260 NATURAL SCIENCE [October 
to other regions, where by means of successive transformations they 
have given rise to the different orders of mammals which live, or 
have lived, in all parts of the earth. But besides these primitive 
mammals, which have left no successors here to reach to our epoch, 
one also meets with the ancestors of those which to-day are charac- 
teristic of our country, such as the hystricomorphous rodents and 
the opossums (Didelphys), which were represented by types more or 
less resembling the living forms, but exceedingly reduced in size. 
Together with the Peltateloidea (Peltephilus), singular armadillos 
with variable, pointed, bony horn-cores above the snout, there were 
already armadillos almost similar to those now living, by the side 
of others very different called Palacopeltis, which gave rise to the 
Glyptodons of more modern periods, and sloths, generally small, but 
similar to those which later were destined to reach the gigantic size 
of the Mylodonts and Megatheria. 
In a sentence, at the end of the Secondary period there lived in 
the Argentine Territory not only the ancestors of the mammals 
which inhabit it now, but also of those which live in all parts and 
all climates of the world. 
The Secondary era closed and the Tertiary opened with a dis- 
turbance and a general change in the orography of the continents, 
and in the distribution of land and water. Great volcanic eruptions 
accompanied the elevation of the large mountain ridges previously 
only indicated, and the oceanic waters were shifted from north 
to south. The northern hemisphere was transformed into a con- 
tinental one, and the southern hemisphere into an insular and penin- 
sular one. The antarctic continent has remained split up, and the 
faunas of its different parts have thenceforward evolved separately. 
South America became reduced to an island of varying outline, and 
the ocean in this tremendous encroachment covered the territory of 
the Republic, rolling over the isolated sierras of the Pampa, reached 
as far to the west as the base of the first spurs of the Andes and 
the great mountain mass of the North-West. This land served as 
the refuge for the terrestrial mammals which were saved from the 
catastrophe. It was in the bottom of this ocean that the beds of 
the marine formation called Patagonian were deposited, which can 
be traced along the greater part of the Atlantic coast to the south 
of the Rio Negro, with a thickness at times of 300 metres, and 
corresponding to the middle or lower part of the Eocene period.? 
During the Upper Eocene period another great upheaval of the 
land or a retreat of the ocean took place, the territory of the Re- 
public rising again with its eastern shores more to the east than at 
‘The Patagonian formation has no species in common with the territory of Chile 
(excepting the system of Lebi), as I have said, but there are some in the formation 
immediately above, which is known as the Santacruzian. 
