[DowLING] THE FORMATION OF COAL 29 
Aquatic plants are, as a rule easily disintegrated and microscopic 
algæ are especially so. The inference that these materials would rapidly 
decay and reach a higher stage of alteration before entombment is 
natural, so that the high hydrogen of cannel coal might be accounted 
for by ascribing its derivation to aquatic vegetation with no doubt 
microscopic animal life associated with it. 
If cannel coals are of aquatic plant and animal origin they would 
be expected to occur largely in the marginal continental coal areas and 
only occasionally in inland lake basins. As aquatic plants preceded 
land forms, it is reasonable to expect that the majority of these beds 
would occur in the early coal measures. The sequence should be (1) 
beds formed from aquatic life both plant and animal, (2) beds in which 
aquatic plant life predominated and (8) beds formed mainly of land 
plants. 
This sequence is borne out in general by (1) the oil shales being 
generally pre-Carboniferous, (2) cannel coals, frequently found in the 
early Carboniferous and (3) the coals which date from Carboniferous 
and later times. 
ALTERATION DUE TO DYNAMIC AND GEOLOGIC INFLUENCES. 
A mass of vegetable matter when covered by subsequent deposits, 
is for the remainder of its history (generally a long time interval), sub- 
ject to alterations due to pressure and possibly also heat. Pressure 
without heat or a great lapse of time, produces merely a physical harden- 
ing or solidifying of the material, mainly by the exclusion of the water 
content. Heat alone, on the contrary, causes the rapid reduction of the 
total amount of carbon by the formation of combustible hydrocarbon 
gases as well as non-combustible compounds of carbon and oxygen. 
The process of eliminating the hydrogen and oxygen from vegetable 
matter by heat is the common one employed in the manufacture of 
charcoal. If the temperatures employed are sufficiently high nearly 
pure carbon remains; but out of the original 50 per cent there would 
then remain but 15, which shows the enormous loss of carbon the process 
entails. If the earthy material which existed as ash in the plants be 
not driven off by the action of heat the original amount instead of being 
part of 100 units would be a fraction of 15, so that the percentage of the 
ash would be raised to a possible six times that originally in the plant. 
But, as the ash of coals seldom shows such a possible increase as this, it 
seems that in the formation of coals, some restraining influence, possibly 
high pressure, had prevented this great loss of carbon. 
If we assume that the most altered coals have passed through all 
the other grades in their formation, the composition then must have 
