638 BEPORT — 1891. 



rocks. These, however, are exceptional cases ; for in the great majority of cases 

 petroleum 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 Carboniferous* rocks. Of late years 

 great 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 undisturbed, 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 AUeghanies. These folds have themselves a gentle 

 inclination towards the S.W. In the AUeghanies, 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 anticlinals, but many are not so 

 placed. In the Trenton Limestone fields of Ohio and Indiana the productive areas 

 are mainly over anticlinals, 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 lime- 

 stone 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 nature, thickness, and extent are taken into consideration. Some of 

 these rocks can contain from j^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 (Up. 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 Trenton Limestone. In the 

 North-West Territories there seem to be great stores of oil in Devonian rocks. Gas 

 and oil now found in Cretaceous strata of the prairies and Athabasca may have 

 been derived from underlying Devonian rocks ; but in the Rocky Mountains, at 

 Crow's Nest Pass, oil is probably native to the Cretaceous beds. 



In Mexico, tlie West Indies, and parts of South America, Tertiary strata seem 

 to be the chief source of oil. The age of the petroleum-bearing unfossiliferous 

 sands, &c., of the Argentine Republic (province of Jujuy) is not certainly known; 

 they have been referred by diil'erent writers to various ages from Silurian to 

 Tertiary ; they are probably sub- Cretaceous. In Europe and Asia the petroleum- 

 bearing beds are of Secondary or Tertiary age, the Palaeozoic rocks yielding only 

 an insignificant supply. 



In North-west Germany we find petroleum in the Keuper Beds, and more or 

 less in other strata up to and including the Gault. As we pass to the south and 

 south-east from this district we find, as a general rule, that oil occurs in newer 

 strata. The various productive horizons of diflerent districts are as follows : — 



North-west Germany ... . Keuper to Gault. 

 Ehone Valley 1 , 



Savoy I Jurassic. 



PvrCDGGS T 



gp^jjj > Neocomian and Cretaceous. 



Elsass Oligocene. 



