STRATIFICATION 265 
there are accumulated thick beds of limestone, sand- 
stone or clay. But at the opposite extreme, there 
are cases in which changes are so frequent, that 
in every few feet new conditions were evidently 
introduced. 
Position of the Strata. — In ocean or lake, the frag- 
ments settle under the action of gravity, and arrange 
themselves in accordance with the outlines of the 
bottom. In most strata there is one portion in which 
the accumulation was more rapid, and here the bed is 
naturally thickest. This place of most rapid deposit is 
that at which, for some reason, the supply was greater 
than at other portions of the stratum. It may be 
opposite a river mouth, or where the waves or currents 
are most active. 
As we recede from this place of greatest accumula- 
tion, the stratum thins out in all directions, often 
rapidly, again more slowly, in every case gradually 
merging into another bed composed of coarser or finer 
material, or of fragments of an entirely different kind. 
For instance, the sand of the seashore imperceptibly 
passes into the clay of the off-shore bottom. There is 
no place where the deposit of sand ends and that of 
the clay begins, the boundary being very indefinite and 
variable, depending upon the force of the currents 
which are moving the particles. 
As the layers are thickest in one part of the stratum, 
