36 STEVENSON— FORMATION OF COAL BEDS. [April.:. 



An additional difficulty is found in the expansion of the Thick 

 and other coal beds toward the north. The expansion of the whole 

 series and the splitting of the beds in that direction seem incom- 

 patible with the idea that the coal beds were formed at or above 

 the surface of the water, while the intervening strata were deposited 

 under it. Of the intervening rocks, those of coarse material are 

 heaped up usuall}' and thin out rapidl}' in all directions, wdiile those 

 of line material have a greater area. This is true of superimposed 

 beds forming a group; when material is fine, the disappearance of 

 a bed is gradual. This law of area and thickness means only that 

 fine materials were spread over a larger area '" in conse(|ucnce of 

 their comparatively light specific gravity, or at least of their being 

 more easil}- and therefore more widely transported by water, and 

 being more generally dift'used through it before finally coming to rest 

 at the bottom. It was pointed out before, too, that beds of coal so 

 far from forming any exception to this general rule, are its most 

 marked example at the one extreme, while coarse sandstones and 

 conglomerates form the most striking example at the other. . . . 

 I wish merely to sa}' as the result of an experience of a good many 

 years, confirmed by the particular instance under examination, that 

 the phenomena of the lamination and stratification of beds of coal, 

 and their interstratification and association with other stratified rocks 

 are explicable solely by the relation of the specific gravity of their 

 materials to the action of moving water, and the consecjuent diffu- 

 sion of their materials through the mass of that water." 



The materials of the clays and sandstones were most largely 

 deposited on the northern side of the coal field and sometimes 

 failed to reach the southern part of the area, whereas the coal beds 

 ''were diff'used e(|uall}', or at least more ecjually, over the whole 

 area." lie finds in the llottom coal l)ed a notable illustration of 

 these condition.^ — and it is only one of many. One '"cannot fail 

 to be struck with the i)bvious ' delta-like ' or ' bank-like ' form which 

 the Coal Measures of South Straft'ordshire must liave originally 

 possessed, and the perfect resemblance they must have had to an 

 undisturbed subaqueous accumulation. It seems to me then impos- 

 sible to suppose otherwise than that the whole series of the Coal 



36 



