700 • PRINCIPLES OF STRATIGRAPHY 



pass upward into a rudyte, whereupon the distinction of strata is 

 a matter of jutlgment. In such cases the stratum of arenyte ter- 

 minates with a layer of rudaceous arenyte where the arenaceous 

 material still predominates, while the stratum of rudyte begins with 

 a layer of arenaceous rudyte, in which the rudaceous material has 

 become most prominent. 



A change in composition is not always of sufficient magnitude 

 to warrant separation into a new stratum. Thus a stratum of 

 silicarenyte or pure quartz sandstone may have interbedded layers 

 of ferruginous, argillaceous, calcareous or glauconite material, 

 where this material is only of sufficient quantity to produce a variety 

 of the sandstone. Where, however, a calcarenyte or a clay rock 

 (argillutyte) succeeds to a silicarenyte, a new stratum is produced. 

 Where deposition is continuous, but the supplied material changes 

 in composition, a gradation from a pure silicarenyte to a pure cal- 

 carenyte may occur, without break of continuity. In this case, as 

 in the case of the gradation in texture, the line of division between 

 the two strata must be drawn on the relative preponderance of ma- 

 terials. The stratum of silicarenyte will terminate with a layer of 

 calcareous silicarenyte, while the stratum of calcarenyte will begin 

 with a layer of siliceous calcarenyte. 



While gradations as here discussed are not of uncommon occur- 

 rence, in the more familiar type of stratification the strata abruptly 

 succeed each other. Thus a stratum of limestone, clastic, organic or 

 chemical and frequently composed of only one layer, may be inter- 

 calated between strata of shales. Again strata of limestones are 

 separated by strata of carbonaceous clay or by sandstones, the sep- 

 arating strata in many cases being mere films. In such cases the 

 stratum of clayey material is represented by only one lamina. Not 

 infrequently strata of clastic limestones (calcarenytes) are sepa- 

 rated by a thin stratum of organic limestone in a single layer, and 

 generally containing an admixture of clayey matter. A decided 

 change in color may readily serve as a basis for division into strata, 

 since such change generally indicates a marked change in physical 

 conditions during deposition. Thus a black shale succeeding a gray 

 or bluish one marks a change in conditions of deposition. A change 

 from a gray to a red sandstone likewise indicates physical changes 

 from conditions preventing to those permitting extensive oxidation, 

 as elsewhere discussed. Finally the occurrence or indications of de- 

 cided physical breaks, such as erosion surfaces and disconformities, 

 serves to separate distinct strata. 



Types of Stratification. Walther (23:(5j/) recognizes two 

 kinds of stratification, direct and indirect. The former is produced 



