TO JUNCTION OF GRAND AND GREEN RIVERS. 105 



Feet 



15. Soft greenish sandstones and shales, alternating with and replacing each other. 140 



Locally, this group is very massive, hard, almost qnartzose, like No. 4; in 

 other places, almost replaced by greenish shales. This is probably the 

 base of the Cretaceous system. Continuing the section down the valley 

 of the Rito to the San Juan, we have— 



16. lied, white, and greenish thin-bedded micaceous sandstones, alternating with 



red shales — the Saurian beds of Camp 26, No. 2 of section on page 91 1 30 



17. Red, massive, cross-stratified calcareous sandstones — No. 3 of section of Camp 



26; No. 1 of section — (p. 95,) to river-bottoms 160 



The last two members of the section evidently represent the upper portion of the 

 Gypsum formation (Trias.) They form the cliffs immediately bordering the valley 

 of the river at Camp 35, and the trough in which it flows from Camp 36 to its mouth. 



The second bench, or higher cliffs bordering the valley at Camp 35, composed of 

 the Lower Cretaceous rocks, contain conglomerates — the equivalents of the upper 

 members of the preceding section — much coarser than those observed farther north; 

 the pebbles being frequently as large as the fist, with only sufficient paste to cement 

 them. Among these pebbles I noticed several composed of silicified wood, probably 

 derived from the erosion of the Triassic rocks. 



In the trough of the San Juan, at this point, are extensive beds and terraces, com- 

 posed of coarse gravel and bowlders, evidently brought down by the river from the 

 mountain-ranges which it drains, and, in some instances, the bowlders must have been 

 rolled by the stream fully two hundred miles from their place of origin. 



The structure of the country about Camp 37 is quite fully given in my journal. 

 My notes are as follows: "That portion of the valley of the San Juan passed through 

 to-day is, in its general aspects, similar to that before described, but the cliffs border- 

 ing it are less continuous and abrupt. In many places they are 300 or 400 feet high, 

 perfect canon walls, but oftener they recede from the river, leaving open areas; the 

 mouths of lateral valleys cutting up through the mesas toward the mountains. 



"Geologically as physically, we have ascended considerablv, and have just sunk 

 the 'Saurian rocks' (Upper Triassic?), leaving the cliffs bordering the valley com- 

 posed exclusively of Lower Cretaceous strata. During the greater part of to-day's 

 .march thin-bedded Triassic sandstones and shales formed the base of the wall along 

 which we passed. These rocks are here more argillaceous than farther north, contain 

 much saline matter, but no beds of gypsum, and no fossils so far as observed.'" By 

 reference to my report on the geology of the Colorado expedition, it will be seen that 

 farther south they are still less sandy; at the Moqui Villages, consisting mostly of soft 

 red and green marls. 



The Lower Cretaceous strata are here more fully exposed, and have a greater 

 thickness than where examined farther north and east ; approaching more nearly the 

 character exhibited by this group at the Moqui Villages. The cliffs near camp are 

 composed of — 



Fei-t. 



1. Yellow, coarse sandstones and conglomerates; floor of Sage-plain 200 



2. Greenish shales and sandstones - - ^ 175 



3. Greenish-yellow sandstones and greenish shales -50 



14 S F 



