108 STRUCTURAL AND FIELD GEOLOGY 
Lamination is typically represented by the finer grained 
sediments, such as argillaceous shales. The laminz of such 
deposits vary in thickness from an inch or so down to the 
finest films, not thicker than ordinary writing-paper (sec 
Fig. 4). As a rule they cohere only slightly, so that a rock 
of the kind is more or less readily separated along the planes 
of lamination. Not infrequently, however, the laminze have, 
owing to pressure, become more adherent. The laminated 
structure being the result of successive depositions of fine 
sediment by periodical river-floods, or by tidal or other 
marine currents, usually indicates accumulation in quiet water. 
These conditions are met with in lakes and estuaries, and over 
such areas of the sea-floor as are not much disturbed by 
currents—that is to say, 
in relatively deep water. 
Although lamination is 
very characteristic of 
argillaceous rocks, it is 
by no means confined 
to these. Laminated 
sandstones are of com- 
mon occurrence, particu- 
larly when the rock is 
very fine-grained and 


Fic. 4.—-STRATIFICATION AND more or less argillaceous. 
LAMINATION. In coarser grained sand- 
8, 8, s, non-laminated beds; J, 7, laminated beds. 
stones the individual 
Jamine are thicker than in argillaceous shales, When they 
exceed an inch or so, however, they are often described as 
layers. 
Led or Stratum is the term applied to any shect-like 
mass which has a more or less definite petrographical 
character, and is separated by well-marked parallel division- 
planes from overlying and underlying rocks. A bed may be 
homogeneous and without any apparent arrangement of its 
constituents, or it may consist of successive layers or lamine. 
It is well to point out, however, that the terms “bed” or 
“stratum” and “layer” are purely relative. A sandstone 
consisting of a series of layers, for example, is often described 
as a thin-bedded rock. Again, a thin sheet of limestone, iron- 

