12 GEOLOGY OF OHIO. 
pany and enclose them, but it is beyond contradiction that these thin 
limestones are decidedly the most widely extended, and the most per- 
sistent of the entire series to which they belong. Ifa heavy deposit of 
conglomerate or pebble rock occurs in a section, it is sure of recogni- 
tion, but if we undertake to trace a series by means of such a stratum, 
we are liable to failure. Conglomerates are the most variable and in- 
constant of all sedimentary formations. A bed of soft shale without a 
pebble may hold the place of a coarse and heavy conglomerate, but a 
mile or two away. 
Limestones are hard and are therefore quite certain to attract at- 
tention in excavations and drill holes, and roadways. They are fre- 
quently replaced by flint, and the indestructible character of this sub- 
stance ensures the ready recognition of the horizon to which it belongs. 
The color of a limestone is often characteristic, so that it can readily be 
distinguished trom associated beds. If fossiliferous, this fact is likely 
to attract attention. Limestones are soluble in atmospheric waters, it 
is true, but even when dissolved at their outcrop, their place is none the 
less distinctly marked by the soils to which they give rise, and by the 
kinds of vegetation which they support. 
Seams of coal have many advantages as guides to a knowledge of 
the true equivalence of sections. ‘They are worked extensively and are 
therefore well known. The character of the coal, the number and 
kinds of the partings, the nature of the floor or roof, the color of the 
ash, and other similar facts often help us to carry coal horizons through 
hills or across valleys with as much confidence as visible continuity 
could inspire. 
Beds of iron ore are worked more largely in some districts than 
even the coal seams, and thus they render a similar service. Occasion- 
ally a stratum of fire-clay has some peculiar character or quality, or some 
unusual volume by which it can be safely used in determining the order 
of two or more disconnected sections. 
The sandstones on which so much popular reliance is placed in the 
identification of distant exposures of coal measure rocks, are in reality 
fallacious guides. Some of them, it is true, have distinctive marks of 
bedding or grain or color, by which they can be safely followed across 
intervals, but many of the identifications that are dependent on them 
are incorrect and misleading. Sandstones and conglomerates owe their 
existence to strong currents, by which their materials have been trans- 
