92 



THE VOYAGE OF EL M.S. CHALLENGER. 



according to the Carlsbad law, and at the same time traversed by polysynthetic lamella? 

 after the albite law. Hornblende occurs associated with the plagioclase ; the crystals of 

 both species are deeply hollowed and corroded. Rather large brownish hornblende 

 sections are seen surrounded by a zone composed of little green grains of augite with 

 granules of magnetite, biotite, and titanite. In this phonolitic mass are embedded 

 heterogeneous clastic fragments, which prove the tufaceous origin of the rocks forming 

 almost the entire explored portion of the island. 



Dr. Klement has obtained the following results from an analysis of this tufa : — 



I. 1'0G76 grammes of substance dried at 110° C. and fused with alkaline carbonates, 

 by Sipocz's method, gave 0*0116 gramme of water, 0'6102 of silica, 0*0285 of titanic 

 acid, 0*2142 of alumina, , 0535 of ferric oxide, 0*0421 of lime, and 0*0459 of magnesium 

 pyrophosphate. 



II. 1'0339 grammes of substance treated with hydrofluoric acid gave 0*1875 

 gramme of sodium and potassium chlorides, and 0'241 of potassium chloroplatinate. 



III. 1*1143 grammes of substance treated in a sealed tube with hydrofluoric and 

 sulphuric acids required 4*0 c.c. of potassium permanganate solution (1 c.c. =0*005439 

 gramme FeO) to oxidise the ferrous oxide. 



Some specimens collected by Mr. Buchanan in a gully prove the presence of 

 eruptive masses of the andesite type at Nightingale. The rock in question is black 

 and massive, with a plane fracture and rather schistose. Crystals of felspar, about three 

 or four millimetres in diameter, and of hornblende of nearly equal dimensions, shine 

 out from the mass. Microscopically there is a glassy ground-mass containing por- 

 phyritic minerals of the first generation. Plagioclase is the most noticeable, and its 

 sections are remarkable in that instead of the usual lengthening along the edge PjM, 

 they show a great extension following yM ; in fact many sections take the form of 

 disymmetric hexagons (sections nearly parallel to M) in which the shortest sides 

 correspond to the edge PjM; this is confirmed by examining the best-marked lines of 



