VERY ANCIENT EARTH HISTORY
(_Archeozoic and Proterozoic Eras_)
We shall now consider the older rocks of the Earth, including those
of Archeozoic, Proterozoic, and Paleozoic ages. What are the salient
points in the very early history of the earth (not including the
evolution of organisms) shown by these very ancient rocks? Beginning
with the oldest known rocks, it will be our purpose to trace out the
principal recorded events of earth history in the regular order of
their occurrence. As in human history, so in earth history the recorded
events of very early times are fewest and most difficult of all to
understand. In spite of this difficulty it is best to begin with the
oldest known rocks or, as Le Conte has said, "to follow the natural
order of events. This has the great advantage of bringing out the
philosophy of the history--the law of evolution." Because of limitation
of space we shall give special attention to the physical history of
North America, but the general principles brought out apply almost
equally well to the other continents.
The Archeozoic rocks contain the earliest known records of geological
history, or, in other words, the oldest recorded ordinary geological
processes such as weathering and erosion, deposition of strata,
igneous activity, etc. Although we are here dealing with the most
obscure records of any great rock system, partly because the rocks
have been so profoundly altered (metamorphosed), and partly because of
the absence of anything like definitely determinable fossils, it is,
nevertheless, true that certain very important conclusions have been
reached regarding this very ancient geological era.
Among the very oldest of all known rocks of North America are the
Grenville strata, so named from a town in the St. Lawrence Valley.
In fact, no rocks elsewhere in the world have been proved to be more
ancient. The Grenville series consists of a great mass of sediments
(strata)--original muds, sands, and limes--which were deposited layer
upon layer under water (Plate 12). The widespread extent and character
of the series in southeastern Canada and the Adirondacks, and more than
likely far beyond these limits, make it certain that the Grenville
strata were accumulated on the bottom of a relatively shallow sea very
much as sediments are now piling up on shallow sea bottoms. Thus,
the most ancient definitely known condition of the region where the
Grenville strata are exposed was an expanse of the sea covering the
whole area. Wherever, in other parts of the world, the Archeozoic rocks
have been studied, stratified rocks also seem to be the very oldest
which are recognizable, but up to the present time no such rocks have
been proved to be any older than, or even as old as, the Grenville
series.
It may occur to the reader to ask, how long ago did the Grenville
ocean exist? There are grave difficulties in the way of answering
this question in terms of years since we have nothing like an exact
standard for such a measurement or comparison. Although we must concede
that not even approximate figures can be given, it can, nevertheless,
be demonstrated by several independent lines of reasoning that the
time must be measured by at least tens of millions of years, a very
conservative estimate of the minimum time which must be allowed
being about 50 million years. In any case, the time is so utterly
inconceivable to us that the important thing to bear in mind is that
the great well-known events of earth history, which have transpired
since the existence of the Grenville ocean, require a lapse of many
millions of years, as shown by revolutionary changes in geographic and
geologic conditions such as the long periods of erosion, the enormous
accumulations of sediment, the repeated spreading out and disappearance
of sea water over many portions of the earth, and the building up and
tearing down of great mountain ranges at various times. The ideas here
expressed will be much better appreciated by the reader after following
through the salient points in the history of North America as set forth
in the succeeding pages.
Again, the reader may ask, by what line of reasoning do we conclude
that these stratified rocks are so exceedingly ancient? All rocks of
Archeozoic Age, including strata as well as certain younger igneous
rocks (see below), invariably occupy a basal position in relation to
all other rock systems. They constitute a complex lot of crystalline
metamorphic rocks, combining certain characteristics which lie below
the base of the determined sedimentary succession. Where rocks with
the characteristics of the Archeozoic are separated from the oldest
Paleozoic (Cambrian) strata by the great sedimentary or metamorphic
system known as the Proterozoic (see below), we may be sure that we
are dealing with Archeozoic rocks. If the series of rocks in question
belongs in the Archeozoic system, all that remains is to determine its
age position in that system. This can usually readily be done because
wherever they have been studied the Archeozoic rocks may be subdivided
into two groups of rocks, a sedimentary and an igneous. Where the
igneous rocks, mainly granites and related types, occur associated with
the sedimentary rocks (e.g., Grenville), they very clearly were forced
or intruded, while molten, into the sedimentary rocks, thus proving
these latter to be the older.
Since the Archeozoic strata of the Adirondack Mountains, southeastern
Canada, and also all, or nearly all, other known districts are mostly
badly disturbed, tilted, and more or less bent or folded, and since
neither top nor bottom of the piles of strata has ever been recognized
as such, it is impossible to give anything like an exact figure for
the thickness of the series. Continuous successions of strata have,
however, been observed in enough places to show that they were commonly
deposited layer upon layer to a thickness of at least some tens of
thousands of feet. A thickness of over 100,000 feet has been reported
from southeastern Canada. The clear implication is that the Archeozoic
sea which received sediments must have existed for a vast length of
time which must be measured by at least some millions of years, because
in the light of all our knowledge regarding the rate of accumulation
of sediments a very long time was necessary for the piling up of such
thick masses of strata. It does not, however, necessarily follow that
the Grenville ocean was many thousands of feet deep where deposition
took place. In fact, the very character of the original sediments
(muds, sands, and limes) clearly indicates that the Archeozoic sea in
which they accumulated was, for most part at least, of shallow water
because such sediments have rarely, if ever, been carried out into
an ocean of deep water. The great ocean abysses of to-day are not
receiving any appreciable amount of land-derived sediment. Thus we are
forced to conclude that in Archeozoic time, as well as many times in
later ages, the shallow sea bottom gradually sank while the sediments
accumulated. Even more conclusive proof of such subsidence has been
obtained from the study of so-called "folded" mountain ranges of
Paleozoic and later time, an excellent example being the Appalachian
Range.
Having established the sedimentary origin and great antiquity of
the Grenville series, we are led to the interesting and important
conclusion that these oldest known rocks are not the most ancient
which ever existed, because the Grenville strata must have been
deposited layer upon layer, upon a floor of still older rocks. If such
still older rocks are anywhere exposed to view, they have never been
recognized as such. Again, the fact that the most ancient known rocks
were deposited under water carries with it the corollary that there
must have been lands at no great distances from the areas of deposition
because, then as now, such sediments as muds and sands could have been
derived only from the wear or erosion of lands, and have been deposited
in layers under water adjacent to those lands. But we are utterly in
the dark regarding any knowledge of the location or character of such
very ancient lands.
The most ancient known strata, as we see them to-day, do not look like
ordinary sediments such as shales, sandstones, and limestones. They
have been profoundly changed from their original condition, that is
to say, they have undergone metamorphism. The Archeozoic strata now
exposed to view were formerly buried at least some miles below the
earth's surface, the overlying younger rocks having since been removed
by erosion through the millions of years of time. Far below the earth's
surface, under conditions of relatively high temperature, pressure, and
moisture, the materials of the strata were completely crystallized into
various minerals. The surfaces of separation of the very ancient layers
of sediment are still usually more or less clearly present (Plate 12).
Original limestone has been changed into crystalline limestone or
marble; sandstone has been changed into quartzite, and shale, sandy
shale, and shaly sandstone have been changed into various schists and
gneisses.
In western Ontario there are also stratified rocks (called the Keewatin
series) which seem to be of about the same age as the Grenville strata
farther east. A point of special interest in connection with the
Keewatin strata is the presence of layers of lava in portions of the
series, thus proving that molten rock materials were poured out on
the earth's surface during the most ancient known era of the earth's
history.
After the accumulation of the very ancient Archeozoic sediments igneous
activity took place on grand scales when great masses of molten rock
were forced (intruded) into the sediments from below. Masses of molten
materials are known to have been thus intruded at several different
times, but of these the most common by far cooled to form a great
series of granite and closely related rocks. The general effect was
to break the old strata up into patches or masses of varying sizes as
clearly shown by the present distribution and modes of occurrence of
these igneous rocks. In most cases the strata were pushed aside by,
or tilted or domed over, the upwelling molten floods--in many cases
the molten materials were, under great pressure, intimately forced or
injected into the strata; numerous large and small masses of strata
were caught up or enveloped (as inclusions) in the molten floods; in
some cases there was local digestion or assimilation of the strata
by the molten materials, while in still other places large bodies of
strata seem to have been left practically intact and undisturbed. Such
igneous rocks, which are very widespread, are all of the plutonic or
deep-seated types; that is, they were never forced up to the earth's
surface like lavas, but they solidified at considerable depths (at
least some thousands of feet) below the surface. We see them exposed
to-day only because a tremendous amount of overlying rock materials
has been removed by erosion. These igneous rocks are generally easily
distinguished from the old sediments of Grenville age because of their
more general homogeneity in large masses, and their lack of sharply
defined bands or layers of varying composition. The fact that the
minerals have always crystallized to form medium to coarse-grained
rocks shows that these rocks solidified under deep-seated conditions,
since it is well known that surface flows (lavas) are much finer
grained commonly with more or less of the rock not crystallized
at all. Slow cooling under great pressure favors more complete
crystallization with growth of larger crystals.
As we have just learned, the very character and structure of the
Archeozoic rocks now exposed to view show conclusively that they were
formerly deeply buried, and the inference is perfectly plain that the
overlying rock materials were removed by erosion. Profound erosion of
any land mass means that the land must have stood well above sea level,
and thus we come to the important conclusion that the great mass of
Archeozoic rocks (both strata and igneous rocks) were upraised well
above sea level. Just when the uplift occurred cannot be positively
stated, but in every region where the matter has been studied it took
place before the strata of the next geological era began to deposit
as shown by the fact that such later strata rest upon the profoundly
eroded surface of the Archeozoic rocks. Such an erosion surface, called
an "unconformity," marks a gap in the geological record of the district
where it occurs. There is much to support the view that the uplift
was concomitant with the great igneous intrusions, especially the
granite. It is reasonable to believe that the same great force which
caused the welling up of such tremendous bodies of liquid rock into
the earth's crust might easily have caused a decided uplift of a whole
large region, but even so the process must have been geologically slow.
In regard to the height of those ancient lands, the character of the
topography, and the drainage lines we are as yet utterly in the dark.
The fact that many thousands of feet in thickness of materials were
removed by erosion to expose the once deeply buried rocks, does not
necessarily imply that the lands at any time had great height, because
it is possible that while elevation slowly progressed, much material
was steadily removed by erosion. In the light of our knowledge of the
origin and growth of mountain ranges of later time there is little
doubt that at least some of the Archeozoic lands were raised to such
mountain heights.
Thus far in our study of the Archeozoic rocks attention has been
mainly directed to southeastern Canada and the Adirondack mountains,
where careful studies have been made. In all parts of the world
where the most ancient known (Archeozoic) rocks have been studied in
detail the same general principles apply. Particular attention has
been given to the Archeozoic rocks south of Lake Superior, and in the
Piedmont Plateau of the eastern United States. In the accompanying
map Archeozoic rocks are widely exposed to view within the areas
shown in black. It has been estimated that Archeozoic rocks appear
at the surface over about one-fifth of the land area of the earth.
Where they are not at the surface it is believed that they everywhere
exist under cover of later rocks. In other words, Archeozoic rocks are
considered to be almost universally present either at or under the
earth's surface. This is true of the rocks of no other age. Special
mention should be made of the fine exhibitions of Archeozoic rocks in
Scandinavia and the Highlands of Scotland.
[Illustration: Fig. 32.--Map showing the surface distribution of
Archeozoic and Proterozoic rocks in North America. (Redrawn by the
author after U. S. Geological Survey.)]
All known evidence leads us to the remarkable conclusion that
the climate of much, or possibly all, of Archeozoic time was not
fundamentally different from that of to-day. There must have been
weathering of rocks, rainfall, and streams much as at present as
proved by the character and composition of the stratified rocks which
formed in that remote era. The presence of graphite ("black lead") in
crystalline flakes scattered through many of the strata shows that the
climate must have been favorable to some form of life, because graphite
thus occurring quite certainly represents the remains of organisms,
this matter being more fully discussed in a succeeding chapter. In
passing it may be stated that climatic zones were then probably
scarcely if at all marked off, as they quite certainly were not even
during Paleozoic time. One of the great contributions of geology to
human knowledge is that during the tens of millions of years from
Archeozoic times to the present the earth's climate has undergone no
fundamental change or evolution. In the earlier ages there was greater
uniformity of climate over the earth, and, during known geologic
time there have been rather localized relatively minor fluctuations
giving rise to glaciers, deserts, etc., but there has been no real
evolution of climate at all comparable to the marvelous evolution of
organisms--both animals and plants.
We shall now turn our attention briefly to a consideration of the
second great subdivision of geologic time--the Proterozoic era. Rocks
of Proterozoic Age comprise all of those which were formed after the
Archeozoic rocks and before the deposition of the earliest Paleozoic
(Cambrian) strata, these latter being rather definitely recognizable
because they contain fossils characteristic of the time. Cambrian
strata are, in fact, the oldest rocks which contain anything like
an abundance of fossils, so that the separation of rocks of either
Archeozoic or Proterozoic Age from the earliest Paleozoic is seldom
difficult. But how may we separate the Proterozoic rocks from the
Archeozoic? Fossils afford us no aid whatever, because no determinable
fossils have been found in rocks as old even as the earlier
Proterozoic. The two great groups of very ancient rocks do, however,
show a number of differences which must be considered together.
Thus, igneous rocks distinctly predominate in the Archeozoic, while
stratified rocks predominate in the Proterozoic. All Archeozoic strata
are thoroughly metamorphosed (changed from their original condition),
while large masses of the Proterozoic strata are only moderately
metamorphosed, or even unaltered, and therefore look much like ordinary
strata of later ages. Archeozoic rocks have almost invariably been
notably deformed by more or less folding, tilting, etc., while the
Proterozoic rocks show relatively much less deformation. Another
important criterion is the fact that the Proterozoic rocks, wherever
they have been studied in relation to the Archeozoic rocks, always
rest upon a profoundly eroded surface of the latter, that is, an
unconformity separates the two great sets of rocks. This erosion
surface is of still further interest because it is the very oldest one
known, none having been recognized within the Archeozoic group itself.
Even where the Proterozoic strata have been considerably metamorphosed
and deformed, this old erosion surface may be recognized, and if the
rocks below that surface possess the characteristics of the Archeozoic
rocks as described above, the two great very ancient rock groups may be
distinguished. One of the triumphs of geology during the last 25 to 30
years has been the recognition of the great rock group (Proterozoic)
between the Archeozoic and Paleozoic, thus bringing to light the
records of an era which lasted many millions of years.
The length of time represented by the Proterozoic era is by many
believed to have been fully as long as all succeeding eras--Paleozoic,
Mesozoic, and Cenozoic--combined. Twenty million years would be a
very conservative estimate for the duration of the era. What is the
nature of the evidence as recorded in the rocks which lead us to
conclude that the Proterozoic era lasted such a vast length of time?
The great thickness of Proterozoic strata (over 30,000 feet in the
Lake Superior region), in the light of what we have already learned
regarding the present rate of wear (erosion) of lands and deposition
of the eroded materials under ordinary conditions, clearly implies
millions of years of time for their accumulation. But the Proterozoic
strata as we now see them are in most places not a continuous pile,
that is they were not accumulated layer upon layer without notable
interruption. Thus, the thick Proterozoic group of the Lake Superior
region has been divided into four distinct, mainly sedimentary series
separated from each other by erosion surfaces (unconformities). Each
erosion surface represents a long time when the area was elevated and
underwent profound wear before the next series of strata accumulated on
the worn surface. That such times of erosion were geologically long is
proved not only by the profound alteration (metamorphism) of one set of
strata before another accumulated, but also by the fact that granite,
which, as we have learned, is never exposed except where much overlying
material has been eroded, actually formed parts of surfaces of earlier
Proterozoic rocks upon which later ones were deposited. In the Lake
Superior region there are not only three great erosion surfaces
(unconformities) within the Proterozoic group, but also one at the base
separating it as a whole from the Archeozoic group, and another at the
top separating it from the Paleozoic group. It is, therefore, fair to
conclude that the amount of time (millions of years) represented by
these great erosion intervals was fully as great as the time needed for
deposition of the existing Proterozoic strata.
In the Lake Superior region the older Proterozoic strata are nearly all
more or less folded and altered (metamorphosed), and they have been
intruded by considerable bodies of molten rock, mostly granite. The
later Proterozoic strata have been much less deformed and in many cases
they are practically unaltered. In this region a very remarkable event
took place in late Proterozoic time. This was volcanic activity on a
grand scale. We may gain some idea of the stupendous and long-continued
volcanic outpourings from the fact that, based upon actual measurements
of thickness, lava sheets, averaging about 100 feet thick, poured out
one upon another until a pile about six miles high had accumulated.
In parts of the Grand Canyon of the Colorado tilted Proterozoic
strata may be seen resting upon the profoundly eroded surface of the
Archeozoic rocks of the inner gorge. The Proterozoic strata, 12,000
feet thick, consist of practically unaltered sandstones, shales, and
limestones, associated with some layers of basaltic lava. An erosion
surface (unconformity) separates the whole group into two distinct
series, and the group is separated from the overlying nearly horizontal
Paleozoic (Cambrian) strata in the walls of the Canyon by another
erosion surface.
More recently the Proterozoic strata so finely displayed in the
Rocky Mountains of Montana and southern Canada have been studied.
These strata, at least two or three miles thick, are mostly
unaltered sandstones, shales, and limestones, associated with some
metamorphic and igneous rocks. As usual, these strata rest upon
the eroded Archeozoic. They were more or less upturned and folded
before deposition of the succeeding Paleozoic strata. Satisfactory
subdivisions have not yet been worked out.
In North America most of the areas shown on the accompanying map
contain more or less Proterozoic rocks. Rocks of this age are known to
some extent in all continents where their general relationships seem
to be much like those of North America. They have perhaps been most
carefully studied in Scandinavia and the Highlands of Scotland, where
the strata portions are about two miles thick.
The climate of Proterozoic time must, for most part, have been about
like that of to-day except, of course, for its much greater uniformity
over the earth. About a dozen years ago very typical glacial deposits
were discovered within the early Proterozoic rocks of western Ontario,
Canada. A climatic condition favorable for the development of glaciers
so early in the history of the earth is, to say the least, directly
opposed to an idea (based upon the nebular hypothesis) long held that
the climate of early geologic time must have been much warmer than that
of the present.