Page:Popular Science Monthly Volume 76.djvu/576

572 and organic modifications of the "drift." The drainage has been greatly interrupted, and new channels, waterfalls and lakes abound. Several types of hills of accumulation appear that we can not find in the southern mountains. The tops of hills and mountains of hard rock have a different aspect—it is the product of the glacial cycle which we are contemplating, and the geographer would only need to cast his eye upon maps of the two regions to recognize their character.

Let us go to the Great Basin. Rather is it a group of basins, flanked by the Wasatch on the east and the Sierras on the west. Its broad and arid expanse is intersected by many north and south mountain ridges, which are the up-thrown edges of great crustal blocks. The upper parts of these mountains show normal erosion. The bordering Sierras show normal and glacial types. Within the basin the summits and slopes of the mountain are shedding down their waste and streams of greater or less vigor are carving a normal topography. At the foot of the mountains, the waste, instead of going in the grasp of streams to the sea, is spread out on the inter-montane floors, raising the surface and building plains of sand, clay, salt and gypsum. The rivers become small instead of growing, and lose themselves in the atmosphere, the soil and in shallow lakes which may be permanent and salt, or intermittent and brackish. The soil is formed mechanically and with but a minor amount of those chemical changes which take place in a more humid climate. Vegetation is scant and this condition retards true soil-making and, by lack of cover, aids the action of winds. Monotony characterizes the arid type, as ceaseless variety belongs to the normal and glacial operations. The ridges that separate adjacent arid basins may be cut away, and both be merged in a single featureless plain, and thus we have a growing lake of waste, somewhat akin to closed basins of water. The only means by which such a basin can lose its material is through the winds. Central Persia offers another land of the same order, where severe conditions of aridity have for an unknown period reflected themselves in the life of man and in the very forms of the land which he makes his home.

Hill and mountain, valley and plain, thus own their dependence, in no remote way, upon the gaseous envelope. As it is wet or dry, hot or cold, so evolves the very physiognomy of our globe, and whether we cross the border of an adjacent township or journey to a far country, its handiwork is before our eyes.

No forms are more characteristic or interesting than those which the ocean makes on its borders. These forms in a minor way are due to the tides, in the major way to waves, and thus, at one remove only, are produced by the atmosphere. Likewise ocean currents, involving transfers of heat, moisture, plants and animals, are believed to be chiefly due to atmospheric movements. If we compare New York with Rome and Constantinople, or Labrador and Hudson Bay with Great