April, 1911.] 
The Ancient Vegetation of Ohio. 
3 1 7 
of the Carboniferous period, and as the latter are much better 
preserved and represented in the Coal flora, a conception of their 
ecological conditions for growth may be deferred with advantage 
until the discussion of that period. 
A renewed expansion of the sea entrapped the fauna and flora 
in beds of sediment of great depth. This organic matter is the 
chief source of the oil and gas in use today. It is impossible as 
yet to state with certainty how these fuels have been formed and 
concentrated. Chemists suggest an inorganic origin for these 
products. It is thought, and the theory is supported by lab- 
oratory experiments, that the great supplies of petroleum were 
produced through the agency of iron carbides within the earth, 
generating the hydrocarbons upon access with percolating water. 
But the quantities traceable to such a source are insignificant in 
comparison with the great repositories containing the oil. Buried 
accumulations either of plants, animals or both can alone account 
for the origin of gas and oil under the observed conditions. The 
production of hydrocarbon compounds has been studied in coal 
mines as the “fire damp,” in bogs and swamps as “marsh gas” 
and in the fermentation of cellulose by anaerobic bacteria. Sea- 
weeds and diatoms arc known to contain globules of oil ; other oily 
substances of organic origin are the “cholesterol” found in plants 
and the fatty parts of animals. The optical phenomena of 
organic oil, that is, the power of rotating the plane of polarization 
of light, is not shown by inorganically formed hydrocarbons. In 
nature an accumulation of organic debris, the exclusion of air, 
and the existence of an impervious protecting sedimentary stratum 
seem to be the essential condition toward rendering the process of 
distillation and transformation possible. It is often surprising 
the quantity of oil which an apparently dense rock stratum can 
hold. Pressure, temperature, viscosity, the nature of surround- 
ing rocks, and a flow of the liquids and gases into porous rocks 
and cavities, no doubt, must all be taken into account when con- 
sidering the changes involved in the origin of gas and oil; but at 
present the organic origin of these fuels seems to have the strongest 
support (2). 
The Sub-carbonieerous or Mississippian period which fol- 
lowed the interval of widespread submergence consists of the 
Bedford shale, Berea grit, the Cuyahoga, Black Hand, and Logan 
formations, and the Maxville limestone. An increased land area 
gave increased contact between the atmosphere and the rocks. 
In the western half of Ohio the period was one largely of sea 
extension. Disintegration and much erosion must have taken 
place to give the sedimentary material of the equivalent forma- 
tions. A gulf which extended east of the great arch-island enabled 
plants as well as animals to flourish in isolation for a period 
sufficiently long to differentiate species of its own. For Ohio the 
