304 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



Turning to the stetch, it will be seen that every junction and impor- 

 tant bend is sighted, and that all the prominent peaks are indicated in 

 their relative positions, as nearly as could be judged. Comparing this 

 •with the map as plotted (see Plate XXI), we find that it is somewhat 

 distorted, as might be expected when we consider that the sketch was 

 made entirely by eye. But at the same time it will be seen, after all 

 the points are actually located, that the remainder of the drainage can 

 be sketched in very correctly. 



Now we have the frame- work of the map upon the paper ; it is inked 

 in and we are ready to commence the drawing of the hill-structure, 

 ■which we indicate by contour lines, each representing 200 feet of 

 vertical distance. By these lines we give the approximate heights as 

 well as forms of the mountains as nearly as we know them, from the 

 numerous height and profile sketches which we have taken from every 

 occupied station. I will state here that I do not attempt in this paper 

 to give all the minor details of tbis work, proposing only to give a gen- 

 eral idea of the methods used in constructing the maps. First, all the 

 heights are calculated and tabulated in some convenient form for refer- 

 ence. 



Having our points and drainage located, we start, for instance, with 

 the heights of the points {a) and (h) (see Plate XX) ; the difference be- 

 tween these two points we find to be about 3,000 feetj the contours 

 being 200 feet apart, we have fifteen to be distributed between these two 

 points ; thej' are accordingly spaced in from the sketch nearer together 

 or farther apart as the slope is greater or less. Next we find the point 

 marked (9) to be about the same height as (&), 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 as we have sketches such as are 

 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 

 XVIII, XIX, and XX will give some idea of the field-sketches, while 

 Plate XXI will show the map as finished in the ofl&ce, only at a much 

 reduced scale. This plate is taken from a proof of the engraved sheet 

 to illustrate 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 XIX, for instance, 



