516 Notices of Memoirs — Dr. Spencer — Structure of Jamaica. 



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. 



Overljdng the Layton beds, whei'e these have not been removed, 

 and other strata formed near the surface of the country, there has 

 been a mantle of stratified loams and gravels 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 of 

 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 600 or 

 600 feet during two distinct epochs since the Mio-Pliocene period. 



The paper describes the broad undulating features characterizing 

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

 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 more than 10,000 feet 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 favourable region 

 for studying the contrast between the undulating topography de- 

 veloped near base-level of erosion during 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 

 moulding 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. 



In contrast with these two features of erosion, that of the post- 

 Liguanea epoch of submergence has been of small proportions ; 

 indeed, the post-Liguanea elevation is so recent that it has not 

 jDassed beyond the stage of making narrow deep canons. On 

 account of this formation overlying the remains of the Layton 

 series, the different features of erosion up to an altitude of 600 feet 

 are geologically preserved, while at greater altitudes they are not so 

 easily distinguishable from those produced before the Liguanea 

 epoch ; yet when one has become familiar with the features of 

 erosion, the respective epochs are generally recognizable. The post- 

 Liguanea canon-making epoch was characterized by an elevation of 

 150 to 200 feet more than at present, for the continuations of the 

 existing rivers are traceable to that depth across the submerged 

 coastal plains. The subsidence which caused the drowning of these 

 valleys reached to an elevation of 10 to 25 feet below the present 

 level ; since which time the coral reefs of the coast have emerged to 

 this amount. 



Numerous as these oscillations appear, all of them, since the 

 post-Layton elevation, have been of comparatively small and 

 diminishing proportions. These changes of level of land and 



