THE CARBONIFEROUS SYSTEM. 115 



tion by the roots of plants, usually Stigmaria, formerly regarded as an en- 

 tire aquatic plant, but now known to be the roots of trees which are char- 

 acteristic elements of the coal flora, Lepidodendron and Sigillaria. Some- 

 times the stumps and spreading roots ot these trees are found in unbroken 

 connection buried in the fire-clay. 



Upon the fire-clay we almost always find a stratum of coal of greater 

 or less thickness. Sometimes this is very thin, sometimes, though rarely, 

 entirely wanting, and in most such instances we can gather proof that 

 it has been removed, either mechanically or by oxidation. This coal 

 throughout its entire mass shows traces of vegetable structure, and it is 

 now agreed among all good authorities that it has accumulated by plant 

 growth in the locality where it is found. Various theories have been 

 proposed to account for the formation of coal, viz., that it is of animal 

 origin ; that it was formed from petroleum ; that it is derived from vege- 

 table tissue transported by river currents and gathered in water basins ; 

 but these theories have already been sufficiently discussed and so clearly 

 disproved that no further reference to them is needed here. All those 

 who have carefully studied the phenomena presented in our coal fields 

 have been satisfied that the beds of coal have been formed where they 

 are now found by the bituminization of vegetable tissue, which accu- 

 mulated precisely as peat does now. Peat beds usually occupy marshes, 

 and are produced by the bituminization of the various plants that grow 

 in water or on moist surfaces. In making a section of a peat bog we 

 almost always find beneath the peat a layer of clay very much like the fire- 

 clay, and by an examination of many of these peat-producing marshes 

 it has been discovered that they have generally been pools of water in 

 which a fine sediment accumulated at the bottom, and that these pools 

 have been invaded by vegetable growth until they are more or less filled 

 up by the accumulation of the bituminized leaves, trunks, etc., of dif- 

 ferent generations of plants. 



The effect of the growth of aquatic plants on the soil in which they 

 are rooted is to abstract the alkalies, sulphur, phosphorus, and a portion 

 of the silica, and leave a fine homogeneous clay containing a large 

 percentage of alumina and highly resistant to fire. This we learn by 

 analyses of clays under our peat bogs. Hence, from the great similar- 

 ity, almost identity, which they exhibit with the fire-clays of the Coal 

 Measures, we may fairly conclude that their histories are essentially the 

 same. The coal sea.ms in our State vary in ; thickness from one inch to 

 twelve feet, and as the. material composing :them has been greatly con- 

 densed by pressure, we may infer that they represent beds of peat of from 

 one to fifty feet in thickness. These were formed by the gradual, perhaps 



