STRATIFICATION 219 



dation. Even with this irregularity in the formation and removal 

 of the stratified rocks, it would be exceedingly difficult, if not 

 impossible, to investigate the entire series of them, if they had all 

 retained the original horizontal positions in which they were first 

 laid down. In many places, however, the rocks have been steeply 

 tilted and then truncated by erosion, so that their edges form the 

 surface of the ground, and thus great thicknesses of them may be 

 examined without descending below the surface. 



Stratification, or division into layers, is the most persistent and 

 conspicuous characteristic of the sedimentary rocks. In studying 

 the sedimentary deposits of the present day (Chapter VIII) we 

 learned that by the sorting power of water, the heterogeneous 

 material brought from the land is arranged into more or less 

 homogeneous beds, separated from one another by distinct planes 

 of division, and the same thing is true of the sedimentary rocks 

 of all ages. This division into more or less parallel layers is 

 called stratification, and the extent to which the division is car- 

 ried varies according to circumstances. 



A single member, or bed, of a stratified rock, whether thick or 

 thin, is called a layer, though for purposes of distinction, exces- 

 sively thin layers are called lamina. Each layer or lamina repre- 

 sents an uninterrupted deposition of material, while the divisions 

 between them, or bedding planes, are due to longer or shorter 

 pauses in the process. A stratum is the collection of layers of 

 the same mineral substance, which occur together and may con- 

 sist of one or many layers. However, the term is not always 

 employed in just this sense and often means the same as layer. 

 The passage from one stratum to another is generally abrupt and 

 indicates a change in the circumstances of deposition, either in the 

 depth of water, or in the character of the material brought to a 

 given spot, or in both. So long as conditions remain the same, 

 the same kind of material will accumulate over a given area, and 

 thus immense thicknesses of similar material may be formed. To 

 keep up such equality of conditions, the depth of water must 

 remain constant, and hence the bottom must subside as rapidly 

 as the sediment accumulates. 



