HISTORY OF THE VEGETABLE KINGDOM. 619 
where the heat and pressure have been very limited, we find an impure lignite; 
in those places, as the Western coal flelds, where these agencies acted with greater 
but yet with moderate energy, the coal is bituminous; while in others, as in the 
Appalachian coal beds, where these metamorphic forces have operated with intense 
power, the coal is of that compact, highly carbonized character known as anthra- 
cite. But where the heat has been intense enough to fuse the rocks and the 
pressure so great as to force out all the lighter gases, leaving only the pure carbon, 
it becomes graphite. That this is the process by which graphite has been formed 
is evident from the fact that in the vicinity of fissures where the heat has been 
intense enough to metamorphose the rock, the bituminous coal has been changed 
into anthracite, and anthracite has passed into plumbago. Thus, at Worcester, 
Mass., a bed of graphite and pure anthracite occurs, interstratified with mica 
schist. It has been employed for both fuel and lead pencils. Thirty miles from 
this, in Rhode Island, an impure anthracite is found, containing impressions of 
leaves of coal plants. This is intermediate between the anthracite of Pennsyl- 
vania and the graphite of Worcester. From these facts it appears that graphite 
is only an extreme metamorphic condition of coal. When, therefore, we find 
large deposits of graphite in highly metamorphic rocks of the Laurentian period, 
the conclusion is irresistible that vast quantities of vegetation must have existed 
at the time they were laid down. 
Still another evidence of the same fact is found in the existence of iron ore in 
these rocks, It appears that iron originally existed in a distributed condition in 
the clays and other rocks of the earth. Thus we find where iron ore deposits 
are found, the iron has been leached out of the adjacent rocks and they are des- 
titute of any red color, while in red colored rocks, the iron still being distributed 
through them, no beds of ore appear. When the iron has been so taken from 
the rocks and gathered into deposits, it appears either in the form of ferric acid or 
carbonate of iron. We see this process in operation at the present day in the 
formation of bog iron ore. The manner in which this iron is collected is of 
much interest in connection with the question of the existence of vegetable mat- 
ter in the adjacent rocks, for it will appear that such matter is an important 
agency in producing such deposits. The iron as it exists diffused through the 
rocks and soils is in the form of peroxide of iron, or ferric oxide, which is insolu- 
ble in water, and consequently cannot be washed out by percolating waters. 
Some deoxodizing agent is needed to bring about this result. This is found in 
decaying vegetable matter. Such decay is an oxidizing process, and when it 
takes place in the presence of peroxide of iron, the effect is to deoxidize it, re- 
ducing it to protoxide, by combining with a portion of the oxygen held in com- 
bination by the iron. Carbonic acid then combines with this protoxide, forming 
carbonate of iron, which, being soluble, is dissolved by percolating waters, which 
come to the surface as chalybeate springs. But when these chalybeate waters 
come to the surface, and are exposed to the atmosphere, the iron reabsorbs Oxy- 
gen, and is converted back to peroxide, which is deposited at the bottom of any 
