542 GEOLOGY. 



the system. Although there are many beds of coal in some regions, 

 and although some of them have great thickness (40 to 50 feet) the 

 proportion of coal in the Coal Measures is rarely one foot in forty, 

 and the proportion of iron ore is much less. 



The Coal Measures are differently divided in different regions. 

 A twofold division, namely a Lower and an Upper, is common. To 

 these series, local names are often applied. In Iowa, for example, 

 the Lower division is designated Des Moines, and the Upper, Mis- 

 sourian. 1 In other regions, as in Ohio, a fourfold division has 

 been made as follows: (1) Lower Coal Measures; (2) Lower Barren 

 Measures; (3) Upper Coal Measures, and (4) Upper Barren Measures. 

 The last is now generally referred to the Permian period. More recently 

 the following classification of the Pennsylvanian and Permian systems 

 of the east has been proposed 2 and somewhat generally adopted: 



Permian Dunkard formation (or series) = Upper Barren Coal Measures 



'4. Monongahela " " " = Upper Productive Coal Measures 



3. Conemaugh " " " = Lower Barren Coal Measures 



2. Allegheny " " " = Lower Productive Coal Measures 



1. Pottsville " " " 



Pennsylvanian 



Distribution and Outcrops. 



Between the Appalachians and the Rockies.— The distribution of 

 the Pennsylvanian system as now exposed is shown in Fig. 240. The 

 black areas represent the areas where the system is not covered, or 

 covered only by drift, etc.; the lined areas represent regions where 

 the system is believed to exist, though concealed; the areas marked 

 by dashes represent regions where the presence of the system is more 

 or less doubtful; while the dotted portions give some idea of the areas 

 whence the system has been removed by erosion. The system is prob- 

 ably concealed over considerable areas to the south of those repre- 

 sented on the map, where the surface is occupied by younger formations, 

 and its equivalent doubtless extends indefinitely under the sea. 



It is not to be understood that the system everywhere contains 

 coal in workable quantity, though within the areas indicated coal 

 is widely distributed (Fig. 241), occurring as far west as the 96th or 



1 Calvin, Bain, Beyer, Leonard, Young, and Keyes, Geol. Surv. of Iowa, Vols 

 XIII, XI, VIII, VII, III, and I. See also Keyes, Am. Geol. , Vol 23, 189.3, pp 298-316. 

 2 Prosser, Am. Jour. Sci., 4th series, Vol. XI, p. 191, 1901. 



