THE NEOZOIC AGES. 277 



This table will suffice at least to reduce tlie great 

 glacier controversy to its narrowest limits, when we 

 have added the one farther consideration that glaciers 

 are the parents of icebergs, and that the question is 

 not of one or the other exclusively, but of the relative 

 predominance of the one or the other in certain given 

 times and places. Both theories admit a great Post- 

 pliocene subsidence. The abettors of glaciers can 

 urge the elevation of the surface, the supposed 

 powers of glaciers as eroding agents, and the trans- 

 port of boulders. Those whose theoretical views lean 

 to floating ice, believe that they can equally account 

 for these phenomena, and can urge in support of their 

 theory the occurrence of drift wood in the inland clay 

 and boulder clay, and of sea-shells in the marginal 

 clay and boulder clay, and the atmospheric decomposi- 

 tion of rock in the Pliocene period, as a source of the 

 material of the clays, while to similar causes they can 

 attribute the erosion of the deep valleys piled with 

 the Post-pliocene deposits. They can also maintain 

 that the general direction of striation and drift im- 

 plies the action of sea currents, while they appeal to 

 local glaciers to account for special cases of glaciated 

 rocks at the higher levels. 



How long our continental plateaus remained under 

 the icy seas of the Glacial period we do not know. 

 Relatively to human chronology, it was no doubt a 

 long time ; but short in comparison with those older 

 subsidences in which the great Palaeozoic limestones 

 were produced. At length, however, the change 



