and contemporaneous Deposits of South America. 419 



and are from one to four feet in diameter ; the outline being irregularly an- 

 gular, with only the edges blunted. In the other places above mentioned, and like- 

 wise at the base of the mountains on the line of coast extending south to Port 

 Famine, boulders are numerous on the sea-beaches. Although I saw only two in 

 the cliffs, yet as the boulders do not, as far as I was able to observe, occur scat- 

 tered on the surface of the ground, and as a large area has evidently been denuded, 

 I concluded that most of the blocks were originally enclosed in the deposit, and that 

 after they were washed out, they had been driven onwards by the surf during gales, 

 and collected at the foot of the retreating cliffs. At St. Sebastian's Bay, however, 

 on the east coast of Tierra del Fuego, this explanation is scarcely applicable, for 

 many gigantic boulders there lie in a protected position at the base of a naked 

 cliff about 200 feet in height, and entirely composed of thin strata of fine-grained 

 sandstone, with a few layers of small, well-rounded pebbles. As it is very impro- 

 bable that the boulders were ever included in a deposit of this nature, we must 

 suppose that they were originally thrown down either on the surface or in a thin 

 superficial bed, which has subsequently been removed. I may specify, that one 

 of these boulders, composed of syenite and shaped somewhat like a barn, was 

 forty-seven feet in circumference, and projected about five feet above the sand- 

 beach. There were many others half this size, and they all must have travelled at 

 least ninety miles from their parent rock. 



The position of the boulders in St. Sebastian's Bay is, in another respect, in- 

 teresting ; for the form of the land clearly shows, that long anterior to the total 

 amount of elevation attested by upraised recent sea shells, a wide channel (indeed, 

 introduced in all the charts before the voyage of the Beagle) connected the middle 

 part of the Strait of Magellan with the open sea. During the same period, a 

 very low neck of land near Cape Negro, now strewed with boulders and bordered 

 by cliffs of the unstratified deposit, must have formed a straight channel between 

 the great land-locked bay called Otway Water, and the eastern arm of the Strait 

 of Magellan. Shoal Harbour, which lies in this line, is scattered over with enor- 

 mous angular fragments of rock, projecting from five to eight feet above the level of 

 the sea, and giving to it a singular appearance. The unstratified beds of hardened 

 mud and whitish earth, containing a few boulders and numerous smaller angular as 

 well as rounded fragments, occur only in the neighbourhood of the Strait of Magel- 

 lan, and are probably connected in their origin with the existence of an ancient 

 channel which had nearly the same direction with the present one : so also it is 

 evident, that the distribution of the numerous great boulders now lying on the sur- 

 face (whether or not all were originally imbedded in the unstratified deposit) is like- 

 wise connected with the course of formerly existing sea-channels. The currents 

 off Cape Horn set almost constantly from the west, as is known to the cost of 



