Murchison' s Silurian System. 219 



fresh water, or in bays in which fresh predominated over salt water) 

 afford convincing proof of the existence of neighbouring dry land, from 

 which rivers flowed, transporting terrestrial vegetable remains, and 

 entombing them with shells, the greater part of which must, unques- 

 tionably, have lived in fresh water. That such streams, however, were 

 near the sea, and that in fact they soon passed into estuaries, will be 

 presently rendered evident by details of the undeniable alternation and 

 intermixture of fresh water, terrestrial, and marine remains in Coal- 

 brook dale, which tract, though only distant a few miles from that un- 

 der consideration, exhibits a vast expansion of the carboniferous strata ; 

 thus leading us to suppose, that whilst the Shrewsbury deposit has been 

 simply formed by streams issuing from the Cambrian and Silurian regions, 

 and giving rise to lakes to which the sea had little or no access, the 

 greater carbonaceous masses of Coalbrook dale have been accumulated 

 by the same waters where they united to empty themselves into an 

 estuary ! The north-eastern edges of this great marine bay were form- 

 ed near Manchester, its western margin being marked by the zone of 

 carboniferous limestone which bounds the coal-fields of Oswestry, Chirk, 

 and Ruabon. An inspection of a general geological map of England 

 will indicate the extent of the area, which now appears as a vast trough 

 of new red sandstone encircled by carbonaceous deposits. Further 

 observations upon the origin of these coal-fields occur in the ensuing 

 pages, particularly in the concluding part of the eleventh chapter, 

 where a small map will be found, explanatory of the probable physical 

 geography of this region during the period at which the accumulation 

 of the carboniferous deposits took place. 



Mr. Murchison here remarks in a note, that these cen- 

 tral coal fields contain fossil fishes, molluscs, and ento- 

 mostraca, identical with, or closely allied to those of Bur- 

 die House limestone, which was once supposed to be of 

 much greater antiquity than the coal measures. The iden- 

 tity of fossils in both strata, and the circumstance of the 

 Shropshire and Lancashire rocks being placed at the upper 

 series of the coal formation, and found to graduate up- 

 wards to the new red sandstone, are good inductive proofs 

 of the beds in both being equivalent, and also of the true 

 age of the Scotch rock, whose isolated position had render- 

 ed its rank doubtful in the series of ancient strata. 



In the next chapter Mr. Murchison passes, as he him- 



2 G 



