200 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



highly probable, therefore, that at some period previous to that at which 

 the river had the course just described, it flowed still more northerly, 

 i. e,, at the time before the subsidence occurred. Counting upon the 

 regularity shown at numerous' other points by the members of volcanic 

 series, the view may gain ground, supported besides by other facts, that 

 tbe subsidence mentioned is owing to a washing out of the soft lower 

 strata. As soon as a sufficient distance was thus eroded, the rocks 

 superincumbent changed, by their falling down and southward, the 

 course of the river. An isolation of the narrow valley lying between 

 the two vertical walls thus formed, would be advantageous to the for- 

 mation of either swamps or lakes, and we do in reality find one of the lat- 

 ter. Its longer axis is parallel to the line of separation of the two ver- 

 tical bluffs. 



Still farther ascending the river we find it running in trachyte No. 2, 

 which forms steep bluffs on either side, and narrows tbe canon very con- 

 siderably at some places. While the elevation of the river-bed is about 

 9,400 feet along there, the hills on the north sid'e rise to nearly 10,500 

 feet, reaching trachyte No. 3. Along some of the ridges this rock 

 shows very decidedly columnar structure, resembling from a distance 

 basaltic columns. In color it is dark, and weathers in steep cUffs. 

 After reaching Lost Trail Creek the higher country begins. Both 

 south and north of the Rio Grande mountain-peaks become more fre- 

 quent and their altitudes more considerable than farther east. On the 

 south side of the river, ascending the main tributaries, we observe the 

 lower numbers two and three in regular succession and typical develop- 

 ment. The Eio Grande Pyramid (station 21) is the highest trachytic 

 point in that direction, 13,773 feet. Beyond that and to the westward, 

 the volcanic rocks begin to thin out owing to the position of underly- 

 ing metamorphics. At the time of the flows, the metamorphic regions 

 must have been sufficiently elevated not to be covered entirely by them. 

 Although, as shown by small local outcrops, quite an extensive area of 

 these rocks is covered by volcauics, the high portions, the Quartzite 

 Mountains, impeded the progress of the eruptive material. Along a 

 line that begins near station 22, and running northwesterly terminates 

 near station 17, the volcanic rocks are overlying the metamori^hic rocks, 

 and while the former show nearly horizontal stratification, the latter 

 have a dip of 12° to ISC' to the northward. Station 24 is a prominent 

 trachytic point projecting south toward the metamorphic area. 



On station 21 a good section of the volcanic rocks was obtained, show- 

 ing the regular order that has been observed at a number of other local- 

 ities. The summit is composed of basalt, capping the other rocks, and 

 having a thickness of GOO feet. Below it there are 200 feet of breccia. 

 This breccia is composed of numerous large and small fragments of the 

 underlying strata, and cemented rather loosely by a cement of feld- 

 spathic character. Then follow 400 feet of a compact gray rock, with- 

 out any distinctly segregated minerals. It resembles a feldspathic ma- 

 trix, without any minerals contained in it, and varies in color from 

 yellowish to gray, and sometimes pink. Between this and the next 

 lower member there occurs a band of porphyritic pitchstone, containing 

 numerous crystals of sanidite, and small decomposing fragments of 

 another feldspar. At the point of observation this interstratum was 8 

 to 10 feet thick, but as it contiuties for several miles in an easterly and 

 southerly direction, variations in thickness occur. Below this pitch- 

 s,tone stratum trachyte No. 3 sets in ; 400 feet of a light pink to reddish 

 rock compose the upper portion of that number, followed, lower dow^n, 

 by 800 feet of the dark-brown laminated trachyte, with much sanidite 



