chap. viii. Elevation of La Plata. 191 



habiting, as I am informed by M. d'Orbigny, brackish 

 water at the mouth of the Plata, nearly or quite as salt 

 as the open sea. The loose sand, in which these shells 

 are packed, is heaped into low, straight, long lines of 

 dunes, like those left by the sea at the head of many 

 bays. M. d'Orbigny has described 1 an analogous phe- 

 nomenon on a greater scale, near San Pedro on the 

 river Parana, where he found widely extended beds and 

 hillocks of sand, with vast numbers of the Azara 

 labiata, at the height of nearly 100 feet (English) 

 above the surface of that river. The Azara inhabits 

 brackish water, and is not known to be found nearer to 

 San Pedro than Buenos Ayres, distant above 100 miles 

 in a straight line. Nearer Buenos Ayres, on the road 

 from that place to San Isidro, there are extensive beds, 

 as I am informed by Sir Woodbine Parish, 2 of the Azara 

 labiata, lying at about forty feet above the level of the 

 river, and distant between two and three miles from it. 

 These shells are always found on the highest banks in 

 the district : they are embedded in a stratified earthy 

 mass, precisely like that of the great Pampean deposit 

 hereafter to be described. In one collection of these 

 shells, there were some valves of the Venus sinuosa. 

 Lam., the same species found with the Mactra on the 

 banks of the Uruguay. South of Buenos Ayres, near 

 Ensenada, there are other beds of the Azara, some of 

 which seem to have been embedded in yellowish, cal- 

 careous, semi-crystalline matter ; and Sir W. Parish 

 has given me from the banks of the Arroyo del Tristan, 

 situated in this same neighbourhood, at the distance of 

 above a league from the Plata, a specimen of a pale- 

 reddish, calcareo-argillaceous stone (precisely like parts 

 of the Pampean deposit, the importance of which fact 



1 ' Voyage dans l'Amerique Mend. : Part. Geolog.' p. 43. 

 8 * Buenos Ayres,' &c, by Sir Woodbine Parish, p. 168. 



