chap. iv. Craters of Elevation. 107 



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, 1 and of MM. 0. Prevost and Yirlet, 

 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 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, 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 con- 

 nection 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 j 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 vol- 

 canos would naturally possess, I can readily believe 

 that this iuclination might have been slowly acquired 

 by that amount of elevation, of which, according* to 

 M. Elie de Beaumont, the numerous upfilled fissures or 



1 'Principles of Geology' (fifth edit.), vol. ii. p. 171. 

 6 



