. The continent we live on. Physical geography; Natural history. This province is so dearly defined in most respects that it is widely recognized as forming a unit within the otherwise complex terrain of the West and has acquired a popular name— the Great Basin. It is not strictly a basin, either geologically or in structure, for it does not slope inward from its subcircular periphery but is nearly level, though its surface is covered with ranks of mountain chains. It is almost entirely ringed by steep, high mountain walls. It can be divided into three not too well defined subregions, and has
. The continent we live on. Physical geography; Natural history. This province is so dearly defined in most respects that it is widely recognized as forming a unit within the otherwise complex terrain of the West and has acquired a popular name— the Great Basin. It is not strictly a basin, either geologically or in structure, for it does not slope inward from its subcircular periphery but is nearly level, though its surface is covered with ranks of mountain chains. It is almost entirely ringed by steep, high mountain walls. It can be divided into three not too well defined subregions, and has an appendage to the north—an area commonly called the Guttered Scablands. Its boundaries are formed on the east by the Utah block of the South Montane and a portion of the North Montane provinces: on the north by the southern face of the North Montane Block plus the Blue Mountains, which lie entirely zvithin its confines; and on the west by the barrier of the Cascades to the northwest and the Sierra Nevada to the southwest. Its southern boundary appears to be arbitrary on a map but may be outlined by the zone of change from the red soils of the Sonoran Deserts of the south to the gray soils of the Scrub Belt of the north. The Great Basin as thus defined is some eight hundred miles long from north to south and five hundred in width. It is shaped like a human heart. This is an odd province in many respects. First, it is geologically rather new as a physiographic unit and is stilt changing somewhat rapidly. Second, unlike all other provinces on this continent, its nature and appearance, although still basically dependent upon its position among the major vegetational belts, is much influenced by secondary forces. Prominent among these is absence of rainfall, due to mechanical factors—namely, the almost complete atmospheric moisture trap formed by the Cascade—Sierra Nevada barrier to the west, which cuts off the prevailing moisture-laden winds from the Pacific. Third, it ha
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