THE HAWKESBURY SANDSTONE 71 
Bischof, there is no venga in which silicate of iran daes not enter. 
Protosilicate of iron or green earth is found in drusy cavities of 
many basaltic abe doleritic amygdaloids, in augite, augitic por- 
a phyry, and forming a coating upon chalcedony. Another source 
of i iron is that much of the carboniferous rocks from which some 
is in the form of a protoxide, and either colourless, bluish, or 
greenish in tint. There is a powerful affinity between silica sod 
pProtoxide of iron. The alkaline silicates, says Bischof,* pe 
te of iron in water into protos silicate of iron. The gree 
earth contains these silicates of iron and water, and gradually 
Scope showed round cavities filled with red ferric oxide. ese 
: of some highly farmnginous mineral, entirely 
by water and ca ic acid, 
: These chemical relations will appear more significant by making 
use of Mr. Sterry Hunt’s beautiful apa a The chemist 
knows that the iron as diffused in the rocks exists chiefly in 
ion with oxygen, with which i forms two principal com- 
e first or protoxide which is readily soluble in waters 
carbonic acid and other feeble acids ; and the 
. the magnetic oxide, which may be looked upon as 
ueerty a the other two, neutral and indifferent to the most 
natural chemi : 
2 matt colourless or bluish or greenish i in tint, while the peroxide 
m-rust. Ordina 
bk “id 
ys are bluish in colour, and contain cobahiinesl iron in the 
4 Mate of protowide, but when burnt in a kiln they become reddish, 
