158 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



In the Caballos Mountains, southern Texas, is exposed an area of Paleozoic 

 rocks from which Udden ^'^ procured Ordovician fossils. Other Paleozoic fornaa- 

 tions are probably represented. 



With reference to the Upper and Middle Ordovician in the district north and 

 east of El Paso, Richardson ^^^^ states : 



The Montoya limestone also has been recognized by its stratigraphic position and fossils 

 in both of the quadrangles [EI Paso and Van Horn]. This limestone contains two distinct 

 Ordovician faunas, the Richmond and Galena, and on paleontologic grounds it is desirable to 

 separate the two, but the small thickness of the formation, only about 250 feet, and the scale of 

 the maps will not admit of it. 



Fossils characteristic of the Galena occur in the lower part of the Montoya linestone, the 

 zone being commonly marked in the El Paso quadrangle by massive dark-colored limestone 

 containing little or no chert. The upper part of the limestone is prevailingly gray, but some 

 of the beds are almost white while others are dark, and the two parts of the formation can not 

 always be distinguished lithologicaUy. The zone which carries the most abundant Richmond 

 fossils in places is seamed with conspicuous bands of chert a few inches in thickness. In the 

 Van Horn quadrangle the base of the Montoya limestone is commonly marked by the presence 

 of thin-bedded earthy yellow and reddish limestone, but otherwise in both quadrangles the 

 contact is apparently conformable. Like the El Paso limestone, the Montoya is characteris- 

 tically magnesian. Mr. Ulricli has identified the following fossils from the Montoya limestone :. 



Fossils from the Galena beds. 



Receptaculites oweni. 

 Maclurina manitobensis. 

 Maclurina acuminata. 



Streptelasma rusticum. 

 Hemiphragma imperfectum 

 Monotryprella quadrata. 

 Strophomena flexuosa. 

 Leptaena unicostata. 

 DinortMs subquadrata. 



Hormotxjma major. 

 Ormoceraa sp. undet. 



Fossils from the Richmond beds. 



DinortMs proavita. 

 Platistrophia acutilerata. 

 Rhynchotrema capax. 

 Orthis whitfieldi. 

 Parastropbia divergens. 



In southwestern United States outside of the areas nere considered few Ordovician rocks, 

 are known. The system apparently is not represented by sediments in either the Grand Canyon 

 or Bisbee districts. The Longfellow formation in the Clifton quadrangle, Arizona, probably 

 should be correlated with the EI Paso limestone as well as a part of the Ordovician limestone 

 in the central Texas region. Recently several small areas of Ordovician rocks have been 

 reported in central New Mexico by Gordon and Graton." Mr. Ulrich reports that the Beek- 

 mantown fauna of the El Paso limestone is of the type prevailing in the Wichita Mountains, 

 Oklahoma, in the upper 1,000 feet or so of the Arbuckle limestone; and that the Galena and 

 Richmond faunas of trans-Pecos Texas are similar to those in the Mississippi VaUey, Oklahoma, 

 the Black HUls, the Bighorn Mountains, and elsewhere. 



I 12-13. ABIZONA AND NEW MEXICO. 



The interval corresponding to the Ordovician appears to be represented through- 

 out most of Arizona only by a parting between two limestones containing Middle 

 Cambrian fossils.®®^- ®®*^ These relations resemble those of the Grand Canyon section. 



"Am. Jour. Sci., 4th ser., vol. 21, 1906, p. 190. 



