Professor Geikie on the " Pitchstone " of Eskdale. 239 



lying sheets of trachyte. Mr Clarence King states his belief 

 that some single sheets of basalt have flowed at very gentle 

 angles for 50 or 60 miles.* The sheets, however, are so flat 

 and so like each other, that unless a given bed were followed 

 without any breach of continuity in the observation, it could 

 not, I fear, be certainly identified at distant points. Here 

 and there small puy-hkQ cones have been formed more 

 recently upon the basalt floor of this region, but these are 

 few in number, and do not much affect the extraordinary 

 monotony of the apparently limitless desert plain. I could 

 nowhere see any trace of tuff between the sheets of basalt, 

 but here and there lay beds of sand and gravel, or a thin 

 seam of red bole appeared, like those which occur in Antrim, 

 Skye, and Mull. 



The absence of any cone or cones which could have sup- 

 plied such a vast flood of basalt and the remarkable horizon- 

 tality and persistence of the successive sheets seem to me 

 strongly to favour Eichthofen's suggestion of fissure eruptions, 

 by which the basalt welled out from many openings without 

 the emission of accompanying ashes and scoriae. The surface 

 of the basalt plain likewise bears its evidence in the same 

 direction. When looked at from above it seems nearly level, 

 but on closer examination is found to be traversed by wave- 

 like undulations of columnar compact basalt, many of which 

 have split along the crest so as to show a trench with ranges 

 of columns on either side. 



That in Britain during Miocene times there were many 

 thousands of fissure eruptions, without accompanying frag- 

 mentary discharges or the production of cones, is certain 

 from the evidence of the innumerable east and west dykes. 

 I am disposed to look upon the great basalt plateaux of the 

 north of Ireland, the west of Scotland, and the Faroe Islands, 

 as representing part of the subserial outflow from some of 

 these fissures. From this point of view the dykes acquire 

 a new interest, inasmuch as they reveal to us some of the 

 features of a remarkable type of volcanic action, and stand 

 to fissure eruptions in the same relation that " necks " do to 

 eruptions from distinct cones. 



* Geological Exploration of the 40th Parallel, vol. i. , p. 593. 



