MEDICINE BOW EANGE. 105 



taken is a fine-grained crystalline rock, made up largely of hornblende and 

 fine acicular crystals of brilliant triclinic feldspar. 



At the head of the northern branches of French Creek, and directly 

 under the great mass of quartzite which forms the summit of the ridge, is a 

 series of thinly-laminated beds, which have not heretofore been met with 

 in either the Archaean series of the Colorado or Medicine Bow Ranges. 

 They consist of dark argillaceous slates and schists, underlying conformably 

 the quartzite, and dipping eastward into the mountain. The upper beds 

 consist of exceedingly fine iron-gray slates or fissile argillites, having a 

 slight wave-like structure, and forming a sort of impure roofing-slate, with 

 an imperfect cleavage. To the unaided eye, they exhibit only minute mica 

 plates. They were estimated at about 400 feet in thickness. Directly un- 

 derlying them, and passing from the one to the other, is a series of harder 

 quartzose argillites, less thinly laminated, less uniform in character, with 

 layers of ferruginous material between the beds, and with occasional nar- 

 row seams of quartz-grains. 



Underlying this in turn is a highly crystalline schist, dififering widely 

 from the last-mentioned beds. There are no cleavage-lines, and the bed- 

 ding appears to be in a great measure obliterated. It is dotted over with 

 rounded nodules of hornblende, similar to the limestone concretions in the 

 "knotenschiefer" of Germany, and is made up largely of highly-altered 

 fibrous hornblende, of a bronze color, highly iridescent, with various hues 

 of green, red, and purple. Very minute quartz-grains are visible in the 

 mass of hornblendic rock ; and in thin sections, under the microscope, may 

 be seen minute crystals of feldspar. In places, the rock has somewhat of a 

 reddish coloration, arising from the decomposition of the altered hornblende. 

 The thickness of these beds and the nature of the underlying rock were 

 hidden by the soil and debris of the mountain-slope, but there cannot be 

 less than 600 feet in the entire series of slates, schists, and argillites. 



Several miles farther down French Creek, on the ridge, just above the 

 bed of the stream, ocour outcrops of quartzose slates and silver- white mica- 

 slates. They strike north 15° east, and dip as high as 70° to 75° to the 

 eastward ; apparently they were underlying a massive quartzite. 



The south wall of French Creek Canon, a high, almost unbroken ridge 



