UNITED STATES OF AMERICA 





UNITED STATES OF AMERICA 



ha> been formed by decay of the underlying 

 rooks or has been deposited by running water; 

 but north of the Ohio and Missouri most of it 

 is of glacial origin, having been deposited in a 

 previous geological age by a great ice sheet like 

 that which now covers the interior of Green- 

 land. In this portion of the interior plain, 

 there are certain districts too rough and rocky, 

 and others too sandy or swampy, for profitable 

 farming. Along the lower Mississippi there arc 

 likewise considerable areas of marsh lands. 

 Near the Rocky Mountains, where the plain 

 gradually rises as it approaches the foothills, 

 it becomes more rolling, because the rivers have 

 cut deeper valleys. In general, however, the 

 larger part of the interior plain is adapted to 

 agriculture, and this consequently is by far the 

 leading agricultural section of the country. 



The C ordillcran, or Western, highland is 

 sometimes said to include the entire region 

 from the Rocky Mountains to the Pacific 

 Ocean. As here defined, however, it extends 

 only to the Sierra Nevada-Cascade range, in- 

 cluding three distinct subdivisions. The Rocky 

 Mountains, forming the continental water part- 

 ing between the Atlantic and the Pacific oceans, 

 present a practically continuous front toward 

 the east, though a tongue of the plains in 

 Wyoming does penetrate some distance into 

 the mountains, forming the route of the Union 

 Pacific railroad. Between the Rockies and the 

 Sierra Nevada-Cascade range lies a lofty pla- 

 teau traversed by numerous lesser ranges. This 

 plateau is drained in part by the Columbia and 

 the Colorado rivers, but in part it forms the 

 Great Basin in Utah and Nevada, which has no 

 outlet to the sea. Finally, the Sierra Nevada- 

 Cascade range, bounding this plateau on the 

 west, is cut through by several rivers, notably 

 the Columbia. Taken as a whole, the Cordil- 

 leran highland is broken by no waterway, and 

 its lowest pass is about a mile above sea 

 level. It thus obstructs commerce far more 

 than do the Appalachians. 



The Pacific slope includes two subdivisions, 

 the Pacific valleys and the Coast ranges. The 

 Pacific valleys comprise the broad and fertile 

 San Joaquin-Sacramento in California, the Wil- 

 lamette in Oregon, and the valley surrounding 

 the Puget Sound. Beyond these valleys lie the 

 Coast ranges, which descend so abruptly to the 

 Pacific as to leave no room for a coastal plain 

 at their base. Only at the extreme southern 

 end of California, where the mountains trend 

 east and west, do they recede from the coast 

 so as to leave a coastal plain. 



Climate. In North and South America the 

 mountain ranges run nearly north and south, 

 while in the Old World they run, in general, 

 east and west. North and south ranges act as 

 moisture barriers, because they lie directly 

 across the course of the prevailing winds, 

 whether in the tropics or in the temperate 

 zones. On the other hand, east and west moun- 

 tain ranges act as temperature barriers, causing 

 quite different types of climate on their north- 

 ern and southern slopes. This is illustrated in 

 t he case of the Alps. 



Owing to the north and south direction of the 

 mountain ranges, North America, though di- 

 vided by the accident of history betv. 

 Canada and the United States, is by nature a 

 climatic as well as a topographic unit showing 

 no abrupt change at any point between the 

 Gulf of Mexico and the Arctic Ocean. On the 

 other hand, the climatic changes are more 

 abrupt from east to west. The Atlantic slope, 

 especially in New England, being exposed to 

 winds from both the land and seas, has .a 

 variable climate. Indeed, Mark Twain always 

 insisted that New England has no climate at 

 all, but merely samples of weather. The great 

 interior plain is subject to extremes of heat and 

 cold, like Russia and Siberia; while the Pacific 

 coast, where western winds prevail, has a mild, 

 oceanic climate like Western Europe. 



The rainfall likewise varies from east to west, 

 being heavy on the Atlantic and Gulf coast-, 

 lighter in the interior, which is farther from the 

 source of moisture, and heavier again on the 

 Pacific slope. From about the 100th meridian 

 (running through the center of the Dakotas) 

 westward to the Sierra Nevada range the rain- 

 fall is usually insufficient for the type of agri- 

 culture found in the eastern part of the coun- 

 try. The north and south trend of the moun- 

 tains, which tends to equalize the temperature 

 of wide areas, thus at the same time checks the 

 moisture-laden west winds and renders nearly 

 half of the continent arid or semiarid in char- 

 acter. 



The United States lies entirely outside the 

 tropics, although the southern tips of Florida 

 and of Texas lack but little of having a tropical 

 'climate. In the United States, as in other 

 temperate regions, the distribution of moisture 

 is influenced by the passage of cyclonic storms 

 (see CYCLONE). It is a familiar fact that air 

 has weight, but this weight is not entirely con- 

 stant, owing to unequal heating by the sun and 

 to other causes. Wherever the air is lighter 

 than the average it tends to rise, while the 



