174 GEOLOGICAL SURVEY OF THE TEEEITORIES. 



smoothed off and are covered with a tliick deposit of debris. In the 

 little valleys of the mountains the gneissic rocks are exposed, and about 

 twenty lodes have been examined to some extent; the crevice matter on 

 the surface is entirely rotten quartz. Tiiese lodes have in most cases 

 well-defined walls, varying from three to six feet in width, and a strike 

 about northeast and southwest. Should these mines turn out to be rich 

 in gold, the ease witli which they can be worked will render them very 

 celebrated. 



From a i)oiut not more than twenty miles north of Santa Fe, to the 

 Sangre de Christo Pass, I was unable to discover any of the older sedi- 

 mentary beds on the western side of the mountaius. Sometimes among 

 the drift boulders, v/hich were very extensive everywhere, a few masses 

 of liinestone would be found which were evidently carboniferous. In 

 Taos Valley slightly worn masses of limestone were found, with well- 

 defined carboniferous fossils. This would seem to indicate that these 

 rocks once existed all along the mountains, even if they cannot be found 

 at this time. I have no doubt that all the sedimentary formations which 

 are found on the e;iStern margins of the mountains once extended unin- 

 terruptedly across the Eio Grande Valley, and some portions may now 

 exist deep beneath the basalt and Santa Fe marls. 



Near the Rio Colorado, the lower ridges or foot-hills of the mountains 

 exhibit the influence of the igneous rocks to a greater extent than south- 

 ward, and continue to do so to the Sierra Bianca. Near the point from 

 which the Eio Colorado emerges from the mountains, the rocks are a 

 bright brick-red over a small area, and I mistook them for remnants of 

 the triassic. A closer examination showed me that high up on the sides 

 of the mountains a great thickness of the recent marls, sands, and clays, 

 have been so changed by contact with the igneous rocks, that they now 

 present that x^eculiar brick-red and variegated ai)pearance which is 

 noticed for several miles. 



At Costilla the main range seems to bend abruptly to the eastward, 

 and a portion of the lower ridges on the western sides of the mountains 

 south of Costilla passes off without interruption in a long basaltic mesa, 

 which extends nearly to Fort Garland. East of this mesa are the " ve- 

 gas" or meadows, which have been carved out of the mesa between it 

 and the foot of the mountains and form a portion of the valleys of the 

 Costilla and Culebra Rivers. North of Culebra the basaltic mesas com- 

 mence again close to the base of the mountains, and continue quite 

 largely cleveloped up to the Sierra Bianca Range. These mesas are 

 capped with a heavy bed of basalt, which always seems to incline eastward 

 toward the mountains at least from three to five tlegrees, and some- 

 times much more. 



On the east side, close to the Rio Grande, near the entrances of the 

 Trenchera and Culebra Rivers, are a great number of ridges and coni- 

 cal peaks or hills, called " Cerillos," all of them basaltic. On the oppo- 

 site side of the Rio Grande these basaltic hills are very abundant, and 

 occupy most of the country. Just north of the Trenchera this range of 

 mountains seems to bend abruptly back to the westward in the form of 

 the Sierra Bianca Mountains, Avhicli have a trend nearly east and west. 

 There is therefore a quadrangular space inclosed on three sides by 

 mountains — the Costilla on the south side, about fifteen or twenty miles ; 

 the principal range on the east, about sixty miles, and the Sierra Bianca 

 on the north, about fifteen or twenty miles. The main range continues 

 northward, bending slightly westward, until it joins the Sierra Madre at 

 the Poncho Pass. The Sierra Bianca is the grandest and most pictur- 

 esque range in Southern Colorado. It is apparently basaltic and is, as I 

 suppose, a gigantic dike. I regard the Spanish Peaks as an enormous 



