72 SURVEY OF COLORADO AND NEW MEXICO. 



stream, show vast carious with nearly perpendicular sides, and the pecu- 

 liar dark somber color of the rocks adds to the gloomy picturesqueuess 

 of the scenery. On each side of the valleys of the little streams as they 

 issue from the mountains are high terraces one hundred to one hundred 

 and fifty feet high, which are here more conspicuous than usual. These 

 are composed of the Santa Fe marls and are not unfrequently covered 

 with a thick bed of basalt. 



The broad intermediate space between the range of mountains which 

 form the east side of the valley of the Eio Grande and the Sierra Madre 

 — a main range of the Kocky Mountains, whicli gives origin to the waters 

 of the Pacific streams — is covered with rounded hills, detached ranges, 

 &c., all of which are basaltic. The two rouuded hills, which are very 

 marked, situated nearly opposite each other on different sides of the Eio 

 Grande, Cerro de las Utas aud Cerro San Antonio, are, it seems to me, 

 old craters or fissures, out of which issued the melted material Avhich 

 overflowed the sides, and time has weathered the whole mass into its 

 present beautiful rounded form. At this time they look like gigantic 

 mammse. 



I am inclined to regard the valley of the Eio Grande as one great 

 crater, including within its rim a vast number of smaller craters and 

 dikes, out of which has been poured at some time a continuous sheet or 

 mass of melted material. All the valleys, small and great, give evidence 

 that they have been worn out of this vast mesa. The Eio Grande, from its 

 source in the San Juan Mountains to Albuquerque, flows along its banks 

 through basaltic rocks to a greater or less extent, and as we go north- 

 ward from it they disappear in part. 



By glancing at a map it will be seen at once that the valley of the Eio 

 Grande belongs to the eastern side or Atlantic slope of the " great di^'ide," 

 and that the ranges of mountains, on the east side of the valley of the 

 Eio Grande, which run out into the plains near Santa Fe, are a series 

 of fragments wliich seem to have broken off from the main rockj- range 

 north of the South Park, and are prolonged southward in a more or less 

 broken condition for over four hundred miles. It will also be seen from 

 tlie map that the parting line or divide flexes over to the west, with a 

 great bend above Middle and South Parks. Now between the base 

 of this mountain prolongation on the east, and the Sierra Madre or 

 western divide on the west, from the head of the San Luis Valley to 

 Gallisteo Creek, at least, an area of over two hundred miles from north to 

 south, and one hundred to one hundred and fifty miles from east to west, is 

 occupied mostly by but two classes of rocks, the basaltic and the mod- 

 ern tertiary or Santa Fe marls. These recent marls are very conspicuous 

 about Santa Fe and north of that point to the Pickaris Mountains, hut 

 north of that point they are not largely developed, though at one time 

 they must have reached a great thickness, but have been removed by 

 erosion. The valley of the Eio Grande, from Santa Fe to Taos, has the 

 appearance of an immature region, much like that of the "Bad Lands," 

 or the tertiary deposits of White and ]!^iobrara rivers on the Missouri. 

 But above and north of Taos the wearing and smoothing process has been 

 applied so that there is a mature appearance of the country, like that of 

 Eastern iSTebraska or Kansas. Still all along the foot of the mountains 

 below Costilla, underneath the mesa which extends from below Costilla 

 to the Sierra Blanca Eange, fifty miles, these marls can be seen in places. 

 At Culebra, the Eio Culebra cuts through the mesa, forming a sort of 

 gorge nearly half a mile in width. On the sides of the mesa these marls 

 are most clearly seen underneath a heavy bed of basalt. Along the little 

 branches of the EioTrenchara, as the Eio de las Utas aud the Sangre de 



