Notices of Memoirs — Dr. Spencer — Structure of Jamaica. 515 



Mississippi, or of some plateau valley. These are separated by- 

 abrupt steps similar to the succession of those descending from the 

 margins of the Mexican tablelands. This point of analogy between 

 the drowned and land valleys, as well as the occurrence of short 

 amphitheatres indenting the edges of the submarine plateaux, when 

 carefully compared, very greatly strengthens the conclusions drawn 

 in the " Keconstruction of the Antillean Continent," namely, that 

 the valleys traversing the submarine Antillean plateaux were of land 

 origin, and indicate the depth to which the West Indian Continent 

 has sunk, even to a depth of two miles or more. 



III. — Late Formations and Great Changes of Level in Jamaica. 

 By J. W. Spencer. (Transactions of the Canadian Institute, 

 vol. V, 1898, pp. 324-357.) 



[Communicated by Professor E. Hull, F.E.S.] 



(PLATE XVIII.) 



THIS paper is descriptive of the physical features of Jamaica 

 which bear upon the evidence of great changes of level in late 

 geological times, and extends the conclusions set forth in the author's 

 work upon the " Reconstruction of the Antillean Continent." ^ 



Speaking in a broad way, Jamaica is a dissected tableland, sur- 

 mounting another but submarine plateau, extending from Haiti to 

 the Yucatan banks, now submerged to depths of 3,000 to 4,000 feet. 

 These banks have the form of old base planes of erosion, but they 

 are traversed by deep valleys more than 2,000 feet below the 

 summit of the platform. Even within the limit of the submarine 

 plateau mass the channels reach to a depth of 9,600 feet, or more 

 than 5,000 feet below the surface of the drowned plains. 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 practically 

 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 formation 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 mechanical 

 deposit of marls with pebbles (made up in part of older fragments), 

 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 horizontal, in contrast to the 

 upturned beds of the underlying white limestone. They contain 



1 Eull. Geol. Soc. Amer., vol. vii (1894), pp. 103-140. 



