20 
OIL AND GAS; OHIO, WEST VIRGINIA, PENNSYLVANIA. 
the sand beach became submerged it received upon its upper surface 
the calcareous deposits cementing it together, and later the full area 
of the sandstone was covered by the great shale deposits. From the 
conditions under which the Berea sandstone was deposited it is prob¬ 
able that this stratum was not level when laid down, but had a gen¬ 
eral dip seaward, the amount of which depended on the rate of sub¬ 
sidence of the land area. 
POSITION OF SEDIMENTARY DEPOSITS WHEN LAID DOWN. . 
From the mode of deposition of sedimentary formations it is evi¬ 
dent that originally the strata consisted of smooth, though not 
necessarily level, sheets. 
It is possible to conceive of a coal bed growing by the rising of the 
water of a sea, which extended the swamp farther inland while the 
outer edge became too far immersed to allow of vegetable growth, 
but coal beds were in all probability level when formed. 
The Pittsburg coal, which covers an area of G,000 square miles, 
carries within itself evidence of the level surface of the swamp in 
which it was formed. Not far from the middle of this bed are two 
small shale partings about 4 inches apart. The space between them 
is called the “ bearing-in ” bench. These partings are characteristic 
of the bed over a large area. They were formed by high water 
which carried fine sediment into and over the swamp at two periods 
not long apart geologically. As a large area received nearly the same 
amount of sediment, the bed must have been formed in one vast 
level swamp. 
As each succeeding sedimentary deposit was laid down in a smooth, 
fairly even sheet upon the bottom of the sea the distance between beds 
remains nearly constant over small areas; in other words, there is a 
certain degree of parallelism between beds composing deposits of this 
character. 
The amount of material laid down by the action of the waves and 
currents is not the same at all points, but may gradually thicken or 
thin in any direction. This difference in deposition from point to 
point prevents succeeding beds from lying perfectly parallel one with 
another, but probably this variation is fairly regular and at a some- 
what uniform rate. I his rate of variation of the interval between 
any two beds of rock may be-determined if a number of measurements 
of the distance between the two beds at different points in the area 
under consideration can be obtained. 
If, however, at any period all or a portion of the area was above sea 
level, it no longer received sediment but was worn down by erosion, 
which in some places may have removed only a little of the surface 
rocks, but in other places completely removed the underlying forma- 
