BITUMINOUS COALS 203 



exceptionally large amounts of it, but layers with equal amounts 

 are not rare in any coal. 



The carbonaceous matter. — All ordinary bituminous coals con- 

 tain certain constituents that are more highly carbonized than 

 the rest of the coal and to which it stands out in sharp contrast 

 on account of their opaqueness. These are well represented in 

 Figure 22, in which these constituents are represented by the 

 more or less irregular black areas. They are also seen in Figures 

 19 and 20, 21, 24, and 25. 



In general, there are two types of carbonaceous matters: one 

 shows definite plant structure and is clearly shown to be more highly 

 carbonized parts of plant cells or bits of woody or other plant tissues, 

 and the other shows no plant structure and is of indefinite origin. 



The former usually have retained the original plant form and 

 characters, such as pores, pits, trabacular and spiral thickenings. 

 These are nothing more or less than smaller bits of mineral charcoal. 

 The different constituents of this class of carbonized matter vary 

 largely in the degree of carbonization and hence also in opaqueness. 

 Its opaqueness varies from that where it is only shghtly more 

 opaque than the normal anthraxylon constituents to that where 

 it is entirely opaque even in the thinnest sections. In general, 

 however, most of it is opaque in the medium thin sections, becom- 

 ing translucent in the thinner sections. Relatively very few 

 appear to be entirely opaque in the thinnest sections. When trans- 

 lucent or transparent, they are of a dark-red color, becoming 

 darker with increase of opaqueness and of a lighter red with decreas- 

 ing opaqueness, approaching a pale yellowish red in color. 



The disorganized opaque matter. — The other kind, the disorgan- 

 ized and more irregular kind of opaque matter, is not so easily 

 defined. Its origin is possibly varied, but most of it is of undoubted 

 organic origin. The shape of most of the particles comprising 

 the matter is irregular, but a considerable number are oval to 

 spherical. In size they vary from the most minute particle to that 

 visible to the naked eye. The more spherical and oval particles 

 suggest carbonized resinous matters. 



Rodlets. — Other constituents that are invariably present in all 

 coals are the so-called rodlets. Generally speaking, they form 



