ART. 17 MOLLUSKS FEOM WYOMING REESIDE AND WEYMOUTH 3 



spicuously marked by abundant white spots from % to 1 or 2 millimeters in 

 diameter. The other two samples are nearly alike — both are very light gray 

 on the weathered surface and medium gray on fresh fracture. 



Microscopic examination shows that the white-spotted sample is a rhyolitic 

 tuff that consists of volcanic glass, angular quartz grains, and calcite. The glass, 

 which constitutes about 75 per cent of the rock, is very pale brownish gray and 

 minutely porous. Its refractive index is slightly less than 1.50 and corresponds 

 to that of a highly silicic glass. Numerous and well-preserved curving plates 

 and cuspate shards of this glass show clearly the vitroclastic texture of the 

 rock. A very small amount of cryptocrystalline material in the glass indicates 

 that it has been slightly devitrifled. The white spots so noticeable in the hand 

 specimen are the larger masses of this porous glass. The crystalline grains, 

 about 10 per cent of the rock, are exceedingly angular fragments and very thin 

 flakes from less than 0.01 to 0.18 (commonly about 0.06) millimeter in diameter. 

 Quartz predominates but mixed with it is some sanidine or orthoclase and a few 

 crystals or fragments of zircon, magnetite( ?), biotite, plagioclase, and horn- 

 blende (?). The rock is distinctly laminated with layers about 0.2 millimeter 

 thick of these crystalline grains alternating with thick and thin layers of the 

 glass. These crystalline grains afford no evidence that they are clastic sand 

 grains, and, although possibly entirely clastic, it seems more probable that they 

 are largely fragments of phenocrysts crystallized before the explosions or of 

 shattered wall rock blown from the sides of the volcanoes. The calcite, about 

 15 per cent of the rock, occurs as large anhedral crystals concentrated chiefly 

 near the quartz grains, but also distributed here and there throughout the 

 glass. No clay minerals were recognized in this sample. 



The two darker gray samples might be termed argillaceous tuffs or tuffaceous 

 mudstoues. Both contain volcanic glass and angular quartz grains, but they 

 also contain clay and organic matter and a considerable amount of cryptocrys- 

 talline material (in part at least devitrifled glass). In neither of them was 

 any calcite recognized. The sample from 80 feet below the top of the forma- 

 tion contains more clay and organic matter and less glass than the one from 

 300 feet below the top. However, in both there is approximately 50 per cent 

 or more of glass, the vitroclastic texture of which is still preserved. The 

 angular quartz grains in both specimens range from less than 0.01 to 0.08 (com- 

 monly about 0.02) millimeter in diameter. These samples are not distinctly 

 laminated, but the small lenses of organic matter and the clay crystals all have 

 a parallel orientation. 



In general appearance in the hand specimen, these two dark gray samples 

 from the Aspen formation greatly resemble the Mowry shale of northeastern 

 Wyoming, and in thin section there is a striking similarity in the constituents, 

 in the size of grain, and especially in the presence of more or less altered 

 rhyolitic ash in both formations. But the relative proportions of these con- 

 stituents are very different — even the more clayey and organic sample of the 

 Aspen contains far less clay and organic matter and much more fresh volcanic 

 glass than samples of the Mowry. Furthermore, the angular quartz grains, 

 though of approximately the same size in both formations, are distinctly more 

 numerous in the Aspen. That is, the two formations appear to be made up 

 of the same constituents, but the Mowry shale contains much less volcanic 

 debris or else the volcanic glass originally in it is now much more thoroughly 

 devitrifled. 



Under the Aspen shale lies conformably the thick Bear River 

 formation of dark shales containing thin beds of sandstone and 



