CARBONIFEROUS FORMATIONS IN NEW MEXICO 149 



tively much poorer representation of the Early Carboniferous 

 sediments, an almost entire absence of the early Mid- Carbonif- 

 erous shales, a very much greater development of middle and late 

 Mid- Carboniferous marine beds, and a very great expansion of 

 the middle Late Carboniferous marine deposits, while the sediments 

 of the closing period are very much the same in both. 



The thicknesses of the various formations are readily deter- 

 mined, usually in single unobscured vertical sections displayed 

 in the fault scarps of the block-mountains which rise 3,000 to 5,000 

 feet above the plains at their bases, giving continuous outcrops 

 that for unbroken extent are nowhere in the world surpassed. 



The marine nature of practically the entire Carboniferous 

 sequence, as represented in New Mexico, contrasts it strongly with 

 the sections of the central and eastern United States. The main 

 body of limestones composing the Manzanan and Maderan series 

 were early recognized by government explorers as "Upper Car- 

 boniferous limestones." These two series and some other beds, 

 taken together, have more recently, especially in the Grand Canyon 

 district, eastern Arizona, and western New Mexico, generally gone 

 under the title of the Aubrey limestone; and in western 

 Texas, under the vaguely defined name of the Hueco limestone. 

 Over the greater part of all of these regions the formation passing 

 under a single title is easily separable into three or four distinct 

 formations having serial rank, and each again is locally subdi- 

 visible. 



The general absence of shales and coal-beds in the Carbon- 

 iferous formations of the New Mexican region is one of its most 

 striking features, particularly to one who has been accustomed 

 to the great beds of shales, shaly sandstones, bituminous beds, 

 and coals of the East. The horizons at which these deposits could 

 be naturally expected are immediately beneath the great limestone 

 plate having the Manzanan series for its base. There is, however, 

 at this stratigraphic level a great plane of unconformity which is 

 of very wide extent, and which represents a profound erosion interval. 



If extensive shales ever existed here, and they evidently have, 

 they have been almost entirely swept away. That there were once 

 important coal-measures deposited on this old erosion surface is 



