthful strength and
powers of recuperation.1 The analyses given on p. 334 will
serve to show the varying character of the deposits included
under this name. Especial attention need only be called to
the relatively high percentages of lime and the alkalies.
Under the head of alluvial deposits we may also consider
i See further on p. 369.
334
THE REGOLITH
those clay accumulations which result from the deposition of
fine aluminous sediments sorted by running streams from gla-
cial debris and like the loess laid down in quiet water, though
usually estuarian rather than lacustrine. These are the well-
known Leda clays1 of glacial regions, and which on genetic
grounds might well be classed as aqueo-glacial deposits.
CONSTITUENTS
I
II
Silica (Si02)
66.69%
44.64 %
Alumina (Al2Os)
14.16
13.19
Ferric oxide (Fe20g)
4.38
5 12
Manganese oxide (MnO)
0.09
0.13
Lime (CaO)
2.49
13.91
Magnesia (MgO)
1.28
2.96
Potash (K2O)
1.21
1.71
Soda (Na2O)
0.57
0.59
Carbonic acid (CO2)
0.77
8.55
Phosphoric acid (P2Os)
0.29
0.94
Sulphuric anhydride (SOs)
0.41
0.64
Chlorine (Cl)
0.34
0.14
Water (H20) .
4.94
3 84
Organic matter
2.00
3.43
99.72%
99.84 %
I. Adobe from Santa Fe, New Mexico. II. Adobe from Fort Wingate, New
Mexico.
Such are very abundant along all the lower valleys of the
principal rivers of New England, sometimes coming to the im-
mediate surface or overlaid with a thin layer of sandy material
which, together with a little organic matter, forms the true soil.
They form, according to Dawson,2 the sub-soils over a large part
of the great plains of Lower Canada, varying in thickness up
to 50 or even 100 feet, usually resting upon the boulder clay.
They are, as a rule, of almost impalpable fineness, unctious, and
extremely plastic. Excepting where superficially oxidized to
buff or brown, they are of a blue-gray color and may show on
analysis considerable quantities of lime carbonate and alkalies,
features whereby they are readily distinguished from the resid-
ual clays, and which are regarded as indicative of an origin by
mechanical rather than chemical means. When dried, they be-
come greatly indurated, and when unmixed with other mate-
rials, bake so hard during seasons of drought, or are so plastic
1 So called from their most characteristic fossil, Leda. 2 The Canadian Ice Age.
THE CHAMPLAIN CLAYS
335
during seasons of rainfall, as to be quite unsuited for cultivation.
Mixed with varing proportions of siliceous sand to counteract
shrinkage, they form the common brick-making materials of the
Northeastern states, burning red and brown.
The materials of the Leda clays naturally vary in different
localities, being dependent on the characteristics of the rocks
from which they were de-
rived. Those of Canada,
according to Dawson,
were derived from the
waste of the Utica and
Quebec groups. This
authority believes that
when the clay was in sus-
pension, it was probably
of a reddish or brown
color from the iron per-
oxide it contained, but
that, like the bottom mud
now forming in the deeper
parts of the St. Lawrence,
the coloring matter be-
came deoxidized by or-
ganic matter so soon as
deposited, the iron being
converted into a sulphide or protoxide carbonate. Inasmuch,
however, as the materials were so largely derived by the grind-
ing action of the glaciers on fresh rocks, it is not impossible
that they may have been again deposited as clay without hav-
ing ever undergone the oxidizing process.
Unlike the till or boulder clays, these Leda clays are dis-
tinctly stratified, as shown in the accompanying illustration.
(PI. 24.) An analysis of a sample from this locality yielded
the author results as given in column I on p. 336. In column II
is given that of the portion (33.26 %) soluble in hydrochloric
acid and sodium carbonate solutions, while in column III is given
the composition of a " semi-assorted glacio-lacustrine " clay
bordering on Lake Michigan near Milwaukee, Wisconsin, and
in IV a glacial pebbly clay underlying II at the same locality.1
1 Analyses II and III from Chamberlain and Salisbury's paper, 6th Ann. Rep.
U. S. Geol. Survey, 1884-85.
FIG. 34. — Showing particles from Leda clays.
1, quartz ; 2, orthoclase ; 3, plagioclase ; 4, mica ;
5, tourmaline ; 6, pyroxene ; 7, chlorite ; 8, horn-
blende.
336
THE KEGOLITH
ANALYSES OF STRATIFIED CLAYS
CONSTITUENTS
I
II
III
IV
Silica (Si02)
66.17 %
10.98%
40.22 %
48.81 %
Alumina (AljjOg)
24.25
8.66
8.47
7.54
Phosphoric acid (PjjOg)
Not det.
Not det.
0.05
0.13
Titanic oxide (Ti02)
Not det.
Not det.
0.35
0.45
Ferric iron (Fe2Os)
Not det.
Not det.
2.83
2.53
Ferrous iron (FeO) • .
3.54
5.191
0.48
0.65
Manganese oxide (MnO)
Not det.
Not det.
Trace
0.03
Lime (CaO)
2.09
1.02
15.65
11.83
Magnesia (MgO)
2.57
2.19
7.80
7.05
Potash (K2O)
4.06
1.12
2.36
2.60
Soda (Na^O)
2.25
0.75
0.84
0.92
Water (H2O)
4.69
3.65
1.95 2
2.02 2
Carbonic acid (CO2)
None
None
18.76
15.47
Organic carbon (C)
None
None
0.32
0.38
Sulphuric anhydride (S08) ....
None
None
0.13
0.05
99.56%
33.26 %
100.21 %
100.46%
Related to the delta deposits already described, but differing
in that their inorganic materials are in large part derived im-
mediately from the sea, are due to the transporting and assort-
ing power of tide and wave action, are the salt-water marsh, or
paludal deposits so common along the Atlantic border of North
America. In discussing the formation of these and their grad-
ual transitions into arable lands, we cannot do better than
follow Professor N. S. Shaler.3
The formation of a sea-coast swamp is due mainly to wave
action and plant growth. It is dependent upon the configura-
tion of the coast. Wave action upon an irregular coast such
as that of New England nearly always results in a breaking or
wearing away of the exposed headlands and the transportation of
the debris from these into intervening inlets, and thrown upon,
or at least in a direction toward, the beaches. On these beaches,
as one may any day observe, the rock fragments are ever being
ground smaller and smaller, and must in time be reduced to
the condition of the finest sand and mud. Each incoming
1 All iron determined as FeO.
2 Contains H of organic matter dried at 100° C.
8 Ann. Rep. Director of the U. S. Geol. Survey, 1884-.85.
SEA-COAST SWAMP DEPOSITS 337
wave hurls more or less of this fragmental material upon the
beach, whence a considerable portion of it may be again carried
seaward by the bottom current or undertow as the wave re-
cedes. One who has stood upon a high rock on the sea-shore
and watched the waves come tumbling at his feet and then go
creeping silently oceanward once more cannot have failed to
notice the continual seething sound due to the constant drag
of the rock fragments one over the other as they are impelled
inward and outward by the alternating currents. A consider-
able part of this mud is taken out to sea by the undertow, or
bottom current, which always sets from a storm-beaten beach
along the bottom, but another part is urged by the movement
of the water caused by the waves and of the tidal flow into the
fjords, where it falls to the bottom. In this process of carriage
the mud is generally conveyed along the shores and is most
commonly deposited in the parts of the inlets near the shore
line. Wherever there is a bay within which the tidal current
is deadened and where the waves have little play, this sediment
is most rapidly laid down. If the process of deposition begins
on a pebbly bottom, it is at first aided by the irregularities be-
tween the stones and the friction of the water among the sea-
weeds, which frequently attach themselves to the stones. As
soon as a sheet of mud is established, it commonly becomes
occupied by a dense growth of eel-grass. This plant, by its
habit of growth, greatly favors the deposition of sediment. The
separate stems are set very closely together, the interspaces not
generally exceeding 1 or 2 inches. A tidal current of 2 miles
an hour, swift enough to carry much sediment, is almost en-
tirely deadened in this tangle of plants.
At half tide on the New England coast these eel-grass fields
are generally covered with water to the depth of several feet ;
at this stage the tidal currents are commonly strongest. The
water above the level of the grass has its usual freedom of
motion and brings much sedimentary matter above the level of
the foliage. As the tide falls, a part of this waste is entangled
and held until it gradually sinks to the bottom, so that each
run of the tide gives a certain contribution of sedimentary
matter, which goes to shallow the water. This process is easily
observed from a boat floating over a field of these plants. The
deadening of the current when the lowered tide brings the tops
of the plants near the surface is very noticeable. The mass of
338 THE REGOL1TH
floating matter — mud, fronds of sea-weed (often with shells or
small pebbles attached to their bases), dead fish, and a mass of
other refuse, is seen to collect in the mesh of foliage and sink
to the bottom. The dead stems of the eel-grass and the bodies
of many small crustaceans and mollusca which live on its stalks
or on the bottom contribute to the deposit, so that it thickens
with considerable rapidity.
When the bed formed on the sea-bottom by the action of the
eel-grass and its associated plants has risen to the point where
it is dry at low water of the ordinary run of tides, the eel-grass
can no longer maintain itself, but gives place to other groups
of sea-weeds and grasses.
These species of plants find their place first near the shore
line, where the eel-grass platform is naturally the highest. At
first their vegetation is quite sparse, owing to the difficulty
with which they endure the depth of water at high tide. There
is often, indeed, a considerable difficulty in establishing the
growth of the second group of plants, and for a while the de-
posit takes the shape of bare mud-flats, dependent in the main
for their accumulation of detrital matter on the growth of
certain mollusca, especially of the genera Mytilus and Modiola.
Sea, T „,.
c LowTi
FIG. 36. — Cross-section of marine marsh, a, original surface of shore line ; b, grassy
marsh; c, mud-flats; d, eel-grass; e, mud accumulated in eel-grass.
When, as is usually the case, the more highly organized plants
have difficulty in establishing themselves over the broad sur-
face of the mud-flat, they win their way to it in the following
manner.
From the vantage ground of the shore line, where these plants
easily find the conditions of submergence which suit their needs,
the plants slowly extend the front of their bench out over the
mud-flats. (See Fig. 35.)
This process of growth can be more easily studied than that
of the earlier or eel-grass stage of the marshes, for it is visible
along miles of our sea-shore. The higher grasses have even
SEA-COAST SWAMP DEPOSITS 339
more thick-set stems than those of the eel-grass flats ; they en-
tangle sediment even more effectively. At first their steins are
covered for a few hours at each ordinary tide ; they gather
waste rapidly, and soon lift the plain which they are construct-
ing up to the point where only at the highest tides are the tops
covered by water. At this stage the growth of the deposit is
practically arrested, there being no means of increase save from
the decay of the grasses themselves.
" On the central parts of the New England shore, as about
Boston, the mud-flat occupies at most 2 or 3 feet in the alti-
tude above mean low tide and the annual addition to its mass
in a year is very small," perhaps not so much as the tenth of an
inch in a year. " On the other hand, in the Basin of Minas,
one of the principal inlets leading from the Bay of Fundy, the
contribution of sediment is so great that vast areas have been
easily reclaimed from the sea by building a rude enclosure
around an area of the higher parts of the mud-flat, so that the
speed of the sediment-laden waters is checked and they are
made to lay down their burdens. In a few years, often in a
few months, this enclosed area is raised to near the level of
high tides. It is then only necessary to erect a barrier suffi-
cient to exclude the tide, with gates for the rain water, in order
to have the land completely reclaimed from the sea. In this
simple way there has been an area of many thousand acres of
excellent arable land created along these shores."1
The lithological and chemical character of deposits of this
nature have been but little studied, and we are here able to give
only two analyses, as below, in which, however, it is probable
that the matter tabulated as insoluble silica includes as well all
silicates insoluble in acid.
Column I of the table is mud from the marshes of Newport
River, a few miles above Beaufort, in Carteret County, North
1 As the total reclaimable area between New York and Portland (Maine)
probably exceeds 200,000 acres, their money value in their best state will
amount to at least §40,000,000,000. The cost of reclaiming these areas and re-
ducing them to cultivation should not exceed the fifth part of that sum. It may
be noted that from the chemical composition of these soils, they are practically
inexhaustible, and that from their position they are often well placed for irriga-
tion. South of the New England shore the marsh area is much more extensive
than in that region. It is probable that the improvable marshes of the Atlantic
coast amount to at least 3,000,000 acres and they may exceed double this amount.
(Shaler, p. 380.)
340
THE KEGOLITH
Carolina. This marsh, formed by the filling up of the old river
channel, several miles wide, is continually enlarging at the ex-
pense of the water surface ; and similar formations, to the extent
of hundreds of square miles, are accumulating in very many
shallow bays and sounds and rivers near the sea.
CONSTITUENTS
I
II
Silica, insoluble
64.42 %
72.70%
Silica, soluble
1.92
Oxide of iron and alumina
16.45
5.69
Lime . . .
1.18
1.39
Magnesia
0.07
0.05
Potash
1.18
1.82
Soda
0.79
0.35
Phosphoric acid .
0.25
0.13
Sulphuric acid
1.46
0.33
Organic matter . . .
10.35
Water
20.92 "I
Oxide of manganese .* ...
0.54 J
3.65
Sulphide of iron
1.09
0.11
Common salt
1.63
1.71
99.98%
100.10%
Column II is the sea mud or slime which is deposited in the
shoal waters of Beaufort Harbor and along the sounds and estu-
aries of the North Carolina coast. It is a fine, dark-colored
salt mud, formed of the silt brought down by the rivers, mixed
with decaying vegetable matter (mostly sea-weed and marsh
grass), and animal remains, — of fish, molluscs, and all sorts of
marine organisms.1
What is described by Whitney 2 as a typical swamp bog or
peat soil, from a rice field near Georgetown, South Carolina,
yielded the results as below, in columns I, II, III, and IV, the
last two being simply recalculated from columns I and II on an
organic and water-free basis. These are the so-called sob-field
soils, in themselves poor, but responding readily to fertilizers.
When exhausted by cultivation, they recuperate quickly through
the aid of silt deposits from the rivers, brought about by the
continual ebb and flow of the tides.
1 Geology of North Carolina, Vol. I, 1875, p. 214.
2 Rice, Its Cultivation, Production, and Distribution, Rep. No. 6, Misc. Series,
U. S. Dept. of Agriculture, 1893.
BEACH SANDS
341
DIAMETER
OF
I
II
III
IV
PARTICLES
mm.
CONVENTIONAL NAMES
SOIL
0-6 inches
SUB-SOIL
6-9 inches
SOIL
0-6 inches
SUB-SOIL
6-9 inches
2-1
Fine gravel
0.00%
0.00 %
o.oo %
0.00 °L
1-.5
Coarse sand
0.71
0.08
1.36
0.14
.6-.-J5
Medium sand
2.70
0.25
6.18
043
.25-.!
Fine sand ....
0.83
0.13
1.69
0 23
.1-.06
.05-.01
Very fine sand ....
Silt
0.37
10.32
0.15
13.97
0.71
19 79
0.26
24 30
.01-.005
Fine silt
5.32
7.10
10.20
14 09
.006-.0001
Clay .
31.90
34.85
61.17
60 65
Total mineral matter . .
Organic matter, water loss
62.15%
47.85
57.53%
42.47
100.00 %
100.00 %
Loss by direct ignition .
100.00 %
47.36
100.00 %
39.65
Beach Sands. — Although differing radically in composition
from the sea-coast swamp deposits already described, we must, on
account of their intimate geological relationship, include here a
brief description of those fragmental deposits formed by wave
action along beaches and in many instances almost absolutely
free from organic matter of any kind. Such are the clean
white beach sands, the delight of the summer visitor at the sea-
sides. These are found here and there in isolated stretches
along the Atlantic slopes, particularly where, as at Old Orchard,
.Maine, they receive the full sweep of wave and tide from the
open sea. In many instances the material forming these beaches
is siliceous sand from glacial deposits which the ocean has
reasserted according to its own liking. In other cases it is
sand brought down by rivers, and which has undergone frac-
tional separation through the varying strength of transporting
agencies. In still others it is material derived immediately
from the shore rocks through the weathering action of atmos-
pheres and the hammering of the waves. In other cases yet,
as along the coasts of Florida, the source is problematical. We
can only say, knowing the character of rocks forming the main-
land, that they could not have here originated, but must have
been transported and probably down the coast, from the areas
of crystalline rocks to the northward. It is sometimes, though
342 THE REGOLITH
not always, possible to gain an idea of the probable source of
these sands through a study of their mineralogical nature and
the physical condition of the individual particles.
Sorby, who devoted careful attention to the microscopic ap-
pearance of granules of quartz sand belonging to various geo-
logical periods, divided them into five types, "which though
characteristically distinct, gradually pass into one another."1
These types are : —
1. Normal, angular, fresh-formed sand, as derived almost
directly from granitic or schistose rocks.
2. Well-worn sand in rounded grains, the original angles
being completely lost, and the surface looking like fine ground
glass.
3. Sand mechanically broken into sharp angular chips, show-
ing a glassy fracture.
4. Sand having the grains chemically corroded, so as to pro-
duce a peculiar texture of the surface, differing from that of
worn grains or crystals.
5. Sand in which the grains have a perfect crystalline out-
line, in some cases undoubtedly due to the deposition of quartz
over rounded or angular nuclei of ordinary non- crystalline
sand.
The material of most beach sands is largely quartz, though
this is not invariably so. Those of the Bermudas are, as a
matter of necessity, calcareous. Those of isolated deep-sea
islands like the Hawaiians, are derived in part from the vol-
canic rocks of the islands, and in some instances are composed
almost wholly of minute shells of the size of a pin's head.
These last from their faculty of emitting a crunching sound
when disturbed, are known as " sounding " or " singing sands."
The beach sand at Diamond Head, Oahu, is mainly of olivine
and magnetite granules, with smaller amounts of calcareous
matter. As usual, the grains in samples from the same level
are of fairly uniform dimension, varying from 0.5-1.0 millime-
tre, the larger forms being often fairly well rounded, while the
smaller may still show crystal outlines. The granules, even
in the same sample, however, vary greatly in the amount of
rounding they have undergone. Like the quartz granules from
the Florida beach, these show conchoidal chippings due to the
shock of impact as one granule strikes against another.
1 Proc. Geol. Soc. of London, Anniversary Address, Session, 1879-80, p. 58.
BEACH SANDS
343
Fio. 36. — Quartz granules in sand from beach,
Santa Rosa island.
The beach of Santa Rosa island, south of Pensacola, Florida,
is composed of clear white quartz sand of almost ideal purity.
The grains, though water-worn and with the lesser angles
rounded, are still in many
cases angular, and of very
uniform size (about .5-
1.0 millimetre), as shown
in Fig. 36. These gran-
ules offer a very beauti-
ful illustration of Sorby's
type No. 2, the surface of
<-;uh one, through abra-
sion, being reduced to the
condition of ground glass.
Kxamination with a high
power brings out minute
fractures and conchoidal
chippings, at once sug-
gestive of the prelimi-
nary stages of manufact-
ure of the quartz spheres
for which the Japanese are so noted. It is as though each
granule had been held in the hand of some pigmy aboriginal,
and its surface reduced by hammering with another pebble,
after the manner known among archieologists as "pecking."
The shape assumed by a rock or mineral fragment subjected
to \\ave action varies somewhat with the nature of the material,
schistose rocks and easily cleavable minerals naturally giving
rise to pebbles or granules of quite unequal dimensions in three
directions. The schist on the coast of Cape Elizabeth, Maine,
for instance, gives rise to pebbles in the form of a greatly flat-
tened oval, while the more homogeneous quartz, with which it
is associated, yields nearly spherical forms. " But of whatever
character the material, the normal shape of a beach-formed
boulder or pebble is oval, and this for the reason that the wave
action is a dragging rather than a carrying one ; the stone is
not lifted bodily and hurled toward the shore to roll back with
the receding wave, but is rather shoved and dragged along.
Gravity tends to hold the fragments in one position so that
the wear is greatest on the side which is down, and this in
itself would cause them to assume an oval or flattened form
344 THE KEGOLITH
even were they spherical and of homogeneous material at the
start."1
(3) JEolian Deposits. — That no sharp lines can in all cases be
drawn between alluvial and oeolian deposits has been made evi-
dent in our discussion of the loess and adobe. We will now con-
sider those deposits which owe their origin and present structural
features almost altogether, if not entirely, to wind action.
The efficacy of the wind as an agent of transportation was
dwelt upon in considerable detail on pp. 184 and 292. The
material thus carried into the air, often to great heights, is
brought to the surface again by gravity, though the normal rate
of descent is not infrequently greatly accelerated by rain or
snow. Indeed, the clearness, limpidity, of the atmosphere after
a rainfall is due simply to the fact that it has been washed, is
cleansed of its suspended impurities.
The very fogs which infest our cities, particularly those of
the soft coal regions, are but indices of the dust particles in the
atmosphere, each globule of fog being condensed about a nucleus
of floating matter.
The amount of this dust brought down even from moderately
clear atmospheres is often sufficiently abundant to attract the
attention of the most casual observer. Professor H. L. Bruner
of Irvington, Indiana, has stated 2 that during a snowstorm in
February, 1895, a layer of snow about one-fourth of an inch in
thickness was colored distinctly brown by the dust it contained.
One sample of snow collected yielded .37% of dust, by weight,
and it was calculated that dust was thus deposited at the rate of
30.7 pounds avoirdupois for each acre. Another observer calcu-
lated the fall as taking place at the rate of 12.77 pounds per acre.
From a gallon of water melted from a snowfall of but 4
inches, which fell in London in January, 1895, there was obtained
10.65 grains of solid matter, 5.75 grains being inorganic and
4.90 grains carbonaceous. Water from a snow collected near
the centre of the city, January 30 of this same year, gave 6.25
grains of mineral and 11.07 grains of carbonaceous matter. It
was also found that 75 % of these impurities were brought
down with the first 2 inches of the snowfall.
Dr. Whitney, who examined samples of the black earth
1 Merrill, Preliminary Handbook, Dept. of Geology, U. S. National Museum,
1889, p. 23.
2 Monthly Weather Review, U. S. Dept. of Agriculture, January, 1895.
PLATE 24
FIG. 1. Section of beds of Leda clay, Lewiston, Maine.
FIG. 2. Beds of volcanic dust, Reese Creek, Gallatin County, Montana.
vEOLIAN DEPOSITS
345
brought down near Rockville, Indiana, during a snowfall of the
winter of 1895, reported1 it as consisting of material almost
identical with the prevailing loess of that region, from whence
it was doubtless derived. The individual particles varied in size
between .10 and .05 millimetre. The results of a mechanical
analysis of the dust are tabulated with those of loess on p. 331.
Samples of the same dust submitted to microscopic examination
were found to consist of fully 96 % silt and 4 % organic matter,
the latter consisting mainly of fresh-water alga}, diatoms, fungi,
cells from decayed grasses, and shreds of woody tissue.
Hilgard, who has examined the so-called "dust soils" of
Oregon, California, and Washington, and which during the dry
seasons are so loose and fine as to rise in clouds at the merest
puff of wind, gives the following tables to show their
chemical and physical natures, and which he regards as fairly
typical for soils of the arid regions of the United States.2
CHEMICAL ANALYSES OF DUST SOILS
I
II
III
CONSTITUENTS
ATATHNAM
PRAIBIE,
Y SKIM \
CO IT NTT,
WASHINGTON
RATTLESNAKE
CREEK, KITTI-
TAS COUNTY,
WASHINGTON
PLATEAU ON
WILLOW CREEK,
MORROW
COUNTY,
OREGON
Insoluble matter
01
/o
71.67 )
%
78.33 1
<y
/o
79.21 \
Soluble silica
6.11 176'78
Vao} 80.53
2 30 i 81'51
Potash (KoO)
1.07
070
089
Soda (NagO)
0.35
0.24
0.05
Lime (CaO) .
2.00
208
137
Magnesia (MgO)
1.34
1.47
1.08
Brown oxide of manganese (MnsOi) .
Peroxide of iron (FejOs)
0.04
6.88
0.07
6.13
0.06
5.63
Alumina (AljOs)
7.91
6.12
6.02
Phosphoric acid (PjOg)
0.13
0.18
0.18
Sulphuric acid (SOs)
0.02
0.02
0.03
Water and organic matter ....
2.82
2.35
2.55
Total
99.33 %
99.90 %
99.35 %
Humus
4.10
0.44
Hygroscopic moisture
4.98
3.20
4.92
1 Monthly Weather Review, U. S. Dept. of Agriculture, January, 1895.
2 Bull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892.
CONVENTIONAL NAME
DIAMETER OF
PARTICLES
I
II
ill
Clay
.0023 mm.
0.93%
3.59%
1.27 %
Fine silt
.005-.011
30.93
13.06
32.29
Silt
.013-.027
3.20
5.82
12.75
Very fine sand
.027-.05
7.18
27.37
37.51
Fine sand
.05-. 122
21.88
43.78
10.92
Medium sand
.122-. 5
32.39
4.57
3.97
96.67%
98.18%
98.72%
Sand Dunes. — The influence of the wind in the formation
of sand hills or dunes, as they are commonly called, has received
attention on p. 184. A few words more regarding their physi-
cal qualities and lithological nature are here essential.
The effect of the single whirlwind or it may be that of the
more constant air current for days, weeks, or even months,
may be from a geological standpoint comparatively insignifi-
cant ; but they are, nevertheless, interesting, and at times
important. In certain regions of the West, and notably in
parts of the Colorado desert, as described by W. P. Blake, in
1853, all the fine loose sand on the surface of the ground is
blown away, leaving every pebble and boulder standing out
in strong relief from the hard sun-baked soil, or ledge of
bed-rock.
Under favorable conditions the material thus blown along
may gather in the form of dunes, which themselves travel
slowly across the country, ever changing their outlines like
drifts of snow. A few miles north of Winnemucca Lake, in
western Nevada, is a belt of these dunes described by geologist
Russell1 as fully 75 feet in thickness and about 40 miles in
length by 8 miles in breadth. These, under the restless
goading of the winds, are constantly varying in shape, and
though moving in mass probably but a few feet a year have
already, in more than one instance, made necessary the splicing
of telegraph poles to prevent the burial of the wires. Another
range of sand dunes, at least 20 miles in length, and forming
188
1 Geological History of Lake Lahontan, Monograph XI, U. S. Geol. Survey,
AEOLIAN DEPOSITS 347
hills 200 to 300 feet high, occurs on the eastern end of Alkali
Lake in the same state. On the eastern shore of Lake Michi-
gan are also dunes of sand sometimes 200 feet in height, and
which at Grand Haven and Sleeping Bear have drifted over
the adjacent woodlands, leaving only the dead tops of trees
exposed. Similar dunes occur frequently on the Atlantic
coast, as at Hatteras, Long Island, and Cape Cod. The island
of Bermuda is made up almost altogether of coral and shell
fragments. These are washed by the waves upon the beach,
dried by the winds, and blown gradually inland, thus forming
hills in some cases, as stated by Professor Rice,1 not less than
250 feet in height. In other instances, as at Elbow Bay, on
the south shore of the main island, the sand, like a huge
glacier, has quite filled a valley, and still progressing in a
mass some 25 feet in thickness, is covering houses, gardens, and
even woodlands, leaving, as at Lake Michigan, only the trunks
of dead trees standing partially exposed in the midst of sandy
plains.
One of the most interesting and remarkable of the many
regions for the observation of sand dunes, lies between Bor-
deaux and Bayonne in Gascony, and which has been admirably
described by Reclus.2 The sea here throws every year upon
the beach along a line 100 miles in length some 5,000,000
cubic yards of sand. The prevailing westerly winds, contin-
ually picking up the surface particles from the seaward side,
whirl them over to the inland or leeward slope, where they
are again deposited, and the entire ridge by this means alone
moves gradually inland. In the course of years there have
thus been formed a complex series of dunes all approximately
parallel with the coast and with one another, and of all alti-
tudes up to 250 feet. These are still marching steadily inward,
though at the rate of but 3 to 6 feet annually, and whole vil-
lages have more than once been torn down to prevent burial,
and rebuilt at a distance, to be again removed within 200
years.3
The lithological nature of the dunes is widely variable,
1 Geology of Bermuda, Bull. 25, U. S. National Museum.
2 The Earth, Atmosphere, and Life.
3 The church of Lege, owing to the encroachment of the sand dunes, was torn
down in 1690, and rebuilt at a distance of 2£ miles from its first site. By 1850
the dunes had traversed the intervening space, and again necessitated its removal.
348 THE REGOLITH
though naturally siliceous sand is the prevailing, constituent
in the majority of cases. J. W. Retgers describes1 the dune
sands of Holland as consisting principally of quartz granules,
together with those of garnets, augite, hornblende, tourmaline,
epidote, staurolite, rutile, zircon, magnetite, ilmenite, ortho-
clase, calcite, and apatite ; and, more rarely, microcline, cor-
dierite, titanite, sillimanite, olivine, kyanite, corundum, and
spinel. The majority of these minerals occur in the form of
well-rounded granules, though many of the garnets, zircons,
and magnetites show quite well-preserved crystal outlines. It
is noticeable that these sands contain no mica, although the
mineral occurs in the sea-sand, from whence the dunes are
derived. Retgers accounts for this on the supposition that
during the transportation of the material the mica folia be-
come so finely shredded as to be sifted out from the heavier
particles of sand, and quite dissipated. It is well to note that
the abrasive power of wind-blown particles is greater than
among those carried by water, since, as noted by Daubree, a
thin intervening film of water may serve to buoy up the gran-
ules, and keep them apart. To this fact is ascribed the angular
nature of many of the wind-blown grains, they having become
shattered through the shock of impact. This same authority
seems to think that with wind-blown sand, as with water-worn
material, there is a minimum limit, beyond which reduction
in size of particles rarely goes. This minimum he places at
about .25 millimetre in diameter. It seems, however, more
probable that attrition may go on to an almost indefinite limit,
but that the finer and lighter materials are driven farther
aAvay — perhaps not collecting in the form of dunes at all —
leaving, as one would naturally expect, the sands of any one
series of dunes of nearly uniform size.
It was noted by Blake during the surveys of the railway
routes to the Pacific that the wind-blown sands of the Colorado
desert were sometimes in the form of almost perfect spheres, all
their sharp edges and asperities having been worn away by
mutual attrition. The grains were composed mainly of quartz,
agate, garnet, and dark granules derived from the debris of vol-
canic rocks. In places there is a black iron sand, and usually
a considerable proportion of lime carbonate, as indicated by its
brisk effervescence when treated with acid. The sand dunes of
1 Neues Jahrbuch fur Mineralogie u. Geologie, etc., 1895, 1 B. 1st Heft, p. 22.
AEOLIAN DEPOSITS
the Bermudas, as elsewhere noted, are composed wholly of cal-
careous material from finely comminuted shells and corals, while
those of the Sevier desert region of Utah, as described by Gilbert,1
are of fine gypseous sand formed by the evaporation of the water
in the neighboring playa lakes.
Volcanic Dust. — The finely comminuted materials ejected
from volcanoes and caught up by atmospheric currents, as de-
scribed on p. 153, are sometimes carried long distances to be
again deposited either on land or in the water, forming loose,
often flour-like deposits of varying thickness. At various points
in Colorado, Kansas, Nebraska, Montana, and other of the West-
ern states, are remnant beds of fine volcanic dust such as must
originally have covered many square miles of territory, and the
materials of which were
derived from sources now
wholly obscured.2 The
illustration given on PI.
L*4 is from a photograph,
taken by the writer, of
one of these beds in
the lower Gallatin val-
ley, Montana. From the
height of the man's shoul-
der to his feet the bed is
of pure glassy dust, very
light gray in color, and
so fine and light that
when thrown into the
air it floats away at the
slightest breath. The fig-
ure given shows the ap-
pearance of this glass as seen under the microscope. Beds of
this nature upwards of 4 feet in thickness occur underlying the
loess or surface soil along the Republican River in Nebraska
and Kansas and even as far east as Omaha in the first-named
state. The source of their materials is problematical.
Deposits of this nature thus far described are of very recent
origin, and the beds loosely coherent. There are, however, good
* Monograph I, U. S. Geol. Survey, 1890.
2 See On -Deposits of Volcanic Dust and Sand in Southwestern Nebraska,
Proc. U. S. National Museum, Vol. VIII, 1885, p. 99.
FIG. 37. — Showing outlines of shreds of volcanic
dust, as seen under the microscope.
350
THE REGOLITH
reasons for supposing that similar processes were carried on in
the earlier stages of the earth's history, but that the peculiarly
susceptible deposits have since undergone such extensive altera-
tion as to be no longer recognizable as wind-drifted materials.
Where the material still exists as a surface deposit, it undergoes
ready decomposition on account of its porosity and easy perme-
ability. The character of the resultant soil is dependent some-
what upon the character of the material, which varies indefinitely.
The volcanic dusts are as a rule siliceous, more nearly allied to
the acid potash rocks than* to the basalts.
The analyses given below show the chemical nature of (I) a
fine, white, almost flour-like pumice dust from Harlan County,
Nebraska, and (II) of dune sands from the Pamlico Peninsula,
North Carolina. This last is described1 as a tolerably fine,
nearly white sand consisting of smooth, well-rounded grains,
mainly quartz, but containing also occasional shell fragments
and black granules of iron ore.
CONSTITUENTS
I
II
Silica (SiOo)
69.12%
92.12%
Alumina (A12O8) •>
17 64 \
5 29
Iron oxide (Fe2Og) /
Lime (CaO)
0.86
1.13
Magnesia (MgO)
0.24
0.03
Potash (K2O)
6.64
0.64
Soda (NaaO)
1.69
0.35
Sulphuric acid (SOa)
0.33
Ignition
4.05
0.60
100.23%
100.49%
(4) Glacial Deposits. — Under this name are included those
drift deposits which are the product mainly of glacial action,
though their immediate deposition may have been brought about
in part through the instrumentality of water. The strictly
aqueo-glacial materials have been noted under the head of
alluvial deposits.
Allusion has been already made to the manner in which gla-
ciers erode and transport. During a comparatively recent
period in geologic history, there appears- to have come over a
1 Geology of North Carolina, Vol. I, 1875, pp. 182-183.
GLACIAL DEPOSITS 351
portion of North America a gradual lowering of the normal
temperature or increase in the annual precipitation, or perhaps
both, until the condition of affairs now existing in northern
Greenland prevailed as far south as the 39th parallel of north
latitude. Now whether the ice sheet extended at any one
time over the aiva outlined below or whether there were
periods of advancement and retreat ; whether the glaciation
was produced by floating ice and local glaciers as argued by
certain Canadian geologists, or by a truly continental ice sheet
thousands of feet in thickness, are for our present purposes mat-
ters of slight concern. We have more to do with results than
methods. Suffice it for the moment, that over the entire north-
eastern part of the United States and eastern Canada, all the ex-
isting loose materials from rock decay that had been gathering
for untold ages was carried bodily northward, westward, or south-
ward, as the case might be. From over a considerable part of
southern New England the original residual soils were stripped
and dumped into the Atlantic, portions of the transported mate-
rial still protruding above sea-level in the forms known now by
the names of Nantucket, No Man's Land, and Block Island. In
process of this transfer the rocks were planed down to hard
fresh surfaces, over and upon which were deposited new mate-
rials from the north. It follows that over this entire glaciated
area, estimated by Upham 1 as some 4,000,000 square miles, with
the exception of a few comparatively insignificant patches here
and there, scarcely a foot of clastic matter is to be found that
is truly native. Wherever road cuts or stream erosion favors,
the regolith in various conditions of compactness may be found
lying directly upon the hard, smooth, and striated rock with
which it has perhaps no affinity in composition or structure.
The rotten and mechanically triturated detritus of many rocks
from many sources more or less admixed by the moving glacier
or commingled by resultant streams, is spread out to form the
soils on lands to which it is as truly foreign as are the emigrants
who land to-day upon our shores. The stone wall, built of
boulders found loose in the field, may consist of granites, dia-
bases, schists, or shales even though the underlying rock may be
a limestone ; or the wall may be of limestone though the coun-
try rock be a gneiss, or slate. A similar distinction exists in
the soil itself, which, while it may in part consist of the material
1 Ice Age in North America, p. 679.
352 THE REGOLITH
of these boulders in a finely divided state, is more likely to con-
sist of detritus of softer rocks which yielded more readily to the
abrasive force. Sand and gravel or clay, dust or mud, black
with organic matter or red-brown from iron oxides, the ad-
mixture is ever varying, dependent only on the nature of the
materials to the north. But the material of the glacial drift
is spread out over the land in a manner far from uniform and
under conditions widely variable. Following Professor Salis-
bury1 and others, we may, according to its physical charac-
ters and method of deposition, separate the deposits into
two general groups: (1) the stratified or assorted drift,2 and
(2) the unstratified or unassorted, the first having been laid
down under the influence of water and hence showing a more
or less stratified condition, while the second, deposited directly
from the ice, consists of a heterogeneous aggregate of coarse and
fine materials without evident marks of stratification. The two
forms are not always readily separable nor is their relative posi-
tion always the same, either one not infrequently occurring up-
permost, and " not rarely they alternate with each other several
times between the surface and the bottom of the drift."
A large part of the drift is composed of this unstratified and
unassorted material, consisting of clay, sand, gravel, and boulders
in ever-varying proportions, and to which the name till or
boulder clay is commonly applied, or from its mode of deposi-
tion, that of ground moraine. As already noted, it is the
material carried along bodily beneath the ice sheet and left
in the position it now occupies on its final retreat. This,
entirely unmodified except upon the immediate surface where
it has become converted into soil through the agencies else-
where described, forms the regolith over large areas of the
northeastern portion of America and of northern Europe as
well. Where as yet unaffected by oxidation, it is of a gray
or blue-gray color, and often so intensely tough and hard as to
necessitate, in process of excavation, recourse to blasting. The
upper portion, through percolation of meteoric waters, is as a
rule of a buff or brownish color, owing to oxidation of the
ferruginous constituents. Through the combined agencies of
this oxidation, of plant and animal life and of cultivation,
considerable contrasts in both physical and chemical properties
1 Ann. Rep. State Geologists of New Jersey, 1891.
2 Here included in large part with the aqueo-glacial deposits.
GLACIAL DEPOSITS 353
are brought about between the superficial and deeper-lying
portion, which are commonly recognized by the terms soil and
sub-soil respectively applied to them, though originally they
may have been one and the same thing. The composition of
tliis till naturally varies with the character of the rocks from
whence it was derived. It may have, and indeed probably Aas,
in most cases travelled but a short distance, and its constituent
particles may be the same as that of the rocks which it overlies,
though in a finely divided condition, only the harder and
tougher rocks retaining their lithological identity, • while the
more friable, like the shales and sandstones, have been ground
to the condition of clay and sand. To attempt to give then
the composition of the till would necessitate its study and analy-
sis in innumerable localities, — an endless and profitless task.
It \\ ill be sufficient to here describe a few representative occur-
rences. In nearly all till the bUnkteTs, consisting of the harder
and more resistant of the materials, are in a more or less rounded
or rhomboidal form, with their surfaces scarred and with other
murks of the rough treatment to which they have been subjected.
They are in fact the tools with which the glacier has done its
work, and these scars are but the signs of wear. Intermingled
with these is an ever- variable amount of finer detritus largely
a result of mechanical abrasion. Professor W. O. Crosby, who
has studied in great detail the physical properties of the till
about Boston, states 1 that, excluding the larger stones, it con-
sist s of 25 % of coarse material which may be classed as gravel ;
20 % of sand ; 40 to 45 % of extremely fine sand, or rock flour,
and less than 12 % of clay. The gravel in these cases consists
mainly of pebbles of the harder and more massive rocks of the
region, such as granite, diorite, diabase, quartzite, and sandstone.
In passing from sand to gravel, there is noted an increase in the
proportional amount of quartz, in clear and angular or sub-
angular forms, due mainly to the disintegration of the granite,
qnartzite, and sandstone pebbles. The "rock flour" also con-
sists essentially of quartz. The most striking feature here
brought out is the very small proportion of actual clay material,
which varies from one-tenth to one-eighth of the total bulk.
The following table, as given by F. Leverett, shows the
approximate physical condition of the till as represented by
the sub-soil in various parts of Illinois.
i Proc. Boston Soc. of Natural History, 1890, p. 123.
2 A
354
THE REGOLITII
<M CO O CO O O
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CHAMPAIGN
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GLACIAL DEPOSITS 355
The till is not, however, always spread out evenly over the
land, but though partaking in a general way of the topography
of the slopes which it covered, lies much deeper in certain
places than others. Indeed, it thickens and thins out very
irregularly and in many places fails entirely either through
having never been deposited, as over many a rocky hillside in
New England, or through having been removed by running
water. Moreover, there are found in certain parts of the drift-
covered areas rounded hills of very symmetrical form, composed
of material identical with the till, but which must have been
deposited under slightly different conditions. These range in
height up to 200 or 300 feet, though rarely more than half that
amount. Such forms are known as drumlins.
The moraines, as already noted, represent those portions of
the ice drift which gathered near the edge of the ice sheet in
the form of submarginal accumulations, to be left as broad belts
or ridges of sand and gravel on its retreat. Such with refer-
ence to their position to the margin of the ice are known as
terminal, marginal, and frontal moraines. The materials of
which they are composed represent (1) that which accumulated
beneath the edge of the ice while it was practically stationary
for a considerable length of time ; (2) that dumped from the
surface at its margin; and (3) that pushed up by the ice sheet,
in front of itself during its forward movement. Such ridges
are not sharp as a rule, but broad and low, it may be from a
fraction of one to several miles in width. Unlike the subgla-
cial drift, — the till, — the materials are but loosely consolidated,
and but a small part, if any, of the boulders show the scarred
and abraded surfaces so characteristic of those of the till proper.
This frontal moraine, occupying the southern and western
margin of the glaciated area, forms one of the most striking
and unique of geological bodies. Composed of materials of a
most heterogeneous nature, ever varying, and limited in range
of variation only by the lithological character of the rocks to
the northward and eastward ; in all degrees of coarseness and
fineness, from boulders of many tons' weight to particles too
small to be visible to the unaided eye, only obscurely and some-
times scarcely at all stratified excepting where subsequently
modified by running water ; in the form of broad low hillocks,
domes, and ridges, — the moraine sweeps in an interrupted, sin-
uous belt from eastern Massachusetts to North Dakota and over
356 THE REGOLITH
400 miles into British America, having a length, in all its wind-
ings and turnings, of not less than 3000 miles.
The water arising from the melting ice sheet flowed off, in
part, over the surface, forming superglacial streams, or in part
upon the surface of the ground beneath as subglacial streams,
of which last the river Rhone of to-day is a good example.
Presumably also a portion of the water became concentrated
and flowed for short distances in the mass of the ice itself,
forming thus englacial streams. In all cases the running water
would collect, reassert, and variously modify the rock debris
found either in immediate connection with the ice itself or at
its extremity, in the terminal moraines. There were thus
formed hillocks and ridges or low fan-shaped masses of " modi-
fied drift." The sand, gravel, and boulders which collected in
the troughs of superglacial streams would, on the final melting
of the ice, be deposited as ridges running essentially parallel
with that of the movement of the ice on which they formed.
Such are known as eskers, or osars. Other deposits closely
resembling these and sometimes confounded with them, but
formed, it is believed, only by swift and changeable currents
near the frontal margin of the ice, present often a rude and
disturbed and distorted stratification, and are known as kames.
They differ from the eskers in their outlines as well as positions
with reference to the glacier from whence their materials were
derived, being as a rule in the form of hills, rather than ridges,
and with their longer axes at right angles with that of the ice
motion.
Beyond the margin of the ice and its terminal moraines are
found still other loosely aggregated deposits of a similar hetero-
geneous nature which are likewise due to swiftly running water
caused by the melting ice. Such, according to their position
and form, are known as valley drift, morainic or frontal aprons,
and overwash plains.
The thickness of these glacial deposits varies greatly, as has
been already indicated. Variations of upwards of a hundred
feet may occur within the limits of even less than one square
mile. Professor Newberry estimated that the area south and
west of the Canadian highlands cove'red with glacial drift was
not less than 1,000,000 square miles, and that its average
depth would not be less than 30 feet. Other estimates on
deposits in Ohio, Indiana, and Illinois give an average thickness
PLATE 25
FIG. 1. Section of glacial till. FIG. 2. Glaciated landscape.
THE SOIL 357
in these states of 62 feet. In extreme cases the deposit
has been found to extend to a depth of 300 to 500 feet. Bell
has stated l that glaciation of the surface of British America has
been almost universal in the regions east of the Rocky Moun-
tains, and all over the Palaeozoic districts west and south of
Hudson and James Bay the average depth of the till is 100 feet,
and perhaps 200 feet in Manitoba and the northwest territories.
The following section is given by James Geikie 2 as showing
the varying character of the glacial drift and its interstratified
interglacial lacustrine deposits : —
FEET INCHES
Sandy clay 5 0
Brown clay and stones (till) 17 0
Mud 15 0
Sandy mud 31 o
Sand and gravel 28 0
Sandy clay and gravel 17 0
Sand 5 0
Mud 6 0
Sand 14 0
Gravel " 30 0
Brown sandy clay and stones (till) .... 30 0
Hard red gravel 4 6
Light mud and sand 1 8
Light clay and stones 6 6
Light clay and whin block 26 0
Fine sandy mud 36 0
Brown clay, gravel, and stones 14 4
Dark clay and stones (till) 68 0
355 0
3. THE SOIL
There remains now to be summarized a few of the character-
istics of those superficial portions of the regolith to which the
name soil is commonly applied, and these, too, only in direct
relation to their properties as soils, since as integral portions
of the regolith they have already been sufficiently touched
upon.
(1) The Chemical Nature of Soils. — The prevailing con-
stituent of any soil, whatever its source, is nearly always silica,
with varying amounts of alumina, oxides of iron, lime, magne-
sia, and the alkalies.3 A small amount of organic matter, from
1 Bull. Geol. Soc. of America, Vol. I, 1890, p. 289.
2 The Great Ice Age, 3d ed., 1894, p. 120.
8 The peat deposits furnish almost the only exception to this rule.
358 THE EEGOLITH
extraneous source, is usually present. This prevalence of
silica, as may be readily understood, is an essential conse-
quence of soil formation through the breaking down of rocks
by the processes of weathering, whereby all but the most in-
destructible portions are lost.
The predominantly inorganic nature of any soil may easily
be shown by fractional separations, made either by washing,
or by sieves of varying degrees of fineness, whereby it is
brought into portions of like size and weight such as can con-
veniently be submitted to microscopical and chemical analyses.
All portions, from the finest dust to particles of such size as to
be classed as pebbles, will thus be found to be but mineral
matter, particles of quartz, feldspar, shreds of mica, and other
silicates in ever-varying proportions and stages of alteration
or decomposition.
Owing to the destructive nature of their formation, it is but
natural that a soil, particularly one of considerable antiquity,
should but slightly resemble the parent rock. This fact was
more than suggested in the chapter on rock-weathering. In
order that its significance may be fully comprehended, the
analyses of fresh rock and corresponding residual material from
various sources are given in the table on p. 359.
The most striking of the dissimilarities shown by this table
are, as is to be expected, those of the limestone soils, as in
columns I and II, where the proportional amounts of silica,
iron, and alumina are increased, roughly speaking, nearly one
hundred fold, while the amount of lime carbonate is corre-
spondingly diminished. This condition of affairs is still further
exaggerated in the case of the red soil of Bermuda (columns
III and IV) and which offers particularly favorable opportuni-
ties for study, owing to the isolated condition of the islands
and the consequent freedom from danger of contamination by
other than local drift.
The shells and corals which in a more or less consolidated
condition form the entire mass of these islands, although es-
sentially of carbonate of lime, are nevertheless not entirely so,
carrying, aside from the magnesia, about 1 % of inorganic im-
purities, chiefly oxides of iron and alumina and earthy phos-
phates, which are practically insoluble in the water of rainfalls,
with which alone we have to do here. As time goes on, the
lime is slowly leached out and carried away into the ocean, the
CHEMICAL NATURE OF SOILS
359
f.
3
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Phosphoric acid (
i
S
S S3
360 THE REGOLITH
insoluble parts remaining. Throughout the centuries of de'cay,
this 1 % of insoluble impurities, representing but one ton of
earth to every 99 tons removed, slowly accumulates until it
forms the common red earth of the islands. Though usually
fertile, in numerous instances where the leaching has been ex-
cessive the resultant soil is so rich in iron and other deleterious
constituents as to be quite barren.
There are few more impressive facts in agricultural geology,
than that each foot in depth of such soil, as it now lies at our
feet, may indicate the removal of at least 100 feet in actual thick-
ness of limestone. In other words, assuming that nothing has
been lost by mechanical erosion, the surface of the ground has
been lowered this much in bringing about the present conditions.
From what has gone before, it is obvious that soils derived
by purely mechanical agencies will, if unmixed with other ma-
terials, show a composition closely resembling the mother rock,
as in the case of that derived from granite as described on p. 207
or those derived from argillites and siliceous sandstones ; others
in which chemical agencies prevailed may by solution and other
changes have so far lost important constituents as to be scarce
recognizable as rock derivatives at all. Obviously a rock mass
containing in itself none of the elements of plant food cannot,
merely through its decay, furnish soil of appreciable fertility.
This fact is well illustrated in the region known as the Bare
Hills north of Baltimore, Maryland, or the Chester County
Barrens in southern Pennsylvania. Both regions are under-
laid by peridotites — rocks rich in iron-magnesian silicates, but
almost wholly lacking in lime, potash, or other desirable con-
stituents. Such rocks not merely decompose very slowly, but
the stingy product of such decomposition consists only of hya-
line forms of silica, magnesian carbonates, or silicates and fer-
ruginous products quite devoid of nutrient matter, affording
food and foothold to scanty growths of grass and stunted
shrubs. That, however, a rock contains all the desired mate-
rials, is no certain indication as to character of its decomposition
product, since in this process of decomposition much desirable
matter may have become lost. Nevertheless most soils retain
what we may call inherited characteristics, and a direct com-
parison whenever possible is by no means uninteresting, as will
be noted later.
It need scarcely be remarked that the value of any soil de-
CHEMICAL NATURE OF SOILS 361
pends wholly upon its capacity for plant growth. Hence a
satisfactory treatise on th£ subject should be written with a
view to showing to what this capacity is due, and what are
the laws governing its fertility and its rejuvenation when that
fertility becomes exhausted. Such a method of treatment is,
however, far beyond the limits of the present work, and we must
content ourselves with merely touching upon a few of the most
salient points, leaving the at present little understood subject
of fertility for other and abler writers. It may be well to re-
mark, however, that a soil left to itself and nature's processes
rarely becomes barren or exhausted except it may be under
changed geological conditions. A growing organism takes
temporarily from the soil that which is essential, but restores
it again with accrued interest in the form of carbonaceous and
nitrogeneous matter derived from the atmosphere, when it dies.
Thus, under normal conditions, the soil grows yearly richer
and richer and capable of supporting larger and more luxuriant
crops. It is only when the husbandman comes in, and by his
improvident harvesting robs the soil not merely of its interest
due, but of a part of the principal as well, that bankruptcy
results.
For a long period the fertility of a soil was felt to be dependent
very largely upon its chemical composition, and older treatises
and reports of geological surveys are filled with tables of analy-
ses which the acquired knowledge of years now shows us to be
almost as worthless as can be, either for the purposes for which
they were first intended, or as indicative of the mineral nature
of the soil itself.1 A soil which, under certain conditions of
climate or moisture, is utterly barren may, under changed con-
ditions, be fruitful in the extreme, as has been repeatedly de-
monstrated in the case of the so-called American deserts, dreary
stretches of aridity given over to sage brush and a few degraded
forms of animal life, but which need only moisture to cause
them to laugh with harvests.
1 The common practice of making soil analyses, whereby the results are tabu-
lated as soluble and insoluble (meaning by soluble the portion extracted by boil-
ing hydrochloric acid) and putting down the latter as silica (or sand) and
insoluble silicates, cannot be too strongly condemned. It means nothing. A
growing plant is capable of extracting only a small, and as yet unknown, portion
of that taken out by the acid, and as to what silica and insoluble silicates may
be, we are left in ignorance. Such analyses can be of use to neither the student
of soils or of geology.
362 THE REGOLITH
Naturally, a soil containing in itself nothing in the way of
available plant food can be made to produce crops only when
the needed constituents are supplied. Investigations have,
however, shown th,at, though varying in different species, the
proportional amount of food demanded by plants which can be
supplied by the atmosphere and meteoric waters is very large.
It seems to be now pretty well conceded that of all the con-
stituents found in soil aside from moisture, only potash, lime,
magnesia, phosphoric and sulphuric acids, can be considered
absolutely essential as plant food. The ash of all plants, to be
sure, contains silica, soda, — and it may be iron and other min-
eral ingredients, — but such are to be regarded as accidental
rather than otherwise.' Of the constituents enumerated as
essential, magnesia and sulphuric acid are almost invariably
present in sufficient quantities, while potash, lime, and phos-
phoric acid, even though sufficiently abundant in a virgin soil,
are liable to exhaustion under the ordinary methods of culti-
vation. The source of these materials has been shown in the
previous pages and need here be only touched upon. The
potash and the lime must have come originally from the de-
composition of potash-lime-bearing silicates, as the feldspars and
micas, amphiboles and pyroxenes. The original source of the
phosphoric acid was undoubtedly the apatite of the eruptive
rocks, though now to be found in bones and skeletons of ani-
mals, whose remains become entombed in sedimentary rocks
of all ages. How small and proportionally insignificant are
the percentages of these constituents in any soil, fertile or
barren, is shown in the table below,1 in which are given the
general average composition of a large number of soils, seden-
tary and transported. The sulphuric acid, which is not given
in this table, rarely amounts to more than from 0.05 % to 0.5 %
when calculated as sulphuric anhydride (SO3).
So small, comparatively, are these percentages, that it is rare,
indeed, to find a soil which v on complete analysis will not be
shown to contain them in sufficient proportion. The varying
degrees of fertility in such cases are due then, not to differ-
ences in ultimate composition, but to difference in combination
of these elements whereby they are or are not available for
plant food, and to physical and climatic differences as well.
iFrom Part A, Vol. II, Part II, Chemical Analyses, Geological Survey of
Kentucky, p. 113.
CHEMICAL NATURE OF SOILS
363
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Sand and insoluble silicates . .
Alumina, iron and manganese oxid
Carbonate of lime (CaCOg) . .
Magnesia (MgO)
I'hosphoric acid (PaOs) . . .
Potash in acid extract ....
Potash in the insoluble silicates
Organic and volatile matters . .
364 THE REGOLITH
Naturally a growing plant can take up only that which is
soluble by the means at its command. A high percentage of
any of the above constituents counts for little when they are
combined in the form of difficultly soluble silicates. A granitic
rock, as has already been noted, contains locked up iu its mass
all the mineral elements necessary for a fertile soil, but remains
barren simply because these are in a condition ,of slight solu-
bility and its physical structure is such that even the soluble
portions are unavailable. Pulverize this rock sufficiently, and
it will become immediately available for soil, though naturally
its fertility is slight, and rendered enduring only by gradual
decomposition. It is of course possible, that by nature's
methods, decomposition and incident leaching may have gone
so far that a soil on the immediate surface, though derived
from rocks rich in essential constituents, has become quite
impoverished and barren. This is especially true with lime-
stone residuals, as has been already noted. It is doubtless to
this fact that is due the enduring qualities of the glacial till
as a soil, though its immediate fertility may not be as great
as one of sedentary origin. The undecomposed feldspathic
and other mineral particles contained by the till, due to its
mechanical origin, yield up slowly but continually their sup-
ply of plant food, and such a soil may long outlast the residual
clays of non-glaciated regions.
The soils derived from deposits of modified glacial drift are
almost invariably sandy or gravelly in their nature. Such, on
account of their easy working qualities, great porosity, and
ready permeability, are commonly known as light soils, even
though their actual specific gravities may be greater than the
so-called heavy soils of the ground moraine.1
1 Mechanical analysis of a glacial soil from an old pasture, Cape Elizabeth,
Maine, yielded results as below. The portion selected was of just the thickness
turned up by the plough, — about 7 inches. In color it was dark gray, at the
immediate surface almost black from organic matter, and penetrated throughout
by grass roots. Fine angular grains of white quartz were the most conspicuous
feature on macroscopic examination. Eight hundred and thirty grammes of this
soil on sifting yielded : (1) 2.5 grammes gravel, which failed to pass a sieve con-
taining 8 meshes to the lineal inch. This consisted mainly of angular quartz and
cleavage bits of feldspar with occasional rounded lumps of impure limonite, and
not completely disintegrated particles of granitic rock. (2) 40 grammes coarse
sand retained by 20-mesh sieve and consisting of clear glassy and white opaque
quartz in angular and sub-angular fragments, the largest forms being some 3
millimetres in greatest diameter ; cleavage bits of white and pink feldspar, rarely
CHEMICAL NATURE OF SOILS 365
There is many an humble homestead throughout the glaciated
areas of North America whose lack of worldly prosperity is due
to the dry and barren soil supplied by these deposits of modi-
fied drift. On the other hand, there are numerous regions, like
those of northern Ohio, where a light, barren, residual soil de-
rived from sandstone has become enriched by an admixture of
glacial clays from the north, and thus brought prosperity m to
thousands of happy homes. Nature works out her own com-
pensations, impoverishing, it may be, here but correspondingly
nir idling there.
II. H. Loughbridge has shown1 that the percentage of soluble
folia of white mica, a few bits of mica schist, and lastly hard, rounded pellets of
indurated silt and organic matter. (3) 170 grammes retained by 40-mesh sieve
and consisting of a clean sand composed of some two-thirds its bulk white quartz
particles and one-third opaque, partially kaolinized feldspathic particles ; rarely
any mica or free iron oxides. (4) 180 grammes retained by 60-mesh sieve and
consisting, like the last, of clean quartz and feldspar sand, the quartz particles in
excess of the feldspar, and rarely a little mica. (5) 82 grammes retained by the
sii-iiifsh sieve. This, very clean sand of quartz and feldspar, in the proportion
of about \ quartz and \ feldspar. (6) 150 grammes retained by a sieve of silk
bolting cloth of 120 meshes to the lineal inch. Like the last, composed almost
wholly of bright quartzes and somewhat kaolinized feldspars with scarcely a
trace of other silicates. (7) 185 grammes which passed the silk bolting cloth.
This was submitted to washing, the lighter finer material being poured off as silt.
By this means were obtained 118 grammes very fine sand and 67 grammes silt.
The fine sand, as before, showed under the microscope only quartz and feldspars,
the quartzes still in excess. The silt to the naked eye consisted of a light brown,
almost impalpable material, which the microscope revolved into quartz and
feldspar particles with shreds of ferruginous products evidently derived from
the decomposition of iron-magnesian silicates, such as micas or amphiboles.
(8) Organic matter, 19.5 grammes.
A bulk analysis of the air dry-soil, excluding all grass and roots, yielded
results as below : —
Ignition (water and organic matter) 2.72%
Silica 76.80
Alumina and iron oxides 14.04
Lime 0.78
Magnesia Traces
Potash 2.87
Soda 1.18
98.39%
Such a soil is plainly little more than a highly quartzose granite or gneiss in a
pulverulent condition and in which the agencies of decomposition have scarcely
begun their work. Its composition could have been almost foretold by the
microscopic examination.
*On the Distribution of Soil Ingredients among the Sediments obtained in
Silt Analysis, Am. Jour, of Science, Vol. VII, 1874, p. 17.
366
THE KEGOLITH
material in a soil rapidly increases with the degree of commi-
nution; i.e. the finer the material the larger the proportional
amount of soluble matter, and hence of matter available as
plant food. This is well brought out in the following table
abridged from the one given in Mr. Loughbridge's original
paper, the figures in the upper space of each column indicating
the. size of the particles, and the percentage amount of each as
determined by fractional separations.
CONVENTIONAL NAME :
CLAY
FINEST SILT
FINE SILT
MEDIUM
COARSE
SILT
SILT
23.56%
12.54%
13.67%
13.11%
21.64%
nun.
mm.
mm.
mm.
DIAMETER OF PARTICLES :
?
.005-.011
.018-.016
.022-.027
.033-.03S
CONSTITUENTS
%
%
%
%
%
Insoluble residue ....
15.96
73.17
87.96
94.13
96.52
Soluble silica
33.10
9.95
4.27
2.35
Potash (K2O)
1.47
0 63
0 29
0 12
Soda (NaaO)
d.70")1
0.24
0.28
0.21
Lime (CaO)
0.09
0.13
0 18
0.09
Magnesia (MgO) ....
1.33
0.46
0.26
0.10
Manganese (MnOg) . . .
0.30
0.00
0.00
0.00
....
Iron sesquioxide (Fe20s) .
18.76 •
4.76
2.34
1.03
....
Alumina (A^Os) ....
18.19
4.32
2.64
1*21
....
Phosphoric acid (l^Os) . .
0.18
0.11
0.03
0.02
....
Sulphuric acid (80s) . . .
0.06
0.02
0.03
0.03
....
Volatile matter ....
9.00
6.61
1.72
0.92
Totals
100 14
99 30
100 00
100.21
Total soluble constituents .
75.18
20.52
10.32
5.16
According to Hilgard,2 the substance which assumes com-
manding importance as controlling the fertility of a soil, aside
from physical conditions, is lime, in the presence of which, in
adequate proportions, smaller percentages of the other plant
foods will suffice for high and lasting productiveness, than
would otherwise be the case. Since lime is the essential con-
stituent of the rock limestone, it follows that, other things
being equal, a "limestone country is a rich country." As else-
where noted, however, a limestone soil may have become so
1 An excess of original amount, due to the addition of sodium chloride to
produce flocculation of clay in suspension.
2 The Relation of Soil to Climate, Bull. No. 3, U. S. Weather Bureau, 1892.
CHEMICAL NATUKE OF SOILS 367
leached of its lime, through prolonged decay, as to be benefited
by artificial applications of this same constituent. Lime is,
moreover, so generally distributed throughout the great major-
ity of rocks that few soils would be lacking in this constituent,
were even a small proportion of the original amount left in the
residue from rock decay, instead of being so largely removed
in solution.
It would follow from this that the composition, and fertility
of a soil is dependent not more upon the character of the rock
mass from which it is derived, than upon the prevalent climatic
conditions under which it originated, the general average tem-
perature and the amount and distribution of the rainfall being
particularly important factors. This branch of the subject has
also been considered in some detail by Hilgard, to whom we are
indebted for the only satisfactory r£8um£. Concerning condi-
tions of temperature, this author says : —
•• Within the ordinary limits of atmospheric temperatures all
the chemical processes active in soil formation are intensified
by high and retarded by low temperatures, all other conditions
being equal. This being true, we would expect that the soils
of tropical regions should, broadly speaking, be more highly
decomposed than those of the temperate and frigid zones.
While this fact has not been actually verified by the direct
comparative chemical examination of corresponding soils from
the several regions, yet the incomparable luxuriance of the
natural as well as the artificial vegetation in the tropics, and
the long duration of productiveness, offer at least presumptive
evidence of the practical correctness of this deduction. In
other words, the fallowing action, which in temperate regions
takes place with comparative slowness, necessitating the early
use of fertilizers on an extensive scale, has been much more
rapid and effective in the hot climates of the equatorial belts,
thus rendering available so large a proportion of the soil's in-
trinsic stores of plant food that the need of artificial fertilization
is there restricted to those soils of which the parent rocks were
exceptionally deficient in the mineral ingredients of special
importance to plants that ordinarily form the essential material
of fertilizers."1
1 While the action of frost in bringing rock masses into the condition of soil
is, in temperate climates, of very great importance, there seems to be a limit
beyond which it accomplishes little in the way of directly promoting decomposi-
368 THE REGOLITH
Concerning the concentration and leaching out of certain con-
stituents by the action of meteoric waters, the same authority
says : —
" When, however, the rainfall is either in total quantity or in
its distribution insufficient to effect this leaching, the sub-
stances which otherwise would have passed into the sea are
wholly or partially retained in the soil stratum, and when in
sufficient amount may become apparent on the surface in the
form of efflorescences of ' alkali ' salts. One of the most im-
portant modifications produced by scantiness of rainfall on soil
formation is the great retardation of formation of clay from
feldspathic rocks (kaolinization) and the sediments derived
therefrom. As a result, it is observed that the soils of the
Atlantic slope are prevalently loams, containing considerable
clay, and even in the case of alluvial lands, oftentimes very
heavy, while the character of the soils of arid regions is pre-
dominantly sandy or silty with but a small proportion of clay,
unless derived directly or indirectly from clay or clay shales.
In the former case, the clay, becoming partially diffused in
the rain water when a somewhat heavy fall occurs, percolates
through the soil in that condition and tends to accumulate in
the sub-soil, the result being that almost without exception
the sub-soils of the humid regions are very decidedly more
clayey than the corresponding surface soils. Not only does
this clay water tend to make the sub-soil more compact and
heavy, making it less pervious to water and air, but it is as-
sisted materially in this by the action which tends to leach the
lime carbonate out of the surface soil into the sub-soil. The
accumulated clay is thus frequently more or less cemented into
a ' hardpan ' by lime partly in the form of carbonate and partly
in that of zeolitic (hydrous silicate) compounds, adding to the
compactness of the sub-soil, and therefore to the usual specific
difference between the soil and sub-soil ; viz. the deficiency or
absence of humus and the difficulty of penetration by an aera-
tion of the roots of plants."
For these reasons the soils of arid regions, even though con-
taining the same materials, are often of uniform physical and
chemical character to great depths. The soluble salts, as car-
tion, and presumably disintegration as well. Collier's (8th Ann. Rep. New
York Exp. Station, 1889) experiments showed that 47 successive freezings and
thawings of a soil did not perceptibly increase the percentage of soluble potash.
CHEMICAL NATURE OF SOILS
369
bonate of lime and salts of potash and soda, which are leached
away in regions of great average humidity, remain in those
where the annual precipitation is less, or where, on account of
its uneven distribution throughout the warmer months of the
year, its permeability and consequent leaching action is less.
Hilgard brings out this fact prominently in tables from which
that below is condensed, the original being compiled from sev-
eral hundred analyses of soils from the humid regions of North
and South Carolina, Georgia, Florida, Alabama, Mississippi,
Arkansas, Kentucky, and the arid regions of California, Wash-
ington, Montana, Utah, Colorado, Wyoming, and New Mexico.
SHOWING THE PROPORTIONAL AMOCNTS OF SOLUBLE SALTS IN SOILS OF ARID
AND HUMID REGIONS
CONSTITUENTS
ARID REGION
HUMID REGION
Insoluble residue
69.681 %
84.472 %
Soluble silica
6.289
3.873
Potash
0.825
0.187
Soda
0.251
0.071
Lime
1.645
0.112
Magnesia
1.384
0.209
Brown manganese oxide
0.056
0.126
Iron peroxide
5.431
3.455
Alumina
7.309
4.008
Phosphoric acid
0.144
0.114
Sulphuric acid
0.035
0.065
Water and organic matter
6.586
3.557
Total
99.978%
100.093 %
Discussing these figures, Professor Hilgard says : " Concern-
ing this table with reference to the lime, a glance at the col-
umns for the two regions shows a surprising and evidently
intrinsic and material difference approximating to the propor-
tion of 1 to 14£. This difference is so great that no accidental
errors in the selection of analysis of the soils can to any mate-
rial degree weaken the overwhelming proof of the correctness
of the inference drawn upon theoretical grounds ; viz. that .the
soils of the arid regions must be richer in lime than those of
the humid countries." These remarks hold good also for the
percentages of magnesia and the alkalies. From the fact that
2 B
370 THE REGOLITH
in humid regions the more soluble constituents are leached
out, we may safely infer a corresponding proportional increase
in the insoluble constituents. This is also made manifest by
the tables, there being a difference of nearly 15% in favor of the
humid regions. The table shows, further, a probably greater
proportion of zeolitic material in the soil of arid regions, the
assumption being based upon the percentages of soluble silica.
Concerning this difference, the author says : —
"Nor should this be a matter of surprise when we consider
the agencies which are brought to bear upon the soils of the
arid regions with so much greater intensity than can be the
case where the solutions resulting from the weathering process
are continually removed as fast as formed by the continuous
leaching effect of atmospheric waters. In the soils of regions
where summer rains are insignificant or wanting, these solu-
tions not only remain, but are concentrated by evaporation to
a point that in the nature of the case can never be reached in
humid climates. Prominent among these soluble ingredients
are the silicates and carbonates of the two alkalies, potash and
soda. The former, when filtered through a soil containing the
carbonates of lime and magnesia, will soon be transformed into
complex silicates in which potash takes the precedence of soda,
and which, existing in a very finely divided (at the outset in a
gelatinous) condition, serve as an ever-ready reservoir to catch
and store the lingering alkalies as they are set free from the
rocks, whether in the form of soluble silicates or carbonates.1
The latter have still another important effect. In the concen-
trated form, at least, they themselves are effective in decom-
posing silicate minerals refractory to milder agencies, such as
calcic carbonate solutions, and thus the more decomposed state
in which we find the soil minerals of the arid regions is intel-
ligible on that ground alone. But it must not be forgotten
that lime carbonate, though less effective than the correspond-
ing alkali solutions, nevertheless is known to produce, by long-
continued action, chemical effects similar to those that are more
quickly and energetically brought about by the action of
caustic lime. In the analysis of silicates we employ caustic
lime for the setting free of the alkalies and the formation of
easily decomposable silicates by igniting the mixture ; but the
carbonate will slowly produce a similar change, both in the
1 See author's remarks on page 374.
CHEMICAL NATURE OF SOILS 371
laboratory and in the soils, in which it is constantly present.
This is strikingly seen when we contrast the analyses of calca-
reous clay soils of the humid region with the corresponding
non-calcareous ones of the same. In the former the propor-
tions of dissolved silica and alumina are almost invariably
much greater than in the latter, so far as such comparisons are
practicable without assured absolute identity of materials."
It is evident from the above that, provided the amount of de-
composition be the same, the soil of an arid region may contain
a larger proportion of desirable constituents than one in a region
of considerable annual precipitation. It may, also, and for the
same reasons, contain a larger proportion of constituents that
are positively deleterious. This is particularly true of arid and
semi-arid regions of poor drainage, like the Great Basin regions
of the United States, where salts of sodium not infrequently
accumulate to such an extent as to render the land sterile and
barren in the extreme.
The primary origin of the sodium in these salts lies in the
soda-bearing silicate minerals forming the rocks of the region
and from which they have been set free through their decom-
position.
It should be stated, however, that the so-called " alkali " is
not composed wholly of sodium compounds, but contains also
salts of magnesia, lime, iron, and potash. Nor is the form under
which the salts exist at all constant. As a rule, the larger por-
tion of the alkali is in the form of sulphate of soda, though a
considerable portion may exist as carbonate or chloride, and
smaller proportions in the form of nitrates. Concerning the
formation of these carbonates, Hilgard says: 1 —
" There seems to be a consensus of opinion that the carbona-
tion of the soda is connected in some way with the presence
of limestone or carbonate of lime, and that an exchange has
occurred in which either common salt or Glauber salt have
transferred their acidic components to lime and have become
carbonates instead. . . . Yet the simple explanation of the
contrary reaction was given and published as early as 1826 by
Schweigger. In 1859 it was again observed by Alex Muller,
in a different form, but neither of these chemists, nor any of
their readers, appear to have perceived the important bearing of
this reaction, not only upon the formation of the natural depos-
1 Bull. No. 3, Weather Bureau, U. S. Dept. of Agriculture, 1892.
372 THE REGOLITH
its of carbonate of soda, but also upon a multitude of processes
in chemical geology. Without going into details ... it may
be broadly stated that the formation of carbonated alkalies oc-
curs whenever the neutral alkaline salts (chlorides or sulphates)
are placed in presence of lime or magnesia carbonates and car-
bonic acid, or of alkali ' supercarbonates ' (hydrocarbonates)
containing even a slight excess of carbonic acid above the nor-
mal carbonates, the latter being the actual condition of all
natural sodas."
We have thus far considered, only those elements of the soil
that are derived directly from the rocks from which they are
formed.
To this list we should add the element nitrogen, not so much
on account of its quantity, as its value as plant food and of the
great economic value of some of its compounds. The common
forms under which this element exists, are (1) atmospheric
nitrogen, a colorless, tasteless, and innocuous gas and which
forms some three-fourth by bulk of the air we breathe, and
(2) the nitrogen of the soil, where it exists in at least three
distinct forms, (1) organic nitrogen, (2) as ammonia or ammonia
salts, and (3) as nitric acid.
The average amount of nitrogen present in agriculture soils
is given by authorities as varying from 0.1% to 0.3 %, though
occasionally, as in certain soils rich in organic matter, 4 or 5 % -
Of these forms only the ammonia salts and nitric acid are of
direct value for plant food. Nitrogen, in the form of nitrate
of soda, forms an important mineral fertilizer, as noted on p. 71.
The extraordinary richness in nitrates of the soils in tropical
countries, and particularly in South America, has often been
remarked since the subject was first broached by Humboldt
and Boussingault. According to Miintz and Maracano, nitrates
occur in the soils of Venezuela, the valley of the Orinoco, and
other localities sometimes to the amount of 30 % of their mass.
These nitrates they show to be due to the oxidation of organic
nitrogen through the agency of bacteria. They state that in
the caverns of the regions, a guano composed mainly of the
excreta of birds and bats, but admixed also with the dead bodies
of these and other animals, has accumulated to the amount of
millions of cubic metres. Through the gradual nitrification of
this guano, and a combination of the nitrogen with the lime
of bones, or existing as a carbonate in the soil, a gradual tran-
MINERAL NATURE OF SOILS
373
sition is brought about wherever there is free access of air or
the temperature is sufficiently high to stimulate the nitrifying
organisms to their fullest activity. There is thus a gradual
change in the character of the nitrogeneous combination from
the interior to the exterior portions of the cave, as shown in the
following analyses : —
CONSTITUENTS
GUANO FROM
INTERIOR OP
CAVB
EARTH FROM
THE ENTRANCE
EARTH FROM
SOME DISTANCE
FROM ENTRANCE
( )rganic nitrogen
11.74%
2.41 %
0.80%
Nitrate of lime
0.00
3.03
10.36
These authorities would account for the presence of extensive
deposits of nitrates as in Chili, on the assumption that the solu-
ble nitrate, originally derived from decomposing organic matter,
as noted above, had been leached out from its place of origin
by percolating water and redeposited elsewhere on evaporation.
The invocation of atmospheric electricity to account for any
part of the nitrates of the soils, they regard as quite unneces-
sary, the same being of indirect influence only, furnishing first
nitrogen for growing plants which in their turn serve as food
for animals. These same authorities give the following figures
relative to the amounts of nitrates and nitrogen in South
American soils : —
CONSTITUENTS
SAN JUAN
Los MORROS
DE PARAPARA
EL ENCANTADO
Nitrate of lime
2.85%
0.15
3.50%
0.27
0.62 %
0.21
Orcranic nitrogen ....
(2) The Mineral Composition of Soils. — This is essentially
the same as that of the regolith of which the soil forms a part.
Fragmental quartzes and feldspars form the larger proportion
of most soils. These are intermingled with shreds of mica,
amphibole, pyroxene, calcite or aragonite, iron and manganese
oxides, and in variable proportions, kaolin and other silicates,
carbonates and oxides. The presence of these constituents is
374 THE REGOLITH
usually somewhat obscured by iron oxides and carbonaceous
matter ; but when these are removed by acids or by ignition,
and the residue submitted to microscopic analyses, the true
mineral nature can be, as a rule, made out with approximate
accuracy.1
From what has gone before, it must be evident that the con-
stituents of any soil are almost universally in a finely fragmen-
ted condition, a few of the smaller more resisting minerals, like
the rutiles, tourmalines, zircons, etc., having perhaps escaped
the comminuting processes. Of the silicate minerals we may
be sure that many are in an advanced stage of hydration and
the ferruginous constituents in a state of peroxidation. It is
possible that under favorable conditions new minerals of fairly
perfect crystalline development may be temporarily formed.
Since the work of Lemberg was made public,2 it has been very
commonly assumed that various minerals of the zeolite group
were present and exercised an important function in the con-
servation of soil fertility. Notwithstanding the somewhat
enthusiastic endorsement by Hilgard, of this idea, as set forth
in the previous pages, the writer can but feel that too much has
been assumed, both regarding their actual presence and their
possible utility.
We must not lose sight of the fact that the actual occurrence
of zeolites in soils, where they have been formed, is as yet not
proven. Their presence is inferred from the fact that weak
acids, such as are known to be capable of decomposing zeolitic
minerals, will extract from the soil, among other constituents,
certain ones which are characteristic of minerals of the zeolitic
group; and it is assumed, purely for lack of a better reason, that
these elements are those thus combined. Even if this be true,
their efficacy as potash holders may well be questioned, since
potash is not as a rule an element of great importance in zeo-
litic minerals. Out of the 23 known species of zeolites (includ-
ing apophyllite), in but five is potash considered an essential
constituent. These five, as already noted on p. 32, are apo-
phyllite, ptilolite, mordenite, phillipsite, and harmotome, of
which phillipsite alone carries upwards of 6 % (theoretically),
1 See Anleitung zur Mineralogischen Bodenanalyse, etc., by Franz Steinriede,
Inaug. Dis. Friedrichs-Universitat Halle-Wittenberg. Halle, 1889.
2 Zur Kenntniss der Bildung und Umbildung von Silicaten, Zeitschrift der
Deutschen Geolischen Gesellschaft, Vols. XXXVII and XXXVIII, 1885 and 1887.
MINERAL NATURE OF SOILS 375
the other smaller amounts, the average for the five being about
4 %. Now assuming that all the zeolites in the soils belonged
to these five groups and none to the 18 potash-free varieties,
and that 10 % of any soil consisted of zeolitic material, even
then we have thus combined only 0.4 % of K2O.
We must remember, further, that the zeolites are invariably
secondary minerals, as already noted, and as such are com-
monly regarded as decomposition products. This does not
necessarily mean, however, that they are products of superficial
weathering. Indeed, in the majority of cases the evidence is
all to the contrary, they being plainly a result of deep-seated
processes going on in the rock masses long before atmospheric
action began to manifest itself. (See under Hydrometamor-
phism, p. 161.) It is even questionable if the conditions preva-
lent in soil are not unfavorable rather than otherwise to the
formation of zeolitic compounds, and if such traces as there
exist are not rather residuals from the breaking down of rock
masses in which they had been previously formed.
In this connection it is well to remember that zeolites as a
whole are characteristic of basic eruptive rocks, such as have
yielded but a proportionately small amount of our soils. Also
that the mutual chemical reactions that may go on in a rock
mass due to close juxtaposition of the various minerals may
largely, if not entirely, cease in a soil where the amount of in-
terspace is so enormously exaggerated.
The researches made during the Challenger Expedition1
show, it is true, that even at so low temperatures as from 2° to
3° C. phillipsite is being formed in the deep-sea muds of the
Central Pacific and Indian oceans. But in these cases the
mud is the finely comminuted debris from basic eruptive rocks,
itself peculiarly liable to decay, and containing all the materials
necessary for zeolitic formation. It is, moreover, in a condition
of continual moisture and under the weight of the thousands
of fathoms of overlying water which is here in a state of ex-
treme quiescence, being beyond the influence of superficial move-
ments, as waves, tides, and currents. These conditions are so
widely different from those which exist in the superficial parts
of land areas, that they can be regarded as merely suggestive.
The same may be said relative to the zeolite (phillipsite and
apophyllite) formations at Plombieres as described by Dau-
1 Rep. on the Scientific Results, 1873-76, Deep-sea Deposits, 1891, pp. 400-411.
376 THE KEGOLITH
bree.1 Another fact which mitigates against the theory of
zeolitic formation in soils, is the almost universal absence of
these minerals in such secondary, unmetamorphosed rocks as
are the product of the reconsolidation of the same class of ma-
terials as in their unconsolidated condition form soils. If they
once existed, it would seem strange they have not in some cases
at least survived. If formed in soils, why should they not be
formed in secondary rocks where the conditions are apparently
so much more favorable?
It would, to the writer at least, seem more probable that the
soluble potash of soils exists, not in zeolitic combination, but
in some of the numerous decomposition products of feldspar,
nepheline, scapolite, etc., to which the name pinite is commonly
applied. Such at least is the case in the potash-rich soils of
Maryland, examined by R. L. Packard.2 It is possible also
that it may exist in compounds allied to glauconite.
The writer has elsewhere3 pointed out that, particularly
among basic rocks, there may be actually a larger percentage
of matter soluble in hydrochloric acid and sodium carbonate
solution in rocks ordinarily designated as fresh, than in the
debris resulting from their decomposition. This fact he has
since emphasized in a paper read at the December (1896) meet-
ing of the Geological Society of America, and from which the
following statements are drawn. Rock-weathering, it must be
remembered, is in the majority of instances accompanied by a
leaching process, whereby original soluble compounds, or new
soluble compounds formed during the process of decomposition,
are gradually removed. The final result is therefore, as already
many times noted, a residue consisting of the least soluble con-
stituents, and which forms the ordinary surface soil. Even in
cases where the actual amount of soluble matter is greatest in
a soil, the apparent excess may be due to water of hydration
and to the large amount of sesquioxide of iron, the latter being
practically insoluble in meteoric waters so long as there is a
free supply of oxygen, though readily soluble in hydrochloric
acid. These conclusions are based upon the following table, in
which the total percentage loss on ignition, minus the ignition
in the insoluble residue, is tabulated with the soluble matter.
1 Geologie Experimental, pp. 180 et seq.
2 Bull. 21, Maryland Agricultural Experiment Station, 1893.
8 Bull. Geol. Soc. of America. Vol. VII, 1895, p. 355.
SOLUBLE CONSTITUENTS OF ROCKS
377
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378 THE REGOLITH
(3) Physical Condition of the Soil. — Chemically, as previ-
ously noted, a soil differs from the parent rock in the amount
of leaching it has undergone, and in the finely comminuted and
more or less decomposed condition of its particles. There are
other distinctions, not the least important of which are its state
of loose coherency and porous condition due to interstitial air
spaces. It has been estimated by Whitney l that the approxi-
mate number of grains in one gramme of soil varies between
2,000,000 and 15,000,000, the lowest estimate being that for a
sandy soil containing only some 4.77% of material in such an
extremely fine state of comminution as properly to be classed
as clay, while the highest number is that in a sub-soil contain-
ing some 32.45 % . Our interest in these remarkable figures is
still further heightened when we are called upon to remember
that these grains are not in actual contact, but each separated
from the other by thin films of moisture, or, in a dry soil, by
actual air spaces. That such spaces exist is easily proven by the
fact that any soil may be greatly diminished in bulk by pressure.
The amount of this empty space is naturally quite variable, but
it is estimated to constitute on an average some 50 %, by volume,
of the soil. That is to say, a cubic foot of soil, in its natural
condition, contains an amount of space between its grains, filled
by air or water, equal to one-half the entire mass.
These extraordinary figures are given, not merely to illustrate
the wonderful degree of comminution reached in rock-weather-
ing, but also, and what is of more importance from the stand-
point of an agriculturist, the amount of surface exposed to the
solvent action of roots and percolating waters. Indeed, it has
been estimated that the total surface areas of the grains in a
cubic foot of soil amounts, on the average, to 50,000 square feet.
The amount is of course greater in a fine than a coarse soil, but
in any case sufficiently large to enable us to understand how,
under the ordinary conditions of cultivation, all the materials
essential to plant growth may in a brief time be removed, unless
renewed by artificial fertilizers.
Further than this, the amount of space between the grains
is of very great importance in determining the circulation of
water in the soil, and its capacity for retaining the right propor-
tion essential to plant growth as noted later.
The experimental work of late years goes to show that fertil-
1 Bull. No. 4, U. S. Dept. of Agriculture, Weather Bureau.
PHYSICAL CONDITION OF SOILS 379
> dependent upon these physical properties perhaps even
more than upon chemical composition. If the structure, i.e.
the manner of arrangement of the soil particles, is such as to be
most favorable to root action and conservation of moisture, there
are few soils but may be made fertile by proper treatment, even
cannot the desired physical properties be imparted by artificial
means. A soil which contains too large a proportion of fine
rlay matter may hold so large a proportion of moisture as to
be quite unsuited for cultivation when saturated, and become
equally unfitted by induration when dry. A light, porous,
sandy soil on the other hand, though fertile during seasons of
abundant precipitation, parts with its moisture so readily as to
be quite barren in seasons of drought. Porosity and capillarity,
two properties dependent wholly on the size and shape of the
soil particles, are therefore very essential items in this consid-
eration. Moisture precipitated in the form of rain soaks into
the ground or flows off upon the surface in varying proportions,
according to local conditions, an open porous soil naturally
Absorbing more rapidly than one that is close and compact.
When, after the rain ceases, evaporation sets in from the
sui' face, the water which has soaked in is brought back again
in part by capillarity, though a part escapes through leaching
downward beyond the reach of capillarity, ultimately coming
to the surface again, at lower levels, in the form of springs.
The capacity of a soil to care for the water it receives from
rains is, perhaps, the most important of any one property.
It has been demonstrated that the soils of the semi-arid regions
of the West will produce abundant crops of wheat and corn,
though receiving but about half the amount of water from rain-
fall that would be requisite in the East. This is accounted
for wholly on physical grounds, and is explained as follows : 1
Water falling upon a perfectly dry soil descends very slowly,
and indeed, in extreme cases, may continue to fall for hours
without wetting the mass for more than a few inches below the
surface, while it will be absorbed very rapidly by a soil already
wet, but not saturated. This is due to the fact, as explained
l»v Whitney, that in a dry soil the tension or contracting power
of the surface of the water is greater than the attraction of the
soil grains. If, on the other hand, there is any appreciable
1 Conditions in Soils of the Arid Region, by Milton Whitney, Yearbook
U. S. Dept. of Agriculture, 1894.
380 THE REGOLITH
amount of moisture in the soil, the tension of the water sur-
face will cause it to contract and pull the water from above
into the sub-soil. It follows, then, that the water of rains fall-
ing in semi-arid regions would not penetrate into the dry sub-
soil, until the overlying portions had become successively so
far saturated that they could no longer hold the water back,
and it would pass downward, therefore, very gradually into the
lower depths, saturating, or nearly saturating, each successive
depth as it progressed. Unless, then, as rarely happens in this
region, the rainfall was so great and so continuous as to saturate
the soil to a considerable depth, the whole supply of moisture
absorbed would remain within a short distance of the surface,
either immediately within reach of plant roots, or where it can
be brought upwards once more by capillarity when evaporation
from the surface begins. With a continuously wet sub-soil,
however, as in the East, a very considerable portion of the
water passes at once to depths beyond the reach of roots or
capillary attraction, and is, so far as our present considerations
are concerned, completely lost until, in the course of nature's
endless cycle, it shall once more be returned as rain. Within
certain limits, a small rainfall, equitably distributed, is more
advantageous to agriculture than are the heavier precipita-
tions which characterize the Atlantic slopes of the American
continent.
The capacity of soils for moisture has been the subject of
experiment, and is found to vary widel}7, being naturally largely
dependent upon the size of the individual particles and the con-
sequent amount of interspace. Whitney states 1 that sub-soils
of Maryland truckland having 45 % of interspace will hold but
22.41 % by weight of water, when this space is completely filled.
The sub-soil of the Helderberg limestone, having 65 % of space,
will hold 41.22%. King2 gives the following table to show
the actual amount of water held by field soils when their sur-
faces are only 11 inches above standing water, this water having
been lifted into them by capillarity : —
1 Some Physical Properties of Soils, Bull. No. 4, U. S. Dept. of Agriculture,
Weather Bureau, 1892.
2 The Soil, p. 159.
KINDS OF SOILS
381
PER CENT
POUNDS OF
INCHES OF
OF WATER
WATER
WATEK
Surface foot of clay loam contained . . .
32.2
23.9
4.59
Second foot of reddish clay contained . .
23.8
22.2
4.20
Third foot of reddish clay contained . .
24.5
22.7
4.37
Fourth foot of clay and sand contained
22.6
22.1
4.25
Fifth foot of fine sand contained ....
17.5
H 19.6
3.77
Total
110 5
21 94
According to Meister, different soils show water-holding
capacities as follows : 1 —
KIND OF SOIL
PER CENT
OF WATER
IMIUBED
KIND OF SOIL
PER CENT
OF WATER
IMBIBED
Clay .
60.0
Chalk
49.5
Loam .
60.1
Gyseous. ... . .
52.4
Humus
70.3
Sandy (82 % sand) . . .
45.4
Peat
63.7
Sandy (64 % sand) . . .
65.2
Garden
69.0
Pure quartz sand ....
46.4
Lime
59.9
(4) The Weight of Soils. — This is dependent upon (1) the
character of the particles composing the soil and (2) their
degrees of compactness. The figures given below are those of
Schubler.2
WEIGHT PER CUBIC FOOT IN POUNDS, OF VARIOUS SOILS
Dry siliceous or calcareous sand 110
Half sand and half clay 96
Common arable soil 80-90
Heavy clay 75
Garden mould, rich in vegetable matter 70
Peat soil . 30-50
(5) Kinds and Classification of Soils. — Being derived from
rocks of all kinds and under greatly varying conditions ; in
1 Handbook of Experiment Station Work, U. S. Dept. of Agriculture, 1893,
p. 317.
2 Handbook of Experiment Station Work, U. S. Dept. of Agriculture, 1893,
p. 315.
382 THE REGOLITH
almost infinitely variable conditions of comminution, decay, and
proportional amounts of their various constituents, no hard and
fast lines for soil classification can be laid down. All things
considered, they are best classed with the regolith of which they
form a part, the general divisions of which are given in tabular
form on p. 299. We thus have the primary divisions of seden-
tary and transported soils, accordingly as they have been formed
in place, or transported. Each of these is again subdivided
according to the agencies involved in its transportation or
original formation.
Many varietal names have been applied to soils, but as a rule
in so loose and ill-defined a manner as to give them only p, very
general significance. A common practice is to name one of
sedentary origin according to the rock from which it was de-
rived, as granite soil, limestone soil, etc. Transported soils, on
the other hand, are often designated either by the agencies in-
volved in transportation, as glacial, or ceolian soils, their position,
as terrace soils, or their physical or chemical characteristics, as
sandy or clayey soils. A loam is usually defined as an admixture
of sand and clay with more or less organic matter, a clayey
loam being one in which clay predominates and a sandy loam
one in which sand prevails. The terms peat, muck, loess, marl,
etc., have been already sufficiently defined. Local names indi-
cative of suitability for particular crops, or sometimes of doubt-
ful or obscure meaning, are frequently met with. The bluegrass
soils of central Kentucky are limestone residuals celebrated for
the luxuriant growths of Poa pratensis which they bear. The
red " buckshot " soils of the Yazoo bottoms, Louisiana, are stiff
clayey alluvial soils mottled with ferruginous spots.
Many names indicative of mode of formation have already
received attention, but a few others may be here noted. The
names Endogeneous and JKxogeneous have been proposed for
soils formed in place (sedentary) or derived from other sources
(transported). It is presumably scarcely necessary to remark
that such terms are quite inapplicable and inappropriate.
The name regur is locally applied to a fine dark argillaceous
soil particularly suited for cotton growing and which has a wide
areal distribution throughout southern India. Its origin ap-
pears to be mainly subaerial, though a part of the material so
called is undoubtedly alluvial. The material is highly plastic
when wet, and expands and contracts -to a remarkable degree
KINDS OF SOILS
383
under varying conditions of moisture and dryness. This soil
is very retentive of moisture and rarely requires to be irrigated
artificially. It is, as a rule, of great fertility and of wonderful
lasting powers, it being stated that in some localities it has
borne crops for 2000 consecutive years, without the aid of ma-
nures. In depth this soil is rarely over 6 to 8 feet. The follow-
ing analyses show the chemical character of the regur (from
near Seoni) on the surface and at depths of (An) 5 feet and
(Bii) 3 feet below. The analyses A, like those given on
p. 306, are instructive as showing the large increase in the
amount of lime from the surface downward. Although not so
stated, the slight differences in Bi and BH are probably due to
the lesser depth below the surface from which Bn was taken.
i
L
I
J
I
II
I
II
Insoluble matter
62.7%
47.61 %
62. 8%
63.7%
Organic matter
9 2
8.4
9.0
8.7
Water
8.4
7.6
8.2
6.5
Oxide of iron
11.0
15.9
10.9
11.8
Alumina
7.5
8.6
7.6
8.4
Carbonate of lime
1.2
11.89
1.5
1.3
100.00%
100.00%
100.00%
100.00%
In many cases this regur is derived directly from basaltic
rocks, by surface decomposition in situ, whilst other varieties
were derived from other aluminous rocks, or are alluvial depos-
its in river vallej^s, lakes, lagoons, and marshes. The dark
color, as is usual, is due to the presence of organic matter.1
The term sub-soil is applied to that portion of the regolith
which immediately underlies the soil proper, and from which
it differs mainly in compactness, and the lesser amount of oxi-
dation and decomposition it has undergone. In a soil which
has never been cultivated, the sub-soil may pass gradually up-
ward into the soil without distinct lines of demarcation. Pro-
longed cultivation may, however, have so thoroughly oxidized
and physically altered the superficial portions down to the limit
of plough and root action, as to bring about a very marked differ-
1 Manual of the Geology of India, 2d ed., by R. D. Oldham, 1893, p. 411.
884 THE REGOLITH
ence, both in color and texture, as well as in actual composition.
At times the sub-soil becomes so thoroughly compacted as to be
almost impervious, forming a so-called hardpan.
(6) The Color of Soils. — The color of soils is due mainly to
carbonaceous matter and iron oxides. To the first are due the
dark gray to black colors characteristic of prairie and swamp
soils. To iron oxides are due the buff, yellow, ochreous-brown,
and red hues, the source of the oxides being mainly the silicate
minerals from whence the soils were derived. It not infre-
quently happens, as abundantly demonstrated in the southern
Appalachian states, that it is possible in passing over any sec-
tion of the country to designate with a fair degree of accuracy
the lithological nature of the underlying rocks from the color
of the residual soils, even though the rocks themselves may
be wholly obscured by decomposition products. In such cases
rocks rich in iron silicates, like hornblende and augite, give
rise to bright red soils, while those poor in these constituents
yield soils of a gray or slightly yellowish hue. Much, however,
apparently depends on extent of decomposition and on climatic
conditions, as noted below.
One of the most striking features of the landscape observed
in travelling southward along the Appalachian belt is the
abrupt transition in color of the soil, as the limit of glacial
action is past. Within the glaciated area, except where de-
rived directly from rocks themselves highly colored, like the
Triassic sandstones, the soils are everywhere dull in color,
some shade of gray, drab, or brown. South of this limit ochre-
ous-red and yellowish prevail. Along the line of the Virginia
Midland railway, south of Washington, these colors prevail in
hues of astonishing brilliancy. Although the soils throughout
the region are residual, their colors seem in many cases quite
independent of the kind of rock to which they owe their origin.
Granite, gneiss, schist, or basic trappean rocks alike give rise to
red and yellow highly tenacious soils of such depth and brill-
iancy of color that every gully, ravine, and roadway stands out
against the green background of the landscape, as though
painted by some Titanic hand with brushes dipped only in
yellow, red, and vermilion ochres. These contrasts are par-
ticularly striking in the early summer and directly after a
rain. But he who wishes to admire had best do so from his
window, and without too much attention to detail.
THE COLOR OF SOILS 385
The soil is plastic and adherent to an intolerable degree.
The grass forms no compact sod, as in the North, and as a re-
sult the walls of houses, fences, feet, legs and clothes of pedes-
trians become uniformly stained a dirty ochreous color equally
trying to the housewife and to ploughman.
The cause of this color variation has been the subject of
speculation by Professors Crosby,1 Dana,2 Russell,3 and others.
So far as our knowledge now extends, it is apparent, as first
stated by Crosby, that the difference is due to a spontaneous
dehydration which takes place in the warmer regions, whereby
the hydrous sesquioxides of the type of limonite and gothite
are converted into the less hydrated or anhydrous forms tur-
gite and hematite with corresponding changes in color from
yellow or brown to red.
This view is rendered the more plausible from the fact that
the most brilliant hues are entirely superficial, and below the
surface gradually fade out into brown and yellow or even gray
hues. Such a transition may be observed in any fresh road
cut, but quickly becomes obscured by the washing down of the
deeply colored material from the higher levels. Sometimes
the brilliant red will be found a mere wash, but a fraction of
an inch in thickness, or again it penetrates to the depth of
a foot or more before giving way to more modest hues. In
such cases the brilliant colors will be found to have penetrated
deepest along joint lines, or the more porous portions, leaving
the intervening compact masses of more sombre hue.
In discussing this matter, there is, however, one point that we
should not overlook, although its importance seems not to have
been fully realized by the authorities quoted, and that is, a
change in color not due alone to a change in the conditions of
the iron, but to the relative greater abundance of this constitu-
ent in the upper portions. The iron oxides, as already noted,
owing to their less soluble nature accumulate in the residues,
and as a rule, the more thorough the decomposition the greater
the proportional amount of iron. A small percentage of free
oxide disseminated throughout a relatively large amount of
detritus imparts but little color ; the more iron, the more color.
1 Troc. Boston Society of Natural History, 1885, p. 219, and Technological
Quarterly, Vol. IV, 1891, p. 36.
2 Am. Jour, of Science, Vol. XXXIX, 1890, pp. 317-319.
» Bull. No. 52, U. S. Geol. Survey, 1889.
2c
386 THE REGOLITH
The residue from the Medford diabase described on p. 220 is
of a deep brown color, as a whole, but the finest silt washed
from it is several shades brighter, of a dull ochreous red. Had
the entire mass decomposed to the condition of this silt, wo
might expect it to have the same color. This change in color,
due to increased proportional amounts of iron oxides, is particu-
larly marked in limestone residuals where the original rock may
contain merely traces of free oxides, or ferruginous silicates.
Neumayer has shown1 that the snow-white Karst limestones
contain only some 0.044 % of ferruginous silicates which them-
selves carry 20 % of iron oxides. Yet the residual soil left by
the decomposition of this limestone is of so pronounced a color,
as to have long ago received the name terra rossa or red earth.
Other things being equal, brilliancy of color may then be
regarded as (1) indicative of advanced decomposition, and
(2) of geological antiquity.
(7) The Age of Soils. — No sooner were the first rocks pushed
above sea-level than the various agencies described under the
head of weathering began the
work of disintegration, de-
composition, and transporta-
tion. Of this we have ample
proof in the entire series of
sedimentary rocks extending
from the Archaean down to
the most recent and which
are but the reconsolidated
FIG. 38. — Trunk of tree still standing in residues of preexisting masses.
soil of Carboniferous age. a, bed-rock ; T|mt SUQ^ ft breaking down
b, under clay or ancient soil ; c, coal ; . i . « r
d, bedded rock ; e, fossil tree. resulted in the production ot
soils is a fair inference, though
we have no absolute evidence of land plants and hence, a
priori, of soils, before the beginning of the Upper Silurian
period, when plants of the lycopod type appeared. Such soils,
as soils, have, however, long since disappeared in the never-
ending cycle of change and it is not until we reach the Car-
boniferous period that we meet with soils which have been
preserved in place and in recognizable form even to the present
day. Even here induration and partial metamorphism has
rendered them no longer fitted for the support of plant life,
1 Verhandl. k. k. Geol. Reichsandstalt, 1875-76, p. 50.
THE AGE OF SOILS 387
but that they once did so serve is amply proven by the occa-
sional finding of still erect, though fossil, trunks with roots
buried in their native soil, as they grew in the marshes and
woodlands of the coal period. But as to the time of the begin-
nings of the formation of such soils as still retain their soil
characteristics, we have not in all cases reliable data. Those
which are but the unconsolidated sediments of recent geological
time, like those of the eastern shore of Maryland, the loess and
alluvium of the Mississippi valley, or the swamp and glacial
soils of the north and east may, of course, be located with a
reasonable amount of accuracy. But as for the residual soils,
those which result from the breaking down in place of rock
masses, we can only say that they must be younger than the
rocks from which they were derived. The writer has shown
that the granite soils of the District of Columbia are post-
Cretaceous ; in other parts of the Piedmont plateau of Mary-
land, they may be post-Tertiary. In but few instances, as at
PoorSoil
FIG. 39.
Medford in Massachusetts, have we evidence of any consider-
able amount of soil formation by decomposition and disintegra-
tion since the close of the glacial period. Obviously the older
a residual soil, the greater the amount of decomposition and
leaching it will have undergone and the less will it resemble
the parent rock. Where horizontally lying strata of varying
character have successively undergone decomposition and a loss
of their soluble constituents, the resultant soil must periodically
vary according to the nature of the rock undergoing decompo-
sition and the inherited characteristics handed down from the
strata earlier decomposed. In such a case as that here figured,
we have a residual soil containing the least soluble constituents
of the hundreds of feet of dissolved and disintegrated rock which
388 THE KEGOLITH
once extended across the entire country becoming commingled
with that now undergoing, in its turn, the soil-making process.
Such a soil may, therefore, in extreme cases, contain materials
of all ages from the first product of disintegration of the upper-
most strata, which may have been Carboniferous, to that which
formed to-day, and may be Cambrian.
It is, of course, true that through the erosive action of water
these soils are continually losing their finer silt and clay-like
particles, it may be almost as fast as formed, especially in hilly
regions, and that as the soil drops lower and lower in the geo-
logical horizons indicated, it becomes more and more impover-
ished in those constituents derived from the upper beds. But
as to what proportion of the material of one horizon is handed
down to become admixed with that from the rocks below, we
have no means of judging, and in fact it must be ever-varying.
The matter of the geological age of any soil, or the age of
the rocks from which it was derived, is therefore of only very
general interest, and may well be dismissed here. The attempt
which has been made by another writer1 to discriminate or
classify soils according to the geological horizons of the rocks
from which they were derived, is believed by the present writer
to be futile and wholly inexpedient.
No attempt should be made, as has been done by at least one
writer, to state the character of soil that may arise from the
weathering of any particular class of rocks, since much depends
upon the extent to which weathering has been carried. The
ultimate product of weathering of rocks of any but the purely
siliceous type is a more or less ferruginous clay, which may
be contaminated or admixed with coarser, foreign particles.
It is the extent of decomposition, more than its lithological
derivation, that determines both the chemical composition and
physical characteristics of any soil.
Rocks of essentially the same type so far as composition is
concerned, regardless of structural modifications induced by
metamorphism, have been formed and re-formed throughout
every period of the earth's history, and the attempt made to
classify those of igneous origin from the standpoint of geologi-
cal age has invariably resulted in failure. As has been already
indicated, the greater portion of the granitic, gneissic, or highly
metamorphosed crystalline schists and calcareous rocks belong
1 See Stockbridge's Kocks and Soils, p. 12.
EFFECT OF PLANT AND ANIMAL LIFE 389
either to the Archaean or older Palaeozoic formations, but this
merely because they, being older, have been longer subjected to
metamorphosing agencies, and not because in themselves they
possess essential differences. It is true that some authorities
lay stress on the supposed abundance of animal remains in cer-
tain Palseozoic formations, but no one but the veriest amateur
would now dream of attempting to discriminate between either
igneous or aqueous rocks of the same nature, but of different
geological ages, on purely chemical grounds.
It is a fact, however, that within certain climatic limits, the
rocks of any one horizon may impart such characteristics to a
residual soil as shall render it adapted to plant growth of a
particular kind. Thus,1 throughout the central portion of
Kentucky, where, within the distance of a few miles, rocks
occur of several distinct geological horizons, each bearing its
mantle of residual soil, each horizon may be traced for long
distances, though the rocks themselves are wholly obscured,
merely by the character of its forest growth.
(8) Soils as affected by Plant and Animal Life. — There are
various forms of animal and plant life the action of which is
worthy of note in connection with the subject of decomposition ;
but since it is probable that they are of greater efficiency in
promoting changes in soils once formed than in bringing about
the preliminary rock disintegration, their consideration has
been left to form a portion of the present chapter.
Ants, by means of their numerous borings, penetrating at
times to depths of man}' feet, bring about not merely a rear-
rangement of soil particles through a transfer of materials from
lower to higher levels, but also a condition of porosity whereby
air and water gain access to the deeper lying portions, there
to promote further chemical and physical changes.
Naturally these insects limit their work to dry and light
soils, where their operations may be compared with that of
earthworms whose operations are confined to moist ones.
Shaler. has calculated2 that over a certain field in Cambridge
( Massachusetts) the ants have made an average transfer of soil
matter from the depths to the surface sufficient to form a layer
each year of at least a fifth of an inch over the entire four acres
under observation. He further mentions a curious effect aris-
1 As the writer is informed by Mr. J. R. Proctor.
2 12th Ann. Rep. U. S. Geol. Survey, 1890-91, p. 278.
390 THE REGOLITH
ing from the interference of the ants with the original condi-
tions, in the separation of the finer from the coarser particles.
In certain parts of New England where sandy soils had laid
for a long time uncultivated, fields were covered to a depth of
some inches with a layer of fine sand without pebbles larger
than the head of a pin, while below the level of perhaps a foot
the soil was mainly pebbles, with very little finer material.
This condition, it is argued, was brought about by the tens of
thousands of ants which each year, over every acre, in the
process of building their dwelling brought up the finer material
and deposited it in the form of a mound about the surface
opening, leaving behind the coarser particles, too heavy for
them to move. The common black and brown ants of the
United States (Formica exsectoides) build upon the surface
mounds not infrequently from 1 to 2 feet in height and 3 to
5 feet in diameter and which are composed of materials brought
up from below intermingled with twigs and shreds of bark and
leaves from the surface. Shaler calculates the mass of some
of these mounds as equal
to 2 cubic yards. Being
of unconsolidated, loosely
coherent material, such
are constantly being de-
FIG. 40. -Effects of ant-hills on soils, aa.sand Sraded *? wind and rain
accumulated in hill; 66, material washed and their particles dis-
down the slopes, mingled with vegetable tributed Over the SUr-
mould.
rounding surf ace. "Where
these structures are numerous, as they are in certain districts
in the United States, by their constant deposits of matter on
the surface of the ground, they bury a good deal of vegeta-
ble waste in the soil ; at the same time the animals are con-
stantly conveying into the earth large quantities of organic
matter which serves them as food, and the waste of this,
including the excreta of the animals themselves, is of con-
siderable importance in the refreshment of the soil." The
geological efficacy of insects of this and other types is un-
doubtedly greater in warmer climes, where not only are they
found in greater abundance, but their period of activity ex-
tends over a larger portion of the year. Messrs. Mills and
Branner, as already noted, are inclined to lay considerable
stress on the work of ants and termites in bringing about soil
EFFECT OF PLANT AND ANIMAL LIFE 391
changes and rocks decomposition in Brazil. Branner states
that in some parts of the Amazon valley, of Minas Goyaz and
Matto Grosso, the soil " looks as if it had been literally turned
inside out by the burrowing of ants and termites." The species
popularly known as saubas excavate chambers and build gal-
leries which are frequently from 50 to 100 feet long, from 10
to 20 feet across, and from 1 to 4 feet high, and contain tons of
earth. The white ants or termites, like the true ants, burrow
extensive channels in the ground, and build up huge nests
upon the surface from the size of which one may gain some
idea of the extent of the underground galleries. In the region
extending from the state of Parana to north of the Amazon
and along the upper Paraguay in Matto Grosso may be seen
places where the nests are so close together that one can al-
most walk upon them for several hundred yards at a time,
while no one of the nests is more than 10 feet from another
over many acres of ground. Such nests vary in size from 1
to 12 feet in height and 1 to 10 feet in diameter, and do not
seem to be confined to any particular kind of country, though
especially noticeable in the interior and timberless regions. The
constant transference of such quantities of soil from below to
the surface, and of organic matter from the surface downward,
cannot fail to bring about marked changes in its physical as
well as chemical condition, while at the same time affording
passageways for air and meteoric waters, as already noted.
Certain animals, like the crayfish, have likewise a habit of
burrowing in the ground, though as they are wholly subterra-
nean or aquatic in their nature, the results are less conspicuous
to the casual observer. In searching for their food, these ani-
mals bore numerous horizontal channels or galleries some-
times an inch or so in diameter and extending for many feet,
usually ending in an upward shaft extending to the surface,
or at the margin of a pond or stream. These form natural
drainage channels and allow a more ready access of air, con-
verting w hat might under other conditions be a heavy, clayey
or even marshy soil, unfit for cultivation, into one light and
fertile.
By burrowing through dams and embankments, they have,
however, in some instances so weakened these structures as to
cause them to give way, whereby large districts have become
inundated and for a time rendered unfit for cultivation.
392 THE REGOLITH
Probably none of the forms of animal life thus far mentioned
produce such wide-spread and beneficial results as have been
ascribed by Darwin 1 to the common earthworm, the angleworm
of the New England disciples of Izaak Walton. These insig-
nificant creatures, as is well known, burrow in the moist rich
soil, and derive their nourishment from the organic matter it
may contain. In order, however, to obtain this comparatively
small amount of nutritive matter, they devour the earth with-
out any selective power, and pass it through their alimentary
canals, rejecting the remainder, which nearly equals in bulk
that first taken in. The numerous holes made, while in part
perhaps to afford passage to the surface, are mainly excavated
in this process of soil eating and actually represent the amount
of material which the worms have passed through their diges-
tive systems.
Darwin states that in certain parts of England these worms
bring to the surface every year, in the form of excreta, more than
10 tons per acre of fine dry mould, " so that the whole superficial
bed of vegetable mould passes through their bodies in the course
of every few years." By actually collecting and weighing the
excretions deposited on a small area during a given time, he
found that the rate of accumulation was at the rate of two-
tenths of an inch a year, or an inch in every five years. The
importance of these worms, then, both as mellowers of the soil
and as levellers of inequalities — by burying stones and filling
hollows — is therefore very great, and we cannot afford to
overlook it here.
While the main influence of the worm is manifested in a
mellowing by burrowing and a transfer of material from a
lower to a higher level, they bring about a slight admixture
of organic matter through a habit of coming to the surface at
night time, and dragging down into their burrows small shreds
of leaves and grass, which, taken into account in connection with
the excrementitious matter of the worms themselves, must tend,
though it may be ever so slightly, to enrich the soil. The sub-
ject should not be dropped without referring to the abundance
of these worms, which in England has been estimated as at the
rate of 53,767 to each acre of garden land, and about one half
that number for pasture land. It is scarcely necessary to re-
mark that their distribution is very unequal throughout the
1 The Formation of Vegetable Mould.
EFFECT OF PLANT AND ANIMAL LIFE 393
world, and that in dry sandy regions they are almost, if not
wholly, unknown.
In northern temperate climates, such as that of New England,
and particularly where the soil is of a clayey nature like the
ground moraine, the burying action of the earthworm, as de-
scribed above, may be wholly overcome through the heaving
action of frost. Every farmer boy who has been condemned
to pick the drift boulders from a field knows through bitter
experience that, however well he may do his work in the fall,
however clean the surface may be when winter sets in, the fol-
lowing spring, after the frost is out of the ground, will find a
new crop in no way distinguishable from the old, and which,
for all that he can see, may have rained down during the win-
ter's storms. The fact is, however, that they have been actually
thrown up, " heaved out," the farmers will say, from below the
surface by the frost which here penetrates not infrequently to
a depth of two or more feet. As the water-soaked clay under-
lying one of these buried boulders freezes, it expands upwards,
since this is the direction of least resistance. The stone is
carried up bodily for a distance dependent on the amount of
expansion. When the frost leaves the ground, the soil sinks
back nearly to its first position; but the boulder never quite
regains its former place, being prevented by particles of soil,
or clay or pebbles which fall into the cavity as the soil shrinks
away from it. The amount of actual lifting for each season
may be but slight, but as the process goes on unceasingly there
is always an abundance of new material at the surface each
succeeding spring. This heaving action of the frost is abun-
dantly exemplified in these clay regions by the throwing out
of fence posts and clover roots ; sometimes, when the winter is
one of frequent freezing and thawing, causing the destruction
of a crop as completely as though it had been pulled up by the
roots. In wet boggy lands this heaving action of frost, as
exerted on partially buried boulders of small size, is sometimes
exemplified in a peculiarly striking manner. The surface of
the ground will be dotted here and there with small hummocks,
each with a comparatively large crater-like opening at the top.
Investigation reveals the fact that at a distance of but a few
inches at most below the surface of this crater-like opening is
a rounded boulder. The heaving action of the frost forces the
boulder gradually upward, causing the turf to first rise with
394 THE REGOLITH
smooth rounded outline, till, through continual pressure from
the boulder, it bursts at the top. When the frost leaves the
ground, the boulder drops back a short distance, but enough
to be quite out of sight, leaving the cavity at the top filled
with mud, and looking — in outline — like a small mud volcano.
So far as the writer's observations go, the heaving action rarely
progresses, in these areas, to the point of actually throwing
the boulder out upon the surface. Each summer the growing
turf makes an attempt at healing the wound, but each winter's
frost opens it once more, the alternating forces so nearly bal-
ancing that little is accomplished after this pseudo- volcanic
stage is reached.
Insects like the boring bee, the burying beetle, or larger bur-
rowing animals, like the " woodchuck " of the Eastern states,
the prairie dogs, badgers, and spermophiles of the West, in the
same way exert powerful though local influences in admixing
the lower with the upper portions of the soil, and through
allowing perhaps a more ready passage of water facilitating
oxidation and decomposition at greater depths. (Fig- 2,
PL 19.)
While the effect of these animals may be comparatively in-
conspicuous in the regions east of the Mississippi, in the drier
regions of the West the surface is not infrequently so under-
mined by burrows as to make travelling on horseback at more
than a very moderate pace a matter of grave difficulty. W. P.
Blake, in the early reports of the Pacific Railroad Survey,
states that the fine, silty soil of the Tulare valley in California
is so undermined that it is almost impossible to travel over it.
" Mules often break through the thin crust and sink to their
shoulders in these holes."
The action of plant life in the accumulation of vegetable
mould has been fully discussed under the head of cumulose
and alluvial deposits. There is, however, one phase of action
which may well be mentioned here. A growing tree, as already
noted, sends its roots deep down into the earth in search of food
and foothold. So long as the tree remains alive and standing,
in firm soil the amount of change in the soil itself, except in the
way of abstraction of certain constituents taken up by the grow-
ing plant, is presumably very small. When, however, the tree
dies, the roots slowly decay, and besides yielding up their con-
tents to form new soil, afford passageway for percolating water
EFFECT OF PLANT AND ANIMAL LIFE
395
Fia. 41.
vith all its attendant results. Moreover, cases are by no means
infrequent in which trees are upturned by the winds, bringing
entangled in their roots it may be tons of soil and boulders
which in part gradually fall back into the hole and in part re-
main to form a mound
which marks the spot long
after the tree has de-
cayed. Into the cavity
thus formed, dead le<aves
and other organic debris
accumulate, which in time
form deep rich loam to be
r« niiiiiingled with the stony
matter of the soil. In sec-
tions of the country where
heavy winds and hurri-
canes are of frequent oc-
currence, the efficacy of
trees in thus burying or-
ganic matter, and produc-
ing a more complete inter-
mingling of the soils, is by
in i means inconsiderable.1
The influence of plants in
adding carbon and inci-
dentally carbonic and other
organic acids to the soils
has been described in previous pages. When plants die and
decay upon the immediate surface, there is left only the inor-
ganic matter or ash behind, the carbonic acid escaping into the
air or being carried by rains into the soil. Hence it would
seem to naturally follow that the -soil where supporting an
abundant vegetation should contain a larger percentage of
carbonic acid than the atmosphere itself. That it does not
contain, in all cases, a greater amount of free carbonic acid is
apparently brought out in the table from the works of Bous-
singault and Lewy, as quoted on p. 178.
1 Some of our archaeologists go so far as to assert that the stone implements
found buried several fee,t below the surface in glacial deposits, and brought for-
ward as proving the existence of pre-glacial man, have been brought into that
position by just such agencies. See Holmes, Early Man in Minnesota, American
Geologist, April, 1893, p. 228.
Foreit Mould.
FIG. 42.
396 THE REGOLITH
Bacteria as agents of nitrification are undoubtedly efficacious
in preparing nitrogeneous matter in the soils for assimilation
by growing plants. Their influence as decomposers of rock
masses was noted on p. 203. According to Wiley,1 it is highly
probable that organic nitrogen in the soil, in passing into the
form of nitric acid, exists at some period of the process in the
form of ammonia. The products of nitrification, he says, are
ammonia, nitrous or nitric acid, carbon dioxide, and water.
The ammonia and nitrous acid may not appear in the soils as
the final products of nitrification, as the organism attacks the
nitrous acid at once, converting it into the nitric form.
It may at first seem strange that man, who prides himself on
being the highest type in the animal kingdom, as well as the
only animal endowed with reasoning powers, should prove the
most destructive ; yet such is the case. Through prodigality,
due in part to thoughtlessness and in part to a wilful disregard
for any but immediate interests, man has, apparently from the
very beginning of his existence, so conducted himself with re-
lation to natural resources as to leave little less than ruin in
his path. This is true not merely with reference to his treat-
ment of the soil, but of the deeper lying rocks and their min-
eral contents. In the name of development he has squandered ;
through careless husbandry he has not merely impoverished
the soil, but in many cases allowed it to run waste and be lost
beyond recovery. So long ago as 1846, when Lyell made his
second visit to America, he was struck by the rapid denuda-
tion of the land in our Southern states due to the reckless cut-
ting away of the forests. He describes near Milledgeville, in
Georgia, a washout in a lately deforested area. " Twenty years
ago," he writes, " before the land was cleared, it [the washout]
had no existence ; but when the trees of the forest were cut
down, cracks 3 feet deep were caused by the sun's heat in the
clay ; and during the rains, a sudden rus*h of water through the
principal crack deepened it at its lower extremity, from whence
the excavating power worked backwards, till in the course of
20 years, a chasm measuring no less than 55 feet in depth, 300
yards in length, and varying in width from 20 to 180 feet was
the result. The high road has been several times turned to
avoid this cavity, the enlargement of which is still proceeding,
and the old line of road may be seen to have held its course,
1 Principles and Practice of Agricultural Analysis, p. 464.
EFFECT OF PLANT AND ANIMAL LIFE 397
directly over what is now the widest part of the ravine. In
the perpendicular walls of this great chasm appear beds of clay
and sand, red, white, yellow, and green, produced by the de-
composition in situ of hornblendic gneiss, with layers of veins
of quartz, which remain entire, to prove that the whole mass
was once solid and crystalline."1
The same lack of foresight or wanton disregard for coming
generations is still manifested, and every muddy stream bears
downward to the sea an increased load of silt from lands im-
properly cultivated and from which every rain removes a por-
tion of the finest and riehest of the soil, leaving behind but the
liarren gravel, channel lei I it may be beyond the possibility of
cultivation. McCJee- lias more recently made observations of
a similar nature in southern Mississippi, where the softer loam
of the Columbia formation, which here forms the soil, has
been allowed to become eroded down to the barren sandy loam
of the Lafayette. •• Old fields are denuded by the acre, leaving
> of pinnacles divided \>\ a Complex network of runnels
glaring red toward the sun and sky in strong contrast to the
rich verdure of the hillsides never deforested ; the plantations,
mansions, and -quarters' are undermined, and whole villages,
once the home of wealth and luxury, are being swept away at
the rate of acres for each year."
"The ravages committed by man," writes Marsh,3 " subvert
the relations and destroy the balance which nature had estab-
lished between her organized and her inorganic creations, and
she avenges herself upon the intruder by letting loose upon her
defaced provinces destructive energies hitherto kept in check
by organic forces destined to be his best auxiliaries, but which
he has unwisely dispersed and driven from the field of action.
When the forest is gone, the great reservoir of moisture stored
up in its vegetable mould is evaporated, and returns only in
deluges of rain to wash away the parched dust into which that
mould has been converted. The well-wooded and humid hills
are turned to ridges of dry rock, which encumbers the low
grounds and chokes the watercourses with its debris, and —
except in countries favored with an equable distribution of rain
1 Lyell, Principles of Geology, 9th ed., 1846, p. 204.
2 12th Ann. Rep. U. S. Geol. Survey, 1890-91.
8 The Earth as modified by Human Action, by Geo. P. Marsh, a new edition
of Man and Nature, pp. 43, 44.
398 THE REGOL1TH
through the seasons, and a moderate and regular inclination of
surface — the whole earth, unless rescued by human art from the
physical degradation to which it tends, becomes an assemblage
of bald mountains, of barren, turfless hills, and of swampy and
malarious plains. There arc parts of Asia Minor, of northern
Africa, of Greece, and even of Alpine Europe, where the opera-
tion of causes set in action by man has brought the face o'f the
earth to a desolation almost as complete as that of the moon ;
and though, within that brief space of time whicli we call 'the
historical period,' they are known to have been covered with
luxuriant woods, verdant pastures, and fertile meadows, they
are now too far deteriorated to be reclaimable by man, nor can
they become again fitted for human use, except through great
geological changes, or other mysterious influences or agencies
of which we have no present knowledge, and over which we
have no prospective control. The earth is fast becoming an
unfit home for its noblest inhabitant, and another era of equal
human crime and human improvidence, and of like dura t inn
with that through which traces of that crime and that improvi-
dence extend, would reduce it to such a condition of impover-
ished productiveness, of shattered surface, of climatic excess,
as to threaten the depravation, barbarism, and perhaps even
extinction of the species."
LIST OF AUTHORS CITED OR REFERRED TO
Agassi z, L., IT-.
<i.T, II. F., 212.
Auuh.-y, S.,331.
Bartl.it. \\ . II . 180.
%8G.
muni, Klir dr. 160.
IWk.-r, <i. F.,-_'.o, 301.
B.-II, K TI, i!»4. 21:;, 211;, 275.
B,-lt.T., I".. -V.I, -'77, 280. .
Bvnl.i.-r. P., 287.
M., 1TK.
•f, c;.. -JO, 27, 191, 192.
Blaaa. J., HL'.
Bl:ik.-f \V. P., 121. 126,185, 247, 25<>, 34!i,
Bult. .n. II. Cairiiigton, 202.
Bouncy. T.G.. 24ii.
Boiissiugauli, J. B., 176.
Braimcr. .1. C., Ill, 175, 179, 188, 203, 2TS.
Brtgger, \v. 0.,«*.
Br..ii-niari. \.. S7, 175,237.
Brown. A. 1'.. •_•:«.
Brunei-. II. l..,:U4.
Biu-liaiiaii. J. V., 204.
I'al.lrleti-Ii. Alexander, 193.
riiaml.crlain, T. C., 278, 301, 303.
<•!>,. iTat. l'...'.V..
Clark, \V. M., LSI.
Clark. \V. ('., 118.
el...-/. iTi;.
Collier, P.. :^'.i.
Comte de la Htire, 188.
Crosby. W. O.. 138, 189, 255, 353, 385.
Cross,* C. W., 35, 62, 71, 81.
Culver, G. E.. '-'TH.
Gushing, H. P., 279.
Dana, E. S., 31, 127.
Dana, Professor J. D., 49, 57, 117, 108,
235,251, 253, 2(i'_'.
Darton, N. L.. 312.
Darwin, E., 175, 233, 292, 392.
DaiilTiM-, A., It!, 197,376.
Davis, W. M., 186.
Davi.lson, C., 287.
Dawson, J. W., 291, 334.
De Luca, 176.
Derby, O. A., 188, 277.
Diller, .1. S., 87, 92.
Dtmon, C. E., 196.
Dwi-lit. -".'7.
1 ».\.-r. I!., 202.
Kbi-li.ien, M., 237.
Egleston, Tbomas, 184.
Kwini;, A. L., I'.M.
Faily.-r. (',. H., 176.
Fernow, B. E., 282.
i. Dr. Max, 243.
Forbes, 184.
Forschammer, J. G., 237.
Fournet, 175, '_' :iT.
Fulton, R. L., 280.
Furlonjre, W. H., 277.
Geikio, A. ,2, Htl, 201,288.
(I. ikie, James, 357.
Gi-klmarlicr, Max, 236.
Gesner, H. S., ::i7.
Gilbert, G. K., 50, 185, 256, 349.
Gordon, C. H., KM.
Griswold, L. S., 111.
(iunibcl. C. W.. 28, 88.
Hall, C. W., and Sardeson, F. W., 161, 250.
Harker, A., 39.
Hartt, C. F., 1T5, 280.
Haw.-s, <;. W., 46, 75,87, 170.
Haworth, E., 25.
Hayden, F. V., 252.
Hayes, C. W., 109, 194.
Heusser and Claraz, 175, 228, 251.
Hilgard, E. W., 333, 346, 3WI, 3G7, 369, 371,
374.
Hitchcock, C. H., 68.
Hitterman, 239.
Hobbs, W. H., 218.
Holmes, W. H., 395.
Hovey, E. O., 230.
Hunt, T. S,, 8(5, 99, 124, 159, 258.
Iddings, J. P., 22, 39, 57, 60, 64, 71, 72, 81.
Irving, R. D., 278. «
Johnson, S. W., 177, 178.
Johnson and Blake, 136.
399
400
LIST OF AUTHORS CITED OR REFERRED TO
Johnstone, Alexander, 189.
Jones, T. Rupert, 317.
Judd, J. W., 284, 321.
Julien, A. A., 190.
Kalkowski, E., 75.
Kemp, J. F., 81, 86, 87, 171.
Kerr, W. C., 286.
Keyes, C. R., 25.
Kidder, J. H., 179.
King, F. H., 381.
King, Clarence, 71.
Klement, M. C., 160.
Kletzinsky, W., 176.
Kulm, M. Levy, 89.
Layard, A. H., 293.
Le Conte, J., 258.
Lemberg, J., 18, 217, 374.
Lindgren, W., 75, 27 J.
Livingstone, David, 183.
Loftus, 293.
Loughbridge, R. H., 365, 366.
Lyell, Sir Charles, 396.
Marsh, George P., 183, 297.
McGee, W. J., 301, 312, 323, 3<J7.
Meister, 381.
Merrill, G. P., 37, 47, 54, 81, 87, 98, 113,
115, 154, 159, 206, 218, 349.
Mills, J. E., 175, 203, 273.
Muller, Alex, 371.
Miiller, R., 192.
Munroe, C. E., 190.
Miintz, A., 203.
Miintz and Aubin, 179.
Miintz and Maracano, 373.
Murakozky, K. V., 238.
Neumayer, M., 302.
Newberry, J. S., 118, 356.
Nordenskiold, A. E., 242.
Oldham, R. D., 311.
Or ton, Edward, 117, 118, 124.
Owen, D. D., 111.
Packard, R. L., 108, 376.
Pallarsen, 89.
Peurose, R. A. F., 231, 232.
Pirsson, L. V., 64.
Pliny, 73, 90.
Porter, J. B., 266.
Potter, W. B., 265, 275, 276.
Prestwich, Joseph, 65, 260.
Prichard, 30.
Pumpelly, R., 275, 277.
Purrington, C. W., 279.
Read, T. Mellard, 194.
Redwood, Boverton, 129.
Retgers, J. W., 349.
Reusch, H., 250.
Richthofen, F. von, 63, 85.
Rogers Brothers, 191.
Rohrbach, C. E. M., 89.
Roscoe and Schorlemmer, 4.
Rose, G., 79, 89.
Roseubusch, H., 57, 62, 70, 72, 74, 82, 93,
97, 98.
Rosiere, 73.
Roth, Justus, 72, 94, 101, 103, 208, 239,
25(i.
Russell, I. C., 112, 201, 266, 279, 280, 284,
2'. Hi, 301, 333, 385.
Rutley, F., Ill, 194.
Safford, J. M., 267.
Salisbury, R. D., 278, 287, 301, 303, 352.
Schlosing, 203.
Schutze, R., 228.
Shaler, N. S., 181, 197, 318, 336, 389, 498.
Smith, Angus, 179.
Sorby, H. C., 26, 38, 199, 243, 342.
Spurr, J. E., 107.
Stanley, H. M., 183.
Stejneger, L., 199.
Stone, G. H., 186.
Storer, F. H., 191, 202.
Strabo, 90.
Streeruwitz, H. von, 182.
Streng, A., 86.
Teall, J. J. H., 24, 74, 90.
Thenard, P., 190.
Thompson, Wyville, 247.
Tornebohm, A. E., 87, 89.
Tschermak, G., 24.
Van Den Broeck, E., 178, 258.
Van Hise, C. R., 106.
Vom Rath, G., 255.
Von Buch, L., 83.
Wadsworth, Dr. M. E., 57, 68, 85, 97, 254.
Weed, W. H., 109.
Werner, A. G., 73.
Whitaker, W., 267.
Whitney, J. D., 68, 127, 278, 328.
Whitney, Milton, 287, 307-309, 313, 340,
344, 353, 379.
Wichman, A., 170.
Widogradsky, 203.
Wiley, H. W., 178, 316.
Williams, G. H., 63, 72, 86, 96, 99, 100,
156, 216.
Williams, J. F., 64.
Willis, Bailey, 52.
Winchell, N. H., 297.
Wolff, Professor J. E., 93.
Woodward, J. B., 186.
Wurtz, H., 127.
Zirkel, F., 38, 57, 68, 80, 87, 89.
INDEX
Abrasive action of wind-blown sand, 185.
Acid rocks, meaning of term, 64.
Acmite, 22.
Adobe, 139, 332.
.!•:.. I i:m deposits, 344.
. K< .1 ian rocks, 153 ; defined, 68.
Aualniatolite, 116.
: soils, 386.
Air in motion, effects of, 189.
Alabaster, 117.
Alaska, rnck-\vcathcring in, 279, 284.
Alhertite described, 127.
Albite as a rock constituent, 16.
Alkalies in soils, .">71.
Alkaline carbonates, when formed, 372;
in soils, ;>71 ; formed during weather-
in-, •_•(».-,.
Alkaline silicates in soils, 370.
Allanite, •_',->.
Allotriomorphic minerals defined, 41.
Alluvial cones defined, 54.
Alluvial deposits, 320.
Alteration defined, 174.
Alum shale, 138.
Aluminum as a constituent of the earth's
crust , ">
Amber, 128.
Amianthus, 11.1.
Ammonia in atmosphere, 177.
Ammonium sulphate, influence in decom-
posing feldspars, 178.
Amorphous, definition of, 40.
Amphiholt's as rock constituents, 19.
Amygdaloidal structure, 34.
Anacostia, deposits of the, ."•:.':!.
Analyses, calculations of, 210; discus-
sion of, 212.
Anamesite, 92.
Audesites, 83.
Andesitic rocks, induration of surface,
255.
Anhydrite described, 118.
Animal life, effect on soils, 389.
Anorthite as a rock constituent, 17.
Anorthit-gesteine, 89.
Anthracite coal, 150.
Antique porphyry, 83.
Ants, effect on soils, 389 ; as promoters of
rock decomposition, 204.
Apatite as a rock constituent, 27.
Apo-rhyolite, 72.
Appalachian Mountain system, material
eroded from, 196.
Appomattox formation, 312.
Aqueo-glacial clays, 334.
Aqueous rocks, 105; defined, 58.
Aragonite as a rock constituent, 26.
Arenaceous group, the, 131.
Argillaceous rocks described, 135.
Argillites, 137; fissile, 170; Harford
County, Maryland, weathering of, 229.
Arkansas River referred to, 289.
Asphaltum described, 125.
Atmosphere, action of, 176.
Augite, molecular alteration of, 39 ; rela-
tive durability of, 235.
Augite porphyrite, 90; Montana, disin-
tegration of, 235.
Augite vitrophyrite, 90.
Augitite described, 101.
Auriferous sands, origin of, 266.
Bacteria, as agents of nitrification, 396;
decomposing action of, 203.
Banding in gneisses, origin of, 165.
Barbadoes Island, volcanic dust on, 298.
Barite described, 118.
Barium as a constituent of the earth's
crust, 7.
Basalt, described, 90; Bohemia, weather-
ing of, 223 ; Haute Loire, France,
weathering of, 223.
Basalts, geographical distribution in
United States, 92 ; weathering of, 262.
Basanite described, 94.
Base, definition of, 40.
Basic rocks, meaning of term, 64.
Beach sands, 341.
Beauxite described, 108.
401
402
INDEX
Bedded. rocks defined, 53.
Bedded structure, 34.
Bermuda, weathering of limestones in,
L'47.
Biliary granite, 68.
Biotite as a rock constituent, 23.
Bitumen, 125.
Bituminous coals, 149.
Bituminous dolomite of Chicago, 145.
Black earth, Russian, .'US.
Bleaching of rocks on exposure, 257.
Bluegrass soil, 382.
Bog of Allen, 317.
Bogs, classification of , 317.
Boss, defined, 50.
Boss-like form accentuated by joints, 245.
Botryoidal structure, 37.
Boulder clay, 138.
Boulder clays, 353.
Boulders, of decomposition resembling
those of the drift, 242; formed by
weathering, 244.
Bowenite, 116.
Breccia, 133.
Brecciated limestones, 139.
Brecciated structure, 38.
Bronzitite, 100.
Brown hematite, 29, 107.
Brownstone, 133.
Cabook, formation of, 242.
Calc sinter, 112.
Calcareous group of rocks, 137.
Calcareous rocks, 143 ; rate of 'weather-
ing, 272.
Calcite as a rock constituent, 25.
Calcium as a constituent of the earth's
crust, 6.
Calcium carbonate, amount annually re-
moved in solution, 194.
Camptonite, 8.
Cannel coal, 150.
Cape Cod, wind action on, 297.
Carbonates of alkalies, influence of, 238.
Carbonates of the alkalies formed during
weathering, 205.
Carbonates, production of, during weath-
ering, 205.
Carbonaceous rocks, 148.
Carbonic acid, influence of, in feldspathic
decomposition, 237, 239; amount annu-
ally brought to the surface, 179 ; in air
of soils, 178 ; in the atmosphere, 178.
Carboniferous soils, 386.
Catlinite, 139.
Cavernous structure, 38.
Cellular structure, 38.
Ceylon, rock disintegration in, 242.
Chalcedony, 110.
Chalk, 143; decomposition of, 267.
Chemical composition of rocks, 44.
Chemical elements constituting rocks, 4.
Chert, 110; of Arkansas, weathering of,
231 ; of Missouri, weathering of, 230.
Chilian nitrates, origin of, 373.
Chlorides, 119.
Chlorite as a rock constituent, 30.
Chrysotile, 115.
Citric acidj solvent property of, 202.
Classification of soils, 381.
Clastic rocks, 129 ; classification of, 130.
Clastic structure, 34.
Clay concretions, formation of, 37.
Clay, defined, 135; effect on soils, 368;
protective influence of, 254.
Clay ironstone, 114.
Clay slates, 137.
Clays, aqueo-glacial, 334.
Climate, influence of, on weathering, 278.
Clinton iron ores, origin of, 266.
Coefficient of cubical expansion of min-
erals, 268.
Coking coals, 150.
Cold, effect on rocks, 180.
Colloidal structure, 33.
Colluvial deposits, 319.
Color ; changes incidental to weathering,
257 ; of rocks, 45 ; of soils, 384 ; of soils,
cause of, 385 ; variation, cause of, 47.
Colorado River, erosion by, 196.
Columbian formation, 312.
Columnar structure, 38.
Complexity of structure favoring disinte-
gration, 250.
Concentric exfoliation, 244; not indica-
tive of an original concretionary struct-
ure, 245.
Concentric structure inevitable to joint-
ing, 245.
Concretionary structure, 35; in granite,
246 ; in crystalline rocks, 37.
Conductivity of rocks, unequal, 184.
Conglomerate, 133.
Conservative action of plants, 202.
Contact metamorphism, 157.
Contours incidental to weathering, 259.
Coprolite nodules, 152.
Coquina, 143.
Coral limestone, 143.
Corroded surfaces, irregularity of, 250.
Corsica, weathering of granite on, 250.
Crayfish, effects on soils, 391.
Creeping of soil cap, 287.
Crenic acid, 190.
Crystalline limestones and dolomites,
162.
Crystalline schists, the, 168.
Crystalline structure, 33.
INDEX
403
< 'rystallites defined, 41.
Cumulose deposits, 313.
Dacite, 84.
Daubree's experiments in rock tritura-
ti-in. 17, HIT.
!>• -ea\ . time limit of, 272; of rocks, how
characterized, L'12.
Di-composition and disintegration, dis-
(•riiniiiatiini between, 283.
Decomposition, depth ui, L'78;' following
disintegration. 1*43; incident to ero-
sion. 1H7; of fragmental rocks. I'L'S; of
uri. t-nstoiie dikes, effects of, 244; of
rocks, chemical processes involved in,
of shells' through the aid of salt,
•_M.;; natural acceleration of, 205.
• •ration of rocks, 174.
i ation of North Ameriea.rate of ,196.
Delia deposits, 320.
Delta of the Nile, section of, 321.
Dcnxidation, 1K7; hy marine animals, 204.
I ).-M-I I varnish. 25G.
Desil's Tower, origin of, 2(51.
Deweylite, 11<>.
Diabase, described, S7 ; mandelstein, 90;
Medfonl. Massachusetts, weathering
of, '_MS: porphyrite, 90; Venezuela,
weathering of, 222.
Diallogite, 100.
Diamonds, origin of, 98.
1 (iatomaceous earth, 141.
Diclite diabase, !tO.
Dike defined, 50.
IHliiriinn, roinjr ct gres, 258.
Diorite, Albemarle County, Virginia,
weathering of , 224.
Piorite-andesite group, 81.
Diorites. S7.
Discoloration, above drainage level, 258;
incidental to weathering. 2.">7.
Discussion of analyses. '2'-'A.
Disintegration of rocks in Lower Califor-
nia, IS.'!; prevented by surroundings,
•J.YJ ; without decomposition, '_'J1.
District of Columbia, rock-weathering
in, 283.
Ditroite, 79.
Dolerite, 92.
Dolomite, as a rock constituent, 26; de-
scribed, 145; origin of name, 163; ori-
gin of, by metasomatosis, 159.
Dolomites, 1C2.
Dolomitic limestones, disintegration of,
250; weathering of, 239.
Drift, extent of, 291.
Drumlins, 355; defined, 55.
Dune defined, 55.
Dune sand, chemical composition, 350.
Dunite, 97.
Dust, in rain and snow falls, 344; vol-
canic, 298, 349.
Dust soils, 345.
Dust storms, 292 ; in Dakota, 293 ; in Mon-
tana and Nevada, 294.
Dynamic metamorphism, 156.
Earth's crust, thickness of, 2.
Earthworms, effects on soils, 392.
Eclogite, 170.
Effacement of characteristics by weath-
ering, 262.
Effusive rocks, characteristics of, 61;
defined, 60.
Ehcolite as a rock constituent, 18.
Elreolite syenites, 78,
Elaeolite syenite porphyry, 79.
Elaterite described, 126.
Elvanite, 70.
Eozoon Canadense, 159, 163; origin of,
11(5.
Epidiorite, 89.
Epidote, as a rock constituent, 25 ; altera-
tion of, 25.
Erosion by rivers, 196.
Eruptive rocks, 59.
Eskers, 290, 356.
Eucrite, 89.
Eulysite, 97.
Eurite, 70.
Exfoliated rocks, shape and size of flakes,
182.
Exfoliation, attended by gun-like reports,
182 : due to heat and cold, 181 ; of rocks
on Cape Cod, 182.
Expansion through hydration, 188.
Extent of weathering, 276 ; in Brazil,
Colorado, District of Columbia, Mis-
souri, Nicaragua, South Africa, South
America, 277.
Fault defined, 53.
Feldspars, as rock constituents, 13; de-
composition of, 17.
Feldspathic decomposition, process of,
237; in Comstock Lode, 235; by fresh
water, 238 ; influenced by ammonium
sulphate and sodium chloride, 178.
Feldspar porphyry of Iron Mountain,
weathering of, 265.
Feldspars, relative durability of, 235.
Felsitic structure, 33.
Felsite pitchstone, 70.
Felsophyr, 70
Felstone, 70.
Ferrous carbonate, solubility of, 239.
Fertility of soil dependent on physical
condition, 379.
404
INDEX
Fichtellite, 129.
Fiorite, 109.
Fissile argillites, roofing slates, 170.
Flagstone, K53.
Flexible sandstone, 134.
Flint, 110.
Flood plain of the Mississippi, 323.
Fluidal or fluxion structure, 34.
Fogs, indices of dust in atmosphere,
344.
Foliated or schistose rocks, 164.
Foliated structure, 34.
Forelleustein, 87.
Forests, buried by sand, 295; influence
of, 2SO ; protective action of, 282.
Fourchite, 79.
Foyaite, 79.
Foyaite-phonolite group, 77.
Fracture of rocks, 48.
Fragmental structure, 34.
Freestone, 133.
Freezing water, disintegrating action of,
198.
Frontal aprons, 356.
Frontal nuiraiiu-s, 355.
Frost, action in accelerating decomposi-
tion, 278; action on soil, 367; disin-
tegrating action, 199; heaving effects
on boulders, 393; supposed protective
action of, 278.
Qabbro described, 85.
Gabbro-basalt group, 85.
Garnet rock, 170.
Garnrerite, formation of, 226.
Garnetite, 170.
Geest, 301.
Gem sands of Ceylon, origin of, 266.
Genetic relationship of rocks, 64.
Geological age, of soils, 389; a basis for
classification, 63.
Geyserite, 109.
Gilsonite, 127.
Glacial deposits, 351.
Glacial detritus, amount of, 201.
Glacial drift, extent of, 291.
Glacial lakes, extinction of, 289; filling
of, 326.
Glacial landscape, 291.
Glacial moraine, 290.
Glacial soil of Cape Elizabeth, composi-
tion of , 364.
Glacier, the, as an erosive agent, 200.
Glaciers, as agents of transportation, 200,
289.
Glass abraded by wind-blown sand, 185.
Glauconite, 31, 134.
Glauconitic marl, 134.
Globulitea defined, 41.
Gneiss, Albemarle County, Virginia, de-
generation of, 213.
Gneisses, the, 164.
Grahamite described, 127.
Granite, described, 05 ; extent of weather-
ing in District of Columbia, 276.
Grauitell, <J8.
Granite-liparite group, 65.
Granite porphyry, 68.
Granite soil defined, 383.
Granitite, 67.
Granofelsophyr, 70.
Granophyr, 70.
Granular structure, 34.
Granulite, 1<>7.
Grauwacke, 133.
Graphic granite, 07.
Gravels superficially oxidized, 258.
Gravity, action of, in transporting de'bris,
2Sli.
Greenland, rock-weathering in, 278.
Greensand marl, 134.
Greenstone, 81.
Greisen, 68.
Greywacke, 133.
Ground-mass defined, 40.
Ground moraine, 352.
Gruss, 301.
Guano, 151.
Gypsum described, 117.
Halleflinta, 167.
Hardpan, 368.
Harzburgite, 97.
Hatchettite, 129.
Hatteras and Henlopen, sand dunes of,
295.
Heat, action on pebbles in Arabia Petrea,
183 ; expansive action on rocks, 180.
Heat and cold, as agents of decomposi-
tion, 180 ; effects of, in Africa, 183 ;
effects limited to surface, 183; most
effective on slopes, 184.
Heavy spar, 118.
Hematite, 106 ; as a rock constituent, 28.
Holocrystalline, definition of, 40.
Hornblende, as a rock constituent, 19 ;
decomposition of, 20; relative durabil-
ity of, 235.
Hornblende picrite, 97.
Hornblendite, 100.
Humic acid, 189.
Humidity, weathering influenced by, 270.
Hyaline andesite, 85.
Hyalite formed during feldspathic de-
composition, 238.
Hyalobasalt, 92.
Hyaloliparite. 72. ""
Hyalomelan, 92.
INDEX
405
Hyalotrachyte, 77.
Hyd ration, 1ST : importance of, 188, 234,
. liT.s ; of micas, 189.
Hydraulic limestone, 145.
Hydrocarbon, compounds, description of,
I2L
Hydro-metamorphism, 101.
Hyperiie. 87.
H\|>erstheiiite, 100.
Hypiicrciiic in-iil, 190.
HypocryBtalline, definition of, 40.
Ice, disintegrating action of, 198; intlu-
••nrc in transporting rock de'bris, 287;
mechanical action of, I'.Ci.
Idiomorphic minerals defined, 41.
Igneous rocks, 59; denned, 57.
llineiiite as a rock constituent, 28.
Induration, cause of, 255; of rocks on
exposure, 254 ; of sandstone by igneous
contacts, 2til.
Infusorial ran li, 141.
Inserts, effects on soils, 394.
Intrusive rocks denned, <>0.
Inundated lands, classification of, 318.
Iron, as aconstituent of the earth's crust,
5 ; removed in form of ferrous sul-
phate, 239; removed in form of pro-
toxide carbonate, 239; variation in
solubility. •_'."•'. i.
Iron Mountain, Missouri, pre-Silurian
weathering of, 27ii.
Iron ores as rock constituents, 27.
Iron pyrites as a rock constituent, 29.
Itacolumite, 133.
Itarohunites, Brazilian, weathering of,
22&
.Jasper, 110.
.loints, as aids to weathering, 244; cause
of, '_'4.~> ; inriuriirr of, in producing
boulders, '_M4: inHueuce in producing
• toss-like forms, 245.
Kalk diabase, 90.
Kames, 290.
Kaolin, 11<;, lliiJ, 2iu; composition of,
.•;o;i: origin of, :i<i8.
Kaolinite distinct from kaolin, 309.
Kaolinization defined, 18.
Keratophyr, 76.
Kersantite, 82.
Kimberlite, 98.
Kinds of rocks, 56.
Kin/igkite. 170.
Konlite. 129.
Krakatoa, dust from, 298.
Ktaadn Iron Works referred to, 107.
Kugel porphyry, 70.
Labradorite as a rock constituent, 17.
Laccolite defined, 50.
Lake Agassiz, deposits in, 290.
Lake Asphaltites, 12(5.
Lakes, filling of, 314: transient charac-
ter of, 326.
Laminated or banded structure, 38.
Landscape, glacial, 291.
Lapilli, 140.
Laterite, 139, 310.
Laurvikite, 79.
Lava defined, 51.
Leda clays, 334.
Leopardite, 70.
Leptinite, 167.
Leucite as a rock constituent, 18.
Leucite basalt, 103.
Leucite-nepheliue rocks, 102.
Leucite rocks described, 102.
Leucitite, 103.
Leucitophyr, 80.
Leucophyr, 88.
Leucoxene, 28.
Lherzolite, 97.
Lichens, action of, 201.
Liebuerite, 79.
Lignite, 149.
Limburgite described, 98.
Lime carbonate, decomposing action of,
370.
Lime in soils, 366.
Limestone, unequal weathering of, 250;
weathering of, 232.
Limestone residuals, character of, 303.
Limestone soils poor in lime, 259.
Limestones, 143; and dolomites, 162;
corroded by acids, 194; corroded by
meteoric waters, 259 ; unequal indura-
tion of, 247; variation in composition,
147.
Limit of diminution in size of particles
by erosion, 197.
Limonite, 107 ; as a rock constituent, 29.
Liparite described, 70.
Litchfieldite, 79.
Lithophysse, 72.
Loess, 139, 290, 327.
Logans, or tors, 252.
Lower California, rock-weathering in,
283.
Lumachelle, 143.
Lustre, 48.
Luxullianite, 70.
Lydian stone, 111.
Magma, definition of, 59.
Magnesian limestones, 145.
Magnesia removed in excess of lime,
239.
406
INDEX
Magnesium as a constituent of the earth's
crust, 6.
Magiiesite, 113.
Magnetite as a rock constituent, 27.
Man, has squandered in the name of
development, 397; ravages committed
by, 397.
Marbles, 163.
Marcasite as a rock constituent, 29.
Marginal moraines, 355.
Marine animals, influence of, on marine
muds, 204.
Marl, 146.
Marmolite, 116.
Marsh gas, 121.
Marsh lands, reclaimable areas, 340.
Martite, 106.
Massive structure, 34.
Material lost through weathering, 208.
Materials lost during decomposition, pro-
portional amounts, 234.
Mechanical action of water and ice, 195.
Mechanical disintegration most active in
regions of extreme temperatures, 182.
Melaphyr described, 90.
Melaphyrs and augite porphyrites, 90.
Melilite basalt, 92.
Menaccanite as a rock constituent, 28.
Metamorphic rocks, 155 ; defined, 58.
Metamorphism denned, 155.
Metasomatosis defined, 158.
Miascite, 79.
Mica, relative durability of, 236.
Micaceous sandstone, cause of weather-
ing, 189.
Micas, alteration and decomposition of,
23; as rock constituents, 22.
Microcline as a rock constituent, 16.
Microcrystalline structure, variation in,
41.
Micro-granite, 70.
Microlites defined, 40.
Microlitic structure, 33.
Micropegmatite, 70.
Microscope used in geology, 38.
Microscopic structure, 38 ; of rocks, 33.
Microscopic study of rocks, efficiency of,
:«).
Mineral caouchouc, 126.
Mineral composition of soils, 373.
Mineral matter, dissolved by water, 191 ;
in solution, removed annually from
England and Wales, 194.
Mineral pitch, 125.
Minerals constituting rocks, 9 ; list of, 11.
Mineral variation of rocks, cause of, 9.
Mineral wax, 128.
Minette, 74.
Minnesota, wind action in, 297.
Mississippi, flood plain of, 323.
Mississippi River, amount of material
transported by, 288.
Missouri River, muddy character of,
288.
Mode of occurrence of rocks, 49.
Monazite sands, origin of, 266.
Mouoclinic feldspars, 14.
Monoclinic pyroxenes as rock constitu-
ents, 21.
Monzonite, 74.
Moraine defined, 55.
Moraines, classified, 355; glacial, 290.
Mosses, action, 201.
Muck, 149.
Muscovite, as a rock constituent, 23 ; rel-
ative durability of, 236.
Natural gas, 121.
Nepheline as a rock constituent, 18.
Nepheline basalt, 107.
Nepheline dolerite, 104.
Nepheline rocks described, 103.
Nepheline syenites, 78; weathering of,
249.
Nephelinite, 104.
Nevadite, 72.
Niggerheads, how formed, 244.
Nile delta, section of, 321.
Nineveh, site obscured by sand dunes,
295.
Nitrates, influence of, in feldspathic de-
composition, 239; in soils, 372 ; source
of, 372.
Nitric acid, in atmosphere, 177 ; influence
of, in feldspathic decomposition, 239.
Nitrogen, in atmosphere, 176; in soils, 372.
Non-coking coal, 150.
Norites, 86.
Noumseite, formation of, 226.
Novaculite, 111.
Nummulitic limestone, 143.
Obsidian, 72.
Oldest known rocks, 49.
Oligoclase, as a rock constituent, 16;
disintegration of, 241 ; decomposition
of, 237.
Oliviue, as a rock constituent, 24 ; altera-
tion into serpentine, 24 ; relative dura-
bility of, 235.
Onyx marbles, 113.
Oolites, English, coloration of, 258.
Oolitic limestone, 143; origin of, 53, 112.
Ophicalcite, 163.
Ophiolite, 89, 116, 163.
Organic acids, action of, 189 ; corrosive
power on marble, 190; solvent power
augmented by nitrogen, 190.
INDEX
407
< >rientai alabaster, 113.
Original constituents of rocks, 10.
Original structures preserved during de-
composition, 2IJ4.
Orthoclase, relative durability of, 236.
Orthoclasu porphyries, 75.
Orthodase as a rock constituent, 14.
« irthophyr, 7<;.
Orthorhombic pyroxenes as rock constit-
uents, 22.
. 2'.Hi, :;:,(;.
Ouaehitite, 79.
< )vcr\vash plains, 356.
Oxidation, how manifested, 187; inci-
dental to decomposition, 234.
Oxides, silica, lu'.i.
, as a constituent of the earth's
crust, 5 ; influence in preventing loss
of iron during rock decomposition, 239;
of the atmosphere as an agent of de-
CMinposition, 180.
0/..kerile, 128.
, 140.
1'aludal deposits, 336.
1'aniellerite, 72.
1'aratlin, native, 128.
Paramorphic minerals, 156.
Peat. Us.
1'cat Im-s, 317.
Pebble, normal shape of, 348.
IV-matite. <J7.
1'clites, i :;.-,.
1'eperino, 140.
Pei idotite, described, 95; weathering of,
235.
Peridotite-limburgite group, 95.
IVrlite, 77.
1'crlitic structure, 35.
Petroleum described, 122.
IVtrosilex, 70.
Plu'iiocrysts defined, 41.
Phlogopite, 23.
I'lioiiolite. weathering of, 217.
Phosphates, 119.
Phosphates of Tennessee, origin of, 267.
Phosphatic sandstone, l.r>2.
1'hosphorite, 119.
Phosphorus, as a constituent of the
earth's crust, 7 ; relative proportion of,
in rocks, 8.
Phyllite, 1G9.
Physical and chemical properties of
rocks, 33.
Physical condition of soils, 378.
Physical manifestations of weathering,
241.
Picrite. <C.
Picrite porphyrites described, 98.
Picrolite, 116.
Pic Pourri, decomposition of, by bacteria,
203.
Piedmontite, 25.
Pike's Peak, Colorado, weathering of
granite, 243, 255.
Pisolitic limestone, 143.
Pitchstone, 77.
Placer deposits, origin of, 267.
Plagioclase feldspars, relative durability
of, 236.
Plagioclases as rock constituents, 16.
Plant and animal life, effect on soils,
389.
Plant life, effect on soils, 394.
Plants and animals, agents of disintegra-
tion, 201.
Plutonic rocks, characteristics of, 60;
defined, 60.
J'uriiil'i >•«/.••>•« antico, 83.
Porphyrites, 83.
Porphyritic structure, 35.
Porphyroid, 167.
Post-Cretaceous decay of granite, 272.
Post-Glacial decay of diabase, 273.
Post-Jurassic weathering of grano-
diorites, 274.
Post-Pliocene weathering of andesites,
274.
Pol ash, in soils, replacing power of, 370;
soluble in soils, 376.
Potassium, as a constituent of the earth's
crust, 6; relative proportion of, in
rocks, 6.
Pot-holes, formation of, 196.
Potomac flats, 323.
1'otomac formation, 313.
Potstoue, 101.
Precious serpentine, 115.
Prc-Palseozoic decay of rocks, 275.
Primary rocks, 51.
Primary constituents of rocks, 10.
Principles involved in rock-weathering,
173.
Propyllite, 85.
Protective action, of plants, 202 ; of soil,
271.
Protogiue, 67.
Psammites, the, 131.
Pseudotuffs, 140.
Psilomelane, 107.
Puddingstone, 133.
Pulaskite, 79.
Pyrite, as a rock constituent, 29 ; decom-
position of, 29.
Pyroclastic rocks, 140.
Pyrolusite, 107.
Pyrophyllite, 116.
Pyrophyllite schist, 168.
408
INDEX
Pyroxenes, alteration and decomposition
of, 22 ; as rock constituents, 21.
Pyroxenite-augitite group, 99.
Pyroxenites, described, 99; weathering
of, 225.
Quarrying by aid of fire, in India, 182.
Quarry water, 199, 254.
Quartz, 110; as a rock constituent, 12;
the most refractory mineral, 234.
Quartz basalt, 92.
Quartz-free porphyries, 75.
Quartz porphyry described, 69.
Quartz veins, influence of contours, 2f>0.
Quartzite, origin of, 158; feldspathic,
disintegration of, 251; polished by
wind-blown sand, 257.
Quartzites, weathering of, in the District
of Columbia, 251.
Quaternary deposits, weathering of, 258.
Quitman Mountains, exfoliation of rocks,
182.
Rainfall, amount reaching the soil, 281.
Rain waters, temperatures of, 193.
Rapilli, 140.
Rate of weathering influenced by texture,
268; by com position, 269; by humidity,
270; by climate, 278 ; by position, 270.
Reaction rims, 240.
Regional metamorphism, 155.
Regolith, classification of, 300 ; origin of
name, 299.
Regur defined, 382.
Relationship between plutonic and effu-
sive rocks, 63.
Relative amount of material lost through
weathering, 284.
Relative durability of minerals, 234.
Relative rapidity of weathering among
eruptive and sedimentary rocks, 271.
Rensselaerite, 116.
Residual clays, 302 ; in caves, 233.
Residuary deposits, 301 ; analysis of, 306 ;
names proposed for, 301.
Results, incidental to weathering, 266;
of weathering due to position, 252.
Retiuite, 70, 129.
Retinolite, 116.
Rhodochrosite, 114.
Rhombporphyry, 76.
Rhyolite, 72 ; weathering of, 255.
Ribbons in slates, 155.
River channels formed by rock-weather-
ing, 243.
River erosion, 196.
Rivers, flood plains of, 289.
Rock, definition of, 1 ; disintegration of
on Bering Island, 199.
Rock-forming minerals, classification, 10 ;
list of, 11.
Rocking stones, 252.
Rock temperatures, in Africa, 183 ; at
Edinburgh, Scotland, 184.
Rock - weathering, 206 ; a superficial
phenomenon, 193; complexity of pro-
cess, 240; early references to, 17f> ; on
Lone Mountain, Montana, 243.
Rocks, absorptive power of, 198 ; chemi-
cal composition of, 44; classification
of, 57; color of, 45; composed mainly
of inorganic tnaterial, 131; composed
of debris from plants and animals, 141 ;
expansion and contraction under natu-
ral temperatures, 181 ; formed through
chemical agencies, 105 ; formed as sedi-
mentary deposits, 129; kinds of, 56;
mode of occurrence, 49; physical ;in<l
chemical properties of, 33 ; specific
gravity of, 43.
Roofing slate, microstructure of, 170.
Root action, how manifested, 202.
Roots, depth of penetration, in caves
and soils, 202.
Rosso de Levante, 98.
Rotteustone, origin of, 267.
Salt, common, 119 ; disintegrating effects
of, 198.
Sand, seolian, 346; Sorby's classification
of, 342; of dunes, sources of, 296.
Sand blast carving, 186; natural, 185.
Sand dunes, 346 ; formation of, 295 ; rate
of movement, 296.
Sand grains, lasting power of, 197.
Sandpipes, formation of, 260.
Sandstone, cause of disintegration, 247;
cementing matter of, 132; induration
of, 256; siliceous, weatherin'g of, 228;
spheroidal, weathering in, 247; un-
equal weathering of, 248.
Sandstone concretions, formation of, 37.
Sandstones, weathering of, 249.
Sanidin, kaolinization of ,238.
Sanidin-oligoclase trachyte, 77.
Saprolite, 301.
Satin spar, 117.
Saxonite, 97.
Scheerite, 129.
Schistose structure, 34.
Schists, the, 168; crystalline, weathering
of, in Brazil, 251 ; of Cape Elizabeth,
weathering of, 248; origin of, 156.
Seacoast swamps, 336.
Secondary constituents of rocks, 10.
Secondary minerals, influence of, 249.
Sedentary materials, classification of,
300.
INDEX
409
Sedimentary nicks, origin of, 52.
itf. 117.
i oil, VI'.',.
Sectarian nodules, 36, 114.
Scricite, 23.
Serpentine, composition, 30; after peri-
dotite, l.>7 ; origin of, 115, 159; origin of
11:11111-, .".1 ; Harford County, Maryland,
weathering of, 226.
Shale. i:.T.
Sheet defined, 50.
Sln-11 limestone, 143.
Shell marl, 14ti.
Shell sand, 143.
Shore ice, transportation by, 292.
Siderite, 114.
Silica, loss of, how accounted for, 237;
lost .luring decomposition, 234; possi-
bility of combination with iron during
rock decomposition, 239; solubility of,
Silicat.-s. 114; most refractory, 235.
Siliceous sinter. 109.
Siliciticil wood, 110.
Silicon as a constituent of the earth's
crust, 5.
Sill defined, 50.
Simplification of compounds incidental
to weathering, 2i>.">.
Sinking sands, 143.
Sink-holes, formation of, 259.
v structure, 34.
Slates, l.-.T.
Slaty cleavage, origin of, 155.
Slickensides defined, 54.
Snow, effect in promoting decomposition,
180.
Snowfall, influence compared with rain-
fall, 280.
Soapstone, Amherst County, Virginia,
weathering of, 226; Fairfax County,
Virginia, weathering of, 227 ; origin of,
101.
Sodium as a constituent of the earth's
crust, 7.
Sodium chloride, influence in decompos-
ing feldspars, ITS.
Sodium salts in soils, 371.
Soil, chemical nature of, 358; capacity
for water, 379; definition, 3: mineral
nature of, 37.°. : nitrates in, 322; nitro-
u'en in, 372; soluble matter of, 365;
water content of, 281.
Soil cap, creeping of, 287.
Soil particles, movements of, 287.
Soil temperatures at Orono, Maine, 184.
Soils, age of, 380; affected by plant and
animal life, .".80: affected by winds.
2'Jlj; as affected by man, 31)6; classifi-
cation, 381; color of, 384; destructive
process of formation, 300; essential
constituents of, ::tiii; fertility of, 361;
fertility dependent on physical condi-
tion, 379; how affected by climates,
367 ; how affected by leaching, 068 ; in-
herited characteristics, 303, ."60, 387 :
mineral composition of, 373; of arid
regions, character of, 368 ; of arid re-
gions, composition of, 369; of humid
regions, composition of, 379; of Kilo
valley, cause of fertility of, 325 ; phys-
ical condition of, 378 ; resemblance to
parent rock, 3(iO ; soluble salts in, 309 ;
the, 357 ; weight of, 381.
Soluble matter in fresh and decomposed
rocks, 377.
Soluble salts in soils, 369.
Solution, 189; rate increased by commi-
nution, 192; relative amount of mate-
rial removed in, 258.
Sounding sand, 143.
South Dakota, rock-weathering in, 279.
Specific gravity of rocks, 43.
Specular iron ore, 28.
Sphaerosiderite, 114.
Sphagnons mosses, rate of growth, 317.
Spheroidal structure, 247.
Spheroidal weathering of sandstone, 247.
Spherulitic structure, 35.
Spilite, 90.
Stalactite, 113.
Stalagmite, 113.
Stamford dike, pre-Palseozoic decay of,
275.
Steatite, 116.
Stone implements, weathered, 273.
Stone Mountain, Georgia, weathering of,
245.
Stratification defined, 53.
Stratified rocks, weathering of, 248.
Stratified structure, 34.
Structure, as affecting weathering, 249 ;
of rocks, 33.
Sub-soil defined, 383.
Succinite described, 128.
Sulphates, 117.
Sulphuric acid formed during rock-
weathering, 205.
Swamp deposits, section of, 317.
Swamp soils, 315.
Swamps,causeof,316;classificationof,317.
Syenite, Little Rock, Arkansas, weather-
ing of, 214.
Syenite-trachyte group, 73.
Syenites described, 73.
Table Mountain structure, how produced,
252.
410
IXDEX
Tachylite, 92.
Talus, defined, 54; slopes, 319.
Temperatures, effect on soils, 367.
Tephrite and basauite described, 94.
Terminal moraines, 355.
Termites, effects on soils, 391.
Termites, or white ants, as promoters of
decomposition, 204.
Tf-ri-'i rossa, 302.
Tescbenit. .
Theralite-basanite group, 93.
Thin sections, preparation of, 42.
Till, 138.
Time considerations, 268.
Time limit of decay, 272.
Titanic iron as a rock constituent, 28. •
Toadstone, 70.
Tonalite, 82.
Trachytes described, 76.
Transportation and deposition of de'bris,
28t>.
Transported materials, classification of,
318.
Trap rocks, 89.
Trass, 140.
Travertine, 113.
Trees, effect on soils, 394.
Triassic conglomerate, weathering of,
264.
Trichites defined, 41.
Triclinic feldspars, 15.
Tripolite, 142.
Trowlesworthite, 68-.
Tufa, 112.
Tuffoids, 140.
Tuffs, 139.
Uintaite described, 127.
Ulraic acid, 189.
Unakite, 68.
Valley drift, 356.
Valleys, formed by decomposition of
greenstone dikes, 244.
Valleys of solution, 253.
Variolite, 89, 90.
Vegetable matter, decomposing action
of, 203.
Veins defined, 54.
Verd antique, 116.
Verde di Genora, 98, 205.
Verde di Pegli, 98.
Verde di Prato, 205.
Vesicular structure, 34.
Viridite, 30.
Vitreous or glassy structure, 33.
Vitrophyr, 70.
Vogesite, 74.
Volcanic ashes, 140.
Volcanic dust, 140, 298, 349.
Volcanic group of fragmental rocks,
139.
Volcanic mud, 140.
Volcanic neck defined, 51.
Volcanic necks, origin of, 261.
Wacke, 139, 311.
Wad, 107.
Water, action of, in dry soil, 379 ; amount
absorbed by rocks, 198; apparent pro-
tective action of, 253 ; chemical action
of, 186; contents of soil, 281: effects
- of freezing, 199; expansive force of
freezing, 198; in cavities of quartz,
199; mechanical action of, 1! '5; solvent
power augmented, 186; solvent power
tested, l!il.
Water and ice, influence in transporting
rock debris, 287.
Wave erosion, rapidity of, 198.
JVaves, erosive action of, 198.
^Veathering, character of, indicative of
climate, 284; defined, 174; difference
in kind in cold and warm climates, 283 ;
effacement of characteristics of, 2H2 :
incidental results, 266; influenced by
crystalline structure, 243; influenced
by mineral composition, 248; influ-
enced by position, 270; influenced by
structure of rock masses, 244 ; irregu-
lar, due to lack of homogeneity, 251 ;
of andesites, 274; of argillite, Har-
ford County, Maryland, 229; of basalt,
Bohemia, 223; of basalt, France, 223;
of calcareous rocks containing silicate
minerals, 249; of chert, 230; of clastic
rocks, 228; of crystalline schists, 251;
of diabase, Medford, Massachusetts,
218; of diabase, Venezuela, 222; of
diabase, Stamford, Connecticut, 275;
of diorite, Albemarle County, Virginia,
224; of dolomitic limestones, 250; of
eruptive and sedimentary rocks, rela-
tive rapidity of, 271 ; of feldspathic
quartzite, 251; of fine-grained homo-
geneous rocks,, 250; of gneiss, Albe-
marle County, Virginia, 213 ; of granite
of the District of Columbia, 206; of
granite, Lake Huron, 275; of granite,
Pike's Peak, 243; of grano-diorites,
274; of limestone, 232, 250; of lime-
stones, process one of solution, 231 ; of
peridotite, 225; of phonolite, 217; of
pyroxenite, 225 ; of quartzite boulders
on deserts, 256 ; of quartzite in the Dis-
trict of Columbia, 251; of rhyolite,
255; of soapstone, Albemarle County,
Virginia, 226; of soapstone, Fairfax
IXDKX
411
County. Virginia, ±.'7; of syenite, Lit-
\rkansas, -_'I4 ; rate oi, •_'•'„, :
• •(, in riuenced by climate, 278; rela-
tive amount of material lost through,
surface contours due to, •_'."; ulti-
mate product of, 388; unequal, of
253.
\Ve:itlier. -i ^tone implements, 273.
rite, 100.
Wehrl:
1 1 of soils, 381.
Whirlwinds. elVi-cis of, 346.
White ailt>, effects OH SOJlS, 391.
Williaiusiie. 115.
Wind action, i .-,:;, 184,292.
Wind action on Cape Cod, 297.
Wind action on Wyoming soils, 296.
Wind-blown sand polisi
Wist'oiisin, rock-weatlu-riiii; in. 'J7>.
Wnrtzilite descrilied.
Zeolites, as conservators of potash. ".74 :
as rock constituents. .".1 : at Plomhiei-es,
375; composition uf.l'.'J; formed in deep-
sea bottoms, :\~:,; in origin
of, 31; products of hydro-nietam.T-
jihism, 375.
/oolitic matter in soils, 370.
Zircon syenite. 7'.'.
/.onal structure. .".7.
/onal structure incident to wcatherinu.
258.
-
.-
. MAI 69
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