STRATIFICATION 109 
stone, or coal, intercalated in a series of shales, might be 
termed either a bed, a layer, or a seam. 
The time required for the formation of any given thickness of sedi- 
mentary materials is necessarily indeterminate. Generally speaking, 
however, a bed of conglomerate may have been amassed more rapidly 
than an equal thickness of sandstone, and a sheet of sandstone may 
have been deposited in a shorter time than one of shale of equivalent 
extent and thickness. It is clear, however, that the rate of deposition 
of any particular kind of sediment must vary indefinitely. Certain 
sandstones, for example, may have been formed more rapidly than 
others of precisely the same character. Usually, however, where the 
rate of accumulation has varied in any marked degree, some evidence of 
this will be visible in the structure of the rocks. Thus, we may reasonably 
infer that a homogeneous sandstone, such as freestone_or liver-rock, has 
been formed in less time than an equal mass of laminated sandstone. 
The liver-rock indicates continuous sedimentation, while the laminated 
sandstone points to a process of intermittent sedimentation. So, again, 
a structureless clay or loam has probably been accumulated more 
continuously, and therefore more rapidly, than a well-laminated shale. 
Nevertheless, it must be admitted that, in comparing separate beds of 
similar character and thickness, we can never be sure that an equal time 
was required for their deposition. Nay, even in the case of beds having 
the same composition and structure, and differing only in thickness, it 
cannot always be assumed that the thickest beds took the longest time 
for their accumulation. Probably, in most cases, they did, but many 
facts conspire to show that mere thickness is no sure test of the relative 
age of individual beds. If this be true of strata having the same 
character throughout, it is certainly not less true of beds which differ in 
composition and structure. A series of limestones and shales, one 
hundred feet in thickness, for example, may well have required for its 
formation a far longer time than a succession of several thousand feet of 
sandstones. 
Intervals indicated by Planes of Lamination and Stratification.—The 
parallel division-planes separating individual strata are always more pro- 
nounced than planes of lamination, z.e. the planes separating individual 
layers or lamine. This naturally suggests that a longer time has elapsed 
between the accumulation of successive strata than between the deposition 
of successive laminz or layers. The length of interval represented by 
planes of stratification, however, is indeterminate. It may be quite 
short or very prolonged. In the case of shallow-water sediments, which 
are apt to show rapid alternations of coarser and finer grained deposits, 
no long intervals need have separated the deposition of the several kinds 
of sediment from each other. Rapid alternations of sediment are quite 
characteristic of alluvial, estuarine, and littoral or shore-accumulations, 
On the other hand, sediments accumulated in deeper water seldom show 
such rapid changes of character. They are usually fine-grained and 
persistent over wide areas. It is justifiable, therefore, to infer that planes 
