CHAP. XLV 
THE ACID ROCKS 
367 
phyric groundmass. But, though the large bosses are usually somewhat 
coarsely crystalline in the centre, and tend to assume finer felsitie textures 
around their borders, as was observed long ago by Oeynhausen and Von 
Dechen , 1 the granitic structure is sometimes exhibited even at the very 
edge, and not only so, but in the dykes that protrude from the bosses into 
the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at its margin 
in Camas Malag, sends a vein into the surrounding limestone, but though 
more close-grained than the main body of the rock, this vein is neither 
felsitie nor granophyric, but truly granitic in structure. 
So far as I have observed, the true granites contain a brown mica and 
also a little hornblende, both visible to the naked eye, but generally some- 
what decomposed. These rocks are thus hornblende-biotite-granites (amphi- 
bole-granitites of Bosenbusch). They may be defined as medium-grained 
aggregates of quartz, orthoclase (also plagioclase), biotite and hornblende, 
with sometimes magnetite, apatite, epidote and zircon. Dr. Hatch found 
that in some instances (Beinn-an-Dubhaich) the quartz contains minute 
inclusions (glass ?), bearing immovable bubbles with strongly-marked con- 
tours : while in others (Beinn-na-Chro, Skye) this mineral is full of liquid 
inclusions with bubbles, sometimes vibratile, sometimes fixed. He remarked 
that the quartz and felspar have consolidated almost simultaneously, but 
that in some instances (Marsco, Glen Sligachan) there are isolated roughly 
idiomorphic crystals, of a white, less turbid orthoclase, which belong to a 
slightly earlier consolidation than that of the more kaolinized felspar of the 
rest of the rock. 
The granite of the island of Arran, in the Firth of Clyde, which 
is here included in the Tertiary volcanic series, has long been 
recognized as consisting of two distinct portions, an eastern or coarse- 
grained, and a western or fine-grained variety. The latter sends 
veins into the former. These granites contain orthoclase, plagioclase, 
quartz and dark mica, the quartz being often idiomorphic with respect 
to the felspar, and a tendency towards a micropegmatitic structure 
being sometimes observable. A distinguishing characteristic of the Arran 
granite is the cavernous or drusy structure which it presents, the cavities being 
often lined with well-crystallized orthoclase and smoky quartz . 2 The granite 
of the Mourne Mountains in Ireland closely resembles that of Arran. Its 
druses, with their beautifully terminated minerals, have long been well 
known. 
Microgranite . — This term is applied to certain intrusive masses, which 
megascopically may be classed with the quartz-porphyries and felsites, but 
which microscopically are found to possess a holocrystalline granitic ground- 
mass of quartz and orthoclase, through which are scattered porphyritie 
crystals of the same two minerals, sometimes also with plagioclase, augite, 
magnetite or apatite. Bocks of this type do not appear to be abundant. 
They occur as dykes and bosses, but occasionally also as sheets. I have 
collected them from Skye, Bum and Ardnamurchan. 
1 Karsten’s ArcMv, i. p. 89. 2 See Mr. Teall’s British Petrography , p. 328. 
