1856.] Physical Geography of North America. 175 



western edge of the Colorado Desert, the rounded water- worn peb- 

 bles which strew the surface are beautifully polished, from the 

 action of the sharp dry sand driven* furiously over the gravelly 

 pav«nient by the prevailing westerly winds ; and in several of the 

 gorges or passes through the Sierra Nevada, in the same neighbour- 

 hood, the fixed rocks of the mountains are themselves smoothed 

 and striated by the same agency. 



E. The Pacific Mountain Chain. 



The third, or western belt, of the great elevated mountain zone 

 of Western North America, is the oceanic chain which runs 

 adjacent to the Pacific from Russian America to the peninsula of 

 Lower California. The northern section of this chain traversing 

 Russian and British America, is sometimes called the Pacific Alps ; 

 the middle section, from Frazer's Kiver to the Klamath, the Cascade 

 Range ; and the southern section, lapping past part of the southern 

 end of the latter, and extending from the Columbia River to about 

 lat. 35®, and becoming extinct in the San Barnadino country, is 

 called the Sierra Nevada. The central ridge of the peninsula of 

 Lower California, a part of the same great Pacific chain, is the south- 

 ward prolongation of a parallel mountain axis, which, in Middle and 

 Northern California ranges parallel to the Sierra Nevada, west of 

 if, and close to the Pacific coast, and is there known as the Coast 

 Mountains. Low in its southern sections, this great mountain chain 

 in Middle California, Oregon, and Russian America is broad, com- 

 plex, and very lofty, its main central crests containing nearly the 

 highest summits upon the continent. The Sierra Nevada, a great 

 watershed insulating the high rainless plateau of Utah from the basin 

 of California ajjd from the sea, carries a very elevated crest, many 

 parts of which reach a height of 10,000 feet ; but it has few insula- 

 ted peaks towering above this line. The Cascade range, on the 

 contrary, wholly different in its geological structure, being largely 

 volcanic, is cleft nearly to the sea level in many places, and yet 

 bears some of the most colossal conical summits to be met with on 

 the globe. The three great volcanic peaks. Mount Jefferson, 

 Mount Hood, and Mount St. Helen's, tower in great masses to the 

 height of 15,500 feet, and even above this. Mount Fair-weather, 

 14,782 feet high, and Mount St. Elias 17,850 feet, are both vol- 

 canoes, believed to be occasionally active ; while Mount St. Helen's 

 and Mount Regnier, though rather torpid, are known to be occasion- 

 ally in eruption. About latitude 35°, tlie Sierra Nevada and the 

 coast range diverge northward, to enclose between them the gold- 

 producing valley of California. 



The geological constitution of the Pacific chain differs in its 

 different sections. The Sierra Nevada, peninsular chain and coast 

 mountains, consist largely of the azoic or semi-crystalline strata, 

 with belts of the gneissic and true plutonic rocks, in some parts of 



Vol. II. N 



