﻿100 Mr. J. A, Phillips on the Chemical Composition and 



schistose ; and it exhibits numerous resinous headings, of the kind 

 which is so characteristic of rocks of this class. Many of these 

 are coated by asbestus, whilst in some of the fissures, and par- 

 ticularly in the cross headings, crystalline calcite is of frequent 

 occurrence. Saponite is also present in the form of reniform 

 concretions, which are most frequently found in a clay, appa- 

 rently resulting from the decomposition of the principal rock. 

 This stone slightly affects the magnet, is only moderately hard, 

 and when recently broken is of a dark green colour with a some- 

 what crystalline fracture. Its chemical composition was found 

 to be as follows : — 



I, II. 



Water* 10-66 10-46 



Silica 38-60 38-80 



Titanic acid . . . trace trace 



Phosphoric acid . . trace trace 



Alumina .... 17'58 17'60 



Ferric oxide . . . 14-98 15-10 



Ferrous oxide . . . 4*62 4*50 



Chromic oxide (Cr 2 3 )f -14 -14 



Oxide of manganese . trace trace 



Lime 5*04 4-92 



Magnesia .... 5-97 6-04 



Potassa trace trace 



Soda -84 -85 



98-43 98-41 



The microscope shows this to be a highly metamorphosed rock, 

 consisting of an amorphous matrix porphyritically enclosing yel- 

 lowish-brown or green patches with indistinct crystalline forms, 

 which are evidently pseudomorphs. There are also many black 

 grains of magnetite, and crystals of some pyroxenic mineral — 

 probably schiller-spar or diallage. 



This appears to afford an illustration of the gradual produc- 

 tion of serpentine, by the absorption of magnesia &c, by strati- 

 fied and other rocks ; in the same way the analyses of the two 

 specimens of killas from Botallack would seem to indicate that 

 some of the clay-slates in that district may have become to a 

 certain extent affected by a serpentinous metamorphism {. 



* Of which 2*17 was lost in the water-bath. 



t Only one estimation made. 



% As an instance of the comparatively recent production of a magnesian 

 mineral by the agency of water, it may be mentioned that at the mining 

 town of Washington, California, some of the pebbles of an ancient river- 

 bed of post-tertiary age have become embedded in asbestus, in the form of 

 long, easily separable crystals. This is probably the result of metamorphic 

 action on a plastic clay in which the boulders were originally enclosed, and 



