140 
THE TERTIARY VOLCANOES 
BOOK VIII 
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. 1 In the 
isle of Arran, according to Necker’s careful measurements, most of the dykes 
range from 2 or 8 to 10 or 15 feet, but some diminish to a few inches, 
while others reach a width of 20, 30, or even 50 feet. 2 3 4 * 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. 8 
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. 1 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 else- 
where 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 Lime- 
stone. Its maximum width is 640 feet. 
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 
1 Trans. Geol, Soc. iii. p. 80. 
2 Trans. Roy. Soc. Edin. xiv. p. 690 et seq. 
3 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. 
4 At Cockfield, where it has long been quarried, it varies from 15 to 66 feet ; at Armatliwatie, 
in the vale of the Eden, it is about 54 feet (Mr. Teall, Quart. Journ. Geol. Soc. xl. p. 211). 
