DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 163 



far beyond the actual base of the cones, for projecting knobs as well as veins and dykes 

 of it rise up among the surrounding rocks. This may be well seen along the northern 

 foot of Beinn na Caillich. But of all the Skye bosses none exhibits its line of junction 

 with the surrounding rocks so well and continuously as Beinn an Dubhaich. This isolated 

 tract of eruptive material lies entirely within the area of the Lower Silurian limestone, 

 and its actual contact with that rock, and with the basalt-dykes that traverse it, can be 

 examined almost everywhere. The junction is usually vertical or nearly so, sometimes 

 inclining outwards, sometimes inwards. It is notched and wavy, the granophyre sending 

 out projecting spurs or veins, and retiring into little bays which are occupied by the 

 limestone. The rock of the boss is massive and jointed, splitting up into great quad- 

 rangular blocks like a granite, and weathering into rounded boulders. It is in some 

 parts a hornblende-biotite-granite, its granitic composition and texture being best seen 

 where the mass is broadest, south of Kilbride. Towards its margin, on the shore of 

 Camas Malag, the granophyric structure appears especially in narrow ribbons or veins 

 that run through the more granitic parts of the rock. 



Immediately to the south of this bay, the junction with the limestone is well displayed, 

 and the eruptive rock, which is there granitic in character, sends out into the limestone 

 a vein or dyke about two feet broad, of closer grain than the main body of the boss, but 

 showing a distinctly granitic structure. The junction on the north side is equally well 

 seen below the crofts of Torran. Here the rock of the boss, for a few yards from its 

 margin, assumes a fine-grained felsitic aspect, and under the microscope presents a curious 

 brecciated appearance, suggestive of its having broken up at the margin before final 

 consolidation. Portions of the already crystallised granite seem to be involved in a 

 microgranitic base. The rock has here truncated a number of basalt-dykes which inter- 

 sect the Silurian limestone. 



On the surface of the mass of Beinn an Dubhaich, a few little patches of limestone 

 occur to the south of Kilchrist Loch. Considering the nearly vertical wall which the 

 granophyre presents to the adjacent rock all round its margin, we may perhaps reasonably 

 infer that these outliers of limestone are remnants of a once continuous limestone sheet 

 that overlay the eruptive rock, and hence that, with due allowance for considerable 

 denudation, the present surface of the boss represents approximately the upper limit to 

 which the granophyre ascended through the limestone. The actual facts are shown in 

 fig, 53. 



All round the margin of this boss, the limestone has been converted for a variable 

 distance of a few feet or many yards into a granular crystalline marble. The lighter 

 portions of the limestone have become snowy white ; but some of the darker Carbonaceous 

 beds retain their dark tint. The nodules of chert, abundant in many of the limestones, 

 project from the weathered faces of the marble. The dolomitic portions of the series 

 have likewise undergone alteration into a thoroughly crystalline-granular or saccaroid 

 rock. The most thorough metamorphism is exhibited by portions of the limestone 

 which are completely surrounded by and rest upon the granophyre. The largest of these 



