Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/606

 Discussion.

Mr. Forbes stated that none of the facts of this communication were new ; but he dissented altogether from the conclusions arrived at by the author in regarding these rocks as originally of sedimentary origin, and for the following reasons : — (1) That this district had been studied in detail by Mr. Scott and Prof. Haughton, who declare the rock to be undoubtedly intrusive, as it not only sends out veins into the neighbouring strata, but also encloses fragments of the rocks through which it has broken. (2) Because the author starts from the idea that if such rocks are found to lie conformably on beds of undoubted sedimentary origin, it is a proof of their being themselves sedimentary or stratified, — a conclusion which is totally unwarranted, since there are innumerable instances, not only of beds of lava or other igneous rocks being conformable to fossiliferous strata, but of their also being found intercalated with such beds even for considerable distances. (3) The strata, so far from being proved by him to be of truly sedimentary origin, are of a most questionable origin, since they are neither in themselves fossiliferous, nor can they be correlated with any containing fossils as proofs of true sedimentary deposition ; and the description of his section is sufficient to show this ; for although it looks well on paper on a scale of 3 feet to the mile, the author has so little confidence in it that he is not even certain as to which is the top or bottom of the section on which so much generalization is based. (4) That a parallel structure equally, if not better, developed than any occurring in the gneiss of Donegal is common to many volcanic rocks, as in a specimen laid before the meeting, in which this parallel foliated structure due to crystallization-layers is so well developed as to make it appear exactly like a stratified rock, and even split along these lines ; and this, although the product of volcanoes still active, is found for great distances both overlying conformably and intercalated between beds of the Cretaceous and Oolite formations.

Mr. Scott was unwilling to accept the section given by the author as satisfactory, but stated that Mr. Green had, without knowing of the existence of his papers on Donegal, confirmed many of his observations. He agreed with him as to the bedded appearance of the granite, and to the masses lying in general conformably with the lines of stratification of the country. The nearest spot at which fossiliferous rocks occurred was separated from the beds described by the whole width of the county of Tyrone, though some presumed Eozoonal forms had been found at a less distance. He was not prepared to believe in the original absolutely fused condition of granite, nor in there being two distinct types of granite, one metamorphic and the other purely igneous.