46 DR GE1K1E ON THE HISTORY OF VOLCANIC ACTION 



built walls of masonry do. Departures from such uniformity may often indeed b« 

 noted, whether a dyke is followed laterally or vertically. The largest amount of varia- 

 tion 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 greatly less than in the case of the great solitary dykes. In the district of 

 Strathaird, in Skye, Macculloch estimated the remarkably abundant dykes there 

 developed to vary from 5 to 20 feet in breadth, but with an average breadth of not more 

 than 10 feet.* 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.t In the north of Ireland, 

 Berger observed that the average breadth of thirty-eight dykes traversing primitive 

 rocks (schist, granites, &c.) was 9 feet; and of twenty- four in Secondary rocks, 24 feet.J 



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 to more than 100 feet, with perhaps an average width 

 of between 70 and 90 feet.§ 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 we trace its course 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 Carboni- 

 ferous Limestone. 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 are available of obtaining information 

 on the subject — (a) from mining operations, and (6) from observations at precipices 

 and between hill-crests and valley-bottoms. 



* Trans. Geol. Soc, iii. p. 80. t Trans. Roy. Soc. Edin., xiv. p. 690 et seq. 



X 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. 



§ At Cockfield, where it has long been quarried, it varies from 15 to 66 feet ; at Armathwaite, in the vale of the 

 Eden, it is about 54 feet (J. J. H. Teall, Quart. Jour. Geol. Soc, xl. p. 211). 



