356 Notices respecting New Booh : — Darwin on the 



and St. Jago the basaltic strata rest on older and probably submarine beds, 

 of different composition. At all three islands deluges of more recent lavas 

 have flowed from the centre of the island, towards and between the basaltic 

 mountains ; and at St. Helena the central platform has been filled up by 

 them. All three islands have been raised in mass. At Mauritius, the sea, 

 within a late geological period, must have reached to the foot of the basaltic 

 mountains, as it now does at St. Helena; and at St. Jago it is cutting 

 back the intermediate plain towards them. In these three islands, but 

 especially at St. Jago and at Mauritius, when standing on the summit of 

 one of the old basaltic mountains, one looks in vain towards the centre of 

 the island, — the point towards which the strata beneath one's feet and of 

 the mountains on each side rudely converge, — for a source whence these 

 strata could have been erupted ; but one sees only a vast hollow platform 

 stretched beneath, or piles of matter of more recent origin. 



"These basaltic mountains come, I presume, into the class of Craters of 

 elevation : it is immaterial whether the rings were ever completely formed, 

 for the portions which now exist have so uniform a structure, that, if they 

 do not form fragments of true craters, they cannot be classed with ordinary 

 lines of elevation. With respect to their origin, after having read the works 

 of Mr. Lyell* and of MM. C. Prevost and Virlet, I cannot believe that the 

 great central hollows have been formed by a simple dome-shaped elevation, 

 and the consequent arching of the strata. On the other hand, I have very 

 great difliculty in admitting that these basaltic mountains are merely the 

 basal fragments of great volcanos, of which the summits have either been 

 blown off, or more probably swallowed up by subsidence. These rings are 

 in some instances so immense, as at St. Jago and at Mauritius, and their 

 occurrence is so frequent, ^hat I can hardly persuade myself to adopt this 

 explanation. Moreover, I suspect that the following circumstances, from 

 their frequent concurrence, are someway connected together, — a connexion 

 not implied in either of the above views ; namely, first, the broken state of 

 the ring, showing that the now detached portions have been exposed to 

 great denudation, and in some cases, perhaps, rendering it probable that 

 the ring never was entire ; secondly, the great amount of matter erupted 

 from the central area, after or during the formation of the ring ; and third- 

 ly, the elevation of the district in mass. As far as relates to the inclination 

 of the strata being greater than that which the basal fragments of ordinary 

 volcanos would naturally possess, I can readily believe that this inclination 

 might have been slowly acquired by that amount of elevation of which, 

 according to M. Elie de Beaumont, the numerous upfilled fissures or dikes 

 are the evidence and the measure, — a view equally novel and important, 

 which we owe to the researches of that geologist on Mount Etna. 



" A conjecture, including the above circumstances, occurred to me when, 

 — with my mind fully convinced from the phaenomena of 1835 in South 

 America't", that the forces which eject matter from volcanic orifices and 



* " Principles of Geology (fifth edition), vol. ii. p. 171-" 

 -f "I have given a detailed account of these phsenomena, in a paper read be- 

 fore the Geological Society in March 1838. [See Phil. Mag. S. 3. vol. xii. 

 p. 584.] At the instant of time when an immense area was convulsed and 

 a large tract elevated, the districts immediately surrounding several of the 

 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." 



