53(1 Proceedings of Philosophical Societies. [MXRC«^ 



GEOLOGICAL SOCIETY. 



Jan. 20. — A paper was read, " On the Geology of Jamaica;'* 

 by H. T. De la Beche, Esq. FRS. &c. Mr. De la Beche's 

 observations are confined to the eastern half of Jamaica, which 

 includes the whole range of the Blue Mountains, the highest 

 eminences of the Island, those of Port Royal, Spanish Town, the 

 Mocko Mountains, and other ridges of inferior elevation. These 

 heights often include or are connected with extensive plains, the 

 principal of which are those of Liguanea, Vere, and Lower Cla- 

 rendon, Luidas Vale, and St. Thomas's. The rocks of oldest 

 formation which presented themselves to the author within this 

 district, he refers to the submedial or transition series. They 

 compose the greater part of the Blue Mountain range, and con- 

 sist of, 1. Graywacke, both foliated and compact, coarse and 

 fine, presenting in short the usual variations common to this 

 rock in Europe, and appearing on some points to pass into old 

 red sandstone ; 2. Transition limestone, apparently destitute of 

 organic remains, compact, of a dark bluish gray colour, and 

 traversed by veins of calcareous spar ; occasionally associated 

 with argillaceous slate, and its upper beads much intermixed 

 .with sandstones. These stratified rocks throughout the Blue 

 Mountains generally dip towards the NE and ENE at a consi- 

 derable angle ; but there are frequent exceptions to this rule, 

 and the strata are on the whole much contorted. They are occa- 

 sionally associated with trap rocks, viz. syenites, greenstones, 

 and claystone porphyry. 



The author observed on one point, viz. the southern slope of 

 St. Catherine's Hill, a series of strata which he conceives to 

 represent the coal measures ; the old red sandstone is, however, 

 developed on a larger scale, and in more numerous localities ; so 

 that the nledial or carboniferous series is certainly not wanting 

 in Jamaica. Resting upon this appears, on many points, a por- 

 phyritic conglomerate, associated with porphyry, and occasion- 

 ally with greenstone and syenite. Similar trap rocks, inter- 

 mixed in the most varied manner, show themselves very 

 extensively, composing the greater part of the St. John's Moun- 

 tains, and the district bordering on the Agua Alta. One 

 variety of porphyry met with by the author is composed of 

 nodular concretions, separated by a soft argillaceous substance, 

 among whiuh strings of chalcedony are sometimes found. It is 

 remarkable that the only instance of a similar structure which 

 has occurred to the author is in an amygdaloidal rock, decidedly 

 of volcanic origin, at Black Hill, on another part of the island. 



These trap rocks are found generally supporting the great 

 . white limestone formation, which occupies a very large portion 

 of the whole island. This formation, from the fossils it contains, 

 ""is referred by Mr. De la Beche to the tertiary series. It is prin- 

 cipally composed of white limestone, most frequently very 



