38 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
carved out of the White Limestone Plateau in preceding erosion epochs 
which took place during the rapid elevation of the island in Middle 
Tertiary time. It is evident that during subsequent epochs of sub- 
sidence these valleys were drowned and filled with alluvium from the 
bocas of the rivers which then debouched at their interior margins, and 
have cut lower streamways through them since the later periods of 
slight uplift which produced the reef levels. 
The frequent occurrence of these plains on the south side and their 
scanty development on the north side of the island gives rise to a 
serios of inquiries concerning the geographic extent of the island at tho 
time of their formation. It might at first appear that a more extensivo 
land area existed to the northward than at present, but on closer 
examination this hypothesis is untenable. The greater development 
of these old valleys on the south is due to the fact that the east and 
west axis of highest elevation lies nearer the north coast than the south, 
and the principal slope has long been in the latter direction. An inti- 
mate topographic relation exists between the aggradational plains of 
the coast and the large central basin valleys, and they present synchro- 
nous and parallel stages of development. In later times the two have 
been united in several places by headwater erosion, as shown in our 
discussion of the basins. 
Drainage. — The drainage of Jamaica is peculiar, that of the Blue 
Mountain distriets being frequent and constant in occurrence and 
copious in run-off, while in the region of the Limestone Plateau it is 
superficially somewhat deficient in streams which are largely of an 
underground nature. As a whole, the island presents two major types 
of streams, one of simple autogenous rivers flowing to either coast, and 
the other of the streams of the interior basins which have no outlet to 
the sea ; a third and more complicated type of drainage, a combination 
of the two preceding, has been produced in certain instances by the 
capture of the second by the first type. 
The rivers of the eastern third of the island divert to either ocean 
from the Blue Mountain Ridge. This drainage is mostly normal to the 
coast, with the exception of the Rio Grande of the north side, the Negro 
River of the south side, Plantain Garden River of the east coast, and 
Yallahs River; these are probably the most ancient members of the 
system, and flow for a large portion of their distances in directions sub- 
parallel to the main trend of the Blue Mountain axis, that is, in north- 
west and southeast directions. The streams of this region are marked 
by deep V-shaped canyons in their upper courses and great deposits of 
