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. Miaskose. Contains anorthoclase, nepheline, sodalite, ægirine-augite, some ægirine, biotite, magnetite, and limonite. Nosite not distinguishable. P. R. C. 616.

B. Phonolite, Mitre Peak. Miaskose. 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. Miaskose. Contains anorthoclase, sanidine, nepheline, sodalite, nosite, analcite, and ægirine, with sometimes ægirine-augite. P. R. C. 612.

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

E. Phonolite, between Florissant and Manitou. Miaskose. Analysis by L. G. 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. 607.

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

G. Trachytic phonolite, dike on west slope of Bull Cliff. Miaskose. 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 Analysis hitherto unpublished.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. Akerose. Contains alkali feldspars, nepheline, sodalite, augite, some ægirine, hornblende, biotite, sphene, apatite, and magnetite. Sp. gr., 2.68, 23°. P. R. C. 626.

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 Analysis hitherto unpublished and not discussed.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. Akerose. 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.

THIRD GROUP.-MISCELLANEOUS ROCKS.

Analyses, elsewhere unpublished, except when otherwise stated, by W. F. Hillebrand, record Nos. 1448, 1453, and 1669. Petrographic data supplied by Whitman Cross. Names marked with a query are provisional designations only.

A. Trachyte?, near Robbins's ranch. Toscanose. Has scattered phenocrysts of alkali feldspar, oligoclase, and biotite in a predominant trachytic groundmass of alkali feldspar, tridymite, particles of residual glass, and ferritic flakes and grains. P. R. C. 1326.

B. Trachyte, Wicher Mountain. Toscanose. Shows biotite and a few glassy feldspars in a groundmass resembling that of A. P. R. C. 1328.

C. Andesite?, Bare Hills. Pulaskose. Composed mainly of plagioclase and alkali feldspar, with small augites and much ferritic matter. Minute prisms of a yellowish brown amphibole (?) and scales of tridymite are also visible. P. R. C. 1327.

D. Pyroxene-andesite?, Wicher Mountain. Monzonose. Contains phenocrysts of plagioclase, a few of augite, minute specks of iddingsite, and flakes of limonite. In the groundmass are plagioclase, orthoclase (!), augite, iddingsite, and hypersthene. In the pores tridymite appears. P. R. C. 1325.

E. Plagioclase-basalt?, mesa east of Mac Gulch. Andose. Contains plagioclase, augite, olivine, iddingsite, magnetite, biotite, and apatite. P. R. C. 1324.

F. Plagioclase-basalt, Saddle Mountain. Andose. Phenocrysts of augite and olivine in a groundmass of plagioclase, orthoclase, augite, magnetite, biotite, and apatite. Very fresh. See Cross, Journ. Geol., vol. 5, p. 684. P. R. C. 1323.

G. Analcite-basalt, from the Basin. Monchiquose. Contains phenocrysts of augite, olivine, and analcite. Also magnetite, with subordinate amounts of alkali feldspars, biotite, and apatite. P. R. C. 1322. H. Portion of the analcite-basalt soluble in hydrochloric acid.

I. Augite separated from G.

J. Analcite separated from G. After deduction of 4.22 per cent of substance insoluble in boiling dilute hydrochloric acid, and later removal of liberated silica with weak potash solution. Sixty-two one hundredths per cent of the water goes off over sulphuric acid. A trace of lithia was found.

The analcite-basalt and its fractions are described by Cross in Journ. Geol., vol. 5, p. 684.

3. SILVER CLIFF AND ROSITA.

Rocks described by Cross, mostly in 17th Ann., Part II, p. 263. Also, partly in Proc. Colorado Scientific Soc., vol. 2, p. 228. Analyses, with one exception, by L. G. Eakins. Those with record numbers attached were made in the Washington laboratory; all others in the laboratory at Denver.

A. Peridotite, Cottonwood Gulch. Custerose. Contains hornblende, biotite, hypersthene, olivine, a little plagioclase, apatite, pyrrhotite, and sillimanite (?). P. R. C. 519.

B. Augite-diorite, Mount Fairview, Rosita Hills. Salemose. Contains augite, biotite, labradorite, and accessory orthoclase, olivine, magnetite, and apatite. Sp. gr., 2.870, 32°. Olivine a subordinate constituent. P. R. C. 526.

C. The same as B, but with orthoclase in much larger amount. Akerose. Sp. gr., 2.768, 34. Record No., 1091. P. R. C. 529. D. Trachyte, Game Ridge. Phlegrose. Contains sanidine, plagioclase, a little biotite, magnetite, apatite, and zircon in a groundmass of orthoclase, with a little quartz. Sp. gr., 2.592, 29°. P. R. C. 70 and

524.

E. Trachyte, dark-colored dike. Pulaskose. Contains more magnetite than D, but otherwise the two are practically identical. Sp. gr., 2.621, 24. P. R. C. 589.