160 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



It will be seen that the Caribbean Sea is almost entirely surrounded 

 on all sides except the east by mountains trending east and west, and 

 by submarine ridges of the Antillean type. The Windward Islands 

 marking the eastern inlet of the sea, are largely old volcanic heaps. 



A distinct class of mountains independent of great lines of folding of 

 the earth crust are the volcanoes. These have grown by extrusion and 

 accumulation. Sometimes they are parasitic upon the folded mother 

 systems, sometimes independent of them. They belong to the great area 

 of igneous eruptivity which, since at least as early as the beginning of 

 Tertiary time, has marked the whole western half of the North American 

 continent, the Caribbean, and the north and west sides of the Andean 

 region. Although blending into each other, the volcanic ejecta of this 

 great belt may be classified for convenience into two distinct age cate- 

 gories, which we may call the quiescent and the active volcanic groups. 



The active volcanic groups occur in four widely separated regions : 

 1. The Andean group of volcanoes of the Equatorial region of western 

 South America, rising above the corrugated folds of the northern ter- 

 mination of the predominant South American Cordilleras. 2. The chain 

 of some twenty-five great cinder cones which stretch east and west across 

 the south end of the Mexican Plateau, protruding parasitic-like upon the 

 terminus of the North American Cordilleras. 3. The Central American 

 group, with its thirty-one active craters, and growing diagonally across 

 the western ends of the east and west folds of the Caribbean corrugations 

 frino'ino' the Pacific side of Guatemala, San Salvador, and Costa Rica. 

 This is separated from the Mexican group on the north by a large non- 

 volcanic area, the Isthmus of Tehuantepec, and on the south from the 

 Andean volcanoes by the Isthmus of Panama, where no active volcanoes 

 are found. 4. The chain of volcanoes of the Windward Islands, marking 

 the eastern gate of the Caribbean Sea, and standing in a line directly 

 across the eastern termini of the Caribbean Mountains, trending east 

 and west, and parallel to the Central American group similarly situated 

 at their western termini. 



The Isthmus of Panama, the Pacific coast of South America west of 

 the Atrato, the north coast of South America, the old volcanic areas of 

 Northern Mexico and the United States, and the Great Antilles, are still 

 other regions in which volcanic activity has long been quiescent. 



The North American Cordilleran region, lying north of the Isthmus 

 of Tehuantepec, is one of north and south folded sedimentaries, plus 

 accumulations of volcanic intrusions and ejecta, and dominates a con- 

 tinental area. 



