ON VEINS. 



II. On Veins. 



Two opinions prevail at present in regard to the 

 formation of veins : According to the one, nearly 

 all veins have been formerly open rents, which 

 have been filled from above with the mineral sub- 

 stances they now contain ; according to the other, 

 these rents have been filled from below by the 

 agency of subterranean heat. The latter opinion, 

 I have always considered as untenable; and the 

 former, although the most plausible, has, I sus- 

 pect, been too much generalized. I am now in- 

 clined to believe, that many veins said to have 

 been filled from above, are of cotemporaneous for- 

 mation with the rocks in which they are contain- 

 ed, and may have been in many instances formed by 

 the mere shooting or crystallizing of the venigenous 

 matter across the direction of the strata, and 

 therefore do not owe their origin to any previous- 

 ly existing rent. This opinion is illustrated by 

 the follov/ing facts and observations : 



1. In the kidneys of granite in gneiss, admitted ^ 

 in every theory to be of cotemporaneous forma- 

 tion with the rock in which they are contained, || 

 we observe the granite shooting from the kidney 

 in the form of veins into the bounding rock. This, 

 then, is an example of the formation of vein^ 

 without previously existing fissures or rents. 



