170 ON THE ORIGIN OF COAL. 



left. Thin cracks filled with sand, also appear in 

 the clay. These seem as if made after the animal 

 had walked. Thus, these American flags present 

 very similar appearances to Weston ones, before 

 described. 



Many of the fine beds of flag show very regular 

 depositions of sand, alternating with clays, such 

 as might, on first view, lead us to suppose them 

 the effects of tidal action ; but they are of small 

 extent, and the direction of the currents which 

 brought the materials of which they are composed, 

 is often very variable, and much more difficult to 

 ascertain than littoral deposits, by the present 

 ocean, appear to be. Many of these beds of flag 

 exhibit impressions of some body having acted 

 upon them, when in a soft state, as a slab of the 

 upper flag rock from Kerridge shows. (See plate 

 I.) All these marks can be traced down- 

 wards, through several successive deposits of 

 one-eighth to one-fourth of an inch each in thick- 

 ness. In some instances a bed of flagstone, eight 

 inches in thickness, will show impressions of the 

 same size as those in the lithographic plate on 

 its upper surface, and corresponding marks, in 

 relief, on its lower surface ; thus showing that 



