LIQUID SUNLIGHT. 139 



naphtha, and others. It is evident that natural gas may wan- j 

 der farther away than oil from the formation in which it origi- 

 nates ; and hence there may be more difficulty in tracing it 

 to its real source. It may become widely separated from ap- 

 parent connection with oil. It may also be distilled from 

 shales not possessing the requisite richness to afford oil. 

 Hence, in some regions, as Fremont, Cleveland, and other 

 localities in northern Ohio, it issues from Cambrian strata 

 which furnish no indications of oil. In western Pennsyl- 

 vania, within the Coal Measures, the great supplies of 

 gas are yielded probably, by the same formations as sup- 

 ply petroleum. This, however, is a question still under 

 investigation. 



Now let us look into the relations of things in some of the 

 principal oil-producing regions. The most famous is that of 

 northwestern Pennsylvania. The surface rocks are Coal j 

 Measures or Lower Carboniferous Sandstones (Waverly or / 

 Catskill sandstones) or Chemung sandstones according to 

 the locality. The oil is found accumulated in the sandstones ; 

 but its source is believed to be the Genesee Black Shale, 

 near the top of the Hamilton Group (See Table, page 73.) 

 There are in all productive situations, shaly strata also, 

 above the sandstone reservoir, which prevent the oil from 

 escaping to the surface. The situations are similar in eastern 

 Ohio and southern New York. 



In Ontario are two kinds of oil, and two different reser- 

 voirs. The thick lubricating oil accumulates in a gravel bed 

 at the bottom of the Drift, and is confined by the clay sheets 

 of the overlying Drift. Its source is probably the Genesee 

 Shale, which immediately underlies, but thins out a half 

 mile further east. The more abundant petroleum is found 

 stored in fissures and cavities of the Hamilton limestone; 

 and its source is probably the black Marcellus shale next 

 below the limestone. These cavities often contain water un- 

 der the oil, and gas above it. If the auger enters the upper 

 part, gas escapes at first, but when this is exhausted, oil may 

 be pumped. When the oil is exhausted, water follows. If the 



