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



395 



the development of spherules and small porphyritic crystals, and 

 may be regarded as a spherulitic condition of sanidine trachyte 

 or rhyolite. 



M. It is seen to consist of a multitude of spherules which possess 

 a radiating, crystalline, or fibrous structure, and consequently show 

 dark crosses between crossed Nicols (fig. 6, PL XX.). The spherules 

 vary in diameter from about inch to extremely minute specks, 

 when the section appears (in these parts) to pass into a microcrystal- 

 line condition, apparently identical with felsite. Indeed, from the 

 microscopic study of this and many other more or less closely allied 

 rocks, it appears to me desirable to abolish any hard lines of classifica- 

 tion which have hitherto been drawn between the trachytes, rhyolites, 

 and felstones. The spherules frequently form continuous and some^- 

 what tortuous strings, which, between crossed Nicols, resemble the 

 chenille put round the bottom of glass shades to exclude dust. The 

 porphyritic felspars frequently fail to show any definite crystal- 

 lographic boundaries, and are occasionally penetrated by creeks of 

 the surrounding substance, which in one or two instances is included 

 and shut off from the matrix. The porphyritic quartz-grains are 

 often irregular in outline, and do not appear to have any good fluid 

 enclosures. 



No. 6 a. Yellowstone Canon. Quartz rhyolite. 



A pale bluish-grey compact rock, showing delicate wavy fluxion- 

 bands, which, on a smooth surface, are seen to thin off, seldom being 

 continuous for more than an inch. Numerous small porphyritic 

 grains of quartz and a very few crystals of sanidine are also visible. 



M. The ground-mass of the rock is seen to be apparently micro- 

 granular, while the bands also show a microcrystalline or crystalline- 

 granular structure, but very much coarser. It is just the same tex- 

 tural difference which is still visible in microscopic sections of our 

 archaean and palaeozoic rhyolites. The ground-mass varies in texture 

 in different parts of the section ; and, where finest-grained, it is 

 studded with little roundish blotches, which seem to depolarize more 

 strongly, and which appear lighter than the ground-mass when the 

 Nicols are crossed, and darker when they are set parallel, or when 

 the section is seen by ordinary transmitted light. When examined 

 with an amplification of about 250 diameters, these spots seem to be 

 the result of devitrification ; while by employing a quartz plate it 

 is seen that the ground-mass is partially isotropic, and the doubly 

 refracting spots appear to become more mixed with isotropic matter 

 towards their margins. The spots, in fact, resemble segregations of 

 doubly retracting granules, which impart a sort of spotty mottling 

 to the ground-mass. The porphyritic quartz- crystals are nearly all 

 rounded in outline, like those which occur in quartz-porphyries ; but, 

 unlike the latter, they seem to contain no fluid lacunae. This rock 

 affords interesting material for the study of devitrification. 



No. 6 b. Lower Geyser basin ; is a pale grey or drab rock, with 

 darker parallel bands. It is a banded spherulite rock, the bands 

 consisting of small and the remainder of larger spherules. It closely 

 resembles No. 6, except in the better definition of the bands. The 

 rock contains some porphyritic felspar crystals. 



