438 CHARLES R. KEYES 



No evidence has yet been found that would indicate that any of 

 the present mountain blocks were produced by folding. All observa- 

 tions go to show on the other hand that only faulting is involved. 

 To be sure the sedimentaries of some of these mountains are often 

 folded and closely corrugated. Thrust-planes are plainly visible. 

 Numerous other indications point to tremendous compression at 

 some time or other. But the period of this compression has been 

 found to be mainly a very different one from that during which the 

 present mountains were formed. The compressive action was exerted 

 long before the existing mountain blocks began to rear their heads 

 above the vast plains. Chronologically this period of compressive 



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Fig. 4. — Old and modern faulting in Caballos Mountains. 



conditions was manifestly subsequent to the Carbonic period because 

 the rocks of this age are involved ; but it was before the Late Cretacic 

 period, since Cretacic strata are as clearly not affected. 



Certain thrust-planes displayed in the Sierra de los Caballos, to the 

 south of the Estancia region, the geologic sections of which have a 

 bearing upon this point are particularly instructive while the pro- 

 duction of others is thought to be somewhat later. Near the 

 highest point of the range, known as Timber Peak, the fault- 

 scarp is over 3,000 feet high and displays an excellent exposure of 

 the rocks throughout this entire vertical distance. The transverse 

 section of the mountain ridge, as shown a short distance to the north, 

 is represented in diagram (Fig. 4). The heavy line, T-P, indicates 

 the position of an exceedingly well-displayed thrust-plane. Along 



