Tlie Geology of Ascension and St. Helena Islands. 155 



Thus, St. Helena, like Ascension, once more shows the closest 

 genetic connexion of alkali-rich rocks with common basalt. Here, 

 too, the solution of the problem of origin may be affected by the fact 

 that nowhere was the efiiisive trachyte of St. Helena found to be 

 vesicular in the ordinary sense of the word when referring to lavas. 



The writer was able to confirm Melliss's discovery of the casts of 

 sizable tree-trunks in the lava flows exposed on the slopes of the 

 arid Jamestown Valley and 800-1,000 feet below the surface of the 

 uppermost flow in the valley walls. The casts occur in two different 

 flows, stratigraphically about 100 feet apart. These facts show 

 that the upbuilding of St. Helena may have been as leisurely as the 

 growth of Hawaii to similar height, and that St. Helen along possessed 

 a climate which was considerably moister than that now ruling. 

 It proved impossible to procure evidence as to the specific nature of 

 the trees represented by the casts. 



No other fossils tending to fix the age of the island were discovered. 

 As at Ascension, no palseontological discoveries of moment were 

 made. In Ascension, except lithified, calcareous beach-sand, a few 

 feet above high-tide level, and the crater-lake deposit at the Riding 

 School, and, in St. Helena, some poorly cemented, calcareous 

 dune-sands at various elevations up to about 700 feet above sea, 

 no sedimentary rocks were found. From the youth of its topography, 

 little, if any, of the visible Ascension is likely to antedate the Recent 

 period. St. Helena has suffered denudation and sea-cliffing to such 

 degrees that a late Tertiary origin seems preferable to one even in 

 the early Pleistocene. 



Except for some possible indications near Georgetown, at English 

 Bay, and at South-west Bay, Ascension shows no evidence of 

 emergence. The obscure features at the three localities named 

 could mean emergence no greater than 20 feet. A eustatic, world- 

 wide fall of ocean level seems to be deducible from a statistical 

 study of " elevated " strand-marks around the different oceans. 

 St. Helena confirms that deduction in a striking way. On its 

 leeward side were found numerous sea-caves and rock-benches at 

 heights above high tide of exactly the same order as among the 

 Samoau Islands, where the hypothesis of this particular eustatic 

 shift first took definite shape.^ This correspondence between 

 islands on opposite sides of the world is manifestly difficult to 

 understand on any other hypothesis. As in Samoa, no evidence of 

 emergence to an amount greater than 20 feet was found at either 

 of the Atlantic islands. 



If the eustatic shift of about 20 feet is to be correlated with the 

 " elevated " Neolithic beach of north-western Europe, which appears 

 to be nearly 5,000 years old, then we may have a possible means of 

 assigning a minimum age to the oldest lavas of Ascension. More 

 certain would be the possibility of gauging the speed of the erosion 



» Geol. Mag., Vol. LVI[, 1920, p. 247. 



