] 06 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Knoxville group are sometimes unaltered, but more frequently silicitied to 

 chert-like masses of green, brown, red, or black colors, intersected by in- 

 numerable veins of silica. These highly altered shales, when very thin- 

 bedded, break into parallelopipedic fragments, but wliere the beds reach ;i 

 thickness of half an inch or more there is a decided tendency to conclioidal 

 fracture. Tlie green varieties are infusible before the blow-pipe, 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 solu- 

 ble in the reddish rocks. The most convenient name for these rocks is 

 phthanite, introduced by Ilaiiy to designate qnartzose, argillaceous rocks 

 with a compactly schistose structure. This term has sometimes been em- 

 ployed in a more special sense to denote siliceous beds intercalated in lime- 

 stone, but this limitation will not be adopted here.' 



Phthanites occur in all the metamorphic districts of the Coast Ranges, 

 and, though the quantitative proportion which they bear to tlie other rocks 

 is not great, the marked contrast between them and the surrounding masses 

 gives them prominence. Geologically it is impossible to dissociate the 

 phthanites from the altered sandstones, holocrystalline, metamorphic rocks, 

 and serpentine. All of these rocks, with transitional varieties, are found 

 together and are often mingled in the confused masses of rubble which 

 have sometimes resulted from intense dynamical action. The metamorphic 

 character of the phthanites is manifest both from their structure and from 

 the ti-ansitions — which exist, for example, at Venado Peak, Newldria — into 

 ordinary shales. Under the microscope, also, the most highly indurated 

 specimens are found to contain fossils 



The peculiar habitus of the phthanites appears to arise from the fact 

 that the shales have offered great resistance to serpeutinization, although 

 the}' have not avIioUv escaped this alteration, while they were admirabl}- 

 adapted both to silicification and to tlie display of a net-work of quartz 

 veins. 



The mass of the phthanites as seen under the microscope consists 

 mainly of fine-grained, crystalline silica, occasionally accompanied by a 



' Compare Jlr. Renard's RccUerches litliologiqucs .stir leg phthanites du calcaire ca^l>OIlif^^e de Bcl- 

 giiine: Bull. Acad. roy. Belgique, vol. 46, 1''78. 



