AGE OF COAL. 215 



strata or seams of various degrees of thickness, between 

 strata of different kinds of rocks. Sometimes the layers 

 are almost as thin as paper, and, on the other hand, they 

 are sometimes 30, 40, and even 60 feet thick. Seldom, 

 however, do they exceed eight feet in thickness. The 

 lower part, or floor, as we may term it, of the coal for- 

 mation is limestone ; but the upper part, where the beds 

 of coal lie between strata of rock, is made up of sand- 

 stones, conglomerates, shales, and limestones. It is this 

 upper portion that is commonly called the coal-measures. 

 The rocks between which the coal is laid down do not 

 differ from many rocks in other formations, but are to be 

 distinguished from them only by their fossils. It is their 

 life-record that indicates their proximity to coal. This 

 fact is of great practical use in searching for localities of 

 coal, and a disregard of it has sometimes occasioned 

 needless expense. On this point Professor Hitchcock 

 remarks : " No geologist would expect to find valuable 

 beds of coal in the oldest crystalline rocks, but in the 

 fossiliferous rocks alone ; and even here he would have 

 but feeble expectations in any rock except the coal for- 

 mation. What a vast amount of unnecessary expense 

 and labor would have been avoided had men who had 

 searched for coal been always acquainted with this prin- 

 ciple, and able to distinguish the different rocks ! Per- 

 pendicular strata of mica and talcose schists would then 

 never have been bored into at great expense in search 

 of coal ; nor would black tourmalin have been mistaken 

 for coal, as it has been." Commonly, the rock that lies 

 directly under a bed of coal is clayey in its character, 

 and is called the under-day. From its composition, and 

 from the fact that roots of certain carboniferous plants 

 are often found in it, it is inferred that it was the dirt- 

 bed in which the plants grew that furnished the mate- 

 rial for the formation of the coal. The amount of rock 

 in the strata of the coal-measures preponderates vastly 

 over the amount of coal, the proportion being generally 

 fiftv or more feet of rock to one of coal. 



