78 CHARLES KEYES 



were obtained at Santa Rita, in 1873, by Professor G. K. Gilbert.* 

 Devonian strata were first recorded in New Mexico by Thomas 

 Antisell f and in the following year the fossils were described from 

 this region by Professor James Hall.^ Mississippian forms collected 

 by Professor E. D. Cope were described in 1881 by Mr. S. A. 

 Miller.'' The most northerly points at which they were recognized 

 recently were in the Magdalena Mountains, west of Socorro,^ and 

 in the Sierra Ladrones 25 miles north. 



So in New Mexico prior to the year 1880 all of the periodic 

 terranes of Paleozoic age had been already fully identified. 



In this tableland region the outstanding feature of the stratig- 

 raphy, and a characteristic which is perhaps nowhere else met with, 

 is a notable segregation, instead of the usual alternation, of the hard 

 and soft strata. Resistant beds are confined chiefly to the bottom 

 half of the vertical section; and nearly all of the weak rocks occur 

 only in the upper part. For a succession more than 10,000 feet in 

 thickness this circumstance is certainly a quite remarkable one. 

 Almost the entire Paleozoic sequence is thus composed of lime- 

 stones of such uniform lithologic texture and aspect that it is not 

 usually possible by casual glance to detect the parts of different 

 geologic age. Only by careful discrimination of the successive 

 faunas at the various stratigraphic levels are even the larger, or 

 periodic, subdivisions rendered determinable. Yet, on the whole, 

 the sequence is one of the most complete on the American con- 

 tinent. Out of twenty-five major terranes holding serial rank only 

 five seem to be missing. 



Both by reason of its completeness and because of its pecuHar 

 continental relationships this general geological section of the New 

 Mexican Paleozoics is for purposes of reference one of the important 

 successions of the country. As determined by various parties from 

 the United States Geological Survey, and the State Geological Sur- 



^ U.S. Geog. and Geol. Surv. W. lOotliMerid., Vol. Ill (1875), p. 117. 

 " Explo. and Surv. Pacific Railroad, Vol. Ill (1856), Pt. II, p. 181. 

 3 United States and Mexican Bound. Surv., Vol. I (1857), Pt. II, p. 104. 

 * Jour. Cincinnati Soc. Nat. Hist., Vol. IV (1881), p. 314. 

 5 Proc. Iowa Acad. Sci., Vol. XII (1905), p. 169. 



