2. Trachyte Dykes.—In the Cowal District of Argyleshire, and in the south of Skye, Mr. Clough has encountered a limited number of dykes of trachyte. On a hasty inspection these are not always readily distinguished from the basalt-dykes with which they agree in general external aspect and in direction. Where their relation to these dykes, however, can be determined they are found to traverse them, and thus to be on the whole later, though one case has been observed where a trachytic dyke is in turn traversed by one of the basic series. Mr. Clough has supplied me with the following notes of his observations regarding the trachytic dykes. They are all characterized by the possession of spherulitic structures near their margins. These features, easily perceptible to the naked eye, afford the readiest means of distinguishing the dykes of this group. So abundant are the spherulites that they not infrequently impinge on each other in long parallel rows forming rod-like aggregates. Thus in a dyke near Craigendavie, at the head of Loch Striven, numerous planes about a quarter of an inch apart, and composed of such close-set rods, may be observed running parallel to the marginal wall for a distance of several inches from the edge. Most of these planes show on their surfaces that the rods are always parallel to each other, but may run in different directions in the different layers, being sometimes horizontal, sometimes vertical, or at any angle between. On examination, each rod is found to be made up of polygonal bodies, the angles of which are quite sharp, but with their sides often slightly curved, as if they had assumed their forms from the mutual pressure of original spherical bulbs. Further scrutiny shows that the polygonal bodies often exhibit an internal radiate structure.
In the central parts of the dyke the spherulitic arrangement is not traceable. About a foot from the margin it begins to be recognizable. At a distance of three or four inches the spherulites are about the size of peas, and gradually diminish towards the edge until they can no longer be seen.
Another characteristic of the trachyte dykes has been found by Mr. Clough to be a useful guide in discriminating them from the basalt-group. While the amygdales in the latter are generally rudely spherical, those in the trachytes are commonly elongated in the direction of the length of the dyke, and are frequently three quarters of an inch, sometimes even an inch and a half, in length, though less than a quarter of an inch in breadth.
A good example of these trachytic dykes, which occurs at Dunans, about the head of Glendaruel, has been examined microscopically and chemically. The central better crystallised portion was found by Mr. Teall to be composed mainly of small lath-shaped crystals of orthoclase, together with scales of brown biotite, a few prismatic crystals of pale somewhat altered pyroxene and scattered granules of magnetite. The chemical analysis of this rock by Mr. J. H. Player gave the following composition:—
| Silica | 56·4 |
| Alumina | 19·0 |
| Ferric oxide | 3·5 |
| Ferrous oxide | 4·8 |
| Lime | 2·6 |
| Magnesia | 1·5 |
| Soda | 4·5 |
| Potash | 5·0 |
| Loss on ignition | 2·6 |
| 99·9 |
4. HADE
In the majority of cases, especially among the great single dykes, the intrusive rock has assumed a position nearly or quite vertical. But occasionally, where one of these solitary examples crosses a deep valley, a slight hade is perceptible by the deviation of the line of the dyke from its normal course. Sedgwick long ago noticed that the Cleveland dyke has, in places, an inclination of at least 80° to its N.E. side.[176] In the coal-workings, also, a trifling deviation from the vertical is sometimes perceptible, especially where a dyke has found its way along a previously existing line of fault, as in several examples in Stirlingshire. But in those districts where the dykes are gregarious, departures from the vertical position are not infrequent, more particularly near the great basalt-plateaux. It was noticed by Necker, that even in such a dyke-filled region as Arran, almost all of the dykes are vertical, though sometimes deviating from that position to the extent of 20°.[177] Berger found that the angle of deviation among those of the north of Ireland ranges from 9° to 20°, with a mean of 13°.[178] The most oblique examples are probably those which occur in the basalt-plateaux of the Inner Hebrides, where the same dyke in some parts of its course runs horizontally between two beds, across which it also descends vertically (see Figs. [251], [252], [374]). But with these minor exceptions, the verticality of the great system of dykes, pointing to the perpendicular fissure-walls between which the molten rock ascended, is one of the most notable features in their geological structure. In the Strath district of Skye Mr. Harker has noticed that while the earlier dykes have sometimes a hade of 45°, those younger than the granophyre are generally vertical or nearly so. In the Blath Bheinn group of hills, however, as already alluded to, he has observed that it is the youngest dykes which are inclined in a north-westerly direction, with a hade of as much as 40° from the horizon.
[176] Cambridge Phil. Trans. ii. p. 28.
[177] Trans. Roy. Soc. Edin. xiv. p. 677.
[178] Trans. Geol. Soc. iii. p. 227.