OF THE WESTEKX ISLES OF SCOTLAND. 355 



volcanoes, has clearly pointed out the important consequences which 

 follow from the extreme liquidity of certain types of basalt ; and in 

 the Western Isles of Scotland, as in Iceland, we find many illustra- 

 tions of the contrast between the features arising from the outflow 

 of viscous lavas of acid and intermediate composition, on the one 

 hand, and extremely fluid basic lava-currents on the other. 



There is every reason for concluding that the ejection of the more 

 acid lavas which constitute thick and bulky currents, and surround 

 the five great centres of igneous action in the Western Isles of 

 Scotland, took place, as a whole, before the appearance of the mass 

 of the ophitic oliviue-basalts which form the mass of the great 

 plateaux. But, as I have already pointed out, we have evidence 

 on the one hand that occasional streams of oli vine-basalt were poured 

 out during the earlier eruptions of the andesites ; and, on the other 

 hand, we find that occasional but very interesting outflows of 

 andesite occurred during the ejection of the olivine-basalts ; and 

 such sheets of andesite-lavas are now found intercalated among the 

 basalts of the plateaux. One such outflow of andesitic lava was 

 referred to by Dr. A. Gcikie in 1871 as occurring in the Island of 

 Eigg, and I have noticed several similar in the same island. In 

 Mull they also occur ; and in the district of Mishnish, which I have 

 somewhat closely studied, a very considerable number of such ande- 

 sitic lavas have been detected by me intercalated with the basalts 

 of the plateaux. The same is also the case in Skye, and about 

 the other centres of igneous activity. 



2. '^Cupolas.'' — Under the name of " Quell-Kuppen," Dr. E. 

 lleyer* and other authors in Germany are in the habit of describing 

 more or less dome-shaped masses of lava, like the domitic-puys of 

 Auvergne, the phouolite-hills of Bohemia, and the Chodi-Bcrg and 

 similar andesitic masses in Hungary. Such " cupolas," '' domes," or 

 " mamelons" may vary in size from mere hummocks with an area 

 of only a few square yards, to mouutaim-masses of the grandest 

 dimensions. Externally such masses may have entirely lost the 

 scoriaceous covering with which, in all probability, they were ori- 

 ginallj' invested. But internally tliey often exhibit a markedly 

 concentric structure ; and there is a striking gradation from the 

 true lava-type (" hyalopilitic '' texture of rock) in the exterior 

 portion to highly crystalline (" hypidiomorphic-granular ") varieties, 

 approaching Plutonic types, in the interior. Dr. lleyer has shown 

 by plaster models how such masses were probably formed by a kind 

 of endogenous growth. Dr. A. Geikie refers to some of those 

 masses under the term " bosses." 



Among the largest and most striking of these " cupolas ■' is that 

 M'hieh constitutes the grand mountain of Beinn Talaidh(BcinnTalla) 

 in Mull, wliich, rising bo the height of 2-19 () feet above the sea, is 

 remarkable for its smooth and graceful outlines. Specimens taken 

 from the flanks and summit of this mountain show the rock to be a 

 hornblendc-andesitc in a more or less altered <*ondition. But examples 



* Thuoretische Geologic, pp. T^J-Di). 



