true coal. Like these, it is used as fuel; but it contains a large 

 amount of moisture, and must be dried or pressed, usually, 

 before it is thus employed. Logs and tmnks of trees that grew 

 in or around the swamp are often embedded in the peat, and 

 undergo a similar change; their color passes to brown or black, 

 and the woody structure, though often beautifully preserved, is 

 ultimately more or less destroyed and an imperfect coal pro- 

 duced in its stead. Such deposits of wood-coal are sometimes 

 quite extensive and important ; they are known as ' ' brown-coal ' ' 

 and also as "lignite." Like peat, they contain a good deal of 

 moisture, which lessens their heating power as compared with 

 true coals. Deposits of peat and lignite are forming now on a 

 large scale in the Great Dismal Swamp of Virginia, the bayou 

 region of Louisiana, and many similar localities. 



Coal proper belongs to a much earlier period in the history'- 

 of the globe, or rather to several such periods. In the course 

 of ages, it has undergone much more alteration and great com- 

 pression, and hence it contains a larger percentage of what is 

 termed ' ' residual carbon. ' ' The plants, moreover, of the earlier 

 ages were different from those of later time, and this fact may 

 have caused some differences, although the general process has 

 been the same. The coals are divided into several varieties 

 according to the extent to which the volatile compounds have 

 been eliminated and the consequent proportion of carbon that 

 is left. The distinction of "hard" and "soft," or anthracite 

 and bituminous, coals is familiar to almost everyone. The 

 latter still retain a considerable amount of the volatile ingre- 

 dients and hence are used for the manufacture of illuminating 

 gas, or when burned in a grate, give off the same gas in a bril- 

 liant play of flame. The hard or anthracite coal, on the other 

 hand, has by the agency of great pressure and of internal heat 

 lost nearly all its hydrogen and oxygen and contains a very 



23 



