Page:Popular Science Monthly Volume 70.djvu/393

Rh 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, 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. The writer has observed transverse faults in the Blue Mountain region, which undoubtedly indicate lines along which fracture may occur.

M. de Ballore coincides with Mr. Hill's ideas regarding an eastwest 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 see 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 Royal 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