44 



THE VOYAGE OF H.M.S. CHALLENGER. 



The rocks under consideration may be described under the general name of Augitic 

 trachytes, and are characterised by the association of three constituents in greater or 

 less amount : monoclinic felspar, augite, and a vitreous ground-mass. Their minera- 

 logical composition is very constant, and the characters well defined, — the slight varia- 

 tions being due to differences of texture, and to the more or less important part played 

 by the vitreous matrix. All stages of transition are to be found between holocrystalline 

 varieties showing an aggregate of augitic and felspathic microliths, with some micro- 

 porphyritic crystals of sanidine, and vitreous varieties, in which there occur a few 

 extremely minute crystals of sanidine and augite. Finally, the vitreous element becomes 

 supreme, and the rock passes into obsidian. 



The trachytes properly so called are whitish grey in colour, sometimes bluish grey, 

 with a rough granular structure. The ground-mass is homogeneous, rarely slightly 

 schistoid. Sometimes they are slightly vesicular, and pass into pumice ; or are more 

 compact, and, according to the predominance of the vitreous element, darker in colour 

 and with a somewhat glossy sheen. The fracture is usually irregular. In some cases 

 the trachytes are friable, in others they are rough to the touch and coherent. Some 

 specimens which have commenced to alter, and are marked with round brownish stains, 

 are impregnated with oxide of iron, which gives them a red or brown colour. These 

 trachytes, when examined with the lens, are found generally to be composed of 

 crystalline grains, but the species could not be made out, except in the case of sanidine, 

 crystals of which are sometimes visible to the naked eye. 



Microscopic examination shows that all the trachytes of Ascension have an almost 



identical microtexture. They possess a ground- 

 mass chiefly composed of confused microliths of 

 sanidine and augite, to which large sections of 

 felspar give a microporphyritic structure ; the sec- 

 tions of augite are less numerous. Sometimes a 

 base is interposed between the microliths of the 

 ground-mass ; the latter is seldom devitrified in 

 spherulites or trichites. A peculiarity of the 

 minerals in the ground-mass is that they are 

 always comparatively small ; this minuteness, and 

 the confused setting of the microliths, makes their 

 determination difficult. The sanidine appears in 

 large crystals with the distinguishing peculiarities 

 of this species. These large individuals are always 

 corroded, their outlines are blunted, they are 

 furrowed by Hues of fracture which sometimes 

 correspond to traces of cleavages oo P, and almost always twinned according to the 



Fio. 6.— Trachyte of Weather Post Hill. Section 

 of sanidine cut almost parallel to P/ilf, Carlsbad 

 twin. The composition plane is 37, and another face 

 probably cc S 3. j' 5 crossed nicols. 



