INTRODUCTION. 



The district surveyed, topographically and geologically, by the San 

 Juan division, during the field-season of 1874, lies between the 107th 

 and 108th degrees of longitude west, and between 37° 15' and 38^ 15' 

 north latitude. Besides this, the party endeavored, by traveling down 

 the Rio Grande to Del Norte, and from there eastward, to connect witli 

 some of the work of 1873, on the southern line of the district then sur- 

 veyed. Two large rivers, the Eio Grande and the Eio Animas, head in 

 the region explored, as well as a number of important smaller streams. 

 Flowing northward, there are, beginning in the east. White Earth Creek, 

 Lake Fork, and Uncompahgre Creek. Eio San Miguel and Eio Dolores 

 flow in a westerly direction ; Eio Animas, and its tributaries, Cascade, 

 Arimosa, Junction, Florida, Vallecito, Pinos, and Piedra, flow south. 

 Numerous small creeks help to complete a very perfect system of drain- 

 age throughout that section of country, admirable, not only so far as 

 horizontal distribution is concerned, but also regarding the amount of 

 water they carry. 



As a rule, the character of the country is very mountainous, with 

 numerous high and rugged peaks studding the mountain groups. It 

 would not be correct to speak of ranges or mountain-chains in that 

 country ; the only appellation that can properly be given is that of a 

 group of large extent. A marked change in the distribution of the 

 higher elevations can be observed as soon as the sedimentary forma- 

 tions are reached, and it is there, that well-defined ridges, of limited 

 extent, however, occur. Toward the north and west the mountains 

 fall off steeply into the plateau country, while to the south the above- 

 mentioned ridges serve as a transitory medium between the high and 

 the low portions of the district. Near the southern limits of the re- 

 gion examined, the well-known " hog-backs " set in, identical in form 

 with those along the Front range. 



Numerous points of great beauty in detail can be noticed throughout 

 the mountain regions just mentioned. Colors in great variety, exhibit- 

 ing maiiy shades, are to be observed in a number of localities, and 

 greatly add to the efl'ect inoduced by the sometimes almost ideal shape 

 of peaks or ridges. The tendency of volcanic rocks, which compose the 

 greater portion of our district, to weather in columns, gives rise to the 

 formation of thousands of little pinnacles, not unlike Gothic architecture 

 in appearance. Eugged and steep is the character of the mountain-sides, 

 while numerous subsidences produced amphitheaters, with perpendic- 

 ular walls, sometimes of considerable extent. Owing to the horizontal 

 stratification of the volcanic flows, and to unquestionable ruptures of 

 the strata, the influence of atmospheric agencies has sharply carved 

 the outlines of i^eaks and small ridges in well-defined forms. 



Geologically speaking, the variety offered is not so great. As stated 

 above, volcanic rocks cover the larger portion of the area surveyed, 

 while on the west and south sides of it the sedimentary beds set in. A 

 group of metamorphic rocks occupies a prominent position, and is well 

 marked by its sharp peaks and deep caiions. 



Altogether, the region is one of very considerable interest to the 



165 



