REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 39 



arising in their turn from the decomposition of the biotite. Without insisting too 

 strongly on this point, these accumulations, to judge from their form, bear a resemblance 

 to those of hornblende called pilite by Dr. Becke, a product which he has pointed out 

 as the result of the decomposition of olivine in certain kersantites of the Waldviertel. 

 In some cases the product of decomposition of the olivine is a greenish substance, the 

 absorption of which is less marked than that of the biotite ; it is more finely fibrous 

 than the latter mineral, and the fibres are more interlaced and less continuous than are 

 the lamellae of black mica ; we regard this green substance as serpentine. Amongst the 

 inclusions of the olivine, we must mention magnetite and some chestnut-brown grains 

 belonging probably, judging from their transparency, to a spinel. 



The ground-mass contains a large number of augite sections which are generally 

 more prismatic than the microporphyritic crystals of this species. The plagioclases, 

 which occur only in the ground-mass and as microliths, yield very elongated sections 

 with polysynthetic striae. The extinctions observed in the sections of the zone 

 P : M are moderately large, and they are included between the angles 5° and 26° ; it 

 is therefore likely enough that this mineral is allied to labradorite. Among the 

 microlithic crystals of the ground-mass one notices very small patches of a vitreous 

 colourless substance. Sometimes this base is coloured slightly yellow, but this tint is 

 secondary, arising from the decomposition of the ferruginous minerals that constitute 

 the rock. To this same decomposition is to be attributed small nests of greenish 

 chlorite which line certain cavities wherein this mineral has crystallised in interwoven 

 lamellae. We may remark, in conclusion, that this basalt approaches the doleritic type 

 in its texture. 



V.— EOCKS OF ASCENSION. 



Darwin in his book on Volcanic Islands has given a very detailed description of the 

 rocks of Ascension; 1 but during the time (almost half a century) which has elapsed 

 since the appearance of that work, no one has, to our knowledge, published any special 

 paper on the petrography of this island. 2 We are now able, in some measure, to fill this 

 gap, thanks to the materials collected during the stay of the Challenger, and by Dr. 

 Maclean, E.N., one of the Challenger officers, who lived for some time on the island. 

 Dr. Maclean has placed at our disposal specimens of the principal rocks that he collected, 

 and also some local information, of which we have availed ourselves in the following 

 notice. We have arranged our material very much in the order adopted in Darwin's 

 Geological Observations, and have recapitulated a good many of his local details. It 



1 Darwin, toe. cit., pp. 34-72. 



2 Murdoch has analysed the well-known obsidian of Ascension (Phil. Mag., 1844, p. 495). Vom Rath described the 

 crystals of hematite of this island, associated with magnoferrite (Zcitschr. d. deutsch.geol. Gesdlsch., Bd. xxv. p. 108, 1870). 

 Ehrenberg has shown the nature of certain siliceous deposits of the " crater of an old volcano" (see p. 68 of this Report). 



