LETTEE TO THE SECEETART. 6 



Eivers, and north by the Bear Eiver. Westward the work extends 

 nearly to* longitude 108°. 



The operations of this division during the field-season of 1874 were 

 directed, first, to the survey of a narrow east and west strip along tbe 

 southern edgeof the North Park, thus extending the work of the previous 

 season in the Middle Park northward to the parallel of 40° 30' north lati- 

 tude,- and, secondly, the extension of the same work westward over the 

 Park range and along the region of the Bear, White, and Grand Eivers, 

 this being the principal field of work. Here, the northern boundary of the 

 survey, 40° 30', is practically the Bear Eiver ; while the southern boun- 

 dary was formed by the Eagle Eiver to its junction with the Grand, and 

 below thispoint by thelattexriver itself; ontheeastthe Parkrange, about 

 in longitude 106° 30', limited the area in question ; while to the west the 

 survey was carried to an irregular border, about touching, at its extreme 

 point, the meridian of 108°. The narrow strip in the North Park prob- 

 ably covers over five hundred square miles, while the principal and more 

 compact area at the west may be considered as averaging nearly seventy 

 miles across east and west and nearly sixty miles north and south, or about 

 four thousand square miles in area. Topographically, this area may be 

 divided into three well-marked divisions : first, the region draining mostly 

 northward into the Bear; secondly, that draining southward into the 

 Grand and Eagle Eivers ; and, thirdly, the basin of White Eiver and its 

 tributaries, which in itself forms a complete drainage-system, trend- 

 ing westward directly between the two i^receding regions. At its source, 

 the Bear, with tributaries of the Grand, quits the sources of the White, 

 which rise in a great isolated mesa-mass of lava, between which and the 

 Park range is the deposited basin of Egeria Park. 



The whole region was examined in the usual manner of the survey : 

 first, such observations were made as to enable a carefully-colored geo- 

 logical map to be constructed, showing the distribution and extent of 

 the rocks, and formations of various ages or kinds, which compose the 

 surface of the region; sections numerous enough to show how these 

 various formations lie upon one another, or how they probably lie beneath 

 the visible surface, or to show the various foldings or fractures to which 

 they have been subjected and yielded; and as many detail- sections as 

 possible, to determine the changes which take place in the character and 

 thickness of these formations in their lateral extension, and to deter- 

 mine, as closely as possible, their relative ages and general paleontolog- 

 ical relations. In this connection, the extent and mode of occurrence of 

 all economical products, as minerals, building-stones, plasters, springs, 

 etc., are noted as far as observed, while collections of specimens of the 

 same, as well as of all rocks, fossils, etc., are made as far as possible. 

 Second, and chiefly to enable some of this knowledge to be more accu- 

 rately represented, such operations are carried on as to enable a 

 map, or representation, of all the surface-features of the country to be 

 prepared, its rivulets, streams, plains, hills, and mountains, its canons 



