TORSA. GEOLOGY. 147 



their superior permanence among the decomposed schists, 

 and their prismatic structure, that they must be considered 

 as parallel veins. But I need not dwell on this subject, 

 as I could only recapitulate the arguments already used 

 in support of this opinion on former occasions. Yet I 

 may briefly remark, that here also, whenever they can 

 be traced to a sufficient extent, they generally betray 

 their true nature by some interference with the strata ; 

 either crossing at some point, or sending through them 

 ramifying veins. It is also worthy of remark, that these 

 parallel veins are more common here than in the northern 

 isles where trap occurs. This appears to arise from the 

 greater facility with which rocks of the schistose struc- 

 ture yield in the direction of their laminae than in 

 the opposite one ; whence have originated the fissures 

 afterwards filled by the trap. To confirm this view, a 

 similar case will be described in Lunga; in which the 

 nature of the rocks seems evidently to have determined 

 the direction of the fissure and that of the subsequent 

 vein. 



The most remarkable of these parallel veins, both for 

 number and magnitude, are to be found at the eastern 

 side of the island. One of a great size, near the sea 

 shore, projects to a considerable altitude above the sur- 

 rounding strata, and is the seat of 'an ancient castle 

 now in ruins.* This vein is not every where parallel 

 to the strata, and seems to be a ramification from the 

 great overlying mass of trap which covers the stratified 

 rocks on the opposite shore of the mainland; the dis- 

 tance being but trifling, and many similar appearances 

 being visible on that shore. The same occurrence has 

 been already shown to be frequent, both in Kerrera 



* Enough of this building remains to show the original design, 

 which consisted of two united square towers with a circular flanking 

 defence at one of the angles. The sides are so contrived as to be con- 

 tinuous with the nearly vertical walls of the vein. 



