HILL: GEOLOGY OF JAMAICA. 39 



ancient alluvium in tlieir lower parts. The run-oflf is constant,ljut 

 variable in quantity owing to torrents. The wide alluvial filled valleys 

 of the Liguanea type, through which the lower portions of the streams 

 now cross to the sea, indicate the following episodes of history : — The 

 bights occupied by the plains were once headwater amphitheatres from 

 which the streams flowed out across a wider coastal border than tlie 

 present. This extended margin was subsequently drowned by sub- 

 sidence, so that the original headwater amphitheatres became estuaries 

 indenting the coast into which the streams then debouched. Finally, 

 elevation extended the headwaters of the streams, partially reclaimed 

 the Coastal Plain, and revived the old lower channels or established 

 newer courses across them to the sea. 



The drainage of the White Limestone Plateau of the western two 

 thirds of the island is both superficial and underground. The former 

 includes the older and longer streams, such as the Cobre, Milk liiver. 

 Black River (St. Elizabeth), Island River, and New Savanna River of the 

 south coast ; Orange River, Island River, Lucea River, Great River, 

 Montego River, and Roaring River of the north coast, which in general 

 present the same history as the streams of the Blue Mountain Ridge. 

 These are ancient streams, which once brought down immense alluvial 

 deposits now forming the material of the Coastal Plains, through which, 

 since the slight elevation of the latter, they have cut newer direct 

 channels to the sea. Others of this class consist of the short, simple, 

 autogenous streams, probably of later origin, which drain from the bluffs 

 of the back coast border, and are especially well shown along the north 

 coast of St. Ann. 



A third and entirely different type of streams, found only in the 

 basins of the Limestone Plateau summit region, include the short local 

 streams which rise from springs or caverns near the contact of the 

 White Limestone and Blue Mountain Series. These flow for short 

 distances, then disappear into the ground without having any visible 

 outlet to the sea. Of this type of rivers are the Minho and Rio Hoe 

 near Moneague ; Great River in the southeast corner of St. Ann ; Pedro 

 River, which sinks at the corner of St. Ann, Clarendon, and St. Cath- 

 erine; Yankee and Cave Rivers, which unite and disappear into a sink 

 on the border of St. Ann and Clarendon (the latter river is ten miles 

 long) ; Hector's River, forming the boundary of Manchester, Trelawney, 

 (about as long as the last stream,) sinking at the northeast corner of 

 St. Elizabeth ; Hicks River in Trelawney ; Pine and Dry Rivers in the 

 northern part of St. Elizabeth ; Niagara, Chester, and Tangle Rivers in 



