Notices of Memoirs — W. Topley — On Petruleum. 509 



Few cases are known in which petroleum occurs in rocks older 

 than the Silurian, and none where the amount is of any importance. 



Petroleum occurs, but not in large quantity, in a trachyte-breccia 

 at Taranaki, New Zealand. In N.W. Hungary it is found in a 

 trachytic tuff of Miocene age. These, however, are exceptional 

 cases ; not only is petroleum not found in volcanic rocks, but in the 

 great majority of cases it is far removed from any known indications 

 of true volcanic action. 



The great stores of petroleum and gas in Pennsylvania and New 

 York are in sandstone beds of the Devonian and Lower Carbonifer- 

 ous rocks. Of late years gi-eat quantities of gas and oil have been 

 obtained, chiefly in Ohio and Indiana, from the Trenton Limestone 

 (Ordovician). 



The oil- and gas-fields of Pennsylvania and New York have a 

 very simple geological structure. The rocks lie comparatively un- 

 disturbed, being only gently folded into a series of anticlinals and 

 synclinals parallel with, and along the N.W. side of, the main axes 

 of the Alleghanies. These folds have themselves a gentle inclination 

 towards the S.W. In the Alleghanies, and to the S.E. of the range, 

 where the rocks are greatly disturbed, neither oil nor gas is found. 

 Some of the larger gas wells are on or near the summits of anti- 

 clinals, but many are not so placed. In the Trenton Limestone 

 fields of Ohio and Indiana the productive areas are mainly over anti- 

 clinals, gas occurring at the crown of the arch, oil on the slopes. 

 , The essential conditions for a largely productive field of gas or 

 oil are — a porous reservoir (generally sandstone or limestone) in 

 which the hydrocarbons can be stored, and an impervious cover of 

 shale retaining them in the reservoir. It is also believed that they 

 only occur where, in or under the porous reservoir, there have been 

 accumulations of fossil remains, the original decomposition of which 

 yielded the hydrocarbons. In the case of the sandstones the original 

 source was probably the fossiliferous shales which underlie them ; 

 in the case of the Trenton Limestone the source was probably the 

 fossiliferous limestone itself. The limestone is only productive 

 under certain circumstances ; in its normal condition it is a compact 

 rock, and then it contains neither gas nor oil. But over large areas 

 the limestone has been dolomitized, and so transformed into a 

 cavernous and porous rock in which gas and oil are stored. The 

 enormous quantities of gas and oil given out from beds of limestone 

 and sandstone can be fully accounted for when their porous natui-e, 

 thickness, and extent are taken into consideration. Some of these 

 rocks can contain from i-o-th to ^-th of their bulk of oil. 



The high pressure under which gas and oil flow from deep borings 

 can in most cases be fully explained by artesian pressure. 



In Kansas gas occurs mainly in the Lower Coal-measures. In 

 Kentucky and Tennessee oil is found in the Ohio shales (Upper 

 Devonian), in Colorado in shales of Cretaceous age. In California 

 it is found in Tertiary strata, mostly much disturbed. 



In Canada the chief source, in Ontario, is in Devonian rocks, 

 along a well-marked anticlinal; but gas and oil also occur in the 



