106 BULLETIN; MUSEUM OF COMPARATIVE ZOÜLOGY. 
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. 
Tun IowEous Rocks or 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 Railway; 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 microscopie 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. 111, 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. Geol. Soc. London, 1860, Vol, X VI. p. 326, 
2 Jamaican Reports, Introduction, p. iv. 
