CHAPTEE I. 



MIJN^IN^a DISTEIOTS. 



THEIE DISTEIBUTION AOT) GEOLOGICAL MODE OF OCCUEEENCE. 



West of the hundredth meridian, and bordering the Pacific Ocean from 

 Southern Mexico to the Arctic Sea, stretches a series of mountain chains and 

 elevated plateaus. From a width of four hundred miles on the Mexican line, 

 it rapidly widens to the north, reaching its greatest expansion on the fortieth 

 parallel, where the actual mountain zone is over one thousand miles wide. 

 From this point it again narrows toward the north, diminishing to four hun- 

 dred miles upon the northwest boundary line. 



The central portion of this area is embraced under the general name of 

 the Great Basin. This term, first applied by Fremont, was intended only to 

 cover a depressed region whose streams never reached the ocean ; but it has 

 gradually been widened to include all that barren middle ground between the 

 California mountains and the Rocky system. 



The Great Basin is walled upon the w^est by the Sierra Nevada Eange. 

 Its eastern boundary is the Wahsatch Chain. A section across the five hundred 

 miles of intervening country, along the fortieth parallel, shows a rapid descent 

 from the Sierra summit, wdiose elevation at this point is about ten thousand feet, 

 to a depressed zone of country which skirts the great range for a thousand miles 

 and spreads out to the east in comparatively level deserts, its surface here and 

 there interrupted by abrupt chains of mountains. From this area of desert 

 lowland, averaging in altitude about four thousand feet above sea-level, the 

 country gradually ascends to the eastward, the surface being occupied by a 

 succession of meridional ranges separated by trough-like valleys. Where the 

 fortieth parallel intersects the hundred and sixteenth meridian of west longi- 

 tude the rise culminates, and passing thence eastward the general profile of the 

 plateau sinks gradually to a second belt of depressed plains, whose aridity rivals 



