191-'.] STEVENSON— THE FORMATION OF COAL BEDS. 507 



ments are from pinhead to 2 inches and the larger ones are angular, 

 the edges at times jagged; the smaller pieces are more rounded. In 

 weathering, the fragments give way first, which leads Roeder to 

 suppose that they must have suffered from subaerial exposure before 

 entombment. He regards the condition as proof of occasional 

 elevation and exposure, when older limestones were broken up and 

 either transported or left in situ. Additional evidence in favor 

 of this conclusion was found in his discovery of angular fragments 

 of the Spirorbis limestone in sandstone at Ardwich. He cites Hull 

 and Williamson to show the wide occurrence of brecciated lime- 

 stone in England. 



De Dorlodot^'^ discussed the great breccia in the upper part of 

 the Carboniferous within the Franco-Belgian basin. He says that 

 some geologists have supposed it to be due to dynamic action, but 

 he believes it sedimentary, due to destructive attack by waves and 

 the rapid accumulation of the products. The elements vary much 

 in size and the paste, filling spaces between large and small blocks, 

 is itself partly limestone. The fragments, large and small, have 

 their angles but slightly rounded. In any event, they could not 

 have come from far. 



Stainier**^ took vigorous exception to de Dorlodot's explanation, 

 because nothing of the sort is known in actual times. The supposed 

 conditions exist at many places along the coast of Great Britain, but 

 no breccia forms. If, by change in conditions, the shallow Carbon- 

 iferous sea should be dried up completely, the exposed limestone 

 would be a desert surface. The contrast in temperature would 

 break up the rock and give material for the breccia. When the sea 

 returned and invaded the area, it would sort the materials. The 

 coarse blocks would be moved little, the less coarse, farther ; by in- 

 sensible gradation one passes to the compact limestone which, east 

 from Namur, occupies the breccia horizon. 



It is difficult to believe that the brecciation was caused by ex- 



"' H. de Dorlodot, " Sur I'origine de la grande breche Viseene," Bull. 

 Soc. Beige de Geol., Vol. XXII., 1908, Mem., pp. 29-38. 



"' X. Stainier, " Du mode de formation de la grande breche du Carbon- 

 ifere," Bull. Soc. Beige de Geol, Vol. XXIV., 1910, P. V., pp. 188-196. 



