mid contemporaneous Deposits of South America. 417 



in all probability it was submerged within, or nearly within, the post-pliocene 

 period. I am induced to form this inference from the presence of existing sea shells 

 in the valley, and from the extension far up it of step-like terraces which on the 

 sea-coast, certainly are of recent submarine origin. Round the estuary-like plain, 

 and between it and the great high plain, there is a second plain, about 800 feet 

 above the sea-level, and its surface consists of a bed of shingle with great boulders. 

 In this part of the valley, namely, between thirty or forty miles from the Cordil- 

 lera, there were, in the bed of the river, boulders* of granite, syenite and con- 

 glomerate, varieties of rock which I did not observe on the high plain ; and I parti- 

 cularly noticed that there were none of the basaltic lava. From this latter fact and 

 from several other circumstances, more especially from the immense quantity of 

 solid matter which must have been removed in the excavation of the deep and broad 

 valley, we may feel sure that the boulders on the intermediate plain and in the bed 

 of the river, are not the wreck of those originally deposited on the high plain. These 

 boulders, therefore, must have been transported subsequently from the Cordillera, 

 and after an interval during which the land was modelled into the form above 

 described. Those on the lowest plain must have been transported within, or not 

 long before, the period of existing shells. 



I have said that the first erratic block which I met with, was sixty-seven miles 

 from the nearest slope of the Cordillera; I must, however, record the case of one 

 solitary rounded fragment of feldspathic rock lying in the bed of the river, at the 

 distance of 1 10 miles from the mountains. This fragment was seven feet in circum- 

 ference, and projected eighteen inches above the surface, with apparently a large 

 part buried beneath it. As its dimensions are not very great, we may speculate on 

 some method of transportal different from that, by which the plain near the moun- 

 tains was strewed with such innumerable boulders ; for instance, of its having 

 been imbedded in a cake of river ice. Its solitary position is, however, a singular 

 fact. 



I met with erratic boulders nowhere else in Patagonia : Captain King, however, 

 states, in his " Sailing Directions," that the surface of Cape Gregory, a headland 

 of about 800 feet in height, on the northern shore of the Strait of Magellan, is 

 strewed with great fragments of primitive rocks. 



2. Tierra del Fuego and the Strait of Magellan. 

 The eastern part of Tierra del Fuego is formed of large outliers of the Patago- 



* I may observe, that it can be clearly shown (Journal of Researches, p. 216) that the river it- 

 self, although large and rapid, has scarcely any power in transporting fragments even of inconsiderable 

 size. 



