CARBONIFEROUS LIMESTONE. 



.105 



« 13. 



In this wood-cut are four figures selected from the mass of fossils to indicate the 

 mixed nature of the deposit, viz. 



The wing of the insect mentioned in the previous pages : its actual size is repre- 

 sented in fig. a. 



A land plant, fig. b., Pecopteris lonchitica, Brongniart, (Lindley and Hutton, Foss. 

 Flor. vol. ii. pi. 153.) A fluviatile shell, fig. c. (Unio acutus), Sow. M. C. t. 33. f. 5, 6 

 and 7. A well-known marine shell, fig. d., the Productus scabriculus, Sow. M. C. t. 69. 

 f. 1. 



Doubtless, therefore, as hinted at in the preceding chapter, this tract of Coal Brook 

 Dale must originally have been a bay of the sea, into which streams of freshwater dis- 

 charged materials derived from those lands, the contiguity of which has been previously 

 inferred from the existence of freshwater limestone in the adjacent coal-fields. This 

 view is also quite in accordance with that of Mr. Prestwich, who is of opinion, " that 

 the alternations of freshwater shells with marine remains, do not prove as many relative 

 changes of land and sea ; but that the coal measures were deposited in an estuary, 

 into which flowed a considerable river, subject to occasional freshes ; and he conceives 

 that this position is supported by the fact of frequent alternations of coarse sandstones 

 and conglomerates with beds of clay or shale," containing the remains of the plants 

 which have been brought down by the river. (See the positions (c, c.) where such 

 estuary accumulations were formed on the shores of the "mare carboniferum," repre- 

 sented in the small map, Chapter XI.) (c*.) is the site of Coal Brook Dale ; (a, a.) the 

 supposed lakes in which the freshwater limestone was deposited. 



Carboniferous Limestone. — It might be expected that as we descend in the series of 

 strata, we should next meet with the great arenaceous formation, called the " Millstone 

 Grit," which forms the substratum of many other coal-fields, and of which abundant ex- 

 amples will be adduced in the ensuing chapters. But in truth, there are here scarcely any 

 rocks which can be referred to that deposit, unless we consider as its representatives, 

 the. grits, conglomerates, and sandstones which form the lower portion of the coal 



