In parts of the Grand Canyon of the Colorado tilted Proterozoic strata may be seen resting upon the profoundly eroded surface of the Archeozoic rocks of the inner gorge. The Proterozoic strata, 12,000 feet thick, consist of practically unaltered sandstones, shales, and limestones, associated with some layers of basaltic lava. An erosion surface (unconformity) separates the whole group into two distinct series, and the group is separated from the overlying nearly horizontal Paleozoic (Cambrian) strata in the walls of the Canyon by another erosion surface. More recently the Proterozoic strata so finely displayed in the Rocky Mountains of Montana and southern Canada have been studied. These strata, at least two or three miles thick, are mostly unaltered sandstones, shales, and limestones, associated with some metamorphic and igneous rocks. As usual, these strata rest upon the eroded Archeozoic. They were more or less upturned and folded before deposition of the succeeding Paleozoic strata. Satisfactory subdivisions have not yet been worked out. In North America most of the areas shown on the accompanying map contain more or less Proterozoic rocks. Rocks of this age are known to some extent in all continents where their general relationships seem to be much like those of North America. They have perhaps been most carefully studied in Scandinavia and the Highlands of Scotland, where the strata portions are about two miles thick. 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. CHAPTER XIV ANCIENT EARTH HISTORY (_Paleozoic Era_)