$56 Pampean Formation pakt n. 



the marine tertiary strata, which (as first shown by 

 M. d'Orbigny) are contemporaneous with those of 

 Patagonia, and which, as we shall hereafter see, belong 

 to a very ancient tertiary stage. "When examining the 

 junction between these two formations, I thought that 

 the concretionary layer of marl marked a passage be- 

 tween the marine and estuary stages. M. d'Orbigny 

 disputes this view (as given in my Journal), and I admit 

 that it is erroneous, though in some degree excusable, 

 from their conformability and from both abounding 

 with calcareous matter. It would, indeed, have been a 

 great anomaly if there had been a true passage between 

 a deposit contemporaneous with existing species of 

 Mollasca, and one in which all the Mollusca appear to 

 be extinct. Xorthward of St. Fe. M. d'Orbigny met 

 with ferruginous sandstones, marly rocks, and other 

 beds, which he considers as a distinct and lower forma- 

 tion ; but the evidence that they are not parts of the 

 same, with an altered mineralogical character, does not 

 appear to me quite satisfactory. 



In Western Banda Oriental, whilst the marine ter- 

 tiary strata were accumulating, there were volcanic 

 eruptions, much silex and lime were precipitated from 

 solution, coarse conglomerates were formed, being de- 

 rived probably from adjoining land, and layers of red 

 mud and marly rocks, like those of the Pampean for- 

 mation, were occasionally deposited. The true Pampean 

 deposit, with mammiferous remains, instead of as at 

 St. Fe overlying conformably the tertiary strata, is 

 here seen at a lower level folding round and between 

 the flat-topped, cliff-bounded hills, formed by the up- 

 heaval and denudation of these same tertiary strata. 

 The upheaval, having occurred here earlier than at 

 St. Fe, may be naturally accounted for by the contem- 

 poraneous volcanic action. At the Barrancas de S. 



