100 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



§ 3. Vents of Eruption. 



In the approximate horizontality and regularly stratified arrangement of their 

 component beds of lava, the plateaux of Britain resemble those of older Tertiary 

 and still earlier date in other volcanic tracts, both in the Old and the New World, 

 where the absence of any obvious vents from which the molten material flowed 

 long presented a difficulty to geological students. I have stated that in no one in- 

 stance have I been able to discover a trace of any central volcano, whence the sheets of 

 basalt in the British plateaux could have proceeded. On the contrary, the uniformity 

 of the beds in petrographical character, thickness, and persistent flatness point, I think, 

 unmistakably to the occurrence not of a few great volcanoes, but of many minor vents 

 breaking out one after another and shifting from district to district. Only by some such 

 distribution of the foci of discharge can we account for the continuity and horizontality 

 of the basalts that have gradually built up the plateaux. It is one of the most interesting- 

 points in this volcanic history that, in spite of the enormous geological revolutions that 

 have passed since they became extinct, the sites of many scattered vents can still be 

 recognised. A far greater number must lie buried under the basalts, and of others the 

 positions are concealed by the sea, which now covers so large an area of the old lava- 

 fields. Nevertheless, partly on the surface of the plateaux, but still more on the 

 surrounding tracts from which the basalts have been removed by denudation, the stumps 

 of unmistakable vents of discharge stand out prominently amid the general wreck. 



Obviously it may be difficult to connect these vents directly with the plateau- 

 lavas. On the one hand, those which project from the surface of the plateaux must, 

 of course, be younger than the basalts through which they rise ; how much younger we 

 cannot tell. They may possibly be later than any of the plateau-sheets ; they may even 

 belong to a subsequent and waning condition of volcanic action. On the other hand, the 

 vents which can now be traced outside of the present limits of the edges of the plateaux 

 may, like those just mentioned, be younger than the basalt-sheets, or, on the contrary, 

 they may be records of a period of eruptivity anterior to the emission of any of the rocks 

 of the plateaux, and may have been deeply buried under a mass of basalt-beds 

 subsequently removed. Positive demonstration is, from the nature of the case, impossible, 

 unless we could find at the foot of the basalt-escarpment a volcanic vent immediately 

 connected with some of the beds of the plateau above it.* When, however, we reflect 

 that the vents which exist are precisely such as the structure of the plateaux would have 

 led us to expect, we may not unreasonably look on them as part of the phenomena of 

 this section of the volcanic period. Besides, in some cases, their connection with the 

 rocks of the plateaux is as nearly proved as many facts in geology which nobody would 

 now dispute. 



The most convenient classification of these vents is according to the nature of the 



* The instance of Carrick-a-raide, to be immediately referred to, is as near such a positive demonstration as could 

 be looked for. 



