34 COAL. 



COAL. 



GEOLOGISTS agree in admitting coal to be of vegetable origin ; impressions of 

 various plants, such as Fern and Calamites, and sometimes trunks of trees, 

 having frequently been found in many kinds : it is classed amongst the Car- 

 boniferous group. It is interstratified with sandstone, limestone and shale, 

 in the south-west of England and in South Wales, resting upon Old Red 

 Sandstone. In Yorkshire and the northern counties a slight intermixture of 

 mountain limestone is found with the coal-measures ; and after passing through 

 the millstone grit, several hundred feet of complex deposit is found of lime- 

 stones, coal bearing sandstones, and shale, below which is the great bed of 

 mountain limestone. " Some of the coal-measures are of freshwater origin, 

 and may have been formed in lakes ; others seem to have been deposited in 

 estuaries, or at the mouths of rivers, in spaces alternately occupied by fresh 

 and salt water*." There are freshwater strata in the coal-field of Yorkshire, 

 some of which contain shells. The great field from which coal for the purpose 

 of gas-making is obtained, is in the neighbourhood of Newcastle-upon-Tyne. 

 This coal has no regular form or structure ; its lustre is resinous, more or less 

 distinct ; colour black, passing in earthy varieties into grayish tints, frequently 

 with an iridescent tarnish. When broken it assumes a cubical or rhombo- 

 prismoidal form : it is lamellar in one direction, sometimes in two. Dr. 

 Thomson gives the analysis of Newcastle coal from several varieties. 



Carbon 75'28 



Hydrogen - - - - 4*18 



Nitrogen - - - - 15-96 



Oxygen ----- 4'58 



Most varieties of coal contain sulphur and other minerals, together with 

 saline matter ; the specific gravity is from 1'271 to T352 water being unity. 

 The varieties possessing the most glassy fracture being compact in one direc- 



* Lyell's Elements of Geology, p. 422. 



