GEOLOGY
_Introduction_
In the preceding chapters an attempt has been made to present outline
sketches of the geography, fauna, and flora of North America as they
exist now. Yesterday, we may say for the sake of emphasis, there were
differences from what exists to-day in each of these great groups of
facts. That is, changes are everywhere in progress. With the recognition
of this idea comes logically the conclusion that similar changes must
have taken place in the past, and that the geography of the earth's
surface, and its flora and fauna, at no very distant time must have been
markedly different from what they are to-day. To test this hypothesis
the geologist studies the records preserved in the rocks in much the
same manner that the historian searches the papyri or the monuments of
Egypt to discover what changes in the affairs of men have occurred since
the days of the Pharaohs. The changes referred to are not essentially
different from those now in progress, but in reality the two are parts
of a single series. For a very long time there have been continents and
oceans, lakes and rivers, and the land has been diversified by mountains
and hills, plains and valleys, in the same general way as at the present
time. When once the idea is grasped that we are living in a geological
age, and that there is no break between the present and the past, it is
evident that the history of the past can be interpreted by means of the
results produced by known causes. Familiar formulas which express this
idea are: "The present is the key to the past"; "Geography is the
geology of to-day," etc. The forces or agencies which are now modifying
the earth's surface, such as the rending of rocks by changes of
temperature and the action of frost, erosion and deposition by streams,
the dash of ocean waves against the land, volcanic eruption, the
chemical action of organic acids, movements producing upheaval and
subsidence, etc., have been in action for geological eras, but their
intensity has varied from time to time and from place to place.
THE GROWTH OF THE CONTINENT
The geological history of North America is, in general, the same as that
of other continents, but claims attention in certain particulars,
largely for the reason that with the exception of Europe it has been
studied more thoroughly than any other comparable land area. In Europe,
throughout much of geological time, there have been numerous islands,
and as a large portion of the records of past changes which have been
presented were formed in the ocean, the results are complex. But in
North America there has been a comparatively steady growth from one main
continental centre or nucleus, and the records of the principal changes
that have occurred are, to a greater degree, simple. Not only in the
major features of the relief of the continent, as already described, but
in its growth and geological history, it is, so far as can be judged
from the present state of our knowledge of the various land areas, the
most typical of all the continents.
Changes in the outlines and area of a continent are brought about
principally by movements of elevation or depression in the earth's
crust. Of less importance is the erosion of the margin of the land by
waves and currents and the deposition of material brought from the land
by streams, together with the spits, bars, and embankments made by waves
and currents. By these and other and less conspicuous processes the
shape of North America has undergone numerous changes in outline and is
still being modified.
General maps have been prepared by J. D. Dana and others, showing the
outlines of North America at various stages in the course of its
development, and from a series of such maps recently compiled by D. C.
Schaffner those here reproduced (Fig. 33) have been selected to
illustrate the growth of the continent. As has been shown by various
geologists, the outlines of the present continents and ocean-basins had
their major features determined at a very early stage in the history of
the earth, and at a time preceding the existence of the oldest known
sedimentary rocks. At the close of the Archean, the earliest geological
era now recognised, and, so far as has been determined, before life
existed on the earth, the principal nucleus of North America was a land
mass some 2,000,000 square miles in area, situated mainly in what is now
the eastern half of Canada, from which there was a southward
prolongation represented by the Adirondack hills of New York (Fig. 33,
A).
The rocks forming this earliest known land in the Western Hemisphere
consist of crystalline schists, gneisses, and granite, which are
considered by some geologists at least as having resulted from the
metamorphism of sedimentary beds. Penetrating and intimately
intermingled with these greatly altered rocks, some of them perhaps
metamorphosed lavas and allied terranes, are many rocks that were forced
upward from deep in the earth into fissures in a molten condition and
have since cooled and crystallized. More than one epoch of metamorphism
has perhaps occurred, and the entire record now accessible is
exceedingly complicated.
The physical conditions at the earth's surface at the close of the
Archean period, as may reasonably be inferred, were not essentially
different from what they are now. The land areas were eroded by streams,
and the _debris_ carried to the sea and deposited, the coarser near
shore and the finer farther seaward. Upward movements in the earth's
crust in various places subsequently laid bare a portion of the
sea-floor adjacent to the former land, and the continent was enlarged.
The outline of the land as it existed previous to the upheaval which
exposed this portion of the ocean's bottom would be defined by the
landward margin of the material deposited. The exposed sediments
would be coarsest near the former coast-line and become finer and finer
seaward from it, and the fossils contained in the consolidated sands and
clays would also supply evidence bearing on the origin of the rocks. It
is by such interpretation of the ancient records in the light of what is
now taking place that the geologist is enabled to map approximately the
outline of North America at several stages in its growth in the manner
shown on the series of maps here presented. Information in this
connection, however, concerning both the northern and southern portions
of the continent is too meagre at present to be largely utilized in
these outline sketches.[5]
[5] The relations of the eras referred to on these maps and the
positions they occupy on the geological time-scale are shown a few pages
later on a chart of the geological history of North America.
The next system thus far recognised, following the Archean, is the
Algonkian, at the close of whose deposition some additions had been made
to the Archean or pre-Algonkian land. Succeeding the Algonkian system
come, in succession, the Cambrian, Ordovician, and Silurian systems. At
the close of the Silurian there was a decided increase in the size of
the main nucleus of the continent. Owing principally to an excess of
elevation over subsidence in the portion of the earth's crust beneath
the northeastern part of the region now occupied by the United States,
portions of the sediments deposited previous to the close of the
Silurian were upraised and important additions made to the extent of the
land southward from the Archean area of Canada. This "Appalachian
peninsula" would be conspicuous in a map representing the outline of the
continent at the close of the Silurian. The eastern margin of the
growing continent was then well to the eastward of its present position,
but how far beyond the present coast we have no means of determining.
Although at the close of the Silurian the continent had greatly
increased in area over that of the nucleus at the close of the Archean,
it bore but little resemblance to its present form. It is worthy
of note, however, that with the exception of the eastward extension of
the land at the time referred to, the growth had been within the present
continental outline.
A later stage in the growth of the continent is shown in Fig. 33, B,
when its eastern margin had much of its present general outline and the
Appalachian Mountains were in their prime. The time indicated is at the
close of the Paleozoic era, and after the great coal-fields extending
from Pennsylvania southward to Alabama and westward to beyond the
Mississippi were formed. The eastern half of the continent was
approximately completed at the time just referred to, and is older than
the western half.
During the Cretaceous period great changes took place in the geography
of the still growing continent, as may be seen by the map illustrating
that period. The conspicuous features in the geography are the submerged
Atlantic and Gulf borders, and the presence of a broad belt of ocean
water in the continental basin which reached from the then much expanded
Gulf of Mexico to the Arctic Ocean, and divided the land into an eastern
and a western continental island.
Following the Cretaceous period came the Tertiary period, during which
the continent assumed very nearly its present outline. During this
period, however, as is indicated in Fig. 33, D, the Atlantic border of
the United States from New England southward and a wide area about the
Gulf of Mexico, were submerged and had deep layers of sediment deposited
on them. During the Tertiary, bodies of fresh water became for the first
time a conspicuous feature on the land, and large lakes and broad
silt-depositing rivers existed particularly in the Pacific mountain
region of the United States, and at its close the continent was
practically completed as we now know it, but several important
oscillations, particularly at the north, have since occurred.
[Illustration: FIG. 33.--Maps showing the growth of the North American
continent.]
With the growth of the continent, briefly outlined above, came greater
and greater diversity in its relief, due principally to the
upraising of various mountains in a somewhat orderly succession from
east to west.
The oldest mountains on the continent are the Laurentian Highlands of
eastern Canada. Although the region referred to--the one mentioned above
as being composed of Archean crystalline rocks--is not now of sufficient
elevation or ruggedness to be termed mountainous, it shows in the nature
and structure of its rocks that deep erosion has taken place. The
inference is that truly great mountains have been removed, but the
evidence may also sustain the interpretation that slow upheaval has been
accompanied by erosion, and that at no time was the land conspicuously
elevated.
Next in age after the Laurentian Highlands come the mountains of New
England and the maritime province of Canada, which were upraised at the
close of the Silurian period. The next great step was the crumpling into
folds and upheaval of the rocks in the Appalachian region at the close
of the Paleozoic era. The Park and Stony Mountains were upraised at the
close of the Mesozoic era, and later came the Sierra Nevada and
Cascades, followed by the Coast Ranges. Youngest of all, and in part for
that reason the boldest and most lofty, are the magnificent mountains of
southern Alaska, with a host of sublime peaks, like Mounts Fairweather,
Logan, St. Elias, and perhaps McKinley. The last-named and highest peak
of all, however, may be of volcanic origin.
In the above list showing the progressive westward movement of the birth
of mountain systems, account is taken only of the elevations produced by
upheaval. The mountains due to volcanic eruptions, which are still
conspicuous, are all young, in comparison with the mountains situated to
the eastward of the Sierra Nevada. The majestic cones of the
northwestern portion of the United States, of which Mounts Shasta, Hood,
Adams, Rainier, Baker, etc., are the most glorious, are of Tertiary or
later age. The same is true, so far as known, of the still more lofty
volcanoes in Mexico. The "pine-tree" forms of steam rising from the
volcanoes of the Caribbees, Central America, southern Mexico, and
southwestern Alaska, proclaim the recency of the birth of the frequently
magnificent craters built of rocks that were once molten, from which
they emerge.
THE ROCKS OF WHICH THE CONTINENT IS COMPOSED
The rocks of which North America is built belong to three classes, which
are world-wide in their distribution. These are: First, rocks produced
by the cooling and crystallizing of formerly molten magmas; second,
those deposited by water; and third, those which previously belonged to
either of the two classes just referred to, but have been recrystallized
and so greatly changed that their preceding condition is no longer
clearly recognisable.
These three classes or subkingdoms, as perhaps they might be termed from
analogy with systems of biological classifications, are in technical
language:
1. _Igneous rocks_, such as the lava of Vesuvius.
2. _Sedimentary rocks_, such as sandstone, shale, limestone,
coal, etc.
3. _Metamorphic rocks_, such as gneiss, schist, some granites,
etc.
These major divisions are based principally on mode of origin, but do
not indicate relative age. While theoretically at least, and in a
general way, the rocks of these three great classes came into existence
on the earth in the order named, it is convenient to consider first
those of sedimentary origin.
[Illustration: PLATE IV.--Leading geological features.]
_The Sedimentary Rocks_ (Plate IV).--Whenever land exists or the waves
and currents of the ocean come in contact with the rocks denudation
occurs. That is, the rocks are broken through the action of mechanical
or chemical agencies, such as the friction of the gravel and sand swept
along by streams, the solvent power of water, etc., and the fragments
thus produced are removed principally through the action of flowing
water and deposited. Resulting from this general process of rock decay
and disintegration, combined with transportation and deposition,
there result mechanically formed sedimentary beds, such as shale,
sandstone, conglomerate, etc.; chemically formed sedimentary beds, such
as the deposits of springs, the saline precipitates from inclosed lakes,
etc.; and organically formed sedimentary beds, as, for example, peat,
coal, and limestone.
Since the first appearance of land in the region now occupied by North
America, sedimentary rocks have been in process of formation, and in
this way the growth of the continent, with the aid of movements in the
earth's crust, has been produced.
The superficial extent of the sedimentary beds in North America is very
great, as is indicated on the map referred to above. By far the larger
portion of the surface of the continent is underlain by them. Their
thickness varies from place to place, but probably reaches a maximum in
the Appalachian region, where a depth of some 40,000 feet has been
measured. Throughout the continental basin their depth is in general
from 3,000 to 4,000 feet. In the Pacific mountains their thickness
embraces tens of thousands of feet, and the same is true in Mexico,
Cuba, and Jamaica. These sedimentary rocks contain fossils which, with
comparatively few exceptions, show that they were deposited in the
ocean; thus sustaining in an important manner the conclusion already
presented in reference to the growth of the continent.
Great as is the area of the sedimentary beds at the present time, it
does not show the entire extent to which what is now land has at some
time been submerged beneath the sea. In certain broad regions,
sedimentary beds which formerly existed have been removed by erosion; in
other extensive areas they are covered by volcanic rocks, and in still
other portions of the continent, embracing thousands of square miles,
they have been metamorphosed and their original characteristics
obliterated.
The system of classification of the sedimentary beds that has been
adopted, as is well known, is based on the relative age of the
formations, determined primarily by the occurrence of one formation
above another, in regions where but moderate disturbances in
position have occurred. Many of the stratified rocks contain
fossils--that is, records of the life of the time they were deposited,
and after the order of succession of a large number of formations has
been ascertained, the life records they contain may be used as a means
of determining the age of a newly discovered terrane.
By grouping the information obtained from the study of the vertical
sequence of the formations in many regions, and also the records of life
contained in them, a composite geological column has been constructed
which shows the relative age of all known formations. The larger
divisions of such a scheme of classification are world-wide in their
application, but the smaller divisions are usually of restricted
geographical extent.
The scheme of classification of general application in North America is
shown in the chart on page 308. The arrangement is in order of age, the
oldest formation being at the bottom. There is some lack of uniformity
among American geologists as to certain of the terms used, more
especially in the lower portion of the column, and in part the scheme is
provisional, but in general it may be taken as expressing the progress
made in the study of the geology of North America up to the present
time.
The names of the larger divisions in this scheme of classification, or
those designating the groups and systems and the eras and periods, have
for the most part been adopted from European geologists. Two important
ones, however--namely, Archean and Algonkian--are of American birth.
While this scheme of classification is based on the succession of
sedimentary beds, igneous and metamorphic rocks have a place in it,
providing their age can be determined.
The Archean period includes the time previous to the deposition of the
oldest known sedimentary beds, and its lower limit is as yet undefined.
The Archean system, or the rocks formed during the Archean period, are
without known fossils, and consist largely of gneisses and foliated
schists, which are metamorphosed sedimentary or igneous terranes,
together with various eruptives. The typical area where these rocks are
exposed at the surface is in the Laurentian Highlands of eastern Canada,
the main Archean nucleus of the continent, but rocks of the same age and
same general character occur in several of the mountain systems of both
the Atlantic and Pacific cordilleras, and underlie the sedimentary beds
throughout a large part of the Continental basin. The Archean system was
named by J. D. Dana, and divided into two portions, namely, the
Laurentian below and the Huronian above. More recent studies, especially
by C. R. Van Hise, have shown the necessity of removing from the system
many of the terranes formerly referred to it, and of placing them in the
Algonkian. The Archean as it remains after this adjustment is termed by
Van Hise the _Basement Complex_. This term, although thus far not
generally adopted, has much to commend it, since the terranes designated
by it are highly complex, and may perhaps be ultimately subdivided into
two or more systems, and besides occupy a basal position lower than any
known sedimentary formation that has escaped metamorphism.
_Outline Chart of the Geological History of North America_
+----------------------------------------+-----------+-----------------+
| Rock-Scale. -->| Group. | System. |
| Time-Scale. -->| Era. | Period. |
+------------+------------+--------------+-----------+-----------------+
|Zoic time: | Time of | Time of |Psychozoic.|Human. |
|embracing | Mammals. | Palms and +-----------+-----------------+
|the history +------------+ Angiosperms. | |Pleistocene. |
|of the | +--------------+Cenozoic. +-----------------+
|earth since | | | |Tertiary. |
|the | Time of | +-----------+-----------------+
|appearance | Reptiles. | Time of | |Cretaceous. |
|of life. | | Cycads. |Mesozoic. +-----------------+
| +------------+--------------+ |Jura-Trias. |
| | Time of | +-----------+-----------------+
| |Amphibians. | Time of | |Carboniferous. |
| | | Acrogens | +-----------------+
| +------------+(Ferns, club- | |Devonian. |
| | Time of |mosses, etc.).| +-----------------+
| | Fishes. | |Paleozoic. |Silurian. |
| +------------+ | +-----------------+
| | +--------------+ |Ordovician. |
| | Time of | | +-----------------+
| |Molluscs and| | |Cambrian. |
| |Crustaceans.| +-----------+-----------------+
| | | Time of | |Algonkian. |
| +------------+ Algae. | +-----------------+
| | | |Eozoic. |(As yet unknown |
| | Time of | | |pre-Algonkian |
| | Protozoa? | | |sediments.) |
+------------+------------+--------------+-----------+-----------------+
|Azoic time: preceding the dawn of life. |Azoic. |Archean or |
| | |Basement Complex.|
+----------------------------------------+-----------+-----------------+
| Solid Earth. |
| Prehistoric Molten Earth. |
| Gaseous Earth. |
+----------------------------------------------------------------------+
The Algonkian series embraces a great thickness of sedimentary beds, in
part metamorphosed, which in certain localities rest unconformably on
the eroded surface of the Basement Complex and in places are overlain
unconformably by Cambrian rocks. Both the upper and lower contacts,
however, in certain localities, have been rendered obscure by
metamorphism. The system derives its name from a tribe of Indians that
inhabited the region about the shores of Lake Superior, where it is well
developed. The Algonkian terranes are exposed in the Grand Canyon of the
Colorado, in the Wasatch and Uintah Mountains, the Black Hills of
Dakota, about the southern shore of Lake Superior, and in many parts of
eastern Canada, as well as in several other localities. The oldest known
fossils occur in these rocks, and consist of a small number of
brachiopods, molluscs, crustaceans, etc. These scanty records are
suggestive, and at least stimulate the hope that an extensive
pre-Cambrian fauna will ultimately be discovered. The few forms found
seem to be not far different from the similar life records of the
Cambrian.
The Cambrian system, although first studied in Europe, has an important
development in North America, and occurs at the surface at a large
number of localities ranging from Newfoundland to California. The known
distribution of the system and the nature of the rocks composing it
indicate that it occurs widely in the Continental basin beneath
subsequent deposits. The most interesting results derived from the study
of the Cambrian, carried on especially by C. D. Walcott, pertain to its
life records. With the exception of a few obscure algae, all the fossils
thus far discovered are marine invertebrates. As regards rank in the
zoological scale, certain molluscan remains are the highest, but
outclassing them in size, abundance, and degree of specialization are
the _trilobites_, the nearest living representatives of which are
certain crustaceans. Of the trilobites about 100 species have been
discovered in the Cambrian rocks of North America, the largest
individual being about 20 inches in length.
The picture of the continent which the facts just referred to enables
one to sketch in fancy includes land areas destitute of animal life, and
probably without vegetation, except perhaps the lichens, the lowest of
the cryptogams. The sea, especially in its shallower portions near land
and over its surface, contains algae, mostly, we presume, of small size,
in fact microscopic, and soft tissued. The animal life subsisting
primarily on the algae are all invertebrates, and nearly all of them,
excepting the crustaceans, simple in organization. None of the animals
the remains of which have thus far been discovered had strong shells or
other well-developed protective or supporting tissues, thus indicating
that they were not subject to the attacks of formidable enemies.
As compared with later faunas, the animals of the Cambrian were
primitive, but their diversity--every subkingdom of invertebrates being
represented--is positive evidence that they were not the first
inhabitants of the waters. Considered from the point of view of
development, this fauna stands at least half-way, and some students of
the ancient history of the earth place it as far as nine-tenths of the
way, up the life column--that is, the time from the first appearance of
life on the earth to the beginning of the Cambrian was at least as long
and possibly nine times as long as the time that has since elapsed. This
is a sufficient promise that many records of life, and it seems safe to
predict as varied an assemblage of organisms as the at present
known Cambrian fauna, will ultimately be discovered in the Algonkian or
lower rocks.
The Paleozoic era witnessed the first appearance of vertebrate life. The
earliest known forms were fish-like in character and were succeeded in
sequence by batrachians and reptiles. In this connection the most
important contribution to the world's knowledge, from the study of the
American records, include the discovery of a large number of fishes, or
fish-like forms, some of them of gigantic size, in the Devonian and
Carboniferous rocks of the Ohio region, by J. S. Newberry; numerous
batrachians in the Coal Measures of Ohio, by E. D. Cope; of batrachians
and probably reptiles in rocks of similar age in Nova Scotia, by J. W.
Dawson and O. C. Marsh.
During the Paleozoic era land plants appeared, and before its close the
continent was densely clothed with forests consisting of flowerless
plants such as ferns and club-mosses, together with a less abundance of
trees related to the existing conifers.
Great additions to the world's knowledge of the varied and beautiful
floras of the swamps in which the coal-beds of Pennsylvania, Ohio, Nova
Scotia, etc., were accumulated have been made by H. D. Rogers, J. S.
Newberry, Leo Lesquereux, J. W. Dawson, I. C. White, David White, and
others.
The Mesozoic era is characterized among other events by the first
appearance and rapid development of flowering plants, the cycads being
especially numerous, and of our ordinary broad-leaved trees, such as the
oak, willow, sassafras, etc., and by the coming in of palms; and in the
animal kingdom by the culmination of reptilian life and the advent of
birds and mammals.
The American Mesozoic rocks have yielded a rich store of fossil plants,
as is well known from the painstaking studies of J. S. Newberry, Leo
Lesquereux, W. M. Fontaine, L. F. Ward, F. H. Knowlton, and others.
These same students of the progress of plant life on the continent have
also made extensive and critical studies of the Cenozoic floras.
The relics of reptilian life brought to light from the Mesozoic rocks of
New Jersey, Kansas, Wyoming, etc., by Joseph Leidy, O. C. Marsh, E. D.
Cope, and others, have astonished the world, even though marvellous
results in a similar direction had previously been made known in Europe.
The reptilian age was marked in America by the presence of such huge
reptiles, and by the strange development and adaptations in various
directions that they surpass the wildest dreams of fable. Lizard-like
reptiles walked the earth that were 40 to 60 feet in length and stood 10
to 14 feet high where the massive hind limbs joined the body. Their
thigh-bones in certain instances measured over 6 feet in length. Some of
these monsters, it is estimated, weighed at least 10 tons. These, the
hugest of all land animals, were vegetable feeders. Others, of less
size, although still gigantic and more active, were carnivorous. Some of
the old lizard-like forms which left their footprints in great abundance
in the sands now hardened into sandstone in the Connecticut Valley and
New Jersey walked on their hind feet, after the manner of birds, and
left three-toed footmarks, some of them 20 inches in length, which are
strikingly bird-like in appearance. Other great reptiles, whale-like in
appearance, inhabited the ocean. Yet more marvellous forms were provided
with wings, resembling those of bats, and in the case of the great
_Pteranodons_ found in the rocks of Kansas had a "stretch of wing" of
fully 20 feet. But the strange menagerie that has been resurrected
contains such a marvellous array of grotesque shapes that not even a
catalogue of the genera can be presented here.
While the Mesozoic era was emphatically the age of reptiles, the coming
of a more highly developed fauna was foreshadowed. Bird life was
represented, and the skeletons of reptilian birds, or birds with teeth
like those of reptiles, have been discovered in the Mesozoic rocks of
Kansas. Important additions to our knowledge of these strange creatures,
which furnish much instructive data in reference to the development of
the higher from the lower forms of life, have been made by O. C. Marsh.
The humble beginning of mammalian life is shown by insectivorous
marsupials, the jaws of which were discovered in the Newark system
(Lower Mesozoic) of North Carolina.
The Cenozoic era is the age of mammals, so called because during that
time brute mammals succeeded reptiles as the rulers of the earth. From
the rocks deposited in North America during this era, principally the
sediments of fresh-water lakes and the gravel-beds laid down by streams
in the Pacific mountain region, a great number of skeletons of truly
remarkable mammals, differing widely from anything now living, have been
discovered by Joseph Leidy, O. C. Marsh, E. D. Cope, H. F. Osborn, and
others. The profound interest attached to this fauna, and the bearings
it has on the study of the geographical distribution of animals,
climatic changes, etc., is indicated by the fact that it includes forms
related to the rhinoceros, elephant, camel, etc., which are not
represented among the animals now living on the continent, although
having relatives in other and principally tropical countries.
During the Psychozoic era mind gained ascendency over brute force, and
man became the leader. The mammals continued to dominate the earth
throughout the Pleistocene period and were then probably more numerous
and of even larger size than during the preceding Tertiary period.
During the Pleistocene great climatic changes occurred, and large
glaciers existed in several regions which now enjoy a temperate climate
and are densely populated.
The presence of man in North America during the Pleistocene has not been
proved, but important contributions to knowledge concerning the brute
mammals, and in reference also to the climatic and physiographic
changes, have been made.
In stream-deposited gravels, caverns, peat swamps, etc., over the
surface of practically the entire continent, the bones of many species
of large mammals have been obtained. These include the mastodon and
elephant, megatherium, megalonyx, mylodon, a large horse, a great bison,
an elk much exceeding the living species in size, a giant beaver,
and many others remarkable for their large dimensions as compared with
their living representatives. Several of these large animals survived
the vicissitudes of climate characteristic of the Glacial epoch, but
have since become extinct.
The chief contributions to Pleistocene history, however, made by
American geologists, are in connection with the records of climatic
changes. During the earlier portion of the period, and beginning perhaps
in late Tertiary time, the continent in large part at least was more
elevated than now and the energetic streams of the mountainous portions
eroded deep canyons. To this Sierran epoch, as it is termed, is referred
the excavation of the larger valleys of the Sierra Nevada, the
world-renowned canyons of the Colorado and Snake Rivers, and probably the
deep Valley of the St. Lawrence and the Hudson.
A climatic change perhaps initiated by the greater elevation of the
land, but not as yet wholly explained, caused glaciers to form about the
higher portions of a number of the ranges in the Pacific mountains, and
continental glaciers of the type of the ice-sheet now covering Greenland
to expand from at least three centres, termed the Labradorean, Keewatin,
and Cordilleran, in what is now Canada. During this time of great ice
accumulation and of glacial advance and retreat, or the Glacial epoch,
as it is termed, fully one-half of North America was buried beneath
ice-sheets of the continental type. A composite map showing the portions
of the continent which were covered with ice at one time or another
during the Glacial epoch is reproduced in Plate V.
[Illustration: PLATE V.--Pleistocene glacial deposits.
NOTE: This map presents what may be termed a composite picture
of the extent of glacial ice during Pleistocene and Recent
time; Greenland, much of the Arctic archipelago, and many areas
in the Pacific mountains are still occupied by ice. The broken
blue lines on the Atlantic and Pacific coasts show
approximately the seaward extension of the Pleistocene
ice-sheets. The detached areas of glaciation in the western
portion of the United States are here assigned to the Wisconsin
stage, but in the Rocky Mountains and Sierra Nevada there are
records of two ice advances. The drift in western Canada here
colored as Wisconsin is perhaps in part of later date.]
During the maximum advance of the ice from the Labradorean centre into
the Continental basin it nearly reached the mouth of the Ohio River
(near Cincinnati). An earlier advance from the Keewatin centre extended
to the Missouri River in Missouri. There is evidence of a succession of
advances and retreats of the ice forming a very complex history. With
its final retreat the Great Lakes came into existence and the continent
reached the stage in its development when man became prominent.
The study of glacial geology in North America was initiated, or at least
given a fresh start and in the proper direction, by Louis Agassiz, and
within recent years energetically carried forward by a large number of
earnest workers. The stage of advance reached in this branch of geology
which serves so admirably to link the present with the past is well
presented in the numerous publications of T. C. Chamberlin and his
associates.
The instructive history of the growth of North America and the
successive appearance of higher and higher forms of life, the records of
which have been discovered in the sedimentary rocks, has been made known
by the combined studies of a large number of investigators, but the
great task has been carried on mainly under the auspices of various
national and State surveys. Chief among these is the present United
States Geological Survey, which has published what may be justly termed
a library of valuable literature and of topographic and geologic maps.
_The Igneous Rocks_ (Plate IV).--Under the at present popular
explanation of the origin of the earth, namely, the nebular hypothesis,
and also the modification of it termed the meteoric hypothesis, the
planet itself is considered to have been at one time in a molten
condition. The starting-point of the study of the rocks composing the
earth should be, therefore, the primitive crust cooled from fusion. In
addition to this there have been throughout history geologic migrations
of molten matter from deep within the earth towards the surface, and a
part of the material thus forced outward, principally through fissures,
has cooled in the rocks it penetrated, forming intrusions of various
kinds, and a part has reached the surface and been extruded, as during
volcanic eruptions.
Probably every known phase of vulcanism is illustrated by the igneous
rocks of North America, and in certain branches of the subject, as the
nature of intrusions and the changes which occur in the cooling of
igneous magmas, marked advances in the world's knowledge have been made
by American geologists.
Examples of volcanic phenomena on a grand scale are furnished by the
still active volcanoes of the Caribbees, Central America, Mexico, and
Alaska. Between southern Alaska and south-central Mexico there are no
active craters, but a large number of volcanic mountains in various
stages of erosion which form an instructive series illustrating the
internal structure of the mode of accumulation of ejected fragment
material and of lava-flows. In this series of mountains built by igneous
agencies belongs the great volcanic piles of the Cascade region, of
which Mounts Baker, Rainier, Adams, Hood, Jefferson, Mazama, Shasta,
etc., are among the leading examples. Many other illustrations in the
same connection, some of them in an advanced stage of erosion and now
revealing only the dikes and necks of resistant rock that cooled and
hardened well below the surface, occur widely throughout the southwest
portion of the United States. The still recognisable volcanic mountains
of the continent, with the exception of those of the Caribbees, are
confined to its western half, and with the exception of certain almost
perfect craters in eastern New Mexico are all within the Pacific
mountains. A great belt of volcanoes, including a large number of both
active and extinct examples, extends from Panama to the Aleutian
Islands, a distance of some 7,000 miles, and is a part of the so-called
"circle of fire" surrounding the Pacific Ocean. This belt is about 1,000
miles broad in its central part, where only extinct volcanoes exist, and
narrows towards both its northern and southern extremities, which are
defined by still steaming craters. The narrow northern portion,
inclusive of the active volcanoes of the Alaskan Peninsula and the
Aleutian Islands, is prolonged westward, and forms a curve concave to
the southward, while the equally narrow southern portion marked by the
energetic craters of Central America forms a curve concave to the
northward. The entire belt has something the shape of a sigmoid curve,
with a wide central portion.
In the preceding sketch of the growth of the continent it was shown that
the Pacific mountain region is younger than the Atlantic mountain
region. In this same connection certain interesting general conclusions
have been reached in reference to igneous activity. In each of the great
cordilleras referred to there have been extensive breaks in the earth's
crust through which molten rocks have been forced upward. Volcanoes and
various intrusions have been formed in each region, but in the eastern
half of the continent the time since the last eruptions has been so
great that all evidence in the relief of the land of the former presence
of volcanic mountains has been obliterated. Erosion has cut deeply into
the rocks on which the ancient volcanoes stood, and revealed in some
instances the dikes occupying the fissures which supplied them. A large
number of dikes of igneous rock occur in the Atlantic coast region from
Prince Edward Island southward to Alabama and Georgia, and vast
lava-flows of ancient date are still preserved about the south shore of
Lake Superior. Volcanic eruptions in the older half of the continent
have long since ceased and the breaks which gave them existence have
been healed. The later movements in the western half of the continent
have caused fresh fractures to form, through which molten matter has
been forced to the surface. Many facts have been observed in each region
which show an intimate connection between movements in the earth's crust
which have produced fractures and the distribution of volcanoes.
The lavas poured out by the more recent volcanoes of North America are
mainly dark basic rocks, among which basalt predominates. An exception
occurs in the case of the Mono craters near Mono Lake, California, which
in recent time extruded a thick, viscous, highly siliceous, rhyolitic
lava, much of which cooled quickly and formed volcanic glass or
obsidian.
In addition to streams and sheets of lava, many volcanoes, and
especially those in a state of explosive eruption, blew into the air
quantities of fragmental material, such as scoria, bombs, volcanic
gravel (lapilli), dust, etc., which was scattered far and wide over the
land. More or less extensive sheets of this material, in many instances
interstratified with sedimentary beds, and especially with the
strata laid down in Tertiary lakes, or separating lava-flows, occur
widely throughout the Pacific mountains. Dust showers of the nature just
referred to have occurred at a recent date, and the fine white material
that fell is now found at the surface in a large number of localities,
ranging from Central America to the Yukon Valley and from Kansas and
Nebraska to Oregon and Washington.
The most remarkable instance of the addition of volcanic rocks to the
surface of North America is in the case of the Columbia River lava,
which covers some 200,000 or more square miles of country in Washington,
Oregon, and neighbouring States. In that region outwellings of highly
liquid rock came from fissures and spread widely over the surface as
veritable inundations, which on cooling became black, basaltic rock, but
without forming mountains or craters. Where the Snake River has
excavated its magnificent canyon in these still horizontal layers of
basalt, a thickness of 4,000 feet is revealed, although the stream has
not as yet cut through the formation, and in Stein Mountain, Oregon, a
similar series of lava-sheets over 5,000 feet thick has been measured.
The Columbia River lava was spread over the surface of a deeply eroded
land in a series of vast overflows of molten material. The liquid rock
covered the broad plains and extended into the valleys in the adjacent
mountains, giving them level floors of basalt. Mountain spurs became
capes and headlands and outstanding buttes were transformed into islands
in the molten sea. The lava since cooled and crystallized has in places
been folded and tilted; streams like the Columbia, Snake, Spokane
Rivers, etc., have carved great canyons in it, and the surface,
especially where it is still nearly horizontal, has decayed and yielded
a wonderfully rich soil. It is the fine, rich residual material of these
lava plains, redistributed in part by the wind, which furnishes the
basis for the immense wheat industry of the northwestern portion of the
United States.
The extrusion of molten rock from deep within the earth so as to form
volcanoes or fissure eruptions at the surface is only a part of a widely
extended and highly varied process. As geologists have discovered,
particularly in deeply eroded regions, by no means all of the fissures
which permit of the forcing upward of molten material in them reach the
surface. Many of them died out before coming to the light and favoured
the production of various forms of intrusion.
A fissure originating deep in the earth's crust and extending upward,
perhaps with many branches and irregularities, if injected with molten
rock from below gives origin to dikes. That is, a dike is a more or less
vertical sheet of igneous rock which has cooled and crystallized in a
fissure. Such sheets of intruded material cutting across the bedding of
stratified rocks, or traversing older igneous or metamorphic terranes,
are of common occurrence and are frequently abundant in deeply eroded
regions. They occur particularly in mountains of upheaval, thus
demonstrating the fact that to a large extent the fissure which became
injected with molten magmas and perhaps gave origin to volcanoes, are
due to movements in the rocks composing the earth's crust. The force
which causes molten rock to rise in such fissures also tends to prolong
and enlarge them. The heat of an intruded magma affects the rocks it
traverses and produces what is termed contact metamorphism. Examples of
dikes in the Newark system have already been referred to, and others are
common throughout the Pacific mountains. Where the Columbia River lava
in central Washington has been removed by erosion, hundreds, and in fact
thousands, of dikes are exposed in the terranes on which it formerly
rested.
When a dike ends above in horizontally bedded rocks it sometimes happens
that the injected magma, especially if highly fluid, is forced in
between the strata and spreads widely between the layers, forming an
intruded sheet, which lifts a broad cover to a height equal to its own
thickness. An example of an intrusion of this nature is furnished by the
palisade trap-sheet in New Jersey and New York, which has a maximum
thickness of about 1,000 feet, and is fully 100 miles in length from
north to south. The portion which remains is but a remnant and is
seldom over 2 or 3 miles wide. This sheet in common with its associated
sandstones and shales has been tilted so as to dip westward at an angle
of about 15 degrees, and its eastern border eroded so as to form the
picturesque Palisades on the west bank of the Hudson opposite New York
city. Many other similar intruded sheets are known in Nova Scotia, the
Connecticut Valley, among the Pacific mountains, etc.
A marked variation in the process just outlined occurs when, as the
controlling condition, the intruded magma is highly viscous instead of
highly fluid, and the friction of contact and of flow is greatly
increased. Under such circumstances the intruded magma expands less
widely than is the case when an intruded sheet is formed, and a thick
intrusion results, which lifts a small cover perhaps to a great height.
Intrusions of this nature are sometimes expanded in their upper portions
into a more or less mushroom shape, and from their fancied resemblance
to cisterns of once molten rock within older terranes have been termed
_laccoliths_. The typical examples are furnished by the Henry Mountains
in southern Utah, described by G. K. Gilbert. Other similar intrusions
in Colorado have been studied by Whitman Cross, and yet other examples
have been discovered in various parts of the Pacific mountains. In the
case of certain of the laccoliths in the Henry Mountains, now laid bare
by erosion, the cistern-like mass of intruded material is 12,000 feet or
more in diameter, some 5,000 feet thick in the central part, and lifted
a cover of stratified rocks fully 7,000 feet thick.
Where a dike ends above in older rocks, and particularly in horizontally
stratified sedimentary beds, in a pipe-like form, similar to the conduit
of a volcano, but without reaching the surface, the unexpanded or but
slightly enlarged summit portion lifts a comparatively small cover into
a dome, and what has been termed a plutonic plug results.
All the various phases of intrusions thus far referred to, it will be
readily seen, are variations of one process. The wide range in
the results produced are dependent on local conditions, either in
respect to terranes invaded, as, for example, whether or not they are
undisturbed sedimentary beds, and on the physical condition of the
intruded material, in reference especially to its degree of viscosity.
There is an intimate and even a genetic connection between intrusions on
the one hand and volcanic and fissure eruptions on the other. If
fissures lead from portions of the earth's crust sufficiently deep to
permit the rocks to become plastic or fused on account of the relief of
pressure due to the opening of the fissure, the magma may be forced to
the surface, becoming more and more plastic or more perfectly fluid as
the weight upon it decreased, and volcanic phenomena result; or if the
fissure fails to reach the surface intrusions of various forms may be
produced. The simplest form of intrusion, the dike, results under
whatever condition the summit portion of the magma comes to rest. A
magma forced upward in fissures in the earth's crust may meet moist
rocks or even reservoirs of water, and in such instances steam or gases
are produced and a new force is added, which may produce explosions.
In addition to the intrusions of the various classes just referred to
there are others on a far larger scale, examples of which occur in North
America, but as yet their mode of origin has been but little studied. I
refer to vast upwellings of molten or plastic material beneath the more
rigid portions of the earth's crust, which elevate domes, perhaps 200 or
300 miles or more in their various horizontal diameters. The great areas
occupied by intrusive granite, as the one from which the Bitter Root
Mountains in Idaho have been sculptured, are of this nature. These
"regional intrusions," as they may be termed, elevate mountains in the
same general manner as in the case of laccoliths, but of far greater
size. To the elevations produced in this manner I have ventured to apply
the name _subtuberant mountains_, in expression of the idea that they
have resulted from vertical uplifts, due to the upswelling of molten
material beneath.
_The Metamorphic Rocks_ (Plate IV).--At the contact of either
sedimentary or igneous rocks with intrusive rocks of whatever form, such
as dikes, sheets, laccoliths, etc., there has been in many well-known
instances an alteration of the terranes penetrated or uplifted which is
most intense along the contact and diminishes at a distance. This change
or metamorphism, as it is termed, consists of an alteration in the
colour, texture, hardness, mineral and chemical composition, etc., of
the rocks affected, and may be manifest throughout a thickness of but a
few feet, or perhaps only a few inches, but near large intrusions is apt
to be traceable for scores or hundreds of feet. In the case of intense
contact metamorphism, the altered rock assumes a new form, and may
exhibit a crystalline and foliated or schistose structure. The changes
referred to are most marked when water is present, and are thought to be
due largely to the influence of heated water percolating through the
rocks and producing changes by solution and deposition. The principal
agencies which take part in contact metamorphism are heat, heated
waters, pressure, and perhaps movements within the rocks.
There are extensive regions throughout which the rocks have been changed
in a manner similar to the alterations commonly found adjacent to
igneous intrusions which, in general, have been brought about in some
other way. This regional metamorphism, as it is termed, has affected the
rocks in certain instances throughout districts measuring many hundreds
of square miles in surface extent, and with a vertical range of many
thousands of feet. The rocks referred to have been changed without
fusion from a previous condition, during which they were either
sedimentary beds or cooled and crystallized igneous magma. This
conclusion has been verified in numerous instances by tracing the
thoroughly altered rocks to regions where the change has been less
intense and finally to where they pass by insensible gradations into
easily recognisable sedimentary or igneous terranes. Common examples of
metamorphic rocks are mica, schist, gneiss, statuary marble,
certain granites, etc. These rocks frequently have a foliated or fissile
structure, such as it is presumed would result from a flowing movement
within the mass while under great pressure. Characteristically also the
rocks are composed of interlocking crystals or portions of crystals,
which are not contained in a glassy base, as is the case with most rocks
that have crystallized from fusion. That is, the metamorphic rocks are
characteristically _holocrystalline_, while igneous rocks are
_porphyritic_, or _cryptocrystalline_.
The analogy between rocks altered by contact metamorphism and those
affected by regional metamorphism had led to the conclusion that the
latter, like the former, have been changed by heat and the passage
through them of heated water bearing mineral matter, and especially
silica, in solution. More than this, the foliation frequently so
characteristic of metamorphic rocks is considered as evidence of a
flowing movement or shearing of the material while under pressure. In
short, rocks are altered by heat, especially if water is present in
them, by motion, and by chemical changes produced by percolating waters,
and perhaps in still other ways. The degree of heat required is not
definitely known, and probably varies according to the nature of the
rocks, the presence or absence of water, etc., but is certainly less
than that necessary to produce fusion, and is thought, in general, to be
in the neighbourhood of 750 deg. F. While heat alone is considered as
sufficient to produce metamorphism, it is probable that in most
instances two or more of the agencies just referred to have been in
operation at the same time. In the case of the foliated rocks motion
within the mass seems to have been the predominating factor, and
dynamical metamorphism is considered as important as heat metamorphism.
In North America, as is indicated roughly on the map forming Plate IV,
metamorphic rocks occur at the surface over a great region in eastern
and northeastern Canada, in Labrador and Newfoundland, in the New
England States, and thence southward along the eastern side of the
Appalachians. Other extensive regions occupied by similar rocks
occur in many of the ranges of the Pacific mountains, from Alaska to
Panama, and are known in the West Indies.
Not only do the metamorphosed rocks outcrop at the surface over large
areas, but, as may be inferred from such outcrops, as well as from the
records of numerous borings, underlies nearly the entire extent of the
sedimentary formations. The basal portion of the continent, with the
exception of certain areas where igneous rocks occur, is formed of
metamorphosed terranes. So generally is this true, that it is safe to
say that if a boring is begun at any locality on the continent where
sedimentary beds occur, and is continued downward until the sedimentary
rocks are passed through, metamorphic terranes will be found beneath.
The same is true also where the surface is composed of lava-sheets. The
exceptions, where metamorphosed rocks do not occur beneath sedimentary
or volcanic beds, are when igneous intrusions or ancient lava-flows are
present at a depth.
In the brief description given of the Archean system on a preceding
page, it was stated that the rocks composing it are largely metamorphic.
But rocks of practically any age may be altered in the several ways
mentioned above, and the resulting gneisses, schists, etc., be
indistinguishable from those of the Archean. In fact, some of the
metamorphosed rocks of North America, as certain gneisses, schists,
etc., of the Sierra Nevada and Cascade Mountains, are known to be of
Mesozoic and even Cenozoic age.
In speaking of the growth of North America, and again in connection with
the distribution of volcanic mountains, it was shown that there has been
a progressive migration of the field of action of the forces which
upheave the rocks so as to form land areas, and also of the movements in
the rocks which produce fractures and lead to the origin of volcanoes.
In a similar way the sphere of influence of metamorphism as indicated by
the age of the transformed rocks in various regions has in a general way
migrated from east to west across the continent.
In the Laurentian Highlands the metamorphosed rocks are of pre-Cambrian
age; in New England and the Appalachian region they are, in part at
least, of Paleozoic age; and in the Sierra Nevada and Cascade Mountains
metamorphosed Mesozoic and Cenozoic rocks occur. As movements in the
outer portion of the earth's crust may produce fractures in any class of
rocks, and as such fractures favour the intrusion of igneous material,
the metamorphic rocks may contain igneous intrusions similar to those
noted above in connection with sedimentary rocks. As the stratification
so marked in sedimentary beds is lacking in metamorphosed rocks, it is
not to be expected that intrusions will take the form of sheets,
laccoliths, etc., but rather appear as dikes with perhaps irregular
branches. As the same region may experience two or more periods of
metamorphism, it is evident that great complexities may arise, as, for
example, when a metamorphosed terrane is penetrated by dikes and
irregular intrusions and again subjected to metamorphosing conditions.
These considerations lead to the suggestion that rocks metamorphosed in
pre-Cambrian time, for example, would be apt to be more complex than
those of Mesozoic date. In general, this has been found to be true, as
is suggested by the fact that to the pre-Cambrian metamorphosed
terranes, as previously stated, the name Basement Complex has been
applied.
_Summary._--The relation of the three great divisions into which the
rocks composing North America, in common with all other portions of the
known lithosphere, are divided, may perhaps be better understood when it
is remembered that the igneous rocks came _from below_ in a molten
condition; that the sedimentary rocks have been formed _at the surface_
from the _debris_ of either igneous, metamorphic, or previously formed
sedimentary beds; and that metamorphic rocks have been produced _within_
the earth's outer crust by the alteration of either igneous or
sedimentary rocks. When the heat which produced certain phases of
metamorphism is sufficiently increased, greater freedom of molecular and
chemical changes occur and the material acted on passes to the
condition of an igneous magma. The three great classes of rocks
considered above are thus seen to be but stages in a cycle which the
material of the lithosphere passes through.
The conditions which bring about these changes are still in action and
are intimately associated with movements in the rocks of the earth's
crust. When elevation raises a portion of the earth's crust above
sea-level, erosion and redeposition ensue and sedimentary rocks are
formed; the greater the elevation the more energetically the forces act
which bring about denudation, transportation, and sedimentation. When
depression occurs of sufficient amount to carry rocks previously at or
near the surface into the zone of metamorphism, alterations follow, and
in general the deeper the depression the greater the changes until
metamorphism culminates in fusion, providing pressure does not
counteract the influence of heat. Dynamical and chemical metamorphism
may occur at less depth than purely heat metamorphism, and it may be
presumed takes place in the axes of mountain ranges, even above
sea-level. Such a broad view of the relations and genesis of the three
great lithologic divisions of the material forming the earth's outer
crust is necessary to the understanding of the conditions observed in
the basal portion of the geological column, as it is termed, in which
the age and order of succession of the sedimentary rocks is indicated.
In certain localities, for example, the Cambrian rocks rest
unconformably on a surface of metamorphic and igneous rocks--that is,
the Basement Complex was raised above sea-level, eroded and subsequently
depressed before the Cambrian sediments were laid upon it. In other
localities the Cambrian rocks pass indefinitely into metamorphosed
terranes beneath, which means that metamorphism invaded the series after
the deposition of the Cambrian, and the characteristics of its junction
with older rocks was obliterated. Similar relations may evidently be
discovered at any horizon in the geological column. Obviously the
chances of a system of stratified rocks becoming metamorphosed or of
being removed by erosion, are greater the nearer their position
to the base of the sedimentary series; in a similar way the chances of a
sedimentary terrane becoming invaded by igneous intrusions is greater
the greater its age; again, the older a sedimentary terrane the greater
the chances of its becoming buried by subsequent deposition and the less
the likelihood of its being exposed for study. The only position in
which a sedimentary formation can maintain its integrity and be safe
from destruction by erosion or transformation by metamorphism is below
sea-level and above the zone of heat metamorphism; but even in this
position it may have its distinctive features, including its fossils,
obliterated by dynamical and chemical alterations. These suggestions are
offered for the sake of indicating, as stated on a previous page, that
the Cambrian and Algonkian rocks should not be considered as the first
formed sediments, and that there is hope of the discovery of a rich
fauna of older date than any at present known. In the search for the
earliest evidence of animal life on the earth, North America holds out
favourable conditions.
THE CONCENTRATION OF MINERAL SUBSTANCES
The most important branch of geology treats of the substances in the
earth's crust that are of direct service to man, as, for example,
building stones, coal, iron, petroleum, gold, etc. Only a glance can
here be given at the conditions which have led to the origin of the
materials of commercial value and to their geographical distribution.
From the mode of origin of the principal classes of rocks it may be
reasonably inferred that certain minerals and ores will be developed or
concentrated in one class of rocks and not in the others. To a great
extent the facts observed during the development of mines, etc., sustain
this prediction.
In the cooling and crystallizing of igneous rocks from a state of fusion
many minerals are formed, the most common being silicates of the
alkaline earths, which are usually inclosed in a glassy or
cryptocrystalline base. The igneous rocks have characteristically
a highly complex chemical composition, and although frequently
containing the metallic element, etc., which are of economic importance,
these are widely disseminated, and in nearly all cases in chemical
combinations, as the minor ingredients of siliceous minerals. Although
the igneous rocks sometimes contain valuable ores, they are in many, if
not all instances, due to secondary enrichment and are not a result of
primary crystallization from fusion. As all the material of the earth's
crust was at one stage in the series of changes it has experienced
consolidated from fusion, it follows that the ores and minerals now of
economic value did not then exist, or were widely diffused and have
since been formed or concentrated.
The processes of concentration referred to are carried on in various
ways through the agency of mechanical, chemical, vital, molecular, and
electrical forces, acting singly or in association. For example,
concentration through the action of mechanical agencies is illustrated
by the manner in which rocks are reduced to fragments in the every-day
process of denudation and the resulting _debris_ removed by streams and
redeposited. In this process an assorting in reference to size, specific
gravity, etc., takes place, and certain substances, as sand, for
instance, is accumulated in one locality, and certain other substances,
as clay, deposited in another locality. During this process gold,
platinum, etc., owing to their high specific gravity, may be
concentrated in stream channels. The accumulation of mineral matter
through the action mainly of chemical agencies, occurs when the waters
percolating through rocks dissolves certain substances, as calcium
carbonate, for instance, and on coming to the surface as springs, or
dripping from the roofs of caverns, deposit calcareous tufa,
stalactites, etc. Silica, iron, manganese, and other substances are
frequently concentrated in a similar manner.
Concentration of previously widely disseminated substances principally
through the agency of vital forces, is illustrated by the manner in
which molluscs and polyps obtain calcium carbonate from water and
deposit it in their shells or skeletons. The part played by plants in
this same connection is shown by the way in which they eliminate carbon
dioxide from the air or from water, and concentrate the carbon in their
tissues. From the carbon accumulated in this manner, under certain
conditions, deposits of peat, lignite, coal, graphite, etc., have
resulted.
What may provisionally at least be termed molecular concentration occurs
when similar molecules are brought together largely by water and
crystallized to form mineral species. In order to simplify this brief
discussion as much as practicable, this phase of concentration will be
included under the chemical processes referred to above.
The three principal methods by which mineral substances are
concentrated, namely, the mechanical, chemical, and vital, have in the
main different fields of action. The mechanical and vital agencies
operate at the surface of the lithosphere, although organic products,
principally certain acids, descend into the earth in solution in water
and play an important part in deep-seated chemical changes, as in the
formation of mineral veins. The chemical agencies bring about the
concentration of mineral substances both at or near the surface and at a
depth.
The intensity with which the several agencies just referred to operate
varies according to conditions. The mechanical agencies, for example,
acting mainly through the aid of flowing water, are in general most
potent in humid regions and where the land is high above sea-level.
Vital agencies depend largely on climate and are most active in warm
humid regions. The chemical agencies are influenced largely by heat, the
presence of water, and by pressure.
It is interesting to note that a high degree of heat leads to the
dissipation and wide distribution of substances previously concentrated;
fusion, for example, permitting of the intimate mingling or
recombination of substances, previously segregated, although during the
dying stages of volcanic activity minerals like sulphur, cinnabar, etc.,
may be directly condensed and thus concentrated from a vaporous
condition.
During the formation of the three main classes of rocks composing the
earth's crust, the agencies leading to the concentration of various
substances now of economic importance have to a great extent been
different, and hence in a marked way the stones, ores, fuels, gems,
etc., to be expected in each of the three classes of rocks,
respectively, are distinct. Certain exceptions to this broad conclusion,
however, arise from the fact that rocks belonging to each of the classes
referred to may have been brought within the influence of the same or
similar concentrating agencies and like results produced in each class.
_Economic Importance of the Igneous Terranes._--The igneous rocks, as
previously noted, are such as have cooled from fusion. On the cooling of
magmas various minerals are formed, most commonly silicates, and except
in a minor way in connection with the weaker stages of volcanic activity
and the slow cooling of the rocks, there does not seem to be any marked
tendency towards the concentration or segregation of metallic minerals
or ores. Although igneous rocks do contain gold, silver, copper, etc.,
and a large variety of the rarer metals, they are widely disseminated.
As is well known, however, igneous rocks are in some instances of value
for the metallic mineral, gems, and ores associated with them, but in
the great majority of instances at least, and as a rule, these minerals
and ores are the result of subsequent changes and owe their origin
mainly to deposition from heated, percolating water. Rich ore bodies
frequently occur on the borders of igneous dikes, and in fissures and
cavities in igneous rocks, but the process by which they have been
formed is similar to that leading to the concentration of mineral matter
in metamorphic rocks, and will be referred to later.
The igneous rocks themselves furnish desirable building stones, such as
granite, diorite, porphyry, diabase, etc. With the exception of granite
and the nearly related diorite, these have not as yet been extensively
utilized in North America. Certain of the igneous rocks have been
altered to serpentine, which on account of its pleasing green colour and
the ease with which it can be cut and polished furnishes a stone
valuable for interior uses. It is also employed, usually with a rough
surface, in the construction of exterior walls of dwellings, gateways,
etc. Large bodies of serpentine occur at a number of localities in the
Atlantic mountains from Pennsylvania and Maryland northward, including
eastern Canada, and also over extensive areas in the Pacific mountains,
particularly in California, Washington, and Alaska.
The principal ores and minerals of commercial importance in the igneous
rocks are native copper, as in northern Michigan; copper pyrites, as at
Butte, Montana; gold, at many localities, including the Treadwell mine,
Alaska; opal, which is mined on a small scale in Idaho and Washington.
In practically all these instances, and numerous others that might be
enumerated, the substances referred to have been deposited from solution
in cavities in the rocks or have replaced other substances, and are due
to what is termed above chemical concentration.
_Economic Importance of the Sedimentary Terranes._--The sedimentary
rocks are composed principally of fragmental material derived from the
disintegration of older rocks transported and deposited mechanically,
and resulting in the formation of sandstone, shale, etc., and of
organically concentrated material, such as shells and corals, which form
limestones. The deposits originating in these ways furnish excellent
building stones, the principal classes being sandstones and limestones.
These occur widely throughout North America, and in formations of all
ages subsequent to the Archean. The sandstones were deposited near the
shores of the seas, or in lakes, and the limestones principally in
moderately deep oceans.
Sandstones occur largely in the Cambrian formation on the south shore of
Lake Superior and about the borders of the Adirondack hills of New York.
They are usually red or reddish-brown rocks, and their pleasing colours,
durability, even grain, and the readiness with which they may be broken
in any direction make them desirable building stones.
The Newark system, extending in detached areas from Nova Scotia
to South Carolina, contains immense quantities of brown and gray
sandstone, which have been extensively quarried, particularly in the
Connecticut Valley, New Jersey, Pennsylvania, and Maryland, and largely
used in Atlantic coast cities. The Carboniferous and Devonian
sandstones, usually of a gray colour, of Pennsylvania, Ohio, and
neighbouring States, are largely used in the cities of the interior
portions of the United States. Extensive deposits of Mesozoic and
Cenozoic sandstones occur throughout the Pacific mountains, and afford a
practically unlimited supply of good building material, which as yet has
been but little utilized. The colours of sandstones vary from bright red
through brown-yellow to gray, and in some cases are nearly white,
depending largely on the condition of the iron present. The red rocks
are dyed with ferric oxide; the brownstones contain iron, frequently in
the cementing material that unites the grains, in various stages of
oxidation and hydration; the gray stones may also contain iron, but if
present it is in union with organic matter, as the ferric carbonate, for
example. The Cambrian and Newark sandstones are prevailingly of some
shade of red, for the reason that not enough organic matter is present
to change the iron to a carbonate.
The sandstones when of an even fine grain and not too hard, are suitable
for sharpening tools, and large quantities of grindstones, whetstones,
etc., are made from them, as on the Lake Huron shore of Michigan, in
Ohio, etc. Other sandstones, practically free from iron, are used in the
manufacture of glass. The best example of "glass sand" is the Sylvania
sandstone of southeastern Michigan. Unconsolidated sand is largely used
in mixing mortars and cements, for smoothing stones used for
architectural and monumental purposes, as foundry sand in making moulds
for casting, and many other ways. Seaward from where sand is being
deposited we find in the present oceans that as a rule fine bluish or
greenish mud occurs, and still farther seaward, except where
coral-polyps thrive, usually at a distance of 100 miles or more from
land, the bottom is composed of calcareous mud or ooze. The sand
and mud are derived from the land, and if consolidated form sandstone
and shale. The calcareous ooze is derived from the life of the sea,
largely minute lime-secreting foraminifera, together with shells of
molluscs, and in the vicinity of coral islands or reefs the hard parts
of coral growth are added. That is, the calcareous oozes are formed by
the concentration of calcium carbonate through the vital action of
animals and to a less extent of plants. Such material, if consolidated,
would form ordinary limestone.
In North America there are terranes scores of hundreds of miles across
in various directions and hundreds and even thousands of feet thick that
have been formed in the manner just indicated. From this mode of origin
it may be truthfully inferred that limestone may have been formed during
any age since organisms having the power of secreting calcium carbonate
existed on the earth. The limestones of North America range in age from
the Algonkian period to the present time, and are still being formed in
the ocean and in a minor way in lakes.
Impure limestones, frequently coloured or clouded with red, due to
ferric oxide, are quarried on an extensive scale in eastern Tennessee,
and are used for decorative purposes. The Tennessee limestones referred
to are of Paleozoic age; in Florida porous rocks, known as coquina,
composed of imperfectly consolidated shells of living species of
molluscs, are used in the construction of buildings. Gray limestones
susceptible of a good polish occur in Ohio and neighbouring States and
are utilized to some extent for columns and interior finish of
buildings, but in the main the stones of this nature when employed for
architectural purposes are rough-faced. Vast amounts of limestone
suitable for masonry occur widely throughout the Mississippi Valley in
many of the ranges of the Pacific mountains, especially in the United
States and Mexico, and are also of immense thickness in the West Indies.
In many instances limestone has been metamorphosed, as will be described
below, and converted into crystalline marble. Commercially, however, all
limestone, whether crystalline or not, which is susceptible of a
polish, is termed marble.
Under certain conditions calcium carbonate is concentrated at or near
the earth's surface by chemical agencies, as about springs where
calcareous tufa, travertine, etc., are precipitated, and in caverns
where stalactites and stalagmites are formed. Stalagmite sheets are
sometimes composed of variegated, laminated layers, and when polished
produce a beautiful decorative stone which passes under the name of onyx
marble. Deposits of this character of commercial importance occur in
Arizona and Mexico.
Calcium carbonate concentrated in lakes through the combined action of
chemical and vital agencies produces the so-called marl, now extensively
utilized in the manufacture of Portland cement. In this mode of
accumulation the calcium carbonate is dissolved by percolating waters
from the rocks and soils and carried to lakes in solution; it is there
precipitated largely through the vital action of certain algae and
deposited as a fine white ooze. Thousands of deposits of this nature,
varying in extent up to several hundred acres, and having a depth of
from a few feet to 40 and even 60 or more feet, occur in the portion of
the continent covered with glacial drift, and especially in the States
from New England to Minnesota. The reasons for the greater abundance of
marl in this region than elsewhere are that the glacial drift is there
highly calcareous, numerous lakes are present, and the climatic
conditions are such as to favour the growth of certain aquatic plants,
and especially the _Characeae_ or stoneworts, which have the property of
eliminating calcium carbonate from ordinary lake waters.
The importance of the vital agencies in concentrating substances of
economic value is illustrated by the manner in which coal, petroleum,
and natural or rock-gas, etc., have been formed.
Land plants have the power, under the influence of light, of decomposing
the carbon dioxide (carbonic-acid gas) of the air and fixing the carbon
in their tissues, the oxygen being liberated and rendered
available for animal respiration. Carbon is thus concentrated, and when
plant remains accumulate and are preserved beneath water in swamps, a
slow change takes place and peat is formed. The essential conditions for
the accumulation of vegetable matter have been present on the earth ever
since a land flora existed, and coal-beds occur at many different
horizons. The earliest date at which land plants seem to have been
sufficiently abundant to furnish material for coal-beds was the
Carboniferous period. Although a similar flora existed during the
preceding period, the Devonian, no coal-beds of workable thickness are
known in the rocks of that age. Since the Carboniferous period coal has
been found at many horizons in the sedimentary rocks, and peat is being
accumulated at the present day.
[Illustration: FIG. 34.--Map showing the distribution of coal in North
America.]
The coal-fields of North America are more extensive than those of any
other continent, excepting, perhaps, the at present but little known
coal-bearing formations of Asia, and are distributed in temperate
latitudes, from tide-water on the Atlantic to tide-water on the Pacific
coasts, where the greatest commercial and intellectual development has
been reached.
Coal of Carboniferous age occurs in large and valuable deposits in Nova
Scotia and New Brunswick; there is a small area of graphitic anthracite,
not now utilized, in Rhode Island; but the great fields are in
Pennsylvania and the States southward to central Alabama, and westward
to beyond the Mississippi. A detached coal-basin containing some 6,700
square miles, but a small part of which is productive, however, occurs
in the central part of southern Michigan. Small coal-fields in Virginia
and North Carolina, the first to be worked in America, are of Jura-Trias
age and form part of the Newark system. Extensive fields of valuable
coal of Mesozoic age, principally in the Laramie system, occur in New
Mexico, Colorado, Wyoming, Montana, and still farther north along the
same great belt in Canada.
Another highly valuable field of Mesozoic coal is now being extensively
worked on Vancouver Island. The coals of the west side of the
Pacific mountains, largely lignites, but in many instances of high grade
and serviceable for steam coal, are mostly of Cenozoic age (Tertiary)
and occur in California, Oregon, Washington, and Alaska. The
distribution of the various coal-fields is indicated on the above map,
and space will not be taken in describing their geographical relations.
Peat is present in innumerable swamps throughout the humid, temperate
portion of the continent, especially from Louisiana and Florida
northward, to the region about the Great Lakes and widely throughout
Canada, but is at present of small commercial importance, although steps
are being taken for its extensive utilization.
The most valuable of the coal deposits are of Carboniferous age, and lie
to the east of the Rocky Mountains. The most of the coal is bituminous,
or soft coal, used principally in generating steam and for manufacturing
gas and coke. The exceptions occur in eastern Pennsylvania and in Rhode
Island. These are considered as metamorphosed coals, although in the
Pennsylvania region there is no evidence of the action of a high degree
of heat. In the Rhode Island field the rocks associated with the coal
are plainly metamorphic in character, and the coal has, in large part,
been changed to graphitic anthracite.
That anthracite may be of any age, however, is indicated by the local
changes that have occurred in Mesozoic and Cenozoic coals, where they
have been penetrated by dikes and other varieties of intrusions, or have
been altered by surface lava-flows. In such situations the coal has lost
nearly all its volatile matter, and in composition and in certain
instances, as in western Colorado, in physical character as well, is
essentially an anthracite.
In addition to the various coal deposits referred to above there is a
second series of organic compounds found stored in sedimentary rocks
which consists of hydrocarbon. This series of substances includes
natural or rock-gas, petroleum, maltha or semifluid hydrocarbon, and
solid hydrocarbons, such as asphaltum, albertite, grahamite,
ozokerite, etc. These substances are usually considered as being of
organic origin and to have resulted from changes which take place in
vegetable and animal tissues when buried and in most cases subjected to
heavy pressure. A large part of the hydrocarbons referred to is thought
to have been derived from animal organisms, an opinion which is
sustained in an important manner by the fact that large stores of both
petroleum and rock-gas have been discovered in rocks which were laid
down before land vegetation is known to have existed. Marine algae were
present, however, so that it cannot be affirmed that the hydrocarbon of
the earlier Paleozoic rocks came entirely from animal organisms. It is
highly probable, however, that a large portion of the hydrocarbons
stored in Paleozoic and later strata was derived from the animals whose
hard parts occur so abundantly as fossils in the same or adjacent beds.
[Illustration: FIG. 35.--Ideal section showing favourable conditions for
the storage of petroleum and gas.]
Besides the concentration of carbon in plant and animal tissues and its
change to hydrocarbons, there is a still further concentration necessary
in order that stores of petroleum, gas, etc., shall be accumulated so as
to be of economic value. This accumulation is dependent largely on
physical conditions. The production of hydrocarbons from organic matter
contained in sedimentary rocks, and particularly in shale, is going on
in many regions, and probably nearly everywhere, especially when the
soft parts of animals are buried in the rocks, but the petroleum, gas,
etc., generated escape at the surface and pass into the air and are
again widely disseminated, unless conditions are present which lead to
their accumulation. The conditions favouring the natural storage of the
substances referred to are cavities, or more usually porous beds, such
as sandstone, beneath impervious beds, such as clay or shale. The
conditions are still more favourable when lateral as well as vertical
escape is cut off, as, for example, when arches or domes occur. The most
favourable conditions result when a bed of shale or other rock, as _a_,
Fig. 35, from which hydrocarbons are being evolved occur beneath a sheet
of porous sandstone or fissured rock of any kind, _b_, above which there
is a close-textured, unfractured stratum, such as shale, _c_, and the
series is bent along certain axes into upward folds or anticlinals.
Under these conditions, as extended experience has shown, a well drilled
at _d_ should yield in succession gas, petroleum, and water.
The conditions for the production of petroleum, gas, etc., have been
present on the earth since the first appearance of life, and reservoirs
may have originated at any subsequent time. The oldest known reservoirs
still charged with these substances that have been discovered occur in
the earlier Paleozoic rocks, just above the formations containing the
oldest known fauna. Important petroleum and gas fields in rocks of the
Trenton period occur in New York, Ontario, Ohio, and Indiana. The
Devonian rocks of Pennsylvania, New York, Ontario, etc., also yield
large supplies of both oil and gas. Mesozoic rocks of Colorado, Wyoming,
etc., are also rich in the concentrated hydrocarbon referred to, and on
the Pacific coast, particularly in California, rocks of Cenozoic age are
highly productive. Petroleum and gas may occur also in rocks more recent
than the Cenozoic, but owing to the absence of reservoirs, and possibly
the lack of sufficient time, no important accumulations are known in
beds more recent than the Tertiary, unless they come from a deeper
source in older rocks. The vast quantity of petroleum stored in the
rocks of various ages in North America is indicated by the fact that in
1900 the yield from the wells of the United States was 63,362,704
barrels, and from Canadian wells about 280,000 barrels, making a total
of nearly 64,000,000 barrels.
The stores of rock-gas are also enormous, as is indicated by the fact
that a single well at Bairdstown, Ohio, yielded over 17,000,000
cubic feet per day. In 1890 the average daily flow of the Indiana
gas-wells was 779,525,000 cubic feet. The value of the natural gas
consumed in the United States in 1900 was $23,606,463.
In the sedimentary rocks of North America there occur also extensive and
valuable deposits of semifluid and solid hydrocarbons, such as maltha,
asphaltum, albertite, grahamite, uintahite, etc., which have arisen,
under the most plausible explanation thus far offered, from the
concentration by evaporation of fluid hydrocarbons such as petroleum.
The evaporation, particularly of heavy petroleum, leads to the formation
of a solid residue, similar to asphaltum. In fact, there is no definite
boundary between the lightest naphtha and the most coal-like asphaltum.
They form a connected hydrocarbon series, analogous to the coal series.
Albertite, a bright, coal-like substance, exceedingly rich in volatile
hydrocarbon, occupies fissures in Carboniferous rocks in Nova Scotia,
and a similar but less lustrous mineral, termed grahamite, occurs in
fissures in rock of the same age, near a rich oil-pool in West Virginia.
Other similar deposits, but usually wax-like and dull, are found in Utah
and neighbouring States. Asphaltum occurs in vast quantities in southern
California, and also in Cuba; these deposits resemble the celebrated
asphaltum of Trinidad and give promise of being fully as extensive and
valuable.
In brief, gaseous, fluid, semifluid, and solid hydrocarbons in great
variety are widely distributed throughout the portions of North America
where the surface is composed of sedimentary beds, and in a few
instances occur in cavities in igneous rocks as well.
The influence of life in leading to the concentration of substances of
commercial value is still further illustrated by the beds of
diatomaceous earth which are found in various portions of North America
and elsewhere, particularly in Cenozoic and more recent terranes. Beds
of diatomaceous earth reported to be 40 feet thick and of wide extent
have been found near Richmond, Virginia, and similar deposits occur at
several localities in Oregon, California, etc. The uses of this fine,
white, flour-like powder, each minute grain of which is a
beautiful siliceous organism, are for polishing powder, as an ingredient
in friction soap, as an absorbent for nitroglycerine in the manufacture
of high explosives, etc.
A class of substances of economic importance which owe their
accumulation to chemical agencies acting at the surface of the earth is
well illustrated by deposits of rock salt and gypsum.
In the Silurian system in New York, Ontario, Michigan, etc., several
beds of rock salt and gypsum occur, indicating that there were formerly
a number of separate evaporating basins in that region. The beds of salt
vary in thickness from a few inches to over 300 feet, as at Tulley, New
York. At Goodrich, Ontario, 6 beds of salt from 6 to 35 feet thick have
been penetrated in a single well. With the salt in this the Salina
formation there are many beds of gypsum. In rocks of Carboniferous age
in Michigan, other extensive beds of salt and gypsum have been
discovered. In Louisiana, Texas, Utah, and other States, salt and gypsum
occur in Mesozoic and Cenozoic rocks. One of the most remarkable of
these deposits is beneath small islands in the Gulf of Mexico off the
Louisiana coast. On Jefferson Island, for example, rock salt was reached
recently at a depth of 260 feet beneath Cenozoic rock, and was
penetrated for over 1,800 feet without reaching the base of the deposit.
The supply of salt stored in the rocks, and the natural brines of the
arid region, such as the waters of Great Salt Lake, afford an
inexhaustible supply upon which comparatively small demands have thus
far been made.
In addition to salt and gypsum there are other substances that have been
accumulated in a similar manner, such, for example, as sodium sulphate,
of which large beds occur in the desiccated lake basins of the arid
region, sodium bromide, which is obtained from some of the ancient
brines pumped from deep wells in Michigan.
Next to the fossil fuels, the most important products of the rocks in
North America are the iron ores. Although certain igneous rocks are rich
in iron, and in some instances contain it even in a pure or
metallic state, none of the rocks that have cooled from fusion carry
iron in any form in sufficient quantities to be of commercial
importance. Most of the iron in igneous rocks is contained in mineral,
usually silicates, and would be difficult to separate. When exposed to
the air and to percolating water, the iron-bearing minerals of the
igneous or other rocks decay and the iron enters into various new
combinations. When organic acids are present, and especially carbon
dioxide, ferrous carbonate is formed, which is quite soluble, and is
taken into solution by percolating water, some of which emerges as
springs, and joins the surface run-off, which may also take up ferrous
carbonate in solution. One of the most common methods by which iron ore
is accumulated is when water carrying ferrous carbonate in solution
forms swamps and lakes, and in many instances as the water is exposed to
the air and aided by evaporation it parts with a portion of its carbon
dioxide, and the hydrated sesquioxide of iron or limonite results. When,
under similar conditions, an excess of organic matter is present, beds
of ferrous carbonate are formed. In other instances iron oxide is
precipitated in swamps and lakes through the action of low forms of
plant life. The ores of iron concentrated in these ways are in many
instances in well-defined layers, or lenticular bodies, which are
thickest in the central portion and thin out in all directions. Their
forms are determined mainly by the shapes of the depressions they
occupy. Both ferrous carbonate and limonite, however, occur in irregular
surface deposits.
In North America, bog-iron ores occur at the surface in many regions, in
existing swamps and about springs, but are seldom of economic
importance, owing in part to the great abundance of better ores.
Limonite occurs at the surface also, having been deposited in cavities
and as a cement for loose fragments, particularly on the weathered
outcrops of formations rich in iron. When rocks contain but a fraction
of 1 per cent of iron, the soil on their weathered outcrops, owing to
the removal of the more soluble ingredients and the leaving of the less
soluble oxidized iron, have a yellow, brown, or red colour, and
in some instances this process of concentration has produced workable
iron ore. Limonite and earthy hematites occur widely throughout the
Appalachian region, in central New York, and westward to the Mississippi
Valley. One of the most productive formations is the Clinton, a division
of the Silurian, the outcrop of which extends in a nearly continuous
band from Alabama, where at Birmingham, etc., it is extensively worked,
northward along the west side of the Appalachians to central New York,
and thence westward to Ohio, and appears again in Wisconsin. At many
localities throughout this belt, some 1,300 miles in length, iron
furnaces have been built, although now mostly abandoned, the ore supply
being the weathered outcrop of the Clinton limestone.
In the Carboniferous rocks of Pennsylvania and neighbouring States to
the south and west, layers of ferrous carbonate, formed when there was
an excess of organic matter present, termed black-band ore and kidney
ore, occur. The former is present as regular strata and the latter in
oval concretionary masses. These ores, although not as rich in iron and
less pure than certain other and more abundant and more accessible
deposits, have been extensively utilized, largely for the reason that
they occur in the same formation which furnishes coal available for
their reduction.
Deposits of iron ore accumulated in the several ways referred to above
may be metamorphosed and changed to hematite and magnetite. The richest
iron ores of North America are of this nature, and will be referred to
below in connection with other substances of economic importance
contained in the metamorphic rocks.
There are various other substances in the stratified rocks of North
America of economic importance which owe their value to some process of
concentration. Certain rocks, as the so-called greensands or marls of
eastern New Jersey, contain from 3 to 10 per cent of potash, which makes
them valuable fertilizers. In this instance the concentration took place
on the floor of the sea, through the action of decomposing
organic matter, and the potash-bearing mineral of the greensand, namely,
glauconite, was deposited in the interiors of the minute tests of
foraminifera. The importance of this material is indicated by the fact
that the greensands of New Jersey have been actively worked for more
than half a century, the annual products during many years being upward
of 100,000 tons.
Extensive areas in the Carolinas, Florida, etc., underlaid by rocks of
Cenozoic age, are rich in phosphatic nodules, which have been derived
from organic matter. The guano deposits of the low arid islands in the
West Indies illustrate another mode of accumulation of organic material
useful as a fertilizer.
The assorting of surface _debris_ by streams and currents has led to the
formation of extensive deposits of clay which occur widely throughout
the portions of North America where the surface is composed of
stratified rock, which is extensively used in the manufacture of
earthenware, bricks, tiles, terra-cotta, Portland cement, etc.
When rocks containing gold in nuggets, grains, scales, etc., are
disintegrated, and the resulting _debris_ removed by streams, mechanical
separation of the heavier from the lighter material takes place and all
but the very finest of the gold is concentrated on the stream beds. In
this manner the rich placers of the Pacific mountain region from
California to Alaska have originated.
The general nature of the ore bodies formed through the action of
chemical agencies in sedimentary rocks, by solution and redeposition, is
illustrated by the lead and zinc ores of Wisconsin, Missouri, the
silver-bearing lodes of the Pacific mountains, etc. In the case of the
lead and zinc deposits the ores occupy the interspaces between broken
sedimentary beds or line caverns. Under the best explanation of the
origin of these deposits that has been offered, although certain
modifications of the general hypothesis have been suggested which it is
not necessary to consider at length at this time, the lead and zinc are
considered to have been at one time widely distributed in the adjacent
sedimentary rocks, mainly limestone, and to have been taken in
solution by percolating waters and carried to cavities where they were
precipitated, together with various other mineral substances, such as
calcium carbonate or calcite, barium sulphate or barite, carbonate of
calcium and magnesium or dolomite, etc. The minerals containing lead are
principally galenite or lead sulphate, cerussite or lead carbonate;
while the zinc is contained in the minerals, sphalerite or zinc
sulphide, calamine or zinc silicate, smithsonite or zinc carbonate, etc.
These minerals, including both those containing lead and zinc, and those
intimately associated with them which are at present of no commercial
value, are such as are known to crystallize from solution without the
aid of high temperatures. In the Missouri lead and zinc districts the
ore deposits occur near the surface, the depth of the present working
seldom exceeding 150 or 200 feet, and, as nearly as can be judged, have
been formed by the downward transfer of mineral matter through the
process of solution and recrystallization, as the surface of the land
has been lowered by chemical and mechanical denudation.
Many of the rich silver-mines of the Pacific mountains occur in fissures
and cavities in sedimentary rocks, mainly limestone. Instances of this
nature are furnished by certain mines in northeastern Mexico, where the
ore is found in cavities in Cretaceous limestone; at Leadville and
Aspen, Colorado; Big and Little Cottonwood canyons, and the Horn
silver-mine, Utah, where the principal country rock is Carboniferous
limestone; the Eureka district, Nevada, where the ore occurs in cavities
in Cambrian limestone. In the case of several of these mines, igneous
rock is near at hand, and the ores are believed to owe their
concentration largely to the action of heated waters.
In other regions deep fissures, occupied in part by dikes of igneous
rock, have permitted of the ascent of water charged with mineral matter
from far below the surface; such waters are heated, in part by the
general heat of the earth's interior, or, if in association with dikes,
by the heat of the once molten intruded rock. The ascending hot water is
an active solvent, and as it rises becomes cooled, and for this
and other reasons precipitates many mineral substances. Veins are thus
formed, which are many times banded--that is, result from the filling of
fissures by the successive deposition of minerals of various kinds on
their walls, each different layer of minerals indicating a change in
conditions. Fissures filled in this manner from below, as denudation
progresses, become exposed at the surface and reconcentration through
the influence of disintegration and decay, and of solution and
redeposition by descending water takes place. Ore bodies of this
character carrying gold, silver, mercury, etc., are of wide occurrence,
especially in the Pacific mountains, but the process of concentration is
independent of the nature of the country rock. Segregated and fissure
veins occur in either igneous, sedimentary, or metamorphic terranes, but
are more commonly of economic importance in the metamorphic rocks than
elsewhere, and will be referred to again in that connection.
_Economic Importance of the Metamorphic Terranes._--The great laboratory
in which rocks undergo important changes in their physical condition and
in mineralogical and chemical composition, is what has been termed on a
previous page the zone of metamorphism. The depth of the upper limit of
this zone is variable, dependent in part on the nature of the rocks and
on movements within them, as is the case of mountain building. In fact,
there is probably no well-defined limit to the zone either above or
below, as in the former direction metamorphism merges by gradations into
alteration produced by the descent of surface water, and in the latter
direction as heat increases passes again, as we imagine, by insensible
and irregular gradations into a region where the rocks are so highly
heated that diffusion rather than concentration results. Whether the
rocks below the zone of metamorphism are fused or not depends on
pressure. They are probably solid, but in a potentially plastic
condition, and become fused and may be forced upward through fissures in
the condition of igneous magmas when pressure is relieved. The zone of
metamorphism lies between a superior zone where alteration by
descending water is dominant, and a lower region where alteration due
mainly to heat is in control. In the zone of metamorphism the influence
of heated percolating waters, combined with movements in the rocks, are
the principal factors which lead to the concentration of mineral
substances.
Under the influence of percolating, heated waters, new minerals are
formed in sedimentary or igneous rocks, and rocks once metamorphosed may
undergo additional changes. Mineral matter previously widely
disseminated through rocks is, under the action of percolating, heated
water, brought together and the regeneration and crystallization of a
large variety of ores and minerals result. The birthplace of a large
variety of ores and minerals is in the zone of metamorphism. It is in
metamorphic rocks that the geologist looks for gems, the precious
metals, crystalline marble, magnetic iron, etc.
For the most part, however, the native metals and ores of the precious
and many of the common metals are too widely disseminated in the
metamorphic rocks to be of commercial importance, and a still further
concentration, principally in fissures and other cavities, is necessary
before they can be of value to man. This secondary concentration is much
the same as in the case of the deposition of lead and zinc ores in
cavities in sedimentary rocks, and results largely from the solution and
redeposition, sometimes by replacement, of mineral matter by heated
waters.
Certain ores and rocks contained in metamorphic terranes owe their
concentration to previously acting processes of concentration, but have
undergone chemical changes in place. Illustrations of this class of
ores, etc., are furnished by the magnetite and hematite contained in the
metamorphic rocks on the eastern border of the Appalachians, in New
England, eastern Canada, and the Lake Superior region. These ore bodies,
frequently of great size, in some instances furnish evidence of having
been originally lenticular masses of bog-iron ore, or ferric carbonate,
associated with sedimentary beds, and originally concentrated, as
already mentioned, at the surface through the action of water charged
with carbon dioxide, but principally on account of the influence of heat
have been changed to a higher degree of oxidation and now appear as
hematite, as, for example, in the iron districts of the northern
portions of Michigan, Minnesota, Wisconsin, and the Ozark Hills, or
still further altered as in the richest of all iron ores, magnetite, so
abundant in the metamorphic rocks of the Appalachian region, about the
Adirondack hills, widely and in extensive bodies in eastern Canada,
about the south shore of Lake Superior, in Texas, etc.
In certain instances, as has been shown by C. R. Van Hise and others,
hematite ore, like that of the Lake Superior region, has resulted from
the alteration of ferrous carbonate which had replaced limestone by a
chemical process of solution and double decomposition.
As bodies of iron ore in the form of the carbonate, or limonite, may
occur in rocks of any age, and as rocks of any age may be metamorphosed,
it follows that hematite and magnetite may be present in any formation
which has been subjected to metamorphosing conditions.
Limestone when metamorphosed is changed to a crystalline marble,
frequently white in colour owing to the dissipation of its previously
contained organic matter. The white marbles so extensively utilized in
Georgia, Vermont, etc., are of this nature. Other similar metamorphosed
layers occur in several of the ranges of the Pacific mountains from
Mexico to Alaska.
The influence of metamorphism on deposits of coal when the heat has been
of moderate intensity serves to drive off a large part of the volatile
matter present and converts the coal into a substance resembling coke,
as has happened adjacent to dikes or intruded sheets of igneous rock in
the Richmond coal-field, Virginia, in New Mexico, Washington, etc. When
the heat is somewhat more intense, the coal is changed to what is termed
graphitic anthracite, as in the Rhode Island coal-fields, and when still
greater or long-continued, results in the production of graphite,
as in the Algonkian rocks about the Adirondack hills and over a wide
region in eastern Canada.
An important result of metamorphism is the production of new minerals in
the rocks acted on. Many of the metamorphic terranes consist essentially
of quartz, feldspar, and mica, which have been formed by the
rearrangement of the mineral matter contained in the rocks during their
previous state. Besides these constituent minerals there are frequently
others present, such as the garnets, tourmaline, emerald, sapphire,
corundum, etc., which are of economic importance. In a large number of
instances the minerals of metamorphic rocks are contained in veins of
one class or another, in part resulting from segregation in the rocks
themselves while yet in a heated condition, and in part deposited in
fissures or other openings as a result of secondary concentration
through the action of heated waters. The principal difference between
the minerals concentrated in the metamorphic rocks and those deposited
in cavities in unaltered sedimentary beds seems to be that in the former
instance the percolating water which carried the material in solution
had a higher temperature than in the latter case.
Among the numerous mineral substances of value in the arts, occurring in
the metamorphic terranes of North America, other than building stones
and the previously concentrated deposits, such as iron ore, graphite,
etc., mention can only be made at present of the following:
Mica, which is used in thin sheets for the windows of stoves and
furnaces, and when ground and mingled with other substances furnishes a
good insulating material for electric wires, fireproofing, and also used
as a lubricant, etc., occurs in large quantities in the metamorphic
rocks of New Hampshire and Ottawa, and less abundantly in North
Carolina, South Dakota, Wyoming, Idaho, etc. It is widely distributed,
but to find transparent colourless sheets of large size is difficult.
Talc and soapstone, consisting of the hydrated silicate of magnesia, and
useful for hearths, mantels, fire-brick, linings for stoves, laundry-,
bath-, and acid-tubs, etc., and when ground, employed as an
adulterant of soap, paper, rubber, and as a lubricant, etc., occurs
widely in the metamorphic terranes on the eastern side of the
Appalachians, in Canada, and at numerous localities in the Pacific
mountains. The chief centres of production at present are in
Pennsylvania, New Jersey, New Hampshire, and Vermont.
Asbestos, valuable on account of its fibrous structure and
non-conductivity of heat, which make it an excellent insulator, and
largely used in the manufacture of fireproof paper, cloth, etc., occurs
in connection with serpentine, in metamorphic terranes, and is
extensively mined in the Thetford district, Quebec.
Corundum, consisting of aluminum oxide, and having essentially the same
composition as the sapphire and ruby, and a less pure variety of similar
composition termed emery, is largely used as an abrasive in polishing
metal, sharpening tools, etc., and also as "sand-paper" in working wood,
occurs in commercial quantities, largely in crystalline limestone, at
Chester, Mass., in Georgia, North Carolina, and several other
localities. Although corundum is next to the diamond in hardness, and
therefore highly favourable, when reduced to a powder, for polishing
various substances, the demand for it has in recent years been
diminished owing to the manufacture of an equally if not superior
material termed commercially _carborundum_.
Among the crystals used as gems, which occur in the metamorphic rocks of
North America but thus far in minor quantities, and as a rule of
inferior quality, may be enumerated sapphires, rubies, tourmalines,
garnets, quartz, etc.
Apatite, a mineral rich in phosphoric acid, and largely used in the
manufacture of fertilizers, occurs associated with limestone in the
metamorphic rocks of Quebec and Ontario in the form of veins, beds, and
irregular pockets, and a few years since was extensively mined, but now,
owing to foreign competition, is held in reserve.
By far the most valuable of the minerals and native metals that occur in
the metamorphosed terranes is gold. Although this metal has been found
in paying quantities in association with nearly every kind of
country rocks and in terranes of all ages, the place of its original
concentration from a previously widely disseminated condition is to a
great extent in the zone of metamorphism. It occurs principally as
native gold, although usually alloyed with silver, but is frequently
contained in iron pyrites. In the crystalline rocks, such as gneiss,
schist, slates, granite, etc., it occurs in flakes and grains, but so
far as its occurrence in commercial quantities is concerned its
deposition has for the most part been secondary, and the metal, usually
in association with quartz, is found in veins, lodes, contact deposits,
etc., and owes its concentration to chemical agencies not well
understood, acting in connection with percolating water. That this
general statement is correct is clearly shown by the fact that gold
occurs in crystals, flakes, grains, etc., most frequently in quartz and
iron pyrites, which, as can be shown in a number of ways, have
crystallized from solution. The gold and its commonly associated mineral
in countless instances occupy fissures and must have been carried to
such localities after the surrounding rock had been fractured. So
intimate is the association of gold with metamorphic rocks that this is
one of the main guides in searching for it, although, as already stated,
it is frequently present in other rocks as well. With the disintegration
of the metamorphic terranes the gold is set free, and may be still
further concentrated by streams so as to form the well-known placers.
A very large proportion of both the quartz and placer mining of North
America is in regions occupied by metamorphic rock. This is true of all
gold-mines, previously quite largely exploited, of the Atlantic mountain
region from Georgia to eastern Canada. The mines of California are also
largely in schistose rocks, as are also those to the northward,
throughout the Pacific mountains, to British Columbia and Alaska,
including the recently established mining district at Cape Nome.
With placer gold, and probably derived largely, if not entirely, from
metamorphic rocks, there are frequently found grains of platinum. The
annual production of this metal in the United States and Canada
has a value of about $5,000.
The study of the distribution of native metals and ores in the
metamorphic rocks of North America indicates that in general the older
rocks, as the Archean, for example, are less rich than the younger
terranes, such as the schist, etc., of the Sierra Nevada and Cascade
Mountains. This seems to indicate that the older rocks were once deeply
buried and their more soluble substances removed by ascending waters,
and in part redeposited in higher terranes. Erosion has since carried
off the rocks which were mineral-charged and laid bare the depleted
terranes beneath. This hypothetical explanation of the general poverty
of the Archean rocks is coupled with another consideration, namely, that
the younger metamorphic terranes, where they have been elevated, as in
the Pacific mountains, are more broken than the Archean rocks, and
afford more cavities in which minerals may be deposited. Whether this is
a complete explanation or not remains to be demonstrated, but
observation shows that the Archean terranes--all of which as yet
discovered are composed of either metamorphosed or igneous rocks--are,
in comparison with younger metamorphosed rocks, relatively poor in
minerals and ores of commercial importance.
Among the economic products of the rocks are included mineral waters.
The direct commercial value of such waters, not including their use for
baths, etc., in the United States, is about $7,000,000 annually. The
demand for these waters depends largely on the mineral substances they
hold in solution, and which in many instances is in process of
transference from one locality to another. Much might be written in this
connection in illustration of the fact that the processes by which
minerals, ores, etc., have been concentrated are still in progress.
LITERATURE
An extensive literature is available concerning the geology, minerals,
ores, etc., of North America, but only a few of the more important
publications can here be referred to. The numerous publications
of the United States Geological Survey, the Geological Survey of Canada,
and the Geological Survey of Mexico contain vast amounts of valuable
information. Several of the States of the United States have independent
surveys and have published numerous reports. Of journals containing
articles of American geology, the more important are: _The Journal of
Geology_, published at the University of Chicago; _The American
Geologist_, published at Minneapolis, Minn.; _The American Journal of
Science_, published at New Haven, Conn. The publications of a large
number of learned societies in Canada and the United States should also
be consulted.
The most useful bibliographies of North American geology are:
DARTON, N. H. _Catalogue and Index of Contributions to North
American Geology, 1732-1891._ Published as Bulletin No. 127 of
the United States Geological Survey, Washington, D. C., 1896.
DOWLING, D. B. _General Index to the Reports of Progress_ [of the
Geological Survey of Canada], _1863-1884_. Published by the
Canadian Geological Survey, Ottawa, Canada, 1900.
WARMAN, P. C. _Catalogue and Index of the Publications of the
United States Geological Survey, 1880 to 1901._ Published as
Bulletin No. 177 of the United States Geological Survey,
Washington, D. C., 1901.
WEEKS, F. B. _Bibliography of North American Geology,
Paleontology, Petrology, and Mineralogy for the Years 1892-1900,
Inclusive._ United States Geological Survey, Bulletins No. 188
and 189, Washington, D. C., 1902.
Of the numerous general treatises on the geology, the following will be
found especially helpful to the student:
DANA, J. D. _Manual of Geology._ Fourth edition. American Book
Company, New York, 1895.
KEMP, J. F. _The Ore Deposits of the United States and Canada._
Scientific Publishing Company, New York, 1900.
LE CONTE, J. _Elements of Geology._ Fifth edition, revised and
partly rewritten by Prof. H. L. Fairchild. D. Appleton and
Company, New York, 1903.
MERRILL, G. P. _A Treatise on Rocks, Rock-Weathering, and Soils._
The Macmillan Company, New York, 1897.
SHALER, N. S. _Outlines of the Earth's History._ D. Appleton and
Company, New York, 1898.