78 



THE VOYAGE OF H.M.S. CHALLENGER. 



Fig. 13.— Felspathic basalt of Tristan da Cunha. 



I. and II. twin of Baveno, the other twins follow- 

 ing the pericline type, or some other analogous 

 twinning. The plane of twinning (n or e) is at 

 the same time the plane of composition. 



Thin sections of certain less decomposed portions of this rock show that it ought to 



he referred to the felspathic basalts, passing, in 

 some cases, to the augitic andesites. The follow- 

 ing minerals — plagioclase, augite, mica, titanic 

 or magnetic iron, and, in certain cases, olivine — 

 give the rock a microporphyritic structure. The 

 crystals of felspar give sections showing plagio- 

 clastic lamellae following the albite type ; some- 

 times they are twinned on the Carlsbad, the 

 pericline, or the Baveno type. Fig. 13 shows 

 a section of plagioclase observed in the rock 

 in question. 



The crystals of augite present no striking peculiarity. Those of olivine, which at 

 first sight somewhat resemble pyroxene, are enclosed in a setting of small augitic 



crystals. The black mica plays a very subordinate part, 



but the ilmenite or hematite is, on the contrary, represented 



by large dark brown or almost opaque sections, furrowed by 



well-marked lines of cleavage intersecting at angles of 120° ; 



these lines run throughout the whole extent of the section, 



and are often parallel to its outlines (see fig. 14). It 



is somewhat rare to find hematite with such clearly 



marked cleavage. Something analogous may be found 



in sections of ilmenite, but then, generally, there are 



needles of rutile, intercalated at constant angles. In the 



mineral we are describing, we have not been able to make 



out any inclusions of rutile. 



The. ground-mass is formed of microliths of the same species, especially of felspar 



and augite ; between these small crystals lies a vitreous base, which plays a wholly 



subordinate part. At certain points a yellowish limonitic matter has been deposited 



as concretionary masses in the pores. 



Some of these decomposed specimens pass almost without gradation into a more 

 compact and harder rock. These compact zones are black with glassy lustre and brilliant 

 fracture ; they exhibit the vitreous modification observed on the contact faces of the 

 dykes in the same island. These black bands resemble- obsidian ; they are only 2 centi- 

 metres thick, and may be looked on as the more quickly cooled surface of the basaltic 

 sheet. This glass shows under the microscope a blackish brown and sometimes nearly 

 opaque isotropic base ; at certain points it passes into the reddish modification, with 

 the resinoid appearance of the palagonitic tufas. In this base crystals of plagioclase are 

 seen, some sections of which give extinctions of 42°, and consist of anorthite ; as usual, 



Fig. 14.— Felspathic basalt of 

 Tristan da Cunha. 



Opaque section of ilmenite or hematite 

 with cleavages intersecting at 120°. 



