PALEOZOIC ROCK8 AND ERA. 



299 



furl, the coal-measures consist of .alternations of sand- 

 stones, shales, and limestones, like other formations ; but 

 interbedded with these are also seams of coal and beds 

 of iron-ore. These five kinds of strata alternate with 

 each other, and are each repeated many 

 times, but without any regular order, as si 

 shown in Fig. 319, which is an ideal 

 column from a coal-field. Thus, the 

 strata of a coal-field may be likened to a 

 ream of sheets of five colors, but arranged 

 without order. Only this may be said, 

 that beneath every coal-seam there is al- 

 ways a thin layer of clay, called the under- 

 day, and above is usually, but not always, 

 a shale, called the Uack shale or roof- 

 shale. It is a rich coal-measure in which 

 we find one foot of coal for fifty feet of 

 rock. 



Subsequent Changes. The strata of 

 coal-measures, like all other strata, were 

 horizontal when first laid down ; but, like 

 other strata also, they have been elevated, 

 and tilted and folded and crumpled and 

 broken and faulted, especially in moun- FlG - 219. ideal sec- 



. . . , . ,, , , tion, showing alter- 



tam-regions. And in all cases, whether nation of different 



horizontal (Fig. 221) or folded (Fig. 220), 



they have been largely carried away by 



erosion, and the strata left outcropping 



on the surface (Figs. 220, 221), and often 



in isolated patches. Since coal-seams, like other strata, 



are broken and faulted, it is very important to remember 



the law of slip mentioned on page 232. 



Thickness and Number of Seams. The thickness 

 of seams varies from a few inches to many yards. The 

 mammoth seam of Pennsylvania is over one hundred feet 

 thick. The best thickness for easy working is about six 



kinds of strata : Ss, 

 sandstone ; Sh, 

 shale ; J, limestone ; 

 , iron ; and c, coal. 



