Fig. 375.—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.
In regard to their modes of occurrence, the dykes of acid material differ in some important respects from those of basic composition. More especially they are apt to assume the irregular venous form, rather than the vertical wall-like character of ordinary dykes. They take the form of dykes, particularly where their material has been guided in its uprise by one or more already existent basic or intermediate dykes, as in the compound dykes, already described. The conditions for their production must thus have been essentially different from those of the great body of the basic dykes. Their intrusion was not marked by any general and widespread fissuring of the earth's crust, such as prepared rents for the reception of the basalt and andesite dykes. They were rather accompaniments of the protrusion of large masses of acid magma into the terrestrial crust. This magma, as we have seen, was often markedly liquid, and was impelled, sometimes with what might be called explosive violence, into the irregular cracks of the shattered surrounding rocks or into pre-existing dyke-fissures. Hence long straight dykes of the acid rocks are much less common than short irregular tortuous veins and strings.
Fig. 376.—Plan of portion of the ridge north of Druim an Eidhne, Glen Sligachan, Skye, showing three dykes issuing from a mass of granophyre.
a, gabbros; b, granophyre; I. II. III., three dykes proceeding from the granophyre. The arrows show the direction of dip of the bands of gabbro.
Much difference may be noticed among the granophyre bosses in regard to their giving off a fringe of apophyses. Thus, along the well-exposed boundary of Beinn-an-Dubhaich in Skye, though the edge of the boss is remarkably notched, hardly any veins deserving the name diverge from it. On the other hand, the ridge of Meall Dearg at the head of Glen Sligachan, already referred to, is distinguished by the number and variety of the dykes and veins which proceed from the granophyre and traverse the banded gabbros. As this locality has been elsewhere fully described, I will give here only the leading structural features which it presents.[432]
[432] Professor Judd (Quart. Journ. Geol. Soc. vol. xlix. (1893), p. 175) described the granophyre dykes of this locality as inclusions of Tertiary granite in the gabbro, and cited them in proof of his contention that the acid eruptions of the Western Isles are older than the basic. Their true character was shown by me in a paper published in the Quart. Journ. Geol. Soc. vol. 1. (1894), p. 212.
Fig. 377.—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.
Within a horizontal distance of less than 100 yards three well-marked dykes issue from the spherulitic edge of the Meall Dearg granophyre, and run in a south-easterly direction in the handed gabbros ([Fig. 376]). The most northerly of these is traceable in a nearly straight line for 800 feet. The central dyke, which can be followed for 200 feet or more, rises as a band six to ten feet broad between the dark walls of gabbro as represented in [Fig. 379].
These dykes are marked by the most perfectly developed spherulitic and flow-structures (Figs. [375], [377]). Numerous detached portions of other dykes and also irregular veins are to be observed cutting the banded gabbros all over the ridge of Druim an Eidhne for a distance of a mile or more. Many of these exhibit the same exquisitely beautiful spherulitic and flow-structure displayed by the dykes which can actually be traced into the main body of granophyre. The lines of flow conform to every sinuosity in the boundary-walls of gabbro, and sometimes sweep round and enclose blocks of that rock. The example of this structure, given in [Fig. 378], shows how these lines, curving round projections and bending into eddy-like swirls, exhibit the motion of a viscous lava flowing in a cleft between two walls of solid rock. Sometimes the laminæ of flow have been disrupted, and broken portions of them have been carried onward and enveloped in the yet unconsolidated material. Certain portions of this dyke are richly spherulitic, the spherulites varying from the size of small peas up to that of tennis-balls. Occasionally two large spherulites have coalesced into an 8-shaped concretion, and it may be observed in some cases that the spherulites are hollow shells.