Various opinions have been expressed regarding the connection between the amorphous eruptive rocks of the hill-groups and the level basalt-sheets of the plateaux. Jameson, though he landed at Rudh' an Dunain, in Skye, where this connection can readily be found, does not seem to have made any attempt to ascertain it. He noticed that the lower grounds were formed of basalt, and that the mountains "appeared to be wholly composed of syenite and hornblende rock, traversed by basalt veins."[336] Macculloch, in many passages of his Western Islands, alludes to the subject as one which he knew would interest geologists, but about which he felt that he could give no satisfactory information, and with characteristic verbiage he refers to the impossibility of determining boundaries, to the transition from one rock into another, to the inaccessible nature of the ground, to the almost insuperable obstacles that impede examination, to the distance from human habitation, and to the stormy climate,—a formidable list of barriers, in presence of which he leaves the relative position and age of the rocks unsettled.[337]
[336] Mineralogical Travels (1813), vol. ii. p. 72.
[337] See his Western Islands, vol. i. pp. 368, 374, 385, 386. With much admiration for the insight and zeal, amounting almost to genius, which Macculloch displayed in his work among the Western Islands, at a time when, with poor maps and inadequate means of locomotion, geological surveying was a more difficult task than it is now, I have found it impossible to follow in his footsteps with his descriptions in hand, and not to wish that for his own fame he had been content to claim credit only for what he had seen. His actual achievements were enough to make the reputation of half a dozen good geologists. It was unfortunate that he did not realize how inexhaustible nature is, how impossible it is for one man to see and understand every fact even in the little corner of nature which he may claim to have explored. He seems to have had a morbid fear lest any one should afterwards discover something he had missed; he writes as if with the object of dissuading men from travelling over his ground, and he indeed tacitly lays claim to anything they may ascertain by averring that those who may follow him "will find a great deal that is not here described, although little that has not been examined" (p. 373). Principal Forbes long ago exposed this weak side of Macculloch and his work (Edin. New Phil. Journ. xl. 1846, p. 82).
Von Oyenhausen and Von Dechen, who wrote so excellent an account of their visit to Skye, and who traced much of the boundary-line between the gabbros and the other mountainous eruptive masses ("syenite"), seem to have made no attempt to work out the connection between the former and the rest of the volcanic rocks.[338]
[338] Karsten's Archiv, i. p. 99. They frankly admit that "the relation of the hypersthene rock to the other trap rocks was not ascertained."
J. D. Forbes, in his able sketch of the Topography and Geology of the Cuchullin Hills, was the first to recognize the superposition of the "hypersthene rock" upon the "common trap rocks"—that is, the plateau-basalts. He was disposed to consider the "hypersthene mass as a vast bed, thinning out both ways, and inclined at a moderate angle towards the S.E."[339]
[339] Edin. New Phil. Journ. xl. (1846), pp. 85, 86.
Professor Judd regarded the bosses of basic and acid rocks that rise out of the bedded basalts as the basal cores of enormously denuded volcanic cones. He believed the granitoid rocks to have been first erupted, and that after a long interval the basic masses were forced through them, partly consolidating underneath and partly appearing at the surface as the plateau-basalts.[340] That the order of appearance of the several rocks has been exactly the reverse of this supposed sequence was fully established by me in the year 1888, and has since been amply confirmed.[341] Professor Zirkel recognized that the gabbros are a dependence of the basalts, that they overlie them, and that on the naked flanks of the mountains they are regularly bedded with them.[342]
[340] Quart. Journ. Geol. Soc. xxx. (1874), p. 249.
[341] Trans. Roy. Soc. Edin. xxxv. (1888), pp. 122 et seq.; Quart. Journ. Geol. Soc. vol. 1. (1894), pp. 216, 645; vol. lii. (1896), p. 384, and Mr. Harker, ibid. p. 320.