4. BUFFALO PEAKS.

Hypersthene-andesite, described by Cross in Bull 1. Contains hypersthene, augite, plagioclase, magnetite, and apatite. Sp. gr., 2.742, 16°. Analyses made by W. F. Hillebrand in the Denver laboratory. A. Hypersthene-andesite. Andose. P. R. C. 86 and 588. B, C, D. Hypersthene separated from the rock. 3.307, 23°. In B and C alkalies were not tested for. the iron is given as FeO.

Sp. gr., of D,

In C and D all

5. LEADVILLE REGION.

Rocks described by Cross in Mon. XII, Appendix A. Analyses made in the Denver laboratory.

A. Porphyry, Mount Zion. Toscanose. Contains orthoclase, plagioclase, quartz, biotite, apatite, magnetite, and zircon. Analysis by L. G. Eakins. P. R. C. 504.

B. White, or Leadville, porphyry. Riesenose. Contains orthoclase, plagioclase, quartz, muscovite, magnetite, apatite, and zircon, with crystals which appear to be rutile and anatase. Sp. gr., 2.680, 16°. Analysis by W. F. Hillebrand. P. R. C. 587.

C. Porphyry, summit of Mount Lincoln. Lassenose. Contains quartz, orthoclase, plagioclase, biotite, apatite, sphene, magnetite, zircon, and allanite. The sample analyzed showed some muscovite, chlorite, and calcite. Sp. gr., 2.670, 16. Analysis by W. F. Hillebrand. P. R. C. 505.

D. Gray porphyry, Johnson Gulch, near Leadville. Yellowstonose. Slightly altered. Contains orthoclase, plagioclase, biotite, and quartz, with decomposition products probably derived from original hornblende. Sp. gr., 2.736, 16°. Analysis by Hillebrand.

E. Pink orthoclase crystals from D. Analysis by Hillebrand. F. Hornblendic porphyrite, lower Buckskin Gulch. Andose. Contains plagioclase, orthoclase, quartz, hornblende, biotite, magnetite, apatite, and zircon, with a little secondary calcite and chlorite. Sp. gr., 2.768, 16°. Analysis by Hillebrand. P. R. C. 89 and 506.

G. Biotite-porphyrite, dike in gneiss in the North Mosquito amphitheater. Tonalose. Composition like F, but with no hornblende, much biotite, and some pyrite. Sp. gr., 2.740, 16°. Hillebrand.. P. R. C. 507.

Analysis by

H. Rhyolite (nevadite), from Chalk Mountain. Liparose. Mainly quartz and feldspar, the latter being sanidine and plagioclase. A little biotite, magnetite, apatite, and zircon are present. Analysis by Hillebrand. P. R. C. 64, 512, and 1289.

I. Sanidine from H. Analysis by Hillebrand. P. R. C. 64.

6. TENMILE DISTRICT.

Rocks described by Cross in 14th Ann., p. 165. Analyses made in the Denver laboratory-A and C by W. F. Hillebrand, B by L. G. Eakins.

A. Quartz-hornblende-mica-porphyrite, Gold Hill. Yellowstonose. Contains plagioclase, hornblende, biotite, and quartz in a groundmass of quartz, orthoclase, and a little plagioclase. P. R. C. 510.

B. Quartz-porphyrite, Sugar Loaf. Toscanose. Contains plagioclase, biotite, and quartz, in a groundmass of quartz and orthoclase. P. R. C. 509.

C. Quartz-porphyrite, Chicago Mountain. Lassenose. plagioclase, orthoclase, biotite, and quartz. P. R. C. 508.

Contains