68 THE JOURNAL OF GEOLOGY. 



recurrent ice epochs, since the second advance of the ice might 

 have destroyed all trace of the preexistent soil and its vegetal 

 life. It is always possible, too, that such beds exist, even if they 

 have not been discovered. It would have been anticipated 

 that they would not be abundant, or wide spread. The'absence 

 of forest beds is therefore at best no more than negative 

 evidence. 



(2) Remains of La?id Animals. Bones of mammalia or 

 remains of other land animals, occurring in relations similar to 

 those in which forest beds occur, may have a like significance. 

 Their value as a criterion of separate glacial epochs is subject 

 to essentially the same limitations as forest beds. 



(3) Inorganic Products formed during a time of Ice Recession. 

 The recession of the ice after a maximum of advance would 

 leave a land surface more or less affected with marshes and 

 ponds. In such situations, bog iron ore might accumulate, if 

 conditions were favorable. Such ore beds, buried by the drift of 

 a later ice advance, would have a significance comparable to that 

 of forest beds, except that they would give less definite informa- 

 tion as to climate, and would be correspondingly less trustworthy. 

 Should such ore beds be found in such relations as to prove that 

 the underlying and overlying bodies of drift were deposited by ice 

 sheets which extended great distances further south, their signifi- 

 cance would be enhanced. From the thickness of the ore beds 

 some inference might be drawn as to the length of time con- 

 cerned in their accumulation. But because of the variable rate 

 at which bog ore may accumulate, such inference should be 

 used with caution. 



Concretions of iron oxide might be formed in the marshes or 

 in ill-drained drift areas where accumulations of greater extent 

 were not made. A subsequent incursion of the ice might incor- 

 porate these nodules with its drift, wearing and striating them 

 as other stones, and depositing them as constituent parts of the 

 later drift. Such iron nodules in the later drift would mean a 

 recession and re-advance of the ice with some considerable inter- 

 val between, although not necessarily an interval sufficiently 



