After the discovery of the Illinois oil fields in 1905 it was but three years until the State had reached its maximum production of 33,686,238 barrels. From that amount a somewhat irregular decline has occurred, and in 1915 only 19,041,695 barrels were produced. For several years it maintained its rank as third in production until 1915, when the discovery of prolific, fields in Texas forced Illinois into fourth place. The decline in production has been offset by the rise in value of the product. Up to the end of 1915 there had been produced from about 230 square miles in Illinois 251,368,311 barrels of oil, with a value of $201,053,017. The new fields that have been discovered in Marion, Clinton, and McDonough counties have not been able to overcome the normal decline in the output of Crawford, Lawrence, and Clark counties.
The producing sands in Illinois range in age . . .