106 EXPLORING EXPEDITION FROM SANTA FE 



Tlie character of the subdivisions will be better seen by the following analysis: 



1st. Yellow sandstone group; here less coarse and containing less conglomerate 

 than at camps 33, 34, and 35. As at those localities, on the Cimarron, in Kansas and 

 Nebraska a portion of this group is here an exceedingly hard, whitish, quartzose sand- 

 stone. Beds of lignite and silicified wood are common. 



2d. Greenish shales and sandstones ; the same group described in my notes on 

 the geology of the vicinity of Camp 34, Nos. 10, 11, 12 of section, page 104, 

 here much less highly colored, consisting of thick beds of greenish shale, locally pink 

 or purple, with bands of yellowish, greenish or purple concretionary silicious rock; 

 no fossils. 



3d. Greenish sandstones and shales; the same group exposed in the lower part of 

 the section at Camp 34. The shales and sandstones frequently replace each other; the 

 group at Camp 37 being nearly all massive, yellowish-green sandstone, while, a few 

 miles below, the shales greatly predominate; no fossils discoverable. 



"Mouth of the Mancos. Camp 38, thirteen miles. We have to-day ascended in the 

 geological scale, have nearly submerged the lowest member of the section at Camp 

 37, and the mesas back from the river on the north side run far up into the Middle 

 and Upper Cretaceous group. On the south side the mesas are not as high; the Lower 

 Cretaceous rocks forming the surface almost to the Sierra Carrizo, near which the 

 Upper Cretaceous beds appear again. The mesas north of our camp, composed of the 

 Middle Cretaceous strata, 'extend north and east to the base of the Sierra Le Late, and, 

 if not cut off by the valley of the Mancos, would connect with the Mesa Verde. As 

 usual in all this region, the shales contain great numbers of Gfyphaea Pitcheri. 



"In the Lower Cretaceous sandstones, I here find impressions of dicotyledonous 

 haves, similar to those found on the Dolores and elsewhere. 



"As in other parts of the valley of the San Juan which we have passed through, the 

 rains of ancient buildings are here very frequently met with; some in the open ground 

 of the valley, some in or on the cliffs ; all built of stone, surrounded by broken pot- 

 tery, evidently the remains of that great Pueblo race which once occupied all this 

 region, but which is now without an inhabitant. 



"Camp 38, 39, 15 miles. In our march of to-day, a marked change has taken place 

 in the valley of the San Juan, which has become more open, is bounded by less high 

 a id abrupt walls ; the bluffs, like the bottom, being covered with fine grass. This 

 change is dependent, mainly, upon the fact that we have ascended from the Lower 

 to the Middle Cretaceous strata, and have passed from a region of coarse massive 

 sandstones to one where almost nothing but soft-bhie shales are visible. 



"Before leaving the Lower Cretaceous sandstones, some interesting local peculiari- 

 ties were noticed in that group. The lignite, which in greater or less quantity they 

 always contain, about and above the mouth of the Mancos becomes a very conspicu- 

 ous feature in their structure. Of one cliff which we passed, not more than 100 feet 

 in height, full one-half was composed of carbonaceous matter. The dip, as before, is 

 gently eastward, and at Camp 39 the Lower Cretaceous sandstones had entire!}' disap- 

 peared beneath the overlying rocks. 



"Here the cliffs near the river are composed of the dark shales of the Middle Cre- 



