234 STRUCTURAL GEOLOGY. 



from solution, the materials of veins are in a purer and 

 more sparry condition than they exist in the rocks. * 



It is evident that, as thus denned, veins must vary 

 greatly in appearance. Sometimes they are fine lines, the 

 fillings of small cracks produced by rock-crushing. Some- 

 times they are the fillings of larger joints. Sometimes 

 they are the fillings of great fissures breaking through the 

 earth-crust. It is these last which are far the most im- 

 portant ; and it is only on these, therefore, that we shall 

 dwell. 



Fissure-Veins. As these are the fillings of those 

 great fissures which are formed by crust-movements, 

 they are of great extent. The fillings of such fissures at 

 once with fused matters are called dikes (page 216) ; the 

 fillings by slow deposit of mineral matter are fissure-veins. 

 These veins, therefore, like fissures (page 229), of which 

 they are the fillings, are often many miles in extent, 

 many feet in width, and of unknown but certainly many 

 thousand feet in depth. Like fissures, they occur in sys- 

 tems, parallel to each other and to the axis of elevation 

 of the mountain where they occur. Between the vein 

 and the wall-rock 011 either side there commonly exists a 

 layer of clay called the selvage. It is very characteristic 

 of true fissure-veins, and probably produced by the solvent 

 effect on the wall-rock of water circulating between the 

 vein and the wall. 



Metalliferous Veins. Metals may occur in beds, for 

 example, iron (page 299), or filling cavities of any kind 

 in rocks, as sometimes lead. But they most commonly 

 occur in veins, especially fissure-veins. The further de- 

 scription of fissure-veins is best undertaken, therefore, 

 under this head. 



Contents. We must not imagine that metalliferous 

 veins are filled with metals. The fillings of fissures are of 

 two kinds, viz., the vein-stuff, vein rock, gangue, or matrix 

 (as it is variously called), and the metallic ore, By far 



