HILL: GEOLOGY OF JAMAICA. 221 
It is apparent that the borders of the region were indicated after the 
Appalachian revolution, at which time protean lands existed to the 
north and south in the then separate Americas. It is even possible 
that corrugations of Pre-Mesozoic rocks now concealed may have at that 
time underlain the North American Coastal Plain, and established the 
fundamental trends of the Floridian, Mexican, Central American, and 
Windward ridges, especially the long east and west Segovian ridge or 
ridges of Guatemala, and its possible extension into Cuba and Haiti, 
which constitute the most northern of the Tropical ranges. 
The Sierra Nevadan revolution, at the close of the Jurassic time, 
undoubtedly played an important part in establishing the northwestern 
barrier of the Mediterranean region, and creating a permanent nucleus 
for the accumulation of subsequent lands against it. The present 
abrupt termination of the trend of the Sierra in southern California, 
and the absence of trends directly continuous therewith in northern 
Mexico, does not, necessarily imply that related uplifts may not have 
occurred in the western Sierra Madre region of that republio concern- 
ing which we now know so little, except that its diminishing summits 
towards the American border, formulated by the later movements, are 
trending towards the Colorado Plateau. There may be nucleal lines of 
the older Nevadan trend in the Mexican region parallel to and en 
echelon with the Californian Sierra. No trace of this revolution can 
even be hypothecated elsewhere to the southward in the Tropical 
American region. 
The Andean and Laramide (or Rocky Mountain) revolutions of late 
Cretaceous and Eocene time, although synchronous in age, have not 
thus far revealed continuity across the Tropical zone. As has been 
shown, no possible protraction of their trends can be connected across 
it. Neither can their trends anywhere be identified in Central America 
or the West Indies, where, on the other hand, the orogenic trends are 
directly across their paths and at right angles to them. The only visible 
effect of the Laramide movement upon Tropical America was to add 
another belt of land, the Mexican Plateau, to the previously existing 
continental mass northwest of the region. 
The Antillean uplifts which occur in the West Indian region (Great 
Antillos and Virgin Islands), Central America, and the Isthmus of 
Panama and the ranges of the Colombian-Venezuelan coasts and islands, 
constitute the members of the latest and most important mountain 
movement, and one which produced the present configuration of the 
region mentioned. : 
