VARIOUS KINDS OF FOSSIL PLANTS 19 



and Yorkshire, where they are extremely common in 

 some of the mines, but they also occur in Westphalia 

 and other places on the Continent. 



In external appearance the "coal balls" are slightly 

 irregular roundish masses, most generally about the size 

 of potatoes, and black on the outside from films of 

 adhering coal. Their size varies greatly, and they have 



Fig. io. Mass of Coal with many "coal balls" embedded in it 



a a, In surface view ; bb, cut across. All washed with acid to make the coal balls 

 show up against the black coal. (Photo by Lomax.) 



been found from that of peas up to masses with a 

 diameter of a foot and a half. They lie embedded in 

 the coal and are not very easily recognizable in it at 

 first, because they are black also, but when washed 

 with acid they turn greyish -white and then can be 

 recognized clearly. Fig. io shows a block of coal with 

 an exceptionally large number of the " coal balls " em- 

 bedded in it. This figure illustrates their slightly 

 irregular rounded form in a typical manner. By 

 chemical analysis they are found to consist of a nearly 

 pure mixture of the carbonates of lime and magnesia; 



