206 ISLAND LIFE 



time had been gradually expanding in each direction from 

 the .northern Azoic, eastward, westward, and southward, 

 and which, after the Palaeozoic, was finished in its rocky 

 foundation, excepting on the borders of the Atlantic and 

 Pacific and the area of the Rocky Mountains, had reached 

 its full expansion at the close of the Tertiary period. The 

 progress from the first was uniform and systematic : the 

 land was at all times simple in outline ; and its enlarge- 

 ment took place with almost the regularity of an ex- 

 ogenous plant."^ 



A similar develoj^ment undoubtedly took place in the 

 European area, which was apparently never so compact and 

 so little interpenetrated by the sea as it is now, while 

 Europe and Asia have only become united into one un- 

 broken mass since late Tertiary times. 



If, however, the greater continents have become more 

 compact and massive from age to age, and have received 

 their chief extensions northward at a comparatively recent 

 period, while the Antarctic lands had a corresponding but 

 somewhat earlier development, we have all the conditions 

 requisite to explain the persistence, with slight fluctua- 

 tions, of warm climates far into the north-polar area 

 throughout Palseozoic, Mesozoic, and Tertiary times. At 

 length, during the latter part of the Tertiary epoch, a con- 

 siderable elevation took place, closing up several of the 

 water passages to the north, and raising up extensive areas 

 in the Arctic regions to become the receptacle of snow and 

 ice-fields. This elevation is indicated by the abundance of 

 Miocene and the absence of Pliocene deposits in the Arctic 

 zone and the considerable altitude of many Miocene rocks 

 in Europe and North America ; and the occurrence at this 

 time of a long-continued period of high excentricity 

 necessarily brought on the glacial epoch in the manner 

 already described in our last chapter. A depression seems 

 to have occurred during the glacial period itself in North 

 America as in Britain, but this may have been due jDartly 

 to the weight of the ice and partly to a rise of the ocean 



1 Manual of Geology, 2nd Ed. p. 525. See also letter in Nature^ Vol. 

 XXIII. p. 410. 



