HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 257 
the drowning of Panama Bay. This resulted in marine littoral sedi- 
mentation within the indentations of both the coasts. This event took 
place in Pleistocene time. 
3. An emergence in Post-Pleistocene or recent times by which the 
former continental outline was partially but not entirely restored. 
Résumé or tam HISTORY or THE TROPICAL AMERICAN MAINLAND. 
We can now compare the biological deductions with the known ge- 
ologic facts. 
Obscurity of the Paleozoic Record. — It is impossible to make any 
serious deductions concerning the relations of the North and South 
American continents during those epochs preceding the Jurassic period, 
owing to the lack of data.! 
Possibility of Continuous Land in Jurassic and Cretaceous. Time, — 
There is really some foundation for an hypothesis that the continents 
were somewhere united shortly after the close of the Paleozoic. We do 
not mean to assert that that was the case, but the following evidence is 
worthy of serious consideration. ٠ 
Probable Absence of Known Jurassic Sediments on the Atlantic Sides of 
the North and South American Continents. — There is a possibility that 
the larger portions of both the North and South American continents 
were land areas during the Jurassic period. Not a single marine Juras- 
sic species has ever been found in the rocks of North or South America 
east of the Rocky Mountain front or Andean Cordilleras. From my 
researches on this subject, — unless the beds of the Wealdan epoch, 
which are classified with the Cretaceous by most authorities, represent 
the uppermost beds of the Jurassic, as Prof. O. C. Marsh maintains, — 
I am convinced that in the North American continent land persisted 
throughout the eastern half of the United States during the Jurassic, 
and that this land, which was subsequently invaded by the Cretaceous 
seas, was far greater than the present continental expanse. Тһе occur- 
rence of Wealdan littorals (Lower Cretaceous shore lines) in Brazil is 
also indicative of large land masses in the South American continent 
during the Jurassic period. 
1 W. В. Scott, in his recently published work entitled “An Introduction to 
Geology,” p. 89, 1897, says: “It has been suggested that [in Devonian time] a 
north and south ridge of land extended from Wisconsin all the way to South 
America, dividing the American seas into Eastern, Interior, and Western. .. . 
This suggestion has not yet been definitely confirmed, but it may represent the 
truth.” 
