TO JUNCTION OF GRAND AND GREEN RIVERS. 81 



"The drift from the moinitiiins drained by tin; Animus shows that their structure is 

 essentially that of the Santa Fe Mountains and the other principal ranges of the Uocky 

 .Mountain system. In this transported material are blocks, often weighing several tons, 

 of red granite, similar to that of Santa Fe; gray granite, not unlike that forming the 

 base of the section in the Colorado Canon at the mouth of Diamond River, described 

 in my former report; Carboniferous limestone, with many of the fossils found at Santa 

 Fe and in the Coal-Measures of the Mississippi Valley; black and white porphyry, 

 trachyte, trap, and metamorphosed red sandstone. No traces of Silurian or Devonian 

 rocks were discovered. The hills just above our camp are composed of red sandstone 

 and conglomerate (Triassic), very much disturbed." 



ISelow the crossing- of the Spanish trail the valley of the Animas is susceptible of 

 cultivation to the junction of the Florido, though the belt of arable land is narrow, and, 

 in part at least, can only be cultivated by irrigation. 



In regard to the origin of the broad valley or basin which borders this part of the 

 course of the Animas, I think \ve may safely say that its features were give:: by the 

 disturbances which this region has sulVered; that, in the breaking up of the table-lands, 

 a basin-like depression was left, into which the Animas flowed, and which it partially 

 filled with gravel and bowlders brought down from the mountains above. Subse- 

 quently, the enclosing walls of this area, were cut down, along the natural line of drain- 

 age, until the river reached its present level, and what was its bed at different epochs, 

 now forms gravel terraces high above it. 



The geology of that portion of our route lying between the Animas and the l\io 

 de la Plata is precisely similar to that of most of the country previously passed through; 

 the only rocks exposed are those of the upper portion of the Cretaceous formation, which 

 compose broken hills flanking the eastern and southern slopes of the Sierra de la Plata. 

 Among these hills the trail winds, following the courses of the picturesque and fertile, 

 though narrow, valleys which separate them. From the headwaters of a small tribu- 

 tary of the Animas we crossed over a divide which rises to the height of about l,ii()i) 

 feet above our Camp 17; thence descending nearly as much, we struck the La Plata, 

 just where it issues from the mountain-gorges in which it takes its rise. Of our camp 

 on the La Plata I find the following description in my notes: "The l\io de la Plata is a 

 beautifully clear, cold, mountain-brook; like the Animas and other streams we have 

 recently crossed, well-stocked with trout. The valley in which it flows, as it issues 

 from the mountains, is exceedingly beautiful, and our camp one, of the most delightful 

 imaginable. Our tents are pitched in the shade of a cluster of gigantic pines, such as 

 are scattered, here and there, singly or in groups, over the surface of the valley, sepa- 

 rated by meadows thickly coated with the finest gramma grass. Stretching off south- 

 ward, a wall of verdure, tinted with the fresh and vivid green of cottonwoods and 

 willows, marks, while it conceals, the course of the sparkling stream whose murmuring 

 How conies softly to the ear. On either side of the valley rise picturesque wooded 

 hills, which bound the view both east and west; between these on the south an open 

 vista reveals, far in the distance, the blue chains of the Sierra del Carriso and Tune- 

 cha. On the north the bold and lofty summits of the Sierra de la Plata look down 

 upon us in this pure atmosphere with an apparent proximity almost startling. Patches 

 11 s F 



