244 Mm M. C. Foley — Enclosures of Glass in Basalt 



apparently been given by Both as to whether it were really molted 

 augite, I commenced a microscopic examination, which brought to 

 light some curious features. The glass in thin sections is of a pale 

 olive-green colour, containing many delicate colourless trichites, and 

 it gives, as one would expect, total extinction. Its junction with the 

 ground-mass of the rock is very thickly fringed with dense masses 

 of microliths of all sizes, for the most part of prismatic shape, their 

 edges in some cases ragged and incomplete, and most of them 

 colourless, with dark inclusions, probably of iron-oxide. They react 

 very markedly on polarized light, showing brilliant colours with 

 crossed nicols. The shape of the prisms, the total absence of 

 pleochroism, and their high extinction angle (ranging up to 43°), 

 prove that they are some form of augite. Here and there they are 

 arranged in rings, and little nests of them lie dotted about in the 

 slide ; the majority, however, seem to cling to the edge of the 

 ground-mass. Lying in the latter and all hut touching the glass 

 are crystals of augite and olivine, which do not appear at all altered, 

 but have preserved their usual shape and appearance ; while in the 

 glass itself, near the ground-mass, are one or two small crystals of 

 augite which appear to be a good deal altered, the edges being very 

 ragged, with tiny microliths clinging to them and, as it were, eating 

 their way into the crystal. In some places the glass appears slightly 

 stained with the dark-brown colour of the ground-mass, but on the 

 whole the latter is separated from the glass by a distinct border. 



Being still unwilling to relinquish my original idea that I was 

 dealing with melted olivine — possibly a lime-bearing variety, which 

 in cooling might have developed partially into some form of augite 

 — I proceeded, at Prof. Bonney's suggestion, to fuse in the oxygen 

 flame pieces of augite and of olivine taken from the rock itself, the 

 fragments being held in each case in platinum forceps. 1 The augite 

 (of the common dark-brown type) fused fairly easily, and turned 

 into a dark glassy-looking bead with a somewhat resinous lustre. 

 This was powdered, mounted, and examined under the microscope. 

 The fused part was then seen to be a pale yellowish-green glass, 

 very similar in colour to that in the Bertrich lava. The olivine, 

 however, did not fuse so easily, although the fragment was held 

 in the flame for about two minutes longer than the augite. 2 It 

 turned into a black bead and adhered very firmly to the forceps ; but 

 on mounting the fragments, microscopic examination showed that 

 a mere outer film had been melted, and the particles fused had 

 turned into an extremely black opaque glass, totally unlike that in 

 the Bertrich lava. The bit of olivine which I had tried to fuse was 

 fairly pure, but contained a few granules of iron. A small chip of 

 the rock itself was treated similarly to the olivine and augite, and 

 it was melted down almost immediately into a rich brown glass. 



1 I am greatly indebted to Miss Aston, B.Sc, for assisting me with these experi- 

 ments, which were carried out in the Chemical Laboratory of University College. 



2 Still, Prof. Daubree states that he fused olivine without much difficulty; this, 

 however, was done under the influence of prolonged heat in a furnace. — " Geologie 

 cxperinientale," vol. ii. Paris (1879), pp. 517-523. 



