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 which 
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 Cordilleran Islands, 
and reaching westward towards the Sierra Nevada. 
The Rocky 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 
pre-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 rhyolitic 
tuffs of Cretaceous age occur in Panama. Near the highest pass of 
Costa Rica, 5,000 feet above the sea, in the neighborhood of San José, 
there are Upper Cretaceous limestones, with fossils of Antillean facies, 
which 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 Platean, 
in San Salvador, Panama, the Andes, and the Great Antilles. All these 
regions but the last were continental. 
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