ROCK-SYSTEM OR COAL-MEASURES. 



347 



measures, it is evident that but a small proportion consists of coal. The 

 coal-measures consist, in fact, of thick strata of sandstone, shales, and 

 limestone, like other formations ; but in addition to 

 these are interstratined thin seams of coal and beds of 

 iron-ore. Even in the richest coal-measures, the pro- 

 portion of coal to rock is not more than as 1 to 50, and 

 the proportion of iron is still much smaller. In some 

 coal-fields, as, for example, in the Appalachian, mechan- 

 ical sediments, shales, and sandstones, predominate ; in 

 others, as in the Western coal-fields, organic sediments 

 or limestone predominate. 



The five kinds of strata mentioned are repeated in 

 the same coal-basin very many times — perhaps 100 or 

 more, as in the accompanying section ; but, in com- 

 paring one coal-field with another,. or in the same coal- 

 field, in comparing one portion of the series with an- 

 other, there is no regular order of sticcession discovera- 

 ble. Except that immediately in contact with the 

 seam, and beneath it, there is nearly always a thin 

 stratum of fine fire-clay. This constant attendant of a 

 coal-seam is called the under-day. Again, immediate- 

 ly above, and therefore forming the roof of the opened 

 seam, there is frequently, though not so constantly, a 

 shale which, being impregnated with carbonaceous 

 matter, is called the black shale or black slate. These 

 accompaniments are, however, usually too thin to ap- 

 pear on sections. 



In different portions, however, of the same coal-field 

 geological horizon, we are apt to find the same order. 



Fig. 450. — Ideal 

 Section, showing 

 Alternation of 

 Different Kinds 

 of Strata : Ss, 

 Sandstone ; Sh, 

 Shale ; I, Lime- 

 stone ; i. Iron, 

 and c, Coal. 



at the same 

 This is the 



necessary result of the continuity of the strata over the whole basin. 

 If we represent coal-basins, with their five different kinds of strata, by 

 reams of variously-colored paper, then, while the order of succession 

 may be different in the different reams, and in the upper or lower por- 

 tion of the same ream, yet at the same level we find the same order in 

 every portion of the same ream. This is a test of a coal-field even 

 when separated by denudation into several basins. It is also a mode 

 of identifying individual coal-seams ; for, if the strata be continuous, 

 then the seam will have the same accompanying strata above and be- 

 low. The Pittsburg seam has been thus identified, with great proba- 

 bility, over an area of 14,000 square miles, and, allowing for removal 

 by denudation, over an original area of 34,000 square miles. Rogers 

 thinks the original area may have been 90,000 square miles.* This 



* Phillips, Geology, p. 217. 



