STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 151 



attained the same degree of enrichment before burial ; the minimum 

 of the Pierre rarely falls below 75, but there are seams with only 

 71 or 72. The condition is well marked in Hannover, where the 

 "black coal " has 89 per cent, of carbon, the brown coal, 73, while the 

 Blatterkohle is almost unchanged — the several types occurring in 

 the same vertical section. 



