REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



81 



it is slightly more schistose, less compact, and not so dark in tint. Under the micro- 

 scope it is found to be composed of the following minerals of the first generation : large 

 crystals of plagioclase, angite, and hornblende. The sections of this last species are 

 encircled by a zone of magnetite. These sections stand out from an almost colourless 

 glassy matrix, containing microliths of plagioclase, augite, and magnetite. 



Another specimen belonging to the bedded rocks consists of a fragment taken from 

 a layer of loose volcanic products, overlaid by a sheet of lava. From the structure of 

 the specimen, it is evident that it is composed of two layers, indicating successive 

 deposits. The one has the composition and texture we have recognised in all the 

 basaltic lavas of the island ; the other is an agglomeration of glassy splinters, plagio- 

 clase, augite, and magnetic iron ; all these minerals are fragmentary, and the layer in 

 cmestion ought to be regarded as a basaltic tufa. 



We have given the lithological characters of the lava-streams and tufa that constitute 

 the greater part of the rocks cropping out on the coast ; it remains to indicate the 

 nature of the transverse dykes injected into these superposed layers. The specimens 

 procured from these dykes look to the naked eye like compact basalts of blackish tint, 

 giving slight indications, also, of a columnar structure. One fragment which was 

 contiguous to the encasing rock exhibits, to a depth of about a centimetre, the black 

 vitreous modification with brilliant lustre, well known in basaltic rocks that have been 

 subjected to sudden cooling. 



To judge from the specimens we have examined, these dykes are felspathic basalts, 

 presenting sometimes a transition into augitic andesites. The minerals of first genera- 

 tion are magnetite, olivine, and plagioclase. The 

 last-named crystals are lamellar ; the extinc- 

 tions, measured symmetrically on two adjacent 

 hemitropic lamellae, are about 36°. This felspar, 

 therefore, approaches labradorite. The ground- 

 mass of this rock is somewhat remarkable (see 

 fig. 18). It is almost entirely composed of augitic 

 microliths, which are grouped in rosettes or 

 twinned crosswise, and sometimes planted almost 

 perpendicularly on the plagioclastic lamellae or 

 between the small prisms of augite, forming a 

 fibro - radiating aggregate. Crystals of olivine 

 with hexagonal or rhombic contours are frequent, 

 and they enclose a nucleus of glassy substance. 

 Magnetite, in more or less irregular grains, fills up the interstices between the various 

 minerals that constitute the matrix. The other specimens from the injected dykes have 

 the same mineralogical composition and the same texture. 



(PHYS. CHEM. CHALL. EXP. — PART VII. 1889.) 11 



Fig. 18. — Dyke of felspathic basalt, Tristan da Cunha. 



Ground-mass composed of augite microliths in rosettes 

 or planted perpendicularly on the plagioclastic 

 lamella?, and crystals of olivine with vitreous 

 inclusions. 



