•T. 



50 K. F. MATHER FRONT RANGES OF THE ANDES 



fossils collected from the beds traversed by Rio Saipuru and above re- 

 ferred to are not diagnostic of the age of the inclosing strata. The collec- 

 tion was submitted to the specialists of the United States Geological Sur- 

 vey for identification. Dr. David White, in commenting on them, states 

 that "according to E. 0. Ulrich, the numerous ostracods apj:>ear to belong- 

 to a single species of Bythocypris. Some of the pelecypod casts suggest 

 Tellina, but are not well enough preserved for positive identification. 

 Both the ostracods and pelecypods are simple, long-lived types that are 

 of little use in age determination." E. 0. Ulrich, in Eastman's trans- 

 lation of ZittePs Paleontology, characterized Bythocypris as "typically 

 recent, but a number of Paleozoic forms have also been assigned to this 

 genus.'" Dr. W. H. Dall, in the same textbook, gives the age limits of 

 Tellina as Jurassic to Recent. The fossils, therefore, are in harmony 

 with the lithologic suggestion that the inclosing beds are of Cenozoic age. 

 Tn all probability the Tatarenda formation will prove to be Tertiary. 



Identical pelecypod remains were found in similar strata north of Rio 

 Grande, 3 miles northwest of Abapo. Doubtless these were from the beds 

 in which fossil shells were observed by Romanes, Madgwick, and Witte- 

 veen, as reported by Bonarelli (1921, page 79). 



QUATERNARY DEPOSITS 



Both the Chaco plain and the intermontane lowlands are quite gener- 

 ally covered with recent deposits of unconsolidated gravel, sand, silt, and 

 clay, which rest unconformably on the eroded surface of the Tatarenda 

 and older beds. These are in part eolian, in part fluvial, and probably 

 in part lacustrine in origin. 



Within the mountains the hillsides are ordinarily so steep that only 

 slight accumulations of such material are present. They rarely offer a 

 serious handicap to the work of the geologist in deciphering the nature 

 and structure of the older beds. The thickness of these surface materials 

 beneath the lowlands is, of course, very variable. The underlying strata 

 are almost everywhere concealed in the flat regions, and the Quaternary 

 deposits probably attain a thickness of two or three hundred feet at many 

 places. 



Geologic Structure 



general statement 



Structurally, the area under consideration consists of a series of closely 

 compressed anticlines extending for long distances north and south and 

 separated by comparatively broad synclinal troughs, some of which are 



