The great coal reserves of Illinois, the developed mines, and the central location of the state assure an industrial develop ment in the future not to be surpassed by any other state.
Tables IV, V, and VI show many facts of interest con cerning the coal production in Illinois.
Petroleum and natural gas.—The Illinois oil fields of Clark, Crawford, and Lawrence counties are associated with the La Salle anticline, an upward fold of bedrock which crosses Illinois from Stephenson County in the northwest to Lawrence County in the southeast. The La Salle anticline crosses the Illinois River at Split Rock between La Salle and Utica. South of La Salle County this rock fold does not appear at the surface, but exists deep underground, and in the oil fields it forms the cap or covering for the oil-bearing rocks. The petroleum has accumulated under the anticlinal fold, and is obtained by penetrating the impervious rocks of the anticline by deep wells from which the oil is pumped to the surface. Small anticlinal folds exist deep underground elsewhere in the state, and some of them confine petroleum and natural gas beneath. The regions of the state where petroleum and gas have been found are shown on the accompanying map. The important commercial output is in southeastern Illinois, and the total productive area is about 250 square miles, or 160,000 acres, an area equivalent to 1,000 farms of 160 acres each.
Prospecting for oil began in Clark County in 1865, only six years after the first successful wells of Pennsylvania had been opened. Some oil was obtained, but not enough to induce further development at that time. Thirty-nine years later, in 1904, successful wells were drilled, and the region which had been tested in 1865 was a fair producer for several years. These were shal low wells having a depth between 400 and 600 feet.
In 1906 the Crawford County field was opened, and the oil industry of Illinois took on large pm portions immediately. The oil in this field is found at depths of 750 to 1,000 feet.
In 1907 the Lawrence County field was opened, the oil being procured at depths of S00 to 1,900 feet.
Table VII shows the progress of the oil-producing industry of Illinois.
Production increased rapidly from 1904 to 1908 and it has declined slowly since 1911. The oil industry has aided in the developfilent of a number of cities in the district, among which are Robinson and Oblong in Crawford County, Lawrenceville and Bridgeport in Lawrence County. A large refinery
has been established at Law renceville. Owners of some farms have been enriched by many times the original value of their holdings.
Natural gas has been found in limited quantities in connection with petro leum. Small quantities of gas have been found where the oil was not obtained in commercial quantities, and in some places where no oil appeared. Usable quanti ties of natural gas have been found in the glacial drift where gas had accumulated under impervious layers of clay. Gas wells have usually given out after producing for six or seven years. The small local sup plies have frequently been used on the farms on which pro duced. Gas has been found chiefly in Mont gomery, Pike, Randolph, and Macoupin counties.
Pig iron and coke.— Pig iron and coke are mineral products of great value in Illinois. They do not figure in the value of mineral products of the state, however, as pig iron is made from iron ore shipped in from other states, and coke is made from coal, which is a primary mineral resource. The steel plants at South Chicago and Joliet produce the pig iron of the state. Coke is pro duced as a by-product in gas- manufacturing plants in various cities. It is also manufactured in the coke ovens of the steel plants to be used in the manufac ture of steel.
Clay products.— Throughout the gla ciated regions of Illi nois, clay suitable for making brick and tile is found. Clays suit able for pottery and sewer pipe are found in more restricted areas. Brick and tile factories are therefore widely scattered throughout the state in order to avoid high transportation charges. Whitehall in Greene County, Macomb in McDonough County, and Monmouth in Warren County are important centers for pottery and sewer pipe. Brick and tile constitute 93 per cent of the clay manufactures of the state and pottery 7 per cent. Cook County makes more common brick than any other county of the United States, producing three-fourths of the brick of Illinois. Knox County leads in the manufacture of vitrified brick for paving purposes, with Livingston County second. Vermilion County leads in the production of front brick, and Kankakee is the only county pro ducing enameled brick.