MODIFICATION OF THEORY EXAMINED. 139 



all the characteristics of deposits which have been laid 

 down in lakes and lacustrine hollows. As some have 

 already yielded organic remains, a more extended 

 scrutiny will probably lead to the discovery of similar 

 fossils in those beds which are at present believed to 

 be unfossiliferous. 



Objection as to the number of Submergences. — It 

 has also been urged as an objection to the Physical 

 theory, that, according to it, there ought to have been 

 a greater number of submergences of the land during 

 the glacial epoch than is known to have taken place ; 

 for according to the theory there ought to have been 

 a submergence to a greater or lesser extent correspond- 

 ing to each ice period. Submergence ought regularly 

 to accompany glaciation, and emergence the disappear- 

 ance of the ice ; whereas geologists have detected only 

 about three such periods. 



In reply to this objection, it will not do to estimate 

 the number of submergences by the number of observed 

 raised beaches or well-marked terraces. These in most 

 cases must have resulted from a subsidence of the 

 land, and not from a rise of the sea due to a displace- 

 ment of the earth's centre of gravity. Oscillations 

 of sea level resulting from an alternate increase and 

 decrease of ice would not likely produce well-marked 

 terraces. In order to cut a terrace the sea must 

 continue for a long period at the same level ; but this 

 could hardly be expected in the case of a rise resulting 

 from the accumulation of ice ; for, according to theory, 

 the mass of the ice gradually increases till a maximum 

 is reached, when it then begins as gradually to decrease. 

 It is true that when the ice is near its maximum it 

 will change but slowly, and when at the turning point 

 it may remain stationary for centuries before any 

 sensible decrease takes place. In this case it might in 



