192 bulletin: museum of comparative zoology. 



1. There is no evidence that vulcanism has in any way affected the 

 Atlantic Coastal Plain east of the Sabine within the periods of geologic 

 history bearing upon our problems, and hence the northern border region 

 of the American Mediterranean may be considered as having been be- 

 yond the zones of volcanic disturbances affecting the Caribbean region. 



2. Vulcanism in Cretaceous time undoubtedly affected all the periph- 

 eral regions of the American Mediterranean except the Coastal Plain 

 of the Gulf, including the Great Antilles and Virgin Islands, in which 

 detrital igneous rocks similar to those we have described as constituting 

 the oldest formations of Jamaica occur under similar conditions. I have 

 seen these old Cretaceous igneous rocks in Cuba, Gabb has described 

 them from San Domingo, and Cleve has pointed out their wide extent 

 in Porto Rico, the Virgin Islands, and St. Bartholomew. In all these 

 Antillean localities, as in Jamaica, the rocks are of a hornblendic nature, 

 occurring largely as conglomerate and tuffs. In fact, the closing days 

 of the Cretaceous were essentially marked by vulcanism in the Great 

 Antilles. In my report upon Panama I have shown that an old rhyolitic 

 or andesitic tuff of probable Cretaceous age, but not hornblendic, there 

 constitutes the oldest discovered formation. 



The occurrence of vulcanism in Cretaceous time in the Andean and 

 Central American region has been shown by many writers. In the 

 Cordilleras and plateaux of Northern Mexico the closing days of the 

 Cretaceous were marked by vast extrusions of volcanic rocks, while 

 volcanic action also sparsely occurred in Trans-Pecos, Texas, and perhaps 

 as far north as Little Rock, along the interior margin of the Coastal 

 Plain during this epoch. 



In Eocene time vulcanism ^vas especially violent in the Isthmian, 

 Central American, and Colombian regions, and along the south margin 

 of the Mexican Plateau. The volcanoes of the Caribbee Islands were 

 also most probably active in this period. 



There is no evidence that vulcanism occurred in the Antilles or Virgin 

 Islands during the Eocene epoch. On the other hand, all data tend to 

 show that the great eruptive activity of Cretaceous time in the Antilles 

 was followed by epochs essentially marked by placid sedimentation. 



The chief Panamic eruptions ceased at or soon after the close of the 

 Eocene, although vulcanism continued in the adjacent Costa Rican and 

 Andean provinces, and along the Mexican volcanic belt, until the present 

 time. On the eastern slope of Costa Rica the Vicksburg fossils of the 

 Guallava formation are interbedded with contemporaneous basic eruptive 

 debris. 



