234: STRUCTURE COMMON TO ALL ROCKS. 



Thus where fissures are formed by lateral pressure or crushing, re- 

 verse faults are formed ; but where they are formed by lateral tension 

 or stretching, normal faults are formed.* 



Section 2. — Mineral Veins. 



All rocks, but especially metamorphic rocks in mountain-regions, 

 are seamed and scarred in every direction, as if broken and again 

 mended, as if wounded and again healed. All such seams and scars, 

 of whatever nature and by whatever process formed, are often called 

 by the general name of veins. It is better, however, that dikes and so- 

 called granite-veins, or all cases of fissures filled at the moment of 

 formation by igneous injection, should be separated from the category 

 of veins. True veins, then, are accumulations, mostly in fissures, of 

 certain mineral matters usually in a purer and more sparry form than 

 they exist in the rocks. The accumulation has in all cases taken place 

 subsequently to the formation of the fissure, and by a slow process. 



Kinds. — Thus limited, veins are of three kinds : Veins of segrega- 

 tion, veins of infiltration, and great fissure-veins. These three, how- 

 ever, graduate into each other in such wise that it is often difficult to 

 determine to which we must refer any particular case. Some writers 

 make many other kinds, but these may be regarded as intermediate 

 varieties. 



1. Veins of Segregation. — In these the vein-matter does not differ 

 greatly from the inclosing rock. Such are the irregular lines of granite 

 in granite, the lines differing from the inclosing rock only in color or 

 texture ; also irregular veins of feldspar in granite or in gneiss. Under 

 the same head belong also the irregular streaks, clouds, and blotches, so 

 common in marble. In these cases there seems to be no distinct line of 

 separation between the vein and the inclosing rock — no distinct wall to 

 the vein. The reason is, these veins are not formed by the filling of a 

 previously-existing fissure, but by the segregation of certain materials, 

 in certain spots and along certain lines, from the general mass of the 

 rock, either when the latter was in plastic condition from heat and 

 water, or else by means of percolating water, somewhat as concretions 

 of lime, clay, iron-ore, and flint, are formed in the strata (p. 188). 



2. Veins of Infiltration. — Metamorphic rocks have, probably in all 

 cases, been subjected to powerful horizontal pressure. Besides the wide 

 folds into which such rocks are thus thrown and the great fissures thus 

 produced, the strata are often broken into small pieces by means of the 



* Reade has shown (Origin of Mountains, chap, viii) that crust-blocks formed by ten- 

 sion and resting on any kind of yielding foundation, whether solid or liquid, would settle 

 so as to form normal faults. It is probable, therefore, that smaller faults of this kind 

 may be formed without a sub-crust liquid. 



