[ 415 ] 



XXVII. — On the Distribution of the Erratic Boulders and on the Contempo 

 raneous Unstratified Deposits of South America. 



By CHARLES DARWIN, Esq., M.A., F.R.S. and F.G.S. 



[Read April 14th, IS^l.] 

 Plate XL. 



CONTENTS. 



1. Description of the Boulder Formation in the Valley of the 



Santa Cruz p. 415 



2. in Tierra del Fuego 



and the Strait of Magellan p. 417 



3. in the Island of Chi- 



loe p. 423 



4. Remarks on the Glaciers of Tierra del Fuego and on the 



transportal of Boulders p. 427 



1 . Boulder Formation in the Valley of the Santa Cruz. 



UURING the survey of the shores of South America, southward of the Rio Plato, 

 by Capt. FitzRoy in H.M.S. Beagle, I did not meet with any boulders on the 

 eastern plains of the continent until we arrived on the banks of the river Santa 

 Cruz, in lat. 50° 10' S. Nor did they occur there near the coast, but were first 

 noticed in ascending the river at the distance of about 100 geographical miles from 

 the Atlantic, and 67 from the nearest slope of the Cordillera. Twelve miles further 

 west, in Ion. 70° 50' W., that is, fifty-five miles from the mountains, they were 

 extraordinarily numerous ; consisting of compact clay-slate, feldspathic rock, a 

 quartzose chloritic schist, and basaltic lava ; and they were generally of an angular 

 form, and many of them resembled fragments of rock at the foot of a precipice. 

 The size of some was immense : I measured a square one of chloritic schist, which 

 was five yards on each side and projected five feet above the ground ; a second, 

 which was more rounded, was sixty feet in circumference, and stood six feet above 

 the ground ; how much of each was buried beneath the surface I could not ascer- 

 tain. There were innumerable other fragments from two to four feet square. The 

 vast open plain on which they lay scattered, is 1400 feet above the level of the 



3 H 2 



