374 The Ainenca7l Geologist. December, 1898 
same fauna and the difference becomes more marked if one 
deposit is marine and the other terrestrial. 
The Cretaceous period in Europe was followed by eleva- 
tion over wide regions, but because the elevation was gradual 
organisms were often able to migrate; but according to our 
present knowledge this place of transference is unknown. On 
the Atlantic slope, however, deposition went on over a grad- 
ually sinking area of the sea floor and the Eocene marls were 
laid down conformably on the underlying Cretaceous. In 
reference to this professor Dana in his revised Manual says 
(p. 821): "The upper Greensand group graduates without a 
break in the stratification into the overlying Eocene Tertiary 
as if its formation were like the upper Laramie, the closing 
work of the Cretaceous period." 
It is quite possible that a cold current from the north may 
have swept along the Atlantic border at the close of the Cre- 
taceous and either exterminated the mollusca or else driven it 
farther south though I am not certain that we have any proof 
of this. It is evident that professor Dana believed in this cold 
current acting as a barrier for on page 877 of his revised 
Manual he states: "If the change had made the arctic waters 
only 15° F. colder than they were during the Cretaceous per- 
iod, the polar waters as they flowed southward, would proba-. 
bly have been exterminating to the greater part of the life of 
coast regions all along the shallower waters, and down to such 
depths as the cold current reached. Such a cause might make 
a complete break in the succession of species in a region, with- 
out any break in the succession of beds, as happened in New 
Jersey." 
The writer believes that elevation of the coast line along 
the western border of the New Jersey Cretaceous early in Eo- 
cene time and a deepening of the waters farther south was 
one of the chief causes for this destruction of life although 
it is quite probable that cold currents coming southward may 
have completed this destruction. A study of the Eocene de- 
posits of Maryland reveals the fact that the formation in- 
creases in thickness toward the southwest like a letter V turned 
sideways. The apex of the V was near the western 
