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



referred to titaniferous magnetite or to titanic iron. Crystalline outlines observed in 

 these may be ascribed to a regular hexagon. These sections are surrounded by a zone 

 of leucoxene, which appears brownish in transmitted, greyish in reflected, light. The 

 ground-mass is formed by a network of small prismatic augites with rather sharp out- 

 lines ; a brownish decomposed glassy susbstance is scattered between these, but this base 

 plays a subordinate part. The almost complete absence of plagioclase, the characters 

 of which have only been recognised doubtfully and then only in a few sections, and 

 the predominance of augite, would tend to class this rock with the pyroxenites. The 



Santa Cruz, Teuerife. 



presence of olivine, on the other hand, connects it with the limburgites, but perhaps 

 this mineral is too subordinate to enable the rock to be classed in the latter group. 



Near the same locality, Puerto d'Orotava, a bluish black rock is found ; it is filled 

 with small circular vesicles, measuring 2 to 3 mm. ; these contain a whitish mealy matter. 

 Its fracture is irregular, and none of the constituent minerals can be distinguished 

 with the naked eye. The specimen in question is a basalt of the felspathic type ; its 

 microscopical structure is that of a dolerite. The plagioclase crystals are lamellar, 

 and grains of augite more or less well crystallised are embedded between them ; larger 

 sections of olivine are also seen. The plagioclases extinguish under rather large angles; 

 they are very probably a mixture approximating to bytownite. This felspar shows 



