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University of California Publications. 



[Geology 



the partial elimination of alkalies and silica and the addition of 

 water, involves a very considerable diminution of volume, and 

 since this process has been general throughout the porphyry, it 

 seems reasonable to make it accountable for the shrinkage mass. 

 The tendency to collapse as the process proceeded would be ad- 

 justed by slipping and shearing on normal fault planes and as 

 these developed the mass would be rendered more and more ac- 

 cessible to permeating solutions. 



The process of kaolinization is probably most commonly in- 

 augurated by the attack of carbonated waters, whereby a portion 

 of the alkalies is removed as carbonate, carrying off with it a 

 portion of the silica in solution, in the form of alkaline silicate. 

 It has been shown that the slipping and shearing of the mass 

 of porphyry was progressive and the process of kaolinization 

 was probably also progressive extending over a long period of 

 time. If this be conceded, there is nothing violent in the assump- 

 tion that the process may have started immediately after the 

 solidification of the porphyry, the source of the carbonic acid 

 being the decomposition of the surrounding limestones by con- 

 tact with the hot intrusive mass. This, at least, is the hypothesis 

 which the writer entertains to explain the chemically altered 

 and mechanically sheared condition in which we now find the 

 porphyry. 



The hypothesis finds support in the fact that it affords at the 

 same time a consistent explanation of certain other important 

 phenomena connected with the porphyry, particularly on its 

 periphery. It also suggests an adequate explanation for the 

 original source of the copper ore and its associated pyrite. Un- 

 der the hypothesis, the carbonic acid would be in the ascending 

 waters arising about and through the recently consolidated por- 

 phyry, the ascending tendency being due to the disturbance of 

 the equilibrium of the ground water of the region by the inva- 

 sion of the intrusive mass. This ascending water charged with 

 carbonic acid would inaugurate the kaolinization of the por- 

 phyry. Lime would be present in these waters in the form of 

 the bicarbonate. Under these circumstances, certain chemical 

 reactions would take place from the recognition of which there 

 appears to be no escape. The potassium silicate formed in the 



