THE MODE OF ORIGIN OF COAL 



223 



Slopes and Watson that true coal balls are formed in situ in the 

 coal by local impregnation of the original peaty substance with 

 lime or magnesium carbonate or both. The minerahzed portion 

 of the peat retains its structure, while that which has not been 

 impregnated is completely devoid of definite organization when 

 transformed into coal. If all coals were derived from ordinary 

 peat they would present the same homogeneous organization as 

 shown in our Fig. 5, since wood and similar formations have 

 their structure obliterated when transformed into coal. In this 

 respect peaty deposits present a marked contrast to the lacustrine 

 mucks which have been the fore- 

 runners of cannels and oilshales. 

 The spores and similar cutinized 

 structures are the only remains 

 of plants, which in general do not 

 become obliterated in organiza- 

 tion when passing into coal. 

 Only petrification can save ordi- 

 nary peat in the long run from 

 the complete obliteration of its 

 original vegetable constituents. 

 It is now possible to consider 

 with advantage the commoner 

 types of coal, those which occur in greatest abundance in all parts 

 of the world where coal deposits exist. Fig. 6 illustrates a coal of 

 this ordinary bituminous type from Illinois. It is at once clear 

 that the structure is not homogeneous as in the Westphalian coal 

 shown in Fig. 5. Running across the figure is a dark band of 

 "mother of coal " or mineral charcoal. This represents wood which 

 was charred by fire before it entered into the substance of the coal. 

 Mother of coal, inappropriately so called, represents the only woody 

 constituent of ordinary coals which retains its structure. Very 

 often the woody elements are in a perfect condition of preservation 

 and the nature of the trees which produced them can be diagnosed. 

 In addition to the dark band of mineral charcoal or mother of coal, 

 there are other narrower dark stripes in the coal, as well as lighter- 

 hued more homogeneous bands. The latter correspond to highly 



Fig. 6 



