ON COAL. 



127 



coal strata are found in the form of basins. Sometimes the strata are 

 so folded as to give rise to series of basins belons^ing to tbe same 



Pig. 2. 



Fi?. 3. 



original field. "Whether, however, the strata retain their original 

 horizontality, or are thrown into basins by igneous agency, seldom or 

 never do we find the whole of the original mass deposited. A large 

 portion has been carried away by aqueous agency. Frrm this cause 

 a large coal field, covering many thousands of square miles, may exist 

 only in the form of isolated mountains or detached basins of coal 

 strata, as in the accompanying figures, where all the mass represented 



Fig. 4. 



Fig. 5. 



FiS. 6. 



by tbe dotted lines has been carried away by denuding agencies. 

 Thus, for instance, nearly the whole of Illinois was originally occupied 

 by a vast coal field, but little disturbed by igneous agency, but by far 

 the larger portion of the coal strata of this immense field was carried 

 away by denuding agencies. 



You will observe, then, the striking difierence in mode of occur- 

 rence between metallic ores and coal. The former are associated with 

 rocks of every age, except, perhaps, the tertiary ; the latter almost 

 exclusively confined to those of a particular age. The former exist 

 in the form of veins intersecting the strata, the latter in the form of 

 seams parallel with the strata. The former extend indefinitely down- 

 wards, the latter horizontally. The former are the result of igneous 

 agency, the latter of sedimentary deposit. Ignorance of this simple 

 but radical difierence has been the cause of much pecuniary loss, and 

 seems not yet entirely eradicated. When, for instance, some years ago 

 it was rumored in the streets of Philadelphia that the bottom of the 

 Mauch Chunk Summit mine was reached, there was an universal panic, 

 and stocks in coal mines went down enormously, not knowing that the 

 continuation of coal seams was to be looked ibr horizontally rather 

 than vertically. 



This simple rule, when taken in connexion with the one previously 

 enunciated, viz: that a coal seam throughout its whole extent is 

 attended both above and below by the same strata, would render the 

 identification of coal seams, and the tracing ot them across valleys 

 from hillside to hillside, a matter of little difficulty, were it not for 



