2l6 



Sierra Club Bulletin. 



A second process developing expansive force is weath- 

 ering, and here again future investigation may discover a 

 true cause; but to cursory and inexpert observation the 

 granites of the Sierra in the glaciated district appear to be 

 unaltered. 



A third process — one as to which we have no direct 

 knowledge — is dilatation from unloading. When the 

 granite came into existence by the cooling of the parent 

 magma it was buried under a deep cover of older rock. 

 Because of that cover it was subject to compressive stress, 

 and that compressive stress was of course balanced by 

 internal expansive stress competent to cause actual ex- 

 pansion if the external pressure were removed. As in 

 course of time the load was in fact gradually removed, 

 the compressive stress was diminished and the expansive 

 stress became operative. Pari passu with this release of 

 expansive stress there was cooling, and the effect of the 

 cooling was to diminish expansive stress; and the result 

 may have been complicated by other stress factors. So 

 long as the pressure of superjacent material was great, 

 the equilibrium of stresses was approximately adjusted by 

 flowage ; but as the descending surface of degradation ap- 

 proached the granite, flowage diminished, and it ulti- 

 mately ceased. The final adjustment was by change of 

 volume, the change being contraction, if lowering of tem- 

 perature was a more important factor than relief from 

 load, and expansion, if relief from load was the more 

 important factor. In the latter case (which I regard as 

 the more probable) the parts of the granite successively 

 exposed at the surface were in a condition of potential 

 . expansion, or tensile strain, and that strain would be 

 relieved by the separation of layers through the develop- 



