86 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
by development in imperfect laths. Carlsbad twinning and visible traces of cleavage 
are less common than in the porphyries. Grains of microperthite are seen here and 
there. Plagioclase is rather abundant, and occurs like orthoclase. Polysynthetic 
twinning is often absent and the mineral must then be identified by its index, 
extinction, or double refraction. In composition it corresponds partly to albite 
and partly to oligoclase-albite, being thus a little more sodic than in the latite- 
phonolites. 
Kaolin, epidote, and sericite result from various processes of alteration of the 
feldspars. 
Nosean occurs as more or less automorpliic grains up to 0.6 mm. in size. It 
sometimes shows the characteristic grills of inclusions, but is commonly decomposed 
into a rather low polarizing aggregate of zeolites. 
Sodalite is often difficult of detection, but in a number of cases appears as 
minute grains between the other minerals, or more commonly included in the larger 
feldspars. In the neighborhood of mineral-bearing fissures sericite seems to form 
with particular ease from sodalite. 
Nepheline, if present at all, is in very subordinate amount and in no case could 
it be identified with certainty. 
Analcite has been found in a few instances as allotriomorphic grains along 
with the other constituents. In such cases it is undoubtedly primary. 
Pyroxene is one of the important constituents, and, aside from its somewhat 
greater abundance and generally poorer crystallographic form, its occurrence cor¬ 
responds to that in the latite-phonolites. It possesses on the whole the best form 
of all the important constituents of the syenites. It ranges from colorless, isochroic, 
to pale green, pleochroic, in the latter case indicating an admixture of a small 
amount of the segirine molecule. Twinning parallel to the orthopinacoid is seen in a 
few instances. A common alteration is to calcite, chlorite, and quartz(?). Other 
decomposition products occasionally seen are epidote, the secondary green mica 
alread} r described, sericite, and perhaps serpentine. 
The hornblende is of similar nature to that in the latite-phonolites, but differs 
somewhat in color. The range in color is as follows: a, light greenish yellow; b, 
yellowish green; c, deep green in some varieties, to a, brownish yellow; b, strong 
brown; c, deep greenish brown in others. In all cases c>b>a. The angle 
c:6 reaches a maximum value of 27°. Hornblende frequently shows poikilitic 
intergrowth with pyroxene and at times completely surrounds individuals of that 
mineral. In a few instances it is possible that slight resorption has taken place. 
Chlorite is a common alteration product. 
Biotite is rather common in irregular plates, usually of reddish-brown color, but 
sometimes greenish brown. Pleochroism is often intense. Like the mica of the 
porphyritic varieties, this mineral may be related to the iron-rich lepidomelane. It 
frequently occurs intergrown with hornblende and sometimes with pyroxene also. 
The most common decomposition product is aggregates of the secondary green mica. 
Apatite is present as numerous small, sometimes corroded, crystals and occa¬ 
sionally holds inclusions of the kind previously described. Titanite, while less 
abundant than in the latite-phonolites, is still of unusual prominence as an accessory. 
Small irregular grains of magnetite are common. 
