348 



PALEOZOIC SYSTEM OF ROCKS. 



rale for the identification of coal-seams of known value is often of 

 practical importance ; but it must be remembered that the strata of 

 coal-measures, both the seams and the accompanying shale and sand- 

 stones, like all other strata, thin out on their edges (page 174). Never- 

 theless, there is a most extraordinary continuity in the strata of the 

 coal-measures. 



Plication and Denudation. — Coal-bearing strata, like all other strata, 

 were, of course, originally horizontal (page 173) and continuous, but, 

 like other strata, they are now found sometimes horizontal and some- 



Fig. 451.— Section of Panther Creek Coal-Basin (after Ashburner). 



times dipping at all angles, and folded in the most complex manner. 



In the Appalachian region, especially in the anthracite region of North- 

 ern Pennsylva- 

 nia, the strata are 

 very much dis- 

 turbed, and the 

 coal-seams inter- 

 stratified with 

 them are often 



Fig. 452.— Illinois Coal-Field (after Daddow). 



nearly perpendicular (Fig. 451), while in Illinois and Iowa the coal- 

 strata are nearly or quite horizontal (Fig. 452). But, whether horizon- 

 tal, or gently folded, or strongly plicated, in all cases denudation has 

 carried away much of the upper portions, leaving them in isolated 

 patches as mountains or basins, as shown in the map of Northern Penn- 

 sylvania (Fig. 454) and in the section (Fig. 453). 



Fig. 453.— Section of Appalachian Coal-Field, Pennsylvania, showing Effects of Erosion on Gently- 

 Undulating Strata (after Lesley). 



By means of the rule for identifying seams given above, it is often 

 easy to trace the same seam from one basin to another, or from one 

 mountain-side to another, with great certainty. 



