100 ('. Schuchert — Problem of Continental Fracturing 



tations are the marsupial or pouch mammals, which in many 

 ways parallel the placental evolution, and the far less diver- 

 sified, more primitive, but more remarkable egg-laying mono- 

 tremes. All seemingly had their origin in the continents of 

 the northern hemisphere. The marsupials were at their culmin- 

 ation in the Pliocene, when forms existed larger than any 

 living rhinoceros {Diprotodon austraUs). From the chrono- 

 genesis of these stocks and their diverse evolution in Australia, 

 we learn that they must have been on that continent long before, 

 and that they had been free from all northern invasions and 

 hence escaped destruction by the higher, more intelligent, 

 carnivorous placental mammals. We must, therefore, conclude 

 that Australia has been an island continent at least since late 

 Eocene time, for it is since then that the placental mammals 

 have elsewhere dominated all other land life. 



The question next arises, When was Australia severed from 

 Asia ? From the paleogeographv as now deciphered, we learn 

 that Asia and Australasia were in complete connection during 

 most of the time from the Cambrian to the close of the Devon- 

 ian. In the Lower Carboniferous, however, southeastern Asia 

 began to be invaded by the Indian and Pacific oceans in the 

 region of what are now the East Indian Islands, and this inva- 

 sion continued into the Pennsylvanian, after which there was 

 again connection lasting into Triassic time. A greater areal sub- 

 sidence then occurred in southeastern Asia, ISTew Guinea, New 

 Caledonia, and elsewhere in Australasia; it began in the 

 Jurassic and probably was repeated in Comanchian and Cre- 

 taceous times. However, from the fact that carnivorous dino- 

 saurs — land reptiles that arose in the northern hemisphere 

 either late in the Permian or shortly afterward — are known in. 

 the Triassic of Australia (none at all occur in ISTew Zealand), 

 we must conclude that there was still at this time an inter- 

 mittent land-bridge connecting this continent with Asia. The 

 time of more complete severance apparently came in the Juras- 

 sic, though intermittent land connections probably persisted 

 to the end of Cretaceous time. The troughs of separation that 

 distinguish the Asiatic land-masses from the Australasian ones 

 seem to be the present Molucca-Banda-Arafura seas, which have 

 depths varying between 4,650 and 21,000 feet (see fig. 1). 



The "Asiatic system" of tectonic structures takes in not 

 only the Asiatic continent, but also the outlying series of island 

 arcs and the greater part of the East Indies, as Sumatra, Java, 

 the islands east of and including Timor, Borneo, Celebes, and 



