Coal Bed of the Lance Formation. 303 



Leverrierite, as described by Termier, has the composition of 

 rauscovite with hydrogen partly or wholly replacing potassium 

 and a little iron equivalent to aluminium ; or, as stated to the 

 writer by F. W. Clarke, it is the vermiculite or end member of 

 the mica series. Its formula then is HAlSi0 4 . However, the 

 above analysis of the parting as recast gives H 4 Al 2 Si 2 9 , which 

 is the composition of kaolin, a fact which seems strange in 

 view of the striking resemblance in physical properties which 

 this material bears to leverrierite. It is possible that the loss 

 on ignition included a lar^e amount of carbon occurring in the 

 sample as mechanically mixed coal, which if allowed for would 

 reduce the water and cause the analysis to approximate the 

 formula HAlSi0 4 ; but unless this be assumed the mineral 

 must be considered a variety of kaolin which for some reason 

 has assumed most of the physical properties of leverrierite. 

 Furthermore, inasmuch as leverrierite has only one molecule 

 of H 2 as against two in kaolin, a difference which is presum- 

 ably due to the conditions of moderate metamorphism which 

 produce it, it would hardly be expected to occur in the practi- 

 cally unmetamorphosed strata of the Lance. 



Genesis. — It has been shown that the parting is persistent 

 and is of nearly constant thickness throughout an area of over 

 800 square miles ; and that it is chiefly an aggregate of crystals 

 which resemble leverrierite in physical properties but have the 

 composition of kaolin. It must have formed in situ, and that 

 it did not greatly disturb plant growth is shown by the pres- 

 ence in it of numerous vertical plant stems. Three theories of 

 the origin of this material at once suggest themselves. It may 

 represent an altered tuff; it may be an alteration product of a 

 pure clay ; or it may be a chemical precipitate. 



The widespread occurrence of this parting at the same hori- 

 zon suggests its derivation from wind-blown volcanic ash. Ash 

 is not uncommon in these rocks, and if this material is of clastic 

 origin it is difficult to imagine another manner in which this 

 thin deposit, of constant thickness over a large area, could have 

 formed. This was the field theory advanced to account for it; 

 but microscopic examination at once revealed the fact that, if 

 so, a thorough alteration and recrystallization must have taken 

 place. It is possible that pure orthoclase might alter to seri- 

 cite and then to kaolin under the influence of swamp waters ; 

 but there is no known tuff approximating so closely the com- 

 position of orthoclase as to yield this product directly. If this 

 hypothesis be accepted, the removal of considerable excess 

 silica, as well as thorough leaching of the potassium silicate 

 formed in the alteration to sericite, must be postulated. 



The next hypothesis, the derivation of this material from a 

 clay deposit, meets at once with strong physical objections. 

 In the Appalachian and interior coal fields, numerous examples 



