648 HISTORICAL GEOLOGY. 



through Ohio, Kentucky, Indiana, and beyond ; but is mostly a sandstone, 

 where present, in the Mississippi basin. But even there, beach-like features 

 are often observed. Like the coal-beds of the Coal-measures the formation 

 was only approximately at a common level. 



In part of western Pennsylvania the Pottsville conglomerate contains 

 one or more coal-beds. Just above the Sharon conglomerate, the base of the 

 Pottsville series in Mercer County, Pa., one coal-bed is two to four feet thick, 

 and has long been worked. The same bed is mined also in Ohio. A bed of 

 similar character occurs in the conglomerate of Kentucky, Tennessee, and 

 Alabama, and that of Alabama aifords excellent coal. These coal-beds, with 

 their alternating beds of shale, prove that slow and varying changes of level 

 were in progress, but that for prolonged intervals portions of the surface lay 

 quiet until deep accumulations of vegetable debris had been made in the 

 marshes. The fact of a general parallelism in the movements over Europe 

 and America favors the view that the changes in level and in deposits were 

 a consequence, in a general way, of oscillations in the sea level, that is, in the 

 crust of the sea bottom ; but at the same time there were other variations 

 in level which were dependent on local conditions and movements over the 

 continents. 



THE COAL-MEASURES. 



The Coal-measures in Pennsylvania are divided into (1) the Lower Pro- 

 ductive Measures, (2) the Lower Barren Measures, (3) the Upper 

 Productive Measures. Above the last there are the Upper Barren 

 Measures, corresponding to the Permian. 



Over the great Appalachian-Arkansas area, the three great Carboniferous 

 or Coal-measure regions are, as shown on the map, page 412, (1) the 

 Appalachian, extending from northern Pennsylvania to Alabama, and having 

 the Anthracite region as a detached portion in eastern Pennsylvania; (2) the 

 Illinois-Indiana, east of the Mississippi, extending south into Kentucky; 

 and (3) the Iowa-Texas, west of the Mississippi. 



The Appalachian area spreads west into Ohio, eastern Kentucky, eastern 

 Tennessee, and northern Alabama. In Tennessee, the Cumberland Table- 

 land hsuS the Coal-measures for the top, and a substructure of Subcarbonifer- 

 ous rocks, 1000 feet or more thick, for the rest of its height. In Alabama, the 

 western portion, constituting the large Warrior coal-fields, is a continuation of 

 the Cumberland Measures, with an extension far westward nearly to the Missis- 

 sippi line — Mississippi having only a small patch of Subcarboniferous beds. 



It is probable that the Coal-measures of Tennessee, and those of Alabama, 

 originally spread across what is now the Mississippi valley and joined the 

 area of southern Missouri. 



The Carboniferous areas are generally much broken, especially so in 

 Pennsylvania and along the Appalachians to the southwest of this state. 

 The following map, by Lesley, illustrates in a general way the condition in 

 Pennsylvania. The Anthracite coal is in narrow isolated strips to the east- 



