The Geology of Ascension and St. Helena Islands. 149 



rising dome, is represented in Wig Hill. Boatswain Bird Island, 

 a spectacular monolith of trachyte, is probably an independent 

 dome. Crags of still another crop up through a veneer of younger, 

 basaltic cinders, a short mile north-north-west of Sisters Peak. An 

 extensive mass of trachyte, exposed in the clifis of Coco-nut Bay, 

 may represent a tenth dome, which has been largely cut away by 

 the heavy surf of this windward side of the island. Finally, White 

 Hill may be the remnant of a dome erupted independently of the 

 adjacent Weather Post body. 



As crater-fillings or domes, the trachytic masses are in wonder- 

 fully clear relations to the basalts. All are younger than the bulk 

 of the basalts. On the other hand, in each of several instances the 

 trachytic effusion is but an interruption in a series of basaltic 

 eruptions. That basalts have followed, as well as preceded, the 

 trachytes is shown at Cross Hill, Green Mountain, and, again, with 

 extraordinary vividness at South-East Head. Quite across the Head 

 an east-west fissure opened, not many centuries ago, and from it 

 streams of black basalt flowed out on both sides, over the bare 

 pale-grey dome of trachyte. The intensity of colour contrast between 

 the two kinds of rock, the gaping crack a mile or more in length, and 

 a row of small craters blown open along the fissure show in perfect 

 fashion elementary principles of volcanism and, as well, the 

 syngenesis of basalt and trachyte. 



Considering Ascension as a whole, one may fairly question that 

 any other equal area displays more eloquent and at the same time 

 more varied examples of volcanic activity. 



Like the similar trachytes of crater-fillings and domes in Samoa, 

 those of Ascension are singularly prone to alteration, rapidly 

 becoming friable to a depth of several feet. These bodies, monolithic 

 as they are, yield to weather and wave, and are ravined and clified 

 at a rate much faster than is the case with the basaltic flows. Large- 

 scale, carious weathering of the trachyte produces characteristic 

 hoodoos, bizarre turrets, and other fantastic shapes. 



The writer was able to add some details to Darv/in's description 

 of the granitic fragments brought to the surface by the volcanic 

 eruptions at Ascension (see Renard for references and discussion). 

 Many fragments of hornblende-granite and probably other plutonic 

 rocks were found in both the basaltic and trachytic agglomerates 

 of Green Mountain, the source of Darwin's collection. Two new 

 localities were found. The basaltic flows issuing from Dark Sloj)e 

 Crater are charged with numerous angular to subangnlar inclusions 

 of granite, the largest seen measuring 2 feet in length. The other 

 locality is three miles to the northward, where the massive trachyte 

 of the monolithic dome north-north-west of Sisters Peak was found to 

 enclose at least one angular fragment of granitic rock. Since the 

 parent body, whence the inclusion was torn, is likely to have been 

 at least 9,000 feet below sea-level, this discovery seems to show that 

 trachytic magma is generated at considerable depth. The existence 



