Discussion.
Prof. Haughton inquired whether Mr. Geikie's attention had been called to the Mourne Mountains in Ireland, which seemed to present some analogous phenomena to those described in the paper. In the Mourne district were dykes of dolerite, pitchstone, glossy basalt, and other volcanic rocks of the same constitution as those of Antrim. He believed that a chemical examination of these rocks in different districts would prove their common origin. The evidence in Antrim was conclusive as to their Tertiary age in Ireland ; and he was glad to find that the view of their belonging to a different age in Eigg was erroneous.
Prof. Ramsay had hitherto believed in the Oolitic age of these trap-rocks in Eigg, but accepted the author's views. The interbedding of volcanic protrusions among the Lower Silurian beds in Wales was somewhat analogous. He was glad to find the history of these igneous rocks treated of in so geological a manner, instead of their being regarded from too purely a lithological and mineralogical point of view. The great antiquity of these Middle Tertiary beds had, he thought, been most admirably brought forward in the paper, as well as the enormous amount of denudation ; and he would recommend it to the notice of those who had not a due appreciation of geological time.
Mr. Forbes hoped that the geologist would remember that his father was a mineralogist. It was refreshing to find a paper of this kind brought before the Society, as it was to be regretted that the details of mineralogy were so little studied in this country when compared with the Continent ; and this he attributed to the backward state of petrology (admitted by Mr. Geikie) in this country. He quite agreed in the view of the Tertiary age of these rocks. With regard to the terminology employed by the author, he objected to the use of the word dolerite, as distinct from basalt ; basalt properly comprised, not only dolerite (the coarse-grained variety) and anamesite (the finely-grained variety), as well as true basalt, but also tachylite (the glassy variety), which was frequently confounded with pitchstone. All four names merely referred to structure, and not to composition.
Mr. Geikie, in reply, stated that he had not examined the Mourne Mountains. He had not in any way wished to disparage mineralogy, but, on the contrary, had attempted to classify the different rocks according to their petrographical characters. He used the term dolerite in the same sense as the German mineralogists, both as the generic name for the whole series, and also for the coarser variety of basalt.
