138 DR GE1K1E ON THE HISTORY OF VOLCANIC ACTION 



which when poured out at the surface became the plateau-basalts, and he represents 

 them in his map and sections of Mull as ramifying through the granitic rocks.'"" 



In Mull some peculiarities in the arrangement of the gabbro are better developed 

 than elsewhere. Instead of forming a huge boss with an amorphous centre and a fringe 

 of intrusive sheets, as in Skye and Ardnamurchan, the rock is distributed in innumerable 

 beds or sheets interposed between the plateau-basalts. The area within which this 

 chiefly occurs is tolerably well-defined by the difference of contour between the long- 

 terraced uplands of the ordinary basalts and the more conical forms of the southern 

 group of hills between Loch na Keal and Loch Spelve. The number and thickness of 

 the gabbro-sheets increase as we proceed inwards from the basalt-plateau. These sheets 

 are specially prominent along the higher parts of the ridge that runs northwards from 

 the northern end of Loch Spelve, and along the west side of Glen Forsa. But they swell 

 out into the thickest mass in the south-western part of the hilly ground, where from 

 above Craig, in Glenmore, they cross that valley, and form the rugged ridge that rises 

 into Ben Buy (2354 feet), and stretches eastward to near Ardara. It is in this 

 southern mass that the Mull gabbro approaches nearest in general characters to the bosses 

 of the other districts. But even there, its true intercalation above a great mass of 

 bedded basalt may readily be ascertained in any of the numerous ravines and rocky 

 declivities. 



One of the best lines of section for exhibiting the relations of the rocks is the declivity 

 to the west of Ben Buy and Loch Fhuaran. Ascending from the west side, we walk over 

 successive low escarpments and terraces of the plateau-basalts with a gentle inclination 

 towards N.E. or E. These rocks weather in the usual way, some into a brown loam, 

 others into spheroidal exfoliating masses. But as we advance uphill, they gradually 

 assume the peculiar indurated shattery character already referred to. The soft earthy 

 amygdaloids become dull splintery rocks, in which the amygdules are no longer sharply 

 defined from the matrix, but rather seem to shade off into it, sometimes with a border of 

 interlacing fibres of epidote. The compact basalts have undergone less change, but they 

 too have become indurated, and generally assume a white or grey crust, and none of 

 them weather out into columnar forms. Strings and threads full of epidote run through 

 much of these altered rocks. Abundant granophyric and felsitic veins traverse them. 

 Sheets of dolerite likewise make their appearance between the basalts, followed further 

 up the slope by sheets of gabbro until the latter form the main body of the hill. 



On the north side of the same ridge, similar evidence is obtainable, though somewhat 

 complicated by the injections of granophyric and felsitic veins and bosses, to which more 

 detailed reference will afterwards be made. But the altered basalts, with their amygda- 

 loidal bands and their intercalated basalt tuffs and breccias, can be followed from the 

 bottom of the glen up to a height of some 1700 feet, above which the main gabbro mass 

 of Ben Buy sets in. Many minor sheets of dolerite and gabbro make their appearance 

 along the side of the hill before the chief overlying body of the rock is reached. Some 



* Quart. Jour. GeoJ. Soc, xxx. (1874). 



