Page:Popular Science Monthly Volume 76.djvu/580

576 spirit in which the agricultural explorer hunts for wheat suited to arid fields or palms for the future orchards of Arizona.

The wind-blown sands are not only materials of accumulation, they are agents of erosion. Deserts abound in bare and unprotected rock surfaces, which occupy thousands of square miles in the Bad Lands, in the ridges of the Great Basin, in northern Africa and in western and central Asia. The impact of sharp-edged grains of quartz, maintained in every wind storm age after age, becomes no small means of wear and destruction, producing a natural sand blast whose principle is now used in many and ingenious ways in the arts. We have interest in Thoreau's quaint story of the clergyman of Cape Cod, frequently setting a fresh pane of glass to preserve the transparency of at least one fragment of window surface, but if we look more widely we find a large and significant phenomenon. Small lake basins have been excavated by the wind, and the sand of desert basins, eroding on its long journey, may come to rest at some remote point, as truly "exported" as if sent across the boundaries of a foreign land.

Before passing from these direct accomplishments of the atmosphere, we must include those peculiar deposits of fine and silty material known to the geologist and the physical geographer as loess. Much has been said of their origin, often in the field of debate, sometimes in the realm of controversy. But these great sheets of material, typically found in the Mississippi Valley, and in the central parts of Asia, have impressed many observers as being in whole or in part the work of winds blowing over vast fields of aridity, or sweeping widely the fine-grained outwash from areas of glaciation.

If we add now the transport of organisms, particularly of seeds, insects and birds, and the influence of winds on the migration of higher animals and man himself, through the medium of ocean currents, we shall see how the face of the organic world gathers its lineaments as broadly and depends on the atmosphere as intimately as the contours of a continent. The organic in turn reacts on the purely physical and we recognize at last that, touch the globe where we will in scientific inquiry we pick up some link in an endless chain.

The climates of the world have not always been what they are to-day. If we go as far back as the records will carry us, we find rocks and fossils that betray the climates of their time. These geologic climates are parts of ancient geographies which, in a long series, lead up to the geography of our own age. Throughout this succession, the atmosphere, its constituents, movements and temperatures held the same influence over the rocks of the crust and the forms of the land, which we now see. The atmosphere has had a history, and its qualities and activities have been among the chief factors in the evolution of the earth's surface.

Geology recognizes many periods of prevailing warmth, in which genial conditions were so wide-spread as almost to amount to a