Murchison s Silurian System. 225 



beds in which they are found. This copious list of fossils, 

 Mr. Murchison observes, enables us to speculate with cer- 

 tainty on the conditions under which the various strata of 

 this coal field were accumulated. Here we find the forms of 

 many terrestrial plants and even of insects, entombed amidst 

 a variety of shells, and some Crustacea, the greater part ma- 

 rine, but others, such as Uniones, and cypris, unquestionably 

 of fluviatile origin. From these circumstances Mr. Murchison 

 concludes that this tract of Coalbrook dale must originally 

 have been an arm of the sea, into which streams of fresh- 

 water discharged materials derived from those lands the 

 contiguity of which has been previously inferred from the 

 existence of fresh water limestone in the adjacent coal fields. 

 This view is also (Mr. Murchison remarks) quite in accord- 

 ance with that of Mr. Prestwich, who is of opinion that the 

 alternation of marine and fresh water shells do not prove as 

 many relative changes of land and sea, but that the coal 

 measures were deposited in an estuary into which a consi- 

 derable river flowed, subject to occasional freshes ; a position 

 rendered the more probable by the frequent alternation of 

 coarse sandstones and conglomerates with beds of clay, and 

 shale containing the remains of plants. 



The rock called millstone grit, which forms the basis of 

 coal-bearing strata in other districts is here wanting, unless 

 we consider the grits, conglomerates, and sandstones con- 

 taining seams of coal and courses of iron stone its equiva- 

 lent. These strata repose on the peculiar limestone of the 

 coal measures, but even this last rock is limited in Coalbrook 

 dale district, so that the greater portion of the coal-bearing 

 strata reposes in the southern part of the district on old red 

 sandstone, and in other situations where this is wanting, on 

 Silurian rocks. The limestone of this coal field is described 

 by Mr. Murchison as occurring in eight beds, having an ag- 

 gregate thickness of eleven yards, but varying individually 

 from 5 to 26 inches each, and are associated with impure lime- 

 stone and shale, amounting in all to upwards of one hundred 



