THE NATURAL HISTORY OF AILSA CRAIG. 137 



Geology. — The form of Ailsa, at least in profile, is well-known, 

 and is concisely expressed in Keats's " Craggy ocean pyramid," 

 with "two dead eternities — the last in air, the former in the deep." 

 Its ground-plan cannot be so well made out by the eye, but the 

 Ordnance Surveyors have shown us that it is somewhat oblong, 

 the long axis being north and south. At the time Ailsa assumed 

 about its present orographical position it was probably nearly 

 circular in plan, but the waves have so nibbled at it, especially on 

 the west and south coasts, as to reduce the original sub-circular 

 to the present form, and this dissection has been of the greatest 

 value to the geologist in bringing out to a large extent its 

 remarkable geological structure. Prof. James Geikie, in his short 

 description of Ailsa, printed in the Rev. R. Lawson's book 

 (B. XV. 68), says that the rock probably rises from Silurian 

 strata, but I think that this is in the highest degree unlikely, 

 and for the following reason. If we extend the line of the 

 great boundary-fault — a " hitch " which separates geologically the 

 southern uplands of Scotland from the lowlands and is thought 

 to have a throw of some twelve thousand (12,000) feet — we will 

 find that Ailsa is situated seven miles north-west of that line, and 

 the only logical conclusion which we can draw from this fact is 

 that it rises from old red sandstone strata. Then as to what 

 Ailsa rock really is (considered as a part or " formation," of 

 the earth's crust) is another question fraught with difficulties. 

 (1) It may be part of the infilled pipe of an old volcano ; or (2) it 

 may be sedimentary rock metamorphosed in position ; or (3) it 

 may be the remnant of an immense sill or lacolite. The first is 

 the favourite theory with geologists, but there are serious objections 

 to it, the most serious being the columnar structure which is 

 nearly vertical, this feature having been well dissected out by the 

 waves on the south and west sides of the island. It is an axiom 

 in geology, well sustained by nearly every writer on the subject, 

 that columns in igneous rocks stand at right angles to the 

 cooling surface, so that, if Ailsa represents a remnant of a filled-up 

 pipe of a volcano, it is but reasonable to expect that the columns 

 would be lying in a horizontal manner. As to the second point, 

 the theory of igneous rocks having been metamorphosed from 

 sedimentary ones finds favour with many geologists, but here there 

 are no data to support it. The third theory answers best to the 



