of the Structure of the Basin Region. 261 



mountains consist essentially of gentle monoclinal slopes ter- 

 minated by fault-scarps. They are in fact a succession of 

 tilted and displaced crust blocks (fig. 10). The simplest ^and 



Fig. 10. Basin structure, (after Gilbert.) 



most beautiful illustration of this structure is found in the 

 northern part of the Basin region in S. E. Oregon. In this 

 region, as Russell has shown, the country rock consists of level 

 sheets of lava outpoured during the later Tertiary period. 

 These have been subsequently broken by parallel 1ST. and S. 

 fissures into rhomboidal and wedge-shaped blocks, which, 

 readjusting themselves by gravity, have tilted or else sunk 

 bodily lower, or floated bodily higher, so as to make a succes- 

 sion of normal faults. Moreover, the event has been so recent 

 and the scale so grand that the uptilted side of each block 

 forms a mountain ridge and the down dropped side a valley, 

 on which has often accumulated a lake (fig. 11). Where the 



Fig. 11. Sketch section showing structure of S. E. Oregon, (after Russell.) 

 W. L. — Warner Lake. A. L. — Albert Lake. Ch. V. — Chewaukan Valley. 



inclination of contiguous fissures are in opposite directions, so as 

 to form weclge-shaped blocks, these, as already explained, will 

 drop down or heave up bodily. Some of the most conspicu- 

 ous valleys are formed in this way — as for example Chewaukan 

 Valley in Oregon, and Surprise Valley in 1ST. E. California. 



Geological age of these events. — I have already shown in a 

 previous article (this Journal, vol. xxxii, p. 167, 1886), that the 

 Sierra Nevada is a great crust-block 300 miles long and 50-60 

 miles wide heaved and slipped on the eastern side, forming 

 there a great fault of 15,000-20,000 feet vertical displacement, 

 and that this took place at the end of the Tertiary, accom- 

 panied with floods of lava. The evidence of this is found in 

 the relation of the new to the old river-beds. The rivers dis- 

 placed from their old beds by the lava have since that time cut 

 far deeper than before, although cutting far less time.* Now, 



* There is another evidence of the comparative recency of the origin of the 

 Sierra in its present form, to which attention is now drawn for the first time. It 

 is well known that slates overlying the axial granite are found forming the very 

 crests of the Sierra. Furthermore, the deepest biting into the granite is found 

 ■twenty miles westward of the crests in the region of the domes about the margin 

 of the Upper Tosemite. This would be impossible if the crest had been in its 

 present position ever since Jurassic times. When these mountains were first 

 formed at the end of the Jurassic, the crest was probably about the region of the 

 Domes. The tilting of the Sierra crust-block transferred it eastward to, or a little 

 beyond, its present position Since that time it has been migrating westward by 

 erosion. But the recency of this event is shown by the fact that these once 

 flanking slates now forming the crest have not yet been removed. 



