OF THE VITREOUS ROCKS OF MONTANA, U.S.A. 393 



almost colourless glass. The yellow fibres depolarize in all azimuths 

 except those coinciding with the directions of the principal sections 

 of the crossed Nicols. There is therefore a marked optical difference 

 between the yellow and the colourless bands, the latter being per- 

 fectly isotropic*. The generally parallel disposition of the bands 

 imparts an appearance to the magnified section which very closely 

 resembles the structure seen in a longitudinal section of wood ; and 

 this resemblance is further heightened by the deflection of the bands 

 around small porphyritic crystals and fragments of quartz, which 

 may be likened to little knots in wood. Although the greater por- 

 tion of the preparation appears dark between crossed Nicols when 

 the general direction of the bands is parallel with one or other of 

 the Xi col-sections, still the deflected bands around the porphyritic 

 crystals transmit a strong brownish-yellow light, because they do 

 not, in this position, coincide with either of the Nicol-sections, so 

 that around each brightly depolarizing porphyritic crystal there are 

 also depolarizing fringes resembling smoothly-combed tresses of hair. 

 Some of the porphyritic quartz-crystals have irregular creeks of glass 

 passing into them, in some cases the yellow fibres being mixed with 

 the included colourless glass, thus clearly showing that these quartz- 

 crystals were actually developed within the rock prior to or during 

 its solidification. The preparation also shows numerous dark opaque 

 granules and a few dark sections, apparently of minute octohedra, 

 which are opaque, except at their margins, where they feebly trans- 

 mit light ; and this imperfect opacity seems to imply that they are 

 not magnetite. The fine banding in this rock is a most perfect ex- 

 ample of fluxion-structure. 



Representatives of this class of rock, similar in structure, but 

 more or less devitrified, are to be found among the lavas of early 

 Palaeozoic age in North Wales. 



No. 3. Yellowstone district. Black spherulitic obsidian. 



A black glassy rock, containing numerous pinkish or pale-grey 

 spherules irregularly distributed and varying from the size of a pea 

 downwards to very small dimensions. 



M. The section is seen to be traversed by bluish-grey and colourless 

 bands, the former consisting of streams of microliths (fig. 3, PI. XX.). 

 In this banded structure there is no depolarization, showing that the 

 banding in this case is quite different from that in the red obsidian just 

 described. The spherules either occur isolated or in little colonies. 

 Porphyritic felspar crystals occur here and there, and they show a 

 few glass enclosures. 



No. 4. Yellowstone district. Spherulitic-banded obsidian. 



A rock consisting of dull dark grey and vitreous black bands, 

 flecked with white porphyritic crystals of sanidine. 



M. The dull bands are seen to consist of radiately crystalline 

 spherules ; while the deep-black bands visible in the hand-specimens 

 appear in thin section as clear and colourless glass. In the drawing 



* Since this paper was read, I have satisfied myself that the double refraction 

 in these coloured bands is probably due to strain, a thick bundle of spun glass 

 exhibiting similar phenomena. 



