164 



DR-GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



overlying patches was many years ago quarried for white marble above the old Manse 

 of Kilchrist. I have recently shown that this limestone, instead of belonging to the 

 Lias, forms a part of the Lower Silurian series, being a continuation of the fossiliferous 

 limestone of western Sutherland and Ross-shire.* 



The generally vertical line of separation between the rock of Beinn an Dubhaich and 

 the contiguous limestone has been taken advantage of for the segregation of mineral 

 veins. On the southern boundary at Camas Malag, a greenish flinty layer, from less than 

 an inch to two or three inches in width, consisting of a finely granular aggregate of 

 some nearly colourless mineral, which polarises brilliantly, coats the wall of the 

 granophyre, and also both sides of the vein which proceeds from that rock into the 

 limestone. But the most abundant and interesting deposits are metalliferous. 

 Fragments of a kind of " gossan " may be noticed all along the boundary-line of the 

 boss, and among these are pieces of magnetic iron-ore and sulphides of iron and copper. 

 The magnetite may be seen in place immediately to the south of Kilbride. A mass of 

 this ore several feet in diameter sends strings and disseminated particles through the 



Fig. 53. — Section across the north slope of Beinn an Dubhaich. aa, Lower Silurian limestone ; 



lb, basalt dykes ; c, granophyre. 



surrounding granophyre, and is partially coated along its joints with green carbonate of 

 copper. 



Relations of the Granophyre to other Members of the Volcanic Series. — It is the 

 connection of the eruptive bosses of acid rocks with the other members of the volcanic 

 series that is chiefly of interest for the purpose of the present memoir. From the Skye 

 area, important evidence is obtainable in regard to the relation of these bosses to (1) 

 earlier eruptive vents filled with agglomerate ; (2) the bedded basalts of the plateaux ; 

 (3) the sheets and bosses of gabbro and dolerite ; and (4) the great system of basic dykes. 



(1) Relation to older Eruptive Vents. — The granophyre of Beinn na Caillich and the 

 two Beinn Deargs has invaded on its north-eastern side the Lower Silurian limestone 

 and quartzite, and has truncated the sheets of intrusive dolerite and gabbro that have 

 there been injected into them. But to the south-west it rises through the great mass of 

 agglomerate already described, and continues in that rock round to the entrance into 



* Quart. Jour. Geol. Soc, vol. xliv. (1888) p. 62. 



