HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 261 
of the Atlantic and Pacific were probably separated in the Tropical 
American region during this epoch. 
While it is probable that a land barrier existed between the two 
oceans in Cretaceous time as now, we do not wish to be committed to 
the assertion that the geographic outline of this barrier is in any way 
recognizable in the present conformation of the Isthmian areas, for cer- 
tain data lead us to believe that subsequent revolutions may have com- 
pletely changed its location, if it ever existed. 
An illustration of the probability of the vast changes of the land 
since the beginning of Cretaceous time is found in Mexico. "The present 
area of this Republic was a great battle ground in Cretaceous time be- 
tween the migrations of the waters of the Pacific and the Atlantic, but 
from the character of the faunas it is apparent that this barrier, though 
migratory, was constantly maintained. The sedimentary formations, 
with the exception of small exposures of the Paleozoic in the extreme 
northwest and a narrow fringe of Tertiary along the shore of the Mexican 
Gulf, are predominantly of the Cretaceous period. Тһе present position 
of the outerop of the oldest Cretaceous formations in, Mexico (those 
recently deseribed as in part Jurassic by Castillo and Aguillera from 
Catorce) shows that the waters of the Pacific in the latitude of the 
Tropic of Cancer reached eastward across the Mexican peninsula at the 
beginning of Cretaceous time, nearly to the locus of the present shore of 
the Gulf of Mexico, and were opposed in that direction by the Jurassic 
land; while in Middle Cretaceous time, however, along the boundary 
region between Mexico and the United States, the Atlantic waters ex- 
tended westward across the Republic far into the State of Sonora and 
within a few degrees of the present Gulf of California. This peculiar 
shifting of geographic relations in Cretaceous time has been well stated 
by Mr. Stanton : — 
** With the occurrence of the Pacific Lower Cretaceous fauna at Catorce, not 
a great distance from the Gulf of Mexico, and of the Texas or Gulf fauna in 
Sonora, much nearer to the Pacific, the question as to how the two faunas were 
kept separate becomes still more difficult. From the data now at hand the 
most plausible hypothesis seems to be that the sea transgressed. the continent, 
first from one side, and then from the other, but never quite crossed the 
shifting barrier,” 1 
So that we may conclude that the waters of the Atlantic and Pacific 
were probably as completely separated by a great continental land bar- 
1 Journal of Geology, Vol. III. No. 7, p. 861, October-November, 1895. 
VOL, XXVIII. — NO. б. 8 
