2G4 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



1873 and 1874. The 104 30' longitude formed the eastern, 

 10S longitude the western, and 36 45' north latitude the 

 southern boundaries. About 12,400 square miles were con- 

 tained in the district. 



A plan for the most rapid and successful completion of 

 the work undertaken was prepared by Mr. Wilson, and sub- 

 sequently carried out as proposed. This district contained 

 the foot-hills sloping eastward from the Front Range, the 

 southern continuation of the Sangre de Christo Range, the 

 southern end of the San Luis Valley, the extension of the 

 La Plata Mountains, and the lower country of the Rio San 

 Juan and its tributaries. A small portion of the sediment- 

 ary eastern foot-hills was first surveyed, and the work then 

 carried westward to the mountainous vicinity of the Upper 

 Rio Grande. Instead of forming a well-defined, sharply lim- 

 ited rano-e, the mountains south of the Rio Grande are form- 

 ed by a high plateau with numerous isolated peaks. Both 

 plateau and the peaks mentioned are volcanic, showing the 

 characteristic regularity of flows prevalent there. From 

 the position of volcanic beds composing the higher peaks, it 

 may be inferred that at one time the summit of the plateau 

 extended to a considerably higher altitude than at present. 

 Toward the southwest it drops off suddenly into the lower 

 country containing Rios Piedra and Pinos. Presenting a 

 line of steep, rough mountains, formed in part by the abrupt 

 termination of the plateau, in part by the peaks above men- 

 tioned, the former contrasts strongly with the rich land in 

 the valleys of the two rivers. Here, as at so many points 

 in the districts surveyed by the southern division, the geo- 

 logical features determine the orographic character. With 

 the plateau end, the volcanic beds and the sedimentaries of 

 the cretaceous age set in. But few stratigraphical disturb- 

 ances have changed the relative position of the beds, and the 

 country, therefore, shows regular features. Long lines of high 

 ridges, abrupt on the north side, sloping more gently toward 

 the south, extend from east to west, and are cut by the drain- 

 age of the San Juan. Eastward the edge of the plateau re- 

 cedes, losing: at the same time some of its rougdmess, and a 

 broad expanse of comparatively low bluff country appears. 

 Rich valleys, partly timbered or covered with grass, follow the 

 course of the larger streams, owing; their formations to the 





