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Mr. De la Beche on the Geology of Jamaica. 169 



seem probable that trap forms the southern part of the mountain range which 

 extends from Monte Diablo towards the north-eastern part of St. Thomas-in- 

 the-Vale. 



Superior or Tertiary Rocks ? 



To this order^ the white limestone formation, which occupies such a consi- 

 derable portion of Jamaica, would seem, if we only take into consideration 

 the character of its fossil organic remains, in some measure referrible. Still 

 however I have many doubts upon the subject; and therefore, while I class it 

 for the present under the above head, I wish it to be understood as merely a 

 temporary arrangement. This formation is principally composed of white 

 limestone, varying considerably in its texture, which is most frequently very 

 close; and, when so, strongly resembling the compact varieties of the Jura 

 limestones^ such as they occur among the Jura Mountains, and in the lower 

 part of the Mont Saleve near Geneva : in fact, specimens from the latter 

 place, and from some of the Jamaica white limestones, present scarcely any 

 difference. The white limestone of Jamaica is not however, taken generally, 

 of an homogeneous composition, some parts of the same stratum being more 

 argillaceous than others ; these latter parts becoming decomposed by the 

 action of the atmosphere, the surface is full of inequalities, from which cir- 

 cumstance the name of hon<^y-comh rock is very generally applied to it by the 

 Jamaica colonists. In consequence of the decomposed surface of the rock, 

 and the thick forests by which it is generally covered, the stratification is often 

 very difficult to determine. The strata are moreover often of very consider- 

 able thickness, — from 10 to 20 feet. This compact white limestone is, in some 

 districts, interstratified with thick beds of red marl and sandstone, as also with 

 white chalky marl. 



The compact white limestone, with its associated beds, constitutes the main 

 or middle part of the formation, and contains organic remains, generally casts : 

 beneath these limestones there are beds of sands and marls, the latter con- 

 taining fibrous gypsum, and being often associated with bluish gray compact 

 limestones, sometimes traversed by veins of calcareous spar. 



In this lower part of the formation there are beds of yellowish white lime- 

 stone, which are more earthy than the others, and contain a greater abun- 

 dance of organic remains, among which a gigantic cerithium may be noticed, 

 together with echinites, spines of echinites, ostreas, &c. Some beds are 

 principally composed of the remains of broken shells. 



The upper beds of this formation are generally chalky, sandy and marly : 

 still however many beds are very compact, and in general texture and appear- 



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