161 



ho small addition to the evils of a cumbrous 

 technology of colouring, that its inefficacy is 

 equal to its deformity. 



Except in a few instances, as in the jaspers, 

 the limestones, and serpentine, the colours of 

 rocks are rarely pure, or decided. The effect of 

 brilliancy, where it exists, is more commonly the 

 result of gradation or contrast. Broken tints 

 prevail ; and, among them, the endless modifica- 

 tions of grey are the most abundant. 



It is almost unnecessary to remark, that dif- 

 ferent colours are frequently intermixed in rocks ; 

 that appearance sometimes occurring in a homo- 

 geneous substance ; at others, depending on the 

 different tints of several substances variously 

 united into one mass. 



In the greater number of .rocks, the colours, 

 under all their variations, have been found to 

 result from the presence of iron in different con- 

 ditions. 



