OBLIQUE BEDDING 273 



different kind of material as was thrown down before. This 

 structure requires only a short pause in deposition, not a long, 

 unrecorded break, and does not necessarily involve movements 

 of elevation and depression. Furthermore, contemporaneous 

 erosion is a local phenomenon, and though in a limited section 

 it may not always be easy to distinguish it from an uncon- 

 formity, the difference becomes apparent when a wider area is 

 examined. If the structure be one of contemporaneous erosion, 

 the two series of strata will be conformable except along the line 

 of the channel or channels. In Fig. 119 is an example of this 

 structure and shows where a channel in an ancient sea-bottom of 

 calcareous material was filled up by a later deposition of similar 

 substance. 



The clay "horses" (as miners call them), which frequently 

 interrupt coal beds, are the channels of streams which meandered 

 through the ancient peat bog, and which were filled up with sedi- 

 ment when the swamp became submerged. The "horses" are 

 usually of the same rock as that which forms the cap or roof 

 of the coal seam. 



Horizontal and Oblique Bedding. — Another kind of deceptive 

 resemblance to unconformity is occasionally caused by the alter- 

 nation of horizontal and oblique bedding, a horizontal bed resting 

 upon a series of inclined layers. A conspicuous example of this 

 is given by the Le Clair limestone of Iowa, which was at one time 

 altogether misunderstood, but the deception is seldom one that a 

 little care will not expose. (See also Fig. 77.) 



