1913.1 STEVENSON— FORMATION OF COAL BEDS. 59 



of 175 to 214 feet.**^ The extreme variations in interval have been 

 proved by continuous workings on the several splits. It is impos- 

 sible to determine the relations of these changes in interval through- 

 out the area, as erosion has been energetic in that contorted region 

 and the coal beds remain only in a few deep troughs. 



Illustrations are abundant in Europe. De Serres,*^ in his descrip- 

 tion of the little basin of Graissessac, says that the coal beds present 

 great regularity as a whole and preserve their parallelism almost 

 constantly. Nevertheless, one finds remarkable anomalies in some 

 parts of the basin. Coal beds approach each other in some localities 

 while in others they are far apart. At times the beds present the 

 appearance of a fan, especially well shown in the mines of one con- 

 cession; in some of those in another concession, coal beds 3, 5, 6, 

 are almost united, though in other mines. No. 3 is most frequently at 

 30 meters from No. 4. When one considers that the whole basin is 

 less extensive than the " outlying area " of Pittsburgh coal in Somer- 

 set county of Pennsylvania, he must be interested by de Serres's 

 loyalty to the orthodox doctrine amid trying circumstances. Gruner** 

 remarks that the parting of the Batardes coal bed is from 50 centi- 

 meters to 8 meters thick. In the middle portion of the Lower Saint- 

 Etienne stage, beds i, 2 and 4 coalesce with 3, which is very thick; 

 but at times, 4 is separated from 3 by 24 meters of rock. Beds 3 and 

 4 are frequently united as are also i and 2. The area of this stage 

 is little more than that of a township in one of the western states ; 

 according to the map, it does not exceed 40 square miles. FayoP" 

 has shown that the Grande Couche of Commentry is one bed at the 

 east side of the sub-basin but on the west side it is represented by 8 

 beds in a vertical section of more than 200 meters. Boulay and 

 others have given illustrations from north France. 



*" The observations on which these statements are based have been sum- 

 marized in "Carboniferous of the Appalachian Basin," Bull. Geol. Soc. Amer., 

 Vol. 17, 1906, pp. 219-221. 



" (M) De Serres, " Des terrains houillers du departement de I'HerauIt," 

 Acad. Sci. Montpelier, Vol. I., 1850, p. 384. 



*' L. Gruner, " Bassin houiller de la Loire," Paris, 1882, pp. 212, 220, 

 225, 226. 



*' H. Fayol, " fitudes," etc., p. 22. 



