CHAP. XXXIV 
TYPES OF DYKES 
I2 3 
In the second; or wliat for brevity may be called the Gregarious type, 
the dykes occur in great abundance within a particular district. They are 
on the whole narrower, shorter, less strikingly rectilinear, more frequently 
tortuous and vein -like, and generally more basic in composition than 
those of the first type. They include the true basalts and dolerites. Illus- 
trative districts for dykes of this class are the islands of Arran, Mull Eigg 
and Skye. ’ 
Ihe great single or solitary dykes may be observed to increase in 
number, though very irregularly, from south to north, and also in Central 
Scotland from east to west. They are specially abundant in the tract 
stretching from the Firth of Clyde along a belt of country some thirty miles 
broad on either side of the Highland line, as far at least as the valley of the 
Tay. They form also a prominent feature in the islands of Jura and 
Islay. 
Dykes of the gregarious type are abundantly and characteristically dis- 
played in the basin of the \ irth of Clyde. Their development in Arran 
formed the subject of the interesting paper by Necker, already mentioned, 
who catalogued and described 149 of them, and estimated their total number 
in the whole island to be about 1500. 1 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 been found by my 
colleague, Mr. C. T. Clough, to contain an extraordinary number of dykes 
(see 1 ig. 257). 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 Maeculloch, who called attention more especially to their extra- 
ordinary development in the district of Strathaird. “ They nearly equal in 
some places,” he says, “ when collectively measured, tire stratified rock through 
w hich 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. 2 This estimate, however, falls much short of the truth in 
,sorQe parts of Strathaird, where the dykes are almost or quite contiguous, 
1 Trans. Roy. Soc. Edin, xiv. (1840), p. 677. 
Ivans. Geol. Soc. iii. (1815), p. 79. This locality is further noticed on p. 164. 
