I9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 449 



pebbles of anthracite have been obtained near the Grande Couche 

 and that in some pebbles the coal has the appearance as well as the 

 composition of lignite. But these last are exceptional — they, evi- 

 dently, are of the type studied by Renault. The distribution of the 

 pebbles is absolute proof that, before the Gres Noirs coal bed was 

 formed in its present area, not only the older rocks with anthracite, 

 but also the Grande Couche itself and its associated beds were ex- 

 posed to subaerial streams, by which the pebbles of coal and car- 

 bonaceous shale were rounded. The writer collected many of these 

 pebbles from a sandstone at less than 60 feet above the Grande 

 Couche. 



Barrois^^ says that pebbles of coal are less numerous in the 

 paralic basins of north France than in the limnic basins of the 

 plateau, but he had opportunity to study some which had been 

 discovered recently. Geologists, in the majority of cases, have be- 

 lieved that coal pebbles had travelled for only short distances and 

 that they prove the process of conversion far advanced when the 

 fragments were detached; but some, objecting to such rapidity of 

 conversion, have preferred to believe that the fragments, when 

 entombed, were merely rolled vegetable matter. He notes 'the 

 statement respecting the Commentry pebbles, that some, at least, 

 show contraction, evidence that the conversion was not complete. 

 The marshes of the Pas-de-Calais, attacked by tides, give ofif blocks 

 of peat, which become rounded and at length ellipsoidal. E. 

 Geinitz has made a similar observation on the Baltic shore near 

 Rostock. The Bruay pebbles occur in hard coarse sandstone, are 

 from mere grains to 14 by 5 by 3 centimeters. They are chiefly 

 cannel, at times have laminae of brilliant coal and, under the micro- 

 scope, they show vegetable structure — they recall the peat pebbles 

 of the Pas-de-Calais. They have suffered contraction, for they 

 are surrounded by a film of calcite which penetrates the pebbles in 

 veinules. 



As the result of his studies, Barrois has found that coal pebbles 

 usually occur in coarse sandstones above coal beds ; that they have 



" C. Barrois, " Observations sur les galets de Cannel-coal du terrain 

 Tiouiller de Bruay," Ann. Soc. Gcol. du Nord., Vol. 37, 1908, pp. 3 et seq. 



