chap. viii. Elevation of Bahia Blanc a. 195 



height than twenty feet ; and therefore the age of the 

 sea-drifted pebbles of pumice, now standing at the height 

 of 120 feet, must remain uncertain. 



The main plain surrounding Bahia Blanca I esti- 

 mated at from 200 to 300 feet; it insensibly rises 

 towards the distant Sierra Ventana. There are in this 

 neighbourhood some other and lower plains, but they 

 do not abut one at the foot of the other, in the manner 

 hereafter to be described, so characteristic of Patagonia. 

 The plain on which the settlement stands is crossed by 

 many low sand-dunes, abounding with the minute shells 

 of the Paludestrina australis, d Orbig., which now lives 

 in the bay. This low plain is bounded to the south, 

 at the Cabeza del Buey, by the cliff-formed margin 

 of a wide plain of the Pa,mpean formation, which I 

 estimated at sixty-feet in height. On the summit of 

 this cliff there is a range of high sand-dunes extending 

 several miles in an east and west line. 



Southward of Bahia Blanca, the river Colorado 

 flows between two plains, apparently from thirty to 

 forty feet in height. Of these plains, the southern one 

 slopes up to the foot of the great sandstone plateau of 

 the Rio Negro ; and the northern one against an escarp- 

 ment of the Pampean deposit ; so that the Colorad, 

 flows in a valley fifty miles in width, between the upper 

 escarpments. I state this, because on the low plain at 

 the foot of the northern escarpment, I crossed an im- 

 mense accumulation of high sand-dunes, estimated by 

 the Gauchos at no less than eight miles in breadth. 

 These dunes range westward from the coast, which is 

 twenty miles distant, to far inland, in lines parallel to 

 the valley ; they are separated from each other by 

 argillaceous flats, precisely like those on the northern 

 shore of Bahia Blanca. At present there is no source 

 whence this immense accumulation of sand could pro- 



