UHYOMTK. 377 



"rains. Tliis i;'r'>iii> <>{' rliyolitts is very poor in accessory minerals, there being only 

 two, wbicli lire of cxci'ijtional ofciirrencc. Zircon in fragments and minute crystals 

 is occasionally met with iii association witli biotite. Garnet in well developed 

 dodecahedrons, and also in irregular grains of a light retl color in thin section, occurs 

 in Nos. Ill, 112. V22, and VS.i. 



The most striking feature ot this variety of rhyolite is its grouudmass, which 

 presents the microgranitic structure, not frecjuently met with. The remarkaljlc thin- 

 ness of the sections prei)ared from this rock ofters a highly satisfactoi-y field for study 

 and leaves no riiasonable doubt of the entire abseuce of glass in the couii)Osition of 

 the groundmass of most of the thin si-ctious. IJosides the granular crystalline develop- 

 ment there are those that are partly cryjitocrystalline aud otlicrs that are siiherulitic 

 and glassy. A microcrystalline structure is common to thin sections 111, 112, 113, 

 114, 115, 116. 120, 119, 12.3, 140, 141, l.")3, 127, l.U, 137. The groundmass of 112 may 

 be taken as representing that of all the first innc sections. It is composed of micro- 

 scopic interpen«^trating grains of quartz and feldspar, through which are scattered 

 larger grains, averaging 0*06 """ in diameter, for tiie most part quartz, with gas cavities 

 like those in the phenocrysts of fel(lsi)ar. A small i)ortion is determinable as ortho- 

 clase and striped plagiodase. The (juartz is often in aggregates of lialf a dozen or 

 more grains and is accompanietl by irregular fragments of light red garnet. There is 

 also a little biotite in microscopic crystals, more abundant in thin section 116. 

 Through it all are innumerable dust-like particles, dark in transmitted light, but 

 retie<'ting incident rays and giving a wliitish gray color to the section. They are 

 prol>ably minute gas cavities. In addition to this are patches of yellow, ill defined 

 grains, corresponding to Vogelsang's fen itc wliicli is only in small quantities and 

 alone indicates tJie flow .structure, best seen in the thin section without the aid of a 

 lens. The groundmass in this section (112) is porous and is tilled with small, irregularly 

 shai)cd cavities. In the others it is more or U'ss dense and varies somewhat in the 

 size of the grains. 



Still more interesting are the changes of structure in the grounduuiss of the 

 rhyolite from the dikes. Thin sections 140. 141. 127, 134, and 137 represent the most 

 crystalline variety, being coarser grained than that just described. They are without 

 any sign of flow structure and ciirry larger grains, which are micropegnuvtitic in thin 

 section 140. The grains are composed of a colorless grain or crystal of quartz with 

 hexagonal outline, inclosing semi-opaque particles, which are white in incident light, 

 and are sometimes arranged radially. The same structure appears as a narrow 

 border around the quartz jihenocrysts. The grains in the groundmass of thin section 

 141 are also mottled in polarized light, but iu 127, where the similarly clouded grahis 

 attain a diameter of d-o."*""", one in the thinnest edge of the section shows a beauti- 

 fully developed niicropegmatitic structure, which near the center of tlie grain is in tri- 

 angular ligures only 0-002 """ in size, and near the edge is iu long, narrow strips. The 



