1897-98]- LATE FORMATIONS AND GREAT CHANGES OF LEVEL IN JAMAICA. 347 



seen in Mexico, where the table-lands more .than 6,ooo feet above the 

 coastal plains demonstrate their own recent elevation, and yet, had the 

 lower coastal plains rising now to 1,700 feet above the sea, been formed, 

 with the high escapement behind them exposed to denudation so far 

 above the base level of erosion, the plateaus would have been dissected 

 by valleys penetrating the highland mass infinitely farther than those 

 of the present border. Thus it is suggested that the plateau region 

 may have received upward thrusts, bringing side b}^ side features of 

 old worn down topography, and others of the most recent date. 

 Accordingly similar features in Jamaica, but with the plateaus more 

 degraded than in Mexico, suggest that the land and submerged 

 plateaus of Mio-Pliocene age were moulded when the base levels of the 

 two regions were more closely related than at present. 



The great deep channels, whether above or below the sea, canyons or 

 valleys, and fjords, alike dissect the plateaus of which those beneath 

 the sea have the greater magnitude with consequently greater incisions. 

 These features represent a period or epoch subsequent to base leveling 

 of the plateau surfaces. These deep valleys upon the land and upon the 

 border of the sea, are of post-Layton Age, as the deposits of that epoch 

 have been penetrated by the subsequent erosion. This feature suggests 

 their origin generally at the more recent date, rather than of pre-Layton 

 Age with the valleys partly refilled and again reopened. This last view 

 in part might be considered, but it would necessitate two epochs 

 of enormous elevations, yet of much shorter duration than the period 

 of Mio-Pliocene base leveling ; one before and one since the Layton 

 epoch of submergence. Moreover, the epoch of greatest elevation on 

 the continent is post- Lafayette, with the consequent enormous amount 

 of excavations of the valleys. Consequently the writer is not inclined 

 to regard the developments of the very deep valleys as reopened pre- 

 Layton channels, as he was disposed to do at an earl}' period in his 

 West Indian studies. 



The Liguanea Formation. — After the epoch of Pleistocene elevation 

 just described, Jamaica sank so that the coastal region was depressed to 

 600 feet more or less, and loams and gravels were deposited upon the 

 floors of the drowned lands. To these accumulations the name of 

 Liguanea, being that of the coastal plain from Kingston for twenty-five 

 miles westward, has been given. This formation is essentially a mantle 

 of 20 feet in thickness, or more in the buried valleys, which is uncon- 

 formably deposited in places upon the Layton ruins, and where these are 

 removed, as is often the case, upon older formations. The Liguanea 



