the United States of America

South of latitude 35° 30', and extending from California to New Mexico, many of the intermontane basins are connected by through drainage or at least by continuous slopes on which water might flow to the sea if the rainfall were sufficient.

Columbia Plateau.

The surface of the Columbia plateau between the Northern Rocky mountains and the Cascades is made by flows of dark lava. These flows submerged an uneven surface of erosion, locally almost mountainous as may be seen in vertical section in the mile-deep canyon of Snake river between Idaho and Oregon. The Snake river basin east of the 115th meridian repre sents the simplest and most youthful phase of lava plains. Here the flat surface, 4,000 to 5,000 ft. high, remains as it was when the lava cooled, without erosion and only beginning to weather and to form soil. It is trenched only by the canyon of Snake river whose depth increases westward to 700 feet. Farther west, in south-western Idaho and south-eastern Oregon, there is more erosion. Near the State boundary the terraced valleys afford valuable irrigable land. The plateau in eastern Washington declines from nearly 3,00o ft. on the east to less than Soo ft. where Columbia river breaks through the Cascades. This north ern section (2oa) consists of older lavas, subjected to long erosion and having a rolling surface covered (at least in the east) with wind-blown soil. Here are the famous wheat fields of eastern Washington and western Idaho, cultivated by methods of "dry farming" by which a crop is raised in alternate years. In eastern Oregon near the middle of the province a group of mountains stood as an island in the floods of lava. Around these a later bulging raised the plateau surface to 7,000 ft., exposing it to greater erosion. This is the Blue Mountain section (2ob). The rise lay across the course of Snake river which succeeded in cut ting down as fast as the land rose, making a canyon 5,000 ft. deep.

Sierra-Cascade Mountains.

From Canada to southern Cali fornia a continuous barrier of mountains separates the Pacific border from the Intermontane plateaux. The Sierra Nevada south of the 4oth parallel presents to the Great Basin one of the highest and steepest mountain fronts on the continent. It is the escarp ment of a great fault, or compound fault, west of which an old mountain range, worn down almost to a lowland, rose again with a westward tilt. Streams descending its long western slope have

greatly increased its ruggedness by cutting gorges. These were later occupied by glaciers which deepened the valleys and steep ened their sides. Yosemite valley is only an extra fine example of the typical glacial trough with its uneven floor whose basins retained lakes until filled by gravel, and its over-steepened walls notched at the top by tributary valleys, the "hanging valleys" which end in falls. These mountains are mainly of granite but in the northern half is a belt of folded and metamorphosed sedi mentaries making the "Gold Belt" which has yielded a large part of California's output of gold.

North of Feather river the range changes completely. There is no evidence of uplift, only a piling up of volcanic materials from hundreds of vents. Lava flows, ash beds and cinder cones are of all ages from those that are fresh and shapely to others almost eroded away. Mt. Shasta with its beautiful cone (14,161 ft.), and Lassen's peak, the only volcano in the country witnessed in action, belong to this southern section of the Cascades (23c). A little north of the Oregon boundary there is evidence that crustal uplift is added to volcanic accumulation. Farther north the range owes its height more and more to uplift and less and less to the piling up of ejecta. The volcanic cover gives out entirely in central Washington, north of which the Northern Cas cade mountains (23a) consist of metamorphosed Paleozoic rocks and granite, intricately dissected into sharp peaks and ridges most of which, in a single view, rise to an almost uniform height. The level thus determined varies from 6,000 to 8,5oo ft., being gently arched. Above this nearly level horizon in both Oregon and Wash ington rise the isolated cones of extinct volcanoes 3,00o to 7,000 ft. higher. The best known are Mt. Rainier (14,526 ft.) in Wash ington and Mt. Hood (11,225 ft.) in Oregon. Crater lake in southern Oregon is the finest example of its class in America. It originated by the engulfment of the upper part of a volcano thus truncating the cone and leaving a pit 5 to 6 m. in diameter at the water-level, which is here 6,117 ft. high. Above it the walls of the caldera rise 500 to 2,200 ft. and below it the basin is 2,000 ft. deep.