194 GEOLOGICAL SURVEY OF THE TERRITORIES. 



the ore-boaring- regions of the San Jnan mining district, and from there 

 over to Mount Sneffels (station 33). While the orographical character is 

 sufficiently precipitous, there is not that regularity which can be observed 

 in the country of long-continued, more uniform flows — in the blufl" coun- 

 try. Although here, too, single strata may be traced without difficulty 

 for miles, as well as in the former, the mountains or groups containing 

 them are mostly separated by deep ravines, and the continuity ol' the 

 stratum is not so apparent at a glance. The flows that form the high- 

 est peaks have been of much greater thickness, occurring at a time, 

 probably, when the bluff country was too high to be reached, but sub- 

 sequent disturbances, upheavals, and depressions have endowed the 

 region with a wild, grand character. No regularity in the arrangement 

 of the higher portions can be observed, no chains or regular systems of 

 mountains. Taking the entire high volcanic country into considera- 

 tion, it cannot be termed otherwise than a group. TJpoit the strati- 

 graphical relations of this group — for the flows are so regular in their 

 succession that we can treat them as strata — more will be said below. 



In the drainage, too, of the district, tlie difference between at least 

 the bluff and the mountain country can be observed. While, in the 

 former, the streams run a more regular course, one more nearly ap- 

 proaching the straight line, the creeks and streams of the latter make 

 numerous turns and curves, probably being forced to do so by the pri- 

 mary distribution of the mountains, and not the reverse, that the 

 mounttiins owe their first form and present condition altogether to ero- 

 sion. ISTumerous places may be found in this volcanic section where 

 large masses of reck have fallen down, at times, for severed thousand 

 feet, and are now lying immediately below the perpendicular cliff that 

 their falling produced. 



In speaking of this volcanic area, it will probably be best to divide 

 it according to its drainage, and after the discussion upon that plan is 

 finished, give the most interesting and instructive points in detail. 

 A consideration based upon the several strata that are defined below, 

 might prove satisfactory were it not for the circumstance that so many 

 streams, mountains, and other localities of the region in question, have 

 thus far not been supplied with names. Accordingly, therefore, the 

 main streams and their tributaries will be utilized as a means facilita- 

 ting classification. 



The liio Grande, from its head-waters eastward to Del Norte, runs 

 entirely in volcanic material, as well as all its tributaries irom the 

 norih, while some of its southwestern ones head in the quartzite regions. 

 White Earth Creek and all its tributaries are within the volcanic area 

 so far as surveyed during 1874, Lake Fork and Uncompahgre Creeks 

 are almost entirely within the limits of this area. Of the Eio Animas 

 only the head-w^aters enter into consideration this time. 



Traveling over so large an area of these formations, it soon became 

 apparent that a certain regularity existed among the various members 

 comprising the entire system of volcanics. Inasmuch as the flows were 

 well defined, the breccias easily recognizable, and both could be traced 

 for, sometimes, considerable distances, the idea presented itself to sepa- 

 rate the best-characterized groups, and giving them numbers (analogous 

 to the numbering of sedimentary formations), thus facilitate both 

 description and subsequent classification. The absence of well-denned 

 jjropylite and andesite, the two oldest eruptive rocks of the Trachor- 

 lieitic gioui), is somewhat astonishing, but it seems, from evidence, that 

 the eruptions of the material in our present district were later than 

 those of the 1873 district. Besides the large, continuous mass we are 



