STRATIFICATION— CROSS-BEDDING 701 



when the changes in sedimentation produce differences in the strata, 

 as when, for example, volcanic ashes are deposited upon a lava 

 flow, or when fine clay deposits are succeeded by deposits of sands 

 from a rising flood, or when in the deep sea, after continuous 

 deposition of Globigerina ooze, a bionomic change brings about a 

 deposition of diatomaceous oozes. Here each stratum corresponds 

 to the physical change which brought about the change in deposi- 

 tion. When, however, a rearrangement of the sediment of the 

 shallow sea occurs, owing to the agitation of this sediment by the 

 waves, a secondary separation of materials results, which was not 

 dependent on original changes in sedimentation. Thus a mixed 

 sand may be assorted into layers, according to grain, or a pebbly 

 deposit, charged originally with sand, may be separated into a 

 stratum of conglomerate and one of sand. Again, a deposit of 

 mixed foraminiferal and pteropodan shells may be separated into 

 two strata, one of Foraminifera, the other of pteropods, on account 

 of the difference in their specific gravities. The cases just cited 

 constitute what Walther has termed indirect stratification. Some- 

 times it finds expression in layers, sometimes in strata. The re- 

 markable alternation of pure limestone and calcareous clays in the 

 Cincinnati series of the Ordovicic of Ohio, etc., has been explained 

 in this manner as indirect stratification. The sharp assortment of 

 the material, the abruptness of contact and freedom from grada- 

 tions w^ould seem to indicate that this interpretation is correct. 

 (Perry-i8.) 



Stratification is often indicated only by the arrangement of 

 pebbles, mica scales, or of fossils in interrupted horizontal lines, 

 within a single stratum or even a single bed. The arrangement of 

 the flints in the chalk suggests the stratification of this deposit, a 

 single stratum appearing often of exceeding thickness, while the 

 material is of uniform texture, thus exhibiting no lamination. The 

 same thing is true of the Ldsspilppchen or concretions of the loess, 

 but in this case it is not always the stratification which is indicated 

 by them, but lines of permeability, which have no direct relation to 

 stratification. This may possibly be the case also in some flint layers 

 of the chalk, as already suggested. 



2. Cross-Bedding. This is most readily seen in elastics of an 

 arenaceous texture, though rudytes sometimes show it on a large 

 scale. It consists of an arrangement of the grains in diagonal lay- 

 ers with reference to the plains bounding the strata. Originally 

 these laminae may have been deposited at a considerable angle from 

 the horizontal. Above and below, these oblique layers are bounded 

 by the planes of stratification, and they are commonly truncated on 



