CHAP. XLVIII 
THE ACID DYKES AND VEINS 
437 
visibly connected with the actual fissure up which its molten material was 
impelled. 
ii. THE ACID DYKES AND VEINS 
Besides bosses and sills, the acid rocks of the Inner Hebrides take the 
form of Dykes and Veins which have invaded the other members of the 
volcanic series. Some of these have already been referred to ; but a more 
particular description of the venous development of the acid rocks as a whole 
is now required. 
As regards their occurrence and distribution, they present two phases, 
which, however, cannot always be distinguished from each other. On the 
one hand, they are found abundantly either directly proceeding from the 
bosses (more rarely from the sills), or in such immediate proximity and close 
relationship to these as to indicate that they must be regarded as apophyses 
from the larger bodies of eruptive material. On the other hand, they present 
themselves as solitary individuals, or in groups at a distance of sometimes 
several miles from any visible boss of granophyre. In such cases, it is of 
course obvious that though not exposed at the surface, there may be a large 
mass of the acid magma at no great distance beneath, and that these isolated 
dykes and veins do not essentially differ in origin from those of which the 
relations to eruptive bosses can be satisfactorily observed or inferred. 
Considered as a petrographical group, these Dykes and Veins are marked 
by the following characters. At the one extreme, we have thoroughly 
vitreous rocks in the pitchstones. From these, through various degrees of 
devitrification, we are led to completely lithoid felsites, quartz-porphyries 
or rhyolites. Micropegmatitic structure is commonly present, and as it 
increases in development, the rocks assume the ordinary characters of grano- 
phyre. Occasionally the structure becomes microgranitic in the immediate 
periphery of a boss wherein a granitic character has been assumed. Viewed 
as a whole, however, it may be said that the dull lithoid rocks of the dykes 
and veins can generally be resolved under the microscope into some variety 
of granophyric porphyry or granophyre. 
A characteristic feature in the granophyric, felsitic or rhyolitic dykes 
and veins is the presence of spherulitic structure (Figs. 375, 377). In some 
cases this structure is hardly traceable save with the aid of the microscope, 
but from these minute proportions it may be followed up to such a strong 
development that the individual spherulites may be an inch or two in 
diameter, and lie crowded together, like the round pebbles of a conglomerate. 
The structure is a contact phenomenon, being specially marked along the 
margin of the dykes, as it is on the edge of sills and bosses. In the Strath 
district of Skye, Mr. Clough and Mr. ITarker have observed that the 
spherulites are apt to be grouped in parallel lines so as to form rod-like 
aggregates along the walls, and that where the rock is fairly fresh the centre 
of the dyke sometimes consists of glassy pitchstone, so that the spherulitic 
felsite or granophyre is probably devitrified pitchstone. Frequently flow- 
structure is admirably developed in these dykes, the streaky layers of 
