LETTER OF THE GEOLOGflST. XXV 



It will be borne in mind that the ultimate design to be accomplished by these sur- 

 veys is the preparation of suitable maps of the country surveyed for the use of the 

 government and of the nation, which will afford full information concerning the aori- 

 cultural and mineral resources, and other important characteristics of the unexplored 

 regions of our Territorial domain. To this end, a general plan for mapping the area 

 of your survey should be followed. Such a plan was adopted by this department last 

 year, and accompanied your instructions for that year. You will continue to conform 

 to said plan, and will make such scientific observations, touching the geology, geo- 

 graphy, mineralogy, and meteorology of the country surveyed by you, as may be neces- 

 sary for the preparation of such maps. In addition thereto, you will obtain the neces- 

 sary information for the preparation of charts, upon which shall be indicated the areas 

 of grass, timber, and mineral lands, and such other portions of the country surveyed 

 as may be susceptible of cultivation by means of irrigation; and will ascertain and 

 report upon the best methods for accomplishing this result. 



The economic map in the "Atlas of Colorado" presents an example 

 of the minuteness of detail with which the economic features of a coun- 

 try may be laid down on a chart. This map, covering an area of over 

 100,000 square miles, shows with remarkable clearness, by means of 

 colors, the agricultural and pastoral lands, the pine and other forests, 

 the barren lands, and those above timber-line — all the valuable mineral 

 deposits, as coal, silver, and gold. An excellent article on the economic 

 resources of Colorado, by Mr. Gannett, is embodied in this report. In 

 all the annual reports of the survey since 1867 more or less attention 

 has been given to this subject. 



Before proceeding to a description of the triangulation in Colorado 

 and its relations to the land surveys, it may be well to give a general 

 description of the method of locating points by triangulation. 



In this method the only direct measurements made are those of one 

 or more base lines ; all other measurements are derived from those by 

 the measurements of angles. 



For the measurement of a base line, a flat extent of country, of a suit- 

 able length, is selected. The ground should be as nearly horizontal as 

 possible, and the two ends, and, if possible, all points of the proposed 

 base, should be intervisible. The length of the line should be measured 

 with all the accuracy possible under the circumstances, as any error in 

 this measurement is increased manifold in the subsequent triangula,- 

 tion. 



In the Coast Survey the lines are measured by metal bars, compensated 

 for temperature, and the contact between the ends of the bars is made 

 by a microscope. In this case the operation of measuring and remeas- 

 uring a base five miles long occupies several weeks, and the error does, 

 not exceed a small fraction of an inch. 



In the work of this survey, which does not admit of the devotion of so 

 much time and expense, these measurements have been made with a 

 100-foot steel tape. The measurements are, of course, corrected for 

 temperature, slope, and error of tape, and are reduced to sea-level. The 

 base measured, a distance of perhaps 5 or 6 miles, the next step is to 

 " expand " it, that is, to obtain from this known distance of 5 or 6 miles 



