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.