48 
FLORIDA GEOLOGICAL SURVEY—I 4 TH ANNUAL REPORT 
dined strata there is no obvious trap by which to retain the petroleum 
and if the hydrostatic pressure is sufficient the oil may be forced to the 
surface and appear as an oil seep. On the other hand, reduction in por¬ 
osity of the rock may limit the movement of the oil and form a trap by 
which it is retained at a definite level. In the case of an inclined stratum 
Fig 2. Illustrating conditions in which oil and gas may accumulate in sands in 
inclined strata. 
of this kind, when the necessary variation in porosity to form a trap is 
lacking, the oil after exuding from the surface for a time as a seep may 
deposit heavy residue in the pores of the sand upon evaporation to such 
an extent as to cement the sand and thus form a trap for the retention 
of the remaining oil. Such a trap under much more complicated condi¬ 
tions has recently been described by Pack as occurring in the Sunset 
Field of California.* (Fig. 3.) 
*U. S. Geol. Surv., Prof. Paper 116. The Sunset Midway Oil Field of California 
by R. W. Pack, 1920. 
