Recent Alluvial Deposits. 113 



alluvial deposits whose age has been indicated to M. D'Or- 

 bigny by human remains. They are, according to him, all 

 posterior to the commencement of our epoch. In the Pam- 

 pas, there are, moreover, over a large extent of surface, me- 

 danos (ancient downs of sand) ; and near the coast, at Bahia 

 Blanca, at San Pedro, &c., beds of shells analogous to those 

 which exist at the present day in a living state in the neigh- 

 bouring seas. 



M. D'Orbigny was for a long time in doubt as to the age of 

 the alluvial matters which cover the Pampean formation at 

 the eastern base of the Andes, but an observation made in the 

 province of Moxos enabled him to determine it. He found 

 on the Rio-Securi a bank 8 yards in height, composed, at its 

 lower portions, of 2 yards of Pampean formation, and above, 

 of 6 yards of alluvial deposit. At a little distance from the 

 Pampean formation, in the lowest beds of the alluvium, he 

 discovered a great number of fragments of earthen-ware, which 

 proved the former residence of the ancient inhabitants ; this 

 discovery afforded certain evidence that these alluvia (if they 

 are all contemporaneous), are posterior to the creation of 

 man. 



At the extremity of the bay of San- Bias, at a place named 

 Riacho-del' Ingles^ M. D'Orbigny found superimposed on the 

 tertiary sandstone, an immense arenaceous bed, containing, 

 along with crystals of gypsum, a great number of shells of 

 Gasteropoda and Acephala identical with those which now 

 live in the bay. This bed, situated about a mile and a quar- 

 ter from the sea, was half a yard above the highest tides. 

 The shells were in the position in which they had lived, and 

 the acephala had their two valves united. The tides in these 

 latitudes rise upwards of eight yards ; these shells occur about 

 half a yard above the highest ; at present they live at a dis- 

 tance of two miles and a half from thence, below the lowest 

 tides. We may hence conclude that the shells in this bed 

 are elevated about ten yards above their present position. 



The environs of Monte Video present hills of gneiss, at the 

 base of which reposes a bed of marine shells, at the height of 

 four or five yards above the La Plata ; the species are indeed 

 different from those which live in the brackish water of the 



VOL. XXXVIl. NO. LXXIII. JULY 1844. H 



