GEOLOGICAL SETTING OF NEW MEXICO 243 



agree on the appearance of the fauna in the West, or Mississippi 

 Valley region, much earlier than in the East, or New York province; 

 so that, although its age in the first-named locality is strictly mid- 

 Devonian, in the second locality it is late Devonian. This being 

 the case, the same migratory fauna may have put in an appear- 

 ance in the southwestern or New Mexican province, considerably 

 earlier even than it did in the Upper Mississippi Valley region — 

 possibly in the middle or latter part of early Devonian time. 



The Mississippian succession in New Mexico comprises only a 

 limited portion of the middle division of the Mississippi Valley 

 section. Both the upper, or Tennesseean, and the lower, or 

 Waverleyan divisions seem to be entirely missing. Because of its 

 restricted representation and its other notable pecuharities the 

 southwestern sequence is designated the Socorran series. The 

 maximum thickness of these rocks is about 400 feet. 



Both the Chouteau and Burlington faunas of the Mississippi 

 Valley are well represented. The list of organic remains is almost 

 as extensive as that of the type localities. Fossils from the Modoc 

 limestone, which extends eastward from Arizona, indicate higher 

 horizons, probably so high as the Keokuk level. Curiously, the 

 huge gastropods which are so rare in the East are very abundant 

 in the Southwest. 



The especial importance of the New Mexican section of the 

 Mississippian rocks is that it directly connects the eastern or 

 Ozark sequence with that of the Far West. Summing up the evi- 

 dence regarding these strata as they are represented around the 

 border of the Colorado High Plateau it appears that there is in 

 the Southeast and South, in southwestern New Mexico and eastern 

 Arizona, the Socorran series; in the West, in northwestern Arizona, 

 the Lower Red Wall formation; in southwestern Colorado the 

 Ouray (upper part) limestone; and east of the southern Rocky 

 Mountains the Millsap limestone. In New Mexico the succession 

 is probably most completely represented and most varied, although 

 perhaps not including all that is representedin Colorado. 



In strong contrast with the Pennsylvanian sequence of the 

 middle and eastern portions of the continent the southwestern 

 succession is almost unbrokenly calcareous. A single plate of 



