26 
METALLIC BASES. 
Gneiss 
Mica slate 
Hornblende rock 
Ctilorite slate 
Talcose slate . 
Compact feldspar 
Granite . 
Schorl rock 
Greenstone 
Hypersthene rock 
Basalt 
Pitchstone 
Serpentine 
Diallage rock 
per cent. 
70 to 71 4- 47 oxygen. 
61 73 + 44 do. 
54 
63 
78 
51 to 60 + 46 oxygen. 
63 74 + 48 do. 
68 
54 + 43 oxygen. 
59 to 61 
44 " 59 4- 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 
(( 
Hornblende rock 
15 
it 
Chlorite slate . 
8 
it 
Talcose slate . 
13 
a 
Compact feldspar 
30 
it 
Granite 
14 
it 
Schorl rock 
17 
a 
Greenstone 
15 
tt 
Hypersthene rock 
10 
tt 
Basalt 
16 
it 
Pitchstone 
11 
it 
Diallage rock • 
13 
a 
