178 
KIRTLEY F. MATHER 
oil films will aggregate because of surface tension into drops 
which will further coalesce into fluid masses separating the 
gas above from the water below. It is, of course, true that the 
volume of gas required is many times the volume of oil which 
it would lift in this way but recognizing the limitations upon 
the process it nevertheless seems potent to accomplish impor- 
tant results in the migration of petroleum. 
Movements of underground water; nature of the movement 
Movement of the water beneath the surface of the earth neces- 
sarily involves movement of the oil or gas which may be in its 
path. Underground water, a term used to comprise all aqueous 
solutions beneath the earth’s surface, includes water from many 
sources and under many various conditions. Much of it is 
seepage, fresh or salt water, which has filtered downward from 
the surface; some is connate, fresh or salt water buried with 
sediments; while a small part of it is juvenile, primitive fluids 
expelled from molten magma or igneous rock by crystallization 
or heat. Above the water-table — below which most openings 
in the rocks are filled with liquids — underground water is in 
more or less constant motion from higher to lower levels. 
Below the water table, motion is in general slower, and great 
volumes of water may for motion periods remain practically 
stagnant. The direction of motion is there determined by 
pressure and is from places of greater to places of lesser pressure 
or hydrostatic head. This frequently results in an upward 
creep and lateral oscillation. 
The chief causes of movement of water below the water-table 
are (a) gravity, expressed in the familiar statement that water 
tends to seek its own level; {h) changes in temperature which 
ma3^ result from the earth’s general heat, from the heat of crus- 
tal movements, involving crushing or friction, from chemical 
reaction, or from igneous intrusion; (c) compacting of rocks by 
burial, or expansion of rocks following erosional stripping of 
2® R. A. Daly, Genetic classification of underground volatile agents, Econ. 
Geol., vol. 12, pp. 487-504, 1917. 
