. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 irregu
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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
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