Ch. IX.] 



VEINS IN PRIMARY ROCKS. 



171 



conclusion, because veins generally taper away and terminate 

 in strings : and, in many instances, by following these strings, 

 they have found them to enlarge again into a workable vein ; 

 or because, many mines having been worked on the con- 

 tinuation of the same line, they have supposed that the vein 

 is one and the same in all these places. On the other hand, 

 veins have been often found entirely to die away, or dis- 

 appear, not only in mines, but also in the rocks on the sea- 

 shore : and even if such examples had not been detected, 

 the extent of mines is comparatively insignificant to warrant 

 the assertion, that veins are indefinite in length and depth. 

 Granting, however, that some veins are of considerable and 

 unknown dimensions, still there will be found no boundary 

 between these and those smaller veins which have been com- 

 monly called contemporaneous. 



An examination of two or three examples of veins on a 

 large scale will, however, convey a better idea of their nature; 

 and, in order to avoid any mistakes, we will select those 

 which traverse both the granite and the slate, and which, 

 therefore, cannot be considered as contemporaneous by the 

 supporters of the prevailing Plutonic theory. 



In the first place, on the S. E. side of St. Michael's Mount, 

 some metalliferous veins may be easily examined when the 



tide is out. 



Fig. 11. 



y////////, 



Quarlx veins at St. Michael's Mount, 

 (Ground Plan.) 



