100 Prof. Sedgwick on the Carboniferous Chain 



the waters of a sea of considerable depth ; and I never saw any shells of 

 decided freshwater genera in the carbonaceous shales alternating with the 

 mountain limestone*; but these shales sometimes contain thin beds of ma- 

 rine shells, chiefly productce. I am of course aware that freshwater shells 

 occur abundantly in some of our rich coal-fields; and in such cases the de- 

 posits maj/ have taken place in lakes, or more probably in shallow bays and 

 estuaries. It is, perhaps, in very few instances necessary to consider them 

 of purely lacustrine origin ; especially when we remember, that in a part of 

 the great Yorkshire coal-field (far above any of the groups represented 

 in the preceding sections) there are beds abounding in pectens and am- 

 monites. 



3. Certain species of marine fossils abound in particular groups of the lime- 

 stone strata, and so far become in some measure characteristic of them ; but 

 I am not sure that any one species is actually confined to one limestone group. 

 Most of the fossils of the Scar limestone may be found, here and there, in the 

 twelve-fathom limestone, and vice versd-\. Wherever there is a sudden change 

 of mineral character, we may, however, remark an equally sudden change in 

 the fossil species. 



Thus, for example, very few of the corallines, encrinites, bivalves, &c, 

 (abounding in the limestone) are found in the alternating beds of shale];; 

 and impressions of coal-plants hardly ever occur in the limestone groups, 

 though they abound in the alternating groups of sandstone and shale. This 

 distribution obviously originated, partly in the habits of the animals above- 

 mentioned, and partly in the mechanical causes by which the beds themselves 

 were produced. 



4. The several groups of limestone, so remarkable above all the other 

 strata for their regularity and continuity, were evidently the result of a slow 

 tranquil deposit, assisted by the action of organic bodies ; and most of the 



* Some geologists have contended, not merely that all coal-beds are lacustrine, but that 

 they once existed as turf bogs, — a supposition surely inapplicable to the carboniferous chains of 

 Northumberland and Yorkshire. To bring such a theory into action, we must suppose that the 

 chains in question were elevated, and again submerged under the ocean, at least as many times as 

 there are beds of coal alternating with beds of encrinite limestone; — a most cumbrous, and, I think, 

 an incredible hypothesis, which never could have been started by any one who had examined the 

 coal-fields in the basin of the Tweed, or even the carboniferous chains of the North of England. 



f I may, however, remark, that I have never seen trilobites, ammonites, orthoceratites, and 

 perhaps some other rare fossils of the mountain limestone, in any part of the series above the great 

 Scar limestone. 



X The bands of calliard or crow-limestone, above described, may seem an exception to this rule; 

 but the number of species they contain is, if I mistake not, very limited. 



