HILL: GEOLOGY OF JAMAICA. 107 



veals the fact that the rocks alluded to are clearly intrusive into the 

 overlying sedimentary section. Our personal observations enable us to 

 say positively that, while the oldest known rocks of the island are un- 

 doubtedly rolled eruptive volcanic debris, no fundamental massifs of 

 plutonic crystalline rocks, granite or other kind, are known to occur as 

 a previously formed basement or axis to the sedimentary section of 

 Jamaica. On the other hand, these rocks, by occurrence, are all erup- 

 tive and intrusive, and in age are contemporaneous in origin with the 

 sedimentary rocks. 



The igneous rocks can be classified by age, occurrence, and rainer- 

 alogic composition into three distinct categories, as follows : — 



I. The andesitic (mostly hornblende) boulders, pebbles, and tuffs of 

 the Blue Mountain Series, of eruptive origin from unknown vents, con- 

 temporaneously deposited with the Cretaceous sediments of the lower 

 beds and occurring as rolled and worked over material in the Richmond 

 beds of the upper part of that series. 



II. The hornblende diorite and granitoid porphyries of the five east- 

 ern parishes of the island, constituting dikes and masses, or laccoliths, 

 intruded through the rocks of the Blue Mountain Series and into the 

 Montpelier formation of the Oceanic Series. These rocks are of Mid- 

 Tertiary age. 



III. Eruptive amygdaloidal basalts of the Low Lay ton stock or vol- 

 canic neck in the north coast of Portland. Mid-Tertiary. 



I. The Boulder Material of the Blue Mountain Series. — In the gen- 

 eral description of the Blue Mountain Series we have noted the pre- 

 dominance of conglomerates, boulders, pebbles, and tuffs, composed of 

 rolled volcanic rock, and the fact that the accompanying shales and 

 sandstones are made up almost, if not entirely, of this material, which 

 has undergone aqueous trituration. This boulder material is the oldest 

 formation exposed on the island, and it has survived by rolling and re- 

 deposition through all succeeding epochs, being especially conspicuous 

 in the Richmond, Bowden, Kingston, and modern aggi'adatioual deposits. 



Particular attention was paid by us to the study of the composition 

 and origin of this material. Specimens from what are apparently the 

 oldest formation of the island, — the Frankenfield conglomerates of the 

 Rio Minho section of Clarendon, — as determined petrographically by 

 Cross, show that the material is almost exclusively composed of horn- 

 blende-andesites and hornblende-andesite tuffs. 



Original masses in situ from which this, boulder material could have 

 derived, are nowhere known to be exposed on the island, and hence its 



