736 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



marked (9) to be about the same height as (6), and the point on the stream 

 marked (5) to be nearly the same as (a) ; consequently we have fifteen 

 contours to space in between these points. But we find from the height 

 of (c) that five of these come between points (c and 5), therefore coming 

 much nearer to each other as the slope is greater than between (c and 

 9), where they are nearly equidistant from each other as the slope is 

 comparatively even and gentle. 



Now we turn to the sketch from Culebra Peak, which will give us the 

 profile of the ridge from (9) to {d), and in the same manner space in the 

 contours. We can now connect these lines, . carefully studying the 

 sketch, to see how deep the ravines are cut, and the general forms of 

 the ridge which lead from (9) to the valleys below. In this manner all 

 of the mountains are carefully drawn. Having sketches such as a^e 

 given from stations 115 and 125 from every point that has been occupied, 

 with the heights of all located points, besides many more that do not 

 appear as located points, such as valleys, passes, benches, &c. Plates 

 36, 37, and 38 will give some idea of the field-sketches, while Plate 39 

 will show the map as finished in the office, only at a much reduced 

 scale. This plate is taken from a proof of the engraved sheet to illus- 

 trate the result of this method of work. 



To give some idea of the amount of work that has been done by the 

 topographical corps in the survey of Colorado, I will state that we have 

 established 1,280 topographical stations within an area of about 70,000 

 square miles, and from each station all the surrounding country was 

 sketched as previously described. My assistant made over one thou- 

 sand pages of profile sketches during the field-season of 1875, each page 

 being 6 by 10 inches, while I myself made some five hundred pages of 

 drainage-sketches, and took the thousands of angles that were necessary 

 to locate all the points. 



In referring to the sketches given, the system of numbering the 

 points may not be clearly understood. Turn to Plate 37, for instance, 

 you will see there such numbers as (16-115), (14-115), (29-116), &c., 

 which mean simply No. 16 from station 115, No, 14 from station 115, 

 and No. 29 from station 116. These numbers are obtained in the fol- 

 lowing manner : — Beginning at station 1, we sight, say, one hundred 

 points ; it will be seen at once that there must be some short and effi- 

 cient method adopted by which these points can be designated when 

 sighting them from thestations that are to follow. To giveeach a descrip- 

 tive name becomes impossible, owing to the number and the time it would 

 take, which is a very important consideration to the topographer, for 

 his time is so much taken up by the ascent and descent of the mount- 

 ain that he is compelled to economize time in every possible manner. 

 Number each point, commencing with one, place these numbers on the 

 sketches over or by the point to be sighted, and when the angles are 

 taken and recorded, place the number of the point after its angle. 

 From the next station as you sight the points, place the same number 



