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



quantities of slates and slabs of good quality, which are exten- 

 sively used by builders in the west of England. 



The roofing-slates of best quality cleave very readily into thin 

 laminse, and are of a grey colour, thickly speckled with a lighter 

 shade of the same. The second-quality slates have a greenish 

 tint, and are spotted with a lighter shade of the same colour ; 

 some of the seams are uncleavable, and rendered utterly worth- 

 less for roofing-purposes by being much twisted and interfoliated 

 with small lenticular deposits of quartz. Some of the surfaces, 

 and particularly those indicating the planes of bedding, are co- 

 vered by a thin crystalline film of calcite. 



A specimen of " best roofing-slate " afforded by analysis the 

 following percentage results : — 



I. II. 



Water* 4*62 4*60 



Silica 58-25 5835 



Titanic acid ... *23 -23 



Phosphoric acid . . trace trace 



Alumina .... 21-74 22*04 



Ferric oxide . . . 7*15 6-96 



Ferrous oxide . . . 2*57 2*57 



Lime -40 -39 



Magnesia .... 1-09 1*10 



Potassa ..... 2-44 2'45 



Soda 1-04 1-23 



99-53 . . 99-92 



This slate, under a low power, does not show any distinct 

 structure, but is seen to be thickly dotted with dark spots. 

 When examined by the help of a high power its structure still 

 remains indistinct, but numerous clusters, about y^o of an inch 

 in diameter, of reddish- brown crystals become distinctly apparent. 

 They are generally opaque ; but some of them, which have been 

 ground very thin during the preparation of the sections, appear 

 to result from the aggregation of several hexagonal plates : these 

 crystals are probably micaceous ironf . 



In addition to the crystalline nodules above referred to, the 

 matrix of this slate is traversed in all directions, but chiefly lying- 

 in planes parallel with the cleavage, by long transparent crystals 



* Of which "35 was lost in the water-bath. 



t Having been unable to arrive at a satisfactory conclusion with regard 

 to the exact nature of the dark crystalline bodies occurring in this rock, I 

 forwarded a section to Mr. H. C. Sorby, F.R.S., with a request that he 

 would be kind enough to examine it. After having done so, he writes as 

 follows : — " The dark crystals in the slate from Delabole are similar to what 

 I have seen in many others. As far as I can make out, they are imperfect 

 hexagonal plates j and since the general character clearly shows that they 

 must at all events contain much iron, it seems very probable that they may 

 be more or less altered specular iron. This, of course, would well agree with 

 the fact of its containing titanic acid." 



