Fig. 373.—Section to show the connection of a sill of Granophyre with its probable funnel of supply, Raasay.
a a, Jurassic sandstones; b, granophyre.
Fig. 374.—Granophyre sill resting on Lower Lias shales with a dyke of basalt passing laterally into a sill, Suisnish Point, Isle of Raasay.
Although the intrusion of the granophyre sills has been subsequent to that of the basalt-sheets with which they are so generally associated, we may expect that as there is a series of post-granophyre basic dykes, so there may be some basic sills later than the injections of the acid sheets. The Raasay granophyre appears to furnish an example of such a later basic intrusion. At the Point of Suisnish on that island I have observed the relations shown in [Fig. 374]. There the dark shales of the Lower Lias (a a) are immediately overlain by the granophyre sill (b), and are cut by a basalt-dyke which, when it rises to the base of the granophyre, turns abruptly to one side, and then pursues its course as a sill (c) between the granophyre and the shales. There can be little doubt that this intrusion is later than the granophyre. Here a basic sill is interposed at the bottom of the acid sheet; and is 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 granophyre. 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. Harker 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 devitrification flowing round the spherulites and any enclosed fragments as perfectly as in any rhyolitic lava ([Fig. 378]).
