tfobinson — Tertiary Peneplain of the Plateau District. 119 



of the pronounced dip of the strata at the localities described, 

 the beveling of their edges was very apparent. It is to be 

 noted, however, that the strata under the Mount Taylor pla- 

 teau lava have a gentle dip north and west, thus making the 

 peneplain coextensive with the plateau. But the peneplain 

 now stands through erosion on an average of 700 feet above 

 the surrounding country, which indicates that it must origi- 

 nally have had a much greater extension than at present. 



Evidence of the existence of the peneplain in the region 

 south of Mount Taylor is found at Tres Hermanos buttes and 

 Mesa Lucera, in addition to the Acoma mesa. It is derived, 

 as elsewhere, from the relation that exists between the eroded 

 strata of folds and faults and the overlying basalt. 



Gilbert has described* this relation at Tres Hermanos 

 buttes, located 50 miles south of Mount Taylor, as follows : 



"From 'the buttes a fold runs 10 miles northwest to the 

 Acoma plateau, beneath which it disappears. . . The throw is 

 between 1,500 and 2,000 feet, and is to the southwest. The 



Fig. 4. Monocline at Tres Hermanos Buttes. 

 C Cretaceous, T Triassic, B Basalt 



fold is older than the basalt of the vicinity. The eroded 

 edges of the strata upturned by it support the lava caps of the 

 Hermanos buttes and of the Acoma plateau. The level line 

 of the Acoma lava shows that the folding has not continued 

 since the eruption, and the antiquity of the eruption is meas- 

 ured by a general denudation of the country of more than 500 

 feet." 



Gilbert's cross-section of the monocline, in which propor- 

 tions were estimated, is given in figure 4. 



It shows that the strata involved in the fold had suffered 

 great erosion before the lava covered them. The situation 

 appears to duplicate that at Black Point (figure 1), except that 

 the lava cap has been largely removed, and as the result the 

 underlying strata have been considerably dissected. As the 

 capping lava may be traced to the Acoma mesa and hence to 

 the Mount Taylor plateau, it is believed that Dutton's descrip- 

 tion of the peneplain at the latter locality may be applied to 

 this locality. 



At Mesa Lucera, 35 miles southeasterly of Mount Taylor, 

 the relation of the basalt to the displacements is also described 

 by Gilbert :* 



*Op. cit., p. 557. 



