TO JUNCTION OP GRAND AND GREEN RIVERS. Ill 



" Since leaving the surface of the great sandstone-bed, the base of the Upper Cre- 

 taceous series, so fully exposed at the mouth of the Animas, we have passed up through 

 three 'strata of soft calcareous sandstone with an equal number of marl-beds; the 

 average thickness of the sandstone being about 50 feet, of the marl, 75." 



At Camp 46 I ascended the cliff on the south side of the canon and found it to be 

 674 feet in height. The section of the strata composing it is as follows : 



Feet. 



1. Coarse soft yellow sandstone, partially conglomerate 55 



2. Greenish- white marl, with silicified wood 50 



3. Soft yellowish sandstone, partly conglomerate, with small quartz pebbles con- 



taining obscure impressions of plants 120 



4. Greenish argillo-silicious rock, breaking into irregular masses above, shaly 



below, shading into purple marl 50 



5. Soft, white, massive, calcareous sandstone, in places traversed by veins of gyp- 



sum 20 



6. Gray or greenish-white marl, with bands of hard bluish-gray limestone, con- 



taining fucoids and silicified wood 80 



7. Yellowish soft sandstone 50 



8. Greenish-white and purple marl, with selenite, bands of concretionary limestone 



and large trunks of silicified wood 80 



9. Soft yellowish, coarse sandstone 60 



10. Greenish and purple marls, with concretionary limestone 109 



From the summit of this cliff I was able to overlook most of the country border- 

 ing Canon Largo, and could see that it was occupied by broad mesas extending east 

 to the Nacimiento Mountain, and north to the bases of the Sierra de la Plata, Sierra 

 San Juan, Navajo, &c., all of which were in full view. South and west higher por- 

 tions of the plateau limited my vision. 



On this plain I noticed islands of higher strata than those on which I stood. 

 These are 300 or 400 feet in thickness, and of the same general character as the walls 

 of Cailon Largo. Adding these to the strata exposed in CafiOH Largo, and those noticed 

 in the San Juan above the mouth of the Animas, and we have an aggregate thickness 

 of about 1,500 feet for this upper series alone. 



In my notes made at Camp 47, 'I find the following memoranda: "In the expos- 

 ures I have now examined of the Upper Cretaceous (?) strata, I have enumerated eleven 

 beds of sandstone, with an equal number of marl-beds. The aggregate force of these 

 two groiips is nearly the same, though each stratum is liable to great local variation ; 

 the section at one point being considerably different from that at others. The sand- 

 stones are quite variable in thickness, both as regards the same stratum at different 

 places, and comparing the different beds with each other. The summit of the cliffs 

 along Canon Largo is formed of a group of sandstones of unusual thickness, in places 

 full 200 feet, with only thin partings of marl. This stratum has protected the beds 

 below and produced the abruptness of the walls. It forms the floor of the adjacent 

 Sage-plain, from which the upper layers, being softer, have been removed by erosion." 



At Camp 48 I write : " To-day left Canon Largo and came out on the plateau 



