REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



85 



augite may be easily separated. The following analysis of these crystals was made by 

 Dr. Klement ; it shows that this pyroxene is akin to chromiferous diopside. 



I. 1*2557 grammes of substance dried at 110° C. and fused with sodium and 

 potassium carbonate gave 0'6504 gramme of silica, 0"0485 of alumina, 0'0071 of 

 chromic oxide, 0'0888 of ferric oxide, 0'2815 of lime, 0'5476 of magnesium pyro- 

 phosphate and traces of manganese. 



II. 1*1195 grammes of substance treated in a sealed tube with sulphuric and 

 hydrofluoric acids required 7 '2 cubic centimetres of potassium permanganate solution to 

 oxidise the ferrous oxide (1 c.c. = 0'005439 gramme FeO) — 



Another rock, which was labelled as a lava, and must have been poured out in sheets, 

 closely resembles that just described. It contains rather large crystals of augite and 

 laniellse of plagioclase, which sometimes measure two or three millimetres, but olivine 

 is not common. The rock is vesicular, and has a bluish grey ground-mass. Microscopic 

 examination shows that the fine-grained paste is formed of small aggregated plagio- 

 clastic lamellse, with augite and magnetite, but free from any vitreous constituent. 

 Sharply crystallised olivines stand out from the ground-mass; some of them are 

 twinned, most probably following a dome. There are also zonary crystals of augite, 

 each of the zones extinguishing at different angles ; these are twinned, following the 

 orthopinacoid, and the twins are frequently repeated polysynthetically. The lamellae 

 of microporphyritic plagioclase are often twinned according to the Carlsbad, pericline, 

 and albite laws. Sections almost perpendicular to P/M, showing very thin and sharp 

 periclinic strias, extinguish at angles between 35° and 39°; this felspar, therefore, 

 approaches anorthite. 



Other basaltic lavas show no porphyritic structure, the only element visible to the 

 naked eye being lamella of plagioclase of three or four millimetres in size, which have 

 lost their glassy sheen. The mass is bluish grey and scoriaceous ; augite and grains of 

 olivine may be distinguished by the lens. Under the microscope the ground-mass is seen 

 to be devitrified by trichites, and to contain augite and magnetite microliths, as well as 

 very slender crystals of plagioclase, sometimes assuming a stellate form. Olivine is 



