28 The American Geologist. January, i904. 
are generally saturated with the oil and may be easily recog- 
nized by their odor or dark oily appearance. 
Petroleum was known to exist in the Appalachian region 
in Pennsylvania and New York for more than one hundred 
years before its value as a source of heat or of light was recog- 
nized. At present it is obtained from wells in Canada, Penn- 
sylvania, West Virginia, Ohio, Tennessee and Kentucky, where 
it first began to be used soon after i860 A.D. It is also profi- 
tably obtained in Wyoming, Colorado, California, Kansas and 
Indian Territory, but is especially abundant in southern Texas. 
Professor Hitchcock enumerates fourteen different geo- 
logical formations from which petroleum has l:)een found in 
the United States, and they include every geological age from 
the Lower Silurian to the Pliocene Tertiary. Limestones are 
sometimes saturated with petroleum, Init more often sand- 
stones. These, on account of their porous condition, may ab- 
sorb and retain a large amount of oil. 
In Pennsylvania oil is found in three principal sandstones. 
In Nova Scotia bitumen occurs in cavities lined with calcite. 
In Canada, large orthoceratites ( of Trenton age) sometimes 
hold several ounces of petroleum in their cavities, and at 
Montmorenci it exudes in drops from the corals of the Birds- 
eye limestone. ' The bituminous limestones of Chicago and also 
of western New York, when heated exude drops of bitumen. 
In Iowa, geodes of Keokuk age enclose bitumen. In Bates 
county, Missouri an Upper Carboniferous limestone, show- 
ing no oil on the surface, upon being broken, shows cavities 
in fossils, where bitumen is found. Salt water was the pre- 
cursor of the discovery of most of the Pennsylvania oil wells. 
Theories Regarding Oil Formation. 
Professor J. D. Dana supposed that oil has been formed 
through decomposition of animal and vegetable substances, 
that the oil-yielding rock was in a state of fine mud. that 
through this mud much vegetable and animal matter was dis- 
tributed. This stratum was afterwards overlaid by other 
strata ; the decomposition of the organic matter went on with- 
out the presence of the atmosphere ; under such circumstances 
either vegetable or animal matter might be converted into 
oil. 
