THE THREE WINDWARD RIDGES 265 



This central Caribbean ridge, from which the islands arise, as circum- 

 scribed by the 1,000-fathom submarine contour, may be divided into 

 two sections, south and north of Martinique, respectively. The south- 

 ernmost of these, made up of Saint Lucia, Saint Vincent, the Grenadines* 

 and Grenada, consists of a simple line or chain of islands. 



The northern half is wider, more complex, and suggestive of an archi- 

 pelago. In addition to the simple line of volcanic islands of Martinique* 

 Dominica, Guadeloupe, Montserrat, Nevis, Saint Kitts, Saint Eustatius, 

 andSaba,it includes Saint Croix and all the outlying Anguillan,Antiguan, 

 and Saba groups of banks and islands. 



In the southern section the 1,000-fathom contour closely follows the 

 base of the volcanic islands. In the northern section it diverges and 

 widens so that it encloses an extensive oval area of a mean breadth four 

 or five times greater than that of the southern section. 



Perhaps the origin of this remarkable submarine configuration will be 

 suggested as our narrative proceeds. 



VOLCANIC ORIGIN AND ARRANGEMENT OF THE MATERIAL OF THE ISLANDS 



The composition of the rock material of the main group or central 

 ridge of the Windward islands is simple. All the islands except Bar- 

 bados, which is different in its lower formation, are primarily composed 

 of volcanic ejecta derived from the earth's interior, supplemented by 

 marginal deposits of lime material extracted from pure sea water b} r 

 marine animal organisms which lived and died around the shore of the 

 preformed volcanic piles. It is important to note that there are no 

 granites, ancient sedimentaries, transported drifts or gravels, or other 

 material whereby any former extensions or continental relations may be 

 established. Hence the islands are oceanic in origin and non-continental.* 



Even in Jamaica, Haiti, Porto Rico, the Virgin islands, Saint Martins, 

 Saint Christopher, and Antigua volcanic tuffs constitute the basement 

 initial rocks of the island, so far as known, and are bordered by a periph- 

 eral fringe of local oceanic sediments. 



Outwardly the islands are either composed almost entirely of volcanic 

 material or of a volcanic basement partially veneered with sea-made 

 lime debris or, in the case of the lowest and least conspicuous, of a pre- 

 sumable volcanic basement entirely veneered with sea debris (calcareous 

 islands). These three types are exemplified in Saba, Antigua, and Bar- 

 buda. The main Caribbee chain, like Martinique and Saint Vincent, 

 consists almost exclusively of extruded volcanic ejecta piled up during 

 ages of intermittent eruption. 



* Oceanic islands never, as a rule, contain any of the typical rocks of continents — i. e., sediment- 

 ary strata, metamorphic rocks, or such acidic rocks as granite. — Sir John Murray, International 

 Geography, p. 62. 



