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CARBONIFEROUS AND PERMIAN FORAMINIFERA. 



We need go no further than the magnesian limestone of the Permian system 1 to find 

 deposits in which this phenomenon is exhibited in an exaggerated degree ; but instead 

 of a compact rock composed chiefly of spheroids of minute, but comparatively uniform size, 

 the constituent masses vary in magnitude from a microscopic smallness to balls many ounces 

 or even pounds in weight, and form a loose " pebbly " bed. Whence the carbonate of lime 

 has been derived, in the case of the oolitic Carboniferous rocks, previous to its solution and 

 precipitation, it is impossible to say, possibly from the calcareous skeletons of animals ; all 

 that it is sought to establish is, that the proximate origin of these particular beds has been 

 dependent in great measure on physical agencies. That the process of precipitation was 

 cotemporaneous with the actual life of marine animals there can be no doubt, from the 

 frequent presence, amongst the spheroids, of perfect, delicate shells, such as would be the 

 first to yield to the power of any active solvent, and the same fact also forbids the idea 

 that the spheroidal structure may have been the result of physical changes at a later period 

 of the earth's history. 



The minute structure of the Carboniferous and Permian rocks only affects the subject 

 incidentally ; but it seemed necessary at the outset to state the great difference which 

 exists between the calcareous beds of the Carboniferous period, as represented in our 

 Mountain Limestones, and those of Cretaceous age represented by the White Chalk. In 

 point of fact, the marine Carboniferous deposits of these islands seem to bear far more 

 analogy to the preceding palaeozoic formations — to the Devonian especially, with its- 

 multitude of Corals and Crinoids, and its scanty evidence of the minuter Protozoa, than to 

 the microzoic rocks of a later epoch. 



The lithological characters of the massive palaeozoic limestones are the cause of the 

 chief difficulties the palaeontologist has to contend with. They are almost invariably hard 

 and generally subcrystalline. They are often largely impregnated with silica, thereby 

 possessing an uneven texture, which renders uniform grinding, whether for the purpose of 

 microscopical sections or for the sake of obtaining a polished surface, almost impossible, and 

 yet disintegration, under ordinary circumstances, cannot be effected by artificial means. 

 When free from siliceous infiltration, it is not more difficult to cut thin slices from them 

 than from other rocks of similar hardness ; but the mere sections of Foraminifera so 

 obtained are of little value, zoologically speaking, unless they can be identified by com- 

 parison with specimens in which the external characters are visible and readily determined, 

 that is to say, specimens freed from the matrix. 



But that which is so difficult to accomplish by artificial means is sometimes done for us 

 on a large scale by natural agencies ; that which chemical solvents, whether rapid or more 

 gradual in their action, and physical processes, such as calcination, the efflorescence of 

 crystallised salts, or treatment by superheated steam under high pressure, effect to only a 

 limited extent when artificially applied, is brought about under favorable conditions by 



1 One of the beds at Fulwell Quarry, near Sunderland, for example. 



