COAL 221 



the " coal " were probably masses of algae, mosses and soft 

 aquatic plants, which were brought down and accumu- 

 lated in swampy, forest-covered ground about the mouths 

 of rivers, the deposit being covered in owing to rapid 

 oscillations of level by beds of sand or clay, followed by 

 new growth and deposit. 



Our British coal and a good deal of foreign coal is 

 found in certain stratified rocks of the earth's crust 

 known as "the Carboniferous System," about 12,000 ft. 

 thick, consisting chiefly of very dense limestone. The 

 " seams," or stratified beds of coal, occur in sandy rock 

 known as the " Coal Measures," and vary in thickness 

 from a mere film to 40 ft. Above the Carboniferous 

 System are later deposits, some 14,000 ft. in thickness 

 the Permian, Triassic, Jurassic, Cretaceous, and Tertiary 

 strata. Below them we find stratified deposits containing 

 fossilized remains of plants and animals, to a depth of 

 another 40,000 ft. : they are the Devonian, Silurian, and 

 Cambrian " systems " or series of strata. Coal of a work- 

 able nature is found in many parts of the world in the 

 beds or strata of later age than our Coal Measures 

 namely, those of Jurassic, Cretaceous, and Tertiary age. 



Coal is so valuable and used in such vast quantities by 

 modern man that, though procured at first from beds 

 lying at or near the surface, it has been found remuner- 

 ative to mine far into the depths of the earth's surface, 

 where its existence is ascertained, in order to procure it. 

 A depth of 4000 ft. is apparently the limit set to such 

 mining by the increase of temperature in mines which 

 penetrate to that extent below the surface. In 1905 the 

 annual output of British coal-mines was in round numbers 

 230,000,000 tons. It is certain that there is a limit to 

 this production, but not possible to calculate what that 

 limit may be, owing to the uncertainty as to the future 

 working of coal-fields as yet unexplored. 



