58 GEOLOGY OF THE COMSTOCK LODE. 



augite-andesites. Of course, neither columnar structure nor fissility, both 

 of which are probably to be regarded as results of tension from cooling, 

 are developed in the comparatively porous breccias, for the fragments of 

 unfused rock in breccia act like the chamotte in a fire-brick in preventing 

 density of structure. 



Decomposition. — The weatlieHng of the hornblende-andesite seems to differ 

 in its nature, as it takes place in direct contact with the air or under ground. 

 Croppings of the rock which on being broken prove internally fresh, are com- 

 monly coated with a very thin, deep-red or brown scale and, to judge by 

 fragments found in the immediate neighborhood of such croppings, the 

 change seems to consist mainly in disintegration by frost and in peroxida- 

 tion of the iron. Under ground, on the other hand, decomposition appears to 

 extend into the body of the rock. One of the first minerals to be affected 

 is the feldspar, which loses its transparency and becomes a dead white. 

 This totally alters the appearance of the rock, which becomes a light-gray 

 porphyry, instead of a dark-bluish and basaltic-looking mass. Ever}- varia- 

 tion in coarseness of grain also becomes apparent. The feldspars lose 

 their transparency when only a very minute portion of their substance 

 (certainly less than one per cent.) is altei-ed. The next stage of decompo- 

 sition is the formation of chlorite from the bisilicates, which soon diffuses 

 itself tlu-ough the groundmass and the feldspars. The chlorite is fiu-ther 

 frequently decomposed into calcite and epidote without an)- special change 

 in the appearance of the rock. All these changes tend to diminish the 

 sharp definition of the porphyritical crystals and give the mass the look 

 rather of an olde)* dioritic porphyry than of a volcanic rock. It is easy to 

 suggest plausible explanations for the different behavior of the andesitc 

 above ground and l)eneath the surface. The presence under ground of 

 water holding carbonic acid in solution is perhaps sufficient to account for 

 the formation of calcite in the feldspars, and the strong oxidizing action on 

 the surface may well explain the direct formation of ferric oxide in tln^ 

 exposed rocks. When the andesites are not in the condition of breccia the 

 subterranean decomposition is conmionl}^ accompanied by a softening or 

 partial disintegration of the mass, though in some cases, as at the South 

 Twin Peak, rock not brecciated preserves great consistency, possibl}' from 



