194 Elevation of Bahia Blanco* part n. 



of immense sand-dunes, resting on gravel with recent 

 shells, and ranging in lines parallel to the shore. These 

 ranges are separated from each other by flat spaces, 

 composed of stiff impure red clay, in which, at the 

 distance of about two miles from the coast, I found by 

 digging, a few minute fragments of sea-shells. The 

 sand-dunes extend several miles inland, and stand on a 

 plain, which slopes up to a height of between one and 

 two hundred feet. Xumerous, small, well rounded 

 pebbles of pumice lie scattered both on the plain and 

 sand-hillocks : at Monte Hermoso, on the flat summit 

 of a cliff, I found many of them at a height of 120 feet 

 (angular measurement) above the level of the sea. 

 These pumice pebbles, no doubt, were originally brought 

 down from the Cordillera by the rivers which cross the 

 continent, in the same way as the river Negro anciently 

 brought down, and still brings down, pumice, and as 

 the river Chupat brings down scoriae : when once de- 

 livered at the mouth of a river, they would naturally 

 have travelled along the coasts, and been cast up, dur- 

 ing the elevation of the land, at different heights. The 

 origin of the argillaceous flats, which separate the 

 parallel ranges of sand-dunes, seems due to the tides 

 here having a tendency (as I believe they have on most 

 shoal-protected coasts) to throw up a bar parallel to 

 the shore, and at some distance from it ; this bar 

 gradually becomes larger, affording a base for the 

 accumulation of sand-dunes, and the shallow space 

 within then becomes silted up with mud. The repe- 

 tition of this process, without any elevation of the land, 

 would form a level plain traversed by parallel lines of 

 sand-hillocks ; during a slow elevation of the land, the 

 hillocks would rest on a gently inclined surface, like 

 that on the northern shore of Bahia Blanca. I did not 

 observe any shells in this neighbourhood at a greater 



