RHYOLITIC PIMICE. :]ft,:] 



sectiou 196. There is much oi)a(iue coloring matter in the base and an abundance of 

 phenocrysts consisting of much quartz and nearly equal quantities of sanidine and 

 triclinic feldspar; in one Carlsbad twin one lialf exhibits an interference cross that is 

 optically negative, while the other half gives a bar parallel to the clinopinacoid, in 

 which case the sectiou must be jjcrpendicular to the negative bisectrix of the first 

 half, having an angle between tiie optic axes of about Qo, and at the same time at 

 nearly right angles to one of the optic axes in the other half having a larg(^ ojjtical 

 angle and the i)lane of the axes parallel to that of symmetry. Still another Carlsbad 

 twin shows the plane of the optic axes normal to that of symmetry. There is rather 

 more hypersthene than is common to these pumices. It is partially decomposed and 

 displays a very striking pleochroism, owing to the thickness of the section. 



Thin section :i()0 is the most interesting of all the alteration products, on ac- 

 count of its undoubted relations to the basalt and its higher degree of nietanior- 

 phism ; it is traceable directly to the same deposit of jjuiuice as lltit, and lies in apparently 

 undisturbed layers directly over basalt, which did not in this instance reach the sur- 

 face, but thoroughly altered the overlying pumice, breaking through it lower down 

 the slope. In thin section it is a whitish gray, fine grained breccia of about the same 

 grain as 19'J. Under the microscope the porphyritical crystals are seen to be angular 

 fragments of quartz, sanidine, and plagioclase of the same size and abundance as 

 those in the last named section; pjToxene, however, is wanting and only a little 

 biotite is present, besides a single grain of garnet. The groundmass lias retained its 

 brecciated character, though the pumice fragments have lost their original form and 

 appear to merge into one another; but the degree of crystallization is far more 

 advanced, hardly any portion of it being without influence on polarized light. As a 

 natural result of its brecciated character the structure is most varied, which is the 

 more pronounced between crossed nicols. It is partly spherulitic and axiolitic and 

 partly cryptocrystallinc and in places it is microcrystalline in irregular grains. 



Thin sections l,'()4 and 205 are ti-om a small outbreak of rliyolite on the south 

 side of the spur about 100 yards from thelocality of 19!), which, tlimigh not traced in the 

 field to unaltered pumice, exhibits under the microscope so close a resemblance in many 

 respects to the last described form as to leave little, if any, doubt that this small flow 

 of porcelain-like rhyolite is a highly altered pumice breccia that has escaped from its 

 place of confinement, probably having been heated under i)ressure to a greater degree 

 than the breccia met with in situ on the surface. In thin section it is whitish gray, 

 204 having a glassy groundmass strongly resembling that of 199, which is filled with 

 faintly i)olariziug particles, and shows as great a diversity of structure, which indi- 

 cates its once brecciated condition. It is in ])laces spherulitic, cryptocrystalline, and 

 microcrystalline. There is a marked flow structure and a smaller amount of frag- 

 mentary crystals, consisting of (piartz and fi'ldsi)ar. with very little biotite and one 

 fragment of greenish brown hornblende. Thin section 20.j has a groundmass of more 



