F. GEOGRAPHY. 



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The direction of the range is about north and south, its 

 length about fifteen miles, and the elevation of the summits 

 12,000 to 12,500 feet. 



The Grand River, from the mouth of the Gunnison to that 

 of the Dolores, is alternately in open valley and low caiion. 

 On the south the river hugs the edge of the plateau closely, 

 while on the north low, open desert country extends about 

 fifteen miles back from the river. This desert country ex- 



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tends down the Grand and across to the Green, forming the 

 Great Plateau in which these streams and the Colorado cut 

 their canons. 



South of the Sierra La Sal are fine valleys, extending near- 

 ly to the head of the Dolores. Farther west the country is 

 a plateau, without water, covered with sage and pinion pine, 

 and cut by numberless dry canons. 



The geological features of the district surveyed by the 

 Grand River Division during the season of 1875 are compar- 

 atively simple, there being no great uplifts nor many local 

 disturbances. The sedimentary formations represented are 

 all included under carboniferous, red beds (triassic?), Juras- 

 sic, and cretaceous. Exposures of metamorphic rocks are 

 seen in several parts of the district, limited mainly to the 

 bottoms of canons, the streams bavins; cut through the over- 

 lying sedimentaries. The eruptive areas are also limited. 

 In the southern part of the district there are the overlapping 

 edges of various trachytic flows, whose sources of origin were 

 in the Tlncompahgre Mountains still farther south. Besides 

 these there are three distinct centres of eruption : viz., the 

 Lone Cone group of mountains on the south, the Abajo 

 Mountains in the southwest, and the Sierra La Sal Mountains 

 toward the northwest. These are of porphyritic trachyte, 

 and have been pushed up through the cretaceous layers 

 which dip gently from them. The greater part of the dis- 

 trict, however, is covered with sedimentary rocks, generally 

 horizontal, or, if dipping, but little inclined. In these beds 

 the drainage is outlined by canons which arc from a few 

 hundred to over a thousand feet in depth. During the sum- 

 mer months these streams are dry. 



Leaving the Los Pinos Indian Agency, the first work was 

 on the south side of the Gunnison River, in a narrow strip of 

 country lying between Mr. Gannett's district of 1874 and 



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