238 



STRUCTURE COMMON TO ALL ROCKS. 



and tore away the horse H, after which the vein was again filled. Also 

 crust-movements may form not only a single clean fissure, but some- 

 times many small, irregular fractures, with wall-rock- between. The 

 filling of these, form irregular veins in which vein-stuff is often inex- 

 tricably mingled with country rock. The vein may thus be filled with 

 a troop of horses. Finally, in some rocks, especially limestone, percolat- 

 ing waters will hollow out passages in the most irregular way. These 

 also may become filled with vein-stuff and give rise to irregular veins. 

 Veins, of course, usually intersect the strata ; but in some cases 

 where strata-planes are highly inclined the opening is between these 

 planes, and the veins are, therefore, conformable with 'them. 



Age. — The relative age of veins in the same region is determined 

 in the same way as that of dikes, viz., by the manner in which they 



intersect each other; the in- 

 tersecting vein being, of course, 

 younger than the intersected 

 vein. Thus in Fig. 215, which 

 is a section of a hill-side in 

 Cornwall, it is evident that the 

 tin vein, a, is the oldest, since 

 it is intersected and slipped by 

 all the others. The copper- 

 vein, Z>, is older than the clay- 

 filled fissure, c. There is a 

 fourth fissure, <:/, newer than 

 a, but its relation to b and c is not shown in the section. 



The absolute age of fissure-veins, or the geological period in which 

 the fissure was formed, can only be determined by the stratified rocks 

 through which it breaks. The lead-veins of Cornwall (b b, Fig. 217) 

 break through the Cretaceous. Their fissures were probably formed by 

 the changes or oscillations which closed the Cretaceous and inaugurated 

 the Tertiary period. The auriferous veins of California break through 

 the Jurassic ; and, as there are good reasons for believing that the Sierras 

 were formed at the end of the Jurassic, it is probable that these fissures 

 were formed at that time by the foldings of the strata consequent upon 

 the pushing up of this range. The filling, of course, was a slow, sub- 

 sequent operation, but commenced then. 



Surface-Changes. — Mineral veins seldom or never outcrop on the 

 surface in the condition we have described them. On the contrary, 

 there are certain changes which they undergo through the influence of 

 atmospheric agencies, which render their appearance along their out- 

 crop quite different from that of the same vein at some depth below. 

 A knowledge of these changes is, of course, of the greatest practical 

 importance. They are, however, extremely various, differing not only 



Fig. 215. 



