HHYOLITK. 377 



grains. This group of rhyolites is very poor in accessory minerals, there being only 

 two, which are of exceptional occurrence. Zircon in fragments and minute crystals 

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

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

 in NOB. Ill, 112, 122, and 123. 



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

 presents the micrograuitic structure, not frequently met with. The remarkable thin- 

 ness of the sections prepared from this rock offers a highly satisfactory field for study 

 and leaves no reasonable doubt of the entire absence of glass in the composition of 

 the grouudmass of most of the thin sections. Besides the granular crystalline develop- 

 ment there are those that are partly cryptocrystalline and others that are spherulitic 

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

 114, 115, 116, 120, 119, 123, 140, 141, 153, 127, 134, 137. The groundmass of 112 may 

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

 scopic interpenetrating grains of quartz and feldspar, through which are scattered 

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

 like those in the phenocrysts of feldspar. A small portion is deterininable as ortho- 

 clase and striped plagioclase. The quartz is often in aggregates of half a dozen or 

 more grains and is accompanied 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 

 reflecting incident rays and giving a whitish gray color to the section. They are 

 probably minute gas cavities. In addition to this are patches of yellow, ill defined 

 grains, corresponding to Vogelsang's ferrite, which is only in small quantities and 

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

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

 shaped cavities. In the others it is more or less dense and varies somewhat in the 

 size of the grains. 



Still more interesting are the changes of structure in the groundmass 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 carry larger grains, which are micropegmatitic in tliiu 

 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 phenocrysts. The grains in the groundmass of thin section 

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

 attain a diameter of 0-O.V" 1 ", one in the thinnest edge of the section shows a beauti 

 fully developed micropegmatitic structure, which near the center of the grain is in tri 

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



