334 Pampean Formation. paet il 



the sea ; therefore Professor Ehrenberg arrives at the 

 important conclusion that the deposit nnust have been 

 of brackish-water origin. Of the thirteen Phytolitharia, 

 nine are met with in the two deposits in Bahia Blanca, 

 where there is evidence from two other species of 

 Polygastrica that the beds were accumulated in brackish 

 water. The traces of corals, sponges, and Polythalamia, 

 found by Dr. Carpenter in the tosca-rock (of which I 

 must observe the greater number of specimens were 

 from the upper beds in the southern parts of the 

 formation), apparently show a more purely marine 

 origin. 



At St. Fe Bajada, in Entre Rios, the cliffs, esti- 

 mated at between sixty and seventy feet in height, 

 expose an interesting section : the lower half consists 

 of tertiary strata with marine shells, and the upper half 

 of the Pampean formation. The lowest bed is an 

 obliquely laminated, blackish, indurated mud, with 

 distinct traces of vegetable remains. 1 Above this there 

 is a thick bed of yellowish sandy clay, with much crys- 

 tallised gypsum and many shells of Ostrea?, Pectens, 

 and Areas : above this there generally comes an arena- 

 ceous crystalline limestone, but there is sometimes 

 interposed a bed, about twelve feet thick, of dark green, 

 soapy clay, weathering into small angular fragments. 

 The limestone, where purest, is white, highly crystalline, 

 and full of cavities : it includes small pebbles of quartz, 

 broken shells, teeth of sharks, and sometimes, as I was 

 informed, large boues : it often contains so much sand 

 as to pass into a calcareous sandstone, and in such 



1 M. d'Orbigny has given ('Voyage, Part. Geolog.' p. 37) a 

 detailed description of this section, but as he does not mention this 

 lowest bed, it may have been concealed when he was there by the 

 river. There is a considerable discrepancy between his description 

 and mine, which I can only account for by the beds themselves 

 varying considerably in short distances. 



