.UNDERGROUND MIGRATION OF OIL AND GAS 
157 
material into ^ 'rocks/’ as that term is popularly used, is accom- 
panied, at least in part, by the conversion of organic substances 
into bitumens. Both processes involve, or may involve, transi- 
tion through a number of stages, none of which is separated from 
its neighbors by hard and fast subdivisions either in time or in 
space. Induration of sediments, and distillation of oil and gas 
progress simultaneously from the time of deposition on sea or 
lake floor or along the river flood plain. Each affects the other, 
sometimes favorably, sometimes adversely; but most important 
is the influence which induration of sediments has upon the 
location of the products of distillation. For purposes of anal- 
ysis it is convenient to consider the processes of consolidation 
of fragmented material into "solid rock” under two heads: 
(1) compacting of sediments, (2) cementation of sediments. 
As a matter of fact, of course, the two processes may proceed 
more or less simultaneously. 
Gom'pacting of sediments 
Much information of great importance to petroleum geolo- 
gists concerning the consolidation of sedimentary material is 
wanting. Careful observations of the changes now going on 
beneath the shallow water of marginal seas are needed to make 
possible accurate interpretation of the results of similar changes 
which have taken place in the past. Such studies have been 
made by few, but will doubtless become a more important part 
of the education of the geologist of the future. 
Sediments deposited by moving water must be at first loosely 
heaped on the basin floor; pore space is at a maximum; open 
spaces are filled with water. With increase of burden due to 
continual deposition of debris the loose accumulation must 
settle and be compressed into smaller dimensions. Shrinkage 
of volume beneath the weight of the overlying debris may greatly 
reduce the thickness of the accumulations. Shaw^ reports, 
for example, that the shore of the Mississippi delta “is in places 
2 E. W. Shaw, The mud lumps at the mouths of the Mississippi; U. S. Geol. 
Survey, Prof. Paper, 85-B, pp. 11-27, 1913. 
