18 CLAYS AND CLAY INDUSTRY. 



disintegrate the surface rocks and wash away the loose fragments 

 and grains. This brings about a general sculpturing of the sur- 

 face, forming hills and valleys, the former representing those 

 parts of the rock formations which have not yet been worn away. 

 The effect of this is to cause phenomena or conditions, which may 

 at first sight appear puzzling, but are nevertheless quite simple, 

 when the cause of them is understood. 



- 2 

 "y- 3. 



Fig. 11. 



Horizontal beds, only the top layer showing at the surface, when the latter is flat. 



Let us take, for example, a section of horizontal clay beds, 

 which originally covered an extensive area and were interstrati- 

 fied with sand beds. In Figs, n and 12, beds 1 and 3 may be 

 taken to represent the clays. In Fig. 11, we have indicated the 

 surface as it originally was, and in Fig. 12, the outline as it 

 appears after the land has been exposed to weathering and 

 erosion for an extended period. Here we see that the upper 



Fig. 12. 



Horizontal beds with several layers exposed, by wearing down of the land surface. 



bed is left only on the highest hills, and has been removed over 

 a large area, while No. 2 caps the smaller knolls and No. 3 out- 

 crops in the sides of the deeper valleys. Many small areas of clay 

 thus represent all that is left of a formerly extensive bed. 



If the beds had a uniform dip, the conditions may be as in- 

 dicated in Fig. 13. Here, bed 1 appears at the summit of 2 hills, 

 a and b, but its rise carries it, if extended, above the summit of hill 



