DIASTROPHICS OF NORTHERN MEXICO 79 



vey, and by others who have worked more or less extensively in the 

 region, the essential features of the section are well epitomized in 

 recently published tables.' 



Since it is with marked unconformity that the Paleozoics rest 

 upon the pre-Cambrian crystallines it is evident that long before 

 Paleozoic deposition in the region set in, the old continental com- 

 plex was beveled off to a smooth plain. This ancient erosion 

 surface cuts evenly the folded, faulted, and altered pre-Cambrian 

 strata, the more or less highly metamorphosed elastics, and the 

 strictly igneous masses and intrusions. It doubtless represents as 

 true a peneplain as ever existed, and one that remained longer and 

 nearer base level than any other one known in geological history. 

 The presence of this once low-lying plain and the near-shore deposi- 

 tion of the vast piles of homogeneous" Hmestones appear strongly 

 to support the idea of the existence of a close genetic relationship 

 between the two phenomena. 



Notwithstanding the fact that such exceptional homogeneity 

 prevailed throughout the Paleozoic succession of the northern Mexi- 

 can tableland, no less than a dozen major unconformities attest the 

 frequency and extent of notable diastrophic movement. Of these 

 by far the most conspicuous hiatus is that at the base of the Penn- 

 sylvanian limestones. In every way it is the most pretentious. 

 Its character and position associate it with the similar phenomenon 

 displayed in the Mississippi Valley. From that it differs in the 

 apparent absence of the Coal Measures. However, this dissimilar- 

 ity fades since remnants of the latter are now known to be actually 

 present. How extensive they originally were is yet a matter of 

 conjecture. 



In the Ladronesian series, exposed in a circumscribed basin 

 near Socorro,^ are represented the all but vanquished coal shales 

 which may be the southwestern extension of the great Arkansan 

 series of the Ozark region. The unconformity plane is comparable 

 in every way with that found at the base of the Coal Measures of 

 Iowa, Missouri, and Illinois. It extends far to the north in Colo- 

 rado; and far to the south in Old Mexico. Although in the Rocky 



* Proc. Iowa Acad. Sci., Vol. XXII (1916), pp. 249-71. 

 ^Journal of Geology, Vol. XII (1904), pp. 250-51. 



