Maack.] 418 [June 1, 



extensive country of South America has increased among natural- 

 ists every year. During the last twenty years, especially, Prof. Her- 

 man Burmeister, the present director of the Museo Publico of Buenos 

 Ayres, and Mr. Aug. Bravard, for many years director of the 

 National Museum of Parana, have devoted themselves to a very 

 careful study of this region. During my stay in that country I 

 had also the opportunity of getting an insight into the nature of 

 those, for a naturalist, very interesting and instructive parts of the 

 world. I owe a good deal to Prof. Burmeister, and to many other 

 friends, for valuable information and assistance. If I therefore try 

 to present to the Society some general outlines of the geology of 

 the Argentine Republic, it is also my duty to return my best 

 thanks to those gentlemen. 



Unparalleled as the fact is, we find hardly another country in the 

 world which possesses in its geographical formation such a sharp cir- 

 cumscription as has the Argentine Republic. In the western part 

 we have the Cordilleras de los Andes, extending nearly from one end 

 of America to the other, and running more or less always in the 

 same meridional direction, that is to say, from the south to the north; 

 while in Europe and Asia, all the greater chains of mountains, with 

 few exceptions, have just the opposite direction, from the west to the 

 east, that is to say, the parallel direction. But this characteristic 

 direction of the Andes is one of those important causes which have 

 a determining influence upon the development of unorganized 

 nature, as well as upon that of plants, animals and even mankind. 



Almost everywhere in Europe and Asia we find the surface of 

 the globe divided by parallel chains of mountains into several 

 transverse sections, which have their own river systems, their 

 special geological, as well as botanical, zoological and ethnograph- 

 ical relations. In South America, and especially in the La Plata 

 States, we find just the contrary; there the meridional direction 

 and the very slightly inclined descent to the east have formed 

 large and extensive plains and rivers; the latter are augmented in 

 volume very much by the atmospheric moisture, which amounts 

 during the year to from 926 to 2920 millimetres; but they are separ- 

 ated from one another by comparatively low watersheds. The dis- 

 tribution of plants and animals also extends there over a larger area 

 than elsewhere, because the climatic and geological influences are 

 of a more uniform character; the general elevation above the 

 level of the ocean has, in this respect, a more direct and active 



