STRATIGRAPHICAL ORDER. 113° 
ported, but such currents have also erosive power, and it has often hap- 
pened that by the removal of the thin beds of coal, clay, limestone, or 
ore that rest upon a stratum of sandstone, another stratum is let 
directly down upon the former in such a way as to defy separation at 
the point where the erosion has occurred. In other words, two sand- 
stones, belonging to two distinct epochs of history, are made to appear 
as one undivided and continuous formation. Many cases of mistaken 
identifications in our series are due to such a line of facts as is set forth 
above. 
It will be well to describe in brief terms the separate elements and 
the combined groups of the Lower Coal Measures, which are most use- 
ful in establishing the order of the great series to which they belong. 
The limestones which are especially serviceable in tracing and 
identifying the various sections of the Lower Coal Measures are the 
following, named in descending order: 
3. The Freeport Limestones (upper and lower). 
2. The Ferriferous Limestone, 
1. The Mercer Limestones (upper and lower). 
These will be separately described. The combined group are the 
various beds of coal, iron ore, and fire-clay that accompany or include 
these several elements. 
I. Tur MeERcER LIMESTONES. 
(a) The Lower Mercer Limestone is a thin but wonderfully 
persistent bed that has long been known and used as a geological guide. 
It received its name (Mercer Limestone) from the geologists of the 
First Pennsylvania Survey, and took quite a conspicuous place in the 
sections reported by them from the northwestern portion of that State. 
(Geol. of Penna., H. D. Rogers, vol. II, part I, p. 476, et al.) 
Much greater use has been made of it, however, by White and 
other geologists of the Second Pennsylvania Survey in establishing the 
order of the same portion of the coal-field. (See Report on Lawrence 
County, Q 2, page xxxi, et al.) 
The value of the same stratum in maintaining the order of the 
lower portion of the series in Ohio was first clearly recognized and 
emphasized by Newberry. (Report of Progress, 1870, page 16, et al.) 
In volume II, page 130, he says of this limestone, that it may be traced 
