STRATIGRAPHICAL ORDER. 9 
The composition of these two sub-divisions of the coal measure 
rocks, as they occur in Western Pennsylvania, has thus been shown. The 
order of arrangement that has here been determined by the most thor- 
ough and the most closely connected stratigraphical work yet done in 
the United States, embodying as it does the harmonious results of a 
considerable number of well-trained geologists, who have scarcely been 
limited by either time or money in the prosecution of their work, may 
be counted settled. 
But the order of the coal measure rocks in Western Pennsylvania 
is in all respects identical with the order of these rocks in Eastern Ohio. 
The sections that have been selected to represent this order were all 
taken from near the boundary line between Ohio and Pennsylvania, 
and the facts illustrative of this order are found equally on both sides 
of the line. 
The true sequence of the various beds of coal, limestone, iron ore, 
clay, shale, sandstone and conglomerate that make up these divisions, is 
a fact of nature, to be determined by a sufficient amount of geological 
exploration and geological sagacity, and when once clearly determined, 
it is determined finally, like the geographical facts of latitude and longi- 
tude, for example. 
But the work of classifying these several facts, and of arranging 
the strata in groups, larger and smaller, is a task of very different char- 
acter. The aim in all such schemes, of course, is to apprehend and in- 
dicate the salient features in the history which the rocks record, but in 
point of fact, all are arbitrary and artificial to a greater or less extent, 
and no geological classification can be counted final in the same sense 
in which a geological section can be so counted. 
The order of sequence of the coal seams that have been enumerated, 
for example, has now been definitely ascertained, and it is not therefore 
liable to be replaced by some other order or to be materially changed, 
but the division of these seams and of the strata associated with them 
into the two great groups that have been named above, viz., the Con- 
glomerate Measures and the Lower Coal Measures, rests on a very 
different foundation, and may well enough be called in question. It is 
quite certain that such a division would never have been made on the 
facts that occur in Western Pennsylvania. It was only by the establish- 
ment or supposed establishment of equivalency between the varied 
series of the lowest coal measures in the western part of the State with 
