PART II. CHAPTER XX. 



245 



Fossils of the Carboniferous Group, 



Freshivater fossils. 

 Fiff. 244. 



■Coal. 



a. Microconchus Cypris inflata, natural 

 carbonarius. size, and magnified. 



h. var. of same, nat. Murchison.* 

 size, and magnified. 



of plants, including a bed of 

 limestone, varying from two 

 to nine feet in thickness, 

 which is cellular, and re- 

 sembles the lacustrine lime- 

 stone of France and Ger- 

 many. It has been traced 

 for 30 miles in a straight 

 line, and recognized at more 

 distant points. The charac- 

 teristic fossils are a small 

 bivalve, having the form of 

 a cyclas, a small cypris, 

 (Fig. 245.) and a miscrosco- 

 pic shell, (microconchus) of 

 an extinct genus. 

 But in the lower coal-measures of Coalbrook Dale, the strata, 

 according to Mr. Prestwich, often change completely within very 

 short distances, beds of sandstone passing horizontally into clay, 

 and clay into sandstone. The coal-seams often wedge out or 

 disappear; and sections, at places nearly contiguous, present 

 marked lithological distinctions. In this single field, in which 

 the strata are from 700 to 800 feet thick, between 40 and 50 

 species of terrestrial plants have been discovered, besides several 

 fishes and trilobites ; the latter distinct in form from those occur- 

 ring in the Silurian strata. Also upwards of 40 species of mol- 

 lusca, among which are two or three of the freshwater genus 

 Unio, and others of marine forms such as Nautilus, Orthoceras, 

 Spirifer, and Productus. Mr. Prestwich suggests, that the inter- 

 mixture of beds containing freshwater shells with others full of 

 marine remains, and the alternation of coarse sandstone and 

 conglomerate with beds of fine clay or shale containing the re- 

 mains of plants, may be explained by supposing that the deposit 

 of Coalbrook Dale, originated in a bay of the sea or estuary into 

 which flowed a considerable river subject to occasional freshes. f 

 In the Edinburgh coal-field at Burdiehouse, fossil fishes, mol- 

 lusca and cypris, very similar to those in Shropshire and Staf- 

 fordshire, have been found by Dr. Hibbert.ij: In the coal-field 

 also of Yorkshire there are freshwater strata, some of which 



* Silurian System, p. 84. 



t Prestwich, Geol. Soc. Proceedings, No. 46. Murchison, Silurian System, 

 p. 150. 



X Trans. Roy. Soc. Edin. vol. xiii. Horner, Edin. New Phil. Journ., April 

 1836. 



