220 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



The slides from all the localities mentioned are so similar that the 

 conclusion is unavoidable that all these quartz glaucophane gneisses 

 were made from the same material, either a chert, or a very acid 

 sandstone. The rock can hardly be called a quartzite, for while it is 

 composed largely of quartz, there is no proof that it was made of 

 rolled quartz grains. 



3. Lawsonite Gneiss. — Lawsonite has long been known as an 

 important mineral in the glaucophane schists of California, but has 

 only recently been found to be a dominant mineral in some of these 

 rocks. The rocks in which it is found as a dominant mineral are 

 massive and gneissic, and although they contain no feldspars their 

 structure is such that they must be called gneisses. They are known 

 at present at only three localities. The first found is at Guerneville 

 in Sonoma County, where bands of massive gneiss are apparently 

 interbedded with lawsonite glaucophane schist. The rock is com- 

 posed of crystals of glaucophane imbedded in a groundmass of 

 lawsonite, the latter mineral showing few crystal outlines. 



The most interesting lawsonite gneiss yet found occurs about 

 three miles southwest of Redwood. It is massive and banded, show- 

 ing a groundmass of lawsonite crystals, with a thick felt of com- 

 pact prisms of glaucophane. Titanite is scattered through the rock 

 in irregular patches, and the whole is seamed with quartz veins. 

 Some layers show many small garnets. No trace of the original 

 feldspars and hornblendes or pyroxenes is left, every mineral in it 

 being secondary. The rock is composed of approximately one- 

 fourth lawsonite, one-third glaucophane, one-third quartz, and one- 

 twelfth garnet, titanite and mica. The total silica in the rock is 

 65.91 per cent., and originally there could have been very little free 

 silica. The original minerals were probably oligoclase, hornblende 

 and a little quartz, but in recrystallizatiou moie basic minerals 

 were formed, setting free a large amount of silica which crys- 

 tallized out in quartz veins. 



An analysis of this rock was made by Mr. W. O. Clark, assistant 

 in mineralogy at Stanford University, and this is quoted below. 



The similarity of the analyses of the completely recrystallized 

 lawsonite gneiss and of the slightly altered quartz diorite shows that 

 the original material was the same in both cases. 



