Mr. De la Beche on Ihe Geology of Jamaica. 187 



abounds^ being mostly filled : those^ however^ which have been exposed to the 

 weather are frequently empty, as are also those within a few inches from 

 the surface. The interior of the empty cells is often incrusted with a 

 little oxyd of iron, while those that are full, and convert the rock into an 

 amygdaloid, contain chalcedony, carbonate of lime, and occasionally some of 

 the zeolitic minerals : in general, chalcedony lines the cavity, and the central 

 part is composed of carbonate of lime. This rock, when the cells are empty, 

 has a very lava-like appearance, and the greater part of it which is exposed 

 is arranged in concretions reminding the observer of the porphyry at Pimento 

 Grove. These concretions are separated from each other by a white argil- 

 laceous substance, which is in considerable abundance in one or two places. 

 On ascending the hill it becomes more rare, or at least is not so easily seen 

 as on the coast, where cliffs of volcanic amygdaloid rise above the sea. 



Near the summit of the Black Hill I found the rock changed in several 

 places into a substance between obsidian and pumice, having however more 

 the character of the latter than of the former, and presenting every appearance 

 of having been subjected to the action of fire at no very distant period (geo- 

 logically speaking). I also found some pieces of rock, changed on the sur- 

 face into this kind of obsidian-pumice, which in the interior were claystone 

 porphyries, having a dark coloured base with white crystals of felspar. 



The connexion of this hill with the sandy marl member of the white lime- 

 stone formation, which surrounds it on all sides but that of the sea, is very 

 obscure : some gravel beds will however be observed close to it, as it is ap- 

 proached from the mouth of the Swift River. 



The Black Hill would seem referrible to what are commonly called extinct 

 volcanos. How many more of them there may be in Jamaica, I had no 

 means of ascertaining, though probably others may exist, more particularly 

 in the western portion of the island, respecting the geology of which I could 

 obtain little or no information, except that the white limestone is very 

 common. 



Explanation of the Sections. (Plate XIX.) 

 In the preceding pages I have given little more than a detailed account of 

 the composition of the Jamaica rocks, and the extent of country occupied by 

 them, touching but slightly on the sections which show their relative positions, 

 conceiving that these would be better thrown together, after the composition 

 of the formations had been described. 



Commencing then with the section from Kingston over St. George's Gap 

 to Buff Bay (Plate XIX. fig. 1.), we find the Liguanea diluvium forming an 



