COAL MEASURES. 



581 



sandstone has been termed " Millstone Grit," because millstones are made 

 from it. We then have limestone 

 shales, and the sandstone beds. Each 

 coal seam indicates a subsidence of the 

 land and a regular series of under- 

 day or soil in which the plants grew, 

 the plants themselves, iron, coal, 

 and then the shale, and so on again, 

 indicating frequent changes and a long 

 lapse of ages. 



These " Coal Measures " occupy 

 an area of five hundred square miles in 

 Great Britain alone ; and as we have 

 already said, the period which elapsed 

 while these deposits were being laid 

 down represents hundreds of thousands of years. The deposit would increase 

 at the rate of about three feet in a thousand years. And this is only one 

 period of the many changes to which our world has been subjected since it 

 first began its revolution in space around the sun. 



Fig. 666. Fern (Pccoptcris ligata) from upper shale, 

 Scarborough. 



Fig. 667. Section across the carboniferous rocks of Derbyshire and Lancashire (After Ramsay). 



Carboniferous Limestone. 

 Coral Measures. 



2. Yoredale Shales. 

 5. Permian Limestone. 



3. Millstone Grit. 



6. New Red Sandstone. 



Coal is usually found in " basins " or depressions a sort of trough 

 owing to the upheaval of surrounding strata which became in time denuded 

 (or washed away) with any coal ; that was there. So it is in places where it 

 is concealed by overlying beds that protect it, that we now find the coal saved 



;F- 



Fig. 668. Labyrinthodoa. 



from disturbance. When we search and come upon red sandstone and 

 grey-wacke, we may be almost certain that we are near coal, particularly if 

 the surrounding rocks form a " basin." 



We have now briefly sketched the Carboniferous system, for in our 



