160 
KIRTLEY F. MATHER 
at vastly different depths below the surface. Indeed it would 
in most cases be extremely difficult if not impossible to draw a 
line between compacting of sediments and rock flow; the one 
grades imperceptibly into the other. The ^^zone of flow’’ is al- 
most at the surface for certain materials, among which the 
most conspicuous are the mud rocks which are pre-eminently 
the birthplace of petroleum. The Mississippi Delta, according 
to Shaw,^ seems to be affected by ‘^a bodily flowage toward the 
sea.” In places the Delta front is bulging outward and upward 
as the materials of which it is composed flow laterally after 
having been compacted by the weight of the shallow over- 
burden. 
Such flpwage of the rocks may further the process of transfer 
of petroleum from shales to sandstone or to limestone, but is 
probably much less important than the universal compacting 
which necessarily must result from the accumulation of any 
considerable amount of sediment in any lodgment basin. The 
movement, both of solids and of fluids, which must accompany 
rock flow may greatly expedite the segregation of oil and gas 
by other processes to which reference is made in the succeeding 
paragraphs. In general, simple compacting precedes rock flow 
and decreases in importance as the later process increases in 
amount, but no sharp distinction may be drawn either between 
the processes or their results. 
Cementation of sediments 
Consolidation of sediments is not solely nor indeed princi- 
pally a matter of compacting; especially are the constituents of 
sandy strata bound together by interstitial cement. Cementa- 
tion may begin immediately after the rocks are subjected to 
the action of ground water. It is through cementation that 
sands become sandstone, and sandstones become quartzite. 
Obviously, the greater the amount of precipitated cements the 
less the effective pore space of the rock. If cementation com- 
mences along a definite plane or at a point and proceeds regularly 
3 Log. cit., pp. 17, 18. 
