1912.] STEVENSON— THE FORMATION OF COAL BEDS. 453 



In some instances, the distance was small or the detached pieces 

 were enveloped in sands and swept along as part of the mass to the 

 place of deposition. This was evidently the condition where one 

 finds irregular chunks and petty lentils of coal as in the sandstones 

 of western Pennsylvania and at many places in other coal fields. 

 These masses, torn ofif by a sand-laden stream, were transported 

 without material attrition and were deposited where the speed 

 slackened on an overflow surface. It must be noted that one rarely 

 finds such masses in localities where the underlying coal bed shows 

 evidence of erosion ; very often the sandstone with coal fragments 

 rests on an uneroded coal bed; while in very many cases the sand- 

 stone resting on a bed with channeled top is wholly without note- 

 worthy fragments. The channel was dug in the vegetable material 

 and was filled with sand of later arrival. It is difficult to conceive 

 how material torn from a bed could be deposited over or very near 

 the place of origin, unless one imagine a whirlpool of modest area; 

 but it is easy to understand how a sand-laden stream flowing irregu- 

 larly across a plain, covered with a thin marsh, could remove the 

 cover and deposit it elsewhere. That condition is approached in 

 considerable areas by streams like those of the Paraguayan region, 

 where the channels are aggraded quickly in flood time and the 

 turbid waters are driven to seek new courses. 



But if the coal fragments be rounded, they point to conditions 

 wholly different from those just considered. In the summary of 

 observations given above, there are references to the process of 

 conversion and to the time required for its completion. Those 

 topics lie, for the most part, outside of the question in hand and are 

 related to it only incidentally ; but the structure and composition 

 of coal in the pebbles make certain that it was already well advanced 

 in conversion when removed from the bed. The Commentry 

 pebbles in some cases were already anthracite while others, those 

 from the Grande Couche, were still flaming coal ; for both types 

 are found in the same sandstone — that above the Grande Couche — 

 and, along with them, are thoroughly rounded pebbles of car- 

 bonaceous shale. 



In not a few^ localities the pebbles are cannel and they have been 



