LETTER TO THE SECRETARY. 7 



4. A map in the vicinity of Colorado Springs, on a scale of one-half a 

 •mile to an inch, made principally for geological purposes. 



The meanders of traveled roads will be plotted on the final maps of 

 Colorado. 



While all this work was looked upon as special work, and done with 

 more detail than the regular work of the survey, the results will, of 

 course, be incorporated in the final maps of the Territory, and form a 

 part of them. 



During the season, Mr. Chittenden made 156 topographical stations, 

 and the total area surveyed was over four thousand square miles. Mr. 

 W. H. Holmes labored with his usual zeal and skill during the entire 

 season, and much of the accuracy and value of the work is due to him. 

 During the sickness of a member of the party at the base of Sopris 

 Peak, I was detained about twenty days. In the mean time, Messrs. 

 Holmes and Chittenden made a careful geological and topographical 

 study of the northwestern portion of the Elk Mountains, the results of 

 which will be found embodied in Mr. Holmes's report. Great numbers 

 of topographical and geological sketches were made by Mr. Holmes, 

 which will serve in a remarkably clear manner to illustrate the 

 structure of the interesting regions surveyed. 



The district assigned to the second division is limited on the north by 

 the Eagle and Grand Rivers, west by the west line of Colorado, south 

 by the parallel of latitude 38° 20', and east by the 107th meridian. The 

 area of this district is about seven thousand square miles, of which the 

 party completed 5,300 square miles. 

 The plan of the geodetic and topographical work is as follows: 

 1st. The latitude and longitude of certain points are determined by as- 

 tronomical observations as accurately as the present state of astronomical 

 science will allow. This work has been done for us, thus far, by the 

 United States Coast Survey. For the prosecution of the survey of Colo- 

 rado, the latitude and longitude of Sherman and Cheyenne, Wyoming 

 Territory, and Denver, Colorado Springs, and Trinidad, Colorado Terri- 

 tory, have been determined by them. 



2d. From a base-line measured as accurately as possible, a system of 

 primary triangulation is expanded and extended to cover the area to be 

 surveyed with a net-work of triangles. By this operation, the positions 

 of a limited number of points are established with accuracy. Connect- 

 ing this system of triangulation with the points whose positions have 

 been established by astronomical observation, the latitudes and longi- 

 tudes of the primary points are established. The first base-line for the 

 primary triangulation of Colorado was measured principally on the 

 track of the Kansas Pacific Railroad near Denver. Its length is between 

 six and seven miles. Check-bases at Colorado Springs and in San Luis 

 Valley have also been measured and connected with the triangulation. 

 The angles are measured by a 15-inch theodolite, reading to ten 

 seconds, using artificial signals. The primary triangulation is carried 



