CHAI’. XXXIV 
DISTRIBUTION OF THE DYKES 
1 2 1 
walls ol which are those of the fissures up which the molten rock ascended 
(Fig. 234). Some good instances of this kind are well known to summer 
a isitors on the eastern shores of Arran. Others, on a large scale, may be 
seen in the interior of the same island along the crests of the granite ridges, 
and still more conspicuously on the jagged summits of Blath Beinn and the 
Cuillin Hills (Fig. 333), and intersecting the Jurassic strata along the 
cliffs of Strathaird in Skye. 
1. GEOGRAPHICAL DISTRIBUTION 
The limits of the region within which the dykes occur cannot be very 
precisely fixed. There can be no doubt, however, that on their southern 
side they reach to the Cleveland Hills of Yorkshire and the southern borders 
of Lancashire, perhaps even as far as North Staffordshire (p. 106), and on the 
northern side to the farther shores of the island of Lewis — a direct distance 
of •> 6 0 miles. They stretch across the basin of the Irish Sea, including the 
Isle of Man, and appear in Ireland north of a line drawn from Dundalk Bay 
to the Bays of Sligo and Donegal. Dykes are of frequent occurrence over 
the north of England and south of Scotland, at least as far north as a line 
drawn from the coast of Kincardineshire along the southern flank of the 
C-rampian Hills, by the head of Glen Shee and Loch Tay, to the north- 
western coast of Argylesliire. They abound all. along the line of the Inner 
Hebrides and on parts of the adjacent coasts of the mainland, from the 
remoter headlands of Skye to the shores of County Louth. They traverse 
also the chain of the Long Island in the Outer Hebrides. So far as I am 
aware, they are either absent or extremely rare in the Highlands north of the 
Fne I have indicated. But a good many have been found by my colleagues 
in the course of the Geological Survey of the northern lowlands of Aberdeen- 
shire and Banffshire. The longest of these has been traced by Mr. L. Hinxman 
lor rather more than two miles running in a nearly east and west direction 
tin ough the Old Bed Sandstone of Strathbogie, with an average width of 
about 35 feet. Another in the same district has a width of from 45 to 90 
leet, and has been followed for a third of a mile. But far beyond these 
northern examples, I have found a number of narrow basalt-veins traversing 
Hie Old Bed flagstones of the Mainland of Orkney, which I have little doubt 
dle a ^‘ so a prolongation of the same late series. Taking, however, only those 
western and southern districts in which the younger dykes form a notable 
eature in the geology, we find that the dyke-region embraces an area of 
upwards of 40,000 square miles — that is, a territory greater than either 
Gotland or Ireland, and equal to more than a third of the total land-surface 
ot the British Isles (Map I.). 
Of this extensive region the greater portion has now been mapped in 
' etail by the Geological Survey. Every known dyke has been traced, and 
ie appearances it presents at the surface have been recorded. "VVe are 
accordingly now in possession of a larger body of evidence than has ever 
e ore been available for the discussion of this remarkable feature in the 
