STRATIFICATION 697 



that, with the exception of the beach cusps, the wave marks and 

 the clay pebbles, they are characteristic of subaerial deposits, while 

 some of them, such as cross-bedding, desiccation fissures, rain prints 

 and footprints, are almost exclusively confined to the formations 

 other than marine or lacustrine. The most pronounced of the char- 

 acters enumerated, stratification, is also of frequent occurrence in 

 the endogenetic formations. 



Concretions are only occasionally original structures, being for 

 the most part secondary. Secretions are always of secondary char- 

 acter, but they are included here for the sake of comparison with 

 concretions which belong here in part. 



I. Stratification. In its broadest sense (Walther-23 :<5i'0 

 et seq.) stratification is the arrangement of rock masses in layers 

 or strata, each one of which was at one time the latest deposit, and 

 the top of each stratum was successively the top of the lithosphere 

 at that point. Stratification thus defined occurs in all rocks, which 

 are deposited in successive layers. Thus a series of lava-flows will 

 show stratification, each flow representing a distinct stratum. These 

 volcanic strata are often steeply inclined, as is also the case in 

 clastic strata along the margins of coral reefs or in the alluvial fan 

 or talus heap. 



Among the pyrogenic rocks stratification is produced where 

 lava streams of different composition succeed each other, or where 

 streams of the same composition are separated by an interval, dur- 

 ing which the surface of the earlier one either hardened or became 

 altered to some extent, or again was covered by a layer of clastic 

 material, before the second flow occurred. Atmogenic snow ice or 

 glacial ice becomes stratified when the succeeding deposits of snow 

 are separated by intervals during which the older layer solidified or 

 was covered by a thin layer of dust or by other clastic material. 

 False stratification is often produced in this rock by the formation 

 of shearing planes, along which some of the subglacial detritus is 

 carried up into the ice. Hydrogenic and biogenic rocks may also 

 be stratified, this being brought about by cessation of deposition, 

 by change in the material, through interposition of elastics, or by 

 alternation in deposition of different classes of endogenetic ma- 

 terials. Examples of this are shown in the alternation of layers of 

 gypsum and salt, or in the intercalation of layers of potash and 

 other salts between the beds of ordinary rock salt. The numerous 

 intercalated silt layers of the salt deposits of the Bitter Lakes of 

 Suez further serve as an illustration. While stratification is thus 

 not confined to the clastic rocks, it finds its most typical expression 

 in this group. All clastic deposits may be stratified, this stratifi- 



