HILL: GEOLOGY OF JAMAICA. 41 
PART II. 
Geological Structure and Sequence. 
The geologic formations of Jamaica belong to four great categories, as 
follows : — 
l. A fundamental series of stratified.shales and conglomerates (of 
terrigenous origin), tuffs and other débris of volcanic material (whose 
Source is not apparent) and, rarely, marine limestones and marls, — all 
9f which have suffered great displacement and deformation. This series 
characterizes the higher mountains and forms tho 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 outerops of the first mentioned Series. 
It constitutos 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, oceanio marls, and coral reef rock, 
Which are adjacent to the present coasts and represent fringing reefis 
and other aceretions 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. 
Tun Bue 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 olay. 
Tho rocks are usually of dark color (black, blue, or dull chocolate), in 
Strong contrast to the glaringly light colors which characterize the sue 
Seeding formations of the Oceanic and Coastal Serios. The material, 
With the exception of occasional limestone beds and a few outcrops of clay 
Marls, can be traced to igneous rocks ; it was first volcanic ejecta and 
Subsequently and successively underwent various degrees of attrition 
