8 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



OH by a specfal party. Using the lines of the primary system as bases, 

 the topographers of the division carry on the secondary triangulation, 

 locating points within the triangles of the primary system. In the 

 secondary system, as in the primary, all three angles of the triangles are 

 measured, and, in most cases, artificial signals are used on the stations. 

 The instrument used for this work is a sort of theodolite, reading 

 minutes. The stations for triangulatiou and topography are, in most 

 cases, the highest and most commanding points, and are so selected that 

 the limits of work from one will reach the limits from those around it. 

 From a station, a sketch-map of all the country within the range of vision 

 is made, as also a prospective sketch. Angles taken on prominent points 

 and recorded on these sketches serve to locate them, and thus to correct 

 the sketch-map. The distance between stations must depend on the 

 character of the country, but the average distance apart is seven to ten 

 miles. For the prosecution of its work during the past season, this 

 division made eighty-six stations, or one station to every eight miles of 

 area. 



The most prominent geographical features occupied by this division 

 are in brief as follows : On the north, the Eagle or Piney Eiver flows, 

 through most of its course, in a broad fine valley, having a course nearly 

 west, interrupted here and there by short canons. At its mouth, it is a 

 large stream, barely fordable at the lowest stage of water. The Grand 

 Eiver, sometimes called the Blue or Bunkara, below the mouth of the 

 Eagle, is in a close caiion about thirty miles, interrupted by a short 

 meadow at the mouth of Roaring Fork. Below this caiion, the river 

 flows sluggishly through a broad meadow, which extends for fully fifty 

 miles, but is narrowed in the middle of its length, where the river cuts 

 through a plateau. Below this meadow, the river enters another caiion 

 about eighteen miles in length, and of no great height, from which it 

 flows into the broad valley in which it meets the Gunnison. The course 

 of the Grand, at the mouth of the Eagle, is about west, which direction 

 gradually changes to southwest, and then near the mouth of the Gun- 

 nison again to the west. 



The drainage of the southern part of the district is by the Gunnison 

 River. This stream takes all the water from the southern slopes of the 

 Elk Mountains, the western slopes of the Saguache range, and the 

 northern slopes of the Uncompahgre Mountains. For twenty-five miles 

 below the mouth of Cochetopa Creek, this river is in a narrow valley, 

 which is diversified tey long tongues of mesa, which separate the 

 numerous streams entering the river on either side. Below this valley 

 is a very heavy caiion cut in a high plateau for fifteen miles. The 

 plateau is horizontal, 10,000 feet high, and the course of the river is 

 nearly west across it, the depth of the caiion increasing with the fall of 

 the river from 3,000 to 4,000 feet. At this point, the plateau breaks off 

 abruptly on the north side, and, while preserving nearly the same height 

 at the edge of the caiion, slopes off gradually toward the north, having 



