106 bulletin; museum of compaeative zoology. 



tween the sea line and the back coast hills, while coral reefs continued 

 to grow upon the shallow portion which remained submerged. These 

 processes are more fully discussed in the chapter dealing with the 

 geological history of the island. 



The Igneous Rocks of Jamaica. 



The following discussion of the igneous rocks of Jamaica is based 

 upon our field observations, including a study of the principal localities 

 enumerated in the Jamaican Reports and many new exposures opened 

 to view by the construction of the Port Antonio Eailway ; the micro- 

 scopic studies of specimens collected by us, and of those collections of 

 the Jamaican Survey which were loaned us by the Museum of the 

 Institute of Jamaica, are by Whitman Cross. No microscopic studies 

 of Jamaican igneous rocks have hitherto been made, so far as we are 

 aware, nor has any systematic discussion of the rocks been attempted, 

 although details of occurrence are given in the Jamaican Reports. 



Barrett, speaking of the porphyry in St. Thomas,^ said in an early 

 paper that it is " evident that the igneous rocks (porphyries) forming 

 the base of this series, and interstratified with some of the shales and 

 conglomerates, were erupted prior to the deposition of the Cretaceous 

 limestones, and at intervals of time sufficient for the formation of inter- 

 bedded aqueous strata." This assertion may have led the geologists 

 in England who wrote the Appendix of the Jamaican Reports to believe 

 these rocks fundamental, but Barrett himself, as well as Sawkins, in 

 their later reports on the eastern parishes of Jamaica, clearly show 

 that the rocks are intrusive and not fundamental. (See description of 

 Blue Mountain Series on pp. 51-53, also pp. Ill, 112, of this work.) 



Sir Roderick Murchison has said that the igneous rocks " are for 

 the most part either of the Miocene age, or posterior to that era ; some 

 of them, as is well known, having been indeed recently erupted." ^ 

 One would also infer a pre-existing foundation of igneous rocks beneath 

 the sedimentary section of Jamaica from the columnar section on 

 page 341 of the Jamaican Reports, and the unqualified statement by 

 Etheridge, page 306 of the same Reports, that " the conglomerate and 

 Cretaceous series rest upon Granitic and Porphyritic rocks," and from 

 numerous references to granites throughout the body of the Report. 

 Careful study of the individual reports upon the different parishes re- 



1 Quart. Jour. Gcol. Soc. London, 18G0, Vol. XVL p. 826. 

 ' Jamaican Reports, Introduction, p. iv. 



