308 GEOLOGICAL SUKVEY OF THE TERRITORIES. 



small patches of fine-graiued grauite, distinct sometimes not only in 

 their texture, but also iu the character of their constituent minerals. A 

 granite found near station 59 showed gray quartz, white feldspar, and 

 black mica; was very fine-grained and compact. On Badger Greek, a 

 short distance east of station 58, a small patch of porphyritic granite 

 crops out that shows differences of texture within a very limited dis- 

 tance. It contains oligoclase, orthoclase in Carlsbad twins, gray 

 quartz, black mica, and chlorite. 



In tlie southern and western part of section a the granite is frequent- 

 ly traversed by dikes of hornbleudic rock, resembling to some extent 

 diorite. Although they will be spoken of under the title of "dikes" 

 hereafter, they form a very characteristic feature of the granite, and as 

 such must be mentioned here. 



Distribution. — Immediately at the eastern slope of the Front range 

 the granite begins, extends westward to station 91, and a few miles 

 beyond stations 93 and 94, bordering the east side of Oil Creek Valley. 

 At the northern boundary of this valley it sets in again, separating two 

 large volcanic areas by a band of three to six miles in width. West of 

 Oil Creek it is found in all the deeper caiions, while trachorheites cover the 

 high plateaus. Along Currant Creek it is exposed in a strip from one 

 and a half to two miles wide, increasing this width toward stations 71 

 and 72. The greater portion of Cottonwood Creek runs in grauite j also 

 a part of Tallahassee Creek. On the Arkansas, 'granite appears again 

 about half a mile west of Canyon City, and continues westward to sta- 

 tion 6, with a single interruption opposite station 10, where Cretaceous 

 rocks form the bed of the river for a short distance. Small patches of 

 it appear at stations 58', 58, and 57. West of Badger Creek several 

 such patches occur. The second extensive mass of it runs approxi- 

 mately parallel to the Arkansas, forming its eastern bank. From sta- 

 tion 52 southward it continues until a point is reached within two miles 

 of station 5, where the volcanic rocks cross the river. 



Besting immediately upon the granite, we find the Silurian character- 

 ized by but a few fossils and the well-known quartzitic formations. 

 From a distance these beds, situated in the western portion of section a, 

 may easily be mistaken for the prevalent trachorheites, as they form 

 steep, although not high, bluffs, rising abruptly from the granite, 

 capping some of its highest points along a line of about 15 miles. 

 Their quartzitic constitution allows them to resist atmospheric influences 

 for a greater length of time than the under and overlying rocks, so that, 

 taking all these points into account, they form a well defined character- 

 istic horizon. At times the rock partakes more of the character of a 

 limestone, but in that case segregations of siliceous matter, mostly in 

 the shape of chalcedony and hornstone, "aiford a welcome lithological 

 feature of distinction. Wherever the Silurian formation occurs in sec- 

 tion a, it conformably underlies the Devonian and Carboniferous as far 

 as could be. ascertained. Beginning in the northwest corner of this 

 section, we find a narrow strip of strata belonging to the Silurian period, 

 running from about one mile north of station 53, down in a southeasterly 

 direction, until it reaches the Arkansas, forming by its course the ap- 

 proximate representation of a large S. At station 53, which is located 

 on a dark-gray limestone, with hornstone segregations, ascribed to the 

 Silurian, the strata dip at an angle of 9° to 12° almost due east, gradu- 

 ally lessening the degree of their dip as they extend eastward. A sec- 

 tion running through station 53, at north 27° east, shows the following 

 strata, which I have referred to the Silurian. 



Eesting immediately^ upon a coarsegrained red granite, we find a blu- 



