536 Geological Society : — Mr. Darwin on the Erratic 



sixty fathoms deep, extending over the flats, and the Cape Promontory 

 must have been an island. To the action of the sea at that period Mr. 

 Clarke attributes the production of the felspathic clay, and its accu- 

 mulation at the Lion's Rum}) ; and to the action of currents at an 

 earlier period, when the summit of the Table range lay as islands 

 and reefs not far above the level of the sea, the removal of the sand- 

 stone and the excavation of the granite at the Kloof, also the denu- 

 dation and rounding of the ridge of the Lion's Hill, the denudation 

 of Robben's Island, and the production of those terraces, which from 

 the summit of Table Mountain appear to stretch gradually down- 

 wards to the Cape of Good Hope. The separation of the Lion's 

 Rump and the Devil's Mountain from the Table Mountain, and 

 the fissures throughout the range, the author conceives were pro- 

 duced during the elevation of the country. Proofs of changes of re- 

 lative level of sea and land are stated to be equally apparent in the 

 interior ; and Mr. Clarke says, that the inspection of an accurate map 

 will convince the inquirer, that Southern Africa must have been an 

 Archipelago. In conclusion, some general observations are made on 

 the great similarity in the geological composition of Southern Africa 

 and New South Wales. 



Mayo. — A Memoir "On the Distribution of the Erratic Boulders, 

 and on the contemporaneous unstratified Deposits of South America," 

 by Charles Darwin, Esq., F.R.S., F.G.S., was read. 



The extensive regions more particularly noticed in this paper are 

 the plains traversed by the Rio Santa Cruz (lat. 50° S.) ; Tierra 

 del Fuego, including the coasts of the Strait of Magellan, and the 

 Island of Chiloe (lat. 43° S., long. 73° W.). 



Patagonia. — Between the Rio Plata and the Rio Santa Cruz, 

 Mr. Darwin did not observe any boulders, and the only one he no- 

 ticed in ascending the first 100 miles of the latter river was a mass 

 7 feet in circumference, about 57 miles from its mouth, or 100 from 

 the Cordillera. At 100 miles from the coast, or 67 from the near- 

 est slope of the Cordillera, transported blocks first occur, and 12 

 miles nearer the chain they are extraordinarily numerous, consisting 

 of clay-slate, felspathic rocks, chlorite schist and basaltic lava. 

 They are generally angular, and some of them are of immense size, 

 one being 60 fe,et in circumference, and projecting from 5 to 6 feet 

 above the surface of the ground. The vast open plain on which 

 they lie scattered, is here 1400 feet above the level of the sea, and 

 its surface is somewhat irregular, owing partly to denudation and 

 partly to the protrusion of hummocks and fields of lava. The plain 

 slopes gently and regularly towards the Atlantic, where the sea- 

 cliffs are about 800 feet high ; but towards the Cordillera it rises 

 more abruptly, attaining near the chain an elevation of 3000 feet. 

 The highest peaks of the Cordillera in this part of its range do not 

 exceed 6400 feet above the level of the sea. The following section, 

 exhibited in the banks of the Santa Cruz in longitude 70° 50' W., 

 is given by Mr. Darwin to illustrate the nature of the plain on which 

 the boulders rest. 



