330 



DYNAMICAL GEOLOGY. 



296. 



vein, where tlie rock in some places becomes a mica schist through the in 

 crease in the amount of mica ; the intervals between the sheets of granite 

 are all of mica schist or gneiss. 



In the making of fissures, portions of the walls have often fallen into 

 them. A foreign mass in a vein is called by miners ahorse; while many 

 masses may make it a brecciated vein. 



Veins are often faulted (Fig. 296). The vein aa' is faulted by bb', and 

 vein 1 is in three parts from other intersecting veins. The faulting shows 

 that the vein bb' is of later origin than 

 aa' ; but not how much later, whether 

 one year or a million. The following 

 figures are from the same region as 

 the large granite veins, Figs. 293-295. 



In the pieces of the vein in Fig. 297, 

 the width varies, but this is owing to 

 an oblique shove in connection with 

 the faulting, and to the fact that the vein-sheet varies in thickness. The 

 parts in Fig. 302 may have connection inside the rock, or they may not. 

 In Fig. 301, two parallel veins, six feet apart, are represented with 

 somewhat similar, but still different, faulting. 



Veins, as well as dikes, derive part of their irregularities from lateral 

 displacements after the fracture is made. In Fig. 303, a fissure is reduced 



297. 



298. 



299. 



300. 



^ 



-^ 



301. 



302. 



-&j:d 



f 



Faulted veins from the vicinity of Valparaiso. D., '49. 



to a series of independent open spaces by the downthrow of the side to the 

 left, bringing the sides at intervals into contact. It may be illustrated in 

 a piece of paper by cutting it across in the direction of the line a in Fig. 

 304 ; then, after opening it a little, and shoving one side first to the right 

 and then to the left, the conditions in b and c will be obtained. The lesson 

 taught is this : that an interruption in a vein, or in a trap ridge, does not 

 prove interruption in the original fracture. 



