FRANCISCAN GROUP 



41 



FIGURE 31. Glaucophane-quartz schist cut and polished at right angles to plane of schlstosity. The light- 

 colored augenlike areas are quartz, as are most of the transverse veinlets. In some specimens there 

 are similar veinlets of glassy albite. The variations in composition and grain size suggest relict 

 bedding. 



structure. Polysynthetic twinning occurs in the law- 

 sonite, but it is so uncommon as to be of little value 

 in identifying the mineral. The stilpnomelane is all 

 of the ferric variety, with intense pleochroism in yel- 

 low to yellow brown. Fuchsite was tentatively iden- 

 tified only by its brilliant green color and blue to 

 green pleochroism, and, if present, it is rare. Garnet 

 also is generally rare, but many small subhedral crys- 

 tals of garnet form a conspicuous part of some quartz- 

 itic schists derived from chert. Calcite occurs chiefly 

 in irregular veins, but it is also found in ragged in- 

 terstitial patches which are thought to have formed 

 during the recrystallization of the rock. 



The relative proportions of the minerals in the glau- 

 cophane rocks vary considerably. Glaucophane makes 

 up almost all of some of the massive rock; it consti- 

 tutes 25 to 60 percent of the common glaucophane- 

 lawsonite schists but only a few percent of the com- 

 paratively rare quartz-lawsonite schists. Lawsonite 

 rarely amounts to more than 40 percent, but some is 

 present in all thin sections of the glaucophane rocks. 

 The micas rarely exceed 5 percent of the rock, and in 

 some sections sphene is equally abundant. 



686-671 O 63 1 



The relationship between the original bedding and 

 the foliation of the glaucophane rocks can be deter- 

 mined with certainty in only a few outcrops, and in 

 these the two are parallel. It seems likely that the 

 planar orientation of the mineral grains and the folia- 

 tion of the rock is controlled by the bedding, as has 

 been suggested by Taliaferro (1943b, p. 168), for the 

 foliated rocks are derived from sediments or tuffs, 

 whereas the nonfoliated rocks seems to be derived from 

 diabasic greenstone or massive graywacke. The glau- 

 cophane needles in some of the schists have a linear 

 arrangement for which no explanation has been found. 



CHLOKITE-IAWSONITE HOCK 



An unusual chlorite-lawsonite rock occurs near the 

 mouth of Almaden Canyon, 10,100 feet N. 57 E. of 

 the top of Mine Hill, as a single 15-foot rounded 

 boulder surrounded by graywacke. This boulder is 

 gray-green, and its surface is studded with lawsonite 

 tablets as much as 15 mm in diameter. As seen along 

 a transverse split in the boulder, the schistosity of the 

 chlorite is well developed and nearly flat in the cen- 

 tral part, but curves to follow the outline of the 

 boulder near its periphery. Although the lawsonite 



