﻿90 Mr. J. A. Phillips on the Chemical Composition and 



I. II. 



Water* 4*16 4*00 



Silica 60*45 60-39 



Titanic acidf *21 *21 



Alumina 20*67 21*00 



Ferric oxide 8*21 8-13 



Ferrous oxide . . . . . 1*91 1*87 



Manganoso-manganic oxide . -43 -39 



Lime 1-86 1*56 



Magnesia ....... trace trace 



Potassa -74 -80 



Soda 1-56 1-54 



100-20 99-89 



Thin sections of this slate, when examined by transmitted 

 light under a low power, exhibit no decided evidence of struc- 

 ture, but appear as a milky-white mass in which are dissemi- 

 nated numerous moss-like semi-crystalline markings of a brown- 

 ish-green colour. They are also observed to be traversed by 

 various fissures, which have become filled by crystalline transpa- 

 rent quartz. When examined by the aid of a high power, this 

 rock is found to be composed of an aggregate of minute granules 

 intimately blended together and without definite outlines. There 

 are also some grains of oxide of iron and other dark markings 

 not sufficiently opaque for this substance ■ some of these are ap- 

 parently fragments of hornblende, whilst others are probably 

 patches of a chloritic mineral. 



This slate has in fact undergone so great a change, that the 

 individual grains, of which it was originally composed, can only 

 be made out by means of the polariscope. When the prisms are 

 crossed, the mass breaks up into a kind of shadowy mosaic of 

 light and dark or variously coloured patches without definite 

 outlines. 



Killasfrom Polgooth Mine, eighty-fathom level Sp. gr. =2*74. 

 —This specimen is much harder than that from the adit-level, 

 and is of a grey colour, strongly marked with reddish brown. 

 Its analysis gave the following results : — 



* Of which l'OO was lost in the water-bath. 



t The amount of titanic acid in these rocks being exceedingly small, it 

 was determined by weighing the total quantity obtained from both ana- 

 lyses. This having been divided by two, accounts for the exact accord- 

 ance of the figures in the two columns of results. 



