chap. iv. Craters of Elevation. 105 



nearest island, namely, Ascension, is very striking. At 

 Ascension, the surfaces of the lava-streams are glossy, as 

 if just poured forth, their boundaries are well defined, 

 and they can often be traced to perfect craters, whence 

 they were erupted; in the course of many long walks, 

 I did not observe a single dike ; and the coast round 

 nearly the entire circumference is low, and has been 

 eaten back (though too much stress must not be placed 

 on this fact, as the island may have been subsiding) 

 into a little wall only from ten to thirty feet high. Yet 

 during the 340 years, since Ascension has been known, 

 not even the feeblest signs of volcanic action have been 

 recorded. 1 On the other hand, at St. Helena, the course 

 of no one stream of lava can be traced, either by the 

 state of its boundaries or of its superficies ; the mere 

 wreck of one great crater is left ; not the valleys only, 

 but the surface of some of the highest hills, are inter- 

 laced by worn-down dikes, and, in many places, the 

 denuded summits of great cones of injected rock stand 

 exposed and naked ; lastly, as we have seen, the entire 

 circuit of the island has been deeply worn back into the 

 grandest precipices. 



Craters of Elevation. 



There is much resemblance in structure and in 

 geological history between St. Helena, St. J ago, and 

 Mauritius. All three islands are bounded (at least in 



1 In the ' Nautical Magazine' for 1835, p. 642, and for 1838, p. 

 361, and in the ' Comptes Rendus,' April, 1838, accounts are given of 

 a series of volcanic phenomena — earthquakes — troubled water — 

 floating scoriae and columns of smoke — which have been observed a* 

 intervals since the middle of the last century, in a space of open sea 

 between longitudes 20° and 22° west, about half a degree south of 

 the equator. -These facts seem to show, that an island or an archi- 

 pelago is in process of formation in the middle of the Atlantic : a 

 line joining St. Helena and Ascension, prolonged, intersects this 

 slowly nascent focus of volcanic action. 



