6 STEVENSON— FORMATION OF COAL BEDS. [ApHi 21. 



terre " and restricts the term " houille " to " the black combustible 

 earthly deposits which are often found over and sometimes under 

 the coal beds." These are simply mold mixed with a small amount 

 of bitumen. The slime deposited in the sea, following the slope of 

 the bottom and extending at times for several leagues along the 

 coast is nothing other than the mold of plants and trees, which is 

 drawn ofif by running water. The vegetable oil of that slime, 

 seized bv acids in the sea, will become in time bituminous coal but 

 always light and friable ; while the plants themselves, drawn off in 

 like manner and deposited by the waters form the true beds of 

 charbon de terre, of which the characteristics are very different 

 from those of houille. the charbon being heavier, more compact and 

 swelling in the fire. 



The dips of the coal are due to the general law of deposit in 

 moving water, while at the same time the materials have taken the 

 inclination of the surface on which they were laid down. Occa- 

 sionally the dip approaches the vertical, but even that great inclina- 

 tion gradually approaches the horizontal more and more as one 

 descends and at last the horizontal plane, the plateur. is reached. 

 A usual feature is that the thickness of a coal bed increases with 

 the depth and the maximum is cii plateur — which is in accordance 

 with the law of deposit of materials carried by water and laid down 

 on a sloping surface. The same law applies to other materials, 

 whereby is explained easily the parallelism of coal beds to each other 

 and to the intervening strata. 



Von Beroldingen" published his work in the same year; it was 

 based on broad field study. Tn it the author maintained that stone 

 coals had originated from brown coals and those in turn from 

 peat. This aj^j^ears to be the first definite assertion of the peat-bog 

 or in situ hypothesis. 



De Luc" published the same theor\- during the next }ear in the 



"v. Beroldingeii. " l>co!)acIilungen, Zwcifel unci I'"ragen, die Miiicralogie 

 bctreffcnd," Erster Versucli, Hannover, 1778. The writer has been nnable 

 to find a copy of this work. It is cited by De Luc (1779), Mietzsch (1875) 

 and by several other authors. 



'J. A. De Luc, " Lettres physiques et morales sur Thistoire de la terre 

 ct de rhomnic," Paris. 1779, Tome V., pp. 213-25. This 126th letter is dated 

 Oldcnlnirg, Scpteml)er 16, 1778. 



6 



