STRATIFICATION 113 
increased, sedimentation was again locally arrested, and the 
process of erosion repeated, the sand being in its turn trenched 
and furrowed, and subsequently covered by arenaceous clay 



















Fic. 8.—CONTEMPORANEOUS EROSION. 
c, cl, sandy clay; s, sl, sandstone, grit, etc. 
(c!), just as this latter became denuded and afterwards over- 
laid by grit and sand (s?). 
Grouping of Strata.— Although almost any diversity of 
strata may be seen in one and the same vertical section, yet, 
as might have been expected, it is usual to meet with rocks 
of similar character associated together. Thus, conglomerate 
is more frequently interstratified with grit and sandstone, than 
with fine argillaceous deposits, while limestone is associated 
rather with the latter than with coarser grained accumulations. 
Alternations of different kinds of sediment are quite char- 
acteristic of the deposits now forming in lakes, estuaries, etc., 
but usually the passage is from gravel to grit and sand, and 
not directly from shingle and gravel to silt and clay. Even 
in the case already mentioned (see Fig. 5), of a limestone 
intercalated between underlying and overlying shales, the 
change from the one to the other is not always so abrupt as 
it may seem to be. Not infrequently it will be found that 
the lower shales become more and more calcareous towards 
the top of the stratum, and that the limestone, in like manner, 
becomes gradually more and more argillaceous above—so 
that there is a sort of passage, as it were, from the one kind 
of rock into the other. 
In the silting up of lakes and estuaries, however, it must 
happen now and again that coarse sediments are laid down 
H 
