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



of sedimentary origin; some resemble Carboniferous rocks and all 

 are in coaly material. These records seem to suggest that pebbles 

 are not abundant in coal and that they are even of comparatively 

 rare occurrence — the instances noted by Orton and Stainier are 

 not exceptions, as they are examples of extreme localization. 



Barrels^* undertook systematic study of the matter in a definite 

 area and presented the results in an elaborate memoir, of which only 

 the merest synopsis can be given here. Most of the fragments w^ere 

 obtained during a four months' exploration of the Vein-du-Nord, a 

 double bed, showing great constancy in the explored area, which is 

 7 kilometers long. The upper bench, 0.25 meter thick, has 14 per 

 cent, of volatile and only 2 per cent, of ash, while the lower bench, 

 0.35 meter thick, has 17.2 per cent, of volatile and 10 per cent, of 

 ash. The rock fragments are coated with soft sooty coal, often 

 pyritous, and the lamination is more, or less distorted about them. In 

 all, 295 specimens were secured, of which 86 per cent, were derived 

 from Coal Measures rocks, a few from Cambro-Silurian deposits 

 and nearly 11 per cent, from the distant Archaean. The largest 

 fragment weighs about 120 kilogrammes or approximately 280 

 pounds. The great preponderance of fragments from the Coal 

 Measures shows that outcrops of those rocks were not far away, 

 so that at the time of the Assise d'Andenne — the Lower Coal 

 Measures — the beds of that epoch were no longer mere muds and 

 sands, but consolidated shales and sandstones ; some fragments 

 show even the jointing of contraction. Many are thoroughly water- 

 worn, others are angular, and both types are mingled indiscrimi- 

 nately. In some other coal beds of this region, fragments have been 

 found in the mur, coated with clay which is marked with lacework 

 of Stigmaria rootlets. 



Fragments were found in all portions of the bed, from bottom to 

 top, but the upper bench yielded 50 times as many as the lower. 

 The number averages only one to each 100 square meters of area, 

 but the distribution is irregular and they occur, as it were, in nests. 

 The more abundant occurrences are associated with contractions of 



" C. Barrois, " fitude de galets trouves dans le charbon d'Aniche, Nord," 

 Ann. Soc. Geol. du Nord, Vol. XXXVI., 1907, pp. 248-330. 



