76 GENERAL COMPARISON OF 



in fact consists in the very nature of these rocks, and 

 in the mode in which they were formed, nor is there at 

 present any prospect of resolving it. It is unnecessary 

 to recapitulate the few points which appeared, in some 

 of the islands, to prove that there were marks of difference 

 in the relative ages of some portions of these rocks; as 

 they cannot be placed in a more luminous point of view. 

 Further acquisitions of knowledge on this subject may 

 possibly hereafter diminish or remove these difficulties; 

 and until that period shall arrive, we must unavoidably 

 remain in doubt respecting many important particulars 

 in the history of the trap series. Yet it has appeared from 

 the examination of Canna, that there have been repetitions, 

 in the same place, of successive formations of trap ; the 

 distinction being there made by the conglomerates inter- 

 posed. It will hereafter be seen that, in the Schistose 

 isles, some phenomena occur which appear to prove the 

 same fact ; a circumstance which is further supported by 

 the evident repetitions of trap veins in one place, of which 

 Airdnamurchan presents some striking examples. As- 

 suming that to be generally true, it may follow that many 

 of the trap rocks of Sky, as well as of the other islands, 

 are of different ages ; that the hypersthene rock of the 

 Cuchullin, for example, differs in time from the syenite and 

 from the stratified traps of the western shore ; but proofs 

 to this effect are not to be obtained, nor is it at present 

 easy to conjecture whence they may ultimately be derived. 

 It has on many occasions been here shown, that substances 

 differing greatly in composition were continuous ; so that 

 no proof of difference in time is contained in difference of 

 character. It has equally been proved that the con- 

 nexion of these rocks with the strata, whether primary or 

 secondary, gave no indications of such a distinction in 

 time. In examining the causes of these difficulties, they 

 will be found to consist partly in the mode in which these 

 rocks have been formed, and partly in the period, rela- 



