191^.] STEVENSON— THE FORMATION OF COAL BEDS. 527 



but decreases until it is the same with that of the lake bottom. 

 The slope at the shore varies greatly. In Lake Geneva, that of the 

 Thonon delta is 30 degrees, but that of the Rhone is much less. As 

 the delta advances, the river's mouth is carried forward, while the 

 stream meanders and covers the Neptunian beds with alluvium, 

 which is horizontal. This formation of the alluvium is sometimes 

 so slow as not to interfere with vegetation, as in the chief deltas of 

 the Alps, those of the Reuss, Aar and Rhone. Fayol describes some 

 natural and artificial sections of lake deltas as illustrating the 

 amount of transported material: he says that, above the confluence 

 of the Aar and the Thiele, all lake basins have disappeared up to 

 the Jura; alluvium from the streams, the growth of peat bogs and 

 the work of man have converted them into prairies. Marine deltas 

 differ from those of lakes as the waves and tides interfere with regu- 

 lar development. The deltas of great rivers differ yet more ; the 

 Neptunian deposits are more extensive and less inclined, there being 

 beds of several thousand square kilometers with inclination of only 

 some centimeters per meter. Little however is known respecting 

 these deltas, information having been obtained from only a few 

 borings. Enough, however, is known to prove that great deltas con- 

 tain deposits of vegetable matter, that the beds are less coarse and 

 less inclined than those of lacustrian origin. The arrangement of 

 the beds is closely dependent on the agitation of the water, which 

 in turn is dependent on the size of the basin. 



After recounting his experiments, which he regarded as amply 

 confirming his conclusions respecting the origin of the Commentry 

 sands, shales and coal beds, Fayol returns to discuss the primitive 

 inclination of sedimentary rocks. As the doctrine of primitive 

 horizontality has had important influence in the formulation of 

 doctrines respecting the formation of coal beds, he thinks useful 

 to examine it to the foundation and to prove its falsity. He cites 

 Steno, Elie de Beaumont and Dufrenoy, Lyell and Credner in favor 

 of the doctrine, following the notes with a long quotation from 

 LyelP^' in which are described the irregular and steeply inclined 

 deposits near Nice, which that author thinks had the present abrupt 



"'C. Lyell, "Elements of Geology," sixth ed.. New York, 1866, pp. 18, 19. 



