80 GEOLOGY OF THE COMSTOOK LODE. 



ical structure. The porphyry is, and seems always to have been, very 

 porous, and has permitt-ed a more rajiid percolation of surface waters than 

 the other rocks. This is not improbably ascribable to the difference in the 

 coefficient of expansion of the quartz grains, and the other mineralogical 

 constituents. 



The chemical aspects of the decomposition of the Washoe rocks will 

 be discussed in a separate chapter. 



