and contemporaneous Deposits of South America. 427 



same districts I did not meet with any beds of till. North of the Strait of Magel- 

 lan, on the sea-shores of Patagonia* and La Plata, neither boulders nor beds of till 

 occur in the same latitudes, under which both are present on the Pacific. With re- 

 spect to the erratic masses, we may infer from what has been shown in the ascent 

 of the Santa Cruz, that their absence is owing to the wide space separating the shore 

 of the Atlantic from the Cordillera. In my Journal I have endeavoured to show 

 in detail, that, in the nortbern parts both of the Old and New World, and in the 

 southern parts of South America, the dispersal of boulders has been limited, in 

 approaching the tropics, to nearly the same latitudes, and that no true blocks of 

 this description have been observed in the inter-tropical regions f ; and we may 

 now be permitted to suspect that beds of ' till ' will be found to be confined to 

 the same parallels of latitude as the boulders. 



4. Remarks on the Glaciers of Tierra del Fuego, and on the Transportal of Boulders. 



In the remainder of this paper I will make a few remarks on the glaciers of Tierra 

 del Fuego, and on the means of transportal of the boulders. I had no opportunity 

 of landing on any glacier, but we passed in the Beagle and Magdalen channels 

 within two miles of several. The mountains were covered with snow, and the 

 glaciers formed many short arms, which descended to the beach, and terminated in 



* I may here mention, that on East Falkland Island, although situated in the same latitude with Tierra 

 del Fuego, and lying only 250 miles eastward of it, and with mountains above 2000 feet in height, I did 

 not observe any erratic boulders. As it may occur to some geologists that the island may have received 

 its chief elevation, subsequently to the period of the dispersal of the boulders on the main-land, I will ob- 

 serve, that the facts are directly opposed to such a view, for I could not find any elevated marine shells 

 on this island ; whereas I did not land on a single point of the coast of Patagonia, or of eastern Tierra 

 del Fuego, without meeting with them. 



f In my Journal (p. 289, and Appendix, p. 615), where I have considered the apparent exceptions to 

 this statement, I accidentally omitted one case. Near Rio de Janeiro I met some large-sized boulders of 

 greenstone, containing iron pyrites ; they were perfectly rounded, and therefore wanted that character of 

 angularity, which, though far from being always a concomitant, may, where it is present, be considered 

 9S eminently distinctive. I could not see the greenstone in situ in the immediate neighbourhood, but 

 the extreme rankness of the vegetation quite precluded accurate investigation. Mr. Caldcleugh (Tra- 

 vels, vol. ii. p. 195) observed greenstone boulders on the road to Villa Rica, and Spix (Travels, Eng. 

 Transl. vol. i. p. 272) observed others on the road to Santa Cruz. Mr. Fox, Minister Plenipotentiary at 

 Rio de Janeiro, informed me that he found similar blocks on the islands of St. Sebastian and St. Cathe- 

 rine, and at Port Alegre on the southern coast of Brazil. Nevertheless, it is not improbable that in all 

 these cases the parent rock was not far distant. I found two greenstone dykes near Rio de Janeiro : Von 

 Eschwege mentions others, and Mr. Fox observed one on St. Sebastian. Besides many obvious means 

 of transportal to moderate distances of large fragments of rock, now that geologists generally admit that 

 most countries have undergone slow oscillations of level, we must not overlook the power which the 

 surf during gales would have, on an exposed and gently inclined surface, of driving onwards blocks of 

 rock from the top of one line of beach to that of another, as the sea gradually encroached on the land. 



