440 SlIIANT ISLES. GEOLOGY. 



disposition, as if each group appertained to different large 

 and separate crystallizations. A similar appearance occurs 

 in the augit rock of Rum, and there, although not nearly 

 so distinct, it is evidently also produced by a crystal of 

 augit. The rock now described displays in other cases 

 on weathering, a structure of which the fracture gives 

 no indication, appearing as if it had been formed of distinct 

 globules; thus offering an interesting resemblance to the 

 results which have been obtained from the fusion and 

 slow cooling of trap. Such are the most common appear- 

 ances of this substance, but other varieties occur, par- 

 ticularly in Eilan Wirrey, of which I shall only notice 

 the most conspicuous. One of these contains minute 

 crystals of splendent stilbite, accompanied by calcareous 

 spar of a remarkably argentine brilliancy. It is, I believe, 

 a very general rule, that the several varieties of zeolite 

 occupy distinct amygdaloidal cavities in the trap rocks ; 

 but, in this case, the stilbite, as well as the calcareous 

 spar, form constituent parts of the stone, which is of a 

 hard and compact nature. The other remarkable variety 

 contains mesotype as a constituent part, equally mixed 

 throughout with the augit and felspar. 



Eilan a Kily, as far as I had access to examine it, 

 displays no beds of rocks associated with the trap: but 

 considerable strata are to be observed in the other two 

 islands, in both cases subjacent to the columnar masses. 

 About the middle of the northern high face of Gariveilan 

 a thick bed of stratified rock appears under the columns, 

 extending to the lower neck which separates it from the 

 peninsular projection formerly described. This bed con- 

 sists of an irregular alternation of different sorts of shale 

 with indurated clay and siliceous schist. The order of 

 their arrangement is much too inconstant to admit of 

 being specified to any useful purpose. The shale is some- 

 times fissile and soft, differing but little from that sub- 

 stance in its most common state of association with the 

 coal strata. In other cases it is compacted into a thick 



