40 bulletin: museum of comparative zoology. 



the southern part of St. James ; Cutting-Grass-Spots and Dean's Rivers, 

 Westmoreland ; and Content River in Hanover. It is supposed that the 

 waters of these streams, after sinking into the ground, in some instances 

 find underground conuection along the contact of the Blue Mountain 

 and White Limestone Series and into the coastal streams. In many- 

 cases the headwaters of the marginal streams have captured the drainage 

 of the great interior basin valleys, especially the Minho, Cobre, Montego, 

 Black River, and Great River. This is shown by the fact that the 

 middle portion of these streams, where they cut the White Limestone 

 Ridge between the plains and the central valleys, have newly formed V- 

 shaped canyons containing no alluvial material such as occurs in the 

 interior valleys which they drain and the lower Coastal Plains through 

 which they flow. The topography of Bog Walk Canyon, shown on 

 Plate X., illustrates the character of the more newly made, interme- 

 diate, connective portions of this compound type of streams. 



Without details of the geologic structure, it is evident from this 

 topographic review that the present land features of Jamaica are of 

 complex origin, and record many past events of uplift and subsidence 

 which have produced different phases of configuration and outline at 

 different epochs of its history. 



Summarized, this history involves : — 



(1) Two periods of mountain making (including the elevation of the 

 plateau in this category) accompanied by greater expansion of the 

 island tlian its present area. The first of these has prevalent north of 

 west trends ; those of the second are east and west. The profiles of the 

 former are angular, of the latter gently arched. 



(2) Two great epochs of subsidence and contraction of the land, 

 alternating with periods of elevation. 



(3) Later uniform elevation which added the narrow modern coastal 

 phenomena. 



Finally, we may add that the configuration of Jamaica does not 

 cease at sea level, but there is everj^ evidence that the visible portion of 

 the island is only the tip of a more extensive foundation below the 

 level of the sea, •which, especially to the south and east, presents 

 terraced features somewhat similar to those of the exposed coast borders, 

 and which indicate that once the island was slightly more extensive 

 than at present. 



