chap. iv. Craters of Elevation. 109 



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. According 

 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 exposed 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 like- 

 wise connected together. On this view the marginal 

 basaltic mountains 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 arching of the surface, but simply by that part 

 having been upraised to a less height. 



informs 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 12°. 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 buen 

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

 tains, 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 extensive lakes at the foot of large volcanos, 

 and the frequent association of volcanic and fresh-water 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 subsidence. 



