PEATS AND LIGNITES. 147 



all that seems necessary to convert them into true brown- 

 coals are the cover and pressure of superincumbent strata, 

 and time sufficient to effect those further chemical changes 

 to which lignites and brown-coals have been generally sub- 

 jected. We see, therefore, in the compressed vegetable 

 matter we call peat, and which has been formed by the 

 growth and decay of certain plants* during many cen- 

 turies, the first stages of coal, and when we come to con- 

 sider the older formations, we shall find that many of their 

 coal-seams have had a similar origin. And just as this peat 

 is sometimes earthy and mingled with stony matter that has 

 been washed into the swamps and hollows by rains and 

 rivers, so we may expect some of the old coals to contain 

 similar impurities, and to be less valuable as fuel. 



The next and older series of coals embraces the lignites 

 or wood-coals, the brmcn-coals and board-coals of the Ter- 

 tiary strata. As these names imply, their woody or vege- 

 table texture is still apparent, and they are generally of a 

 brown or earthy hue, compared with the black and glisten- 

 ing lustre of the coals of the older formations. Alternating 

 with clays, marls, sands, and gravels, they have evidently 

 been formed partly in fresh-water lakes and swamps, and 

 partly in areas that have been submerged and covered over 

 by marine deposits. In some instances they are earthy, 

 and composed of the drifted trunks and branches of trees ; 

 and in others the submerged and fallen forest-growth can 

 be traced as clearly as it can be in some of the shallower 

 peat-beds of Scotland. Most of these lignites, whether as 

 once worked at Bovey in Devonshire, or as still worked in 

 Germany, Prussia, Austria, New Zealand, and other coun- 

 tries, may be described as coal in its second stage of 

 consolidation and mineralisation. In the mine they are 

 * See Sketch entitled "Recent Formations." 



