316 H. T. Hill — Cretaceous Formations of Mexico. 



Extent and History of the Comanche Sedimentation. 



The Lower Cretaceous deposits of Mexico extend from the 

 Pacific to the Atlantic and clearly show that during the 

 Comanche sub-period the waters of the two oceans were united 

 and separated the north and south continents into two islands. 

 The southern shore line cannot be minutely traced at present 

 owing to our indistinct knowledge of South American geology, 

 but there is abundant evidence to show that it covered much 

 of the western coast of Peru, Chili, Bogota, and the northern 

 States of South America reaching far into the present area of 

 Andean uplift against the nucleal Archaean and Paleozoic area 

 of that continent, as shown by the occurrence of the typical and 

 peculiar Comanche series, Schlambachia peruvianus, Buchoce- 

 ras jpedernalis, Oryphcea pitcheri, Exogyra texa?ia, Patellina 

 (Orbitulina) texana and other forms. The Comanche sedi- 

 mentation extended into northeastern Brazil where its typical 

 fauna has been described from Sergipe by Hyatt* but it does 

 not appear in more southern faunas described by White. 



The North American border of this Comanche sedimenta- 

 tion can be definitely located around the coastward margin of 

 the old Appalachian region to near Marietta south of central 

 Indian Territory, but west of this point it can be located 

 at only widely separated places because of the overlapping of 

 the Great Plains and basin formations and the disturbances of 

 the Cordilleran region. In northeastern New Mexico the thin 

 littoral beds of the Comanche are exposed in the strata of 

 Tucumcari Mountain and the adjacent Llano Estacado.f 

 They are next revealed at El Paso where the attenuated and 

 highly tilted beds again outcrop against the foot of the Organ 

 and Monument Mountains on both the Mexican and American 

 sides of the Rio Grande. The most western localities recorded 

 are Ariviche, and Sahuaripa in Senora from where Gabb de- 

 scribed its characteristic uppermost or Denison fauna. Except 

 a locality described on the southern line of Kansas no trace of 

 the great Comanche sedimentation has been found and every 

 possible interpretation of the littoral sediments shows that an 

 adjacent land area must have existed over all the Appalachian 

 Great Plains and Cordilleran areas north of a line connecting 

 these points during that epoch, as shown upon the accompany- 

 ing map. 



The central portion of the Mexican plateau where the 

 Comanche Series attains a thickness of more than twenty 



* Mentioned on pages 385-393 of Hart's Geology and Physical Geography of 

 Brazil. 



f See " On the Occurrence of Artesian and other Underground Waters," etc., by 

 Rob't T. Hill. Pinal Report Artesian and Underflow Investigations. Senate 

 Doc. 41, 52d Cong., vol. iii, p. 129, Washington, May, 1892 ; also Third Annual 

 Report Texas State Geological Survey, pp. 152-154. Austin, Sept., 1892. 



