16 GEOLOGY OF OHIO. 
large and hitherto unknown Placoderms have been procured from the 
Huron shale within the last year. These are too imperfect for descrip- 
tion, but they indicate the existence of a varied ichthyic fauna in the 
Huron epoch, and afford reason for expecting that the Huron shale will 
yet contribute largely to our knowledge of the life of the Devonian age. 
It will be remembered by those who have read the first volume of this 
report, that the Huron shale is regarded by the writer as the most impor- 
tant source of supply of petroleum in this country, and also that the 
greater part of the gas-wells of Ohio and Pennsylvania derive their flow 
of carburetted hydrogen from this formation. The arguments in favor 
of this view are briefly stated in Volume I, Geology, (page 158, e¢ seq.,) and 
most geologists have accepted them as affording a rational and satisfac- 
tory explanation of the problem of the origin of petroleum. There are 
some, however, who still cling to the theory that the petroleum and gas 
which fill the cavities and interstices of the sandstones and conglom- 
erates in the Oil Creek region, are indigenous in these rocks. This 
hypothesis is certainly untenable. Sandstones and conglomerates are 
made up almost altogether of comminuted quartz, which could contribute 
nothing to the formation of hydrocarbons. All chemists agree that these 
are of organic origin, and must have emanated from some source foreign 
to the sandstones, and that these latter can only act as reservoirs to hold 
them, or as channels through which they flow. 
That the hydrocarbons have not descended to saturate the sandstones is 
certain, as they are always thrown upward by hydrostatic pressure, and 
are, in fact, found working to the surfaceeverywhere. They must, there- 
fore, have risen from some source beneath to fill the reservoirs that hold 
them. The organic sediments which underlie the oil-bearing sandstones 
in Ohio and Pennsylvania are bituminous shales and limestones. In 
making a choice between these, as the possible source of oil and gas, the 
following facts deserve consideration, some of which have been already 
noticed : 
1. No rock can furnish that which it does not contain, and none of 
the limestones that underlie the oil regions of Ohio and Pennsylvania 
hold more than one or two per cent. of organic matter. They are, there- 
fore, entirely inadequate to supply the enormous flow of hydrocarbons 
which has continued through ages past and will continue through ages 
to come. In the argillaceous shales, sandstones, and conglomerates, the 
amount of organic matter is even less than in the limestones; hence the 
theory that they can furnish petroleum or carburetted hydrogen in any 
quantity is untenable. 
