56 THE VOYAGE OF H.M.S. CHALLENGER. 



Basaltic Rocks. 



"VVe have said that almost the entire surface of Ascension is covered by streams of 

 black, scoriaceous, basaltic lava, through which the trachytic escarpments crop out. 

 According to Darwin, 1 this lava is sometimes vesicular and at other times massive. It 

 is black in colour, and sometimes contains many crystals of felspar, olivine predomi- 

 nating in rare cases. The streams appear to have been not very fluid ; the lateral 

 walls are extremely steep, and attain a height of 20 or 30 feet. The surface is very 

 scoriaceous, and from a little distance it appears covered with small craters. These 

 mounds are heaps of scoriaceous lava of the same kind as that forming the mass of the 

 stream ; their form is more or less regularly conical, and they are traversed by fissures, 

 which give them a columnar appearance. These hillocks rise to 10 or 20 feet above the 

 stream, and Darwin attributes their formation to the accumulation of viscous lava at 

 points where some obstacle presented itself to the flow. At the base of these conical 

 heaps, and at other points on the stream, blocks of lava are to be seen, resembling 

 arches in appearance. Fantastic masses of scorise rise up over the whole surface, 

 occasionally, according to Darwin, presenting such an extraordinary appearance as 

 hardly to be distinguished from trunks of trees. Some of these lava-flows may be 

 traced to their point of origin at the base of the great trachytic mass, or to the isolated 

 conical hills of reddish rock situated in the north and west of the island. Darwin 

 counted twenty or thirty of these cones of eruption from the central eminence. Most 

 of them have their summits truncated obliquely, the steepest slope being on the south- 

 east side facing the prevailing wind, as Lesson 2 points out. Hennah remarks, in addi- 

 tion, 3 that in Ascension the most extensive beds of ashes are always found in the lee of 

 the wind. 



This arrangement of the volcanic hillocks may be explained by taking account of 

 the fact, that during eruptions the incoherent products would be carried in the direction 

 to which the prevailing winds blew. 



The basalts collected by the Challenger Expedition at Ascension are almost always 

 of the felspathic variety ; dolerites rarely occur. 



Amongst the rocks of the type of ordinary basalt we may describe the specimens from 

 Eed Hill. They are completely penetrated with oxide of iron, and present a porphyritic 

 structure by reason of crystals and grains of plagioclase — attaining a maximum diameter 

 of a centimetre — embedded in a slightly vesicular ground-mass. Olivine is very rarely 

 seen, and augite more rarely still. Microscopically the rock is formed of a ground-mass in 

 which plagioclase microliths predominate, almost always twinned according to the Carlsbad 

 law, and associated with little crystals of augite. Larger sections of magnetite, augite, 



1 Darwin, Geol. Obs., p. 3-4. 2 Lesson, Voyage de la " Coquille," p. 490. 



3 Hennah, Proc. Geul. Soc. Zond., vol. ii., p. 189, 1835 ; cited by Darwin, he. cit., p. 35. 



