ENDLICH.J CATALOGUE OF COLORADO MINERALS. 147 



Nateolite. — Golden, Colo. Table Mountain. In cavities in the 

 Basalt near Uneompahgre Peak. 



Obsidian. — Porphyritic, in a dike, at station 27 of 1873; Buffalo Peak, 

 Arkansas Yalley, and Union Park. Under the trachyte, on Gunni- 

 son River (porphyritic and spherulitic). A heavy vein of porphyritic 

 obsidian occurs near the Rio Grande Pyramid, and continues from 

 there southward in the trachytic beds. l!fodules occur in the lower 

 members of the trachytic series. A dike of obsidian, light gray, clear 

 sets across the Colorado Central lode near Georgetown. North of 

 Saguache Creek with concectric structure. 



Oligoclase. — Occurs in many of the granites and in the volcanic 

 rocks of Colorado. Good crystals are rare. 



Olivine. — Transparent, green in the basalts of San Luis Yalley. 



Onyx.— >S'ee Quartz. 



Opal. — Aguas Calientes, Gilson Gulch ; Idaho Springs, here the Opal 

 occurs in narrow seams in the granite ; mostly it is brownish ; milk- 

 white at Colorado Springs. 



Semi opal. — Found together with the Chalcedonies at the Los 

 Pii5os agency. North of Saguache Creek in trachyte. 



Wood opal. — On Cherry Creek, near Florissant, South Park. 

 Hyalite. — In the trachytes, near the Los PiSos agency. At the 

 Hot Sulphur Springs, Middle Park. Basalt of North Mam. 

 Sometimes occurring in very fine specimens in the trachorheites 

 of the Uneompahgre groups. 



Orthoclase. — Occurs in very fine, though small crystals, on Bobtail 

 and Gregory Hills at Central. Crystals there are either simple or 

 Carlsbad twins. It is found in very large pieces in some of the 

 coarse-grained granites. Near Mount Ouray this is particularly the 

 case. Large tablets of flesh-colored orthoclase can there be found. 

 Crystals of large size, simple and twins, occur in the porphyritic 

 dikes at Gold Hill, Boulder County, at the head of Chalk Creek, 

 interlaminated with oligoclase in the porphyritic protoginyte. 

 Crystallized in Jefferson County ; greenish in South Park, west of 

 Pike's Peak; reddish on Elk Creek; brown and gray at various 

 localities near Central City. Beautiful, green crystals of orthoclase 

 are found on Bear Creek, near Pike's Peak, associated with smoky 

 quartz. An analysis by Dr. Oscar Loew* of this orthoclase furnishes 

 the following result: 



(1) (2) 



Silicic acid 67.01 63.12 



Alumina 19.94 19.78 



Protoxide of iron 0.89 1.51 



Soda 3.15 2.11 



Potassa 8,84 12.57 



Lime trace. 0.66 



Magnesia trace. 0.13 



99. 83 99. 88 



The coloring of this orthoclase, therefore, is due to a small percentage 

 of protoxide of iron. 



Dr. G. A. Koenigt regards the coloring-matter of this green 

 orthoclase as dependent upon a ferric compound, probably an 

 " organic salt." 

 Ozocerite ? — From head Cherry Creek. 



*AT)n. Rep. Expl. and Surv. West 100th Mer., App. L. L., 1875, p. 111. 

 tProc. Ac. Nat. Sci,, Phil., Part II, 1876, p. 155. 



