20 



I Yin dha usen — The Cretaceous- Tertiary 



u 



m 





L^ 



WW 



K 



fc 



P3 w 1 



3 «* 

 O 3 





05 +-> +3 



™ M 



o l- , 



ft ^b l 



2 p S§ 

 g'.Sw'S 



* « 





r 1 X *-> 



r/^ it-" k ~ H 



(D - ~-« 



f on 

 S- 3 ^ 







M 



ft 



In reviewing the principal strati- 

 graphic features of the region from 

 Canada Colorada to the Sierra Auca 

 Mahuida, there can be distinguished 

 a chalky-sandy gypsum-bearing divi- 

 sion with marine as well as brackish 

 fossils and above these a complex of 

 variegated marls with sandy interca- 

 lations and no fossils. At the foot of 

 the Sierra Auca Mahuida clayey sedi- 

 ments with gypsum of great thickness 

 begin to interwedge at the base of the 

 chalky-sandy division, and this clayey 

 horizon can be observed as far as the 

 village of General Roca. At the bot- 

 tom as well as at the top of the whole 

 complex in the region of the Rio 

 Grande, Groeber observed obvious 

 unconformities. 



The geologic conditions of the sur- 

 roundings of General Roca may be 



summarized 



investigations 



last 

 the 

 the 

 the 



according to my 

 as follows : On 

 north bank of the Rio Negro, 

 marine fossiliferous strata of 

 Roca Beds are well exposed at various 

 localities, especially on the heights of 

 a long spread terrace situated about 

 10 miles north of Roca. But nowhere 

 can the base upon which these strata 

 are resting, be seen. In these sec- 

 tions a clayey gyp sum- bearing hori- 

 zon with Gryphaea Burckhardti 

 Boehm and fish teeth ("Capas del 

 Jaguel") is followed by a layer with 

 Nautilus Valencienni Hupe, and upon 

 this layer rest chalky and marly sedi- 

 ments with the known fauna of Roca- 

 invertebrates. This complex may be 

 approximately 100 feet thick. 



Upon these marine sediments lie 

 clear tuff-bearing sandstones contain- 

 ing opalized trunks of trees and fur- 

 ther testifying to a terrestrial origin. 

 On the south bank of the Rio Negro 

 these terrestrial sediments are of great 



