Russell.] PETROLEUM. 73 
petroleums evaporate, they become more and more dense and change 
to heavy oils; still greater loss by evaporation leads to the production 
of highly viscous substances, like mineral pitch ; and a further change 
in the same direction furnishes a large variety of solid hydrocarbons. 
In a general way,. then, petroleum furnishes natural gas as a " dis- 
tillate" and viscous and solid hydrocarbons as residues. This is as 
far as we need attempt at present to penetrate the wide and but par- 
tially explored field of the origin and chemistry of petroleum and its 
derivatives. 
If we assume, as stated above, that petroleum is derived from the 
organic matter contained in rocks, then it follows that we may expect 
to find it in sedimentary rocks of any geological age which were 
deposited after life appeared on the earth. This conclusion is abun- 
dantly sustained by the many discoveries of petroleum and gas that 
have been made in rock ranging in age from the lower Paleozoic to 
the Tertiary and even recent time. Any series of stratified rocks, 
therefore, so far as its geological age is concerned, may be expected 
to yield hydrocarbons in one form or another, and, as has already been 
suggested, even igneous and metamorphic rocks may serve as reser- 
voirs. 
Fluids and gases contained in subterranean chambers, or in the 
interstices of porous rocks, are first of all subject to the force of 
gravity. Water in porous rocks will descend under the pull of gravity 
and displace lighter fluids, as well as gases, thus causing them to rise 
if avenues for escape are available. Petroleum alone, in a porous 
rock, will descend and seek the lowest attainable position, although the 
thicker varieties, owing to their high degree of viscosity, would move 
sluggishly. 
Hence, water, petroleum, and gas in a porous rock will, as alread}^ 
stated, arrange themselves in the order of their respective specific 
gravities, the water being at the bottom and the gas at the top, and 
the water will be subjected to the pressure of the lighter substances 
resting on it. When porous rocks charged with the three substances 
named are confined by impervious beds, so that an escape for the gas 
is impossible, the water and the oil must be pressed on by the gas, 
owing to its tension or expansive force; and should gas continue to 
be generated, it will acquire such a tension as to exert a great pres- 
sure on the fluids imprisoned with it. The fluids and gases contained 
in the rocks may therefore be subjected to gas pressure which, as is 
well known, may equal thousands of pounds to a square foot. 
When a porous rock is inclosed between two impervious beds and 
the series is inclined, the natural arrangement of the three substances 
under consideration will be the same as just stated, but under certain 
conditions the petroleum and gas may be brought under hydrostatic 
pressure. To illustrate the requisite conditions for producing hydro- 
static pressure, reference is made to the following diagram, which 
