156 
KIRTLEY F. MATHER 
It may be conveniently referred to as transverse migration, for 
the direction of movement is in general across the bedding planes 
of the rocks from one stratum to another. The second phase 
involves the concentration of mobile hydrocarbons in certain 
areas within the extent of the more porous beds. It must 
depend upon lateral movement of the oil and gas within a stra- 
tum or series of strata and may be called parallel migration, 
for the direction of movement is more or less parallel to the 
bedding planes. Both sorts of movement are involved in the 
origin of most accumulations of oil or gas of economic value. 
Before drawing any conclusions as to the probable causes of 
these migrations in any particular case, it will be well to review 
all possible motive forces which may have been in any way re- 
sponsible. Having assembled the data it will then be less diffi- 
cult to select the more important agents and observe how they 
may have cooperated in the local accumulation of oil or gas. 
THE MOTIVE FORCES 
Strictly speaking, there are but two forces which in the last 
analysis may be the ultimate causes of underground migration 
of fluids or vapors; these are (a) gravitational force and (6) 
molecular force. Both of these forces, however, may operate 
in several ways. The direct action of either may affect oil and 
gas indirectly through a long chain of intermediate causes and 
results. Interaction and reaction between the two introduce 
a number of complications. A somewhat arbitrary grouping 
of the many proximate causes of migration must therefore be 
adopted.! 
Induration of sediments 
The genesis of oil and gas depends upon the entrapping of 
plant and animal tissue in the midst of accumulations of inorganic 
debris, which whether deposited by water or by wind, are at 
first loosely heaped together. The consolidation of fragmented 
1 For a classification of causes of oil migration different from that adopted 
here, see V. Zeigler, The Movements of Oil and Gas Through Rocks, Economic 
Geology, vol. 13, pp. 335-348, 1918. 
