580 TIMOTHY WILLIAM STANTON 
laid down the sea had retreated from most of the continental 
area and it has never since invaded it beyond the coastal border 
regions. It is a noteworthy fact, as Neumayr* and others have 
pointed out, that there was a similar invasion of the sea upon 
the other continents in mid-Cretaceous time. 
The occurrence of contemporary marine, fresh-water and 
brackish-water formations has greatly complicated the classifica- 
tion and correlation of the Cretaceous beds. Deposits formed 
under such diverse conditions naturally have few features in 
common, either lithologically or paleontologically, and their 
correlation must usually depend on similarity of stratigraphic 
and structural relations with formations of known age. Ina few 
cases, however, the same flora and other land organisms have 
been preserved in both marine and fresh-water beds, and have 
thus demonstrated their practical contemporaneity. 
Besides the sharp contrasts between marine and non-marine 
beds there are several distinct facies within the marine Creta- 
ceous formations. Whether the paleontological differences are 
due to climate, to isolation, to differences in depth or in the 
nature of the sea bottom, are questions that should be solved 
independently in each case, but their solution is usually diffi- 
cult. The first essential is to determine that the formations 
compared are actually contemporaneous or homotaxial. A 
failure to do this has led to serious errors in the past. For 
example, Roemer? noticed that the Cretaceous fauna of New 
Jersey is very different from that found in Texas in beds that he 
supposed to be contemporaneous. He also noticed that the 
former corresponded rather closely with the Cretaceous fauna of 
northwestern Germany, while the Texan fauna found its nearest 
analogues in the Cretaceous of southern Europe bordering on 
the Mediterranean. He concluded, therefore, from these geo- 
graphic relations that the faunal differences were due mainly to 
climate, that the present climatal zones were already established 
in Cretaceous) time and. even {that “the ocean ‘currents had 
t Erdgeschichte, Bd. 2, p. 377. 
2 Am. Jour., Sci. 2d. ser., Vol. II, 1846, p. 364. Kreideb. v. Texas, 1852, p. 25. 
