38 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



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 

 series of inquiries concerning the geographic extent of the island at the 

 time of their formation. It might at first appear that a more extensive 

 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 districts 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 wliole, 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 typo. 



Tlie rivers of the eastern third of the island divert to either ocean 

 from the Blue Mountain llidge. 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 largo portion of tlieir distances in directions sub- 

 parallel to the main trend of the Blue Mountain axis, that is, in north- 

 west and southeast directions. Tiic streams of this region are marked 

 by deep V-shaped Amyous in their upper coui-ses and great deposits of 



