rtjssell.] PETROLEUM. 77 
east, at a rate of 20 to 30 feet to the mile, but they suddenly cease their 
descent and for about 3 miles there is no appreciable fall. The amount 
of territory included in the terrace appears to be 15 or 20 square miles. 
Beyond this the regular dip is resumed. It is difficult to understand 
how the petroleum and gas are retained in a "terrace" if it is really 
horizontal, but the conditions regulating the storage of these sub- 
stances are so delicately adjusted that a difference of a very few feet 
in the height of the top of the porous layer may determine the success 
or failure of a well drilled to it. The difficulty of ascertaining the 
actual position of the surface of the porous layer when gently inclined 
is shown by the fact that a thickness of a thousand feet or more of 
overlying strata has frequently to be penetrated to reach the necessary 
rock. It is well known, however, that the nearly flat areas, as the 
drill shows them to be, in certain regions where the rocks are gently 
inclined in the same direction on each side, sometimes contain com- 
mercial quantities of liquid and gaseous hydrocarbons. 
In the above consideration of the influence of structure and texture 
on the storage of petroleum and gas we have assumed that water is 
also present in the rocks. While this is the normal and practically 
universal condition, yet it may happen that water is absent from a 
porous layer containing the other two substances named. In the 
absence of water, petroleum would accumulate in the downward folds 
or would descend until arrested by an impervious stratum, and gas 
would occupy the remainder of the porous bed. An increase in the 
amount of gas would be accompanied by an increase in pressure. 
Under these scarcely to be expected conditions, a well penetrating a 
syncline would yield oil under gas pressure which might be great, while 
a well drilled in the course of an adjacent anticline would yield gas 
only, the oil not rising unless the relation of the well to the inclina- 
tion of the porous layer was such that the oil itself could produce 
"hydraulic" pressure. In such a case the flow of petroleum would 
not be followed by water, and when the outflow ceased the well would 
still be filled with petroleum. 
Certain modifications or limitations of the general principles above 
stated need to be kept in mind by persons searching for commercial 
supplies of petroleum and gas, lest valuable pools of exceptional 
character be overlooked. 
One of the exceptional conditions referred to is the absence of a 
pervious stratum to act as a reservoir or receiver. For example, a 
bed of shale which is impervious to water may be of such a texture as 
to permit of the slow percolation of petroleum and the ready passage 
of gas. Should such an oil-charged shale occur in thick masses, as in 
the Cretaceous system in the great plateaus, without interbedded 
porous beds and not having an impervious cover, the gases generated 
would pass off, leaving heavy petroleum, which would descend to the 
bottom of the generating layer or until it was arrested by an impervi- 
