PLANT MIGRATiONS ACROSS THE NORTH ATLANTIC OCEAN 187 



large eruptions took place in an area extending from Scotland to east Green- 

 land and a huge basalt plateau was formed. Fossiliferous deposits between the 

 basalt layers permit its dating. Since no deposits of Mid-Tertiary age are 

 known from this region, it is believed that the plateau existed as a land area. 

 The basalt plateau was later broken up by renewed volcanic activity in 

 Pliocene or Pleistocene times and only some remnants are preserved. 



Barth (1941, p. 8) has given this account of the geologic history of Iceland; 



Across Iceland, from the south to the north, there is a broad belt with mountains and 

 plateaus with sharp peaks and jagged crests. The rest of the country to the east and 

 to the west consists of flat-topped mountains made up of layers of solidified lava which 

 came up from the interior of the Earth abouc 50 million years ago, long before man 

 appeared on Earth. The material making up these mountains was molten lava, stream 

 after stream gushed out, gave ofl" the heat to the Cosmos, and became solid rock; 

 layer upon layer were formed and huge flat plateau mountains built up. In this way the 

 Old Iceland was born, a huge stone plateau which filled almost the whole of the North 

 Atlantic Ocean; Old Iceland was much larger than the Iceland we know today; it 

 stretched northwards and eastwards with a land connection to both Jan Mayen and the 

 Faeroes. 



After this, all became quiet for a while; the eruptions stopped and the lava retreated 

 slowly into the interior cf the Earth. There it remained for millions of years, until it 

 once again came back up to the surface. As the spring flood breaks the ice so that the 

 floes are broken and raised on end, thus were the plateau mountains of Old Iceland 

 broken up by new volcanic activity and the molten lava pressing upwards; from 

 innumerable cracks and crannies the lava again flowed out over the surface. The 

 foundations of the stone plateau gave way, and, like another Atlantis, Old Iceland sank 

 into the sea. What remains is chiefly the Faeroes and the large island now known as 

 Iceland. This happened less than a million years ago, and volcanic activity has con- 

 tinued with undiminished force ever since. (Translated from the Norwegian.) 



As may be seen, Earth's outline, based on geologic evidence, fits well the 

 results obtained by biogeographic reasoning. The idea that a land connection 

 existed between Europe and America as late as the Pliocene or the Pleistocene 

 must be said to rest upon strong foundations. 



Many questions remain unanswered, however. The most important con- 

 cerns the time when the connection was broken. The close relationships in 

 biota between Iceland, Scotland and Scandinavia suggest that this might have 

 happened in relatively recent times. The connection probably did not exist 

 during the Post-glacial climatic optimum (the Hypsithermal). Marine deposits 

 along the coast of Norway from this period contain molluscs of a southern 

 distribution type. If a connection between Iceland and Scotland had existed, 

 the Atlantic warm water could not enter the Polar Basin, and the effect of this 

 should be noticeable in the marine fauna. The connection between eastern 

 America and western Europe was probably not operative during the Glacial 

 Ages since the ice masses on Iceland and Greenland then ought to have formed 

 obstacles to plant and animal inigration. It seems unlikely that the connection 

 existed after the Last Glacial Age, but precisely when it was broken remains 

 an open question. The answer is of considerable evolutionary interest, since 

 it could afford us with a means to gauge rates of evolution. Also, the question 



