EARLIER TERTIARY (EOCENE AND OLIGOCENE). 717 



E 17-18. Jamaica; 



Hill's classification of the rocks of Jamaica is given in Chapter XV (pp. 639-642) . 

 From his account "^^ we take the following description of the deposits assigned to 

 the early Tertiary : 



The Richmond beds constitute the upper subdivision of the Blue Mountain series. Their 

 arrangement and composition, consisting of shallow marine deposits of worked-over and water- 

 assorted terrigenous material, indicate a succession of more quiet sedimental conditions than 

 those which marked the preceding epoch. 



The rocks are mostly black bituminous laminated clays and ferruginous sandstones with 

 occasional beds of loose conglomerate. They occur in uniform alternations of thin, regular, 

 and evenly bedded strata, varying from an inch to a foot or more in thickness. They are dull 

 blue-black on fresh exposure, but undergo excessive oxidation and hence are ordinarily of dark- 

 brown ferruginous colors. In general texture, arrangement, color, and stratification they resemble 

 the Eo-Lignitic (lower Eocene) beds of the southern United States. The clays contain many 

 small flakes of carbonized vegetal matter, and silicified wood has been found in the gravel. The 

 gravel is mostly derived from the antecedent beds of the lower division of the Blue Mountain series. 

 The so-called "sandstones" are composed of cemented grains of waterworn hornblende andesite 

 derived from the underlying igneous rocks, and the shales are the same material more finely 

 triturated and mixed with vegetal matter. The conglomerates consist of rounded pebbles of 

 various dimensions and in places attain a thickness of 50 feet. They are almost entirely of 

 the same material as those of the lower subdivision. Rounded fragments of the Rudistean 

 limestone also occur in them. * * * j^^ addition to these rocks of the conglomerate, former 

 observers have noted, from the bluff at Port Maria, specimens of gneiss and crytalline slates, 

 "rocks of which no trace either in situ or otherwise have hitherto been found in Jamaica; also 

 a fine-grained granite to which nothing analogous has been noted on the island. * * * Ju 

 this unique collection are many instances of rocks which have totally disappeared from the 

 ' surface of Jamaica but which must have existed during former epochs, either in the formations 

 of this country or in adjacent lands that have been destroyed." " 



At the same locality, as also noted in the Jamaican reports," the ordinary sandstone is 

 rapidly transformed in its seaward extension into a promiscuous assemblage of large pebbles 6 or 

 8 inches in diameter. This fact indicates that some of this material came from the area to the 

 north, now occupied by the sea. In this same bed of conglomerate were found the Eocene 

 corals described by Duncan, and a few species of moUuslffi. * * * 



The Richmond formation outcrops in many places a short distance back of the sea along 

 the north coasts of the parishes of St. Ann and Trelawney. It is well exposed beneath the 

 Cambridge beds south of Cambridge, along the highway on the west side of Great River, as seen 

 by the writer. It also occurs on the south side of the Blue Mountain Ridge in St. Andrews and 

 St. Thomas. According to Sawkins, in the latter parish at Blue Mountain Valley it consists of 

 "alternate bands of red clay, yellow "sandstone, and light-gray shales, 1,000 to 1,200 feet in 

 thickness." 



In general, this formation underlies nearly aU the later rocks, and, in our opinion, prior to 

 the Montpelier subsidence it occupied an area as large or larger than that of the island of to-day. 



From data presented in the paleontological chapter of this work the age of these beds is 

 undoubtedly old Eocene, although it is impossible to draw an exact line between these beds and 

 those of the lower division which we have termed Cretaceous, and they are no doubt stratigraph- 

 ically continuous. 



The uniform alternations of the Richmond beds indicate that they were rapidly deposited 

 over a considerable shallow area of deposition; since much of this area was the present locus 



t" Jamaican Reports, p. 130. (Entitled " Reports on the geology of Jamaica, or Part II of the West Indian Sur- 

 vey," by James G. Sawkins, with contributions by others.) 



