14i8 Reginald A. Daly — 



been to make clear the structural and age relations of the trachytes 

 to the basaltic types. The trrchytes cover the remaining one- 

 eighth of the island, being most developed in the higher, south- 

 eastern parti In apparently every instance the trachyte has risen 

 at central vents, drilled or melted up through older, basaltic flows 

 and pyroclastic beds. This relation is illustrated at the Devil's 

 Riding School, where the old crater of a basaltic cone was filled 

 with highly viscous trachytic magma, which flowed over the rim 

 on the eastern side. Following the outflow, the surface of the 

 trachytic crater-filling sank, giving a basin probably more than 

 100 feet deep at the centre. The area of the basin is nearly equal to 

 that indicated within the crater-rim sketched in the chart. For a 

 considerable time the basin contained a freshwater pond, in which 

 were deposited the fossiiiferous beds described by Darwin and 

 Ehrenberg. A similar basining has affected a trachytic crater-filling, 

 one quarter of a mile north-west of the Thistle Hill. 



In most cases, however, the viscous trachyte assumed the form of 

 great domes projecting high above the old crater rims. Ideal 

 examples are found in Ragged Hill, Little White Hill, and Weather 

 Post. Cross Hill is of greater complexity. Its core is a broad dome 

 of trachyte reaching a height of 600 feet above sea-level. The dome 

 rose about 400 feet above the rim of ar older basaltic crater and 

 later became veneered with basaltic agglomerate, tuff, and driblet 

 flows to a depth of 200 feet or more. Green Mountain (elevation, 

 2,817 feet), the highest cone in Ascension, is another composite. 

 Its visible base is a large basaltic cone truncated by a major 

 explosion. Into the resulting caldera a trachyte dome rose, to a 

 height of nearly 2,400 feet above sea, and sent short, thick flows 

 toward the Riding School and the Bear's Back. The eastern part 

 of the dome and the trachyte flows alongside were blown away by 

 a second explosion, forming another caldera, nearly a mile long and 

 1,000 yards wide. This caldera in its turn was completely filled with 

 basaltic tuff, which from a central vent was built up to a height some- 

 what greater than 2,817 feet, the present elevation of " The Peak " 

 of Green Mountam. 



The most voluminous effusion of trachyte was that of the Weather 

 Post dome, rather more than 2,000 feet high and the source of a 

 northbound flow ; this flow is more than a mile wide, nearly two 

 miles long, and several hundred feet in maximum thickness. Three 

 major explosions followed the solidification of the Weather Post 

 dome. The resulting calderas are the Devil's Cauldron, Cricket 

 Valley, and the even grander, more irregular basin above Spire 

 Beach . Most impressive is the heavy burden of coarse debris spread 

 over basalts and trachyte alike, and as yet almost unaffected by 

 erosion or by a concealing cover of vegetation. 



Another trachyte dome composes nearly all of South-East Head. 

 A smaller dome, of the same type of pale-grey trachyte, capped 

 with black basalt, which seems to have been pushed up ■ by the 



