The rocks of Pre-Cambrian time have been buried deeply under the
accumulation of younger sediments, and the resulting pressure in many
places has been tremendous. In addition to the effects of pressure there
also is recorded in these ancient formations the repeated movements of
the materials since they were first deposited. Vertical and side
adjustments of parts, with relation to other parts, have distorted the
original arrangement of the rock particles to such an extent that
ordinary fossils would eventually become unrecognizable. These crushing,
grinding, and kneading forces working through millions of years alone
would account for the absence of fossils from the older deposits.
Frequently the rocks have become so changed in form that their original
character can only be conjectured, and because of this change they are
known as metamorphic rocks.
A few beds of Archeozoic age remain in nearly their original condition,
but they are either without fossils or they have produced very
questionable and unsatisfactory specimens. The existence of life during
these early stages of earth history is indicated largely by chemical
rather than fossil evidence. Much of the ancient limestone has been
converted into marble, but it is not unreasonable to believe that plants
and animals were instrumental in the production of this type of rock as
they are today. Certain varieties of iron ore deposits are now being
built up by the aid of plants, and similar ores in the ancient rocks may
have had a like origin. The presence of great quantities of carbon, in
the form of graphite, may be regarded also as a sign of life, for this
substance is accumulated on a large scale by living plants, and may be
retained in a solid form after the partial decay of the plant tissues.
So far as the direct evidence goes, there is no sign of any creature of
large size or of such complicated structure as the common plants and
animals of today. The chemistry of the mineral deposits is not entirely
convincing as to the presence of life, but it is regarded as highly
probable that microscopic, single-celled plants and animals, comparable
to modern algae and protozoa, were in existence during Archean time.
Throughout later eras there is unmistakable evidence of gradual
development from simpler to more elaborate life-forms and the Archeozoic
is commonly regarded as a time of preparation during which simple
organisms of some kind were becoming adapted to early conditions which
could not support life on a higher plane. The importance of the work
done by such lowly creatures in the preparation of suitable environments
for more advanced modes of living is overlooked almost entirely.
During the next era, the Proterozoic, the record of life becomes
somewhat clearer. Fossils are hardly to be regarded as abundant but
there were several well-defined types of animals which left shells and
other parts composed of mineral matter. Among these may be mentioned the
Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced
delicate, often lace-like shells of many patterns adorned with the
radiating filaments or spines which have suggested the name for this
group. Foraminifera produced minute shells, sometimes many chambered,
and often bearing a confusing resemblance to the work of snails. Common
chalk is composed almost entirely of such shells and fragments of them.
Sponges and Bryozoa are animals of slightly higher organization. They
are many-celled instead of one-celled and the cells have special work to
perform, which is a most important step in the direction of the
specialization which characterizes the structural and life pattern of
later arrivals. The Bryozoa lived in moss-like colonies which have been
important rock-makers; the fossil forms bear some resemblance to corals.
Sponges are too well known to require description although the familiar
article of commerce is merely the framework of once-living animals. They
represent the earliest organization of true animal bodies even though in
appearance they may have a resemblance to plants.
Actual plants of this era were of the algae class, aquatic in habit as
were their animal neighbors, the first to leave a record in the form of
fossils. This record, obscure and distorted, has long been a source of
perplexity to investigators. Without well-defined floras and faunas to
guide them, and with rocks frequently in chaotic relationships, early
geologists were content to regard it all as a “Pre-Cambrian complex.”
Recent studies have contributed a great deal of information not
available some years ago. It is quite possible that more advanced types
of life were in abundance before the close of the second era, but
material on which to base sound opinion is still scarce.
Rocks of Pre-Cambrian age are plentiful in the foothills region west of
Denver. The schists, gneisses, and quartzites exposed for some miles
immediately beyond the red-beds are part of this great complex. The
Idaho Springs formation is known to be one of the oldest in this
district, although its exact age has not been determined. Other
formations are recognized among the metamorphic rocks of the region but
none has contributed to our knowledge of early life.