ON V£INS. 



227 



2. Veins that issue from larger masses of gra- 

 nite, and cut across strata of gneiss, grey-wacke, 

 or other rocks, are of the same nature, i, e. are 

 veins formed without previously existing rents ^, 

 because these masses are kidneys on the great scale. 



3. Beds of limestone sometimes alternate with 

 beds of trap, and consequently they are to be con- 

 sidered as of cotemporaneous formation. These 

 beds of limestone sometimes send off branches or 

 veins into the surrounding trap ; consequently these 

 veins are to be considered as of cotemporaneous 

 formation with the limestone, and therefore may 

 have been formed without any previously existing 

 rent. 



4. Beds of porphyry, sienite, greenstone, &c. 

 which terminate in the surrounding strata, some- 

 times, during part of their course, cross the direc- 

 tion of the strata of the rocks in which they are 

 contained, and thus acquire the character of veins : 

 but these beds are of cotemporaneous formation 

 with the bounding rocks ; hence this fact may be 

 viewed as illustrative of the formation of veins 

 without previously existing rents, and also of the 

 cotemporaneous formation of porphyry, greenstone, 

 and other veins. 



p2 



* These masses have been sometimes viewed as fragments ; 

 but that idea is inconsistent with their geognostic relations^ 

 ^nd particularly their magnitude, for there are well ascertaine4 

 instances of their extending for miles. 



