304 Boyers — Persistent Parting in a Coal Bed. 



of the persistency of partings of constant thickness are known, 

 but the coal in those fields is generally believed to have formed 

 in swamps which were only a few feet above sea level, so that 

 a very slight oscillation might submerge large areas. The 

 swamp in which this parting was laid down was probably some 

 distance above sea level, however, and it is difficult to imagine 

 how a slight change in level, or a flood, in such a swamp could 

 produce a deposit of constant thickness throughout the whole 

 district. Although the parting varies slightly in thickness, it 

 seems to vary irregularly ; it does not appear to be thicker in 

 any one general locality than in another, as would be expected 

 if deposited by a freshet. If this objection be disregarded 

 there are no chemical difficulties involved other than that which 

 any hypothesis must meet, namely, the peculiar recrystalliza- 

 tion of the material. 



The last hypothesis is satisfactory from a physical standpoint, 

 but involves obscure chemical reactions whose validity ma} 7 

 well be questioned. Silica and aluminium hy T droxide, the prin- 

 cipal constituents of the parting, are both known to exist in 

 solution in the colloidal state. Whether the two can exist 

 together in the colloidal state without being precipitated is an 

 open question ; assuming that they can, it is conceivable that 

 an influx of water carrying colloidal silica and alumina entered 

 the swamp, and that the colloids were slowly precipitated by 

 the organic acids in the swamp water. If they cannot, it is 

 necessary to suppose that waters carrying alumina encountered 

 those bearing silica in the swamp, and that the product was 

 thereupon precipitated, a supposition that is rather improbable. 

 This hypothesis best explains the complete crystallization of 

 the mineral, in assuming that it is a primary crystallization of 

 gelatinous material rather than a recrystallization of tuff or 

 clay. 



iSTone of the above hypotheses is thus wholly satisfactory. 

 There are grave chemical objections to the first, and the physi- 

 cal difficulties in the deposition of a thin layer of constant 

 thickness over a large area militate strongly against the second. 

 In the present state of our knowledge, the third hypothesis 

 appears the most tenable, although the evidence is not at all 

 conclusive. Despite the uncertainty which attaches to the 

 origin of this parting, its occurrence is of scientific interest ; 

 and if it is, as the writer is inclined to believe, a not uncom- 

 mon development, its importance in connection with the 

 deposition and chemical alteration of certain sediments is mani- 

 fest. It is hoped that in the future a wider application of 

 petrologic and chemical methods will be accorded to sediment- 

 ary rocks and that the significance of this and other obscure 

 deposits will be more fully understood. 



