224 



University of California. 



IVOL. 3. 



out. This is undoubtedly the mineral which Turner first identi- 

 fied as tremolite and afterwards referred partly to edenite and 

 partly to gedrite. It is a colorless monoeliuie amphibole with 

 extinction angles not exceeding 18°, showing the oblique 

 emergence of an optic axis on (100). Turner does not give the 

 chemical and optical reactions which induced him to change his 

 identification. The only other mineral present is magnetite, and 

 this is secondary, being found only in shreds in the serpentine 

 areas of the sections. 



Chemical Composition of Peridotite. — The rock when digested 

 in a pulverized condition with strong acids fails to dissolve and 

 yield gelatinous silica, and on this account a doubt arose as to 

 the character of the mineral which on optical grounds had been 

 determined as olivine. To resolve this doubt, and also to obtain 

 information as to the chemical character of the colorless amphi- 

 bole, the rock was chemically analyzed by Dr. W. C. Blasdale, 

 to whom the writer desires to express his obligations. Dr. 

 Blasdale' s analysis is as follows: 



Si0 2 41.49 



A1 2 3 2.22 



Fe 2 3 1.07 



FeO 7.11 



MgO 39.63 



CaO 1.89 



Loss on ig. 5.56 



98.97 



From this analysis the mineralogical composition of the rock 

 has been computed to be as follows: 



Olivine 2(MgO,FeO),Si0 2 44.97 per cent. 



Serpentine 2H 2 0,3MgO,2Si0 2 33.12 " 

 Magnetite Fe 2 3 .PeO 1.39 " 



f 2H 2 0,CaO,3MgO, 4Si0 2 15.36 " 

 LMgO,Al203,Si02 4.24 " 



Edenite 



99.08 " " 



The analysis thus leaves no doubt of the peridotitic character 

 of the rock notwithstanding its failure to yield gelatinous silica 

 on digestion with acids. It reveals, moreover, the presence of 

 an aluminous molecule which is supposed, in accordance with 

 Turner's observations, to be combined with the tremolite molecule 



