ENDLicH.] GEOLOGY SAN LUIS DISTRICT, SECTION A. 307 



a briglit-red color ; the quartz, Tellowish or white ; the mica, gray to 

 browu. Another point where the granite assumes a similar character is 

 immediatelj' west of Canon City, which may perhaps partially account 

 for the deepening of the steep caiion from which that city has derived 

 its name. This caiion forms the bed of the Arkansas Eiver for a dis- 

 tance of about six miles. Station 11 is located directly north of this 

 canon, in an air-line about three-quarters of a mile distant, and about 

 2,000 feet above the river-bed. Allowing all dne consideration for the 

 immense effect produced by erosion, it still seems improbable that the 

 Arkansas should have forced its way through almost six miles of solid 

 granite, when a slight deviation to the northeast would have afforded 

 it a by far easier passage, unless some existing favorable condition 

 induced this course. It seems reasonable, and warranted by the facts 

 observed, to suppose that, by some agency or other, possibly by the grad- 

 ual rising of the granitic area and consequent cracking or splitting of 

 strongly-tensioned portions of it, some facility may have been afforded 

 to the river in shaping its course. This view may receive some support 

 in the fact that the line pursued by the Arkansas through that section 

 of granite is a comparatively straight one. The suggestion that the 

 river may gradually have cut through during the continuation of the 

 rise along the former coast cannot be entertained, because at the time 

 before that took place the now Cretaceous portions must have been so 

 low as to compel it to flow out over the area they at present occupy. 



To the northwest of station 11 the granite is mostly covered by the 

 trachorheites, and only exposed in depressed localities. Along the 

 Arkansas, westward, toward station 10, it maintains its character, wea- 

 thering in more or less angular masses. At the west side of the bridge 

 built across the river, a short distance south of station 10, is stationed 

 an isolated granitic sentinel, dark red or brown, with a very large per- 

 centage of feldspar, but little quartz, and still less mica. This monu- 

 ment-like rock shows a pretty illustration of the intersection of two 

 veins; a narrow quartz-vein running up and down has been broken and 

 displaced by a broader younger one, crossing it at an almost right angle. 

 Speaking more correctly : the two halves inclosing the smaller vein have 

 been displaced by the formation of a broad Assure, in which the mate- 

 rial composing the younger vein was deposited. North of station 10 

 granite mainly crops out in the valleys, as in those of Currant, Cotton- 

 wood, and Tallahassee Creeks, the beds of which are mostly formed by 

 it. Proceeding in this northerly direction, we find the lithological char- 

 acter of the granite undergoing some change ; the feldspar more fre- 

 quently shows yellow and flesh-colored tints instead of the red and 

 brown ; the quartz turns from browu and reddish to rose-colored and 

 yellow, even gray and white ; black mica begins to predominate over 

 the white, gray, and brown. At the upper end of Cottonwood Creek, 

 near station 68, the texture of the granite begins to resemble closely 

 that of gneiss, and this change is carried out also in the appearance of 

 weathering, although it characterizes no newly-entering formation, but 

 merely an increase and accumulation of the micaceous constituents. 

 North of this station the granite widens out, being exposed over larger 

 areas as the trachorheites recede. West and southwest of station 10 

 it forms lower hills, considerably cut by the drainage. The western por- 

 tion of section a bordering on the Arkansas is formed by granite again, 

 appearing this time in' a strip of about twenty-lour miles in length and 

 three to four miles in breadth. It is coarse-grained, and very similar, if 

 not identical, with that of the eastern half of the section. Distributed 

 tJiroughout its southern halfj although not occurring frequently, are 



