hill: geology of Jamaica. 201 



its maximum during the Jurassic period.^ No marine formations cf 

 Atlantic origin representing the intervening periods of time between the 

 Permian and Wealden Cretaceous iiave anywhere been found east of the 

 Rocky Mountain front in the North American complex, south of the Black 

 Hills, where boreal Jurassic forms are found, which probably came from 

 the northwest.^ From similar data it is also evident that the northern 

 part of the South American continent likewise had eastward expansion in 

 Jurassic time. 



The distribution of fossiliferous marine Jurassic formations on the 

 Pacific slope of North America also shows that the border of the 

 Pacific Ocean at that time extended far eastward of its present posi- 

 tion. It is probable that the continental mass as a whole, practically 

 equivalent in area to the present one, occupied a position slightly east 

 of its present locus. In my opinion, the submerged bench of the 

 Atlantic coast of the United States represents approximately the eastern 

 expansion of the North American Jurassic land. How and in what 

 manner this theoretical eastern expansion of the Western Hemisphere 

 affected the Antillean and Caribbean regions is a question of great im- 

 portance, which can be only hypothetically answered. 



That the waters of the two oceans were completely separated along 

 the American Mediterranean region in Jurassic time by a narrow land 

 area connecting North and South America is indicated by the entire dis- 

 similarity of the Pacific and Atlantic faunas in the oldest Cretaceous 

 sediments, as has been often shown. Furthermore, the Pacific faunas 

 transgressed eastward in late Jurassic time far across the present site of 

 the Mexican Plateau, having been found in the longitude of Cuba. This 

 indicates that the continental bridge was then flxr east of its present 

 location. If the Jurassic fossils in Western Cuba, as reported by Lea, 

 should upon further study prove to be of a Pacific type, the Jurassic 

 Isthmus must have been situated east of the longitude of Havana. 



Another line of evidence indicates that the present isthmian region 

 presented no barrier between the oceanic waters prior to late Cretaceous 

 time, and that if a continental bridge then existed it must have been 

 located towards the Windward side of the American Mediterranean. 

 This is the testimony of the deep sea fauna of the Caribbean Sea. Ac- 



1 If the Wealden epoch is the top of the Jurassic instead of the base of the 

 Cretaceous, as asserted by Marsh, it does not materially alter this proposition. 

 The greater time of the preceding Jurassic is unrepresented so far as known by 

 fossils or sediments on the Atlantic side of the continent. 



2 The Jurassic rocks of Mexico and Trans-Pecos, Texas, all occur, so far as 

 known, to the westward of the east front ranges of the Cordilleras. 



