DIASTROPHICS OF NORTHERN MEXICO 8i 



amounting to hundreds of miles, instead of very short distances as 

 at first glance seems probable. 



Paleogeographical maps commonly show the southern Rocky 

 Mountain region as a huge island persisting throughout Paleo- 

 zoic times. Orographic arching of the tract appears to have taken 

 place only late in the era. For the first time since pre-Cambrian 

 days general peneplanation does not appear until the Mississippian 

 or Pennsylvanian period. 



That the peneplanation at the beginning of Pennsylvanian times, 

 when Coal Measures were being deposited elsewhere around the 

 growing American continent, was extensive is strongly supported 

 by many facts. Since farther north in the Rocky Mountains area 

 the older Paleozoics are present in a few limited and isolated belts, 

 where they are preserved through infolding with more ancient 

 rocks, it is presumed that Cambrian, Ordovician, Silurian, Devonian, 

 and Mississippian strata as they are represented in the South doubt- 

 less once extended entirely over the province before its epeirogenic 

 uprising. Over the Mexican tableland district last lingering traces 

 of the old formations remained until the grand erosional 

 period represented elsewhere to the eastward by the Arkansan 

 (Pennsylvanian) deposition. It may be that the Pennsylvanian 

 peneplanation epoch of the southwestern region is to be exactly 

 paralleled with that of Iowa where it is designated as the Arkan- 

 san hiatus. 



Another reason why in the Cordilleran region north of the Mexi- 

 can tableland the Paleozoics do not appear more frequently than 

 they do is that Triassic peneplanation was also profound. This 

 surface is largely covered by mid-Cretaceous sediments before the 

 eastern front of the Rockies is reached. Along this border the more 

 ancient rocks are thus not open to inspection. 



The rather abrupt termination of the several periodic terranes 

 of the Paleozoic toward the north in central New Mexico does not 

 appear to be altogether a direct result of successive advances of the 

 ancient sea in that direction over a low-lying even coast. If any 

 part of the abrupt thinning is thus to be ascribed it is entirely lost 

 through repeated and profound planation effects. After the lay- 

 ing down of the great Pennsylvanian limestones no less than two of 



