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 

 reefs 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 time, 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 

 volcanic 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 we 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 

 crystallines 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 

 elaaped ; 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 Archoean 

 regions throughout the world ; that the products of alteration are 

 similar to those in other Archcean districts, etc. ; 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. 



