ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixix 



tions to our knowledge of the geology of South America, contained 

 in his account of his long residence and widely-extended observations 

 in that country, the recent work of Mr. Darwin contains a more 

 detailed and elalDorate description, a more critical examination, if I 

 may so express it, of the nature and probable origin of these modern 

 tertiary formations, than any we have yet had. 



The Pampean formation is throughout of a very uniform character, 

 consisting of a reddish, slightly indurated earth or mud, often, but 

 not always, including, m horizontal lines, calcareo-argillaceous con- 

 cretions or marl. Except in a few detached localities, it is unmixed 

 with gravel, and the traveller may pass over many hundred miles of 

 level surface without meeting with a single pebble, or discovering any 

 change in the nature of the soil. These marly concretions often unite 

 into irregular strata, and over very large tracts of country the entire 

 mass consists of a hard, but generally cavernous marly rock, resembling 

 the less pure freshwater limestones of Europe, and called by the in- 

 habitants Tosca rock. A microscopic examination has disclosed in it 

 fragments of shells and corals ; and Professor Ehrenberg, having ex- 

 amined specimens of it from diiferent localities, discovered twenty 

 different forms of infusoria, the greater proportion being of fresh- 

 water origin, but five identical with such as are fomid in brackish 

 water. It is remarkable that, except in some detached localities near 

 the coast, the Pampean deposit is almost entirely devoid of shells, 

 either marine, fluviatile, or land. Mr. Darwin states, that with the 

 exception of the Azara labiata, a living estuary shell, occasionally, 

 but rarely found, and sometimes imbedded in the tosca rock, this 

 formation, within the true limits of the Pampas, although of such 

 vast extent, aifords, as far as he knows, no instance of the presence of 

 shells. It exhibits here and there changes of colour, indicating 

 regular lines of stratification, always horizontal ; and although it has 

 been subjected to great and powerful elevatory forces, it nowhere 

 exhibits any irregular movements, nor is there any appearance of 

 much superficial denudation. 



It extends, uninterruptedly, nearly 800 miles from N.to S.,and about 

 400 miles from E. to W. In depth it varies from 30 to 100 feet. A 

 range of mountains, attaining a height of 3340 feet, rises in the midst 

 of the plain near the Rio Colorado, the Sierra Ventana, and the Pam- 

 pean deposit comes up nearly horizontally to the northern and southern 

 foot of these mountains, insinuating itself between the parallel ranges, 

 at a height, in this place, of 840 feet above the sea, indicating an 

 upward movement of the land in mass. The high plain round this 

 range sinks quite insensibly to the eye on all sides. Mr. Darwin 

 states that "round the Sierras Tapalguen, Guitru-gueyu, and be- 

 tween the latter and the Ventana, the tosca rock forms low, flat-topped, 

 clifp-bounded hills, higher than the surrounding plains of similar com- 

 position. From the horizontal stratification, and from the appearance 

 of the broken cliifs, the greater height of the Pampean formation 

 round these primary hills ought not to be altogether or in chief part 

 attributed to these several points having been uplifted more energeti- 

 cally than the surrounding country, out to the argillo-calcareous mud 



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