74 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
sometimes taken place and resorption, or magmatic alteration into pyroxene and 
magnetite, is a usual phenomenon. The borders of pyroxene grains sometimes have 
the same orientation as the hornblende core, but are as frequently without definite 
arrangement. They seem to have about the same composition as the original 
augite. Magnetite is less plentiful in the rocks containing considerable unchanged 
hornblende. Chlorite, serpentine, and the secondary green mica often result from 
decomposition. 
Biotite is frequently absent, but in many specimens occurs sparingly. In a few 
varieties it becomes the predominant ferro-magnesian constituent owing to a slight 
variation in the chemical character of the rock, as already noted (p. 71). It is 
probably related to the variety lepidoinelane. It is reddish brown, with strong 
absorption and sensibly parallel extinction. Corrosion is common and frequently 
the foils are bent or broken. Resorption with the production of abundant magne¬ 
tite, a little pyroxene, and probably some feldspar, occurs in most rocks which contain 
it. Chlorite and the green mica are common alteration products. 
Nosean, sodalite, and analcite .—As has been stated, these minerals are of inter¬ 
est as indicating a close relation of these rocks to the phonolites of the district. It 
may also well be repeated that in those rocks in which these minerals are at all 
plentiful the richness in alkalies which they indicate is offset by a greater abundance 
of the lime and ferromagnesian minerals than in the phonolites. In a very few 
specimens none or only a slight amount of these minerals has been detected. 
Nosean is present in many rocks and occurs as in the phonolites in phenocrysts 
of good size, usually with the grill-like inclusions. It is often decomposed into a 
more or less faintly polarizing aggregate, probably of zeolites. Other decomposition 
products are analcite and serieite. The occasional presence of grains of calcite 
among the decomposition products in rocks which do not elsewhere contain much 
calcite may indicate that some hauyne is present. 
Sodalite is found in all the nosean-bearing rocks and in a few others. It is 
almost always in the form of minute grains or crystals between the other ground- 
mass crystals, or included in the larger feldspars, but occasionally attains sufficient 
size to be regarded as a phenocryst, and then shows a clear border and a brown 
center crowded with inclusions. 
Analcite is of still more general distribution and occurs usually in irregular 
interstitial grains of small size. In a very few cases it occurs in individuals which 
may be idiomorphic, but with this exception its mode of occurrence and properties 
are exactly as in the phonolites. Small, sharp crystals of augite and of titanite 
which occur embedded in it sometimes entirely separated from other constituents 
necessitate the same conclusion as to its origin as was reached in the treatment of the 
phonolites—that is, it is a primary or original constituent of the rock. The analcite 
derived from the decomposition of feldspar is of course wholly distinct in origin and 
mode of occurrence. 
Apatite .—This mineral is always present to some extent, usually associated with 
iron ore, and often occurs in numerous crystals of notable size. It forms stout hex¬ 
agonal prisms up to 2 mm. in cross section, and is usually terminated by the unit 
pyramid and sometimes also by the basal pinacoid. Shapeless, irregular grains are 
not common, but well-formed crystals have sometimes been corroded. In many 
