ON COTEMPORANEOUS VEINS 



5 



beds and strata, and are observed to wedge out in 

 every direction, and consequently have no out- 

 going above, below, or laterally, intimating, that 

 they have not been rilled from above or below, 

 but are, as it were, a secretion from the rock 

 itself. These veins are denominated cotempora- 

 neous, because they appear to have been formed 

 at the same time with the rock in which they 

 are contained. 



Cotemporaneous veins occur in every moun- 

 tain-rock. Granite, which is the oldest rock hi- 

 therto discovered, contains different kinds of co- 

 temporaneous veins. Thus, some are entirely fil- 

 led with quartz ; others contain only felspar or 

 mica ; others are composed either of felspar and 

 quartz, forming a granular rock, or of felspar, 

 quartz, and mica, in the usual granitic proportion, 

 therefore forming a true granite. Thus, it ap- 

 pears, that all the constituent parts of granite oc- 

 cur either singly or together in the form of co- 

 temporaneous veins. Gneiss has the same consti- 

 tuent parts as granite ; hence we find it contain- 

 ing similar cotemporaneous veins. These veins 

 do not present the slaty structure which is one of 

 the discriminating characters of gneiss when it oc- 

 curs in strata ; hence cotemporaneous veins, filled 

 with common granular, or what may be called 

 granitic gneiss, have been confounded with true 

 granite. Mica-slate, the rock next in age to 

 gneiss, is composed of quartz and mica ; and 

 these are granular in the small, and slaty in the 



