16 GEOLOGY OF OHIO. 
beds associated with them, if is seen how admirably adapted the lime- 
stones are as single elements, and the limestones with their ores and 
coals are, as combined groups, to become plain and trustworthy guides 
to the order of the Lower Coal Measures. When either of the lime- 
stones is found, it is hard to misinterpret the section in which it belongs, 
but when the six or more elements of the combined groups occur in a 
single section as they often do, there is no excuse for going wrong in 
the determination of its place in the general scale. 
Too much stress cannot well be laid on these elements. The Lower 
Mercer horizon, in particular, is the one undisputed and indisputable 
element in the Lower Coal Measures of Pennsylvania and Ohio. New- 
berry asserts in words that have been already quoted, that it can be 
followed almost uninterruptedly from the Ohio river to the Pennsy]- 
vania line. At this point, the Pennsylvania geologists take it up, and 
follow it in unmistakable continuity through the western and northern 
counties of the coal field. In any system of correlation or coordina- 
tion of the different portions of the Lower Coal Measures, this horizon 
must be taken as the common and accepted basis. 
Il. Tue FERRIFEROUS LIMESTONE. 
The second of the series of widely extended strata that can be 
made to-serve as a basis of identification throughout the Lower Coal 
Measures, to a greater or less degree, is the Ferriferous Limestone. This 
limestone is also the center of a group of beds, all of which are exten- 
sively worked, and therefore widely known, on account of their economic 
value. The group comprises, beside the limestone, the best iron ore, 
the largest clay deposit, and several of the most widely worked coal 
seams of the Lower Measures. 
The limestone derives its name, Ferriferous, from the fact that it 
carries upon its upper surface an iron ore of great excellence, the basis 
of the old charcoal iron manufacture of Western Pennsylvania. (Geol. 
of Penna., Rogers, vol. II, part I, p. 491.) It has long been counted 
as a chief landmark in the geology of the districts in which it occurs. 
It was constantly used by Rogers in the construction of his sections 
(vol. II, part I, pp. 476, 484, 488, et al). It has held since that time 
the most important place in the determination of the order of every 
