52 THE CHICAGO ACADEMY OF SCIENCES. 



piped the escaping gas to his kitchen stove, and has for ten 

 years been using the gas for cooking purposes. Occasionally 

 small pockets of gas are found while boring artesian wells or 

 making large excavations. Passengers on the C. & N. W. R'y 

 between Evanston and Chicago saw, during the summer of 

 1899, a gas well burning for several weeks. The presence of 

 small quantities of gas is to be expected in the limestone of 

 this region, since the residual products in the decomposition 

 of organic matter (asphalt, etc.) are so much in evidence. 

 Further, the limestone is similar in character to the Trenton 

 limestone, which is the great repository of the Ohio and Indi- 

 ana oil fields, inasmuch as it is a porous magnesium limestone 

 and is slightly elevated in a long monocline. Underlying it are 

 bituminous shales. Thus we have lacking only one structural 

 condition necessary for an oil field, and that is a protecting 

 cover. Natural gas is composed almost wholly of marsh gas 

 (CH 4 ). It contains also olefiant gas (C 2 H 2 ), besides free 

 hydrogen, nitrogen, oxygen, carbon dioxide, carbon monoxide, 

 and sulphuretted hydrogen. 



Petroleum. 



Diffused and distributed through the underlying Niagara 

 is a visco'us reddish brown oil of rather high specific gravity. 

 One one- thousandth of one percent of oil distributed through 

 the rock of a section as thick as the Niagara limestone (300 

 feet) according to the method of calculation of Orton (Annual 

 Rep. U. S.-G. S., 1886, Part II. p. 507) would yield 150,000 bar- 

 rels of oil to the square mile, and Hunt (Chem. and Geo 

 Essays, p. 173) estimated that the limestone underlying this 

 area contains "even more oil than the above figures represent. 

 According to his calculation there are over seven million bar- 

 rels per square mile. This oil is indigenous. 



Asphalt and Maltha. 



The presence of asphalt in the limestone of the region has 

 long been noted. Twenty-five years ago it was the subject of 

 comment by Hunt (loc. cit.}. It gives to the stone a soiled 

 appearance which quarrymen have tried to turn to a virtue 

 instead of a blemish by claiming that when in a building it 

 softens the color and gives an air of stability and dignity to a 

 stone which otherwise would show tod strong a glare of white. 

 Visitors at the Stony Island quarry often think that the black 

 soft masses which, heated by the sun, run down the sides of 



