5 4 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
markedly—and is acutely bisected by the axis of least refraction. Alteration 
products are of several kinds. Very commonly the mineral is changed along 
cracks and cleavages to feathery serpentine which is of exceptionally strong color— 
deep bluish green—and noticeably pleochroic, probably because of richness in 
iron. The serpentine is not always confined to the olivine boundary, but often 
fills cracks and spaces throughout the rock. A less frequent decomposition produces 
pseudomorphic, mixed aggregates of limonite and chalcedonic silica, much as in 
the decomposition of iron-rich garnets. Still another product of alteration is a 
material of brownish-yellow color, forming scaly aggregates. When crystalliza¬ 
tion is more complete the mineral appears in good-sized foils showing intense 
pleochroism—strong brownish yellow across the cleavage and deep green or some¬ 
times brown parallel to it. The transformation from olivine is beautifully shown 
in some cases. This mineral has the cleavage, high double refraction, and charac¬ 
teristic surface of mica, which it is beyond cpiestion. Not uncommonly a chlorite 
of beautiful deep-blue color accompanies the mica. Occasionally the olivine is 
changed to talc and limonite. The brown color, distinct cleavage, comparatively 
small optical angle, negative character, and the nature of the alteration products 
all point to an olivine rich in iron, like hyalosiderite . a 
Apatite is very abundant in well-formed prisms of noticeable size. Black iron 
ore is likewise plentiful in large, irregular grains. A very little zircon is present in 
grains which reach a size of 3 mm. 
The combination of alkali feldspar, olivine, and diallage is unusual, and justifies 
the application of the name olivine syenite. Somewhat similar rocks have recently 
been found in Wisconsin.® This rock also resembles in some respects the perth- 
itophyres of Chrustschoff/' 
Of equal interest are the segregations which have taken place in this magma. 
In the southern half of the mass covered by it occur small inclosed areas, connected 
by gradual transitions with the main mass, where an increase in the amount of 
feldspar and a corresponding decrease in olivine give a pyroxene syenite with 
accessory olivine. In such cases the rock is red, usually of finer grain than the 
olivine syenite, and then sometimes porpliyritic through the development of a 
few large feldspar crystals. In a few cases the olivine nearly or quite disappears 
and the prominence of quartz becomes such that the rock is a pyroxene granite. 
Along the eastern edge, and particularly at the north end of the mass, the rock 
gradually becomes much darker in color—-almost black. Feldspar is less promi¬ 
nent than in the olivine svenite and is of green color. Olivine is more abundant, 
but in less characteristic macroscopic grains. Iron ore is very plentiful, and fre¬ 
quently apatite occurs in beautifully - sharp green needles up to half a millimeter 
in cross section. Occasional glistening flakes of mica are seen. In thin sections 
feldspar is found still to be the most important constituent. It belongs to the 
plagioclase series and ranges from oligoclase to labradorite inclusive, the most 
abundant variety being a sodic labradorite. Albite, Carlsbad, and pericline 
twinning are often beautifully exemplified. Diallage occurs as in the main rock, 
but has the purple or lavender tint indicative of appreciable titania content. 
oCf. Weidman, S., Jour. Geol., vol. 12, 1904, pp. 551-561. 
b Tschermaks Min. u. petrogr. Mittheilungen, vol. 9, 1888, p. 476. 
