CHAP. XXXIV 
HADE AND BREADTH OF THE DYKES 
139 
Silica .... 56 - 4 
Alumina . . . 19'0 
Ferric oxide .... 3'5 
Ferrous oxide . . . 4'8 
Lime . . . . 2’6 
Magnesia . . . . l - 5 
Soda .... 4-5 
Potasli . . • . 5-0 
Loss on ignition . . . , 2 -6 
99-9 
4. HADE 
Iii the majority of eases, especially among the great single dykes, the 
intrusive rock has assumed a position nearly or quite vertical. But occa- 
sionally, 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 X.E. side. 1 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 infre- 
quent, more particularly near the great basalt-plateaux. It was noticed by 
Neckcr, 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°. 2 Berger found that the angle of deviation among those of the north 
of Ireland ranges from 9° to 20°, with a mean of 13 0 . 3 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 granopliyre 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. 
5. BREADTH 
An obvious characteristic of most dykes is the apparent uniformity of 
their breadth. Many of them, as exposed along shore -sections, vary as 
little in dimensions as well-built walls of masonry do. Departures from 
such uniformity may often indeed be noted, whether a dyke is followed 
1 Cambridge Phil. Trans, ii. p. 2S. 2 Trans. Roy. Soc. Edin. xiv. p. 677. 
3 Trans. Geol. Soc. iii. p. 227. 
