GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 129 



of Catamarca, where the beds are much indurated and were 

 involved in the Andean uphft, the other, of unconsohdated 

 materials, is at Monte Hermoso near Bahia Blanca on the 

 Atlantic coast. The very small proportion of northern ani- 

 mals in the Araucanian beds is surprising, but not more so than 

 the almost complete absence of South iimerican types in the 

 upper Miocene and lower Pliocene of the United States. Inter- 

 migration between the two Americas would seem to have been 

 a much slower and more difficult process than between North 

 America and the Old World, and the reason for the difference 

 is probably the greater climatic barriers involved in a migration 

 along the lines of longitude. Upper Pliocene is found in the 

 Tarija Valley of Bolivia and probably also in Ecuador, in both 

 of which areas the proportion of northern animals was very 

 greatly increased. 



II. Quaternary Period 



The Quaternary period was a time of remarkable geo- 

 graphical and climatic changes, which had the profoundest 

 and most far-reaching effects, partly by migration and partly 

 by extinction, upon the distribution of animals and plants, 

 effects which are naturally more obvious than those of earlier 

 geological events, just because they were the latest. It is cus- 

 tomary to divide the period into two epochs, (1) the Pleis- 

 tocene or Glacial, and (2) the Recent, which continues to the 



present day. 



1. Pleistocene Epoch 



When Louis Agassiz first suggested (1840) the idea of a time, 

 comparatively recent in the geological sense, when northern 

 and central Europe was buried under immense sheets of 

 slowly moving ice, like the "ice-cap" of modern Greenland, 

 the conception was received with incredulity. Nearly thirty 

 years passed before this startling theory gained the general 

 acceptance of geologists, but now it is one of the common- 

 places of the science, for no other hypothesis so well explains 

 the complicated phenomena of Pleistocene geology. One great 



