SUBMARINE DEPOSITS 199 



Ocean was formed during Tertiary times by successive 

 sinkings of large areas of a pre-existing land surface. The 

 present isthmus of Panama was formerly a waterway 

 between the Atlantic and the Pacific, and a great sea once 

 extended through an enlarged Mediterranean across what is 

 now the south of Asia and northwards along the line of the 

 Caspian Sea through Russia to join the Arctic Ocean. The 

 mountains of Tjrrol, now 10,000 feet above the sea, once lay 

 submerged beneath it bearing coral reefs and shallow lagoons ; 

 and many other extensions of the sea into what are now 

 continental areas have come and gone. 



Restorations of the distribution of land and sea, more 

 or less well established, have been made by geologists for 

 each great geological period, and they show that portions of 

 the continents have one after another sunk beneath the waves 

 and then reappeared as dry land. This has happened time 

 after time, and so although sizes and shapes and land 

 connections have varied through the ages, the main contin- 

 ental masses have persisted in parts and in some form. 

 Similarly, notwithstanding repeated oscillations, extensions 

 and restrictions, some parts of the great ocean basins 

 have probably remained as permanent depressions on the 

 earth's surface since very early times, and may possibly be 

 relics of the original wrinkles on the cooling and contracting 

 skin of the molten globe. 



The most recent speculation bearing on the possible past 

 history of the oceans is Wegener's hypothesis of the wander- 

 ing or drifting apart of the present continents from an 

 original continuous land mass which covered about half the 

 globe in Carboniferous times. Suess had previously shown 

 that there was reason to believe that the crust of the earth 

 may be divided into a more superficial and lighter but more 

 rigid layer (the "Sal"), which forms the continental areas, 

 and a deeper and denser but more plastic mass (the " Sima ") 

 which underlies the continents and comes close to the surface 

 on the fioors of the oceans. Wegener supposes the present 



