Not only do dykes cross each other, but still more is this the case among the narrower tortuous intrusions known as Veins ([Fig. 252]). Among the illustrations which the dykes of the Inner Hebrides supply of these features one further characteristic example may be culled from the shore of Skye, near Broadford, where the gently-inclined sheets of Lias limestone are traversed by three systems of dykes ([Fig. 253]). One of these systems runs in a N.W. or N.N.W. direction, a second follows a more nearly easterly trend, while the third and youngest runs nearly north and south.
| Fig. 252.—Basalt Veins traversing bedded dolerites, Kildonan, Eigg. | Fig. 253.—Ground-plan of intersecting Dykes in Lias limestone, Shore, Harrabol, East of Broadford, Skye. |
16. DYKES OF MORE THAN ONE IN-FILLING
The intersections of dykes prove that the process of fissuring in the earth's crust took place at more than one period, and prepare us for the reception of evidence that the same line of fissure might be again re-opened, even after it had been filled with molten material. Numerous instances have now been accumulated in which dykes are not single or simple intrusions, but where the original dyke-fissure has been re-opened and has been invaded by successive uprisings of lava.[197] Compound dykes have thus been formed, consisting of two or more parallel bands of similar or dissimilar rock.
[197] See an example figured by Macculloch, Western Isles, plate xviii. Fig. 1.
While it is not difficult to conceive of the re-opening of a vertical fissure during terrestrial strain, and the injection into it of later intrusions of a volcanic magma, it is not so easy to understand the mechanism where the line of weakness has been slightly inclined or horizontal, and where, consequently, there has been the enormous superincumbent pressure of the overlying part of the earth's crust to overcome. Yet gently inclined compound dykes exhibit their parallel bands with hardly less regularity than do those that are vertical. The difficulty of explanation is felt most strongly in the attempt to realize the origin of the compound sills described in Chapter xlviii.
In the re-opening of dyke-fissures the later intrusions have generally taken place along the walls, or where the dykes were already compound, between some of the component bands. Less frequently the first dyke has been split open along the middle, and a second injection has forced its way along the rent.
Of the first of these two types, numerous instances have now been observed in the West of Scotland. If the portion of a compound dyke exposed at the surface be limited in extent, we may be unable to determine which is the older of two parallel bands of igneous rock, though the fact that they present to each other the usual fine-grained edge due to more rapid cooling, shows that they are not one but two dykes, belonging to distinct eruptions. So far as I have noticed, where one of the dykes can be continuously traced for a considerable distance, the other is comparatively short. I infer that the shorter one is the younger of the two.
In the Strath district of Skye, Mr. Harker has recently observed that many of the basic dykes, both those older and those younger than the granophyre protrusions, are double, triple or multiple. Thus in a conspicuous dyke, more than 100 feet wide, to the south-east of Loch Kilchrist, belonging to the older series, he has detected at least six contiguous dykes which as they are traced south-eastward, in spite of their interruption by the Beinn an Dubhaich granite, can be seen to separate and take different courses, or successively die out. He remarks, further, that "many cases of apparent bifurcation of dykes are really due to the separation of distinct dykes which have run for some distance in one fissure. Sometimes apparent variations in the width of a dyke are to be explained by this dying out of one member of a double dyke. These multiple dykes are less easily detected in the newer than the older set, owing to greater uniformity of lithological type in the prevalent kinds and to the frequent absence of chilled selvages."[198] An example of a compound basic dyke cutting the crest of the gabbro-mass of the Cuillin Hills is shown in [Fig. 333].
[198] MS. notes supplied by Mr. Harker.