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University of California Publications in Geology [Vol. 11 



A further fact is that the Knoxville shales in the region are often 

 black and thin bedded. In Bagley Canon at Mount Diablo, Knoxville 

 shales of this sort are exposed in the lower part of the canon. Fur- 

 ther up stream, in the Franciscan mass, above the plane of the thrust 

 fault there is a large rib of chert several hundred feet long and 

 seventy-five feet thick included in a mass of intrusive basalt. This 

 chert is well exposed by the dissection of Bagley Canon. It so hap- 

 pens that the chert horizon contains, as the cherts rarely do, some 

 black, carbonaceous, thin bedded shales, much like those in the Knox- 

 ville. Altogether Whitney's conclusion was natural in view of the 

 facts he found in Bagley Canon, and his failure to recognize the 

 nature of the structure there. 



Whitney did his detailed work at Mount Diablo and used the 

 results of his work there for the determination of Coast Range 

 sequence. He never seems to have questioned his determinations at 

 Mount Diablo, nor to have critically studied other sections. It is pos- 

 sible that careful study in other localities would have shown him his 

 error. 



In 1888, Geo. F. Becker 111 described these rocks in connection 

 with his report on the quicksilver deposits of the Pacific Coast. He, 

 also, regarded the Franciscan rocks as metamorphic equivalents of the 

 Knoxville. He speaks of the cherts of the Franciscan group as 

 "phthanites, or schistose rocks which have been subjected to a pro- 

 cess of silieifieation, retaining their schistose structure. ' ' He says : 



. . . The shales of the Knoxville Group are sometimes unaltered, but more fre- 

 quently silieified to chert-like masses of green, brown, red, or black colors, inter- 

 sected by innumerable veins of silica. These highly altered shales, when very 

 thin-bedded, break into parallelopipidic fragments, but where the beds reach a 

 thickness of half an inch or more there is a decided tendency to conchoidal frac- 

 ture. The green varieties are infusible before the blowpipe, while the brown 

 specimens are more or less fusible. The only essential difference appears to be in 

 the state of oxidation of the iron, which is partially soluble in the reddish rocks. 

 The most convenient name for these rocks is phthanite, introduced by Haiiy to 

 designate quartzose, argillaceous rocks with a compactly schistose structure. 

 This term has sometimes been employed in a more special sense to denote siliceous 

 beds intercalated in limestone, but this limitation will not be adopted here. 



As proof of their metamorphic origin he pointed to certain rather 

 abrupt transitions which he thought he could establish : 



. . . The metamorphic character of the phthanites is manifest both from their 

 structure and from transitions — which exist, for example, at Venado Peak, New 



in Becker, G. F., Quicksilver Deposits of the Pacific Coast, IT. S. Geol. Surv. 

 Monograph 13, 1888. 



