] 
Cycles of Sedimentation vn the Eocene. LEY 
paleontological horizons as time planes, one can trace the relative 
time of commencement of each cycle in different places. 
In an enclosed or partly enclosed basin it is not difficult to see that 
intermittent earth movements would cause oscillations in the water- 
level and give rise to a series of cycles of sedimentation. As will 
be seen later, this is precisely the condition of formation of the 
Kocene strata of the Anglo-Franco-Belgian region. Hach move- 
ment caused a marine transgression, which was followed naturally 
by silting up and retreat of the sea. 
In a broad sense and on a large scale the importance of the 
sedimentary cycle has long been recognized. It is the raison 
détre of most of the systems before they received more exact 
paleontological definition. Professor Watts, in his Presidental 
Address to the Geological Society in 1911,1 has dealt exhaustively 
with the sequence of deposition. Commencing with abysmal 
deposits he has traced the changes through shallow-water and 
littoral sediments to beds deposited during emergence, then the 
beds formed during a succeeding submergence, passing finally back 
to abysmal deposits. This is a simple way of presenting the sequence, 
but it must be borne in mind that the natural period commences 
with the unconformity at the base of the marine beds (i.e. coinciding 
with a submergence of the land), and the abysmal deposits occupy 
the middle period of the cycle, its termination being marked by the 
continental deposits and finally abruptly limited by the succeeding 
marine invasion. 
The application of these principles to detailed classification on 
a small scale has been given primary consideration on the Continent. 
One may quote the work of the Geological Survey of Belgium. The 
nomenclature and grouping used on the maps indicate at once the 
importance attached to it. Hach formation is given an index letter, 
and the deposits in each formation are marked by a second letter. 
Taking, for example, the Ypresian (London Clay) Series, one has the 
following divisions :—? 
Ye. Littoral deposits (‘‘ gravier d’émersion’’). 
Yd. Sand indicating shallowing conditions. 
Ye. Clay (main mass). 
Yb. Sand of basement bed. 
Ya. Basal conglomerate (‘‘ gravier d’immersion ’’). 
The bed “‘e” in most cycles is removed before the deposition of 
succeeding strata. In the Belgian classification the continental 
deposits generally receive a separate index letter. In some cases, 
however, a bed which does not fit in the main series is given the 
distinguishing letter ““m” or ““n’’, etc. For example, at the base 
of the “ Paniselian” there is a stiff blue unfossiliferous clay, 
1 Watts, “ Geology as Geographical Evolution’: Quart. Journ. Geol. Soc., 
vol. Ixvii, 1911, p. Ixii. 
2“ Légende de la Carte géologique de Belgique ...a& I’Echelle du 
40,000° ”, 1891, etc.: Review in Ann. Soc. géol. Belg., vol. xix, 1891, p. 107. 
