132 J.W.SPENCER — rf:construction op antillean continent. 



not cross the line of Central America the conclusion as to Antillean 

 oscillations will remain. If this inference be true, then the modern islands 

 of the West Indies formed an elevated plateau bridge between the two 

 Americas during the two epochs of elevation, namely, in the Pliocene 

 and in the Pleistocene periods. This conclusion is supported by all the 

 geomorphic structure except perhaps the lower depths of the sea of 

 Honduras, lying between the mountain folds of Cuba, Haiti, and Jamaica. 

 These mountains rise to from 7,000 to 9,000 feet above the modern sea- 

 level ; also on the west this basin is bounded by plateaus with ridges 

 and peaks rising to from 6,000 to 14,000 feet above tide, with valleys 

 as low as 2,956 feet (railway survey). The higher peaks are mostly 

 modern or late volcanic, but there are older crystalline rocks, which have 

 been elevated to 12,000 feet since the deposits of Miocene age, which have 

 been forced up by the granite masses to an elevation of 3,000 feet and 

 now form the oldest sedimentary accumulations in Costa Rica.* Dr 

 Gabb concluded that the Miocene formations extended several thousand 

 feet higher than now on the flanks of the granite masses, but have since 

 been degraded. These observations of the deceased geologist are highly 

 suggestive, as his other work has been. It seems to be the key to the 

 orogenic obstructions crossing the Antillean basins, which have been also 

 partly modified by the epeirogenic movements. The latter movements 

 per se are not rudely deformatory, but along with them there have been 

 some modern faults and tilting of even the later strata, yet there has not 

 been enough disturbance to obliterate the geomorphic features. A gentle 

 epeirogenic undulation raises the floor of the Caribbean basin, so that its 

 submergence is only 9,000 feet in its western portion. From Gabb's 

 observation, cited, a suggestion appears that in it there is evidence that 

 the Miocene strata have not only been raised 3,000 feej^ above the sea? 

 but they have been further raised 9,000 feet by the orogenic movement 

 from the floor of the sinking Caribbean basin. Thus there appears a first 

 step in separating the Central American orogenic and epeirogenic move- 

 ments, the latter being over 6,000 feet. 



The epeirogenic deformation of a few feet per mile has a zone across 

 the sloping plateau of from 150 to 250 miles wide on which to expend its 

 forces between the ridges of Central America and the floors of the Gulf of 

 Mexico and the Caribbean sea. How much of the obstruction has been 

 raised in Central America since the time of the Pliocene elevation and 

 how much after that of the Pleistocene period can scarcely be determined 

 at present. However, the difference in the elevation of the Miocene and 

 the upper Pliocene limestones may give a rude measure, and this difference 



*See Dr Gabb's papers, " Notes on the Geology of Costa Rica," Am. Jour. Se., vol. vin,1874, p. 387, 

 and vol. ix, 1875, p. 198. 



