112 



WOOD-COAL. 



wood-coal and common coal bear in other respects too close a resem- 

 blance, to allow us to ascribe to them a different origin, though they 

 were probably formed from different tribes in the vegetable kingdom, 

 and under different circumstances. 



IVood-coaP is found in considerable quantities at Bovey Heath- 

 field, near Exeter. Several beds of coal are separated by strata of 

 clay and gravel : the lowest is seventeen feet thick, and rests on a 

 bed of clay, under which is sand resembling sea sand. The coal in 

 contact with the clay has a brown colour, and appears intermixed 

 with earth. In other parts the laminae of the coal undulate, and re- 

 semble the roots of trees : in the middle of the lowest stratum the 

 coal is more compact, and is of a black colour, and nearly as heavy 

 as common coal. 



A great repository of this kind of coal exists near Cologne : it 

 extends for many leagues : it is fifty feet in thickness, and is covered 

 with a bed of gravel, from twelve to twenty feet deep. Trunks of 

 trees deprived of their branches are imbedded in this coal ; which 

 proves that they have been transported from a distance. Nuts 

 which are indigenous to Hindostan and China, and a fragrant resinous 

 substance, are also found in it. A similar resinous substance occurs 

 in the Bovey coal, and was also discovered with fossil wood, in cut- 

 ling through Highgate Hill. Mr. Hatchett, by whom it was analyzed, 

 has given it the name of retinasphaltum. 



In wood-coal we almost seize nature in the act of making coal, 

 before the process is completed. These formations of coal are of 

 far more recent date than that of common coal, though their origin 

 must be referred to a former condition of the globe, when the veg- 

 etable productions of tropical climates flourished in northern latitudes. 

 The vegetable origin of common mineral coal, appears to be estab- 

 lished by its association with strata abounding in vegetable impres- 

 sions ; by its close similarity to wood-coal (which is undoubtedly a 

 vegetable product) ; and, lastly, by the decisive fact, that some min- 

 eral coal in the Dudley coal-field is composed entirely of layers of 

 mineralized plants. 



But though the vegetable origin of mineral coal may be satisfacto- 

 rily established, there is considerable difficulty in conceiving by what 

 process so many beds and seams of coal have been regularly ar- 

 ranged over each other in the same coal-field, and separated by strata 

 of sandstone, shale, and indurated clay. It will tend to simplify the 

 enquiry, if we examine a coal-field of very limited extent ; such as 

 those which occur in small coal-basins called sivilleys on the hills in 

 the West Riding of Yorkshire, and which are not more than one 



* The description of wood-coal ought to be given in the account of the tertiary 

 strata and diluvia, but it offers many circumstances which tend to elucidate the 

 formation of mineral coal. 



