
476 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Book IV. 
the alternation of thin bands of laminated rock with others, coarser 
in texture and non-laminated, indicates considerable oscillation of 
currents from different quarters bearing various qualities and 
amounts of sediment. 
Strata or Beds are layers of rock varying from an inch or 
less up to many feet in thickness. A stratum may be made up of 
numerous lamine, if the nature of the sediment and mode of deposit 
have favoured the production of this structure, as has commonly 
been the case with the finer kinds of sediment, In materials of 
coarser grain, the strata, as a rule, are not laminated, but form the 
thinnest parallel divisions. Strata, like lamina, sometimes cohere 
firmly, but are commonly separable with ‘more or less ease from each 
other. In the former case we may suppose that the lower bed 
before its consolidation was followed by the deposit of the upper. 
The common merging of a stratum into that which overlies it must 
no doubt be regarded as evidence of more or less gradual change in 

Fic. 183.—Srcrion or STRAtTiriep Rocxs. 
a, conglomerate; b, thick-bedded pebbly sandstone ; c, thin-bedded sandstone; d, shelly 
sandstone; ¢, shale with ironstone nodules; f, limestone with marine organisms. 
the conditions of deposit. Where the overlying bed is abruptly 
separable from that below it, the interval was probably of some 
duration, though occasionally the want of cohesion may arise from 
the nature of the sediment, as for instance where an intervening 
layer of mica flakes has been laid down. A stratum may be one of 
a series of similar beds in the same mass of rock, as where a thick 
sandstone includes many individual strata, varying considerably in 
their respective thicknesses ; or it may be complete and distinct in 
itself, as where a band of limestone or ironstone runs through the 
heart of a series of shales. As a general rule, the conclusion appears 
to be legitimate that stratification, when exceedingly well-marked, 
indicates slow intermittent deposit, and that when weak or absent it 
points to more rapid deposit, intervals and changes being necessary 
‘for the production of a distinctly stratified structure. | 
Lines due to original stratification must be carefully dis- 
tinguished from other divisional planes which, though somewhat like 
them, are of entirely different origin. Three distinct kinds of 
