1912] 



Louderback : Pseudostratification 



25 



furnish the proof. In a number of cases where the lied in 

 which the pseudostratification had developed was itself without- 

 original visible structure lines, a recent slide or wash, or an 

 excavation made for the purpose, showed distinct structure lines 

 in an under- or overlying layer, especially in shale streaks, or in 

 layers with arranged micaceous materials. A test area of about 

 two square miles, where the only exposures appeared to be those 

 of pseudostratification, was worked over, and it was found that 

 on the careful examination of the deeper slides and washes, and 

 particularly of a series of excavations, a consistent group of 

 observations could be obtained determining the original strati- 

 fication and the attitude of the original beds. It showed a very 

 open syncline while the more evident exposures of pseudostrata 

 gave consistent indications of an anticline (a pseudoanticline), 

 the axis of the syncline being quite a distance south of the axis 

 of the pseudoanticline. 



Subordinate secondary layers. — All of the pseudostrata do 

 not show within themselves distinct minor layers, but in some 

 they are very well developed, and vary from two or three milli- 

 meters in thickness up to ten or more centimeters. Layers of 

 this latter thickness down to those of two or three centimeters 

 are more common than the very thin ones, which may be called 

 pseudolaminae. They are often bounded by separation planes 

 and samples may be removed from their exposures as thin plates 

 or approximately parallel bounded blocks. Such a block is shown 

 in plate 6, figure 3. These features are more common in pseudo- 

 strata occurring with dips of ten to twenty degrees or more. 



Sometimes these minor layers within the pseudostrata are 

 quite parallel and continue for a number of yards — perhaps 

 throughout the whole exposure. Sometimes they may be curved 

 or may cut obliquely across the pseudostratum, or one division 

 plane may cut obliquely across the others. Such oblique and 

 curved arrangements may give rise to appearances that may be 

 called pseudo-current bedding. Typical cases are illustrated in 

 plate 4 and plate 5. 



Pseudofolds. — It has already been described how the ex- 

 posures over an area may consistently give the impression of 

 a folded arrangement of the strata. Plate 5 illustrates a some- 



