The following rocks were also analyzed in the Denver laboratory, but the analyses are unpublished elsewhere. Petrographic data furnished by Whitman Cross.

A. Granite-porphyry, NeNulty Gulch. Toscanose. Contains phenocrysts of orthoclase, oligoclase, andesine, quartz, biotite, and altered hornblende, in a groundmass of quartz, orthoclase, and magnetite. Accessory sphene, allanite, apatite, and zircon, and a little secondary · chlorite are also present. Analysis by W. F. Hillebrand. P. R. C. 586. B. Granite-porphyry, Jefferson tunnel. Toscanose. Contains orthoclase, oligoclase, quartz, and biotite, in a groundmass of mainly quartz and orthoclase; also accessory magnetite, apatite, zircon, and allanite. Chlorite appears as a decomposition product of biotite, and calcite and magnetite are present in small amounts. Analysis by Hillebrand.

C. Same as B. Toscanose. Analysis by L. G. Eakins. P. R. C. 583. D. Diorite-porphyry, Copper Mountain. Yellowstonose. Contains oligoclase, hornblende, and biotite, in a groundmass of quartz, orthoclase, plagioclase, and magnetite; also accessory zircon, sphene, and apatite, and a little secondary chlorite and epidote. Analysis by Eakins. P. R. C. 585.

E. Diorite-porphyry, McNulty type. Lassenose. Contains oligoclase, andesine, hornblende, biotite, and magnetite, in a groundmass of orthoclase, plagioclase, quartz, magnetite, apatite, allanite, and sphene; also secondary chlorite, epidote, and calcite. Analysis by Eakins. P. R. C. 584.

7. ELK MOUNTAINS.

Analyses made by L. G. Eakins in the Denver laboratory, and hitherto unpublished. Petrographic data supplied by Whitman Cross.

A. Rhyolite, East Mountain, Crested Butte district. Alsbachose. Phenocrysts of orthoclase, oligoclase, quartz, and biotite, in a microspherulitic and cryptocrystalline groundmass. P. R. C. 514.

B. Rhyolite, Round Mountain, Crested Butte district. Toscanose. Small phenocrysts of quartz, sanidine, biotite, and oligoclase, in a groundmass of quartz and orthoclase. P. R. C. 513.

C. Diorite, Brush Creek, Gunnison County. Tonalose. Contains several varieties of plagioclase, with orthoclase, quartz, hornblende, biotite, augite, sphene, apatite, and magnetite. P. R. C. 93.

8. WEST ELK MOUNTAINS.

Rocks described by Cross in 14th Ann., p. 165.

A. Hornblende-mica-porphyrite, Cliff Creek. Adamellose. Contains plagioclase, hornblende, and biotite, in a groundmass of quartz, feldspar, and mica. Analysis by W. F. Hillebrand, record No. 1429.

B. Porphyrite, Storm Ridge. Yellowstonose. Contains plagioclase, biotite, hypersthene, hornblende, and augite, in a groundmass of quartz and orthoclase. Analysis by L. G. Eakins, record No. 1238. P. R. C. 517.

C. Porphyrite-diorite, Mount Marcellina. Yellowstonose. Contains plagioclase, hornblende, and a little biotite, in a groundmass of quartz and orthoclase. Analysis by T. M. Chatard, record No. 1238. P. R. C. 516.

D. Quartz-porphyrite, Mount Carbon. Amiatose. Contains plagioclase, orthoclase, biotite, hornblende, augite, and quartz. Analysis by Chatard, record No. 1238. P. R. C. 518.

E. Quartz-porphyrite, Crested Butte. Lassenose. Contains plagioclase, orthoclase, hornblende, biotite, quartz, and a little augite. Analysis made by Eakins in the Denver laboratory. P. R. C. 515.

9. SAN JUAN REGION.

Rocks A to O collected by Whitman Cross, who supplies the petrographic data. Hitherto unpublished, except when otherwise stated.

A. Rhyolitic vitrophyre, near Del Norte, Rio Grande County. Toscanose. Reported by Cross as containing phenocrysts of oligoclase, quartz, biotite, and augite in a dark, fresh, glassy groundmass, the latter being predominant. Analysis made by Eakins in the Denver laboratory. Sp. gr., 2.423, 14°. P. R. C. 164.

B. Rhyolite, Summit district, Rio Grande County. Toscanos. Large phenocrysts of sanidine, with smaller ones of oligoclase and biotite, in a groundmass of orthoclase, quartz, oligoclase, biotite, and magnetite. Analysis by Eakins, made in the Denver laboratory. Sp. gr., 2.489, 14. P. R. C. 8.

ose.

C. Quartz-monzonite, Sultan Mountain, San Juan County. AmiatContains plagioclase, orthoclase, quartz, augite, biotite, magnetite, and apatite. Some chlorite and epidote as alteration products. Analysis by L. G. Eakins in the Denver laboratory. Sp. gr., 2.751, 14a. Described by Cross in 21st Ann., part 2. P. R. C. 204.

D. Augite-diorite, Stony Mountain, Ouray County. Hessose. Analysis by Eakins in the Denver laboratory. Sp. gr., 2.891, 13.5`. P. R. C. 199.

E. Monzonite, La Plata Mountains. Akerose. Contains augite, hornblende, plagioclase, and orthoclase in large amount, with biotite, quartz, sphene, apatite, and magnetite as subordinate constituents. Also secondary chlorite, muscovite, and calcite. Analysis by W. F. Hillebrand, record No. 1640. Sp. gr., 2.79, 21°. Sp. gr., 2.79, 21. Described by Cross in 21st Ann., part 2. P. R. C. 1284.

F. Diorite-porphyry, Deadwood Gulch, La Plata Mountains. AkerContains phenocrysts of hornblende, plagioclase, occasional quartz, sphene, apatite, and magnetite in a groundmass of orthoclase,

ose.

plagioclase, and quartz. Also secondary epidote, chlorite, and calcite. Analysis by Hillebrand, record No. 1636. Sp. gr., 2.677, 24°. A trace of sulphur is present. Described by Cross in 21st Ann., part 2. P. R. C. 1283.

G. Syenite, between Tirbircio and Schurman gulches, La Plata Mountains. Monzonose. Contains much alkali feldspar, some oligoclase, augite, biotite, and hornblende, with a little titanite, magnetite, and apatite. Analysis by H. N. Stokes, record No. 1764. Sp. gr., 2.704, 25°. P. R. C. 1286.

H. Augitic monzonite, Babcock Peak, La Plata Mountains. Andose. Contains orthoclase and plagioclase in about equal amounts, with augite and hornblende, and a little quartz, titanite, magnetite, and apatite. Analysis by Stokes, record No. 1764. Sp. gr., 2.767, 26°. Described by Cross in 21st Ann., part 2. P. R. C. 1285.

I. Porphyritic lamprophyre, allied to camptonite, Snowstorm Peak, La Plata Mountains. Kentallenose. Contains numerous phenocrysts of green hornblende, augite, and plagioclase, in a groundmass of plagioclase, orthoclase, augite, magnetite, and apatite. Some secondary calcite. Analysis by Hillebrand, record No. 1640. Sp. gr., 2.906, 21°. P. R. C. 1287.