1 86 REINHARDT THI ESSEN 



breaking with a rather irregular fracture. The former is generally 

 designated as "bright coal" or "glanz coal," and the latter as "dull 

 coal" or "mat coal." On a more careful examination, it is seen 

 that the "bright coal" consists of lenticular masses, greatly var3dng 

 in breadth and thickness and entirely surrounded by or imbedded 

 in the "dull coal." 



It is further found that the "dull coal" is extensively sublami- 

 nated into thinner sheets of "bright coal" and "dull coal"; and 

 again on a more minute examination, the bright coal layers are 

 found to be embedded in the dull coal. 



The distinction between "bright coal" and "dull coal" has 

 been recognized since the close of the eighteenth century, and many 

 theories have been advanced since that time to explain the phe- 

 nomenon, but a satisfactory explanation and a true meaning of 

 the alternating dull and bright layers and laminae has never been 

 given, 



A condition prevaihng generally in all ordinary bituminous 

 coals is well illustrated in Figure i, representing a small chunk of 

 IlHnois coal. In this lump, the "bright coal" is represented by 

 uniform black bands a, while "dull coal" is represented by the 

 lighter, finely striated bands d. In this respect all ordinary, 

 bituminous coals, no matter from what locality they may be chosen, 

 are similar. Some, of course, like the coal from the Vandalia 

 mine, near Terre Haute, Indiana, possess a larger proportion of 

 "bright coal." Others again, like the coal from the Pittsburgh 

 seam, show a larger proportion of "dull coal"; some seams are all 

 "dull coal," but these are only differences in degree and not in 

 kind. 



The sublamination of the "dull coal" is much better illustrated 

 in a vertical fracture slightly magnified, say ten diameters, and 

 especially when such a surface is first smoothed and pohshed 

 (Fig. 2). The very best illustration, however, is to be had from 

 a thin section, by means of transmitted light and at a low magni- 

 fication, provided a large enough section is available (Figs. 7 and 10). 



Figure 2 represents a part of the surface of the block just shown, 

 limited by the intersecting lines x-x' , y-y' , m-m , and n-n' and 

 magnified 10 diameters. The bands a-i, a-3, <i-5, and a-7 represent 



