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



Sections prepared from specimens of rock taken from this 

 depth very closely resemble those from the eighty-fathom level ; 

 no fragments of hornblendic crystals, however, were observed, 

 and chloritic patches were less abundant. 



Killasfrom Polmear Mine, forty fathoms below surface. Sp. gr. 

 = 2*68. — This specimen of clay-slate is much contorted, and 

 readily divides into curved laminae with glossy surfaces, on which 

 were numerous wavy lines resembling minute ripple-marks. Its 

 colour is a medium shade of grey, in places slightly marked with 

 yellow. Its composition, as found by analysis, is as follows : — 



Water* 6*66 6-50 



Silica 49-33 49*20 



Titanic acid .... trace trace 



Alumina .... 18*00 18'00 



Ferric oxide . . . 12*63 12*73 



Ferrous oxide . . . 8*56 8*54 



Ferric persulphide . *80 '82 



Lime 2*14 212 



Magnesia .... trace trace 



Potassa *57 *56 



Soda '75 *74 



99*44 99*21 



Sections prepared from this rock do not differ materially from 

 those obtained from the Polgooth specimens. The ultimate 

 fragments of which it is composed are exceedingly minute; but 

 granules of quartz of an appreciable size, and affording colours 

 with polarized light, are disseminated throughout the fine-grained 

 matrix. A greenish tint is also imparted to it by chlorite; and 

 it is minutely divided by a system of markings made up of pairs of 

 nearly parallel lines, each about yooq °f an mcn m l en g tn an d 

 eoVo °^ an * ncn a P ar t« These cross each other so thickly as to 

 form a kind of close network, and give no colours by polarized 

 litgh: they were at first taken for amorphozoa; but my friend 

 Mr. R. Etheridge, who has kindly examined them for me, is of 

 opinion that they are <c certainly not organic/' 



Slate from "Sanctuaries" near St. Austell. Sp. gr. = 2*52.— 

 This, in common^with many other Cornish slates lying above the 

 natural drainage-level of the country, is very light in colour, the 

 prevailing tints being light grey and buff, and bears the appear- 

 ance of weathering. It adheres strongly to the tongue, has a 

 decidedly clayey smell when freshly broken, and divides with 

 difficulty into laminae, of which the surfaces are entirely without 

 polish. Its specific gravity is low ; and it is divided by headings 

 variously coloured, apparently by infiltration of water containing 

 oxide of iron. 



* Of which *93 was lost in the water-bath. 



