Geology and Mineralogy. 271 



Here as everywhere, when studied, the valleys have in all respects 

 the features of those of the plateau regions of Mexico and other 

 countries. And they head in embayments of the land, receiving 

 as tributaries the principal rivers of the district. 



The modern topographic features of Jamaica date back, practi- 

 cally, only to the middle Miocene period; for the larger part of 

 the island is covered by old Miocene white limestones. But the 

 subsequent denudation has been enormous, for although the for- 

 mation still reaches a thickness of 2,000 feet in some places, yet 

 in others the dissection of it has penetrated the whole mass. 

 Upon this old Miocene surface no Mio-Pliocene formations occur 

 until those at the close of the period, showing it to have been one 

 of long-continued elevation. 



Upon these white limestones there was a subsequent mechani- 

 cal deposit of marls, with pebbles (made up in part of older frag- 

 ments), and in other localities there were gravels and loams 

 (according to the source of the materials). These accumulations 

 rise to a height of 500 feet in stratified beds, still nearly horizon- 

 tal in contrast to the upturned beds of the underlying white lime- 

 stone. They contain a few shells of modern species. The 

 formations have been found to correspond, in position, with the 

 Lafayette of the continent, or the Matanzas of Cuba, which have 

 been provisionally placed at the close of the Pliocene period. 



Overlying the Layton formation, where this has not been 

 removed, and other formations found near the surface of the 

 country, a mantle of stratified loams and gravels has been 

 laid down. This occurs up to an elevation of 600 feet. It has 

 been named the Liguanea formation, and has been correlated 

 with the Columbia ot the continent and the Zapata of Cuba. 

 While no fossils have been found in this fragmental deposit, yet 

 its stratified beds, occurring adjacent to the coast high above the 

 sea, indicate its origin at sea-level. Thus it appears that the 

 island was submerged to 500 or GOO feet during two distinct 

 epochs, since the Mio-Pliocene period. 



The paper describes the broad undulating features characteriz- 

 ing the Mio-Pliocene period. These have since been dissected by 

 great deep valleys, extending from the land to the submerged 

 plateau, formed subsequent to the Layton epoch; and from the 

 depths to which they reach in the submerged plateau, the inference 

 drawn is that the land stood 10,000 feet, or more, higher in the 

 early Pleistocene period than to-day. The Layton formation, 

 during this elevation, was enormously degraded, so that in many 

 localities only remnants are found in protected places. Jamaica 

 affords a favorable region for studying the contrast between the 

 undulating topography developed near base-level of erosion dur- 

 ing the Mio-Pliocene period of more extensive lands than to-day, 

 and the great and enormously deep valleys of the post-Layton or 

 early Pleistocene epoch. The molding of the submarine plateau 

 is supposed to have occurred during the Mio-Pliocene period, 

 while the deeply-drowned valleys are continuations of those of 

 the land, which are of post-Layton age. 



