JURA. GEOLOGY. 



appears to present no peculiarities requiring further 

 detail. 



The trap veins of Jura are conspicuous both for their 

 great size and continuous extent. It has so often been 

 observed in the description of the preceding islands, that 

 these veins were chiefly predominant in the vicinity of the 

 large overlying masses of trap, and diminished in number 

 as they receded from these, that the present exception 

 appears unaccountable. It will shortly be seen that they 

 are equally conspicuous and abundant in Isla ; and it is 

 further apparent that the same veins are prolonged between 

 both the islands, their general correspondence being visible 

 on the opposite sides of the sound that separates the two. 

 It is further important to remark, that they are compara- 

 tively rare in the northern extremity of Jura, as they are in 

 Scarba. Thus, in tracing them from that extremity of the 

 island towards the south, they at first appear to diminish, 

 according to all former experience in these islands, as they 

 recede from the great mass of Mull and the Slate isles. 

 It is not therefore without some surprise that they are 

 again found suddenly to increase at the division between 

 Isla and Jura; and to be there accumulated, as if diverging 

 from some common central mass ; both the opposed shores 

 presenting a continued succession of them, very conspicu- 

 ous for their size and for their permanence among the sur- 

 rounding strata. An expectation is thus naturally excited 

 of finding, either in one or the other of these islands, some 

 extensive mass of this rock. None such, however, exists 

 in either. But in the sound between the two, is found a 

 small green islet, named Glas island, which might easily 

 have passed unnoticed, as a fragment of the stratified rocks. 

 This spot consists of trap, chiefly of an amygdaloidal 

 character, containing nodules of calcareous spar and of 

 heliotrope ; a variety that does not occur among the veins. 

 In examining the sea rocks in the vicinity, with the view 

 of ascertaining the possible extent of this mass, they 

 are all found to be of the same composition ; and it is* 



