1397) THE SOURCE OF THE TERTIARY MAMMALIA 261 
the present time. Freshwater and atmospheric agencies accumu- 
lated on this newly-raised land the great Santacruzian formation, 
which, with a thickness of more than 200 metres, appears exposed 
in different parts of Patagonia, and especially in the region of the 
Rio Santa Cruz. 
The mammals which had taken refuge in the heights 
turned to descend to the plain, but already many had become 
extinct. The MHyracoidea, the Condylarthra, the Pyrotheria, 
and the Tillodontia had disappeared. Of the Notohippidea, pre- 
viously so numerous, there scarcely remained any trace. The 
Ancylopoda had diminished remarkably in size and number. The 
Notopithecidea of the Cretaceous (Notopithecus, Eupithecops, etc.) had 
been transformed into the Homunculidea, which are the direct 
ancestors of the monkeys of both continents. The Typotheria and 
‘Astrapotheria had also begun to decline. On the other hand, the 
rodents, the Plagiaulacoidea, the Sparassodonta, the Nesodonta, and 
the Litopterna (Theosodon, Proterotherium, etc.) had increased in an 
extraordinary manner, the same as the armoured and unarmoured 
edentates. The groups of the Glyptodons and the Megatheria were 
already perfectly developed, but with representatives of a com- 
paratively small size. 
The data concerning the period in question are still much 
confused, but we know that at the beginning of the Oligocene epoch 
the Argentine Territory suffered a fresh submergence, accompanied 
by new volcanic and tectonic disturbances. The sea flowed back to 
cover the greater part of the plain, while the lava streams thrown 
out by the submarine volcanoes formed the sheets of basalt which 
cover like a black shroud the older formations of the Patagonian 
slates. Later, during the beginning of the Miocene, impetuous tor- 
rents brought down from the rugged, rocky heights granite and 
porphyritic blocks, rocks of all kinds, which, beaten by the waves of 
the sea, formed that great deposit of boulders which covers the sur- 
face of Patagonia without break from the Rio Negro to the Straits 
of Magellan.? The inhabitants of the plains migrated again to the 
heights, many of them perishing, others adapting themselves to the 
new conditions. 
At the end of the Oligocene period the ocean made a retrograde 
movement, and took up the position it occupies more or less to-day, 
and the mammals returned to live on the plains, but again fewer 
than they had been. The Nesodonts, the greater part of the 
17The Santacruzian formation exhibits a considerable number of species of fossil 
mollusca which are also met with in the Tertiary system of Navidad in Chile, which 
proves that both formations belong more or less to the same geological period. 
2 Tt is this formation which has been designated under the name ‘‘ Tehuelche Forma- 
tion.” There have been recently found in it beds of fossil shells, which show that it is 
a marine formation, probably of the same epoch as the Tertiary system of Coquimbo in 
Chile. 
