We are living at a time when the earth has marked climatic differences, varying between icy polar climates and hot moist or dry tropical conditions. This has, however, not always been the case, for not long ago geologically the temperature of the entire earth was even colder than it is now, although most of the climates of the past have been warm and fairly equable the world over (see Fig. 3). The ancient plants and animals are "self-registering thermometers" with regard to the climates of the past, and they indicate that warm climates persisted during long geological ages, and that even though there were at all times zonal belts and fluctuations in the temperature, the polar areas were usually inhabited, as is now well known, by plants and animals of kinds that were adjusted to winterless environments. These temperature fluctuations were greatest during the closing and opening stages of the periods and eras.

The very long warm times were separated by short periods. of cool to cold climates. Geologists now know of seven periods of decided temperature changes (earliest and latest Proterozoic, Silurian, Permian, Triassic-Lias, CretaceousEocene, and Pleistocene), and of these at least four (those in italics) were glacial climates). Cooled climates occur when the lands are largest and most emergent, during the closing stages of periods and eras, and cold climates nearly always exist during or immediately following the times when the earth is undergoing most marked mountain making (see Fig. 3).

Origin of the earth's waters. There was a time when the earth was too small in mass to hold a hydrosphere, the envelope of water that lies beneath the atmosphere and above the rocky surface of the planet. As the earth grew in mass, it became more and more possible for it to have standing bodies of water and clouds of water-vapor floating with the winds of the atmosphere. All of this water, the newer geology

thinks, came forth out of the earth itself, through volcanic activity and thermal springs. Even though some of the surficial waters vanish through soaking into the cold earth-shell, their volume has increased throughout geologic time; the greatest amount was added during the later part of the long growing period of the earth and during the Archeozoic era, when from 25 to 50 per cent of the present volume is believed to have come into existence. The rest has been added during subsequent geologic time.

Source of the salts of the oceans. It is well known that the seas and oceans are salty, and that on the average there are 3.5 pounds of saline matter to every 100 pounds of marine water, and in each 100 pounds of sea salts there are nearly 78 pounds of sodium chloride or our table salt. Clarke has estimated that if all of the saline matter of the oceans were concentrated, and the volume placed on the United States, its surface would be completely covered to a depth of 1.6 miles. As all of this saline matter has been leached out of the rocks of the dry lands since the earth has had rains, and as very little of it, comparatively, has been taken out of the ocean by the accumulating rocks, it has been further estimated that it represents the breaking down of a mass of average igneous rock equal to at least 6,900 feet in thickness over all the continental platforms. Probably it is more correct to state that the continents have suffered erosion of igneous rocks amounting to between 1 and 2 miles of average depth. Of course all erosion throughout geologic time was far greater, perhaps, as Barrell states, from 50 to even 100 per cent higher. It included the reworking of older materials, igneous and sedimentary. Furthermore, "more than a half, perhaps fourfifths, of the erosion of igneous rocks was accomplished before the beginning of the Paleozoic" (Barrell).

Various computations have been made as to how long it has taken the salts in the oceans to accumulate and the best

FIG. 6.-Glacial bowlder beds beneath thick formations of dolomite on the upper Yangtse River, China, in latitude 31° N. Age of till, late Proterozoic. Photograph by Bailey Willis. Published in "Researches in China,' Publication No. 54, Vol. I, 1907, of the Carnegie Institution.

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