THE PENXSYLVANIAN PERIOD. 



573 



west of the Appalachians, the number is often less than a dozen (Figs. 254 

 to 257). Thus in Illinois the number of workable beds is nine. 1 



p ft /* M f fiS/e- J%^*zo&Za/ Scci/e 



jro soo /so -zoo.jfeeZ Y&t=j?xc-<a/ «_Jc&Z? 



1 ii 1 



Fig. 255. — Section showing coal beds (heavy black lines) in Bay County, Mich., 

 Amelith shaft. (Lane, U. S. Geol. Surv.) 



If the foregoing sequence of events was repeated for each coal- 

 bed, it is manifest that the surface of great areas of the continent or 

 else the surface of the sea, was in an unstable attitude during the Car- 

 boniferous period. The second hypothesis (b) has the merit (so far 

 as it is a merit) of greater simplicity than the first, but is probably 

 not applicable to all cases. The fact that local unconformities abound 

 in the Coal Measures (Ind., 111., la., et ol, Figs. 258 to 260) shows that 

 the surface was locally out of water and subject to erosion at vari- 



Fig. 256. — Section showing coal belt (heavy black lines) along the Pere Marquette 

 Ry., Mich., east from Alma. (Lane, IT. S. Geol. Surv.) 



ous times and places in the course of the period. This points to changes 

 in the relative level of land and water (partly the sea and partly land 

 waters), though it does not show which was the changeable element. 

 When it is remembered that many relative changes of level may have 

 taken place without giving origin to coal seams, it is not too much 

 to sav that the number of fluctuations in the relative level of land 



1 Reports on coal have been published by all States containing coal, where there 

 have been surveys. For local details see reports of Pennsylvania, Ohio, Kentucky, 

 Tennessee, Alabama, Indiana, Illinois, Iowa, Missouri, Arkansas, Kansas, and Texas. 



