hill: geology of Jamaica. 205 



Isthmus of Panama was fully established at this epoch, as indicated by 

 the stratigraphy of the succeeding Upper Cretaceous epoch, during wliich 

 time events assume sufficient clearness to be more clearly interpretable 

 in the Antillean and Caribbean regions. 



In Upper Cretaceous time another subsidence ensued in North Amer- 

 ica. This produced the greatest known expansion of the Gulf of Mexico. 

 The Dakota littoral of the Cretaceous Gulf of Mexico transgressed the 

 Great Plains region from eastern Texas northward towards the British 

 line, almost if not quite connecting with the waters of the Pacific and 

 nearly separating the continent into Appalachian and Cordillerau Islands, 

 and reaching westward towards the Sierra Nevada. 



The Eocky Mountain or eastern area of the North American Cordil- 

 leran region, as far west as Utah, then became a submerged oceanic 

 region, with ridges and islets of the older formations. The deepest 

 deposits — the Niobrara chalks — could not be interpreted to indi- 

 cate greater depths in the United States than a thousand fathoms, 

 although the thickness of the sediments would indicate a subsidence of 

 thirteen to fifteen thousand feet (not counting the Laramie) in the 

 Rocky Mountain region, marked by deposition of littoral sands, car- 

 bonaceous shales, and the conspicuous Niobrara chalk horizon. All 

 these strata, except the latter, indicated the degradation of a vast 

 IDre-existing land to the westward. 



The known facts of paleontology indicate that a Central American 

 bridge existed during the latest Cretaceous epoch. There is no evidence 

 that the life of either ocean then passed that barrier, and old rhvolitic 

 tuff's of Cretaceous age occur in Panama. Near the highest pass of 

 Costa Rica, 5,000 feet above the sea, in the neighborhood of San Jose, 

 there are Upper Cretaceous limestones, with fossils of Antillean facies, 

 wdiich show that the Caribbean Sea at that time had encroached at this 

 locality far across the present Central American barrier. Similar lime- 

 stones have also been reported from Guatemala by Sapper. These facts 

 indicate Caribbean conditions in late Cretaceous time in portions of what 

 are now the summit regions of Central America, and that the Cretaceous 

 land barrier, if one existed, was then situated in a region now covered by 

 the waters of the Pacific to the south of the present Central American 

 land. 



Vulcanism was active in the Coastal Plain and Cordilleras of western 

 Texas, northern Mexico, along the southern end of the Mexican Plateau, 

 in San Salvador, Panama, the Andes, and the Great Antilles. All these 

 regions but the last were continental. 



