REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 95 



to the ordinary law, and this twinning is shown in the most slender crystals, where it 

 appears in the sections as two extremely thin lamellae that give the mineral a fibrous 

 aspect. The colour is brown, and the pleochroism is very well marked — 



y > /3 > a 



deep brown. yellowish brown. pale yellow. 



These crystals extinguish at a less angle than is usual for hornblende. Magnetite 

 occurs in the preparations as irregular grains or sections of octohedra. 



The ground-mass is composed of crystals of secondary consolidation showing distinct 

 fluidal structure. When examined under very high powers the paste is seen to contain 

 sections of plagioclase usually much lengthened following the edge PjM, and twinned 

 according to the albite law; the little crystals are often grouped in rosettes. A 

 series of extinctions measured from the trace of M gave values between 16° and 32°, 

 the polysynthetic lamellae giving for one side 20°, for the other 30°, 26°-30°, 13"-! 6°, 

 36°-44°. These crystals accordingly differ little in composition from the felspar of first 

 generation. Microliths of augite are also present in the form of greatly lengthened 

 prisms, sometimes broken in several pieces and of a very pale green colour ; 40° is the 

 maximum angle of extinction. The part played by hornblende in the ground-mass 

 ought to be noted here. From the minute dimensions and brownish colour of its 

 crystals this mineral might be taken for a glassy base devitrified by microliths, and 

 interposed between the larger sections of plagioclase and augite. A high power, 

 however, brings out the individual crystals as small, fibrous, brownish prisms, some- 

 times lying in parallel lines or grouped in bundles, sometimes interwinned so as to 

 form a network. They often show distinct pleochroism and extinguish at small 

 angles, while their fibrous structure and elongated form complete their analogy with 

 the larger individuals of the same species. Magnetite appears in very definite sections 

 of octahedra. A network of trichites is occasionally observed closely resembling that 

 of hornblende crystals referred to above ; the trichites may perhaps be magnetite, but 

 more probably they are altered hornblende. 



All the rocks seen on the coast of Middle Island, which lies a little to the 

 north of Nightingale, are composed of the tufaceous mass now to be described, and 

 according to Mr. Buchanan's observations the entire islet is probably an accumulation 

 of the same formation. The rock is a yellowish, pumiceous, almost earthy, substance, 

 enclosing lapilli and very distinct hornblende crystals. Microscopic examination shows 

 that it is formed of cemented fragments. The most important rock occurring in this 

 tufa will be briefly described. Under the microscope it shows a very compact ground- 

 mass surrounding fragmentary microporphyritic crystals of hornblende, plagioclase, 

 sanidine, and augite, the splinters of the last-named mineral being smaller than those 



