66 



THE VOYAGE OF H.M.S. CHALLENGER. 



Under the microscope the structure of olivine gabbro is brought out. Elongated 

 lamellse of plagioclase, containing between them sections of augite moulded on the 

 associated elements, do not appear here ; the felspar is in large sections of irregular 

 outline, in very rare cases assuming a form more or less resembling a parallelogram. 

 The symmetrical extinctions, measured on sections more or less nearly parallel to the 

 face h, give values of from 36° to 40° on each side of the albitic lamellse. These 

 extinctions have been measured on sections showing at the same time lamellse of albite 

 and of pericline crossing at an angle of about 80°, the sections being thus sensibly 

 parallel to k For sections of the zone P : k, which give symmetrical extinction, the 

 ano-le only varies from 12° to 20° on the average. These values indicate in each case a 

 very basic felspar, almost a mixture of bytownite and anorthite. This determination 

 ao-rees both with the form this felspar assumes here and with the nature of the rock in 

 which it occurs. It is known that the plagioclase of the Neurode gabbro, for instance, 

 is anorthite. 



Plagioclase isolated from the rock has been analysed by Dr. Element, with the 

 following result : — 



I. T2166 grammes of the substance dried at 110°C, and fused with the carbonates 

 of soda and potash, gave 0'6203 gramme of silica, 0'3708 of alumina, 0'0134 of ferric 

 oxide, 0'1751 of lime, and 0-0030 of magnesium pyrophosphate. 



II. - 5398 gramme of the substance treated with hydrofluoric acid gave - 0405 

 gramme of potassium and sodium chlorides, and - 0058 of potassium chloroplatinate. 



Percentage Composition. 



101-06 



The results of analysis given above confirm the optical determination. The mixture, in 



fact, corresponds in composition to 30 per cent, of albite and 70 per cent, of anorthite, 



which is — 



Silica, Si0 2 , 50-68 



Alumina, A1 2 3 , 31-73 



Lime, CaO, 14-05 



Soda, Na 2 0, 3-54 



100-00 



