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NATURE 



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BRITISH TERTIARY VOLCANOES} 



DURING the last twenty-seven years, the study of the 

 volcanic rocks of the British Isles has been a 

 constant and favourite pursuit with Dr. A. Geikie. It is 

 now seventeen years since he read before the Geological 

 Society of London the most important of the numerous 

 memoirs which he had from time to time up to th.it date put 

 forth on this subject. It was the well-known paper on the 

 Island of Eigg, and was intended to be the first of a series 

 of papers descriptive of localities where the characteristic 

 features of the British volcanic rocks are well displayed. 

 But man proposes : for the promised continuation of the 

 series geologists have waited long and anxiously, no 

 further instalments having till now appeared. The delay 

 however, though trying while it lasted, has been pro- 

 ductive of good result in the end ; for we have now the 

 long-wished-for consummation, not scattered through 

 a long string of isolated papers, but in one connected 

 whole. Dr. Geikie has garnered his harvest, and has 

 summed up the results of the labours of more than a 

 quarter of a century in a memoir which may fairly be 

 looked upon as one of the most important of the contribu- 

 tions to the geological history of Britain which have seen 

 the light since the days of William Smith. 



And it gives to this elaborate communication a further 

 importance that it is controversial as well as descriptive. 

 In January 1S74, Prof J. W. Judd read before the Geo- 

 logical Society a paper " On the Ancient V^olcanoes of 

 the Highlands." It is a singularly fascinating production ; 

 its storv is concisely and graphically told, and hangs well 

 together ; and I shall not easily forget the interest with 

 which I read it for the first time, and which frequent 

 reference to it subsequently has not abated. But Dr. 

 Geikie's study of the subject has led him to conclusions 

 directlv in the teeth of two at least of the most important 

 of those arrived at by Prof. Judd. Both authorities 

 are agreed that the great basaltic plateaus of the Western 

 Islands of Scotland and the North-East of Ireland are 

 formed of sheets of sub-aerial lava piled one above 

 another, the products of a long series of eruptions. 

 Prof. Judd holds that the lavas were poured out from 

 great central volcanoes of the type of Etna and Vesuvius, 

 and he has endeavoured to fi.x the sites, and form an 

 appro.vimate estimate of the size of these volcanoes. 

 Dr. Geikie is unable to find evidence for the former 

 existence of central volcanic piles, and he believes that 

 the lavas were emitted from fissures and numerous 

 scattered vents of inconsiderable size. Again Prof. 

 Judd thought he had established a threefold order of 

 events during the period of volcanic activity. The first 

 series of eruptions was marked by the discharge of lavas 

 belonging to the acid class. Then came an abatement or 

 cessation of the volcanic energy, and during a quiescent 

 interval the cones and products of this period were largely 

 denuded. Volcanic activity was resumed during the second 

 period, but its products were of the basic class, and now 

 form the basaltic plateaus. During the third period the 

 volcanic energy was dwindling down, and had so far 

 spent itself that it was equal to the production only of 

 sporadic cones of small size, which are paralleled with the 

 " Puys " of Auvergne. 



And Prof. Judd further maintained that the great 

 intrusive masses of granite and gabbro, which now form 

 some of the boldest heights of the district, are the 

 hardened contents of the reservoirs which fed the 

 volcanoes of the first two periods. They had originally 

 been buried beneath the cones that were heaped over 

 them by the eruptions, and have been bared by denudation. 

 The granites belong to the volcanoes of the first period, 



' "The History of Volcanic Acti ,n during the Tertiary Perod in the 

 British Isles." By Archibald (ieikie. LL.D,, F.K.S., Director.General of 

 the Geoloiiical Survey of the Unitci Kingdom. Transactions of the Royal 

 Society of Edinburgn, vol. *x.\v.. Part 2. (Edinburgh : R. Grant and Son, 

 1888.) 



and Prof Judd maintained that a gradual passagve, 

 could be traced from them into sub-aerial lavas of acid 

 composition that were emitted during that period. 

 Similarly the gabbros were relegated to the secoad period, 

 and pass gradually into its basic lavas. Dr. Geikie on 

 the other hatid brings an overwhelming mass of evidence 

 to show that the intrusive masse i of granite and other 

 acid rocks are younger than the plateau-basalts. He 

 mentions nothing that can possibly correspond with the 

 sub-aerial sheets of acid lava which Prof. Judd states 

 were poured out during his first period ; he shows, indeed, 

 that the one solitary known instance of a true superficial 

 stream of acid lava is that of the Scuir of Eigg, which 'v=> 

 unquestionably considerably younger than the plateau- 

 basalts. 



Though Dr. Geikie does not express a positive opinion 

 on the subject, it seems to me that there is nothing to 

 forbid our looking upon some at least of the smaller vents 

 which he describes as Puys as belonging to Hrof Judd's 

 third period. The vent, for instance, at Faskadale (p. 106) 

 must be later than the great acid protrusions. 



It is a serious matter for one whose acquaintance with 

 the field of dispute is but slight to endeavour to hold the 

 balance fairly and evenly between the conflicting views of 

 two such eminent authorities, who have both made a study 

 of the ground itself. Nor is the delicacy of the task 

 diminished by the fact that the disputants have been for 

 many years among the writer's most valued brethren of 

 the hammer, and that to their teaching and example he 

 owes more than can be put into words. This circum- 

 stance has fortunately however a certain advantage, for 

 in attempting to decide between the conclusions of two 

 equally valued and equally respected friends he will at 

 least be free from any suspicion of partiality. 



Some general considerations may be noticed before 

 coming to detailed criticism. Dr. Geikie has known the 

 ground ever since he was a boy : he has roamed over it 

 again and again ; he has had opportunities without num- 

 ber of reviewing, and in some cases of correcting, his first 

 impressions. He has had, to some exient, the assistance 

 and CO- operation of his colleagues on the Geological 

 Survey, and has had free access to all the details of their 

 elaborate surveys. I believe 1 am right in saying that 

 Prof Judd was able to devote to his examination of the 

 district the summer months of not more than two or three 

 years. Without in the least implying that his observa- 

 tions were hasty, it must be clear that his opportunities 

 for going into detail were very inferior to those of Dr. 

 Geikie. 



Further I am bound to confess that, though I was fairly 

 carried away by the charm of Prof J udd's paper, he did 

 not succeed in bringing conviction to my mind to the 

 same extent as the perusal of Dr. Geikie's memoir has 

 done. His story had on the face of it an air of reaUty, 

 but his statements were broad and general, and I could 

 not help wishing that he had interpolated among his 

 sweeping conclusions some details of the evidence on 

 which those conclusions were based. I should have been 

 sorry to miss the bold and strikingly graphic sections of 

 his folding-plate ; but I should have liked to have had, in 

 the text, woodcuts, such as those which crowd Dr. Geikie's 

 memoir throughout, of the actual exposures out of which 

 those generalized representations had been constructed. 

 It is easy to imagine good reasons for the omission of 

 these details in Prof Judd's paper, but all those who have 

 made it a business of their life to cultivate a healthy tone 

 of scepticism must have regretted their absence. No 

 such charge can be brought against Dr. Geikie ; more 

 than sixty woodcuts, most of them representing actual 

 sections, give ample opportunity for deciding for or 

 against tiie sufficiency of his evidence. 



We may now examine more in detail the main points 

 of difference between the two readings ; and, first of all, 

 as to the vents from which the lava-flows were discharged. 



