450 STEVENSON— THE FORMATION OF COAL BEDS. l-Vw. i. 



the coal of the area, not differing matet-ially in volatile content ; that 

 they belong to some coal bed not far away, for one never finds 

 maigre pebbles in a gras area, or the reverse; that pebbles of cannel 

 occur more frequently than those of ordinary coal. These condi- 

 tions hardly justify the supposition that the pebbles were brought 

 by streams flowing over beds, which, already coal, alternated with 

 shale and sandstone. The Bruay pebbles are all from one coal bed 

 and that must have been near by. The presence of lamina of bril- 

 liant coal in the sandstone, identical with the laminae in the cannel, 

 leads to the same conclusion, because > in the Nord basin, as in 

 South Wales, cannel is in the roof of tne beds. When a coal bed 

 is eroded, the cannel is the first portion to be removed, which 

 explains the frequent occurrence of cannel pebbles in areas where 

 that type of coal is rare. Barrois concludes that the Bruay pebbles 

 came from one bed, in which the material was not wholly converted. 

 The erosion occurred simultaneously ovef an extensive area and 

 was due to changes in course of a stream, loaded with sediment, 

 which inundated the bog abruptly. It liltlst have come from a 

 distance, for the deposit contains elements of crystalline rocks, 

 larger and more abundant than in any other clastic deposit within 

 the basin. 



Petrascheck'* investigated the mode of occurrence of coal pebbles 

 in a sandstone within the Galician area. The deposit is thick, 

 usually only moderately coarse, but in the portion carrying the 

 pebbles it becomes irregular and conglomerate. Some fragments of 

 coal are distinctly rolled, some have merely rounded angles, while 

 others have the edges sharp. The minutely Jiitted surface of some 

 fragments led him, at first, to imagine that they had been peat when 

 entombed, the pitting being due to pressure by sand grains. But 

 the conditions throughout compelled him to abandon this conception. 

 The form and substance of the pebbles favored belief that at the 

 time of burial the material was already hard. Glance coal breaks 

 into angular pieces and can endure little chafing, while cannel and 



^' W. Petrascheck, " Das Vorkommen von SteinkohlengcroUen in einem 

 Karbonsandstein Galiziens," Verhandl. k. k. Geol. Rcichsaiit., 1909. Wien, 

 1910, pp. 380 et seq. 



