6 GEOLOGICAL SURVEY OF THE TERRITORIES. 



observed. Thirty-two more points were located from the primary sta- 

 tions by cuts. 



Forty-seven closed triangles were used in the adjustment of the sys- 

 tem. Their mean error of closure, after reduction for spherical excess, 

 is 10'^3. 



Two of the principal stations were accurately located in latitude and 

 longitude by the United States Coast Survey. 



Azimuths were observed at five of the stations. 



The system of triangles rests upon a base about six miles long near 

 Denver. This was twice measured with a steel tape under twenty 

 pounds strain and the temperature taken every five minutes. A check- 

 base will be measured this year. A secondary triangulation, resting 

 . upon this primary, was carried by the topographers over the same area. 

 The primary triangles range from thirty to sixty miles in the length 

 of their sides, while the secondary average eight miles. 



Mr. A. D. Wilson, a topographer of large experience, directed the San 

 Luis division. His method of work was, on arriving at his field of labor, 

 to select a peak from which, in his judgment, he could see the surround- 

 ing country to the best advantage, and from this also to choose other 

 points in advance. These points or stations were carefully located by a 

 system of secondary triangles, connecting directly with the primary- 

 triangulation points, several of which were visited. From each station 

 were taken angles, both vertical and horizontal, to all peaks, passes, 

 ends of spurs, streams, junctions of roads, and all recognizable features 

 of the country. The drainage-sketches and instrumental observations 

 were made by Mr. Wilson, while the assistant topographer, Mr. Chit- 

 tenden, was employed in making profile-sketches and observing the 

 barometer. Barometric observations were made at all stations, camps, 

 passes, valleys, and places of note. The district thus surveyed em- 

 braced an area between longitude 104° 30' and 107° and latitude 37° 50' 

 and 38° 45', of about eight thousand four hundred square miles. Within 

 this district ninety-nine regular stations were made by Mr. Wilson, be- 

 sides ten or twelve by the assistant, which give an average distance 

 from station to station of seven or eight miles. 



According to instructions received. Doctor Endlich, geologist for the 

 San Luis division, visited, on May 17, 1873, the mining-regions of Gilpin, 

 Bowlder, and Clear Creek Counties in Colorado, remaining there until 

 July 1. During that time all the important mines in operation were 

 personally inspected, with a view of determining their geological and 

 niineralogical relations among themselves as well as with reference to 

 the geognostic formations surrounding them. On July 3, the San Luis 

 division, to which he was attached as geologist, took the field, and 

 remained in active field-service until October 5. Eight thousand four 

 hundred square miles were surveyed topographically and geologically, 

 and particular attention paid to the agricultural and mineralogical 

 resources of the country traversed. 



