MINERAL VEINS. 



237 



thus formed was filled with cupriferous quartz. The same is well shown 

 in Fig. 212, where a, b, c, d, e, /, are successive quartz-combs, separated 

 by 2, 3, 4, 5, 6, which are clay selvages, and therefore old walls. 





l 



II 



1 t 



l=-= 



— _- 





~-~< 



ml 







-_-ij 



'1 



i nr 



i 



~—~ 



^ 3 



Fig. 211. 



Fig. 212. 



Irregularities. — Although more regular than other kinds, yet fis- 

 sure-veins are also often quite irregular — sometimes branching, some- 

 times narrowing or pinching out in some parts and widening in others 

 (Fig. 213), sometimes dividing and again coming together, and thus 

 inclosing a portion of the wall-rock (Fig. 214). Such an inclosed mass 



Fig. 213.— Irregularities in Veins. 



Fig. 214. —Irregularities in Veins. 



of country rock in the midst of a vein is called a "horse" Many of 

 these irregularities are probably the result of movements after the fis- 

 sure was formed, or even after it was filled. Thus, if a b c d (Fig. 213) 

 be one wall of an irregular vein, then it is probable that a' V d d' was 

 the original position of this wall ; but, before it was filled, it slipped 

 up to its present position. Or, an open fissure may pinch together in 

 places by what is called creeping of the strata of the wall, i. e., a mash- 

 ing and filling in by pressure of superincumbent weight. Again, 

 movements may reopen a fissure after it is filled. In such cases, if the 

 adhesion of the filling to the wall is strong, portions of the wall-rock 

 are torn away ; and, if a second filling takes place, a " horse " is formed. 

 Thus a a a and bbb (Fig. 214) represent the two original walls of an 

 irregular vein ; but subsequent movement reopened the fissure to V V V 



