C— GEOLOGY. 49 



merely replaced, bulk for bulk, a mass of solid crust which had foundered 

 and has completely disappeared. Although not perfectly symmetrical, the 

 relations of the successive intrusions to each other, and to the country- 

 rock, are typically those of ring-dykes. The granites show to perfection 

 their roofs as well as their bounding sides, and demonstrate how the 

 vertical or steeply inclined walls turn abruptly into a slightly domed roof. 

 In this respect the intrusions closely resemble the granophyres of Beinn 

 a Ghraig and Knock in Mull, which have replaced a great mass of lavas 

 and minor intrusions without disturbing the arrangement of the adjoining 

 country-rock. Mr. Richey states that no floor can be detected to the 

 Mourne intrusions. He postulates that overhead piecemeal stoping has 

 played no part in allowing the rise of magma, and that the space occupied 

 by the various granites has probably been provided by a subterranean 

 cauldon-subsidence of the pre-existing rocks. 



In 1900 Gunn discovered a large oval area, about four miles in 

 diameter, in central Arran, which consisted of fragmental rocks in close 

 association with numerous igneous masses, and which he described as a 

 volcanic vent. Dr. Tyrrell, as the result of recent work in the island, 

 clearly recognises in this vent a ring-complex, which, though less perfectly 

 preserved than those of Mull, Ardnamurchan and Skye, he considers to 

 have had an equally long and complicated history. The main complex 

 is outlined by masses of explosion-breccia, of which the formation was 

 concomitant with the intrusion, and possible eruption, of an acid magma 

 that ascended an arcuate fissure bordering the vent. It probably occupies 

 the position of, or intersects, an older complex, for gabbros and other basic 

 plutonics of older date are cut by it and also enter into the composition of 

 the explosion-breccias. 



It is interesting to find within the region of the complex isolated 

 masses of plateau basalt preserved from denudation. Their presence 

 points to two facts : firstly, that before the initiation of the central area, 

 plateau basalts had been erupted as in other districts ; and secondly, 

 that their preservation can only be accounted for by assuming subsidence 

 of central type. 



In the north of Arran a granite mass, some eight miles in diameter, 

 occupies an almost circular area outlined by Dalradian schists. This 

 mass also is regarded as being of Tertiary age, although definite proof is 

 lacking. Its relation to the surrounding sedimentary rocks and its 

 general likeness to the granites of the Mourne Mountains and of the central 

 complex of Arran are certainly strong evidence for its intrusion in Tertiary 

 times. 



Although a feature usually associated with laccolitic intrusions, it 

 has been proved in Mull and elsewhere that individual ring-dyke or stock- 

 like masses may arch their roofs when they develop an excess of pressure — 

 either hydrostatic or due to dissolved gases. This arching, as in the 

 Mournes, is usually quite gentle ; but if carried to a greater height than 

 usual, Mr. Bailey considers that the centrifugal forces acting within the 

 unconsolidated magma in the higher portions of a cylindrical intrusion 

 will exert an outward pressure on the containing walls. The relief of 

 this pressure, he argues, would be accomplished by the arcuate folding 

 or wrinkling up of the surrounding strata, and be accompanied by a 

 1927 E 



