STRATIFICATION ill 
and gravel accumulate close inshore, while grit and sand are 
carried further off, and the lightest or most readily trans- 
ported sediment further still, the finer deposits invariably 
extending over the widest tract of sea-floor. Beds of shale, 
therefore, will generally have a greater lateral extension than 
beds of sandstone, grit, and conglomerate occurring in the 
same series of strata. Marine limestones, even when thin, 
often range over a very wide area. For their formation 
somewhat clear water is required, and, unless they be of the 
nature of coral-reefs, they will usually have accumulated at 
some distance from any land, and consequently often in 
relatively deep water. Under such conditions, therefore, we 
might have expected them to have a wide extension. All 
this is in keeping with the broad fact that accumulation of 
sediments proceeds with least interruption over those parts 
Fic. 6.—DISTRIBUTION OF MARINE ACCUMULATIONS. 
s, gravel and sand; c, clay, mud, etc. ; 0, organic accumulations. 
of the sea-floor which are not strongly swept by currents. 
Where there is much stir in the waters deposition of sediment 
is frequently interrupted, the sediments are coarse-grained, 
and show constant alternations of gravel, grit, and coarse 
sand. Where the sea-floor is not so liable to the scouring 
action of tidal currents, finer sand is spread far and wide, 
and passes out, as greater depths are reached, into mud and 
silt, which extend over still wider tracts of undisturbed sea- 
floor. At last a zone is approached, beyond which little 
or no terrigenous material is carried. Here the most import- 
ant oceanic accumulations are of organic origin, calcareous 
and siliceous oozes'(see Fig. 6). 
Each particular stratum in a sedimentary series may be looked upon 
as a lenticular sheet, which, seen in section, begins at zero, thickens out 
regularly or irregularly as the case may be, until it reaches its maximum 
development, and then thins off in the same way. This lenticular 
‘structure can often be seen in one and the same quarry, where the whole 
group of beds may consist of a series of short, imbricating, overlapping, 
