I9I2.J STEVENSON— THE FORMATION OF COAL BEDS. 509 



tinctly not marine. The best summary of the conditions was given 

 by Barrois/"^ who was led by the apparently contradictory modes 

 of occurrence to make thorough investigation of the whole subject. 

 Some of his conclusions will find place in another connection but 

 others are of interest here. He found Spirorhis shells attached to 

 living plants, to plant debris, to brackish and even to essentially 

 salt water animals. In the larval condition, the type is free but in 

 the adult, fixed. It is allied to Vermetus and Spirorhis of the 

 present time, all marine. To reconcile the existence of such forms 

 on plants essentially terrestrial, one must admit that the original 

 Carboniferous Spirorhis lived fixed on marine shells; that the de- 

 scendants became habituated to brackish water, where they attached 

 themselves to Carhonicola; and at last to fresh water, where they 

 became fixed on ferns. It is very evident that the conditions noted 

 by Barrois exist in the Appalachian basin, so that the presence of 

 Spirorhis cannot be regarded as evidence for or against any 

 hypothesis respecting the character of the water. 



It is quite possible that the occurrence of Naiadites may be evi- 

 dence of brackish water invasions. 



Reference has been made to very dark carbonaceous or bitumi- 

 nous limestones, of which the Brush Creek may be taken as type. 

 They are not numerous. Their fauna is marine, they are often 

 fetid and, at times, have some vegetable matter. The Brush Creek 

 limestone, as has been shown, occupies a long narrow area, bordered 

 by shales and sandstones and, at some localities in both Pennsylvania 

 and Ohio, it suffered severely from erosion prior to deposition of 

 the overlying rock. De Dorlodot^"^ has discussed the origin of 

 sapropelic limestones and his conclusions seem to require examina- 

 tion at this point. Some crushed polyps, exhibited at a meeting 

 of the Geological Society, were in a gangue which had undergone 

 slow compression, such as that which Potonie had recognized as 

 characterizing sapropelian muds. A. Renier had accepted sapro- 



^"" C. Barrois, " Sur les Spirobes du Terrain Houiller de Bruay (Pas-de- 

 Calais)," Ann. Soc. Geol. du Nord., Vol. XXXIII., 1904, pp. 50 et seq. 



"* H. de Dorlodot, " Sur les conditions de depot des marbres noirs dinan- 

 tiens et des sapropelites marines en general." Bull. Soc. Beige de Geol., 

 Vol. XXV., 191 1, P. v., pp. 146 et seq. 



