1897] THE SOURCE OE 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. 1 



The mammals which had taken refuge in the heights 

 turned to descend to the plain, but already many had become 

 extinct. The Hyracoidea, the Coudylarthra, 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 

 iSTotopithecidea of the Cretaceous {Notopithecus, Ewpithecops, etc.) had 

 been transformed into the Homuuculidea, 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 (Thcosodon, Proterothcrium, 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 

 bv 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 Eio Negro to the Straits 

 of Magellan. 2 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 



1 The 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 It 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. 



