UNDERGROUND MIGRATION OF OIL AND GAS 
183 
lighter distillates from the petroliferous substances is, however, 
so delicate that it is doubtful if folding movements have con- 
tributed largely to the accumulation of oil in economic amounts. 
Indirectly, folding and faulting of the earth’s crust may have 
materially assisted in the migration and segregation of oil by 
providing that element of motion which seems essential to the 
effective operation of such motive forces as capillary action and 
gravitative assortment. The frequency of earthquake vibra- 
tions is suggestive of the repetition of jarring shocks which may 
have operated to overcome adhesion and inertia which were 
militating against gravitative assortment of oil and water. 
Similarly, the development of fault fissures may afford chan- 
nels for the escape of compressed gasses, and thus be the indirect 
cause of migration and accumulation of oil impelled by expand- 
ing gas, as noted in the preceding section. 
Body tides 
Regularly recurrent oscillatory movements within the body 
of the earth are caused by the attraction of sun and moon. 
Experiments undertaken to determine the rigidity of the earth 
by tidal observations indicate^^ that the water tides are of much 
less amplitude than they would be if the earth were absolutely 
rigid, and that in fact three- or four-tenths of the tida] strain 
is compensated by tides within the earth’s body. These move- 
ments may be compared to the kneading to which a baker sub- 
jects dough. They would in general result in an upward migra- 
tion of earth fluids because as the liquids are squeezed out they 
would move, if possible, in the direction of least pressure. This 
kneading of the earth, like the less regular folding and faulting 
movements, has probably been an important contributing cause 
of oil and gas migration, notably assisting the operation of 
other forces. 
THE PROBABLE HISTORY OF MOST OIL AND GAS ACCUMULATIONS 
Having thus reviewed the possible causes of the migration 
of oil and gas, the probable history of the majority of the great 
A. A. Michelson, Preliminary results of measurements of the rigidity of the 
earth, Journ. GeoL, vol. 22, pp. 97-130, 1914. 
