and contemporaneous Deposits of South America. 429 



extremity of the glacier, and is in parts covered by old trees : hence we must 

 infer, that the glacier formerly extended considerably further than it now does. 



It would be useless even to allude to the difficulties which affect every theory 

 of the transportal of erratic boulders, excepting that by the agency of ice ; 

 but after the remarkable discoveries of Venetz, Charpentier, Agassiz, and others, 

 of the great extension in Europe of moraines formed by ancient glaciers, it is 

 necessary to observe, that neither the ' till ' beds of eastern Tierra del Fuego, 

 which pass into and are regularly interstratified with a great formation of horizon- 

 tally laminated sandstone, containing marine remains ; nor the stratified gravel 

 and till, which form low plains on the shores of Chiloe, and cap in regular beds 

 the tertiary strata, can have been produced like ordinary moraines ; and, therefore, 

 that the imbedded boulders cannot have been propelled by the glaciers them- 

 selves. I am led to the same conclusion with respect to the till of southern 

 Tierra del Fuego, which forms a level plain and a fringe around several islands, and 

 which in one part passes into a regularly stratified deposit. The boulders on the 

 lower levels at the head of the Santa Cruz river are strewed on land, which cer- 

 tainly has been modelled by the action of the sea. Those on the 1400 feet plain 

 are sixty-seven miles from the Cordillera, of which the highest pinnacle is only 

 6400 feet, and the general range considerably lower ; this little inclination of the 

 surface, with the absence of mounds or ridges on it, and the angularity of the frag- 

 ments, are opposed to the notion that the blocks have been pushed to this great 

 distance by glaciers. Hence I conclude, that in the two first-mentioned districts it 

 is quite certain, and in the three latter highly probable, that the boulders were 

 transported by floating ice. 



The fact of many of the blocks on the northern end of Chiloe being different 

 from those thirty miles southward, where there must anciently have been a chan- 

 nel across the island, is not opposed to the foregoing conclusion : for the tidal 

 currents must have drained, according to the number and position of the seaward 

 channels, determinate spaces of the area between Chiloe and the Cordillera ; and 

 according to the situation of the spot whence the iceberg with its cargo of rock was 

 first launched, so would it be swept towards one or the other channel. The varying 

 winds, no doubt, would partly influence the course of icebergs, but, from their 

 floating very deeply, the currents would act far more powerfully on them. Nor is 

 the circumstance of the boulders on the high and low plain of Santa Cruz being 

 of different kinds of rock any difficulty ; for after the change of level in the land, 

 necessary to account for the existence of the lower plains, we might have anticipated 

 that some of the glaciers which formerly debouched on the coast would cease doing 

 so ; and that rocks hitherto submerged beneath the sea would become exposed, 

 and their fragments falling on the glaciers would be transported with the icebergs. 



VOL. VI. — SECOND SERIES. 3 K 



