CHALK MOUNTAIN. 197 



made. The porphyries exposed here are quartz-porphyries, not closely 

 allied to any particular type; -but on the slope of the hill about one hundred 

 yards above the cut is an outcrop of lavender-colored rock, which in its fresh 

 fracture shows characteristic features of the Eagle River Porphyry. In the 

 railway-cut on the western side of the gorge both sandstones and porphyries 

 are thoroughly decomposed, but it can be seen that the latter both spread 

 out between and cut across the beds. On the spur which extends from 

 Chalk Ranch up to the Buckeye Peak ridge the sedimentary beds dip con- 

 formably to the northeast at an angle of 20 to 25. This dip continues 

 across the creek and up to the Nevadite at the southwest extremity of Chalk 

 Mountain, the disturbance produced by it being slight at this point. A blue- 

 gray limestone 6 feet thick is seen in the third railway-cut east of Chalk 

 Ranch, where it has an apparent northwesterly dip. On the eastern side 

 of the Arkansas Valley, just below and also a little above Chalk Ranch, 

 the beds dip 45 to the west and southwest. The evidences thus afforded 

 in regard to the structure of the sedimentary formations in this region are 

 not very satisfactory, showing simply that the beds are much broken up 

 and indicating that the influence of the intrusive Nevadite mass has not 

 been felt beyond narrow limits. 



Upper Ten-Mile Valley. The remaining area of sedimentary rocks is em- 

 braced between Chalk Mountain on the west, the Arkansas Valley on the 

 south, and the Mosquito fault on the east. Through it passes the conti- 

 nental divide, separating the waters of the Ten-Mile from those of the 

 Arkansas. The low, wooded Fremont's Pass has an elevation above sea- 

 level of 11,350 feet, and over it passes the Blue River branch of the Denver 

 and Rio Grande Railroad, the steep rise of 500 feet from the Arkansas 

 Valley being overcome by means of a long loop, as shown by the map. 

 Ten-Mile Creek has its head in a small rugged amphitheater in the Archean, 

 just east of the Mosquito fault, whence it flows directly west to the base of 

 Chalk Mountain, then turns abruptly northwest, and soon after passing the 

 limits of the map bends to the north and then to the northeast. The gentle 

 slopes near the creek are wooded, and outcrops are rare until the neighbor- 

 hood of the great Mosquito fault is reached. 



