76 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Many of the apatites thus filled with inclusions exhibit a noticeable pleochroism. 
E is violet and O is smoky brown, and the absorption is E>0. 
Inclusions somewhat analogous in character and arrangement have been 
described in rock-forming apatites from Persia 0 and from Montana, 6 but in neither 
case were fluid-filled cavities mentioned. As observed in both these descriptions 
and also by Washington, c the abundance of inclusions in the apatite in these rocks 
appears to bear a definite relation to the degree of resorption of the hornblende, and 
in this case of biotite also. If the above assumption as to the origin of the inclusions 
in the Cripple Creek rocks is correct, there may be some connection between the 
excess of iron indicated in the inclusions and the later separation of magnetite in the 
resorption process. 
In a specimen of the biotite trachyte from the Portland mine well-formed 
crystals of apatite, reaching a maximum diameter of about 1 mm., are rather com¬ 
mon and on casual examination under the microscope appear to be of normal 
structure. In parallel light, however, indistinct traces of a fibrous structure are 
seen, and when the nicols are crossed the mineral polarizes in a peculiar manner, 
giving aggregate, feathery extinction due to minute fibers confusedly radiating 
from numerous centers or nuclei. The appearance is similar to that which often 
results from the filling of a cavity by quartz deposited from solution. The single and 
double refraction and other optical properties show- that apatite is the only mineral 
present and no explanation of the phenomenon has been found. 
Aside from these pecularities, the occurrence of apatite in these rocks is of 
considerable interest. The number and size of the crystals is rather unusual. A 
noticeable fact is that in the sodalite-bearing rock specimens of this group the 
amount of apatite is considerably less than usual. This may be due to some adjust¬ 
ment or equilibrium between soda, chlorine, and phosphoric anhydride. It is 
possible that there is a concentration of chlorine in the soda-rich (sodalite-bearing) 
varieties and of phosphoric anhydride in those holding less soda. 
The presence of \v r ater (the liquid can hardly be anything else) in the inclusions 
is also of interest when taken in connection with the occurrence of primary analcite. 
Titanite .—Although not unfailingly present, this mineral is a common accessory 
of the latite-phonolites, and sometimes is noticeably abundant. It occurs at times 
as irregular grains, but is more frequently idiomorphic in the diamond-shaped 
forms or the narrow laths, as in the plionolites. Some of the crystals reach a size 
of over 1 mm. The mineral has a brown color and the strong absorption some¬ 
times amounts almost to pleochroism. Twinning is occasionally seen. 
The manner of decomposition of the titanite of these rocks is rather interesting. 
At the outer edge of the crystal is formed a narrow rim of material, only slightly 
transparent, of very high refractive index and double refraction, which corresponds 
to rutile. Narrow arms of this mineral sometimes also extends across the inner 
portion of the crystal. Within this shell of rutile cal cite is plentiful and frequently 
fills the entire cavity, but in many cases there is also present a small amount of a 
mineral of rather low double refraction and an index near that of Canada balsam, 
n Blaas, J., Tschermaks Min. u. petrogr. Mittheilungen, vol. 3, 1880, pp. 477-479. 
t Merrill, G. P., Proc. U. S. Nat. Mus., vol. 17, 1894, pp. 642-643. . 
c Washington, H. S., Magmatic alteration of hornblende and biotite: Jour. Geol., vol. 4, 1896, p. 279. 
