MOUNTAIN ORIGIN AND STRUCTURE. 265 



type — including the greatest mountains, and long supposed the only 

 kind. But there is another type only recently brought to light by the 

 United States Geological Survey, the most conspicuous if not the only 

 examples of which are found in the Basin and part of the Plateau re- 

 gions. These mountains are formed in an entirely different way, viz., 

 by the tilting or else the lodily uplifting of great crust-blocks, sepa- 

 rated by parallel fissures. Mountains of the usual type may be called 

 anticlinals ; for, although often made up of many anticlines and syn- 

 clines, yet, taken as a whole, they may be regarded as a grand anticline 

 (see Fig. 229, p. 257). Mountains of this second type are called 

 " monoclinals." In mountains of the first type the faults are usually 

 reverse, in the second type they are normal. Normal faults are ex- 

 tremely common everywhere, but they are rarely great enough to give 

 rise to earth-features which deserve the name of mountains ; but in 

 the Basin region — as we have already seen, page 233 — their scale is so 

 enormous and their formation so recent that they give rise to very 

 conspicuous orographic features. We have already, under faults, (p. 

 233), sufficiently explained the mode of formation of the Basin ranges. 

 We now wish to explain how the Sierra has been modified — in fact, 

 received its present form and altitude — in this way. 



The Sierra, as we have already seen, was formed by crushing and 

 folding of thick sediments at the end of the Jurassic. It is probable 

 that, by the enormous erosion of the Cretaceous and Tertiary periods, 

 it was subsequently cut down to very moderate altitude. At the end of 

 the Tertiary this great mountain-block — 300 miles long and 50 to 70 

 miles wide — was heaved up on its eastern side, forming there a normal 

 fault, with a displacement of probably not less than 15,000 feet.* The 

 range was thus greatly elevated, and its crest transferred to its extreme 

 eastern margin. The movement was attended with lava-flows, which 

 ran down the western slope, filling up the old river-beds, and displacing 

 the rivers. The displaced rivers immediately commenced cutting new 

 beds (Fig. 8, p. 16, and Fig. 218, p. 248). That this event took place at 

 the end of the Tertiary is shown by the fact that even the most recent 

 Tertiary beds were covered by the lava. That the slope and therefore 

 the height of the mountain were greatly increased at that time is shown 

 by the fact that the rivers, seeking their base-level (p. 21), have cut 

 their new beds 2,000 feet below their old beds, even though the time of 

 cutting was very much less. Evidently, therefore, the present form 

 and height of the Sierra date from the end of the Tertiary. 



Coincidently with this last great modification of the Sierra, the 

 Basin ranges were also formed by crust-block-tilting. On the other 



* The fault-scarp is 10,000 feet, and the summit slates are deeply buried beneath 

 Quaternary deposits at its foot. 



