272 The Realm of Nature CHAP. 



be looked on as a general result of innumerable minor 

 elevations and depressions. The following hypothesis of 

 the growth of continents is not to be looked on as an 

 established theory, but as a probable conjecture of the 

 relative order in which the various land - masses were 

 formed. Plate XIV., adapted from Professor J. Geikie's 

 maps, shows in the deepest tint the areas of the 

 World Ridges that are believed (although the evidence 

 is far from complete) to have projected above the 

 hydrosphere during the greater part of the period when 

 Palaeozoic rocks were being formed. They composed groups 

 of great islands clustered on the northern and scattered over 

 the southern parts of the World Ridges, between which warm 

 ocean currents would flow from the equatorial seas, and an 

 equable climate would reign over the whole land. In the 

 Mesozoic period the lands (shown in the second tint) were 

 far more extensive, but insular conditions still prevailed. 

 The deepened Abysmal Area drained the oceans from the 

 summits of the World Ridges, and the up-ridging of the 

 Continental Area raised wide tracts far above the sea. The 

 western and eastern edges of the great Eastern World Ridge 

 were clearly outlined, but the sea spread across its central 

 portion from east to west, and from north to south. The 

 Western World Ridge was developed similarly, land extend- 

 ing along its western and eastern edges in North America, 

 separated by a wide sea-channel from south to north, while in 

 the South American portion the central part of the existing 

 continent had appeared running almost from north to 

 south. In the Tertiary period there was an enormous 

 increase of upheaval over the World Ridges, and the 

 crests of them (lightest brown on map) everywhere emerged. 

 The sea still swept over the central* part of the Eastern 

 World Ridge from north to south and south-west, so that the 

 Indian Ocean was united with the Arctic Sea, and through 

 the wide Mediterranean with the Atlantic. Africa and 

 Australia were almost as extensive as at present. Britain 

 was separated from Scandinavia, and the south of Europe 

 formed a mountainous archipelago, amongst the islands 

 of which the Alps and Balkans were conspicuous. The 



