150^ Reginald A. Daly — 



of these granitic rocks in Ascension has manifest bearing on the 

 origin' of the Atlantic basin. 



Another observation, perhaps of importance to the genetic theory 

 of trachyte is, that, while there are numerous thin beds of trachytic 

 pumice, tufi, and breccia (enclositag small masses of obsidian) in 

 the island, the great domes and flows of trachyte, at both floor and 

 original surface, are practically free from gas-vesicles of the ordinary 

 bubble form. The same is true of the visible parts of the trachytic 

 bodies in Hawaii and Samoa. The contrast with the dominant, 

 highly vesicular basalts, thus repeatedly shown, is a problem 

 deserving attention. Do the magmatic gases escape upwards, while 

 femic crystals or their non-consolute equivalents sinh during the 

 prolonged differentiation of basaltic magma, leaving a gas-poor, 

 alkali-rich, siliceous melt — a derived, trachytic magma ? 



Pending laboratory study, it is impossible to state the range 

 of composition represented in what have been above described as 

 " basaltic " rocks. Flows of true olivine basalt are not uncommon, 

 and a beach-sand, very lich in olivine, is found at Crystal Bay. 

 Olivine-poor and olivine-free basalts of various textures are more 

 abundant. Both andesitic and trachydoleritic tyjDes may be 

 demonstrable. 



Five narrow dykes represent the only intrusive bodies discovered. 

 In view of the island's extreme youth and the absence of important 

 faulting, the rarity of visible intrusives is not surprising. 



St. Helena. 

 Seven hundred miles south-eastward of Ascension is St. Helena, 

 covering nearly 50 square miles, and thus a little the larger. The 

 last month of 1921 was spent on this Gibraltar of the South Atlantic. 

 It is essentially an older Ascension, submaturely dissected by 

 streams, ancient and modern. On the steep valley walls, reaching 

 1,000 to 2,000 feet in depth, as on the magnificent sea-clifis of similar 

 maximum heights, one can see well down, and with unusual clearness, 

 into the sub-surface mechanism of volcanic vents. The one 

 exemplifying constructional forms so brilliantly, the other 

 exemplifying the correlated structures underground, the two islands 

 are, therefore, complementary to each other. From them alone 

 most of the essentials of a textbook of volcanic geology could be 

 well illustrated. In St. Helena also the advantage given the geologist 

 by the absence of a forest cover was specially appreciated by one 

 who had recently tried to make out the history and volcanic 

 mechanism of the Samoan Island of Tutuila, most of v/hich is covered 

 with a smother of jungle. Accessible by regular steamers, covered 

 with good paths and roads, its rocks exposed with a completeness 

 seldom matched, and, above all, intrinsically important, St. Helena 

 should be visited, if possible, by every sjDecialist in igneous geology. 

 The publication of the reconnaissance map shoald at least help to 

 advertise the worth of this choice place of the earth to the student of 

 volcanoes and of erosion. 



