COLORADO.

1. DENVER BASIN.

Rocks described by Cross in Mon. XXVII. E by L. G. Eakins, C by W. F. Hillebrand. in the Denver laboratory.

Analyses A, B, D, and

All but A were made

A. Dolerite, dike near Valmont. Contains augite, plagioclase, olivine, orthoclase, and biotite, with accessory magnetite and apatite. Record No. 1145. P. R. C. 534.

B. Augite separated from A. P. R. C. 105.

C. Basalt, Table Mountain, lower capping sheet. Contains plagioclase, orthoclase, augite, magnetite, and apatite, with olivine much serpentinized. Sp. gr. 2.83, 22.5°.

D. Basalt, earlier flow, south slope of North Table Mountain. Contains augite, olivine, plagioclase, probably orthoclase, magnetite, apatite, and a little biotite. P. R. C. 535.

E. Augite-mica-syenite, from north fork of Turkey Creek, Jefferson County. Contains orthoclase, augite, biotite, rhombic pyroxene, hornblende, plagioclase, quartz, apatite, and magnetite. Sp. gr. 2.857, 29.5°. P. R. C. 532.

The following rocks from the Denver Basin were analyzed by L. G. Eakins in the Denver laboratory, but the analyses do not appear in the monograph. The subjoined data have been supplied by Whitman Cross.

A. Enstatite-diabase-porphyry, Mount Morrison. Contains labradorite and enstatite in a groundmass of considerable amount, which is colorless and cryptocrystalline, probably feldspathic, and carries magnetite and other indistinct ferritic matter. P. R. C. 533.

B. Augite-andesite, Table Mountain. Contains plagioclase (andesine), with rare augite and biotite, in a groundmass of plagioclase, augite, magnetite, and minor accessories. This rock contained ptilolite in its vesicular equivalent. Described in Proc. Colorado Sci. Soc., 1886, p. 72.

C. Augite-andesite, a pebble from the Denver beds, Table Mountain. A few augite and andesite phenocrysts in a groundmass of oligoclase, orthoclase, and quartz (?) grains, with some augite and magnetite.

Described by E. B. Mathews. Analyses by W. F. Hillebrand, record No. 1470.

A. Granitite, Sentinel Point, western part of Pikes Peak massif. Contains microcline, microcline-perthite, quartz, biotite, a little oligoclase, and accessory fluorite, apatite, zircon, sphene, magnetite, and allanite. P. R. C. 600.

B. Granitite, near road between Florissant and the Platte River. Consists chiefly of microcline in perthitic intergrowth with albite, quartz, and biotite. P. R. C. 606.

C. Porphyritic granitite, south side of Pikes Peak, ridge between Middle and North Beaver creeks. Contains microcline, perthite, orthoclase, oligoclase, quartz, biotite, and accessory apatite, fluorite, zircon, and magnetite. P. R. C. 602.

D. Sheared granite, Currant Creek Canyon, north of Twelvemile Park. Contains perthitic microcline, quartz, muscovite, and sericitic aggregates replacing plagioclase and a part of the microcline. P. R. C. 604.

E. Granite, Currant Creek Canyon, north of Twelvemile Park. Consists chiefly of perthitic microcline, quartz, greenish biotite, muscovite, and plagioclase altered to a sericitic mass. Also flakes of limonite. Accessory minerals rare. P. R. C. 603.

F. Granitite, Middle Beaver Creek, south side of Pikes Peak. Contains microcline, orthoclase, perthitic albite, oligoclase, abundant quartz, biotite, and a little accessory magnetite, fluorite, and zircon. P. R. C. 601.

G. Granite-gneiss, north of Twin Creek. Contains microcline, orthoclase, quartz, biotite, abundant fluorite, and a little sphene and apatite. P. R. C. 605.

SECOND GROUP. ROCKS OF THE CRIPPLE CREEK DISTRICT.

Described by Cross. Descriptions published, except when otherwise stated, in 16th Ann., Part II, pp. 38-50. Analyses, with two exceptions, which are properly noted, by W. F. Hillebrand, record Nos. 1448, 1453, and 1524.

A. Phonolite, dike in granite northeast of Big Bull Mountain. Contains anorthoclase, nepheline, sodalite, ægirine-augite, some ægirine, biotite, magnetite, and limonite. Nosite not distinguishable. 616.

P. R. C.

B. Phonolite, Mitre Peak. Contains sanidine, nepheline, sodalite, ægirine, nosite, and colorless particles which may be låvenite. P. R. C. 608.

C. Phonolite, hill 1 mile south of Straub Mountain. Contains anorthoclase, sanidine, nepheline, sodalite, nosite, analcite, and ægirine, with sometimes ægirine-augite. P. R. C. 612.

D. Phonolite, Rhyolite Mountain. Contains nepheline, nosite, analcite, ægirine, and some ægirine-augite, in a feldspathic groundmass. Sp. gr. 2.52, 23°. P. R. C. 618.

Analysis by L. G.

E. Phonolite, between Florissant and Manitou. Eakins, made in the Denver laboratory, inserted here for comparison with the Cripple Creek samples. Described by Cross in Proc. Colorado Sci. Soc., vol. 2, p. 167. Contains sanidine, nepheline, hornblende with accessory pyroxene (?), magnetite, apatite, and sphene. Sp. gr. 2.576, 13°. P. R. C. 637.

F. The portion of E soluble in hydrochloric acid. This portion amounted to 25.39 per cent, and is reduced here to 100. Analysis by Eakins.

G. Trachytic phonolite, dike on west slope of Bull Cliff. Contains alkali feldspars, nepheline, nosite, sodalite, augite, scanty ægirine, brown hornblende, magnetite, sphene, apatite, and sometimes låvenite. P. R. C. 624.

H. Altered phonolite, Washington shaft, Victor. Contains potash feldspars, with some alteration to muscovite. Crystals of pyrite and fluorite are visible, but nepheline and ægirine have disappeared. Analysis hitherto unpublished. P. R. C. 623.

I. Nepheline-syenite, near the Longfellow mine. Contains alkali feldspars, nepheline, sodalite, augite, some ægirine, hornblende, biotite, sphene, apatite, and magnetite. Sp. gr. 2.68, 23°. Sp. gr. 2.68, 23°. Contains 0.02 V2O3.

J. Nepheline-basalt, Appie Ellen shaft. Much altered. Rich in olivine, augite, and magnetite. Also contains nepheline, feldspars in small amount, and biotite. Sp. gr. 2.99, 23°.

K. Altered nepheline-basalt, Anna Lee mine. Analysis hitherto unpublished and not discussed.

L. Local facies of a phonolitic mass, Bull Cliff. Contains abundant augite, plagioclase, alkali feldspar, magnetite, and a little red-brown biotite, with a colorless isotropic base in the darker spots. Analysis hitherto unpublished. P. R. C. 627. Contains 0.03 V2O.