chap. vin. Gravel Formation of Patagonia. 223 



were brought me from the western extremity of the 

 Falkland Islands. 1 The distribution of the pebbles of 

 this peculiar porphyry, which I venture to affirm is not 

 found in situ either in Fuegia, the Falkland Islands, 

 or on the coast of Patagonia, is very remarkable, for 

 they are found over a space of 840 miles in a north and 

 south line, and at the Falklands, 300 miles eastward of 

 the coast of Patagonia. Their occurrence in Fuegia 

 and the Falklands may, however, perhaps be due to 

 the same ice-agency by which the boulders have been 

 there transported. 



We have seen that porphyrinic pebbles of a small 

 size are first met with on the northern side of the Rio 

 Colorado, the bed becoming well developed near the 

 Rio Negro : from this latter point I have every reason 

 to believe that the gravel extends uninterruptedly over 

 the plains and valleys of Patagonia for at least 630 

 nautical miles southward to the Rio Gallegos. From 

 the slope of the plains, from the nature of the pebbles, 

 from their extension at the Rio Negro far into the 

 interior, and at the Santa Cruz close up to the Cor- 

 dillera, I think it highly probable that the whole breadth 

 of Patagonia is thus covered. If so, the average width 

 of the bed must be about 200 miles. Near the coast 

 the gravel is generally from ten to thirty feet in thick- 

 ness ; and as in the valley of Santa Cruz it attains, at 

 some distance from the Cordillera, a thickness of 214 

 feet, we may, I think, safely assume its average thick- 



1 At my request, Mr. Kent collected for me a bag of pebbles from 

 trie beach of White Eock harbour, in the northern part of the sound, 

 between the two Falkland Islands. Out of these well-rounded 

 pebbles, varying in size from a walnut to a hen's egg, with some 

 larger, thirty-eight evidently belonged to the rocks of these islands; 

 twenty-six were similar to the pebbles of porphyry found on the 

 Patagonian plains, which rocks do not exist in situ in the Falklands ; 

 one pebble belonged to the peculiar yellow siliceous porphyry ; thirty 

 were of doubtful origin. 



