19".] STEVEXSOX— FORMATION OF COAL BEDS. 95 



The Pittsburgh coal bed thickens toward the southeast and the 

 slate partings, as well, thicken in that direction. The evidence favors 

 the assumption that the organic as well as the inorganic materials 

 came from the land surface in that direction. The absence of 

 Stigmaria casts reasonable doubt upon the hypothesis of formation 

 in situ, and this doubt is increased by the discovery of an aquatic 

 fauna in the underclay of the bed, which Gresley has found to be a 

 calcareous shale. 



The extraordinary uniformity of the Pittsburgh coal bed in 

 purity and structure, the evenness and geographical extent of its 

 several divisions make it the most remarkable known. In explana- 

 tion of its phenomena, about all that can be said safely is " that, 

 everything being horizontally stratified, every part of it was most 

 likely accumulated under water. I have therefore come to the con- 

 clusion that this coal is the accumulated remains on the bottom of 

 a lake or sea of vegetable growth of aquatic forms (though much 

 of it did not necessarily grow iu the water) living afloat and dying 

 and decaying, falling through the water." All the familiar phe- 

 nomena can only be explained by an aqueous origin for the coal. 



The problem of coal accumulation attracted Potonie's attention 

 in 1886 but he published no results of direct study until 1895. ''•' In 

 that year he had opportunity to study a core obtained in the Upper 

 Silesian coal field. This core, 750 meters long, one to 2 decimeters 

 in diameter, begins in Saarbruck beds and ends in the Upper Ostrau 

 deposits. As submitted to Potonie, it was complete and it was 

 studied by him in company with C. Gaebler of Breslau. The core 

 shows not less than 2"/ coal beds, each of which is in direct contact 

 with a Stigmaria underclay ; in most of them, remains of Sigillana 

 are present and some contain Lcpidodcndron — particularly in the 

 accompanying carboniferous shale. 



Ochsenius, who urged the allochthonous origin of coal beds, 

 explained cases, such as are present in the core, as due to local 

 subsidences and thought them of rare occurrence. But Potonie, as 

 an outgrowth of broad observation, asserts that these cases are 



"^ H. Potonie, " Ueber Autochthonie von Carbonkohlen-Flotzen und des 

 Senftenberger Braunkohlen-Flotzes, Jahrh. d. k. preuss. geolog. Landesanstalt 

 fiir 1895, PP- 31, pl- 2. 



95 



