24 ANCIENT PLANTS 



few inches to many feet in thickness and is called the 

 " underclay ". This fine clay is generally free from 

 pebbles and coarse debris of all kinds, and is often 

 supposed to be the soil in which the plants forming the 

 coal had been growing. The line of demarcation be- 

 tween the coal and the clay is usually very sharp, and 

 the compact black layers of hard coal stop almost as 

 abruptly on the upper side and give place to a shale or 



I LIMESTONE 



2. SHALY ROCK -- 



3. COAL 



4- CLAY 



5. SHALY ROCK--- 

 e- COAL 



CLAY- 



8. SHALY ROCK -- 



9- COAL 



"0. CLAY 



n. COAL 



12. CLAY 



'3. SANDSTONE - - 



Fig. 13. Diagram of a Series of Parallel Coal Seams with 

 Underclays and Shale Roofs of varying thicknesses 



limestone "roof"; see fig. 13, layers 5, 6, and 7. Very 

 frequently a number of small seams come together, 

 lying parallel, and sometimes succeeding each other so 

 rapidly that the "roof" is eliminated, and a clay floor 

 followed by a coal seam, is succeeded immediately by 

 another clay floor and another coal seam, as in fig. 13, 

 layers 10, u, and 12. The relative thickness of these 

 beds also varies very greatly, and over an underclay of 

 seven or eight feet the coal seam may only reach a 

 couple of inches, while a thick seam may have a floor 

 of very slight dimensions. These relations depend on 



