DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



157 



group of small vents, from which, besides the usual basalt-debris, there were ejected many- 

 pieces of different felsitic (or rhyolitic) rocks, and that these eruptions of fragmentary 

 material took place during the accumulation of the plateau-basalts. The existence of 

 these volcanic funnels occasioned a line of weakness of which, in a long subsequent 

 episode of the protracted volcanic period, the acid rocks took advantage, forcing them- 

 selves upwards therein, and leaving only slight traces of the vents which assisted their 

 ascent. 



The second or Glen More boss, instead of rising into hilly ground, is confined to the 

 bottom of the main and tributary valleys, and has only been revealed by the extensive 

 denudation to which these hollows owe their origin. It begins nearly a mile below 

 Torness and extends up to Loch Airdeglais — a distance of almost four miles. Though 

 singularly devoid of topographical feature, it exhibits with admirable clearness the 

 relation of the granophyres to the gabbros, and thus deserves an important place among 

 the tracts of acid rocks in the Western Islands. Its petrographical characters change 



'^^W THTitiYj 



Fig. 48. — Section of junction of south side of Loch Ba' Granophyre boss, with the Bedded Basalts, Mull, a, bedded 

 basalts; bb, basalt-tuff and breccia; c, granophyre; d, black felsite; e, coarse dolerite dyke, 30 or 40 feet wide. 



considerably from one part of its body to another. For the most part, it is a true grano- 

 phyre, sometimes with orthoclase, sometimes with plagioclase as its predominant felspar. 

 At Ishriff, as already stated, it is sprinkled with long acicular decayed crystals of horn- 

 blende ; but at the watershed the ferro-magnesian mineral is augite. The surrounding 

 rocks are mainly the plateau-basalts, with their intruded sheets of dolerite and gabbro. 



This strip of granophyre sends abundant apophyses from its mass into the dark basic 

 rocks around it. Some of the best sections to show the nature of these offshoots are to 

 be found on the steep hill-slope which mounts from the watershed in Glen More 

 southward into the Creag na h-Iolaire (Eagle's Crag), and thence up into the great 

 gabbro ridge of Ben Buy. From the main body of granophyre a multitude of veins 

 ascends through the basalts and gabbros from 2 feet or more in breadth down to mere 

 filaments. Even at a height of 300 feet up the hill some of these veins are still 3 inches 

 broad, and present the usual granophyric structure, though rather finer in grain than the 

 general mass of the boss, and sometimes assuming a compact felsitic or spherulitic texture 



VOL. XXXV. PART 2. X 



