118 THE STOEY OP THE EAETH AND MAN. 



simplest way is to compare under the microscope a 

 transverse section of recent pine-wood with a similar 

 section of a pine trunk compressed into brown coal 

 or jet. In the one the tissue appears as a series of 

 meshes with thin woody walls and comparatively wide 

 cavities for the transmission of the sap. In the other 

 the walls of the cells have been forced into direct 

 contact, and in some cases have altogether lost their 

 separate forms, and have been consolidated into a 

 perfectly compact structureless mass. 



With regard to its mode of occurrence, coal is 

 found in beds ranging in vertical thickness from less 

 than an inch to more than thirty feet, and of wide 

 horizontal extent. Many such beds usually occur in 

 the thickness of the coal formation, or "coal measures," 

 as the miners call it, separated from each other by 

 beds of sandstone and compressed clay or shale. 

 Very often the coal occurs in groups of several beds, 

 somewhat close to each other and separated from other 

 groups by " barren measures " of considerable thick- 

 ness. In examining a bed of coal, where it is exposed 

 in a cutting or shore cliff, we nearly always find that 

 the bed below it, or the " underclay," as it is termed 

 by miners, is a sort of fossil soil, filled with roots and 

 rootlets. On this rests the coal, which, when we 

 examine it closely, is found to consist of successive 

 thin layers of hard coal of different qualities as to 

 lustre and purity, and with intervening laminae of 

 a dusty fibrous substance, like charcoal, called 

 "mother coal" by miners, and sometimes mineral 



