26 



METALLIC BASES. 



Gneiss 

 Mica slate 

 Hornblende rock 

 Chlorite slate 

 Talcose slate . 

 Compact feldspar 

 Granite . 

 Schorl rock 

 Greenstone 

 Hypersthene rock 

 Basalt 

 Pitchstone 

 Serpentine 

 Diallage rock 



per cent. 



70 to 71 -j- 47 oxygen. 

 61 " 73 -f 44 do. 



54 



63 



78 



51 to 60 + 46 oxygen. 

 63 " 74 4- 48 do. 



68 



54 -f- 43 oxygen. 

 59 to 61 



44 " 59 + 43 oxygen. 



72 " 73 

 42 " 43 

 58 " 60 



In pure rock crystal or quartz, silica is nearly 

 the sole ingredient. In limestone we often find sil- 

 ica disseminated ; and this is the reason why such 

 limestone does not form good lime ; the silica, by 

 burning, combines with the lime, forming silicate of 

 lime* Silica, in the form of rounded flint nodules, is 

 often met with in beds of chalk ; in many places these 

 nodules constitute one third of the whole mass. 



Aluminium is the next most important base of the 

 earths on the surface of the earth. It is diffused as 

 widely, though not so abundantly, as silicium. It 

 constitutes the base of the various clays and argil- 

 laceous slates, and often exists in limestone, ren- 

 dering it very valuable, in consequence, for water- 

 setting purposes. It exists in the rocks enumerated 

 beneath in the following proportions : 



Gneiss 



15 per cent. 



Mica slate . 



15 



a 



Hornblende rock 



15 



a 



Chlorite slate . 



8 



it 



Talcose slate . 



13 



« 



Compact feldspar 



30 



c< 



Granite 



14 



u 



Schorl rock 



17 



u 



Greenstone 



15 



it 



Hypersthene rock 



10 



it 



Basalt 



16 



it 



Pitchstone 



11 



«< 



Diallage rock . 



13 





