hill: geology of JAMAICA. 41 



PART II. 



Geological Structure and Sequence. 



The geologic formations of Jatnaica belong to four great categories, aa 

 follows : — 



1. A fundamental series of stratified shales and conglomerates (of 

 terrigenous origin), tuffs and other debris of volcanic material (whose 

 source is not apparent) and, rarely, marine limestones and marls, — all 

 of which have suffered great displacement and deformation. This series 

 characterizes the higher mountains and forms the nucleus of the 

 island structure upon or around which all subsequent formations have 

 accumulated. 



2. A series of organically derived oceanic material — marls and lime- 

 stones — which rest unconformably against and upon but do not com- 

 pletely overlap the more elevated outcrops of the first mentioned series. 

 It constitutes piedmontal formations of great thickness around bases of 

 the higher mountain summits. 



3. Laccoliths, dikes, and sills of igneous rocks, which penetrate the 

 first series and the lower portion of the second, and are, therefore, of 

 later age than both. 



4. Certain deposits of alluvium, oceanic marls, and coral reef rock, 

 which are adjacent to the present coasts and represent fringing reefs 

 and other accretions around the island's border after it had almost 

 attained its present area and outline. 



These four series of rocks are uncomformable to one another, and are 

 the products of the greater events in the geologic history of the island. 

 Together with the formations composing them, they are tabulated on 

 the following page. 



The Blue Mountain Series. 



This series comprises the oldest rocks of the island. It consists of 

 loose or slightly indurated beds of gravel, clay, boulders, and tuffs, with 

 exceptional beds or bosses of hard indurated limestones and yellow clay. 

 The rocks are usually of dark color (black, blue, or dull chocolate), in 

 strong contrast to the glaringly light colors which characterize the suc- 

 ceeding formations of the Oceanic and Coastal Series. The material, 

 with the exception of occasional limestone beds and a few outcrops of clay 

 marls, can bo traced to igneous rocks ; it was first volcanic ejecta and 

 subsequently and successively underwent various degrees of attrition 



