102 QUICKSILVER DEPOSITS OF THE I'AOIFIC SLOPE. 



The hand specimens of this rock do not greatly resemble eruptive 

 masses and the nature of the occurrence clearly indicates their metamorphic 

 c-haracter, but the slide is indistinguishable from thin sections of eruptive 

 gabbros. 



There appears to be no reason to consider the above-described rock 

 ;ts anything more than a variety of the pseudodiabase. A similar rock 

 is found at tlie Grreat Western, and at New Almaden a gabbro occurs as 

 pebbles, apparently derived from the mountains to the south. 



OLAUCOPHAXE SCHISTS. 



cha.-actar. — Accompauyiug the granular, liolocrystalline metamorphics, in 

 nnicli smaller quantities than these, are somewhat s;"histos9 rocks, which 

 are sometimes evidently micaceous and sometimes appear to the naked eye 

 chloritic. All of these are found to carry glaucophane, usually accompa- 

 nied by zoisite and Inica. Some of them are macroscopically indistinguish- 

 able from specimens from S3'ra. They are so related structurally to the 

 granular rocks as to show them to be members of the same series, and, as 

 has been shown, glaucophane and zoisite both occur in the granular rocks. 

 It is worthy of note that the plagioclase of the granular rocks and the glau- 

 cophane of the schists each imply the presence of sodium in the solutions, 

 by which metasomatosis of the sandstone series was effected. The zoisites 

 also, at least in part, contain alkalis. Though glaucophane rocks are not 

 infrequent in the Coast Ranges they usually occur only in small patches, 

 and it is seldom possible to trace them to their imaltered form. At Mt. 

 Diablo, however, they certainly pass over into slightly altered shales, and 

 there is also evidence elsewhere that the schistose structure is an original 

 feature, not a result of raetamorpliism. The predominant cleavage in these 

 schists is marked by a prevailing similarity of direction of the glaucophane 

 prisms and mica foils, although by no means all of the crystals of either 

 mineral are similarly placed. The structure and association of minerals 

 will best be described Sby examples. 



Examples. — Xo. 31, Sulphur Bauk, is a schistose, gueiss-like rook, in 

 which lavers of greenish mica, in small foils, travei'se a fine-grained, 

 reddish or greenish grav, granular mass. Bluish-grav grains of glauco- 



