GEOLOGIC RECONNAISSANCE OF TAOS RANGE 733 



where they are finally encroached upon by the basalt flows of the 

 Rio Grande Valley. 



In Stevenson's report' the mountains north and west of Pueblo 

 Creek are assigned to the "Taos axis," those south and east of that 

 creek to the " Mora axis." It was beheved that the Taos axis does 

 not continue beyond Pueblo Creek, but that a new one, the Mora 

 axis, begins at the head of the Red River and runs southward, 

 parallel to the Taos axis on the west, until the latter vanishes. 

 No such structural division could be noticed by the writer between 

 the Taos Range and Mora uplift south of it. The misconception 

 was probably due to the belief that the Pennsylvanian strata exist 

 also on the west side of the range.^ The sedimentary outliers 

 on the main range, to be described later, when viewed from the 

 distance, easily give such an impression. No new axis begins in 

 this district, but the Taos axis pitches steeply toward the south 

 and the pre-Cambrian rocks disappear at the junction of Pueblo 

 Creek and Indian Creek beneath the Pennsylvanian strata, which 

 form here an uninterrupted anticline across the range. 



In the region mapped this anticlinal structure is absent. No 

 Pennsylvanian sediments were found on the western slope north 

 of Pueblo Creek. The mountains present a bold fault scarp facing 

 west. Whether sedimentary rocks of Pennsylvanian age underhe 

 the thick debris fans and basalt flows or not is unknown at the 

 present time. But farther north, in Colorado, Siebenthal mentions 

 their occurrence on the west side of Culebra Peak and the anticlinal 

 structure of the Sangre de Cristo Range at that latitude.^ 



PRE-CAMBRIAN CRYSTALLINE ROCKS 



Ancient gneisses and schists. — The most ancient rocks are 

 amphibolite and chlorite schists and gneisses that grade into green- 

 stone in places. They cover the larger portion of the northwestern 

 half of the area mapped, form an almost continuous outcrop along 

 the western scarp of the range, and cap all of the high peaks with 

 the exception of Old Mike. Lack of space wiU not permit to 



* Op. cit., pp. 41-42. ^ Op. cit., p. 42. 



3 C. E. Siebenthal, "Geology and Water Resources of the San Luis Valley, Colo.," 

 U.S. Geol. Survey Water Supply Paper 240 (1907), p. 34. 



