THEORETICAL DISCUSSION. 
19 
shells, were probably laid down in still water and in smooth, even 
sheets over large areas. For this reason they are probably the best 
strata to be used as geologic markers for the formations. 
1 he coal beds represent the remains of vast swamps in which moss, 
ferns, and trees grew. These plants, on dying, fell into the water 
and formed great beds of peat that later was compressed into coal. 
Adjacent to the coal beds and at other horizons are found clays 
and fire clays. Clay is of the same composition as shale, without its 
bedding planes. F ire clays are clays from which has been extracted 
the more fluxible materials, presumably by the action of plant life, 
llie degree to which the easily fusible material has been extracted 
determines the refractoriness of the clay. 
In studying a region of sedimentary deposits it is possible to reason 
out with a fair degree of exactness what movements were taking 
place in the earth s crust at the time some particular deposit was 
laid down. As as example, it is interesting to consider what were 
the conditions during the time of the forming of the great sand¬ 
stone known in Ohio as the “Berea grit'' and in Pennsylvania as 
the “ Thirty-foot shells.” 
This sandstone extends from the vicinity of Wheeling, W. Va., 
to the west fully a third of the way across Ohio, to the north nearly 
to the Great Lakes, and to the northeast almost to the line of New 
\ ork State. 1 he thickness of the sandstone remains nearly con- 
stant, being from 30 to 40 feet. It is composed of clean, fine-grained 
sand of nearly uniform texture. The upper portion of the rock to 
the depth of 18 to 20 feet is cemented probably by calcareous matter 
into an impervious rock. This cap is generally present and in some 
places has thickened to the full depth of the sandstone, making the 
complete stratum impervious to oil and gas. Directly above the cap 
to the Berea sand is a black shale, above which is shale of various col¬ 
ors, which extends for some hundreds of feet to the next great sand- 
stone, the Pocono or Big Injun, as named by the drilling fraternity. 
A sandstone of the extent and uniform thickness of the Berea 
could not‘have been laid down at one time. This sandstone must 
have grown, being extended on its outer edge by the sea gradually 
encroaching upon the land, the waves washing down and cleaning 
the material of the shores, depositing as a beach the heavier par¬ 
ticles and carrying the finer portions out into deeper water. The 
rate of encroachment of the sea upon the land must have been slow 
and regular, as the sand is found thoroughly cleaned and of almost 
uniform thickness. The shore from which this sand was derived 
was probably low and consisted of previously worked-over deposits. 
From a bluff or rugged shore the broken pieces of rock would not be 
of the same.uniform size as the grains of the Berea sandstone. When 
I 
