PALEOZOIC ROCKS AND ERA. 303 



without tiny volatile matter, and, although it is not called 

 coal, because incombustible, yet it is but the last term in 

 the above series of varieties. 



Cause of these Varieties. Vegetable matter decay- 

 ing out of contact with air, i. e., beneath water or buried 

 in mud, loses a large portion of its material in the form of 

 gases (CO,, CH 4 , and H 2 0). These (C0 2 and CH 4 ) are 

 the gases which escape in bubbles when we stir the bottom 

 of a stagnant pool in which plants are growing. They 

 are also the gases which are constantly escaping in every 

 coal-mine, and form the deadly choke-damp and the still 

 more dreaded fire-damp of the mines. Now, the relative 

 proportion in which these are given off determines most 

 of the above varieties. 



Anthracite and graphite may be regarded as metamor- 

 phic coals. The reasons for so thinking are mainly the 

 following : 1. Coal is often made locally anthracitic by 

 a lava-flow or dike. 2. In the same coal-field, wherever 

 the strata are crumpled and metamorphic, as in eastern 

 Pennsylvania, the coals are anthracitic ; and where the 

 strata are flat-lying and unchanged, as in Ohio, the coal 

 is bituminous. 



Plants of the Coal 



In no other strata have the remains of plants been 

 found in so great abundance and variety as in the coal- 

 measures. We could expect nothing else when we re- 

 member that a coal-seam is a mass of vegetable matter, 

 and that, on account of their economic value, these seams 

 are continually explored. The remains of plants are 

 found in the form of leaves, flattened stems and branches, 

 and sometimes fruits, in connection with the black roof- 

 shale ; and as stumps and roots, in connection with the 

 under-day or floor of the seams. 



Principal Kinds. The plants belong mainly to four 

 or five great orders, viz., Conifers and probably Cycads, 



