OF IRON-STONE. VICKERY. 215 



These figures show that considerable variation in com- 

 position may occur and the low proportion of volatile matter 

 and sulphur and the high silica are significant of the effect of 

 the granite intrusion upon the slate in its immediate vicinity. 



Although the microscope reveals the presence of feldspar, 

 it is in such small quantities that no effort was made to 

 estimate the alkalis. 



The following tables give the analyses of typical slates. 



A black roofing slate from Vermont gave:* 



Silica 59.7 



Combined Oxides: Alumina 17.0 

 Ferrous Oxide 4 . 9 



Ferric Oxide. . .5... 22. 4 



Magnesium Oxide 3.2 



Calcium Oxide 1.3 



Alkalis 5.2 



Water 4.1 



Other Oxides and Carbon 4.3 



100.3 



A slate from Wales gave: 



Silica 60.5 



Combined Oxides: Alumina 19.7 

 Ferrous Oxide. 7 . 8 ... 27 . 5 



Magnesium Oxide 2.2 



Calcium Oxide 1.1 



Akalis 5.4 



Water 3.5 



100.2 



* Pirrson's Rocks and Rock-Minerals, page — 



