hill: geology of JAMAICA. IGl 



Blue Mountain ridges, was elevated some 500 feet or more, bringing up 

 with it, not only the present marginal border composed of the new made 

 sediments of the Bowden epoch, but a now slightly submerged area 

 towards the south, which extended at least as far as the present 500 

 fathom line, embracing the Pedro banks and keys. This upward move- 

 ment, while accompanied by slight deformation, was less erogenic than 

 those of the preceding epoch, probably representing the last throes of 

 the Antillean uplifts. It is very certain that the large area of Jamaica 

 was expanded during this epoch (late Miocene) beyond its present borders 

 at least as far as the present 500 fathom line. 



Accompanying or closely following the time of this elevation in late 

 Miocene or early Pliocene time was a period of great erosion and de- 

 nudation, which largely produced the minor relief of the surface con- 

 figuration of to-day. During the Pre-Bowden elevation the headwaters 

 of the marginal drainage of the south coast, which then flowed out to 

 sea across the present submerged banks, were actively engaged in cut- 

 ting out, as headwater amphitheatres, the embryo embayments of the 

 present Liguanea type of plains, and the older sink-holes in the summit 

 of the limestone arch by dissolving through the soluble limestone. In 

 the Post-Bo wden epoch the sink holes were cut downward to the insolu- 

 ble strata, underlying the limestone, producing the first of the present 

 great interior basins, which also commenced to expand laterally by 

 erosion of their soluble margins. Meanwhile, the coastal drainage 

 rapidly extended interiorward by headwater erosion, and the country 

 adjacent to its lower portions became extensive base-levelled plains, and 

 included then what are now the submerged benches of the island, and 

 which were veneered with the aggradational Kingston formation dur- 

 ing a subsequent subsidence. This period of erosion was late Miocene 

 or early Pliocene, corresponding in time almost exactly with the great 

 Pre-Lafayette erosion epoch of the North American continent. 



During this late emergence the middle series of terraces (now from 

 200 to 700 feet high) were cut around the coastward face of the back 

 coast country. 



Possibly there was another subsidence in late Pliocene time, — the 

 first of a cycle of epeirogenic oscillations that characterized the later 

 history of the island. The island once more underwent partial subsi- 

 dence ; the pre-eroded coastal plains and base-levelled bights bordering 

 the island became in their seaward extension submerged platforms, 

 while their interior borders constitute deeply indented estuaries at the 

 mouths of the great rivers of the south coast and Montego Bay of the 



VOL. XXX IV. 11 



