LETTER. 7 



Geological work and topographical observations were carried on in 

 harmony with each other, so that the geologist was able to prepare a 

 map showing, with a considerable degree of accuracy, the horizontal 

 distribution of formations throughout the district assigned. 



The topographical stations were almost invariably accepted by the 

 geologist as suitable for his examination, and a co-operation was thus 

 secured that must render better results than can be obtained by any 

 other method which may claim to cover an equally large area. 



During the winter of 1873-'74, the report upon the sections examined 

 was worked up from the notes taken in the field. A large map, showing 

 the distribution of formations, was prepared, and a report submitted. This 

 report is divided into four chapters and an appendix; the first chapter 

 treating of the mining-regions explored; the three next of the geology 

 of the San Luis district, which was divided into three sections in order 

 to facilitate descriptions and the comprehension of localities. In the 

 appendix are contained " Mineralogical notes," describing two new spe- 

 cies of mineral, and the occurrence of native tellurium — being the 

 second locality in the world where it occurs — and a " Catalogue of 

 minerals found in Colorado Teiritory,'' enumerating over one hundred 

 and fifty species, by far more in number and locality than had ever be- 

 fore been reported from that Territory. 



Experience in the field has shown most dearly the necessity of com- 

 bining topographical observations with geological research. Of all maps 

 that are of importance and great use to the geologist, contour-maps 

 must be preferred. He may note by his observations, and express by 

 means of vertical sections, the arrangement of strata throughout a cer- 

 tain mountain, ridge, or range, and the contour given on the map will 

 then greatly facilitate his work by enabling him to define more correctly 

 than in any other way the limits of the successive strata. 



Prom the stations selected, the geologist can indicate the points 

 important for his work ; the topographer can locate these points and 

 reproduce them on the maps which are plotted during the winter follow- 

 ing the field-work, thus giving to the former data that must be in- 

 valuable to any one who appreciates precise work, even when done on 

 so large a scale. 



Mr. W. H. Jackson performed his duties in the field with his usual 

 success. His triumphs in the mountain regions of Colorado are already 

 well known all over the country. The panoramic views of the mountain- 

 peaks have been of great value to the topographer as well as the geolo- 

 gist, and have proved of much interest to the public generally. 



Mr. W. H. Holmes also made numerous panoramic sketches from 

 the high peaks used as primary stations. The value of the present 

 report is greatly increased by the beautiful and accurate sketches and 

 sections from the results of his skill in the field and in the ofQce. His 

 knowledge of structural geology is such that he merits the position of 

 assistant geologist. 



