62 GEOLOGY ANT) GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Owing to their similar properties and development they are not readily distinguish¬ 
able. But careful study under the microscope, in connection with a consideration 
of the chemical analyses, leads to certain conclusions, which accord with micro- 
chemical tests performed by Cross. The variety bearing sulphuric anhydride is 
principally nosean, though an occasional decomposed crystal incloses a small grain 
of calcite when the remainder of the rock is almost carbonate free, and this may 
indicate the presence of hauyne. It occurs in clear-cut dodecahedral crystals from 
0.1 to 2 mm. in size and often has a pale-bluish or greenish color, particularly when 
inclosed in feldspar. It is characterized by exceedingly small, dark inclusions 
arranged in two or more series of definite, parallel lines. These inclusions tend to 
be most numerous near the periphery of the crystal and sometimes fail entirely in 
the interior. At times, however, they are so abundant throughout that the mineral 
becomes almost opaque. Embayments due to corrosion are frequently observed. 
Sodalite is on the whole considerabty more abundant than nosean and is sometimes 
a prominent constituent. It also occurs in dodecahedrons, but fully as commonly 
in forms which give square or elongated rectangular sections. Occasionally the 
crystals attain the size of the largest nosean individuals, but usually they are mucn 
smaller, sinking to less than 0.001 mm. in dimension. Regular gradation in size 
aids in making certain the identification of the smallest particles. The crystals 
usually show a clear outer border surrounding a dull-brownish interior whose color 
is probably due to inclusions. 
These two minerals were among the very first to crystallize from the phonolite 
magma and in consequence occur as inclusions in all the other important constitu¬ 
ents, though they are not common in nepheline. They lose their isotropic character 
, on weathering and are transformed into a faintly polarizing aggregate of, probably, 
a zeolitic. mineral. Sericite forms occasionally. 
*/ 
Analcite. —Lindgren a was the first to regard analcite as a primary constituent 
of igneous rocks, and with that view numerous later observers have concurred. 
Cross considered the analcite of the Cripple Creek plionolites to be primary, and 
this seems the only possible conclusion. Beside its occurrence in the drusy cavities 
as already described, this mineral is seen in irregular masses up to 1 mm. in size, 
which suggest filled cavities. In such places it is apt to be intimately associated 
with sharp, fresh crystals of nepheline and of tegirine. It also occurs in even the 
densest rocks as small veinlets, and here too legirine is unusually abundant. In a 
few instances, as already remarked, it appears to have crystallized at about the 
same time as the nepheline, but it is generally seen to have been the last product to 
solidify. These rocks seem to have crystallized under pressure sufficient to retain 
water in the magma, and Cross concludes 5 that the analcite formed “when the 
excluded water contents of the magma had been concentrated by a kind of differen¬ 
tiation, in spots or along certain planes.” Soda also appears to be concentrated 
in these places, as shown by the development of large nephelines and the especial 
purity and abundance of the aegirine. On account of the chemical composition of 
these parts of the magma, it appears that on holocrystalline development analcite 
is practically the only mineral known to exist in igneous rocks which could utilize 
the water and excessive soda. 
a Tenth Census, vol. 15, 1886, pp. 727-729; Proc. California Acad. Sci., 2d ser., vol. 3, pp. 51-57. 
b General geology of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 36. 
