142 ON THE CHARACTERS AND DISPOSITION 



that this is the case in Barra. On the contrary, the 

 predominant veins of this substance are generally of 

 considerable, and sometimes of enormous dimensions, 

 while they continue to hold extensive courses without 

 ramification; a fact rarely occurring in the veins of 

 granite. This circumstance implies one, or both, of 

 two things in which the generation of granite and 

 trap veins have differed; gamely, a different condition 

 of the invaded strata, and a force exerted in a different 

 manner. Perhaps both of these are correlative pro- 

 positions. Numerous circumstances, elsewhere de- 

 scribed, prove a flexible condition once existing in 

 many strata; and most conspicuous in the older ones. 

 These are the rocks which granite has especially invad- 

 ed; and such limited and tortuous fissures are precisely 

 what might be expected from an imperfect rigidity 

 of the strata. The strata invaded by trap, on the 

 contrary, rarely contain indications of flexibility; and 

 thus are accounted for the greater decision and 

 straightness of the fissures into which the liquid rock 

 has flowed. It is interesting to remark how well the 

 several parts of a system hold together : and how 

 readily a true theory is applicable to phenomena 

 which had not entered into that mass of facts on which 

 it was founded. 



Most apposite confirmations of this view of the dif- 

 ferent effect of fissures on flexible and on rigid strata, and 

 on those which yield most easily compared to those that 

 oppose the greatest resistance, are contained in some 

 of the veins of the Western Islands, as in Lunga and 

 elsewhere. One of them was already necessarily no- 

 ticed, in the remarks on the contortions of rocks; but 

 while they show the different effects produced on 

 different strata, they serve to illustrate differences in 

 the forms of trap and of granite veins, which are of a 



