66 CHARLES R. KEYES 



the present mountain ridge; a second was coeval with its formation; 

 and the third was long subsequent to its uprising. 



Were it not for exceptionally clear evidence to the contrary, 

 casual examination could very easily lead to the conclusion that the 

 Caballos mountain range was produced by sharp folding and that 

 the crest of this asymmetric fold was removed through erosion. 

 This deduction is a quite natural one, especially when, in a view from 

 the summit of the range, there are clearly shown the strata dipping 

 eastward forming a broad syncline and coming up again with westerly 

 dips in the great San Andreas Mountain block, 30 miles beyond. 



It so happens that in the instance under consideration we now 

 know enough of the general geological history of the region to give 

 us a good insight into some of the actual conditions that have existed. 

 It has recently been shown 1 that the Upper Cretaceous of central 

 New Mexico rests in marked unconformity upon the older rocks. 

 The period during which conditions existed for folding of the strata 

 was, already stated, later than the laying down of the Carboniferous 

 limestones and later than the deposition of the "Red Beds," as these 

 were all involved. 



A number of observations lately made emphasize the character of 

 the events which took place in the region during early Cretaceous 

 times, and the great importance of the unconformity at the base of 

 the Cretaceous of the region. That during this period the older 

 rocks were greatly disturbed over wide areas is amply attested by 

 the almost vertical "Red Beds" (Carbo-Triassic) overlain by hori- 

 zontal Cretaceous, as is seen at Tejon, south of Santa Fe, in Sandoval 

 county; 2 the highly inclined Carboniferous limestones, on either side 

 of high trachyte dikes on which recline nearly horizontal Cretaceous 

 sandstones on the Chupadera Mesa, in eastern Socorro county; 3 

 and the position of the Cretaceous on the older formations in the 

 Caballos Mountains in Sierra county; as well as in other localities. 

 The unconformity represents a great land surface; and during the 

 interval for which it stands the strata of the region were folded and 

 eroded off to a plain-like surface before being later covered by 

 sediments. 



1 American Journal of Science (4), Vol. XVIII (1904), pp. 356-58. 

 'Ibid., p. 357. 3 Ibid., p. 358. 



