In the great majority of examples hitherto observed in Skye the two lateral dykes consist of some basic rock (diabase or basalt), while the central and thickest band is of some acid material (granophyre or quartz-felsite). This triple arrangement occurs both in dykes and sills.
Fig. 254.—Compound dyke, Market Stance, Broadford, Skye.
a, Granophyre; b b, Basalt; c c, Torridon sandstone.
As an illustration of the association of the two kinds of rock in dykes I may cite an example which appears on the southern edge of the Market Stance of Broadford ([Fig. 254]). Here the characteristic triple arrangement is typically developed. A central light-coloured band, about eight to ten feet broad, consists of a spherulitic granophyre in which the spherulites are crowded together and project from the weathered surface like peas, though they do not here show the curious rod-like aggregation so marked in some other dykes. On either side of this acid centre a narrow basalt dyke intervenes as a wall next to the Torridon sandstone which here forms the country-rock. Such compound dykes have sometimes a total width of 100 feet or more.
In this instance, and generally throughout the district, there is nothing to indicate that the different bands of the dyke have any relation to each other as connected uprises of material from the same original magma which was either heterogeneous or was undergoing a process of differentiation beneath the terrestrial crust. On the contrary, the several parts of each dyke are as distinctly marked off from each other as they could have been had they been injected at widely separated intervals of volcanic activity.
Mr. Harker, in the course of his survey of this Skye ground, has observed that "where evidence is available, the central acid dyke is found to be newer than the basic ones. It has not split a single basic dyke, but has insinuated itself between the two members of a double dyke. This is more clearly seen when the acid magma has been forced into a triple or multiple basic dyke; the perfect symmetry of arrangement may in this case be lost. For instance, on the shore north-east of Corry, Broadford, a 13 feet dyke of granophyre occurs in a multiple dyke of basalt, but it has taken its line so as to leave only a one-foot dyke on one side, and a group with a total width of 12 feet on the other. Also it has not accurately kept its course, but has cut obliquely across one of the group of dykes alluded to. In some cases it is certain that the acid magma has to some extent dissolved a portion of the wall of a basic dyke with which it has come in contact. This may account for the magma finding its easiest path along, and especially between, pre-existing more basic dykes." This subject will be again referred to in Chapter xlviii., when the phenomena of compound sills are discussed.
Before closing this account of compound dykes, I may remark that no examples have yet been observed among the ordinary Tertiary dykes of Britain where, by a process of differentiation between the walls of a fissure, successive zones have been developed in the dyke, differing from each other in structure and composition, but becoming progressively and insensibly more acid towards the centre, such as have been described from the older rocks of Norway and Canada. Among the Tertiary gabbro bosses, indeed, there occur sheets or dykes which present a remarkably banded structure, to which full reference will be made in later pages. But I have never seen anything at all resembling such a structure among the dykes of andesite, dolerite, or basalt.
17. CONTACT-METAMORPHISM OF THE DYKES
A geologist might naturally expect that such abundant intrusions of igneous rock as those of the dykes should be accompanied with plentiful proofs of contact-metamorphism. But in actual fact, evidence of any serious amount of alteration is singularly scarce. A slight induration of the rocks on either side of a dyke is generally all the change that can be detected.
