THE PRE-CAMBRIAN SYSTEMS
THE EARTH'S BEGINNINGS. The geological record does not tell us of
the beginnings of the earth. The history of the planet, as we have
every reason to believe, stretches far back beyond the period of
the oldest stratified rocks, and is involved in the history of the
solar system and of the nebula,--the cloud of glowing gases or of
cosmic dust,--from which the sun and planets are believed to have
been derived.
THE NEBULAR HYPOTHESIS. It is possible that the earth began as a
vaporous, shining sphere, formed by the gathering together of the
material of a gaseous ring which had been detached from a cooling
and shrinking nebula. Such a vaporous sphere would condense to a
liquid, fiery globe, whose surface would become cold and solid,
while the interior would long remain intensely hot because of the
slow conductivity of the crust. Under these conditions the
primeval atmosphere of the earth must have contained in vapor the
water now belonging to the earth's crust and surface. It held also
all the oxygen since locked up in rocks by their oxidation, and
all the carbon dioxide which has since been laid away in
limestones, besides that corresponding to the carbon of
carbonaceous deposits, such as peat, coal, and petroleum. On this
hypothesis the original atmosphere was dense, dark, and noxious,
and enormously heavier than the atmosphere at present.
THE ACCRETION HYPOTHESIS. On the other hand, it has been recently
suggested that the earth may have grown to its present size by the
gradual accretion of meteoritic masses. Such cold, stony bodies
might have come together at so slow a rate that the heat caused by
their impact would not raise sensibly the temperature of the
growing planet. Thus the surface of the earth may never have been
hot and luminous; but as the loose aggregation of stony masses
grew larger and was more and more compressed by its own
gravitation, the heat thus generated raised the interior to high
temperatures, while from time to time molten rock was intruded
among the loose, cold meteoritic masses of the crust and outpoured
upon the surface.
It is supposed that the meteorites of which the earth was built
brought to it, as meteorites do now, various gases shut up within
their pores. As the heat of the interior increased, these gases
transpired to the surface and formed the primitive atmosphere and
hydrosphere. The atmosphere has therefore grown slowly from the
smallest beginnings. Gases emitted from the interior in volcanic
eruptions and in other ways have ever added to it, and are adding
to it now. On the other hand, the atmosphere has constantly
suffered loss, as it has been robbed of oxygen by the oxidation of
rocks in weathering, and of carbon dioxide in the making of
limestones and carbonaceous deposits.
While all hypotheses of the earth's beginnings are as yet unproved
speculations, they serve to bring to mind one of the chief lessons
which geology has to teach,--that the duration of the earth in
time, like the extension of the universe in space, is vastly
beyond the power of the human mind to realize. Behind the history
recorded in the rocks, which stretches back for many million
years, lies the long unrecorded history of the beginnings of the
planet; and still farther in the abysses of the past are dimly
seen the cycles of the evolution of the solar system and of the
nebula which gave it birth.
We pass now from the dim realm of speculation to the earliest era
of the recorded history of the earth, where some certain facts may
be observed and some sure inferences from them may be drawn.
THE ARCHEAN.
The oldest known sedimentary strata, wherever they are exposed by
uplift and erosion, are found to be involved with a mass of
crystalline rocks which possesses the same characteristics in all
parts of the world. It consists of foliated rocks, gneisses, and
schists of various kinds, which have been cut with dikes and other
intrusions of molten rock, and have been broken, crumpled, and
crushed, and left in interlocking masses so confused that their
true arrangement can usually be made out only with the greatest
difficulty if at all. The condition of this body of crystalline
rocks is due to the fact that they have suffered not only from the
faultings, foldings, and igneous intrusions of their time, but
necessarily, also, from those of all later geological ages.
At present three leading theories are held as to the origin of
these basal crystalline rocks.
1. They are considered by perhaps the majority of the geologists
who have studied them most carefully to be igneous rocks intruded
in a molten state among the sedimentary rocks involved with them.
In many localities this relation is proved by the phenomena of
contact; but for the most part the deformations which the rocks
have since suffered again and again have been sufficient to
destroy such evidence if it ever existed.
2. An older view regards them as profoundly altered sedimentary
strata, the most ancient of the earth.
3. According to a third theory they represent portions of the
earth's original crust; not, indeed, its original surface, but
deeper portions uncovered by erosion and afterwards mantled with
sedimentary deposits. All these theories agree that the present
foliated condition of these rocks is due to the intense
metamorphism which they have suffered.
It is to this body of crystalline rocks and the stratified rocks
involved with it, which form a very small proportion of its mass,
that the term ARCHEAN (Greek, ARCHE, beginning) is applied by many
geologists.
THE ALGONKIAN
In some regions there rests unconformably on the Archean an
immense body of stratified rocks, thousands and in places even
scores of thousands of feet thick, known as the ALGONKIAN. Great
unconformities divide it into well-defined systems, but as only
the scantiest traces of fossils appear here and there among its
strata, it is as yet impossible to correlate the formations of
different regions and to give them names of more than local
application. We will describe the Algonkian rocks of two typical
areas.
THE GRAND CANYON OF THE COLORADO. We have already studied a very
ancient peneplain whose edge is exposed to view deep on the walls
of the Colorado Canyon. The formation of flat-lying sandstone
which covers this buried land surface is proved by its fossils to
belong to the Cambrian,--the earliest period of the Paleozoic era.
The tilted rocks on whose upturned edges the Cambrian sandstone
rests are far older, for the physical break which separates them
from it records a time interval during which they were upheaved to
mountainous ridges and worn down to a low plain. They are
therefore classified as Algonkian. They comprise two immense
series. The upper is more than five thousand feet thick and
consists of shales and sandstones with some limestones. Separated
from it by an unconformity which does not appear in Figure 207,
the lower division, seven thousand feet thick, consists chiefly of
massive reddish sandstones with seven or more sheets of lava
interbedded. The lowest member is a basal conglomerate composed of
pebbles derived from the erosion of the dark crumpled schists
beneath,--schists which are supposed to be Archean. As shown in
Figure 207, a strong unconformity parts the schists and the
Algonkian. The floor on which the Algonkian rests is remarkably
even, and here again is proved an interval of incalculable length,
during which an ancient land mass of Archean rocks was baseleveled
before it received the cover of the sediments of the later age.
THE LAKE SUPERIOR REGION. In eastern Canada an area of pre-
Cambrian rocks, Archean and Algonkian, estimated at two million
square miles, stretches from the Great Lakes and the St. Lawrence
River northward to the confines of the continent, inclosing Hudson
Bay in the arms of a gigantic U. This immense area, which we have
already studied as the Laurentian peneplain, extends southward
across the Canadian border into northern Minnesota, Wisconsin, and
Michigan. The rocks of this area are known to be pre-Cambrian; for
the Cambrian strata, wherever found, lie unconformably upon them.
The general relations of the formations of that portion of the
area which lies about Lake Superior are shown in Figure 262. Great
unconformities, UU' separate the Algonkian both from the Archean
and from the Cambrian, and divide it into three distinct systems,
--the LOWER HURONIAN, the UPPER HURONIAN, and the KEWEENAWAN. The
Lower and the Upper Huronian consist in the main of old sea muds
and sands and limy oozes now changed to gneisses, schists,
marbles, quartzites, slates, and other metamorphic rocks. The
Keweenawan is composed of immense piles of lava, such as those of
Iceland, overlain by bedded sandstones. What remains of these rock
systems after the denudation of all later geologic ages is
enormous. The Lower Huronian is more than a mile thick, the Upper
Huronian more than two miles thick, while the Keweenawan exceeds
nine miles in thickness. The vast length of Algonkian time is
shown by the thickness of its marine deposits and by the cycles of
erosion which it includes. In Figure 262 the student may read an
outline of the history of the Lake Superior region, the
deformations which it suffered, their relative severity, the times
when they occurred, and the erosion cycles marked by the
successive unconformities.
OTHER PRE-CAMBRIAN AREAS IN NORTH AMERICA. Pre-Cambrian rocks are
exposed in various parts of the continent, usually by the erosion
of mountain ranges in which their strata were infolded. Large
areas occur in the maritime provinces of Canada. The core of the
Green Mountains of Vermont is pre-Cambrian, and rocks of these
systems occur in scattered patches in western Massachusetts. Here
belong also the oldest rocks of the Highlands of the Hudson and of
New Jersey. The Adirondack region, an outlier of the Laurentian
region, exposes pre-Cambrian rocks, which have been metamorphosed
and tilted by the intrusion of a great boss of igneous rock out of
which the central peaks are carved. The core of the Blue Ridge and
probably much of the Piedmont Belt are of this age. In the Black
Hills the irruption of an immense mass of granite has caused or
accompanied the upheaval of pre-Cambrian strata and metamorphosed
them by heat and pressure into gneisses, schists, quartzites, and
slates. In most of these mountainous regions the lowest strata are
profoundly changed by metamorphism, and they can be assigned to
the pre-Cambrian only where they are clearly overlain
unconformably by formations proved to be Cambrian by their
fossils. In the Belt Mountains of Montana, however, the Cambrian
is underlain by Algonkian sediments twelve thousand feet thick,
and but little altered.
MINERAL WEALTH OF THE PRE-CAMBRIAN ROCKS. The pre-Cambrian rocks
are of very great economic importance, because of their extensive
metamorphism and the enormous masses of igneous rock which they
involve. In many parts of the country they are the source of
supply of granite, gneiss, marble, slate, and other such building
materials. Still more valuable are the stores of iron and copper
and other metals which they contain.
At the present time the pre-Cambrian region about Lake Superior
leads the world in the production of iron ore, its output for 1903
being more than five sevenths of the entire output of the whole
United States, and exceeding that of any foreign country. The ore
bodies consist chiefly of the red oxide of iron (hematite) and
occur in troughs of the strata, underlain by some impervious rock.
A theory held by many refers the ultimate source of the iron to
the igneous rocks of the Archean. When these rocks were upheaved
and subjected to weathering, their iron compounds were decomposed.
Their iron was leached out and carried away to be laid in the
Algonkian water bodies in beds of iron carbonate and other iron
compounds. During the later ages, after the Algonkian strata had
been uplifted to form part of the continent, a second
concentration has taken place. Descending underground waters
charged with oxygen have decomposed the iron carbonate and
deposited the iron, in the form of iron oxide, in troughs of the
strata where their downward progress was arrested by impervious
floors.
The pre-Cambrian rocks of the eastern United States also are rich
in iron. In certain districts, as in the Highlands of New Jersey,
the black oxide of iron (magnetite) is so abundant in beds and
disseminated grains that the ordinary surveyor's compass is
useless.
The pre-Cambrian copper mines of the Lake Superior region are
among the richest on the globe. In the igneous rocks copper, next
to iron, is the most common of all the useful metals, and it was
especially abundant in the Keweenawan lavas. After the Keweenawan
was uplifted to form land, percolating waters leached out much of
the copper diffused in the lava sheets and deposited it within
steam blebs as amygdules of native copper, in cracks and fissures,
and especially as a cement, or matrix, in the interbedded gravels
which formed the chief aquifers of the region. The famous Calumet
and Hecla mine follows down the dip of the strata to the depth of
nearly a mile and works such an ancient conglomerate whose matrix
is pure copper.
THE APPEARANCE OF LIFE. Sometime during the dim ages preceding the
Cambrian, whether in the Archean or in the Algonkian we know not,
occurred one of the most important events in the history of the
earth. Life appeared for the first time upon the planet. Geology
has no evidence whatever to offer as to whence or how life came.
All analogies lead us to believe that its appearance must have
been sudden. Its earliest forms are unknown, but analogy suggests
that as every living creature has developed from a single cell, so
the earliest organisms upon the globe--the germs from which all
later life is supposed to have been evolved--were tiny,
unicellular masses of protoplasm, resembling the amoeba of to-day
in the simplicity of their structure.
Such lowly forms were destitute of any hard parts and could leave
no evidence of their existence in the record of the rocks. And of
their supposed descendants we find so few traces in the pre-
Cambrian strata that the first steps in organic evolution must be
supplied from such analogies in embryology as the following. The
fertilized ovum, the cell with which each animal begins its life,
grows and multiplies by cell division, and develops into a hollow
globe of cells called the BLASTOSPHERE. This stage is succeeded by
the stage of the GASTRULA,--an ovoid or cup-shaped body with a
double wall of cells inclosing a body cavity, and with an opening,
the primitive mouth. Each of these early embryological stages is
represented by living animals,--the undivided cell by the
PROTOZOA, the blastosphere by some rare forms, and the gastrula in
the essential structure of the COELENTERATES,--the subkingdom to
which the fresh-water hydra and the corals belong. All forms of
animal life, from the coelenterates to the mammals, follow the
same path in their embryological development as far as the
gastrula stage, but here their paths widely diverge, those of each
subkingdom going their own separate ways.
We may infer, therefore, that during the pre-Cambrian periods
organic evolution followed the lines thus dimly traced. The
earliest one-celled protozoa were probably succeeded by many-
celled animals of the type of the blastosphere, and these by
gastrula-like organisms. From the gastrula type the higher sub-
divisions of animal life probably diverged, as separate branches
from a common trunk. Much or all of this vast differentiation was
accomplished before the opening of the next era; for all the
subkingdoms are represented in the Cambrian except the
vertebrates.
EVIDENCES OF PRE-CAMBRIAN LIFE. An indirect evidence of life
during the pre-Cambrian periods is found in the abundant and
varied fauna of the next period; for, if the theory of evolution
is correct, the differentiation of the Cambrian fauna was a long
process which might well have required for its accomplishment a
large part of pre-Cambrian time.
Other indirect evidences are the pre-Cambrian limestones, iron
ores, and graphite deposits, since such minerals and rocks have
been formed in later times by the help of organisms. If the
carbonate of lime of the Algonkian limestones and marbles was
extracted from sea water by organisms, as is done at present by
corals, mollusks, and other humble animals and plants, the life of
those ancient seas must have been abundant. Graphite, a soft black
mineral composed of carbon and used in the manufacture of lead
pencils and as a lubricant, occurs widely in the metamorphic pre-
Cambrian rocks. It is known to be produced in some cases by the
metamorphism of coal, which itself is formed of decomposed vegetal
tissues. Seams of graphite may therefore represent accumulations
of vegetal matter such as seaweed. But limestone, iron ores, and
graphite can be produced by chemical processes, and their presence
in the pre-Cambrian makes it only probable, and not certain, that
life existed at that time.
PRE-CAMBRIAN FOSSILS. Very rarely has any clear trace of an
organism been found in the most ancient chapters of the geological
record, so many of their leaves have been destroyed and so far
have their pages been defaced. Omitting structures whose organic
nature has been questioned, there are left to mention a tiny
seashell of one of the most lowly types,--a DISCINA from the pre-
Cambrian rocks of the Colorado Canyon,--and from the pre-Cambrian
rocks of Montana trails of annelid worms and casts of their
burrows in ancient beaches, and fragments of the tests of
crustaceans. These diverse forms indicate that before the
Algonkian had closed, life was abundant and had widely
differentiated. We may expect that other forms will be discovered
as the rocks are closely searched.
PRE-CAMBRIAN GEOGRAPHY. Our knowledge is far too meager to warrant
an attempt to draw the varying outlines of sea and land during the
Archean and Algonkian eras. Pre-Cambrian time probably was longer
than all later geological time down to the present, as we may
infer from the vast thicknesses of its rocks and the
unconformities which part them. We know that during its long
periods land masses again and again rose from the sea, were worn
low, and were submerged and covered with the waste of other lands.
But the formations of separated regions cannot be correlated
because of the absence of fossils, and nothing more can be made
out than the detached chapters of local histories, such as the
outline given of the district about Lake Superior.
The pre-Cambrian rocks show no evidence of any forces then at work
upon the earth except the forces which are at work upon it now.
The most ancient sediments known are so like the sediments now
being laid that we may infer that they were formed under
conditions essentially similar to those of the present time. There
is no proof that the sands of the pre-Cambrian sandstones were
swept by any more powerful waves and currents than are offshore
sands to-day, or that the muds of the pre-Cambrian shales settled
to the sea floor in less quiet water than such muds settle in at
present. The pre-Cambrian lands were, no doubt, worn by wind and
weather, beaten by rain, and furrowed by streams as now, and, as
now, they fronted the ocean with beaches on which waves dashed and
along which tidal currents ran.
Perhaps the chief difference between the pre-Cambrian and the
present was the absence of life upon the land. So far as we have
any knowledge, no forests covered the mountain sides, no verdure
carpeted the plains, and no animals lived on the ground or in the
air. It is permitted to think of the most ancient lands as deserts
of barren rock and rock waste swept by rains and trenched by
powerful streams. We may therefore suppose that the processes of
their destruction went on more rapidly than at present.