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980 [October 
VI 
South America as the Source of the Tertiary 
Mammalia ' 
F the Argentine Territory during the Archaean era there only 
existed the frame of the massive mountains of the north-west 
and a few points and islets, which to-day form part of the various 
isolated mountain chains which rise from the plain of the Pampa, 
from Salta to Patagonia. 
The oldest fossiliferous deposits of the first Palaeozoic epochs 
rest on these Archaean rocks: all the organisms are marine. In 
the latest times of the Palaeozoic era, during the Carboniferous and 
Permian periods, these small islands served as a nucleus for a 
greater extension of the land, and then great numbers of terrestrial 
organisms appeared, of a uniform aspect, precisely as the temperature 
in all parts of the globe was uniform. 
The deposits of the greater part of the Mesozoic era, with rare 
exceptions, are found in the Cordillera, where they appear on either 
side in the form of narrow bands running north and south, proving 
that then as now the Cordillera of the Andes already existed as a 
long and narrow land which separated the Atlantic from the Pacific. 
Both oceans reached the foot of the Cordillera, but in the Atlantic 
the mountain chains of Tandil, Ventana, Cordoba, San Luis, and 
various others formed large islands. At this time the geographical 
differences of temperature began to be felt, causing climatic zones, 
the most active of the factors which operate in the differentiation of 
organisms—a differentiation which allows us to determine the rela- 
tions of the floras and faunas of different regions, and to restore the 
routes which they followed in their migrations across the lands of 
other times, which are not the same as those of to-day, furnishing 
us with the data to reconstruct the ancient connections of the lost 
continents. 
We have now reached the latest time of the Cretaceous period, 
the most recent of those which constitute the Mesozoic era. Water 
predominated in the northern hemisphere, and land in the southern 
—the reverse of what happens at the present day. The European 
1Translated by Mrs Smith Woodward from ‘‘ La Argentina al través de las altima 
épocas geoldgicas,” an address delivered at the inauguration of the University of La 
Plata, April 18, 1897. (8vo., pp. 35. Buenos Aires: P. E. Coni & Sons, 1897.) 
