Cjl XXTV.] CLAY-IRON-STONE. 405 



In the Edinburgh coal-field, at Birdiehouse, fossil fishes, mollusks, 

 and cyprides (?), very similar to those in Shropshire and Staffordshire, 

 have been found by Dr. Hibbert. In the coal-field also of Yorkshire, 

 there are freshwater strata, some of which contain shells referred to 

 the family Unionidce ; but in the midst of the series there is one thin 

 but very widely spread stratum, abounding in fishes and marine shells, 

 such as Goniatites Listeri (fig. 550), Ortkoceras and Avicula papyracea, 

 Goldf. (fig. 551). 



Fig. 551. 



Goniatites Listeri, Martin sp. Avicula papyracea, Goldf. 



(Pecten papyraceus, Sow.) 



No similarly intercalated layer of marine shells has been noticed in 

 the neighboring coal-field of Newcastle, where, as in South "Wales and 

 Somersetshire, the marine deposits are entirely below those containing 

 terrestrial and freshwater remains.* 



Clay-iron-stone. — Bands and nodules of clay-iron-stone are common 

 in coal-measures, and are formed, says Sir H. De la Beche, of carbon- 

 ate of iron mingled mechanically with earthy matter, like that con- 

 stituting the shales. Mr. Hunt, of the Museum of Practical Geology, 

 instituted a series of experiments to illustrate the production of this 

 substance, and found that decomposing vegetable matter, such as 

 would be distributed through all coal strata, prevented the farther 

 oxidation of the proto-salts of iron, and converted the peroxide into 

 protoxide by taking a portion of its oxygen to form carbonic acid. 

 Such carbonic acid, meeting with the protoxide of iron in solution, 

 would unite with it and form a carbonate of iron ; and this mingling 

 with fine mud, when the excess of carbonic acid was removed, might 

 form beds or nodules of argillaceous iron-stone.f 



* Phillips ; art. " Geology," Encyc. Hetrop., p. 592. 

 f Memoirs of Geol. Survey, pp. 51, 255, &c. 



