ORIGIN OF THE FAULTS. 



73 



ments and by many geological observations. It is also a manifest fact that 

 erosive action removes masses of material from some limited areas and de- 

 posits them upon others. To restore isostacy these masses must sink in their 

 new positions. If the earth's crust has a fluid substratum they will sink 

 vertically, unless some force independent of erosion and gravity is brought 

 to bear upon the transported material. But if the earth is a solid though 

 viscous mass, the sedimented areas will not sink vertically and the denuded 

 areas will not rise vertically. 



In the following diagram (figure 12) let the dotted line represent in verti- 

 cal cross-section a range of mountains and a valley in an isostatic state (so 

 that the irregular surface is wholly due to difference in density). Let erosion 

 remove a mass, a, from the range and deposit it in the valley at b, so that 

 the outline becomes that shown by the full line. Then, provided the earth 



Figure 12 — Analysis of Uphearal and subsidence. 



is viscous, diagonal forces c and d will act on the two areas. These may be 

 resolved into vertical and horizontal components, as shown in the diagram. 

 The two horizontal components then cooperate to produce a compressive 

 horizontal thrust on the range. The two vertical components, on the other 

 hand, form a couple, tending to tilt the range towards the valley.^ 



If now there is a resistance to this tilting, as there surely would be on a 

 solid globe, this resistance would also form a couple. Then the system of 

 forces would be as shown in figure 13 on page 74. 



Applieation to the Sierra. — This system of forces is precisely that which my 

 analysis of the fissure system in the Sierra shows must have existed there. 

 Figure 12 also represents such a configuration as the Sierra and the great 

 valley of California must have had during the Pliocene. One point, how- 

 ever, requires a few words of explanation. The trend of the Sierra and of 



*In any such deformation of the, glohe there will be certain strains failing entirely within the 

 elastic linnitsof the rocks involved. The principles of viscous tlow and isostacy are, of course, in- 

 applicable to these strains, which are probably small. See Mr. G. K. Gilbert's " Lake Bonneville," 

 U. S. Geol. Survey Monograph I, 1890. 



XI— Bui.L. Geol. Soc. Am., Vol. 2, 1890. 



