April, 1911.] The Ancient Vegetation of Ohio. 317 



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 are 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 accvimulation 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-carboniferous 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 fomia- 

 tions. A gulf which extended east of the great arch-island enabled 

 plants as well as animals to flourish in isolation for a period 

 sufficienth^ long to differentiate species of its own. For Ohio the 



