1897-98]- LATE FORMATIONS AND GREAT CHANGES OF LEVEL IN JAMAICA. 327 



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Figure i. — Section illustratingf a succession of base levels of erosion : — in m, an old plain now represented 

 bv only fragments of plains and ridges of similar elevation ; p p^ a. newer base level less defaced ; 2, a later 

 plain only incised by a canyon (c) of a stream cut since last elevation of the district. 



canyons a mile or two in length which have been excavated since the last 

 elevation of the island. The different phases of erosion will be further 

 considered in connection with the characters bearing upon this physical 

 investigation. 



HYDROGRAPHY OFF THE COAST. 



The northern side of the insular mass of Jamaica descends as 

 abruptly beneath tide level as the mountains drop down to the sea. 

 With a few exceptions the sea bottom increases to a depth of 600 feet 

 within the distance of a mile from the shore, so that even the delta 

 deposits cover small areas and have not filled the broader bays formed 

 by the washings of the coastal regions when the land stood at higher 

 altitudes than now. From the mouths of existing rivers (while 

 some have changed their courses), channels though sometimes partly 

 obstructed can be traced across the submerged shelf of the island,^ 

 whether of coral reefs or of mechanical deposits. Even where the 

 Insular fringe is not covered with more than from 50 to 250 feet of 

 water, the transverse trenches or canyons may be 600 or 700 feet deep. 

 The descent of these submerged channels has a similar slope to that of 

 the valleys which may reach to 500 or even 1,000 feet in a mile. The 

 same narrow fringe extends around both the eastern and western ends 

 of the island, but it becomes broadened to eight or ten miles upon the 

 southern side of Jamaica. 



A few examples of the deep channels crossing the insular fringe may 

 be given : — Thus the submerged channel of Plantain Garden river has 

 a depth of 612 feet at a mile from the shore, where the adjacent shelf 

 is covered with only 156 feet of water; so also at the mouths of the 

 streams entering Hope, Buff, Falmouth, Montego, Lucea, Port Morant, 

 and Morant bays, channels over 600 feet can be traced across the sub- 

 merged fringe. Beyond that depth the successions of soundings have 

 not been made by which the trenches can be followed to the deep sea 

 of 12,000 feet and more to the north of the island. 



Annatto is one of the widest bays along the northern coast, with a 

 breadth of four miles between the outer headlands. Into this indenta- 



