Geological Society of London. 281 



leritic lavas had long ceased to flow in this district. Their succes- 

 sive beds, widely and deeply eroded by atmospheric waste, were here 

 hollowed into a valley traversed by a river, which carried southward 

 the drainage of the wooded northern hills. Into this valley, slowly 

 scooped out of the older volcanic series, the pitchstone and porphyry 

 coulees of the Scuir flowed. Vast, therefore, as the period must be 

 which is chronicled in the huge piles of volcanic beds forming our 

 dolerite plateaux, we must add to it the time needed for the excava- 

 tion of parts of those plateaux into river- valleys, and the concluding 

 period of volcanic activity during which the rocks of the Scuir of 

 Eigg were poured out. 



5. Lastly, from the geology of this interesting island we learn, 

 what can be nowhere in Britain more eloquently impressed upon 

 us, that, geologically recent as that portion of the Tertiary periods 

 may be during wliich the volcanic rocks of Eigg were produced, it is 

 yet separated from our own day by an interval sufficient for the 

 removal of mountains, the obliteration of valleys, and the excavation 

 of new valleys and glens where the hills then stood. The amount 

 of denudation which has taken place in the Western Islands since 

 Miocene times will be hardly credible to those who have not ade- 

 quately realized the potency and activity of the powers of geological 

 waste. Subterranean movements may be called in to account for 

 narrow gorges, or deep glens, or profound sea-lochs ; but no sub- 

 terranean movement will ever explain the history of the Scuir of 

 Eigg, which will remain as striking a memorial of denudation as it 

 is a landmark amid the scenery of our wild western shores. 



Discussion. — Prof. Haughton inquired whether Mr. Geikie's attention had been 

 called to the Morne Mountains in Ireland, ■which seemed to present some analogous 

 phenomena to those described in the paper. In the Morne district were dykes of 

 dolerite, pitchstone, and other volcanic rocks of the same constitution as those of 

 Antrim. He believed that a chemical examination of these rocks in diiferent 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 difi'erent 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 interbedding of volcanic beds among the Lower 

 Silui'ian 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 re- 

 garded 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 as 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 termin- 

 ology 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 anamezite, the finely-grained variety, and the true basalt, but also trachylite, 

 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 Morne Mountains. He 



