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. [Footnote 176: _Cambridge Phil. Trans._ ii. p. 28.] [Footnote 177: _Trans. Roy. Soc. Edin._ xiv. p. 677.] [Footnote 178: _Trans. Geol. Soc._ iii. p. 227.] 5. BREADTH