368 
THE TERTIARY VOLCANOES 
BOOK VIII 
Granophyre . — Under this name may be grouped the large majority of the 
acid rocks which play an important part in tire geology of the "West of Scotland. 
They are typically developed in the islands of Mull and Skye. Generally 
pale grey or huff in colour, they range in texture from the true granites, into 
which, as above stated, they graduate, to exceedingly close-grained varieties 
like the felsites of Palaeozoic formations. In the great majority of them the 
micrographic intergrowth of quartz and felspar, known as micropegmatite, 
is their conspicuous structure, and even constitutes most of their substance. 
They may thus he classed generally as granophyres, in the sense in which 
this term is employed by Eosenbusch, but without his limitation of it to 
pre-Tertiary rocks. 
The specific gravity of these rocks has been determined from a series of 
specimens by Mr. A. Harker to range from about 2 - 3 among the felsites to 
2-7 among the granites. No chemical analyses of these rocks have yet been 
made, but they have been subjected to microscopical examination, and their 
general structure and composition are now known. 
The typical granophyre of the Inner Hebrides outwardly closely 
resembles an ordinary granite of medium grain, in which the component 
dull felspar and clear quartz can be readily distinguished by the naked eye. 
Throughout all the varieties of texture there is a strong tendency to the 
development of minute irregularly-shaped drusy (miarolitic) cavities, which 
here and there give a carious aspect to the rock. That these cavities, how- 
ever, are part of the original structure of the rock, and are not due to mere 
weathering, is shown by the well-terminated crystals of quartz and felspar 
which project into them. On a small scale, it is the same structure so 
characteristic of the granite of the Mourne Mountains and of parts of that 
of Arran. 
Examined under the microscope, a normal specimen of the granophyre of 
the Western Isles presents a holocrystalline grouudmass, which fills all the 
interspaces between the crystals of earlier consolidation. This groundmass 
consists of an aggregate of clear quartz and turbid orthoclase, arranged as 
micropegmatite, but also in more or less idiomorphie crystals. In some 
parts, the two dominant minerals are grouped in alternate parallel fibres, 
diverging from the surface of the enclosed crystals, which are thus more 
or less completely surrounded by a radially fibrous mass. The felspatliic 
portion of the micropegmatite which usually surrounds the orthoclase 
crystals, when viewed between crossed Nicols, is found to extinguish 
simultaneously with the central crystal. 1 In other parts, the felspar forms 
a kind of network, the meshes of which are filled up with quartz. Through 
the groundmass, besides the clear quartz and dull orthoclase, some ferro- 
magnesian or other additional constituent is generally distributed, but 
usually somewhat decomposed. In certain varieties Hr. Hatch found an 
abundant brown mica, as in the rock at Camas Malag, Skye. In others, a 
p 3 ’roxene occurs, which he observed in minute greenish grains, sometimes 
completely enclosed in the quartz. In a third variety, the dark constituent 
1 3Ir. Teall, Quart. Journ. Choi. Soc. vol. 1. (1894) p. 219. See also liis British Petrography, p 327. 
