GEOLOGICAL SETTING OF NEW MEXICO 241 



interstitial deposition of silica disposed in optical continuity with 

 the separate sand grains. The enormous thickness of the shales is 

 especially noteworthy. It is probable that eventually they will 

 yield an extensive fauna, or a succession of faunas. Should the 

 rock section prove fossiliferous the opportunity for determining 

 faunal sequence would certainly be as favorable as among any 

 Paleozoics of the continent. 



Where the crest of the great Tijeras fold of the Proterozoics is 

 deeply beveled off flat-lying limestones of latest Paleozoic age 

 recline directly upon it. This notable unconformity plane repre- 

 sents a period of time of vast duration, one almost coextensive 

 with the Paleozoic era. No less than ten great erosion intervals 

 are superposed one upon another. 



Although the early Paleozoic strata are entirely absent over the 

 northern half of New Mexico they are extensively developed in the 

 southern part of the state. There some of the major terranes were 

 identified and correlated with the European sections almost as 

 soon as they were in the Upper Mississippi Valley, or within a 

 decade of the first appearance of Murchison's and Sedg^\ick's 

 classic works. 



When in 1874 undoubted Cambrian beds were first recognized 

 by Jenney in New Mexico only unimportant sandstones were dis- 

 closed in the Franklin Mountains north of El Paso. Since that 

 date the extent of the section has been greatly expanded, until 

 now over 1,000 feet of sandstones, quartzites, and limestones are 

 known. Westward these formations connect with the great 

 Cambrian sections of Arizona. 



As indicated by the contained fossils both mid- and late- 

 Cambrian sections are fairly well represented, the former by 

 about 700 feet of strata and the latter by 400 feet. There are no 

 evidences of the presence of early Cambrian beds within the 

 borders of the state, and it does not seem likely that any ever 

 will be found. 



Ordovician strata were the first Paleozoic rocks recognized 

 within the limits of New Mexico. As early as 1848 Wislizenus 

 recorded the finding of Ordovician (lower Silurian) fossils in rocks 

 west of El Paso. A few years later both Shumard and Antisell 



