34 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



layers of undecomposed Glossopteris leaves, not brittle but retaining 

 their original substance; soaked in glycerine and water, they can be 

 unrolled and laid flat. A large number of the specimens were 

 mounted and placed on view in the museum of the Department of 

 Mines at Sydney. 



Stone coal marks a still greater advance in chemical change. 

 With rare exceptions, it is laminated, black or grayish black, more or 

 less lustrous and with a black streak. In nearly all stone coals, 

 there are alternations of bright and dull laminae, the Glanz- and the 

 jMattkohle of von Giimbel, which may be extremely thin or several 

 inches thick. Usually, there is little macroscopic evidence of plant 

 structure, aside from the mineral charcoal, mother of coal, fusain, 

 Faserkohle of authors, which resembles charred tissue. This is the 

 ordinary coal of the Carboniferous and it is present in many localities 

 of later Cretaceous age. The difficulty encountered in the effort to 

 define a limit between brown and stone coal is increasingly great, as 

 the determination is of commercial importance in the western United 

 States, especially in areas where both types occur in the Mesozoic. 

 Stone coals have been divided commercially into bituminous and 

 semi-bituminous on the basis of volatile content, but this does not 

 suffice for distinction from the brown coals. The latter have been 

 termed hydrous coals because they contain much water, apparently 

 combined, and break up rapidly on exposure to the air. But many 

 so-called anhydrous coals break up with equal readiness on exposure 

 to dry air. It is quite certain that typical Carboniferous coals have, 

 for the most part, a definite prismatic cleavage and that many brown 

 coals lack that feature, while some have it. Alany methods of dis- 

 tinguishing the types have been suggested, but none is satisfactory; 

 the exceptions are too numerous to prove the rule. No hard and 

 fast line between brown and stone coals exists except in generalized 

 tables ; but, as a rule, the older coals are more advanced in chemical 

 change than those in later deposits. 



Anthracite resembles stone coal in structure and often in appear- 

 ance, but it is more brittle and more brilliant. The volatile content 

 is small, often approaching a trace. Like the stone coal, it often 

 contains much mineral charcoal, thus showing relationship to the 



