I9I3.] STEVENSON— FORMATION OF COAL BEDS. 77 



with other constituents in small proportion ; the fossilized wood like 

 the mass of the concretion is composed chiefly of carbonate of 

 calcium and sulphide of iron. The analysis of Stur's specimen 

 differs somewhat ; it is 



Carbonate of calcium 56.52 



Carbonate of magnesium 10.02 



Carbonate of iron i5-6o 



Clay 0.89 



Insoluble matter 0.17 



Organic matter, water, loss 16.80 



but, like the other analyses, it shows the great freedom from clay 

 and silica, which are so important in roof balls. This difference led 

 Stur to distinguish the latter as clay-sphaerosiderites. 



Except at the Bacup locality, dolomite is not the important con- 

 stituent of the coal ball. Strahan's notes respecting the Wiral col- 

 liery in Cheshire seem to have some bearing on this matter. The 

 coal there was 4 feet thick and of good quality where opened ; but 

 within a short distance bands of stone, i to 10 inches thick, ap- 

 peared, some of them consisting of spherical pellets. Within 250 

 yards, the coal was replaced with this rock, but the roof and floor 

 remained unchanged, save that the former had become reddened — 

 this change, however, being unrelated apparently to that in the coal. 

 The rock is black and hard, but weathers gray; the structure is 

 pisolitic and the concretions are sometimes united, at others inde- 

 pendent and separated by coaly matter. They consist of dolomite 

 with some coaly material, iron, silica and clay. Some fragments are 

 composed of small masses or irregular crystalline layers, separated 

 by fine mud containing quartz and flakes of mica ; while others, con- 

 sisting partly of woody tissue filled with dolomite, may be regarded 

 as wood fragments, impregnated with and cemented by dolomite. 

 When this dolomite has been removed by acid, a copious residue of 

 carbonized fibers is obtained. 



These balls or sphaerosiderites are concretions formed in the 

 coal and shale after the deposits had been made but before consoli- 

 dation. The laminae of coal and shale curve around them and some 

 of the concretions were broken during the later compression. 



