HILL: GEOLOGY OF JAxMAICA. 33 



belong to the lower group of levels. On the south coast the 650 feet 

 level may be seen in the Yallahs topography, the summit of Round 

 Mountain, at Vere, and the Healthshire liills west of Port Royal en- 

 trance. The 300 foot bench, or Bowden level, is very conspicuous around 

 the east end of the island from Yallalis Mountain on the south to Port 

 Antonio on the north, as well as on the west coast of the north side, at 

 Cinnamon Hill and elsewhere in St. Ann, Trelawney, and St. James. 



The back coast benches enumerated have all been cut out of the land 

 by gradational processes (base levelling and marine erosion) and represent 

 pausation stages in two long continued periods of elevation. Those be- 

 tween 700 and 2,000 feet were carved out of the white limestone matrix 

 during the first period of emergence from the sea. The benches, from 

 100 to 700 feet in altitude are also cut out of the old limestone matrix, 

 but were probably made during a second period of emergence and erosion 

 following a period of subsidence as is explained later on. 



This abrupt ending of the land, considered in connection with its ac- 

 companying terraces, the arch of the summit region, and the narrow 

 submerged platform around the island, strongly suggests, as outlined in 

 the geologic chapters of this paper, that the Limestone Plateau was once 

 more extensive land, which, after its first elevation, underwent marginal 

 erosion, drowning of its coastal plains by partial subsidence, and re-ele- 

 vation into its present outline. 



JFrom the data given in the geologic portion of this paper, the time of 

 these events was between the beginning of the Miocene (late Oligocene) 

 and Pliocene time. There is much evidence that old gradational terraces 

 of this type continue lower down upon the submerged slopes of the 

 island, and that the phenomena of the Coastal Plain, next to be described, 

 represent veneerings of organic, littoral, and terrigenous deposits upon 

 old erosion planes of this character. 



Benches of the Coast Plains. — A narrow strip of low land extends 

 more or less interruptedly around the island, between the sea and the 

 back coast border. In some places this is an old beach only a few feet 

 wide ; in others it has greater width and indents the back coast border 

 for miles. This coastal strip is composite in character, being of three 

 types of formations, such as elevated reef rock, marginal sea debris, and 

 land derived alluvium. It presents distinct features of relief, including 

 several benches of different height and origin, and a long and gentle 

 slope known as the Liguanea Plain. The basements of these benches 

 are old erosion levels which were submerged, covered with a veneering 

 of constructional material, and re-elevated into land. 



VOL. XX XIV. 3 



