64 CHARLES R. KEYES 



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

 these numerous mountain blocks were produced by folding. All 

 observations go to show most conclusively that only faulting is 

 involved. The sedimentaries of some of these mountains, however, 

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

 visible. Numerous other indications point to tremendous compres- 

 sion at some time or other. But the period of compressive action 

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

 present mountains were formed. The compression took place long 

 before the existing mountain blocks reared their heads above the 



Casallos Mrs 



Ca-rh. Lx 



1800' 



Qunvfaite [H 

 Schists ' " 



Fig. i 



vast surrounding plains. Chronologically this period of compressive 

 conditions was manifestly after the Carboniferous, because the rocks 

 of this age were involved; but it was before the late Cretaceous, 

 since Cretaceous strata are as clearly not affected. 



In the Sierra de los Caballos, in south-central New Mexico, the 

 geological sections are particularly instructive. Near the highest 

 point of the range, known as Timber Peak, the scarp is over 3,000 

 feet high and displays an excellent exposure of the rocks throughout 

 this entire vertical distance. The transverse section shown a short 

 distance to the north is represented in the diagram, Fig. 1. The 

 heavy line T. P. indicates the position of an exceedingly well-dis- 

 played thrust-plane. Along it the beds are highly contorted. The 

 entire limestone is badly shattered and traversed by large and small 

 crevices which are now cemented by calc-spar. The inclination of 

 the thrust-plane is now rather steep, but, as will appear subsequently, 



