THE CARBONIFEROUS SYSTEM. 115 
tion by the roots of plants, usually Stegmaria, 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 of 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. ‘T’he coal seams 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 
