CHAP. XLV 
THE ACID ROCKS 
369 
is hornblende, the most remarkable example of which is one to be seen at 
Ishriff, in the Glen More of Mull, where the ferro-magnesian mineral takes 
the form of long dirty-green needles, conspicuous on a weathered surface of 
the rock. A fourth variety is distinguished by containing plagioclase in 
addition to or instead of orthoclase. In the rock of the sheet forming Cnoc 
Carnach, near Heast, in Skye, Dr. Hatch observed both orthoclase and 
plagioclase scattered through a fine micropegmatitic groundmass, and in a 
part of the boss at Ishriff he found the rock to be composed mainly of 
plagioclase, in a micropegmatitic groundmass of quartz and felspar, with a 
few scattered grains of a pale brown augite and grains of magnetite. A fifth 
variety is marked by the prominence of the crystals of quartz and felspar 
of earlier consolidation, and by the fineness of grain in the surrounding 
micropegmatitic groundmass, whereby a distinct porphyritic .structure is 
developed. Rocks of this kind are megascopically like ordinary quartz- 
porphyries. Still another variety has been detected by Mr. Teall in the rock 
of Meall Dearg, at the head of Glen Sligachan, Skye, in which, besides 
irregular patches which may represent decayed biotite, and others which are 
possibly ilmenite, the rare mineral riebeekite is present . 1 
Fdsite . — The close-grained rocks into which the ordinary granophyres 
frequently graduate may be conveniently grouped under the general name 
of Felsite. They differ in no essential feature from the felsites of the 
Pakeozoic formations. They are more particularly developed, as might be 
expected, in those places where the conditions have been most favourable 
for rapid cooling, while the more coarsely crystalline granophyres occur 
where the material may be supposed to have consolidated most slowly. 
Where the acid magma has been injected into chinks and fissures so as to 
take the form of veins or dykes, it is sometimes felsitic, sometimes grano- 
phyric, in texture. Along the margin of large bosses, like those of Mull 
and Skye, it frequently though not invariably has assumed a fine texture, 
with even spherulitic and flow-structures. But in the centre of large 
bosses it usually appears as coarse granophyre or as granite. 
The felsites vary in texture from flinty or horny to dull finely- 
granular, and in colour from white through shades of grey, buff and lilac, 
to black, generally with porphyritic felspars and blebs of quartz. Where 
these porphyritic enclosures increase in size and' number, the rocks cannot 
be distinguished externally from ancient quartz-porphyries. In general the 
groundmass of these rocks has been completely devitrified. But in some 
dykes enough of the glassy base remains to show their original vitreous 
condition. A gradation can thus be traced from thoroughly glassy pitch- 
stone into completely lithoid felsites and crystalline granophyres. 
A characteristic feature of the felsitic varieties of acid rock is their 
flow-structure, which they often display in great perfection. Sometimes, 
indeed, this structure has been so strongly developed as to cause the rock to 
weather along the planes of flow and to break up into thin slabs. 
Many of these rocks also present admirably developed spherulitic struc- 
1 Quart. Journ. Geol. Soc. vol. 1. (1894), ]>. 219. 
2 B 
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