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 was long held 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 also held 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.
Such a spiral nebula might be formed by the close approach of one star
to another,--of a passing star to our own sun, for example, before the
birth of the solar system. As the pull of the moon raises the tides on
opposite sides of the earth, so, it is supposed, the pull of the
passing star released the explosive forces of the sun, and two streams
of matter were flung out from it. The knots in the arms formed the
nuclei of the planets. The gaseous matter scattered outside the knots
cooled into small stony masses, revolving about a central mass and
hence called planetesimals (little planets). Like the meteorites which
still fall upon the earth, the planetesimals were gradually gathered
in by the nuclear knots, which thus grew to the present planets.
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 (p. 268);
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 (_nu´_, Fig. 207). 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 (_b_, Fig. 207). 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
(_nm´_, Fig. 207) 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 (p. 89), 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.
[Illustration: Fig. 262. Ideal Section in the Lake Superior
Region]
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.
[Illustration: Fig. 263. Successive Stages in the Development
of the Ovum to the Gastrula Stage]
=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 subdivisions 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.