RHYOLITIC PUMICE. 381 



an opaque grain from which they radiate in all directions, frequently resembling the 

 down of a thistle and suggesting in some instances a bunch of ravelings. There are, 

 besides, other indeterminable, smaller microlites and a few gas bubbles. 



Closely related to the rhyolite of Purple Hill, both in their field occurrence and 

 mineral composition, and in the latter respect allied to the purple rhyolite of Rescue 

 Canyon, are the tuffs and pumice found in the vicinity of the town of Eureka, on the 

 west of Richmond Mountain, and also on the south slope of the same mountain. 

 These are specially interesting because of numerous alteration products which have 

 resulted from outflows of basalt that have broken through them. Thin sections from 

 a scries of specimens representing different stages of alteration naturally exhibit the 

 same character of phenocrysts, which have not been affected by the remelting and 

 may therefore be considered in one general description, the modifications of the pumice 

 having been confined to the glass base. Thin sections 185, 188, 189, 193, 191, 192, 196, 

 are from the quarry and hill slope east of the town. The phenocrysts consist of augu 

 lar fragments, seldom of perfect crystals of quartz and feldspar with a small amount 

 of hypersthene, hornblende, and biotite, together with magnetite, apatite, zircon, and 

 garnet as occasional accessory minerals. The quartz is of very pure substance carry- 

 ing only glass inclusions, one of which, in thin section 192, is brown, a neighboring 

 inclusion being colorless. There are two instances in the same thin section of quartz 

 and feldspar grown together with micropegmatitic structure. Of the feldspar, sani- 

 dine is the predominating species, many of the unstriated sections being optically deter- 

 minable as such. Triclinic feldspar is always present in greater or less amount. 

 A zonal structure is frequent and some individuals bear inclusions of glass in the form 

 of the most beautifully defined negative crystals; the feldspar is everywhere per- 

 fectly fresh. The strongly pleochroic hypersthene is in places crowded with apatite 

 and glass inclusions. It is precisely similar to that in the pyroxene andesite and ande- 

 sitic pearlites; while the dark green hornblende without black border and the biotite 

 correspond exactly to the same minerals in the pearlite. The accessory minerals ha ve 

 also similar characters to those found in the pearlites. Small fragments of allanite 

 are abundant in Nos. 189, 191, 192. 



Thin sections 185 and 188 are from unaltered portions of the pumice breccia; 

 185 is from the quarry back of the engine house and 188 from a spot 6 feet distant 

 from the plane of contact with the basalt a little to the north. They are essentially 

 the same rock, 188 being the better section. It consists of a fine grained mixture of 

 colorless pumice fragments full of elongated fluid inclusions with variously sized gas 

 bubbles, sometimes looking like welded glass threads, together with a projwrtionately 

 smaller amount of crystallized minerals, in a matrix of yellow glass that appears to be 

 made up of minute glass particles held together by glass, in which are much fewer 

 gas cavities, and which is partly cryptocrystalline. There are also occasional frag- 

 ments of glass of other kinds, some brown and others microfelsitic and in part crypto- 



