TO JUNCTION OF GRAND AND GREEN REVERS. SI 



"The drift from the mountains drained !>v the Animas shows that their structure is 

 essentially that of the Santa rY Mountains and the other principal ranges oi the Rocky 

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

 of red granite, similar to that oi' Santa Im'; gray granite, not unlike that forming the 

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

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

 Ke 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 ot red sandstone 

 and conglomerate (Triassic), very much disturbed." 



Below the crossing of the Spanish trail the valley of the Animas is susceptible of 

 cultivation to the junction of the Florido, though the bell of arable land is aarrow, and, 

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



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

 course of the Animas, I think we may safely say that its features were given by the 

 disturbances which this region has suffered; 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. Suhse- 

 quently, the enclosing walls of this area were (ait down, along the natural line oi drain- 

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

 now forms gravel terraces liigh above it. 



The geology of that portion of our route lying between the Animas and the Rio 

 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 ( Yetaceoiis formation, which 

 compose broken hills flanking the eastern and southern slopes of the Sierra de In 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 <a-ossed over a divide which rises to the height of about 1,000 

 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. ( )f our camp 

 on the La Plata 1 find the following description in my notes: "The Rio 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 Hows, 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 (duster of gigantic pines, such as 



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

 rated by meadows thickly coated with the finest gramma _ii-rass. Stretcliing 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 

 flow comes softly t«> 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 a F 



