260 J. LeConte — Origin of Normal Faults and 



are more or less inclined, as is more commonly the case, then it is 

 evident that the crust-blocks will be either rhomboidal {a, b,f, g,) 



A—- 



Z 



/c\ J- / e\ 



/ \ / 



A. Crust broken into blocks. B. Crust arched and blocks separated, 

 re -adjusted by gravity. 



C. Crust 



or wedge shaped (c, d, e, fig. 9 A). These in the arching of the 

 crust would be separated from one another, fig. 9 B. But after the 

 relief of tension by outpouring of lava or by the escape of steam, 

 they would of course readjust themselves by gravity in new 

 positions. ]\ T ow by the laws of flotation how would such blocks 

 adjust themselves ? It is quite evident that every rhomboidal 

 block would tip over on the overhanging side and heave up on 

 the obtuse angle side producing in every case normal faults, and 

 every wedged-shaped block would sink bodily lower or float 

 bodily higher according as the base of the wedge were upward 

 or downward, producing again in every case normal faults (fig. 

 9 C). A thick board sawn in the manner represented in fig. 9 A 

 and the separated blocks placed together and floated on water 

 would take exactly the positions represented in fig. 9 C. The ex- 

 planation is complete. Of course erosion will modify the fault- 

 scarps thus formed, by sculpturing their faces and by reducing 

 their heights and slopes or even in some cases effacing them 

 altogether. But if the fracturing and faulting have been geolog- 

 icallv recent and on a large enough scale they may still remain 

 and give rise to very conspicuous orographic features. 



It is in this way that the orographic features of the Basin 

 region have been formed ; and the scale has been so grand and 

 the features are so conspicuous that the resulting structure has 

 been appropriately called Basin structure. The Basin region, 

 according to the researches of King, Gilbert and Russell, is 

 traversed by numerous north and south ridges several thou- 

 sand feet high, with intervening valleys which are now or have 

 been occupied by lakes. Although greatly modified by con- 

 emporaneous igneous ejections and subsequent erosion, these 



