INTERSECTION OF VEINS. 407 



Intersection of Veins. The most simple case is when two straight 

 veins cross without any change of direction, or any lateral displace- 

 ment; and the order of effects appears to be the production of a 

 fissure, and the filling of this by a vein which was afterwards broken 

 through by another fissure, and this, in its turn, received another 

 mineral vein. It seems difficult to doubt the truth of this explana- 

 tion ; for if the vein which cuts through the other be subsequent to 

 the fissure in which itself lies, it must also be subsequent to the vein 

 which that fissure divides. The occasional complication of the 

 problem by the number of intersections does not at all change its 

 nature. 



Appearances at the Crossing. It is sometimes observed that 

 the vein which upon this theory is the oldest, suffers a particular 

 kind of accident at its junction with the other. It is divided 

 into several branches on one or both sides of the cross veins, and 

 these branches enclose portions of the neighbouring rocks. There 

 is some difficulty in this case, however it be considered, but the 

 coincidence of this splitting of a vein with the crossing of another 

 vein may often be only accidental ; for such splitting frequently occurs 

 in a wide vein, far from any cross course, as in a fault. 



The fissures which have received the mineral veins are in most 

 cases accompanied by slips or dislocations of the strata in a vertical 

 direction, and the veins are of course subject to the same accidents of 

 displacement. When two veins cross, and both are vertical, the lines 

 of bearing of the two portions of the displaced vein must remain 

 coincident after the fracture. If the divided vein be not vertical, its 

 separated portions will have their lines of direction parallel, but not 

 coincident ; and in any horizontal plane they will appear to have 

 sustained a lateral movement. 



Thus in the diagram (fig, 79), & ' * 



the cross vein a and the divided I /^ \\ 



vein b are both vertical; but the ^ II // *" Jff- 



divided vein c is inclined in the " *"'* I // \\ 



direction of the arrows, and its 



apparent lateral displacement is 



really due to a vertical movement. Fi g . 79- 



If two divided veins are inclined 



in opposite directions, and be dislocated by the same cross vein, they 



will appear to have moved laterally in opposite directions, as c and d. 



Were we to include the cases of the inclined cross veins, and also 

 those where the inclination of these veins varies both in amount and 

 direction, the results would become too complicated for explanation 

 without mathematical symbols ; and we must, besides, remember that 

 the displacement of the strata is really very seldom in a vertical direc- 

 tion, but generally accomplished by an angular movement from some 

 fixed point, or round a virtual centre. 



Geographical Relation of Veins. Werner long ago indicated that 

 mining districts are almost entirely confined to the vicinity of moun- 

 tains or elevated land, because in these situations the rocks were most 



