ii. RELATIONS OF THE GABBROS TO THE OTHER MEMBERS OF THE VOLCANIC SERIES

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]

[Footnote 336: _Mineralogical Travels_ (1813), vol. ii. p. 72.]

[Footnote 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]

[Footnote 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]

[Footnote 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]

[Footnote 340: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 249.]

[Footnote 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.]

[Footnote 342: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871), pp. 58, 92.]

Up to the time of the publication of my memoir in 1888 no one had traced out in more detail the actual boundaries of the several rocks on the ground, so as to obtain evidence of their true relations to each other as regards structure and age. Some of the numerous impediments recorded by Macculloch no doubt retarded the investigation. But, as Forbes so well pointed out, there is really no serious difficulty in determining the true structural connection of the amorphous rocks with each other and with the bedded basalts of the plateaux. I have ascertained them in each of the districts,[343] and have found that there cannot be the least doubt that the amorphous bosses, both basic and acid, are younger than the surrounding bedded basalts, and that the acid protrusions are on the whole younger than the basic, I shall now proceed to show how these conclusions are established by the evidence of each of the areas where the several kinds of rock occur.

[Footnote 343: In two of my excursions in Mull, and once in Skye, I was accompanied by my former colleague Mr. H. M. Cadell, who rendered me great assistance in mapping those regions.]