ON MINERAL VEINS. 381 



common cause, or had been produced by the repe- 

 tition of similar actions. This also is sometimes the 

 case where more than one set of veins exists, and 

 where the posteriority of the one class is proved by 

 their intersecting the other, as in Cornwall, where the 

 more antient veins are directed, in a general sense, 

 from east to west, and the more recent from north to 

 south. 



Their longitudinal extent must evidently be limited, 

 but is often considerable. They have been traced for 

 two, and even three miles, in Cornwall ; and it is said 

 that one vein in South America has been ascertained 

 to extend for eighty miles. But such observations as 

 this excite distrust, when we advert to the comparative 

 length and breadth of such a supposed continuous 

 fissure, and to all the circumstances under which 

 these must have been formed. Inaccuracy and hypo- 

 thesis unite to produce such assertions. 



The breadth of veins is extremely uncertain, varying 

 from less than an inch to many yards. It is believed, 

 by some, that their depth is indefinite : it is at least 

 said that this has never been reached by miners. If 

 that were even true, it would not prove the truth of an 

 opinion so improbable, when we consider the circum- 

 stances under which fissures must have been formed. 

 When the separated or dislocated strata preserve an 

 accurate parallelism, the same relative disposition 

 must exist between the opposite sides of the vein ; 

 and we might thus imagine it interminable. But if 

 these have lost their parallelism after separation, it is 

 evident that under one modification of this, they must 

 come into contact in some part of the series, and that 

 the vein will therefore disappear. This reasoning 

 takes only a simple view of the consequences resulting 

 from the appearances ; but if it should be admitted 



