Ixxii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



power, upon careful examination, pebbles of compact basalt could not 

 be found in the bed of the river at a greater distance than ten miles 

 below the point where the stream rushes over the debris of the great 

 basaltic cliffs forming its shores : fragments of the cellular varieties 

 have been washed down twice or thrice as far." Mr. DarAvin is of 

 opinion, that the cause of the rounding of the fragments and the 

 spreading out and levelling of the gravel is to be ascribed to the 

 action of the sea, as it gradually receded from the foot of the Cordil- 

 lera to the present coast, by the slow upheaval of the land. He 

 admits, however, that it is a problem of great difficulty. " By what- 

 ever means," he says, " the gravel formation of Patagonia may have 

 been distributed, the vastness of its area, its thickness, its superficial 

 position, its recent origin, and the great degree of similarity in the 

 nature of its pebbles, — all appear to me well-deserving the attention 

 of geologists, in relation to the origin of the widely-spread beds of 

 conglomerate belonging to past epochs*." It is seen on the coast to 

 rest on horizontal beds of older tertiary strata, which in some places 

 form cliffs from 800 to 900 feet in height : as it is seen in the inte- 

 rior capping terraces formed of deposits containing shells, and as the 

 gravel with its sandy covering is often strewed with recent marine 

 shells, there is no doubt of its belonging, like the Pampean formation, 

 to the pleistocene age ; and in all probability they were nearly con- 

 temporaneous. In the valley of Santa Cruz, at a distance of 100 

 miles from the sea, and at an elevation of 1400 feet, the gravel is 

 covered with numerous angular erratic blocks, some as much as 60 

 feet in circumference. These were described by Mr. Darwin in a 

 paper read in this room, and published in the sixth volume of our 

 Transactions, and he there attributes their position to the transport- 

 ing action of icebergs, the probable origin of the erratic blocks of 

 Northern Europe. 



Elevation of the land. — ^We know that the land of the western 

 coast of South America has risen considerably in our own time : we 

 have proofs of considerable elevations in the recent period of geolo- 

 gical time, when the country was inhabited by man, and we can trace 

 back the continuance of the same operation of subterranean force to 

 far earlier periods, and upon a greater scale, in various parts of that 

 same coast ; nor is there wanting evidence to show that there 

 have been partial subsidences of the land within the historical period. 

 But these changes of relative level of sea and land during the pleisto- 

 cene period are more distinctly seen on the eastern coast : they were 

 described by Mr. Darwin in his ' Journal,' but in his recent work he 

 has gone into far greater details respecting them, — -into a minuteness 

 of description that was not admissible in the plan of his * Journal,' but 

 which is far more interesting and satisfactory to the geologist. 



For a space of more than 1200 miles, from the 33rd degree of S. 

 latitude southward, the land has been gradually elevated, as shown 

 by a succession of terraces one above the other, with abrupt escarp- 

 ments facing the sea, and separated from each other by gently sloping 

 plains. On the coast of Patagonia, between Santa Cruz and Port 



* Page 24. 



