Volcanic Islands visited during the Voyage of the Beagle. 357 



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 volcanic district or island, in the 

 centre of which one or more orifices continue open, and thus relieve the 

 subterranean 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*, but that they are 

 separated from it by curved faults. We might expect, from what we see 

 along ordinary faults, that the strata on the upraised side, already dipping 

 outwards from their original formation as lava-streams, would be tilted 

 from the line of fault, and thus have their inclination increased. Accord- 

 ing to this hypothesis, which I am tempted to extend only to some few cases, 

 it is not probable that the ring would ever be formed quite perfect ; and 

 from the elevation being slow, the upraised portions would generally be ex- 

 posed to much denudation, and hence the ring become broken ; we might 

 also expect to find occasional inequalities in the dip of the upraised masses, 

 as is the case at St. Jago. By this hypothesis, the elevation of the districts 

 in mass, and the flowing of deluges of lava from the central platforms, are 

 likewise connected together. On this view, the marginal basaltic moun- 

 tains of the three foregoing islands might still be considered as forming 

 'Craters of elevation;' the kind of elevation implied having been slow, 

 and the central hollow or platform having been formed, not by the arch- 

 ing of the surface, but simply by that part having been upraised to a less 

 height." 



Other subjects scarcely inferior in interest are discussed in the 

 chapter on the Galapagos Archipelago, as the fluidity of different 



* " 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 tuflf, 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 in- 

 forms me, that the inwardly dipping strata appear to extend as far as this 

 line, and that their inclination usually corresponds with the slope of the 

 surface, from the high coast-mountains to the low land at the head of these 

 creeks. In the section described by Sir G. Mackenzie the dip is \2°. The 

 interior parts of the island chiefly consist, as far as is known, of recently 

 erupted matter. The great size however of Iceland, equalling the bulkiest 

 part of England, ought perhaps to exclude it from the class of islands we 

 have been considering ; but I cannot avoid suspecting that if the coast- 

 mountains, instead of gently sloping into the less elevated central area, 

 had been separated from it by irregularly curved faults, the strata would 

 have been tilted seaward, and a ' crater of elevation,' like that of St. Jago,, 

 or that of Mauritius, but of much vaster dimensions, would have been 

 formed. I will only further remark, that the frequent occurrence of ex- 

 tensive lakes at the foot of large volcanos, and the frequent association of 

 volcanic and freshwater strata, seem to indicate that the areas around 

 volcanos are apt to be depressed beneath the general level of the adjoining 

 country, either from having been less elevated, or from the effects of sub- 

 sidence." 



