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.
ANCIENT EARTH HISTORY
(Paleozoic Era)
B
BEGINNING with the earliest Paleozoic, the legible records of events of earth history are far more abundant and less defaced than those of earlier times. Stratified rocks of the ordinary kinds greatly predominate over the igneous and metamorphic rocks, and the strata are in general far less disturbed than those of the Archeozoic and Proterozoic groups. From the earliest Paleozoic we have also the first abundant records (fossils) of the life of the earth, so that the ordinary methods of subdividing and determining the relative ages of the Paleozoic and later strata, as well as correlating the subdivisions (formations) in widely separated regions, can be used. From here on in our discussion of earth history we shall be able to trace the salient features of the changing outlines of the face of the earth, the coming and going of the seas over the lands, and the evolution of animals and plants with a considerable degree of definiteness and satisfaction.
First, we shall trace out, in the regular order of their occurrence, the main physical history events of Paleozoic time, leaving a consideration of the evolution of life for other chapters. Because of limitation of space, our attention will be almost wholly centered upon the continent of North America, but the reader should bear in mind that the general principles and facts set forth apply with about equal force to most other continents. In Europe the wonderful records of Paleozoic history are found in strata, whose estimated maximum thickness is about 100,000 feet! It must not be thought, however, that all these strata are piled up in a single locality, but the figure does actually represent the sum total of the greatest thickness of the many subdivisions (formations) of the Paleozoic rocks in different portions of the continent. In North America the maximum thickness of all Paleozoic rocks seems to be no less than 50,000 feet. More than 25,000 feet of strata may actually be observed piled layer upon layer in the highly folded and deeply eroded central Appalachian Mountains. The great thickness of the strata, combined with the facts that the fossils show that many marvelous, mostly progressive, changes took place among living things, that seas came and went repeatedly over many parts of the continent, and that great changes took place in the configuration of the land, force us to conclude that Paleozoic time must have lasted for many millions of years.
Just before the opening of the Paleozoic era practically all of North America appears to have been dry land, which had undergone so much erosion that it was low and far less rugged in relief than at present. This we know, because the rather widespread early Paleozoic (Cambrian) strata almost everywhere rest upon deeply eroded rocks of either Archeozoic or Proterozoic age. Considering both the time involved and the wide area affected, we have no record of anything like such a profound erosion interval since the beginning of the Paleozoic era. It seems that the constructive or upbuilding forces within the earth were then remarkably quiescent, while the destructive forces (erosion) were almost unhampered in their work of cutting down the land.