86 



University of California Publications. [Geolcxjy 



colorless muscovite. Basal sections of the mineral show fine 

 cleavage lines parallel to the prism faces, the lines being very 

 numerous and much resembling twinning striations, but no real 

 twinning was observed. This highly developed cleavage parallel 

 to the prism is brought out prominently in thin sections, and as 

 a. matter of fact the prismatic cleavage of lawsonite is not an 

 easy cleavage because the plates break usually quite independent 

 of the cleavage. The average hardness of these plates seems to 

 be about 6, instead of the extreme hardness of 8. 



The chloritic outcrop upon which the boulder rests shows no 

 lawsonite, and is different in structure and composition from the 

 boulder. It is an isolated boulder, and may have been trans- 

 ported and curiously found lodgment here, although there is the 

 possibility that it is a lawsonitic phase of this particular outcrop. 

 Chloritic masses are common about the bay which have been de- 

 rived from the alteration of the actinolite-garnet schists, and the 

 origin of this lawsonite-chlorite mass may have been due to the 

 alteration of such a rock containing lawsonite, and in the change 

 the lawsonite may have become recrystallized into the platy form. 



A similar mass of chlorite occurs near San Luis Obispo. Spec- 

 imens M ere collected by Dr. Fairbanks and given to the writer 

 to determine the numerous thin glassy plates they contained. 

 The rock is identical with the one above described, being essen- 

 tially composed of chlorite, muscovite, and lawsonite. The plates 

 are thinner and much more impregnated with chlorite than in 

 the Berkeley rock. 



In a paper read before the Cordilleran section of the Geolog- 

 ical Society of America, W. O. Clark of Stanford University 

 mentioned the occurrence of lawsonite in several localities in 

 California, and in particular described a lawsonite-gneiss from 

 Eedwood. The rock consisted of quartz, glaucophane, and law- 

 sonite. with some titanite or leucoxene, and the original rock was 

 presumably a quartz diorite which occurs in the vicinity. P. 

 Thelen 10 also mentions lawsonite as a constituent of some of the 

 glaucophane schists of North Berkeley, lawsonite forming from 

 5-30 per cent, of the rock. 



"Bull. Dept. Geol. Univ. Calif., 1905, 4, 221. 



