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 laterally or vertically. The largest amount of variation is, of course, to be found among the dykes of the gregarious type, the thinner examples of which may diminish to a width of only one inch or less, while their average breadth is much smaller than in the case of the great solitary dykes. In the district of Strathaird, in Skye, Macculloch estimated that the remarkably abundant dykes there developed vary from 5 to 20 feet in breadth, but with an average breadth of not more than 10 feet.[179] In the isle of Arran, according to Necker's careful measurements, most of the dykes range from 2 or 3 to 10 or 15 feet, but some diminish to a few inches, while others reach a width of 20, 30, or even 50 feet.[180] In the North of Ireland, Berger observed that the average breadth of thirty-eight dykes traversing primitive rocks (schist, granites, etc.) was 9 feet; and of twenty-four in Secondary rocks, 24 feet.[181]
[179] Trans. Geol. Soc. iii. p. 80.
[180] Trans. Roy. Soc. Edin. xiv. p. 690 et seq.
[181] Trans. Geol. Soc. iii. p. 226. He believed that dykes in Secondary rocks reach a much greater thickness than in other formations. My own observations do not confirm this generalisation.
But when we pass to the great solitary dykes, that run so far and so continuously across the country, we encounter much thicker masses of igneous rock. Most of the measurements of these dykes have been made at the surface, and the variations noted in their breadth occur along their horizontal extension. The Cleveland dyke, which is the longest in Britain, varies from 15 feet to more than 100 feet, with perhaps an average width of between 70 and 90 feet.[182] Some of the great dykes that cross Scotland are of larger dimensions. Most of them, however, like that of Cleveland, are liable to considerable variations in breadth when followed along their length. The dyke which runs from the eastern coast across the Cheviot Hills and Teviotdale to the head of the Ale Water, is in some places only 10 feet broad, but at its widest parts is probably about 100 feet. The Eskdale and Moffat dyke is in parts of its course 180 feet wide, but elsewhere it diminishes to not more than 40 feet. These variations are repeated at irregular intervals, so that the dyke alternately widens and contracts as its course is traced across the hills. Some of the dykes further to the north and west attain yet more gigantic proportions. That which crosses Cantyre opposite Ardlamont Point has been measured by Mr. J. B. Hill, of the Geological Survey, who finds it to be from 150 to 180 feet broad on the shore of Loch Fyne, and to swell out beyond the west side of Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo, in Fife, is about 400 feet wide. The broadest dyke known to me is one which I traced near Beith, in Ayrshire, traversing the Carboniferous Limestone. Its maximum width is 640 feet.
[182] At Cockfield, where it has long been quarried, it varies from 15 to 66 feet; at Armathwatie, in the vale of the Eden, it is about 54 feet (Mr. Teall, Quart. Journ. Geol. Soc. xl. p. 211).
Unfortunately, it is much less easy to get evidence of the width of dykes at different levels in their vertical extension. Yet this is obviously an important point in the theoretical discussion of their origin. Two means are available of obtaining information on the subject—(a) from mining operations, and (b) from observations at precipices and between hill-crests and valley-bottoms.
(a) In the Central Scottish coal-field and in that of Ayrshire, some large dykes have been cut through at depths of two or three hundred feet beneath the surface. But there does not appear to be any well-ascertained variation between their width so far below ground and at the surface. In not a few cases, indeed, dykes are met with in the lower workings of the coal-pits which do not reach the surface or even the workings in the higher coals. Such upward terminations of dykes will be afterwards considered, and it will be shown that towards its upper limit a dyke may rapidly diminish in width.
(b) More definite information, and often from a wider vertical range, is to be gathered on coast-cliffs and in hilly districts, where the same dyke can be followed through a vertical range of many hundred feet. But so far as my own observations go, no general rule can be established that dykes sensibly vary in width as they are traced upward. Every one who has visited the basalt-precipices of Antrim or the Inner Hebrides, where dykes are so numerous, will remember how uniform is their breadth as they run like ribbons up the faces of the escarpments.[183] Now and then one of them may be observed to die out, but in such cases (which are far from common) the normal width is usually maintained up to within a few feet of the termination.