Joseph LeConte— Genesis of Metalliferous Veins. 138 
chemical reactions by which deposit may take place. These 
may occur in any part, and determine exceptional richness in 
that part. Taking all these causes into account, we easily see 
why many veins grow poorer, some hold their own, while a few 
grow richer, in depth. 
5. Origin of the alkaline and metallic sulphides.—The ques- 
have been formed in the same way, i. e., have been reduc 
from metallic sulphates by organic matters. It is probable, 
therefore, that we never see either alkaline or metallic sulphides 
which have not been reduced from the corresponding sulphates. 
But, on the other hand, it is certain that we never see any alka- 
line or metallic sulphates which have not been oxidized from 
Corresponding sulphides. The original form was doubtless sili- 
cates. 
sulphates, sulphides, to be again changed into sulphates in per- 
yel 
worked over and over again many times, passing thus through 
4 perpetual cycle of changes. : ‘ 
6. Heat not always necessary.—We have said that the solutions 
