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. The present position 

 of the outcrop of the oldest Cretaceous formations in Mexico (those 

 recently described 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- 



* Journal of Geology, Vol. III. No. 7, p. 861, October-November, 1895. 

 vol. xxvm. — no. 5. 8 



