gilbert.] PHYSICAL GEOGKAPHY OF THE REGION. 265 



from Grand Junction to Canyon city, and thence to Denver skirts the 

 base of the Front range. The branch route to the Colorado canyon 

 crosses the southern extension of the chain in New Mexico between 

 Las Vegas and Albuquerque. 



The Plateau region lies between the southern Rocky mountains on 

 the east, and the Great basin on the west. Structurally, it consists of 

 stratified rocks of all ages, in chief part conformable, and usually Lying 

 nearly level, but dislocated in great orogenid blocks which stand at 

 different altitudes. With them are associated eruptive rocks, in part 

 injected as laccolites. and in part heaped in conical mountains. Its 

 physiographic character is further determined by the fact that the re- 

 gion stands at great altitude above the sea. and has been profoundly 

 dissected by the streams constituting its drainage system. It thus 

 consists of a number of tables, standing at various heights, and sepa- 

 rated partly by cliffs and partly by deep canyons. The return route 

 traverses the region from Soldiers Fork in Central Utah to Grand 

 Junction in western Colorado. On the branch route to the Colorado 

 canyon it is traversed from Albuquerque westward. 



The Great basin is a district without drainage to the ocean, it is 

 bounded on the west by the Sierra Nevada, and on the north, east, and 

 south by the basins of the Columbia and Colorado. At the north its 

 lowlands have a general elevation of 4,000 to 6,000 feet (1,200 to 1,800 

 m.), and they descend southward to the level of the sea. Cpon this 

 sloping plateau are set a multitude of mountain ranges, for the most 

 part of moderate height and of moderate length. 4 They are in gen- 

 eral parallel with one another, the prevailing trend at the north being 

 north and south, and at the south, northwest and southeast. The 

 intervening valleys are tlooded by alluvium derived from the erosion of 

 the ranges, and are usually L6 to 20 miles (24 to 32 km.) broad. The 

 ranges are party volcanic, hut consist chiefly of Paleozoic strata, some- 

 times with folds, but nearly always profoundly faulted. The typical 

 structure is in contrast with that of the Appalachians. In the Appa- 

 lachian mountains are thrust faults, indicating compression of the 

 strata; in the Great basin the faults exhibit hade to tin' downthrow. 

 The ridges of the Appalachian mountains, as exhibited to day, are due 

 to differential erosion; the mountain ridges of the Great basin are due 

 directly to uplift. The excursion enters the Great basin at lied Rock 

 pass, in Idaho, and continues within it to the summit at the head of 

 Soldiers fork, in eastern Utah. 



Iht type of mountain structure of the Great basin, which has been 

 called the "Basin Range type," is not restricted to the region of interior 

 drainage, but prevails throughout a belt extending southward and east- 

 ward, about the margin of the plateau region, in Arizona and New 

 Mexico. 



