Page:Popular Science Monthly Volume 76.djvu/581

Rh of latitude. These temperate and subtropical conditions in high latitudes belong not only to ancient, but to the middle and modern ages of the world, and geologists long ago surrendered the notion that they could be due to supposed stores of the earth's primal heat.

There have been periods of notable dryness, so that deposits of rock-salt were formed through the evaporation of marine waters. From New York westward occur beds of salt, due to a dry climate, in a region where the rainfall is now abundant and where the basins of the Laurentian lakes are filled to their brims.

Not many years ago, the ice invasion of the Pleistocene was regarded as simple in character and unique in time. We now accept, among the commonplaces of glacial geology, that the late ice invasion was composite, marked by great advances and recessions, and by interglacial times of genial climate. And we recognize further, among the accepted facts of the science, the existence of vast glacial sheets in Permo-Carboniferous times, in India, Australia and South Africa, in regions which are now either warm or warm-temperate, and in lands of no great elevation.

Yet more remote, in the Cambrian, in an age of early life forms, an age recognized by the older geology as having almost ubiquitous warmth, the evidences of extensive glaciation have been brought to light. We must remember that humidity, precipitation, great or slight, and variations of temperature, are intimate questions of meteorology, whether we raise them in Cambrian or Miocene or present times. The meteorology of the passing age is related to geologic climates precisely as the physiography of existing lands is related to the rocks and rock structures of the past. The atmosphere has undergone a prolonged evolution in close association with the progress of the solid earth. As a part of the earth's history the study of the atmosphere is somewhat belated, but its importance is now recognized, and it will fill a large place in the geology of the future.

The present atmosphere therefore has not always existed and is but the latest term in an evolutionary series. We find two leading assumptions concerning this history.

There is a widely prevalent geological doctrine that our atmosphere is a residuum from a more dense or a more extensive body of gases. On this view it once contained all, or much, of the carbon dioxide whose carbon is now wrapt up in the coal and the limestones of the earth's crust. Thus Dana refers to the "purification of the air and waters through the making of limestone" as commencing in later Archean time and continuing through the Cambrian. Accepting, as he does, the idea that all the carbon of the coal and of many rocks was originally in the air and the waters, he still finds difficulty, for in early Paleozoic time life shows an atmosphere not too heavily charged