COM, FIELDS. 261 



have coal seams directly above them. In addition to the persistent 

 sheets of limestone, there are many sporadic deposits of calcareous 

 character. 



Always associated with the limestones, and often replacing them, are 

 ten or twelve deposits of iron ore, some of which have been of large 

 economic value. 



Distributed throughout the entire series, and giving name and char- 

 acter to the whole, are fifteen or more seams of coal, ranging from a 

 black mark to a dozen feet in thickness. The valuable phases of the 

 seam are uncertain and unsteady, but the horizons are persistent and 

 distinct, to a wonderful degree. Each, as we know, stands for a land 

 surface. With the seams of coal and far more extensive and steady 

 than they, are beds of fire clay, of varying degrees of purity. Almost 

 every coal seam is underlaid by clay, and the limestone and ores are also 

 covered or supported by them in many cases. 



How are these elements distributed? By no means at hap-hazard 

 throughout the scale, as it might appear at first sight. A well-marked 

 order of arrangment comes into veiw when the series is properly studied. 

 The three elements of those already named that stand for life, that repre- 

 sent the agencies and forces of life, are always found in close proximity 

 to one another. Coal, standing for the life of the land, limestone, for 

 the life of the sea, iron ore, equally dependent on life for its separation 

 and concentration, but blended with both limestone and coal, these form 

 vital nodes in the series, relatively of small amount but containing most 

 of its economic interest and value. The intervals between the nodes 

 are occupied by the sandstones and shales already named as forming the 

 bulk of the series. They are, for the most part, barren of life, and owe 

 their accumulation to inorganic forces. Measured against the products 

 of life, these sandstone intervals have a thickness of five or ten feet to 

 one. But these intervals are approximately equal. In every part of 

 the field, some normal measure will be found that will occur again and 

 again in the sections we shall meet, as we rise with stair-like regularity 

 from coal to coal, or from one limestone to another. 



The eastern, northern and western boundaries of the Ohio coal field 

 are well defined and well known. We're the lowest coal seams formed 

 over this entire area? Have we a right to expect their presence within 

 the central portions of the basin if we descend deep enough? Were the 

 earlier seams progressively covered with the swamps of all the latest 

 coals, and were the last the widest in extent? These questions, and 

 others of like import, have been variously answered, but the affirmative 

 replies are so irreconcilable with the facts o the field, and with the first 

 principles and established laws of coal geology that they must ultimately 

 be abandoned and discarded. A few propositions embracing these points 

 that could be abundantly expanded and supported if time and space 

 allowed, will be stated here. 



