1052 The American Naturalist. [December, 
neighborhood of Silver Cliff only the rhyolites are present. The gran- 
ites intrude the gneisses, both being early Cambrian or pre-Cambrian. 
The voleanic series dates from the early Eocene; in this and in some 
other respects being like similar rocks at Cripple Creek and at other 
volcanic centres in the State. Both the Rosita and the Cripple Creek 
voleanoes are regarded as small outlying vents connected in origin 
with the much larger eruptions of the San Juan and South Park re- 
gions. 
The gneisses of the Silver Cliff-Rosita Hills district are described by 
Cross‘ as variable in composition, no one type persisting over any great 
area. They consist of microcline, orthoclase, quartz and biotite, mus- 
covite, hornblende or augite, and by variation in the amounts of either 
one or several of these constituents they grade into quite different types. 
The quartz and the bisilicates are often segregated in thin layers or 
streaks, thus giving the gneiss a banded aspect. Among the principal 
types represented in the complex, the augite-hornblende-gneiss is the 
most definite. It is a basic gneiss composed of a basic plagioclase and 
numerous intergrowths of augite and hornblende. The old gneisses 
and granites are cut by dykes of syenite, diabase and peridotite.® 
The earliest member of the volcanic series is a hornblende-mica-an- 
desite which occurs principally as a tuff. This was followed by mas- 
sive effusions of an augite-biotite-andesite containing large phenocrysts 
of orthoclase and small crystals of plagioclase surrounded by orthoclase 
rims. The andesites are cut by dykes of an augite-diorite containing 
biotite and altered olivine, and in some facies of the rock considerable 
orthoclase. 
A later lava than the andesite was the dacite, which differs from the 
augite-andesite in the presence of quartz and in the subordination of 
the dark silicate to the feldspars. Orthoclase is absent from the 
dacites, and the plagioclases present are nearly all andesine. 
The next in age and the most abundant of the lavas are rhyolites. 
These vary greatly in character, but nearly all phases are devoid of 
dark silicates. They occur in massive and in banded ledges, and in 
others remarkable for the great abundance in them of very large spher- 
ulites. These have been carefully studied by the author and reported 
upon in another place. In addition to the lavas tuffs, agglomerates and 
other evidences of explosive voleanic activity are noted in the vicinity 
of Rosita. Following the rhyolite came another biotite-augite-an- 
desite, and finally a trachyte of normal character. 
Rises Ann, Rep. U. S. Geol. Survey, p. 269. 
Cf. Proc. Colorado Sci. Soc., Vol. II, “1887, p- 223. 
