394 



F. RUTLEY ON" THE MICROSCOPIC CHARACTERS 



(fig. 4, PI. XX.) two portions of one and the same sanidine crystal, 

 which between crossed Nicols undergo extinction synchronously in 

 the same azimuth, are separated by spherulitic matter; and this 

 seems to indicate that the spherules and crystals were probably 

 developed at the same time. 



No. 5. Yellowstone district. Black porphyritic obsidian. 



A black glossy rock, with white porphyritic crystals, some of them 

 | inch long. A smoothly-ground surface shows fine, continuous, 

 greyish, parallel bands and porphyritic crystals in a deep-black 

 ground-mass. The fractured surfaces of the specimen are very irre- 

 gular or small conchoidal. 



i¥. The bands are seen to consist of closely aggregated spherules ; 

 and the surrounding glass is filled with streams of microliths, which 

 impart a finely banded appearance to it. The porphyritic crystals 

 lie with their longest axes in the same direction as the bands. One 

 crystal (part of which is shown in fig. 5, PI. XX.) is completely sur- 

 rounded by a border of small spherules ; and in another instance a 

 little crystal lies immediately within a rudely concentric series of 

 perlitic cracks. The section is fissured in all directions, the cracks 

 traversing the glassy matrix, the spherulitic bands, and the por- 

 phyritic crystals ; they must therefore have been produced subse- 

 quently to the formation of those bodies. A few of the porphyritic 

 crystals in this rock bear a close resemblance to olivine. One 

 transverse section of a rhombic prism gives an angle of 94°, which 

 corresponds with the angle in olivine. The face of the section shows 

 the granulated surface so frequently seen in olivine-sections ; the 

 angles of the crystal are slightly rounded ; and the fringed cracks 

 which traverse the crystal closely resemble those seen in olivine. 

 The extinctions, moreover, in this crystal coincide with the directions 

 of the crystallographic axes, assuming the section to be transverse 

 to a rhombic prism. If this crystal be olivine (as I believe it to be), 

 we have a curious instance, possibly the only one yet recorded, of 

 olivine occurring in an obsidian. Zirkel, however, describes the 

 occurrence of olivine in a trachyte from the top of Whitehead Peak 

 in the Elkhead mountains, which he says " presents, beside sanidine, 

 very many cracked quartzes as large as a pea, hornblende, and augite, 

 and, what is remarkable, not very numerous but doubtless charac- 

 teristic half-serpentinized olivines, the sections of which, measuring 

 as high as 0*75 mm., are visible even to the naked eye in the slides. 

 The peculiar quartz occurring here is therefore accompanied by a 

 mineral which has never before been observed in a sanidine rock " *. 



No. 6. Yellowstone district. Spherulite rock. 



A pale bluish-grey rock, slightly cellular and with yellowish-brown 

 stains, with numerous little porphyritic felspar crystals (chiefly 

 sanidine) and, exceptionally, some roundish grains of quartz. On 

 a smoothly-cut surface the rock is seen to consist almost wholly of 

 closely aggregated spherules, with a little dark glassy interstitial 

 matter. It is, in fact, an obsidian almost completely devitrified by 



* "Microscopical Petrography," p. 159 (U.S. Exploration of the Fortieth 

 Parallel: Washington, 1876). 



