OF THE WESTERN ISLES OF SCOTLAND. 



373 



I 



Xot less interesting and remarkable are the sheets of lava with 

 the ii;reat masses of underljing tuffs that form the mountain of 

 Ben Hiant (Beinn Shiant) in Ardnamurchan. The general struc- 

 ture of this mountain will be understood from the accompanying 

 map and section, which have been constructed on the basis of the 

 recently published 6-inch maps of the Ordnance Survey (figs. 1 & 2), 

 see pp. 374 and 375. 



Lying in part upon the much eroded basalt of the plateau, in part 

 on the underlying Jurassic strata, and in part on the fundamental 

 crystalline schists, we find thick masses of volcanic agglomerate. 

 These volcanic agglomerates can be especially well studied on the 

 northern face of the mountain, and on the southern sea- washed 

 promontory known as Sron Mhor, or Maclean's jSTose. They 

 vary in character from ordinary andesitic tuffs, to very coarse 

 breccias made up of fragments derived from all the underlying rocks 

 (crystalline schists, liassic shales, sandstones, and limestones, and 

 andesitic and basaltic lavas), with varying quantities of other vol- 

 canic materials. 



The preservation of these agglomerates has been clearly due to 

 the fact that they were covered by currents of a peculiar lava, often 

 of a columnar habit, which cap all the spurs of this singularly 

 outlined mountain ; only the lower and more crystalline portions of 

 these lava-currents having in most cases escaped removal bj^ denu- 

 dation. These lavas are found by careful study to be an andcsite 

 presenting many remarkable varieties, to which I propose to call 

 especial attention in the sequel *. 



Tlie rocks of Ben Hiant find their closest analogues in the 

 augite-andesites of the Tertiary dykes of the north of England 

 ("tholeites" of Rosenbusch), so well described b}^ Mr. Teall, and in 

 the rock of Eskdalemuir, for a very careful and accurate account of 

 which we are indebted to Dr. A. Geikie. Mr. Grenville Cole, P.G.S., 

 has kindly supplied me with a series of specimens collected by him 

 from the Eskdalemuir rock, which have proved of great service 

 to nie in my comparisons. 



Dr. A. Geikie has shown that in the case of the Eskdalemuir 

 dyke a marked separation has often taken place between the more 

 acid, vitreous parts of the rock and the more basic, crystalline 

 materials. In consequence of this, as shown by Mr. Grant 

 Wilson's analyses, different portions of the dyke come to present 

 wndc divergencies in composition and appearance f. 



* Tlie main features of the remarkable mass of Ben Hiant were accurately 

 described by me in 1874. Dr. A. Geikie, as the result of what must surely have 

 been a superficial examination of tlie locality, asst^rts that the lava of Beii Hianr 

 is a single intrusive sheet, and that it cousists of the ordinary ophitic dolerite 

 of the "sills" that were erupted at the same time as the plateau-basalts. My 

 shitemeut of 1874 that the rocks consist of a very remarkable au<;ite-audesite 

 is borne out by the microscopic study of a very large series of specimens 

 derived from various parts of the mountain ; and the statement that the rock 

 is neither glassy nor vesicular is contradicted by the study of this large series 

 ofr(jcks(see Plate XV.). 



t Pro.-. Roy. Phvs. Soc Edinb. vol. v. .^1880) p. 2oG: and Trans. Roy. Soc 

 Edinb. vol. xxx. (1888) pp. 40-44. 



Q. J. G. S. No. 183. 2 E 



