REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 51 



These crystals are often infinitesimal, appearing as mere lines, which it would be 

 impossible to identify were it not that they merge by insensible gradations into well- 

 characterised crystals of these species. It is only by following the gradually 

 diminishing size of these minerals, step by step, from the augitic trachytes, in which 

 they are easily recognised, to the obsidians, that they can be determined in the latter 

 rocks. 



Sanidine is, as we have said, the only constituent attaining any size. Sections of 

 this felspar cut parallel to the face M, and showing the traces of P, y, T, give positive 

 extinction. Carlsbad twins are sometimes seen, but never hemitropic striae ; the latter 

 observation holds good of the large crystals as well as of the numerous microscopic 

 sections of felspar in the ground-mass. These very minute colourless microliths are 

 probably also sanidine ; the mode of their development, their form, and their twinning 

 relate them to the larger crystals of these species. Only the faces P, y are usually 

 to be seen ; but in certain cases x is also represented. Like the larger specimens of 

 sanidine, these are tabular, extremely thin, and elongated following PJM. They are 

 often twinned according to the Carlsbad law, as we have already described in the case of 

 trachytic rocks. Two of these thin lamellse are often superimposed with oblique axes, 

 and this mode of composition recurs so persistently that there is no doubt of its being a 

 twin, although the extreme minuteness of the crystals makes it impossible to ascertain 

 the law. The felspathic crystals grow smaller as the vitreous ground-mass becomes more 

 developed, but they are always distinguishable from augite, being colourless, and 

 generally rather larger than those of pyroxene. The augite crystals never attain the 

 proportions of those of sanidine ; they are always prismatic, but with ill-defined margins ; 

 the colour is greenish, and the angle of extinction rises from 35° to 40°. This is also 

 the angle of extinction of the little microliths, but when these assume the form of 

 capillary lines, their optical properties cannot be observed, and their identity is only 

 arrived at by considering the transitional forms. 



The obsidians are sometimes devitrified, and exhibit a finely granular texture ; some 

 of the vitreous rocks of the obsidian series show perlitic structure, and have the shining 

 appearance of pitchstone. 



The following is an analysis of a specimen from Green Mountain, which presented 

 all the appearances of an unaltered volcanic glass. An early analysis by Murdoch ' is 

 given for comparison. 



I. 1-0752 grammes of substance dried at 110°, and fused with sodium-potassium 

 carbonate, gave 07818 gramme of silica, 0'1376 of alumina, 0-0461 of ferric oxide, 

 0'0062 of lime, 0'0029 of magnesium pyrophosphate and traces of manganese. 



II. 0-7699 gramme of substance treated with hydrofluoric acid gave 0"1415 

 gramme of sodium and potassium chlorides, and - 1538 of potassium chloroplatinate. 



1 Murdoch, Mil May., 1844, p. 495. 



