112 Geology of South America. 



It appears that these pebbles increase in size towards the 

 south, and at last pass into erratic blocks. These blocks, dis- 

 tributed in great abundance over the southern extremity of 

 the American continent, as they are over the northern extre- 

 mity both of the new world and of Europe, were not observed 

 by M. D'Orbigny, but they have afforded Mr Darwin a num- 

 ber of curious observations. The most northern point at which 

 they were noticed by this distinguished traveller, on the plains 

 of the eastern portion of South America, was on the banks of 

 the river Santa Cruz, in S. lat. 50°, 10', a latitude corresponding 

 to that in which the phenomenon of erratic blocks derived 

 from the north becomes much less intense in the northern 

 hemisphere. Erratic blocks are not met with in Patagonia 

 near the coast ; in ascending the river of Santa Cruz, they have 

 not been remarked nearer the coast than 112 English miles, 

 and that 74 English miles from the foot of the Andes at the 

 nearest point ; they consist of compact clay slate, a felspathic 

 rock, a very quartzose chlorite slate, and basaltic lava. They 

 are generally angular, and their dimensions are frequently 

 gigantic* 



What are the relations of these erratic blocks to the Pam- 

 pean loam \ The question is here the same as in regard to 

 Europe and North America, because the blocks and the loam 

 succeed each other in the same order of succession as we ad- 

 vance from the pole to the equator, and the blocks cease where 

 the loam commences.*!- 



Becent Alluvial Deposits. — The Pampean loam, although 

 very recent, is, nevertheless, not the newest of the deposits 

 which are spread over the surface of South America. It is 

 itself covered by deposits of two different kinds, but which 

 M. D'Orbigny regards as contemporaneous. On the great 

 Bolivian Plateau, and in the province of Moxos, there are thick 



* Darwin, on the distribution of the erratic boulders, and on the con- 

 temporaneous unstratified deposits of South America. ( Transactions of 

 the Geological Society ^ 2d Series, vol. vi. p. 415.) 



t Vide Report on the Memoir of M. de Castelnau, Comptes Rendus, 

 t. xvi. p. 535. 



