192 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



accumulated in great thickness, as in southern France. It is evident 

 that some were made close to estuaries, for several observers have 

 recorded that thick deposits, crowded with freshwater fossils, are 

 interrupted by layers containing forms which are unquestionably 

 marine. The intervals between Tertiary coal beds vary so greatly 

 and so abruptly in thickness that, where natural exposures are the 

 only dependence, correlation of beds in any area is difficult, often 

 impossible. 



The roof of a Tertiary coal bed may be composed of any kind of 

 rock, transported or formed in sitit. There are abundant illustra- 

 tions of transition from the underlying coal to the roof rock, from 

 accumulation of coal to final destruction of plant life on the bog 

 surface, alternating laminae of vegetable and mineral material testi- 

 fying to the struggle between silt overflows and the dwindling bog. 

 This faux-toit is a characteristic feature ; at times it is merely a 

 carbonaceous shale ; at others it is a very impure coal. Very often 

 the immediate roof is distinctly transported material with leaves, 

 twigs and broken bits of wood, even fragmentary trunks of trees. 

 It may be marine shale, rich in fossils, or it may be a marine lime- 

 stone, as at Haring, containing bits of water-loving plants, such as 

 Salix, Erica and palms. Or it may be sand enveloping erect trunks 

 of trees, which grew on the dried surface of the bog, as at Senften- 

 berg or near Friesdorf in the Cologne area. Grand'Eury-*^^ remarks 

 that the fossil forest in the roof of the great coal bed at Petroszeny, 

 in Hungary, rivals that of Purbeck and that the trunks, erect, are 

 rooted on the top of the coal, while around the stems at the bottom 

 of the deposit are branches of Taxodium distichnm, clearly fallen 

 from the stumps. A similar condition is reported from localities in 

 the United States and in Europe. At times, the roots of trees grow- 

 ing in the faux-toit, pass downward into the coal as described by D. 

 White for a mine in Texas and by other observers in places where the 

 stumps are rooted in the coal. Grains of coal are not rare in the 

 roof or the accompanying rocks, showing clearly that a coal deposit 

 had been exposed to erosion. In much of the Oligocene areas of 

 Prussia, the original roof has been removed and has been replaced 

 with late drift material. The gradual disappearance of coal-forming 



-^^ C. Grand'Eury, Comptes Rendus, T. 130, 1900, p. 1689. 



