152 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



to rest against them, and sometimes lie in outliers on their sides and summits. From 

 the margins of these bosses veins are given off into the surrounding rocks, sometimes 

 only rarely and at wide intervals, in other places in prodigious numbers. 



The rock of which the bosses consist is generally granitoid in texture, passing on the 

 one hand, particularly in the central parts, into a truly granitic character, and on the 

 other, and especially towards the margin, into granophyre, quartz-porphyry, and various 

 compact felsitic varieties, and sometimes exhibiting along the outer edge a more or less 

 developed flow-structure. 



Decided contact metamorphism is traceable round the bosses, but is by no means 

 uniform even in the same rock, some parts being highly altered, while others, exposed 

 apparently to the same influences, have undergone little change. The most marked 

 examples of this metamorphism are those in which the Lower Silurian limestone of Skye 

 has been converted into a pure white saccaroid marble.* But the most interesting to the 

 student of volcanic action are those where the altered rocks are older parts of the volcanic 

 series. As the bosses of each volcanic area offer distinctive peculiarities they will here 

 be described geographically. 



a. Mull. — Though of comparatively small extent the bosses of the island of Mull 

 probably afford to the geologist a larger amount of instruction than those of any 

 other district. Especially important is the evidence which they contain of the true 

 relations of the acid and basic groups of rocks. They have been laid bare in many 

 natural sections, some of which, forming entire hill sides, are among the most astonishing 

 in the whole wonderful series which, laid open by denudation, reveal to us the structure 

 of these volcanic regions. They lie in two chief areas. One of these extends along the 

 northern flanks of the mountainous tract from the western side of Beinn Fhada across 

 Loch Ba' to the west side of Glen Forsa. The other occupies for over three miles the 

 bottom of Glen More, the deep valley which, skirting the southern side of the chief group 

 of hills, connects the east side of the island by road with the head of the great western 

 inlet of Loch Scridain. There are other minor areas. One of these extends for about a 

 mile along the declivities to the south of Salen, across the valley of the Allt na Searmoin ; 

 another occurs at Salen, a third runs along the shore at Craignure. In the interior also, 

 many isolated areas of similar rocks, besides thousands of veins, occur in the central group 

 of hills and valleys which form the basins of the Glencannel and Forsa rivers. 



The chief northern boss, which for the sake of convenience of reference may be 

 called that of Loch Ba', has a length of nearly six miles, with a breadth varying from a 

 quarter of a mile to about a mile and a quarter. It descends to within fifty feet of the 

 sea-level, and is exposed along the crest of Beinn Fhada at a height of more than 1800 

 feet. It chiefly consists of a grey crystalline rock which might readily be identified as 

 a granite, but which when examined microscopically is found to possess the granophyric 

 structure. But with this distinctly granular-crystalline rock are associated various 



* This marble was 1 elieved to be altered Lias ; but I have proved it by lithological, stratigraphical, and 

 pah-contological evidence to be Lower Silurian (Quart. Jour. Geol. Soc, xliv. (1888) p. 62). 



