THEORETICAL DISCUSSION. 
15 
PLACES OF ACCUMULATION. 
(1) In dry rocks the principal points of accumulation of oil will 
be at or near the bottom of the synclines or at the lowest point of the 
porous medium, or at any point where the slope of the rock is not 
sufficient to overcome the friction, such as structural terraces or 
benches. (2) In porous rocks completely saturated the accumula¬ 
tion of both oil and gas will be in the anticlines or along level por¬ 
tions of the structure. Where the area of porous rocks is limited the 
accumulation will occur at the highest point of the porous medium, 
and where areas of impervious rocks exist in a generally porous 
stratum the accumulation will take place below such impervious stop, 
which is really the top limit of the porous rock. (3) In porous rocks 
that are only partly filled with water the oil accumulates at the upper 
limit of the saturated area. This limit of saturation traces a level 
line around the sides of each structural basin, but the height of this 
line may vary greatly in adjacent basins and in different sands of the 
same basin. 
Partial saturation is the condition most generally found, in which 
case accumulations of oil may occur anywhere with reference to the 
«/ «/ 
geologic structure; it is most likely, however, to occur upon terraces or 
levels, as these places are favorable to accumulation in both dry and 
saturated rocks. 
Under all conditions the most probable locations for the accumu¬ 
lation of gas are on the crests of anticlines. Small* folds along the 
side of a syncline may hold a supply of gas, or the rocks may be so 
dense that gas can not travel to the anticline, but will remain in 
volume close to the oil. 
APPLICATION OF THEORY IN THE APPALACHIAN FIELDS. 
The accompanying illustration, PI. I, is a diagrammatic section 
through the oil-producing formations of Ohio and Pennsylvania 
based on data from actual surveys. It shows those sands which are 
found under the Steubenville quadrangle and extends to those that 
are known to exist in Pennsylvania. In it is represented the uncon¬ 
formity which has'been shown by David White a to exist at the base 
of the Pottsville formation throughout the entire western edge and 
probably the central part of the Appalachian coal field, evidence in 
favor of which has been found in the well records examined in the 
course of this investigation. The Salt sand represents the base of the 
Pottsville formation. Below this is the Big Injun or Pocono sand- 
n white, David, Deposition of the Appalachian Pottsville: Bull. Geol. Soc, America, 
yol. 15, 1904, pp. 267-282. 
