132 



NATURE 



{Dec. 6, 1888 



Dr. Geikie relies on the absence of any obvious vent from 

 which the molten matter flowed. But surely the huge 

 orifice of Strath, in Skye, was large enough to have 

 served such a purpose. True there are appearances 

 which seem to show that some of the plateau-basalts once 

 extended right across the mouth of this funnel, but Dr. 

 Geikie himself admits with perfect candour that the 

 relation of this neck to the plateau-basalts does not admit 

 of satisfactory treatment, owing to the destruction of the 

 evidence by later intrusion of masses of granophyre in its 

 immediate neighbourhood, and likewise to enormous de- 

 nudation. I see nothing unlikely in the supposition that, 

 from this enormous funnel, basaltic lava may have flowed 

 in a manner to be shortly described ; that the chimney 

 became afterwards choked by agglomerate, too coarse to 

 be spread far over the neighbourhood ; and that, above 

 all, basalt emitted from some new adjoining vent may 

 have afterwards extended itself. Dr. Geikie further lays 

 stress on the uniformity of the plateau-basalts in petro- 

 graphical character, thickness, and persistent flatness, 

 and on the almost total absence of interbedded frag- 

 mental deposits ; and he maintains that these distinctive 

 characters lead us to seek the modern analogues of the 

 volcanic phenomena, not in large central cones like 

 Vesuvius and Etna, but in the vast basalt-fields of West- 

 ern America, where the lavas have issued from innumer- 

 able minor, and sometimes almost imperceptible, vents. 

 With the first part of this opinion everyone must, I 

 think, side with Dr. Geikie ; but the method of formation 

 which he advocates is by no means the only one possible 

 or likely. Ever since I read Captain Dutton's account 

 of the Hawaiian volcanoes,-' it has seemed to me that it 

 is to them we must look, if we are to understand the 

 machinery by which great lava-plateaus have been pro- 

 duced. Speaking of the enormous flow which issued 

 from Mauna Loa in 1855, he says : " As I looked over 

 this vast expanse of lava, I was forcibly reminded of the 

 great volcanic fields of the western portion of the United 

 States, where the eruptions are of such colossal propor- 

 tions that they have received the name of massive erup- 

 tions." After noticing Richthofen's view that these lavas 

 had been poured forth through great fissures, and stating 

 that the volcanic rocks of Western America, well as they 

 are laid open to view, would be considered relatively 

 obscure by one who has had an opportunity of inspecting 

 the recent lavas of Mauna Loa, he goes on thus : — " I 

 am by no means certain that Richthofen's conclusions 

 are wrong. But here is a lava-flow, the dimensions of 

 which fully rival some of the grand Pliocene outbreaks 

 of the West, which demonstrably differs in no material 

 respect, excepting in grandeur, from the much smaller 

 eruptions of normal volcanoes" {Joe. cit., p. 156). But 

 the differences between the modes of action of vol- 

 canoes of the Vesuvian and the Hawaiian types, whether 

 we designate them as material or not, are striking 

 enough, and they are just those which seem to have 

 accompanied the discharge of the plateau-basalts we are 

 now engaged with. Captain Dutton has well described 

 them. " Mauna Loa and Kilauea," he says, " are in many 

 important respects abnormal volcanoes. Most notable is 

 the singularly quiet character of their eruptions. Rarely 

 are these portentous events attended by any of that ex- 

 tremely explosive action which is characteristic of nearly 

 all other volcanoes. The lava wells forth like water from a 

 hot, bubbling spring ; but so mild are the explosive forces 

 that the observer may stand to the windward of the 

 grandest eruption, and so near the source that the heat 

 will make the face tingle, yet without danger. A direct 

 consequence of this comparatively mild and gentle beha- 

 viour is the absence of those fragmental products which 

 form so large a portion of the products of other vol- 

 canoes " {loc. cit., pp. 84, 85). Fissure-eruptions are, to 

 say the least, hypothetical ; but here we have a way in 



' United States Geological Survey, Fourth Annual Report, 1882-S3. 



v^hich huge lava-fields, of the type of basaltic plateaus, 

 are being produced before our eyes. The universally 

 adopted canons of geological reasoning leave us no 

 alternative as to which of the two explanations we should 

 favour. 



But if the view just expressed be correct, we ought 

 certainly to find some indications left, even among these 

 ruined volcanoes, of the position of the vents from which 

 the lavas issued. And here I cannot help going a long 

 way with Prof. Judd in thinking that the great eruptive 

 bosses of gabbro in Skye, Rum, Ardnamurchan, and 

 Mull, are plugs filling in some of the main orifices of dis- 

 charge. Prof. Geikie lays stress on the facts that the 

 gabbros send off intrusive sheets into the plateau-basalts, 

 and even ovei-lie them. But this proves merely that the 

 plugs which now fill the vents are later than the plateau- 

 basalts : the vents themselves may be older. There must 

 be some reason why the great intrusive bosses cluster 

 j thickly round a few centres, and are elsewhere conspicuous 

 by their absence, and the following seems not unlikely. 

 It was at these spots that vents were opened early in the 

 volcanic period ; from them there flowed, in the mild 

 undemonstrative fashion of the Hawaiian volcanoes, the 

 lavas which now build up the basaltic plateaus ; as sheet 

 was laid down upon sheet, the chimney gradually rose in 

 height ; and when, for this reason, and perhaps also on 

 account of a temporary abatement of volcanic energy, 

 the lava was no longer able to flow out at the top, it 

 solidified in the vent, and, being under pressure, hardened 

 into gabbro instead of dolerite. And indeed, though 

 Dr. Geikie speaks of the eruption of the gabbro bosses 

 as an event sufficiently marked and independent to cha- 

 racterize a distinct epoch in the volcanic period, he at the 

 same time expresses himself in a way that shows he 

 shares in the view I have just put forward, for he says : 

 I " We must remember, however, that the gabbro in many 

 places found its readiest ascent in vents belonging to the 

 j plateau-period." 



So far then the views of Dr. Geikie and Prof. Judd 

 may admit of modifications which render them less con- 

 flicting than they seem at first sight. But there is one 

 1 point on which reconciliation is impossible, viz. the 

 nature and relative date of the eruptions of acid compo- 

 I sition. Prof. Judd recognizes not only acid eruptions of 

 the massive type — granites and their allies— but he speaks 

 ; of thick bodies of felstones, disposed in regular sheets 

 and of amygdaloidal structure, which alternate with beds 

 of scoriae, lapilli, and ashes, that lie upon the skirts of 

 I the central bosses of granite. These he believes to be 

 I the remnants of a volcano formed mainly of acid lavas, 

 j which was piled up and largely ruined by denudation 

 I before the discharge of the plateau-basalts began. The 

 existence of the granite bosses admits of no doubt ; but 

 Dr. Geikie has depicted numerous sections which leave 

 no doubt that these rocks intrude into the basalts and 

 gabbros, and are therefore of later date than them. Now 

 that all these details are before us, the question of relative 

 age can admit of only one answer, but it is evidently a 

 point on which observers, who had not opportunities of 

 entering minutely into details, were apt to go wrong. 

 Both Principal J. D. Forbes and Prof. Zirkel seem to 

 have come to the same conclusion as Prof. Judd, and Dr. 

 Geikie has supplied the explanation. " That there should 

 ever have been any doubt," he says, " about the relations 

 of the two eruptive masses is possibly explicable by the 

 facility with which their junction can be observed. Their 

 contrasts of form and colour make their boundary over 

 j crag and ridge so clear that geologists do not seem 

 I to have taken the trouble to follow it out in detail. And 

 as the pale rock (granophyre or granite) underlies the 

 dark (gabbro), they have assumed this infraposition to 

 1 mark its earlier appearance." All this is graphically 

 i brought out in Fig. 43 of Dr. Geikie's memoir, which is re- 

 I produced here (Fig. i). Anyone trusting to surface-feature 



