Boulders and Unstratified Deposits of South America. 537 



feet. 



1. Gravel, or well-rounded shingle, coarsely stratified, bear- "I 212 

 ing chiefly on its surface great angular erratic blocks . . J 



2. Basaltic lava 322 



3. Variously coloured thin strata, the lower ones contain- "| 



ing minute pebbles of the same nature as the boulders, > 588 

 with the exception of the lava J 



1122 

 Bed of the Santa Cruz, above the level of the sea 280 



1402 



The shingle bed (1.) extends uninterruptedly to the coast, where 

 it is certainly of submarine origin ; and from the general similarity 

 of its nature, Mr. Darwin is of opinion, that it was all accumulated 

 under the same circumstances. The contrast in the means of trans- 

 port between the deposits (3.) and (1.), the former consisting of fine 

 particles and the latter of large pebbles and immense blocks of the 

 same rocks with the former, is noticed by Mr. Darwin as an inter- 

 esting circumstance. 



The valley of the Santa Cruz widens, on approaching the Cordil- 

 lera, into an estuary-like plain, which has an elevation of only 440 

 feet ; and it is believed by Mr. Darwin to have been submerged 

 within the post-pleiocene period, because existing sea-shells were 

 found near the mouth of the plain, and because terraces, which, near 

 the coast, certainly are of recent submarine origin, extend far up the 

 valley. Around this estuary-like plain, and between it and the 

 great high plain, is a second plain, 800 feet in height, the surface 

 of which', as well as the bed of the river in this part, consists of 

 shingle with great boulders. Some of these are of granite, sienite 

 and conglomerate, rocks, which were not observed by Mr. Darwin 

 on the high plain ; and on the contrary, the boulders of basaltic 

 lava which were so numerous there, were entirely absent from this 

 lower plain and the river-course. From these circumstances, and 

 likewise from the immense quantity of solid matter which must have 

 been removed in excavating the valley of the Santa Cruz, the author 

 infers that the boulders on the intermediate plain and in the bed 

 of the river, between 30 and 40 miles from the Cordillera, are not 

 derived from the wreck of the high plain, but were transported from 

 the Cordillera subsequently to the modelling of the country, and 

 within, or not long before, the period of existing shells. 



Mr. Darwin did not observe erratic blocks in any other part of 

 Patagonia, but he states, on the authority of Capt. King, that large 

 fragments of primary rocks occur on the surface of the great plain 

 which terminates at Cape Gregory, in the Strait of Magellan. 



Tierra del Fuego, and Strait of Magellan. — The eastern portion of 

 Tierra del Fuego is formed of large outliers of the Patagonian form- 

 ation, fringed by deposits of more recent origin. These lower 

 plains, varying in height from 100 to 250 feet, have been elevated 

 within the post-pleiocene period ; and thev consist of finely grained 



Phil. Mag. S. 3. Vol. 19. No. 127. Suppl. Jan. 1 842. 2 N 



