Tlie Geology of Ascension and St. Helena Islands. 147 



like tliat of St. Helena, is highly satisfactory to a field-worker. 

 Hence it proved possible to make a fairly comprehensive sttidy of 

 Ascension, notwithstanding the shortness of the available time-- 

 twenty-seven days, chiefly in the month of November, 1921. 



Ascension presents an ideal case of a complex volcanic island 

 in its constructional stage. A half-dozen extensive lava-flows and 

 a third or more of the sixty vents mapped look as if they had been 

 formed not many centuries ago. So young are they, that one cannot 

 but be surprised when one learns that no fumarole or hot spring 

 is to be found. Apparently, too, no emanation of the kind is recorded 

 in the early sixteenth century accounts of the island by the 

 Portuguese. With marvellous directness most of the surface flows, 

 cones, and craters tell their story. This is specially true of structures 

 composed of basaltic or closely allied rocks. In all they cover 

 about seven-eighths of Ascension. 



Most of the cones shown on the chart are red, or red-brown, 

 steep piles of scoriaceous agglomerates, tuffs, with steep-angled, 

 stubby flows of basaltic type interbedded. Examples are : East 

 Crater, Holland's Crater, the Three Sisters, Perfect Crater, Traveller's 

 Hill, Lady Hill, South-East Ked Hill, Dark Slope Crater, Horseshoe 

 Crater, Eound Hill, Saddle Crater, Gannet Hill, Spoon Crater, 

 South Red Crater, and Mountain Red Hill. Some of the crater 

 rims, such as those of the first two mentioned, have been breached 

 by outrunning basaltic streams, the breach being characteristically 

 on the windward side. Mountain Red Hill, 1,786 feet in elevation, 

 is the highest of these purely basaltic cones. The smallest are true 

 driblet cones, illustrated in one series of three which are aligned on 

 a fissure running north from the main crater of Sisters Peak, and in 

 another line of a half dozen, strung out east of the breached basaltic 

 crater of Booby Hill. Bear's Back, a craterless, basaltic plateau, 

 with gently inclined surface, has no parallel in the island. 



The many basaltic flows are rugged, often recalling the typical 

 aa or blocky lava of Hawaii. Tumuli and hornitosof diagrammatic 

 perfection are features of some of the flows. Ropy or fahoelioe 

 lava and lava tunnels are rare. Pressure ridges and teiisional 

 gapings in the flows many times suggested the query whether those 

 common features of low-angled flows are not analogies to the 

 cordflleras and ocean-basin of the Atlantic type respectively, 

 explained on the Taylor-Wegener theory of continental creep. As 

 usual, the greater flows have issued from the flanks of the cones, 

 rather than from the craters. Where they reach the sea they have 

 been nipped by the waves, but the cliffs are generally low, witnessing 

 to the recency of the eruptiors. An exception to the rule is found in 

 a 200-foot cliff at South- West Bay, probably an old sea-cliff, which 

 has been largely covered by younger, thin, cascading flows. A new 

 shore-line has there been formed, a few hundred yards to the west. 



Darwin, Renard, and Reinisch have shown that alkaline trachytes 

 are important in Ascension, and a leading purpose of the writer has 



