1871. j geikie — tertiary yolcanic eocks, 311 



Discussion'. 



Prof. Hattghton inquired whether Mr. Geikie's attention had been 

 called to the Mourne Slountains 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. Eamsay had hitherto believed in the Oolitic age of these 

 trap-rocks in Eigg, but accepted the author's views. The interbed- 

 ding 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 j)oint 

 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 geolo- 

 gical 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. AU 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 do- 

 lerite 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. 



