STRUCTURAL RESULTS OF FAULTING. 183 



apparent. The surfoces of the sheets in the same locality are not similar 

 to those commonly formed by bedding, and are indistinguishable from frac- 

 tures, nor is the persistence of the sheets comparable with that of sedi- 

 mentary strata. The McKihhen Titmiel in Spanish Ravine passes through 

 diorites in part somewhat porphyritic, in part of the dark, highly horn- 

 blendic variety. A quartz seam is cut by the tunnel, but no dikes of later 

 rocks. There is a greater superficial resemblance to a bedded structuie 

 here than on Mount Davidson, but close examination shows that most of the 

 apparent differences in color and texture are referable to degrees of decom- 

 position. Decomposition has set in from the partings of the sheets of rock, 

 often leaving the central portion of a sheet less affected than its faces. In 

 the diabase, hornblende-andesite, and augite-andesite of the east country, 

 the phenomena are similar. There is ample evidence of fracture and of 

 decomposition following lines of fracture. Sometimes individual sheets or 

 portions of sheets have in a measure escaped decomposition on account of 

 the presence of protecting clay seams and the like, and these have been 

 mistaken for dikes, or flows of andesite or other rock; but careful examina- 

 tion shows that they differ only in the degree to which they have yielded to 

 decomposing agencies, and in no other respect. The partings are not such 

 as we should expect in bedded flows. There is no trace of lamination 

 except the irrelevant local occurrences mentioned, and while it might well 

 be that the greater part of the seams had been reopened by upheaval, it 

 cannot be supposed that no adherent laminte would escape separation. In 

 short, my observations wholly fail to accord with the hypothesis that these 

 rocks were laid down in horizontal beds, and afterward tilted. Even if 

 observation furnished considerable grounds for such an interpretation of the 

 facts, I should hesitate to accept an explanation which appears to me wholly 

 at variance with what we know of the occurrence of similar rocks else- 

 where.' The deposition of a single igneous rock over several square miles, 

 in thin horizontal beds, implies a watery fluidity and a very high specific 

 heat. So far as I know only one or two of the later volcanic rocks are 



' Mr. Church, indeed, states (.1. c, p. 153) that "diorite is one of the fiue-graiued, thiu, ruuuing 

 lavas." But he cites uo authorities for, or instances in proof of, this statemeut, which is at variance 

 with the coiuuionly accepted opinion, and with the indications of its composition and micro- structure. 



