70 G. F. BECKER — STRUCTURE OF THE SIERRA NEVADA. 



Semite region seem to me simply the other end of the series. These, when 

 their surfaces are perfect, show no fissures, and wherever cracks are formed 

 in them rounding of the edges and corners made by the cracks commences at 

 once. 



The gigantic domes are found only near the Yosemite and to the south- 

 ward, but there are domes of considerable size between the Truckee and the 

 Stanislaus. 



Origin of the Forces. 



Thesis of a solid Earth maintained, — Thus far I have discussed the fis- 

 sures, their immediate cause and their effects ; but I have made no reference 

 to the origin of the inclined thrust to which the existence of the fissures was 

 traced. I now desire to show that this thrust can be satisfactorily explained, 

 provided the earth be regarded as a solid mass of extremely high viscosity 

 which would yield slowly to relatively moderate forces of constant terrestrial 

 direction and long duration, but which would probably yield almost imper- 

 ceptibly to any force of brief duration or rapidly changing direction. 



Direction of Subsidence. — Without assuming any hypothesis as to the flu- 

 idity or solidity of the earth's interior, let it be assumed that the globe tends 

 to a condition of hydrostatic equilibrium and that at one particular period 

 of time it had reached such equilibrium. Now when the equilibrium is per- 

 fect, imagine a certain mass removed from the surface of one region, A, 

 and piled upon some other region, B '=^. 



Figure 9 — Disturbance of terrestrial equilibrium. 



Then a tendency to reestablish hydrostatic equilibrium would at once 

 assert itself, and material would begin to flow from beneath B towards A. 

 The mass at B would siuk into the globe and the cavity at A would fill up. 



Now let it be supposed that the earth's mass below the dotted line of the 

 figure were a substantially perfect fluid. Then, as the fluid was displaced 

 by the sinking at B it would flow over towards A by the shortest possible 

 path. In doing so there would be a tendency to frictional resistance be- 

 tween the moving fluid and the supernatant mass. But the fluid being sub- 

 stantially a perfect one, the friction which its motion would induce would be 



* Precisely this ease does not occur in nature, but erosion produces results which are equivalent 

 to it so far as the disturbances of equilibrium are concerned, as will be pointed out a little later. 



