136 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



of elective affinity inducing plant materials to flock by themselves. 

 He did find mud containing much vegetable and other organic matter. 

 His observations indicate only that much plant material carried into 

 the sea does become waterlogged and does sink to the bottom ; but 

 they give no suggestion respecting formation of coal beds, although 

 they certainly explain well some features of carbonaceous shale. 



The doctrine of allochthony accounts satisfactorily for many 

 phenomena observed in the coal deposits, such as the fragmentary 

 remains of plants in the roof, the presence of drifted trees in sand- 

 stones, the occurrence of marine limestones and many others. But 

 these are explicable quite as easily by autochthony, so that they need 

 no further note at this point. Allochthony, as the writer under- 

 stands it, offers no adequate explanation of the lamination in coal, 

 which does not resemble that of sedimentation ; it fails wholly to 

 account for a structure such as that of the Pittsburgh coal bed, 

 whose thin partings of mineral charcoal and mostly impalpable silt 

 persist in an area of several thousand square miles ; its assumption 

 of distribution of sediments under influence of gravity fails when 

 applied to the Appalachian basin, for there the coal, as in Commen- 

 try and in other areas disappears with decreasing coarseness of sedi- 

 ments ; it aft'ords no means of explaining the remarkable purity of 

 some beds which yield coal of very high grade throughout continuous 

 areas of 2,000 to 7,000 square miles. 



The fundamental assumption of allochthony is that rain and 

 floods can remove the vegetable cover, living or dead, from land 

 areas and can convey it to a water basin, there to be deposited and 

 to become coal. This conception seems to be without foundation in 

 actual conditions and to be based upon study of erosive processes in 

 unprotected or disintegrated rocks. The effects of running water 

 on a cover of vegetation were examined in Part H. of this work. 

 It remains only to present the matter synoptically with reference to 

 statements made in defense of allochthony. 



It is well to restate the opinions offered by prominent defenders 

 of allochthony, that there may be no misapprehension. Bischof 

 thought that in the earlier times the land was more densely forested 

 than now and that the streams carried off a much greater quantity 



