Thus far in our study of the Archeozoic rocks attention has been mainly directed to southeastern Canada and the Adirondack mountains, where careful studies have been made. In all parts of the world where the most ancient known (Archeozoic) rocks have been studied in detail the same general principles apply. Particular attention has been given to the Archeozoic rocks south of Lake Superior, and in the Piedmont Plateau of the eastern United States. In the accompanying map Archeozoic rocks are widely exposed to view within the areas shown in black. It has been estimated that Archeozoic rocks appear at the surface over about one-fifth of the land area of the earth. Where they are not at the surface it is believed that they everywhere exist under cover of later rocks. In other words, Archeozoic rocks are considered to be almost universally present either at or under the earth's surface. This is true of the rocks of no other age. Special mention should be made of the fine exhibitions of Archeozoic rocks in Scandinavia and the Highlands of Scotland. [Illustration: Fig. 32.--Map showing the surface distribution of Archeozoic and Proterozoic rocks in North America. (Redrawn by the author after U. S. Geological Survey.)] All known evidence leads us to the remarkable conclusion that the climate of much, or possibly all, of Archeozoic time was not fundamentally different from that of to-day. There must have been weathering of rocks, rainfall, and streams much as at present as proved by the character and composition of the stratified rocks which formed in that remote era. The presence of graphite ("black lead") in crystalline flakes scattered through many of the strata shows that the climate must have been favorable to some form of life, because graphite thus occurring quite certainly represents the remains of organisms, this matter being more fully discussed in a succeeding chapter. In passing it may be stated that climatic zones were then probably scarcely if at all marked off, as they quite certainly were not even during Paleozoic time. One of the great contributions of geology to human knowledge is that during the tens of millions of years from Archeozoic times to the present the earth's climate has undergone no fundamental change or evolution. In the earlier ages there was greater uniformity of climate over the earth, and, during known geologic time there have been rather localized relatively minor fluctuations giving rise to glaciers, deserts, etc., but there has been no real evolution of climate at all comparable to the marvelous evolution of organisms--both animals and plants.