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 (Didclphys), which were represented by types more or 

 less resembling the living forms, but exceedingly reduced in size. 

 Together with the Peltateloidea {Pcltephilus), 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 Palaeopcltis, 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. 1 



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 



1 The Patagonian formation lias no species in common with the territory of Chile 

 (excepting the system of Lebu), as I have said, but there are some in the formation 

 immediately above, which is "known as the Santacrnzian. 



