OTHER CLYDE ISLANDS. 353 



AILSA CRAIG. 



154. This majestic rock rises steeply from the sea to the 

 height of 1114 feet. It is 13 miles S. of Pladda, and 10 

 miles "VV. of Girvan, and has an elliptic base 3300 feet by 

 2200. Its form is nearly that of a right cone, but it is 

 somewhat more elongated from north to south than in the 

 opposite direction. This is distinctly seen on climbing to 

 the summit, which presents a flattened ridge in the former 

 direction, and is comparatively narrow from east to west. 

 In shelter from the prevailing winds, a high bank of shingle 

 has accumulated in the course of ages against the originally 

 steep face on the east side, and is now perfectly stable. 

 From the top of this bank the rock rises in broken terraces, 

 \ip which the ascent is not difficult; on other sides it is 

 inaccessible. An old stone keep, with vaulted apartments- 

 one of the line of watch-towers is perched upon a terrace 

 on this side, at about one-fifth of the height. With the 

 exception of the shingle bank, the isle consists wholly of a 

 reddish-coloured, close-grained syenitic greenstone, quite un- 

 like any other of the Clyde traps. On the Ayrshire coast, 

 opposite to Ailsa, a similar syenite is said to occur amid 

 strata of silurian age. The rock consists of red felspar and 

 hornblende, and occasional grains of quartz, and is inter- 

 mediate between a true syenitic or hornblendic granite and 

 syenitic greenstone. It is thus of igneous origin and a 

 member of the trap family, though not a basalt, as often 

 stated. The rock, however, affects the columnar form, and 

 the precipices on three sides exhibit magnificent ranges of 

 pillars, which reach the height of fully 400 feet ; but these 

 are much less perfect than in basalt, the angles are less 

 sharp, and the concavo-convex joints are wanting, the 

 divisions being simple seams. The upper 700 feet consist 

 of amorphous rock; but there is no perceptible difference 

 between the mineral structure of this and the columnar 

 part. The precipices are traversed by numerous basaltic 



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