OF THE WESTERN ISLES OF SCOTLA.ND. 353 



Y. Relations or the Scottish Pkopylites to the othek 



lioCKS OF THE DISTRICT. 



A. Geological Age. 



That the great mass of " felstones " or propylites (as I have now 

 shown they ought properly to be called) were the earliest erupted of 

 all the rocks in the Western Isles of Scotland, I have already pointed 

 out in my memoir of 1874. 



That the propylites are older than the granitic masses of the 

 district (" granophyres " of Dr. A. Geikie) is shown by the fact that 

 the latter are seen to send off numerous veins into them, and to enclose 

 portions of them in their mass, producing all the phenomena of 

 contact-metaraorphism where in apposition with them. These are 

 all facts that I strongly insisted upon in the memoir referred to. 



That the gabbros are younger than the propylites is equally ob- 

 vious. Sheets and dykes of gabbro and doleritc connected with the 

 mountain-like masses can be traced traversing the propylites in all 

 directions, and also giying rise to the phenomena of contact-meta- 

 morphism. 



That the " felstones "' of the AVestern Isles of Scotland arc invaded 

 by the extrusions of granite (" granophyre '') and of gabbro is con- 

 firmed by many sections described by Dr. A. Geikie * : but, in con- 

 sidering the descriptions given by this author, it must be borne in 

 mind that under the same general name " bedded basalts " he has 

 confounded two totally distinct petrographical types, namely, the 

 ophitic olivine-basalts of the i)lateaux, which I described in detail 

 before this Society in 1886, and the andesites and associated rocks 

 of the central areas, of which I am treating in the present memoir. 

 He has supposed that the rocks which wc are now considering are 

 really basalts which have acquired their peculiar and distinctive 

 characters as a consequence of the metamorphism they have been 

 subjected to through contact with great intrusive igneous masses f. ' 



It will thus be seen that the great cause of the conflict of 

 opinion between Dr. xV. Geikie and myself, concerning the relations 

 of the igneous masses of the AVestern Isles of Scotland, is to be found 

 in the different interpretation we place on these propylitic rocks. 

 Dr. Geikie has clearly noticed these propylites, which he describes in 

 such a way as to avoid possibility of doubt concerning what he 

 refers to. He states that they weather, not like the basalts, but 

 with a " thin white crust, beneath which the rock appears dull, 

 black, and splintery. They are generally veined with minute thready 

 and strings of calcite, epidotc, and (juartz, which form a yellowish- 

 brown network that projects above the rest of the weathered surface. 

 Where they are amygdaloidal, the kernels no longer decay away or 

 drop out, leaving the empty, smooth-surfaced cells, but remain as if 

 they graduated into the surrounding rock by an interlacing of their 

 crystalline constituents "+. Unfortunately, however, Dr. Geikie 



* Trans. Eoy. Soe. Edinb. vol. xxxv. pp. 151-] 81. 1 Ibid. pp. 1G7, U)t<. 



+ Ibid. p. U>7. mul ])]). 77 i*s: 1G8. 



