54 E. Desor on the Post-Pliocene of the Southern States. 
St. Lawrence they reach a height of 390, and at Montreal as 
much as 470 feet. It is evident therefore, that the changes of 
level by which the shells were raised to their actual position in 
these various localities must have embraced a much wider range 
at the north than at the south. oe. 
Another and more striking difference between the two deposits — 
is afforded by the erratic boulders. It is well known that over 
all New England and the greater part of New York, including 
Long Island, there occur a quantity of boulders scattered over 
the surface or buried within the detrital formations; and more- _ 
over these boulders can all be traced back to some northern origin 
more or less remote, showing that they have travelled from north 
tosouth. But in proceeding farther south, they vanish gradually. 
There are very few in the vicinity of Philadelphia and Baltimore, 
or if they occur occasionally, as for instance along the Potomac 
at Washington and in the neighborhood, they are of a different 
kind and can be traced to some rock within the Alleghanies. 
Some of them contain even fossils characteristic of formations 
which occur beyond the Blue Ridge. 
Proceeding still farther south, to North and South Carolina, 
we lose sight of the boulders entirely ; Mr. Conrad states that he 
never saw a single boulder on the surface of the state of Alabama. 
But the stratified deposits with which they are associated at the 
north continue uninterruptedly. It is indeed not a little remark- 
able that the only difference which M. Tuomey recognizes be- 
tween the Post-pliocene and the diluvium consists in the absence 
of boulders in the former. “TI have,” says he, “nowhere in the 
state (South Carolina) seen one (boulder) a foot in diameter, not 
have I met with a single bed that I could refer to this formation 
(diluvium) along the tertiary plane from the Mississippi to the 
Potomac.”* The same author adds that, as we proceed south- 
wards, the “ materials become finer and finer. The large pebbles — 
are suc y gravel and coarse sand, and towards the coast 
they are replaced by the fine sands blown about by the winds.” — 
The question then will be asked how it happens, that both 
formations (Laurentian and Post-pliocene) being contemporane-— 
ous, the one contains boulders, whilst the other contains none? — 
Was there some barrier which prevented the northern boulders 
from proceeding farther south than New York and New Jersey? 
And if such a barrier existed, why do we not find any trace of 
it? Ordo the last boulders about New York indicate that the — 
agency by which they were transported had exhausted its power? 
But in that case they ought to decrease in size from north to 
south whereas we know that the boulders on Long Island are as 
voluminous and heavy as those occurring farther north through- 
tn pilntnaitnempintainin Secllntcamiisak sess ig . 
* Ibid, p89. 
