Reviews—Geological Survey of Ohio. 85 
retted hydrogen gas and petroleum in limestone strata, about 1100 
or 1200 feet beneath the surface. The rocks proved to be of 
Trenton age, 7.e. Ordovician, about the horizon of our Llandeilo 
Flags. The existence of these products in workable quantities at 
this low horizon was an entirely new and unexpected fact to 
American geologists, and as the geological structure of Ohio is 
extremely simple, without breaks or important disturbances, and it 
was known that the Trenton formation would be found at depths 
varying from 1000 to 2000 feet beneath the surface, throughout the 
western areas of the State, wells were rapidly drilled in all 
directions. The great majority of these were unproductive, but in 
certain limited areas the yield of gas and petroleum has been on a 
marvellous scale. The facts brought to light by these borings were 
carefully collated by the Geological Survey of Ohio, and they are 
presented by Professor Edward Orton, the State Geologist, in a 
detailed and lucid manner in the present volume. Owing to the 
simple and undisturbed character of the geological series in Ohio, 
the questions affecting the origin and accumulation of natural gas 
and petroleum are more likely to be solved here than in areas where 
rock disturbances tend to make the problem more complicated. 
In the north-western part of Ohio, beneath the mantle of boulder 
clay, the rocks outcropping at the surface belong for the most part 
to the Lower Helderberg series, the upper portion of the Silurian. 
These are mainly bituminous limestones, with an average thickness 
of about 300 feet. Beneath these in conformable succession, and 
with dips so small as to be scarcely appreciable, are the Niagara 
series of limestones and shales, and the Clinton series, together about 
400 feet thick ; then a thin deposit of Medina shales; beneath these 
the Hudson River or Cincinnati group of shales and limestones, with 
an average thickness of 600 feet, and the Utica shales 300 feet in 
thickness; under which the Trenton limestones are found. Both 
gas and petroleum are present in small and variable amounts in each 
of these different rock-series, but not sufficient to be of economical 
value. Even in the Trenton rocks, the accumulation of these 
products in workable quantities seems to depend on two conditions ; 
first, a rise or elevation of the strata in the productive areas above 
the beds of those adjoining, and next, that the upper portion of the 
Trenton beds consists of a porous dolomite or magnesian limestone. 
This dolomite appears to have resulted from the alteration of true 
limestones; at all events, it occurs in patches surrounded by the 
normal limestones containing the usual fossils. The Trenton lime- 
stone itself has been penetrated for 500 or 600 feet, but it has been 
ascertained that the profitable accumulation of gas or oil was limited 
to the upper beds. 
The facts point to the inevitable conclusion that the Trenton lime- 
stone is the source of the inflammable products ; that resulting from 
artesian pressure they accumulate in the areas which are slightly 
elevated and where the rocks have become dolomitized and porous. 
The petroleum and gas can only be derived from the decomposition 
of the animal organisms, the skeletons of which now compose the 
