This secondary limestone is accompanied by the magnesian limestone, but I cannot say that the one overlies the other. They rather seem to occur by separate beds, or sometimes by patches, the one within the other. The magnesian limestone does not put on that regular stratified appearance which is so conspicuous in the other, nor have I observed in it any organic remains but in one single instance, and they are similar in their composition to the stone itself.
Mr. Parkinson has most obligingly furnished me with a list of the organic remains contained in the limestone of the Isle of Man, and he farther remarks, that the whole much resemble those found in Westmoreland, Cumberland, Durham, and, as far as he can judge, those of Kilkenny in Ireland.
The following are the organic bodies contained in the specimens that were submitted to his examination.
A large madrepore from two to three inches in diameter with distinct branches, a small madrepore with distinct branches, minute madrepores and entrochitæ, large trochitæ and terebratulæ, the latter varying from two inches to half an inch in diameter, large entrochi, small fragments of minute trochitæ.
The limestone of Castle-town, Scarlet, Pool-vash, and Ball-Fhallack is of a grey, or more generally of a dark grey colour: the texture compact with some lamellar concretions. It is traversed by slender veins of calcareous spar, and the dip-joints are coated
- This is an expression I beg leave to introduce, and refer to the judgement of Geologists. All the stratified rocks, but particularly those that have been deposited in a rather horizontal situation, are divided by rents perpendicular to their direction, corresponding therefore with their dip, into solids of dimensions more or less regular, and usually similar in the same individual strata. Thus if the structure of a rock be middle thick-slaty, it will divide itself spontaneously into rectangular solids. When, the thickness of the strata bears a more equal proportion to the length or respective distance of the dip-joints, the natural divisions will come to a cubical form. If on the contrary, the structure be thin-slaty, the rocks will rise spontaneously under the form either of flags or slates. Whatever may be the shape of these natural blocks, their two contiguous sides are constantly coated with the predominating ingredient of the rock itself in its present state. Thus if the rock be quartzose, the coatings of the dip-joints will be veins of pure white quartz; if it be calcareous, the coatings will be crystallized calcareous spar, but unless we cause artificially the splitting of the rocks to take place, seeing but a vertical or a horizontal section of the coatings, we are apt to adopt the wong opinion they are veins instead of layers.