io8 St. Helena. 



PAET I. 



dikes are the evidence and the measure, — a view 

 equally novel and important, which we owe to the re- 

 searches of that geologist on Mount Etna. 



A conjecture, including the above circumstances, 

 occurred to me, when, — with my mind fully convinced, 

 from the phenomena of. 1835 in South America, 1 that 

 the forces which eject matter from volcanic orifices and 

 raise continents in mass are identical, — I viewed that 

 part of the coast of St. Jago, where the horizontally 

 upraised, calcareous stratum dips into the sea, directly 

 beneath a cone of subsequently erupted lava. The 

 conjecture is that, during the slow elevation of a vol- 

 canic district or island, in the centre of which one or 

 more orifices continue open, and thus relieve the sub- 

 terranean forces, the borders are elevated more than 

 the central area; and that the portions thus upraised 

 do not slope gently into the central, less elevated area, 

 as does the calcareous stratum under the cone at St. 

 Jago, and as does a large part of the circumference of 

 Iceland, 2 but that they are separated from it by curved 



1 I have given a detailed account of these phenomena, in a paper 

 read before the Geological Society in March, 1838. At the instant 

 of time, when an immense area was convulsed and a large tract 

 elevated, the districts immediately surrounding several of tne great 

 vents in the Cordillera remained quiescent ; the subterranean forces 

 being apparently relieved by the eruptions, which then recommenced 

 with great violence. An event of somewhat the same kind, but on 

 an infinitely smaller scale, appears to have taken place, according 

 to Abich (' Views of Vesuvius,' plates i. and ix.), within the great 

 crater of Vesuvius, where a platform on one side of a fissure was 

 raised in mass twenty feet, whilst on the other side, a train of small 

 volcanos burst forth in eruption. 



2 It appears, from information communicated to me in the most 

 obliging manner by M. E. Robert, that the circumferential parts of 

 Iceland, which are composed of ancient basaltic strata alternating 

 with tuff, dip inland, thus forming a gigantic saucer. M. Robert 

 found that this was the case, with a few and quite local exceptions, 

 for a space of coast several hundred miles in length. I find this 

 statement corroborated, as far as regards one place, by Mackenzie, 

 in his ' Travels ' (p. 377), and in another place by some MS. notes 

 kindly lent me by Dr. Holland. The coast is deeply indented by 

 creeks, at the head of which the land is generally low. M. Robert 



