Boundary in South America. 19 



With regard to the region east of the Rio Grande (36° 

 and 37° S. L.), Bodenbender expressed the opinion that 

 the vast plateaus which constitute the characteristic 

 physiographic element of this region, consist chiefly of 

 sediments of the Roca-Beds lying under thick sheets of 

 andesitic rocks. The investigations of Groeber in this 

 portion of Argentina have confirmed Bodenbender 's 

 assumption, as sediments and fossils have been found in 

 numerous localities to the north, west and south of the 

 Cerro Payen. The fossils found by Groeber have been 

 partly identified with well-known forms of the Roca- 

 Beds ; partly they will considerably enlarge the paleon- 

 tological knowledge of this formation. 



A rapid change of facies and the lack of constant and 

 easily recognizable horizons of fossils are the most 

 characteristic features of deposits originating like these 

 in a shallow sea. In general, according to Groeber, the 

 entire series of Roca-sediments in this region may be 

 divided into a lower chalky or sandy part with fossils 

 and local intercalations of gypsum and in an upper part 

 with gray reddish nonfossiliferous marls. The thick- 

 ness, in accordance with the frequent change of facies, is 

 very variable. The sandy facies with gypsum shows in 

 general a thickness reaching 450 feet, whereas in the 

 chalky facies the thickness is frequently reduced to 30 

 feet. The whole complex rests unconformably upon the 

 upper marly formations of the Upper Cretaceous Varie- 

 gated Sandstones. Sometimes brackish deposits with 

 Paludina appear at the base. The breccias, tuffs and 

 sheets of the so-called Andesitic Series (a very thick 

 complex of volcanic material to which Groeber is inclined 

 to assign an approximately OligoCene age) have been 

 laid down upon the eroded surface of these sediments. 



To the south of the Rio Colorado, widespread deposits 

 of the Roca Beds have been observed by the writer in the 

 surroundings of the Sierra Auca Mahuida and may be 

 divided in two horizons, just as in the region north of 

 the Rio Colorado. 49 In the lower horizon of clayey sedi- 

 ments with much gypsum ("Capas del Jaguel") frag- 

 ments of Perna have been found and, a little higher up, 

 the typical fauna, especially Ostrea Ameghinoi Ih., bryo- 

 zoa and corals. An upper nonfossiliferous horizon is 

 formed by marls with sandy intercalations. 



49 See Windhausen, El yaeimiento de Rafaelita de Auca Mahuida etc. and 

 Einige Ergebnisse zweier Eeisen in den Territorien Eio Negro und Neuquen, 

 pp. 343 etc. 



