•J> DR GEIK1E ON THE HISTORY OF VOLCANIC ACTION 



an address to the Geological Section of the British Association, I reiterated these views, 

 and more particularly emphasised the importance of the system of dykes, which in my 

 opinion was possibly the most striking manifestation of the vigour of Tertiary volcanic 

 action.* In 1871, after further explorations in the field, I gave a detailed account of 

 the structure which had led to the mistake as to the age of the Tertiary volcanic rocks 

 of the Western Islands ; and in a description of the island of Eigg, I brought forward 

 data to show the enormous duration of the Tertiary volcanic period in the west of 

 Britain. t It was my intention that the paper in which these views were enunciated 

 should be continued in a subsequent series of memoirs. Before the preparation of the 

 second of the series was completed, Mr J. W. Judd read before the Geological Society 

 (21st January 1874) a paper " On the Ancient Volcanoes of the Highlands. "I The most 

 novel feature of this paper was the announcement that the author had recognised 

 the basal wrecks of five great central volcanoes in the Western Islands, among which 

 that of Mull was inferred by him to have been at least 14,500 feet high. He was led 

 to the conclusion that the volcanic period in these regions was divisible into three 

 sections, — the first marked by the outburst of acid rocks (felspathic lavas and ashes, 

 connected with deeper and more central granitic masses); the second by the extrusion 

 of basic lavas and tuffs (the basaltic plateaux) ; the third by the appearance of small 

 sporadic volcanic cones (" felspathic, basaltic, or intermediate in composition ") after the 

 great central cones had become extinct. It will be seen from the present communication 

 that the views adopted by Professor Judd are not those to which my study of the 

 subject has led me. I have not been able to discover evidence of any great central 

 volcanoes, and have found the order of outflow of the successive groups of rocks to have 

 been the reverse of what he believed it to be. The appearance of his memoir, however, 

 led me to postpone the continuation of the series of papers which I had begun. The 

 conviction that, in some way or other, as yet wholly inexplicable to me, the dykes, 

 which at so early a period of my researches arrested my attention, had played a leading 

 part in the volcanic phenomena of the Tertiary period, became every year stronger. 

 At last, in the year 1879, during a traverse of some portions of the volcanic region of 

 Wyoming, Montana, and Utah, I was led to perceive the meaning of what had hitherto 

 been so puzzling. Eiding over those great plains of basalt, and looking at the sections 

 cut by the rivers through the thick series of horizontal basalt-beds underlying them, 

 I appreciated for the first time the significance of Richthofen's views regarding 

 " massive " or " fissure-eruptions," as contradistinguished from those of central volcanoes 

 like Etna or Vesuvius, and I saw how completely the structure and history of these 

 tracts of Western America explain those of the basalt-plateaux of Britain. § Since that 

 year, at such intervals of leisure as I could command, I have renewed the investigation, 

 and now at last, after a quarter of a century of more or less continuous labour in the 



* Brit. Assoc. Report (Dundee), 1867, sects, p. 49. t Quart. Jour. Geol. Soc, xxvii. (1871) p. 279. 



I Quart. Jour. Geol. Soc, xxx. (1874) p. 220. 



§ Geological Essays at Home and Abroad, pp. 271, 274; Nature, November 1880. 



