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 uot entirely restored. 



Resume of the History of 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. 1 



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. 



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. 0. 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. The 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. B. Scott, in his recently published work entitled " An Introduction to 

 Geology," p. 39, 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." 



