REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 49 



variety. Obsidian is simply the last term of this series, and its external characters are 

 then sharply defined. We shall describe here some of the more or less vitreous 

 varieties of trachyte, but as the texture and the mineralogical composition are always 

 fundamentally the same, it is unnecessary to follow all the stages of transition. We shall 

 accordingly say a few words about the highly vitreous trachytic rocks, and afterwards 

 enlarge upon the well-characterised obsidian of Ascension. 



Vitreous augitic trachyte sometimes appears as a greyish mass, soft and very 

 friable, somewhat scoriaceous and passing into pumice, but more homogeneous in the 

 fracture. Its macroscopic characters are like those of a tufa, but microscopically the 

 ground-mass is seen to contain no heterogeneous fragments, being composed of microliths 

 and a vitreous mass. In this matrix microporphyritic crystals of sanidine appear ; the 

 crystals of augite are always smaller than those of the felspar with which they are 

 associated. 



Obsidian. 



All the obsidians of Ascension are closely related to the trachytic rocks which have 

 just been described. Before discussing the mineralogical characters of these volcanic 

 glasses, it will be well to give a resume of Darwin's very detailed observations 1 upon 

 them. He first describes the transition of the rocks into zonary 2 beds between which 

 the obsidian is intercalated. These outcrops of the beds of obsidian in the middle of 

 the trachytic region west of Green Mountain are highly inclined, and partially 

 covered by more recent eruptions ; for this reason Darwin could not observe their 

 contact with the trachyte, nor satisfy himself as to whether they had been poured out 

 like lava, or injected like the veins in the adjacent rocks. At the point explored by 

 the author three beds of obsidian appeared, the largest at the base of the section. 

 These alternating rocks attracted the particular attention of Darwin, and he described 

 five varieties which passed into each other by all gradations. We refer the reader for 

 particulars regarding these varieties to the complete description given in the chapter of 

 Darwin's book dealing with the subject. 



The transition of these zonary rocks to beds of true obsidian takes place in several 

 ways. At first angulo-nodular masses of obsidian of varying size appear isolated in a 

 schistoid or massive felspathic rock of a light colour and conchoidal fracture. Then 

 irregular nodules of obsidian are seen, isolated, or grouped in layers not more than the 

 tenth of an inch thick, which alternate repeatedly with thin strata of a zonary felspathic 



1 Darwin, Geol. Observ., pp. 54-C2. 



2 We employ "zonary" instead of Darwin's term "laminated" in this description. He explains his meaning 

 of the latter word in a note at the foot of p. 54 loc. cit. : " This term might be misunderstood ; it is applied to rocks 

 which divide into thin leaves of the same composition, or are formed of closely united layers of different mineral 

 species without a tendency to split up, but distinguished by special colours. The term laminated is employed here in 

 the latter sense. When a homogeneous rock has a cleavage plane along which it may be readily split, like slate, I 

 apply the term^ssife." 



(PHYS. CHEM. CHAIX. EXP. — PART VII. — 1889.) 7 



