F. GEOGRAPHY. 275 



line of faulting of the west side of the plateau, and enters the 

 red sandstones which incline westward. In these it joins the 

 Dolores River. North of the caiion, between it and Grand 

 River, the Dakota group, which prevails to the southward, 

 is almost entirely absent, the red beds forming the greater 

 part of the surface, which is here a maze of dry canons. The 

 country gradually tails ofi toward Grand River ; the western 

 line of faulting becomes a fold, and the eastern fold, which is 

 also faulted in places, gradually becomes less. North of 

 Grand River beds of upper cretaceous age appear, probably 

 succeeded by tertiary as we go north. On the San Miguel 

 and Dolores Rivers, and extending westward, the rocks are 

 sandstones. There are broad folds extending across the coun- 

 try, whose axes are parallel, the general direction being north 

 and south. Between the San Miguel and Dolores, the Da- 

 kota group forms the floor. Beyond the Dolores the red 

 beds prevail, capped with isolated patches of Jurassic shales, 

 and underlaid with beds of carboniferous age. The latter 

 show but in few places. The drainage here has two general 

 courses at right angles to each other. The main streams 

 flow in a general northerly direction. 



In the Sierra La Sal the prevailing rock is a beautiful por- 

 phyritic trachyte, which in some places has included masses 

 of cretaceous shales. One of the most prominent peaks has 

 a capping of sandstone, which was lifted up by the eruption 

 of the mass, the base of the peak being entirely of trachyte. 

 There are evidences of glacial action here. Northwest and 

 west of the group the red beds have the Roches Moutonnes 

 form, beautifully seen from the summits of the mountains. 



The Abajo Mountains are of porphyritic trachyte similar 

 to the Sierra La Sal, as are the mountains about Lone Cone, 

 which properly belong to the district assigned to the San 

 Juan Division. 



The work of the Fourth Division, directed by G. R.Bcc-h- 

 ler, extended over a large area, situated between meridians 

 104 30' and 106 30', and parallels 3S 40' and 40 30': or 

 from the foot-hills of the Rocky Mountains to the Upper 

 Arkansas and Eagle Rivers, and from a point six miles south 

 of Pike's Peak to within fifteen miles of Long's Peak. 



In this district the entire Middle and South Parks are lo- 

 cated, and three of the large rivers of the west the Arkan- 



