V. 



Physical Geolog)^ in the Basin of the Colorado. 



THE basin of the Colorado River offers a wonderful series of 

 object-lessons to the physical geologist. The two operations 

 of erosion and crust-movement, in particular, are there exhibited on 

 the grandest scale and in the simplest manner. The region to be 

 noticed comprises the southern and eastern portions of Utah, with 

 parts of the neighbouring States and Territories, Wyoming, Colorado, 

 New Mexico, and Arizona. Utah itself is divided by an ill-defined 

 water-shed, the northern and western parts being included in the 

 Great Basin, which drains only into inland salt lakes. The Colorado 

 River is formed by the union of two great branches, the Green River, 

 which rises in Wyoming and runs south, and the Grand River, which 

 heads in the Rocky Mountains of Colorado, and has a general 

 westerly course. The united waters flow through northern Arizona, 

 in the famous Grand Canon, and finally reach the Gulf of California. 

 Physically, the region is a remarkable one. It has, indeed, 

 mountain-chains, some like the Wasatch, the westerly front range of 

 the Rocky system, rising ii,ooo or 12,000 feet above sea-level; but 

 essentially it is a country of high plateaux having more or less of 

 the character of deserts. The highest plateaux, bordering the Grand 

 Canon, sometimes reach an elevation of nearly 9,000 feet. 



The phenomena of erosion are intimately connected with the 

 climate, which is an extremely dry one, owing to the great range of 

 the Sierra Nevada, cutting off the moisture from the Pacific. Not 

 only is the annual rainfall very small, but it is concentrated in 

 a few violent storms, leaving the country absolutely parched during 

 the greater part of the year. Except on the highest plateaux and 

 mountains, vegetation is generally restricted to a few hardy forms. 

 The only permanent streams are those fed by the snows of the Rocky 

 Mountains. It would be a great mistake, however, to suppose that 

 aridity of climate implies slowness in denudation. There is every 

 evidence that the general surface of the country is undergoing 

 degradation and removal at a rapid rate. Over large tracts the strata 

 from the Permian to the Eocene inclusive, with an average thickness 

 estimated by Captain Dutton at 10,000 feet, have been completely 

 removed, and the beginning of the process cannot date back farther 



