S28 



ON VEINS. 



5. In mica-slate, also in clay- slate, we observe 

 cotemporaneous kidneys of quartz, from a few 

 inches to many fathoms in extent : frequently 

 these masses have an elongated form, and at length, 

 by a series of gradations, become tabular ; in which 

 form, they are to be considered as cotemporaneous 

 veins, formed without any previously existing fis- 

 sure. Similar appearances occur in trap rocks, where 

 cotemporaneous kidneys and veins of greenstone 

 occur in greenstone ; and cotemporaneous kidneys 

 and veins of basalt occur in trap-tulF. 



6. In some veins, even in those of a metalliferous 

 nature, the seams of the strata are not interrupted 

 by the vein, but fairly cross it. This fact is also 

 in support of the opinion of the formation of co- 

 temporaneous veins without any previously exist- 

 ing rent. 



7. Cotemporaneous veins frequently cross each 

 other. This fact may be alleged as a proof in favour 

 of the opinion of these veins having been formed 

 successively, and in open fissures. But if these veins 

 have been formed in the manner of crystals, the 

 crossing maybe accounted for on the same principle 

 as that used for explaining the crossing observed in 

 groups of crystals. If this mode of explanation 

 should be thought satisfactory, then it would fol- 

 low, that the different venigenous, metalliferous 

 and stony formations in a district, may be of co- 

 temporaneous formation. 



8. Cotemporaneous veins are observed to occa- 

 sion shifts in the veins they cross. This also is ^ 



