CHAPTER XVI. 
REMARKS ON THE GEOLOGY OF RIO NEGRO. 
A watkx of five or six hours about the shores of Rio Negro could ac- 
complish little in the way of geological investigation, and a few brief 
remarks are, therefore, all we have to present. The works of Mr. 
Darwin* and D’Orbigny+ have already well illustrated the region. 
The mouth of the Rio Negro is situated about six hundred miles 
south of Buenos Ayres, on the confines of Patagonia, in latitude 42° 
south. The country is a part of the Pampas or prairies of La Plata, 
and lies so low that the land did not become visible till we had ap- 
proached within twelve miles of the coast. We then barely distin- 
guished an even line, unvaried by a single eminence. As we came 
to anchor, a range of low bluffs was seen to border the sea to the 
south. One point of these bluffs, the South Barranca, was after- 
wards measured by the writer, and ascertained to be ninety-four feet 
above high water level: they vary from ninety to one hundred feet. 
Nearer the river, the surface was covered with hillocks of rounded 
contour, which were found, on examination, to be mostly heaps of 
sand that had been blown up by the winds, or thrown together by the 
sea. They were partly protected by tufts of tall grass; but frequently 
the summits only were covered with these tufts, and as the winds 
wore away the sides, it left them with a grassy head, presenting a sin- 
gularly grotesque appearance. ‘These tussucks occur partly in lines 
of half a dozen or more along the river near its mouth. 
The sea-beach to the north and south of the river consists of fine 
sand and pebbles, and is generally so hard as to receive but little 
impression from the foot. The pebbles are mostly either quartz, 
jasper, porphyry, chert, or chalcedony, and the last mentioned has 
* Geological Observations on South America. 
T Voyage, Partie Gcologique, p. 57 to 65. 
