THE JAMAICA EARTHQUAKE 389 



dance and variety of its tropical and subtropical products, due to the 

 fertility of the limestone soil and the abundance of the rainfall, which 

 varies largely, however, in the amount, from 10 inches at Port Royal 

 to 126 inches some years in the higher regions. 



Geologically, Jamaica is of comparatively recent age, 3 for its basal 

 Blue Mountain series of sediments and intrusives is of late Cretaceous 

 and Eocene times. This series makes up the mountainous backbone 

 of the island, while the later Oligocene limestone overlaps the former 

 series in a thick piedmontal formation covering two thirds of the island. 

 The more recent alluvial and littoral formations were deposited during 

 the period of uniform elevation following, and constitute the fringing 

 plains of the island. 



In the structural geology of Jamaica, the earliest axis of folding 

 now evident is the northwest-southeast line of the Blue Mountains, 

 with later east-west foldings along the more ancient line of orogenic 

 movement which outlined the Greater Antilles in early Mesozoic times. 4 

 The writer has observed transverse faults in the Blue Mountain region, 

 which undoubtedly indicate lines along which fracture may occur. 



M. de Ballore 5 coincides with Mr. Hill's ideas regarding an east- 

 west folding for the Antilles in postulating his theory of an anticlinal 

 axis that marks the line of the Greater Antilles and a parallel synclinal 

 belt immediately to the north of Jamaica, which coincides with the 

 Bartlett Deep. In the photograph of a relief map (Fig. 2), the east- 

 west elevation and depression are brought out strongly. 



The bathographic relations of Jamaica are significant. We sec 

 that Jamaica and the other Antillean islands are but the higher peaks 

 of a lofty and precipitous, but submerged, mountain chain. The 

 tremendous differential relief of over 38,000 feet that exists in places 

 in the Caribbean region apparently coincides with a zone of seismic 

 and volcanic frequency. We know that the crust of the earth is always 

 in a state of tension. This stress may come from the shrinkage of the 

 earth, from the loading or unloading of the earth's surface through 

 erosion or deposition, or from other sources. The resistance is lessened 

 on a relatively steep slope (Fig. 1, b) where the points of application 

 of this lateral pressure at the ends, not falling in the same plane, tend 

 to produce a fracture. When a sudden slip in the adjustment occurs, 

 the resulting jar is transmitted through the earth as earthquake waves. 



Port Eoyal is at the western tip of a narrow seven-mile sand-spit 

 that makes a natural breakwater to one of the finest harbors in the 



3 ' The Geology and Physical Geography of Jamaica : a study of a type of 

 Antillean development,' Robert T. Hill, Bull. Mus. Comp. Zool., Vol. XXXIV., 

 Geol. Series, Vol. IV., September, 1899, p. 421. 



4 Ibid., p. 164. 



5 ' Tremblements de Terre,' F. de Montessus de Ballore, 1906, Fig. 63. 



