THE REVOLUTIONS OF THE CRUST OP THE EARTH 325 



would give a stratum two meters in thickness (G^ feet) and 56 square 

 kilometers (21 square miles) of extent. As the volume of the coal-de- 

 posits is known, as well as the annual quantity consumed, it is calculated 

 that this store of combustible material will not be exhausted for thirty- 

 six thousand years. 



The formation of coal is not easy to explain. Are these deposits col- 

 lected in the hollow of valleys, at the bottom of lakes, at the mouths of 

 rivers or in turf-bogs "? Modern research inclines toward the latter sup- 

 position. The nature of the plants the debris of which forms coal im- 

 plies the presence of marshes, on the borders of which these plants 

 grew ; if such was the case, coal is a formation analogous to peat. The 

 climate must have been damp and temperate as in our day. The trees 

 which bordered the swamp fell into it as they were uprooted by the vio- 

 lence of the winds, and thus increased the quantity of vegetable matter 

 which formed the coal. For a long time the carbonization of the remains 

 of plants was attributed to the influence of terrestrial heat ; but Gcep- 

 pert has shown that it is rather due to the enormous pressure and the 

 action of chemical agents, which produced a peculiar decomposition, the 

 consequence of which was the metamorphosis of wood into coal. A 

 piece of Pinnites succinifer transformed into black coal, while some amber 

 adhering to it underwent no alteration, is a proof in favor of this view, 

 for otherwise the co-existence of the coal and the amber would be alto- 

 gether inexplicable. 



Some geologists consider that anthracite is a deposit of vegetable 

 matter, which preceded the formation of [bituminous] coal. But in 1831 

 Featherstonehaugh showed that the anthracites and [bituminous] coals 

 were deposits formed at the same time, and were often contained in the 

 same stratum. The identity of fossil plants in the two substances 

 proves the same thing.* The chemical difference of the two combusti- 

 bles must, then, be attributed to stronger pressure, to the presence of 

 more energetic chemical agents, and in every case to a greater degree 

 of terrestrial heat. 



The lignite deposits are collections of vegetable matter, encountered 

 in more recent formations. The thickness of these strata rarely exceeds 

 5 meters (16.^ feet). It is a combustible not much sought after, for 

 when it comes out of the mine it contains as much as 45 per cent, of 

 water, which is disengaged with difficulty, by exposure to the air. It 

 contains only about 50 per cent, of coal, and its calorific power varies 

 between six and eight thousand calories. It is found in the secondary 

 and lower strata of the Tertiary deposit. Its quality diminishes in pro- 

 portion as it exists in more remote periods. 



The formation of deposits similar to the lignite is taking place to-day 

 in the neighborhood of the mouths of large rivers. We may give as 

 example the large quantity of wood floating in the Mississippi, forming 

 often regular moving islands of matter. A portion of these vegetable 



* Lyell, Travels in North America, vol. 1, ]i. 8ti. 



