HILL: GEOLOGY OF JAMAICA. 39 
ancient alluvium in their lower parts. The run-off is constant, but 
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 the 
present. This extended margin was subsequently drowned by sub- 
sidenco, so that the original headwater amphitheatres became estuarios 
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 River, 
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 
hear 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 
9n 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 
s . » * . . . . 
St, Elizabeth ; Hicks River in Trelawney ; Pine and Dry Rivers in the 
horthern part of St, Elizabeth ; Niagara, Chester, and Tangle Rivers in 
