chap. xi. Banda Oriental. 341 



Pampas, and tliis appeared to underlie the crystalline 

 limestone ; but the section was not unequivocal like 

 that at P. Gorda. These beds now form only a narrow 

 and much denuded strip of land ; but they must once 

 have extended much farther ; for on the next stream, 

 south of the S. Juan, Captain Sulivan, K.N., found a 

 little cliff, only just above the surface of the river, with 

 numerous shells of the Venus Munsterii, d'Orbig., — 

 one of the species occurring at St. Fe, and of which 

 there are casts at P. Gorda : the line of cliffs of the 

 subsequently deposited true Pampean mud, extend 

 from Colonia to within half a mile of this spot, and no 

 doubt once covered up this denuded marine stratum. 

 Again at Colonia, a Frenchman found, in digging the 

 foundations of a house, a great mass of the Ostrea Pata- 

 gonica (of which I saw many fragments), packed to- 

 gether just beneath the surface, and directly superim- 

 posed on the gneiss. These sections are important : 

 M. d'Orbigny is unwilling to believe that beds of the 

 same nature with the Pampean formation ever underlie 

 the ancient marine tertiary strata ; and I was as much 

 surprised at it as he could have been ; but the vertical 

 cliff at P. Gorda allowed of no mistake, and I must be 

 permitted to affirm, that after having examined the 

 country from the Colorado to St. Fe Bajada, I could not 

 be deceived in the mineralogical character of the Pam- 

 pean deposit. 



Moreover, in a precipitous part of the ravine of 

 Las Bocas, a red sandstone is distinctly seen to overlie 

 a thick bed of pale mud, also quite like the Pampean 

 formation, abounding with concretions of true tosca- 

 rock. This sandstone extends over many miles of 

 country : it is as red as the brightest volcanic scoriae ; 

 it sometimes passes into a coarse red conglomerate 

 composed of the underlying primary rocks ; and often 



