34 DR GE1KIE ON THE HISTORY OF VOLCANIC ACTION 



interesting paper by Neckeu, who catalogued and described 149 of them, and estimated 

 their total number in the whole island to be about 1500.* As the area of Arran is 165 

 square miles, there would be, according to this computation, about nine dykes to every 

 square mile. But they are far from being uniformly distributed. While appearing only 

 rarely in many inland tracts, they are crowded together along the shore, particularly at 

 the south end of the island, where the number in each square mile must far exceed the 

 average just given. The portion of Argyleshire, between the hollow of Loch Long 

 and the Firth of Clyde on the east and Loch Fyne on the west, has recently been found by 

 my colleague, Mr C. T. Clough, to contain an extraordinary number of dykes (see fig. 17). 

 The coast line of Renfrewshire and Ayrshire shows that the same feature is prolonged into 

 the eastern side of the basin of the Clyde estuary. But immediately to the westward of 

 this area the crowded dykes disappear from the basin of Loch Fyne. In Cantire their 

 scarcity is as remarkable as their abundance in Cowal. Both in the north of Ireland and 

 through the Inner Hebrides, dykes are singularly abundant in and around, but particularly 

 beneath, the great plateaux of basalt. Their profusion in Skye was described early in 

 this century by Macculloch, who called attention more especially to their extraordinary 

 development in the district of Strathaird. " They nearly equal in some places," he says, 

 " when collectively measured, the stratified rock through which they pass. I have 

 counted six or eight in the space of fifty yards, of which the collective dimensions could 

 not be less than sixty or seventy feet." He supposed that it would not be an excessive 

 estimate to regard the igneous rock as amounting to one-tenth of the breadth of the 

 strata which it cuts.t 



Among the districts where dykes of the gregarious type abound at a distance from 

 any of the basalt-plateaux, reference should be made to the curious isolated tract of the 

 central granite core of Western Donegal. In that area a considerable number of dykes 

 rises through the granite, to which they are almost wholly confined. Again, far to the 

 east another limited district, where dykes are crowded together, lies among the Mourne 

 Mountains. These granite hills are probably to be classed with those of Arran, as 

 portions of a series of granite protrusions belonging to a far more recent period than that 

 to which the youngest granitic masses of the Highlands are to be assigned. 



Though the dykes may be conveniently grouped in two series or types, which on the 

 whole are tolerably well marked, it is not always practicable to draw any line between 

 them, or to say to which group a particular dyke should be assigned. In some districts, 

 however, in which they are both developed, we can separate them without difficulty. 

 In the Argyleshire region above referred to, for example, which Mr Clough has mapped, 

 he finds that the abundant dykes belonging to the gregarious type run in a general N.W. 

 or N.N.W. direction, and distinctly intersect the much scarcer and less basic dykes of the 

 solitary type, which here run nearly E. and W. (fig. 17). Hence, besides their com- 

 position, distinction in number, breadth, rectilinearity, and persistence, the two series 

 demonstrably belong to distinct periods of eruption. 



* Trans. Hoy. Soc. Edin., xiv. (1840) p. 677. t Trans. Geol. Soc, iii. (1815) p. 79. 



