CHAP. XLVI 
THE ACID BOSSES OF SKYE 
389 
rock by an interlacing of their crystalline constituents. They then look at 
a distance more like spots of decoloration, and even when seen close at hand 
would hardly at first betray their real nature. 
From the specimens collected by me among the Inner Hebrides up to 
the year 1888, 1 selected two dozen which seemed to be fairly typical of these 
altered rocks, and placed thin slices of them for microscopic examination 
in Dr. Hatch’s hands. His notes may be condensed into the following 
summary. One of the most frequent features in the slides is the tendency 
in the component minerals to assume granular forms. In one specimen from 
Loch Spelve, Mull, the rock, probably originally a dolerite, shows only a few 
isolated recognizable crystals of plagioclase and augite, the whole of the rest 
of the rock consisting of roundish granules embedded in a felspathie matrix. 
The felspar crystals are sometimes broken up into a mosaic, though retaining 
their external contours. Besides the granules, which are no doubt augite, a 
few grains of magnetite are scattered through the rock, aggregated here and 
there into little groups. In another specimen, taken from the junction with 
the granophyre in Glenmore in the same island, parts of the augite crystals 
are converted into granular aggregates associated with large grains and 
patches of magnetite. The latter mineral also assumes in some of the rocks 
granular and even globular shapes suggestive of fusion. 
The felspars, which in most of the basic rocks are usually remarkably 
clear and fresh, show marked kaolinization in some of these altered masses. 
Minute dusky scales of kaolin are developed, sometimes also with the separa- 
tion of minute grains of quartz. The augite shows frequent alteration to 
hornblende, proceeding as usual from the exterior inward. In some cases 
only an envelope of uralite appears round the augite, while in others only a 
kernel of the original mineral is left, or the whole crystal Inis been changed. 
In many cases the altered substance appears as minute needles, blades and 
fibres of actinolite. Occasionally, besides the green hornblende, shred-like 
pieces of a strongly pleoehroic brown hornblende make their appearance. 
Serpentinous and chloritic substances are not infrequent. Epidote is some- 
times abundant. The titaniferous iron has commonly passed more or less 
completely into leucoxene. Here and there a dark mica may be detected. 
Since the year 1888 I have continued the investigation of this subject, 
and have especially studied the metamorphism of tire bedded basalts on the 
western shores of Loch Seavaig, where, as already described, they are trun- 
cated by vertical beds of gabbro, and are traversed by basalt-dykes and by 
abundant veins of fine-grained granophyre. The alteration here effected 
affords excellent materials for study, as the very same sheets of basalt can be 
followed from the normal conditions outside to the altered state within the 
influence of the metamorphic agent. The alternations ol amygdaloidal and 
more compact sheets can still be recognized, although their enclosed amyg- 
dales have in places been almost effaced. They show the dull, indurated, 
splintery character, with the white weathered crust, so distinctive of this 
type of contact-metamorphism. They are traversed by numerous sills and 
veins of gabbro. As has been already suggested, although no large .mass of 
