1912] Louderback: Pseudostratification 29 



layer, and that it produces a structural appearance closely 

 simulating stratification. It might readily happen that similar 

 phenomena would arise from the deposition of a calcareous or 

 ferruginous instead of a siliceous cement. 



Necessary conditions for production of observed phenomena. — 

 An essential condition for the formation of pseudostratification 

 as exhibited in the localities studied is a porous, moderately even- 

 grained sediment, massive over the areas exposed. That is, it 

 must be without definite segregation of the material into layers 

 of different grain, or alternations of more porous and less porous 

 layers, or other definite marks or conditions resulting from 

 original stratification that might influence the percolation or 

 deposition of cement, and disturb the formation or appearance 

 of the new and independently oriented lines of structure. 

 Furthermore, the climatic conditions would have to be such that 

 while water would be supplied to dissolve, transport and deposit 

 the cement, it would not be humid enough to wash all such 

 solutions into a general ground-water system, and so prevent 

 cementation near the surface. 



The rainfall, in other words, should mainly be by short, dis- 

 connected showers, giving but a moderate water penetration. 

 This degree of aridity is commonly realized in the western states. 

 The fact that the dip of the pseudostrata is often greater than 

 that of the surface is probably due to the greater penetration 

 of the rain lower down the slope, where the ground may be 

 supplied not only by the direct dropping of water from the 

 atmosphere, but also by the surface movement of the rain sheet 

 from the higher slopes. 



The predominance of silica in the cement is probably due to 

 the composition of the sands here involved. Clay substance is 

 very slight in amount, calcium and magnesium unimportant 

 because there is a dearth of decomposable basic feldspar or lime- 

 magnesium-iron minerals, while silica can be derived from the 

 plentiful orthoclase and acid plagioelase, and perhaps from the 

 abundant quartz. 



An important auxiliary condition which renders the pseudo- 

 stratification the more easily mistaken for true stratification is 

 the disintegrate nature of the original formations which yield 



