£. Desor on the Post-Pliocene of the Southern States. 59 
they occur in a bed of mud,” seems also to justify the supposition 
that they may belong to a later period. 
nthe other hand, we know that the remains of mastodon | 
and elephants are found associated with land and freshwater 
shells in the blufis of the lower Mississippi, and that on that ac- 
count these bluffs have been parallelized by geologists with the 
river terraces of the North which we know to be more recent 
than the Laurentian and to belong to the alluvium. Should it 
be proved that the deposits with mastodon bones throughout the 
states of Georgia and South Carolina do not belong to the true 
Post-pliocene, but are, like those of the North, of a more recent 
origin, there would no longer be any difficulty in the way o 
identifying the Laurentian and the Post-pliocene as one and the 
same formation, whereas the beds containing mastodons at the 
South, together with the river terraces of the North and* the 
valley drift of the West, including the loam bluffs of the Lower 
Mississippi and the celebrated deposits of Big-Bone Lick, would 
represent another more recent period, that of the alluvium, which 
passes gradually into the historical period, if it does not actually 
ong to it. 
But should it ever be shown that the mastodon remains of the 
State of Georgia are really associated with true Post-pliocene 
Shells, it does not by any means follow that it would contradict 
our views as to the parallelism between the Laurentian of the 
North and the Post-pliocene of the South. The question might 
. then be asked whether the absence of remains of land animals at 
the North could not be accounted for, merely by supposing that 
the changes of level which were going on at that time were too 
considerable, and the extent of dry land at the time of the greatest 
depression of the Laurentian sea too small to support such huge 
animals, whereas ‘the extensive tertiary plains of the southern 
States, which were then dry land, might well have afforded a con- 
Venient home for them. In this case it would appear that they 
subsequently migrated to the North, where at the beginning of 
the alluvial period, the area of dry land began to be sufficiently 
extensive, 
hese are problems which are well worthy the attention of 
American geologists. 
