EOTLicH.] GEOLOGY— SAN LUIS DISTEICT, SECTION C. 337 



Until the hornblendic rocks near station 3 are reached, the Sahwatch 

 range retains its own peculiar kind of granite, and then the older 

 predominating one again appears. On Chalk Creek, one of the two 

 feldspars (oligoclase) decomposes quite readily, and the bluffs are conse- 

 quently of a white, almost chalky appearance, whence the name of the 

 creek. This decomposition appears to be only local, extending not very 

 far, and only on the northern side of the creek. Two warm springs 

 are fonnd near the bluffs, bubbling up through the deposit of drift- 

 material, and, as long as we remained encamped there, observations of 

 their temperature were taken. 



August 27, 1873, the smaller one, farther from the graaite-blufis than 

 the other, within about 80 yards of the creek : 



6 a. m. 12 m. 7 p. m. 9 p. m. 



108^.5 F. 112O.0F. lllo.OF. 109O.5F. 



It remains to be stated that the waters of this spring were exposed to 

 the action of the sun's rays during the entire day, which may account 

 for the rise of temperature during noon and its gradual cooling toward 

 evening. Two observations of the larger one were taken, which is 

 located nearer to the granite-bluffs, farther removed from the creek. 

 One was taken at G a. m., the other at 12 m., and in both cases the tem- 

 X^erature was found to be 130° F. 



Besides these younger granites, we have still another one in section h, 

 at station 38. Analogous to the granite that has given cause for the 

 formation of the Sangre de Christo range, station 38 presents a small 

 granitic cove, very similar to the former in lithological character, with 

 Silurian and Carboniferous strata dipping off from it on all sides. This 

 is the only instance of the kind that we have here, but it is certainly a 

 very striking one. A section cutting through station 38, given under the 

 head of Carboniferous, will more clearly show its position. 



Closely related to the granites we find a rock composing the mount- 

 ain upon which station 45 was located, at the head of Chalk Creek. A 

 crystalline aggregate of orthoclase, oligoclase, quartz, chlorite, and horn- 

 blende, (very sparingly,) constitutes the rock. Orthoclase is contained 

 in it as Carlsbad twins, reaching an inch and a half in length, of light 

 flesh-color, inclosing laminae of white, transparent oligoclase. The lat- 

 ter is dispersed throughout the rock in small portions, of white color, but 

 not so transparent as when in the orthoclase. Quartz is colorless, trans- 

 parent, not very plentiful ; sometimes found as rounded grains. Chlo- 

 rite of a pale muddy green to dark green and blackish green occurs 

 in small crystals and crystalline masses replacing mica. Hornblende 

 . was observed rarely in thin prisms. For this rock I use the term Por- 

 phyritic Protogine. 



Hornblendic rocks are found at several localities in section c, invariably 

 forming peaks of considerable elevation. The masses are never contin- 

 uous, but occur in isolated patches, in every instance serving as topo- 

 graphical stations — stations 2, 3, 43, and 44 being located on them — and, 

 without exception, they are surrounded by granite. A stratified struc- 

 ture of the rocks will always be found, at times parallel to the stratoid 

 texture, again making various angles with it. On station 2, Mount 

 Ouray, we find the granite breaking through this rook ; find it interstrati- 

 fied in a most characteristic manner ; and find on the most precipitous 

 side tha,t the very interior of the mountain must be made up to a con- 

 siderable extent of granite, which is coarse-grained, of light-red color, 

 wit]^! but little mica. The shape of the mountain is approximately that of a 

 22 GS 



