There is much resemblance in structure and in geological history between St.Helena, St.Jago, and Mauritius.All three islands are bounded (at least in the parts which I was able to examine) by a ring of basaltic mountains, now much broken, but evidently once continuous.These mountains have, or apparently once had, their escarpments steep towards the interior of the island, and their strata dip outwards.I was able to ascertain, only in a few cases, the inclination of the beds; nor was this easy, for the stratification was generally obscure, except when viewed from a distance.Ifeel, however, little doubt that, according to the researches of M.Elie de Beaumont, their average inclination is greater than that which they could have acquired, considering their thickness and compactness, by flowing down a sloping surface.At St.Helena, and at 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 ("Principles of Geology" fifth edition volume 2 page 171.), 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 difficulty in admitting that these basaltic mountains are merely the basal fragments of great volcanoes, 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, that 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 connection 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 thirdly, 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 volcanoes 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 phenomena of 1835 in South America, 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.
