CHAP. XLIV 
THE GABBRO OF MULL 
357 
defined from tlie matrix, but rather seem to shade off into it, sometimes with 
a border of interlacing fibres of epidote. The compact basalts have under- 
gone less change, but they too have become indurated, and generally assume 
a white or grey crust, and none of then 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 amygdaloidal 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 of these can be distinctly seen breaking across or ending off 
between the bedded basalts which here dip gently into the hill (Fig. 342). 
A conspicuous band of coarse basalt-agglomerate, containing blocks of com- 
pact and amygdaloidal basalt a yard or more in diameter, shows by the 
excessive induration of its dull-green matrix the general alteration which 
the rocks of the basalt-plateau have here undergone. An almost incredible 
number of veins of fine basalt, porphyry and felsite has been injected into 
these rocks — a structure which is precisely a counterpart of what occurs 
under the main body of gabbro in Skye, Ardnamurchan and Eum. 
The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge over- 
lying sheet, which prob- 
ably reaches a thickness 
of at least 800 feet. It 
seems to descend rather 
across the bedding into 
the hollow of Glen More, 
and possibly its main pipe 
of - supply lay in that 
direction. Being enor- 
nrously thicker than any 
other sheet in the island, 
it exhibits the crystalline 
peculiarities which are so 
well developed in the 
central portions of the 
larger bosses of gabbro. It presents more coarsely crystalline varieties than 
appear in the thinner sheets, some portions showing crystals of diallage and 
felspar upwards of an inch in length. It likewise contains admirable examples 
of banded structure, which, as in Skye and elsewhere, is best developed where 
the texture becomes especially coarse. Veins or bands, in which the con- 
Fig. 342. — Altered Plateau-Basalts invaded by Gabbro, and with 
a Dyke of prismatic Basalt cutting both rocks, north slope of 
Ben Buy, Mull. 
a a, amygdaloidal basalt, much altered ; b, gabbro ; c, finely prismatic 
basalt. 
