HILL: GEOLOGY OF JAMAICA. 201 
its maximum during the Jurassic period. No marine formations of 
Atlantic origin representing the intervening periods of time between the 
Permian and Wealden Cretaceous have anywhere been found cast 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 Caribbeán 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 far 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 oceanio waters prior to late Cretaceous 
timo, 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 Tf 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. 
