Cycles of Sedimentation vn the Eocene. 109 
retreat (regression) of the sea is marked by a recurrence of coarser 
sediments—silts and sands—culminating possibly in conglomerate 
(“ gravier d’émersion ’’). 
The succeeding marine invasion may remove a portion of the 
continental or coastal deposits, the coarse or hard materials being 
rearranged to form a new “ gravier d’immersion”’ or basal con- 
glomerate. This frequently includes rolled fragments of fossils, ete., 
derived from the underlying beds. Where the latter are fine-grained 
or unconsolidated, the “‘ gravier d’immersion’’ may be feebly 
developed and inconspicuous, but the plane of erosion between the 
new marine deposits and the underlying beds, whether of continental 
or marine origin, is almost invariably well-marked. In other words 
there is a ‘‘ravinement”’ at the base of each marine formation.t 
The period between two successive “ ravinements”’ constitutes 
a “cycle of sedimentation”. In other words, a “cycle of 
sedimentation ’’ comprises the deposits of a complete oscillation of 
the basin, each oscillation including a positive phase of marine 
invasion and a negative phase of regression. 
The deposits of the second marine invasion form a series closely 
comparable with the first. The invasion may be more or less 
extensive than the earlier, and the seaward range of the continental 
deposits more restricted or less so. 
Fig. 1 is a diagrammatic representation of the deposits of three 
complete cycles of sedimentation. The section is drawn from the 
landward edge of a basin towards the centre. The relative extent 
of marine and continental deposits is seen at a glance. Taking the 
upper or lower limit of the zones as representing approximately time 
planes, it is apparent that each basal conglomerate is not of the same 
age over its whole extent. That is, the cycle does not commence 
at exactly the same time at each point. Yet, as many authors 
have recognized, the only possible grouping of such deposits is by 
cycles. The changes at any particular locality during a series of 
cycles may be represented graphically. Taking a straight line to 
represent sea-level and regarding, for the sake of clearness, all 
continental deposits as having been formed above sea-level, the 
oscillations of the land surface at any one point may be represented 
by a curve. This graphical representation is important in that it 
emphasizes the time factor. Two examples, for the points A and B, 
1 A ‘‘ravinement ’’ may be defined as an irregular junction which marks 
a break in sedimentation. The break may be due to a period of denudation 
consequent on movements of masses of water, but not necessarily accompanied 
by earth-movements. Although one of the commonest of geological 
phenomena, there is no English word which expresses quite so aptly the 
relationship. One may say that there is an ‘‘ unconformity ” at the base of 
the marine formation, but that term rather suggests discordance of stratifica- 
tion and earth-movements of a more pronounced character. The term “ plane 
of erosion ” is bad, since the basal conglomerate tends to occupy an irregular 
or hummocky surface of the underlying deposits rather than a plane surface. 
““Non-sequence”’ is most frequently applied to a break in sedimentation 
which can be detected only by a study of successive faunas. 
