HILL: GEOLOGY OF JAMAICA. 189 
formation above the adjacent waters, in which similar reefs with the 
same kinds of escarpments are now growing. The altitudes of these 
recfs in Barbados are so distinctly greater than those of the rest of the 
West Indian region that they can be accounted for only on the theory 
that the synchronous movement which has produced this result was 
there of much greater amplitude than elsewhere, as will be more fully 
discussed in Part VI. 
A discussion of the formations of Tropical America would be incom- 
plete without a consideration of the igneous extrusions which, from time 
to timo, have assisted in producing the radical changes in the geography 
of the land and sea bottom, and broken into the sequence of sedimentary 
events. Yet there has been so little systematic study of the various 
voleanie and intrusive rocks that I take up the subject with great diffi- 
dence. Since the time of their intrusion is only determinable by their 
association with fossiliferous sedimentaries, it is evident, in the light of 
the facts wo have given concerning the latter, that we have some data 
for at least approximating with more accuracy than has hitherto been 
attempted the history of the vulcanism. 
Dr. Persifor Frazer has asserted! that there is strong reason to 
believe that the axial range of the entire islands, and of Cuba, Jamaica, 
San Domingo, Porto Rico, and the Windward Islands, instead of 
being igneous extrusions of the Tertiary period, and later, are in reality 
erystallines of much earlier date, and may not be entirely volcanic. 
The considerations which he advances to support his view are as 
follows : that microscopic analysis “of the rocks which form the nucleus 
of the spurs of the Sierra Maestra of Cuba shows immense alteration 
to have taken place, and consequently a very long period to have 
elapsed ; that the complexity of the congeries of rocks forbids the 
hypothesis of their having been derived from one mass; that the asso- 
ciated characters are those which one finds united in very many Archeean 
regions throughout the world; that the products of alteration are 
similar to those in other Archean districts, ete. ; and that the rocks are 
diabases or diorites with epidote, porphyry, actinolite, felsite, orthofelsite, 
and porphyry like that of the South Mountain of Southeastern Pennsyl- 
Vania. Professor Frazer adds that a number of the first petrologists of 
Europe who have examined his slides are disposed to consider the speci- 
mens of not later than Paleozoic age, while none are willing to deny that 
they may be earlier. 
1 British Association for the Advancement of Science, Bath, 1888, pp. 654, 
655, : 
