576 THE POPULAR SCIENCE MONTHLY 



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 sig- 

 nificant 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 atmos- 

 phere, 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 disre- 



