Eastward from the crest, the plateau slopes gently in a long even incline down to the Mississippi Valley, forming what is known as the plains.

On the westward the general features are more complex. The great Colorado River of the west heads near the crest line of this general plateau, and, pursuing a course generally somewhat west of south, enters the Gulf of California at the southern boundary of the States.

a

This great river, with its long tributaries from either side, indicates great general depression in the surface, with a corresponding rise on the west. This rise, which may be considered as a spur from the main crest, joins it near the head of the Colorado River. The Wahsatch range of Utah stands upon this summit line.

West of the Wahsatch range, extending thence to the Sierra Nevada of California, is a great area of

ON THE COLORADO RIVER.

country known as the Great Basin, the waters of which have no communication with either ocean, and consequently no outlet save evaporation. This area has an irregularly elliptic form, extending northward nearly to the Columbia. River in Northern Oregon, and southward into Southwestern California. Of the details of this curious region more will be said farther on. Near the middle of this Basin there is a well-marked rise of the general level, on a meridional line, while east and west of it there is a depression, the eastern being at the base of the Wahsatch range, and the western at the eastern foot of the Sierra Nevada.

In the northern section, the plateau westward from the crest appears to have a more uniform slope downward.

In British America the eastern line of the plateau trends rapidly off to the west, narrowing the plateau, and at the same time it decreases in elevation.

The ranges which crown this plateau are many and complex, ranging widely in height, in orographic forms, and in mode of formation. One great generalisation, however, includes nearly all of them. Their trends, with very few exceptions, whether of main or secondary chains, are nearly north and south, ranging but a few degrees on either side. The only known exceptions to this general law are the Uintah range of Wyoming and Utah, and the Sweetwater range of Wyoming, which trend very nearly east and west.

We find the main streams of this region conforming to the general slopes, regardless of such minor obstacles as mountain ranges, which may have arisen to dispute their path. The branches of the Missouri, Arkansas, and Red Rivers flow generally east, down the long inclined plane of the Great Plains. While the Colorado occupies and flows nearly south, its branches flow nearly at right angles to it and to the

the bed of a depression,

ranges, many of which are encountered and cut through without deviating from their direct courses. Another well-marked example is the Humboldt River, in the western portion of the Great Basin, which pursues a course somewhat south of west, cutting across a number of ranges which stand across its path. This apparently inexplicable conduct of the larger streams, in cutting their way across seemingly impassable obstacles, when easier paths are in many places offered to them, is simply explained when one reflects that the rivers had the prior right by possession, and that streams are extremely tenacious of their courses,-that, in short, streams live longer than mountains. The streams were there first, and as the mountains were slowly evolved from the plateau, the streams, like huge saws, cleft their way through the rising mass, preserving thus not only their general courses, but, in many cases, even the details of their meanderings. Of this class of orographic phenomena the Cordilleran region affords the physical geographer very many fine examples. Very rarely has a stream been driven from its original course except by violent catastrophic action.

2. Climate of the Cordilleran Plateau.

To obtain a good general idea of the climate of this region, it must be considered as a whole, disregarding, for the moment, the local effects of the many mountain ranges which diversify its surface.

Starting at the 100th meridian, in the United States, and ascending the gradual slope of the plains, we shall find the climate gradually growing more and more arid, and the rainfall correspondingly less. Still going westward, we see this aridity continually increasing, and its attendant phenomena correspondingly being intensified

until it reaches a maximum at the eastern base of the Sierra Nevada and Cascade Range of the Pacific States. This aridity is necessarily modified by the temperature, whether differences of the latter be caused by latitude or altitude; hence we find that points in the same longitude, and having the same elevation, but differing in latitude, have different degrees of aridity of atmosphere and soil, due mainly to the difference in amount of evaporation. The mean annual temperature of this region is abnormally low, but by no means as low as the elevation would presuppose. There is a great range of temperature between day and night, the maximum thermometer often indicating 80° to 90° Fah. in the shade, while twelve hours later ice may be forming. Still, owing to the dryness of the atmosphere, there is little or no dew deposited at night. Decomposition of animal or vegetable remains is almost unknown. Dead carcases dry up. Meat may be preserved for an indefinite time, without the use of antiseptics, simply by drying in the sun ("jerking"). A large part of the snows of winter evaporate directly (apparently without going through the process of melting).

The rainfall, while light, and constantly decreasing westward, changes in character, from the long storms of the Mississippi Valley and the Atlantic slope, to that of sudden explosive showers, of short duration and great violence. These characteristics of the climate have modified the surface, through erosion, in a peculiar and marked manner, of which more hereafter.

These are the general characteristics of the climate. In detail it is modified by the local topography to a marked extent. Every valley differs from its neighbour

in some respects.

The two sides of a valley may differ from one another distinctly in rainfall.

The causes of these peculiarities of climate are simple.

and are similar to those in many other parts of the surface of the earth. The prevailing air currents over this region are from the west, primarily from the Pacific Ocean. They come laden with moisture, and meet, not far from the Pacific Coast, that great continental barrier, the Sierra Nevada, and the Cascade Range. Forced upwards by this, they reach higher and colder regions, where they are chilled, and forced to disgorge their stores. of moisture on the western slopes of these mountains. The Coast Ranges, which, within the United States, follow the Pacific Coast closely, lying west of the above-mentioned ranges, though much lower, also play no inconsiderable part in inducing precipitation, especially in the cold season. The result is that the western coast of the continent, especially in the winter, is extremely—nay, proverbially-well-watered; while the country to the eastward, the Cordilleran Plateau, suffers in consequence. The air currents, thus deprived of their moisture, blow as dry winds over the arid regions. Whatever moisture they may accumulate thereafter is taken from them in passing the numerous ranges which they successively encounter, producing on the mountains a more or less moist climate, especially on the western side of the crests; while the valleys, even at a distance of but a very few miles, may be utter deserts, where rain seldom or never falls.

3. Natural Vegetation of the Cordilleran Plateau.

The natural vegetation of this region is that peculiar to one placed under such climatic conditions. The plains are treeless, except along the few water-courses. For the most part they are covered with buffalo grass—a short, tufted, very nutritive grass, on which have pastured the vast herds of buffalo which formerly ranged over this