GEOLOGY Illinois rocks valuable.—The succession of the geologic processes of the past are extremely significant in the formation of the natural resources now available for the use of the people of Illinois. Soil, which is formed from decayed rock, is the most valuable mineral resource of the world, and Illinois has been well favored by the kinds of rock which, when broken into fine particles, produce fertile and lasting soils. Soil, however, is not usually included among the mineral resources, as its importance and its widespread presence over the earth's surface require separate discussion. The mineral products of Illinois consist of those useful materials obtained from the solid rock beneath the covering of loose earth known as mantle rock, or from the mantle rock itself, if these products are used for purposes other than the growing of crops. Among the mineral resources of Illinois are coal, petroleum, lead, zinc, fluor spar, and building stone which come from the solid rock, or bedrock; clay, sand, and gravel which come from the mantle rock; and road-building materials which are secured from both solid rock and mantle rock.
The annual output of Illinois minerals ranks next in impor tance to farm crops among the products of the state. In 1910 the value of the farm crops of Illinois was $370,000,000, and the mineral output was valued at $125,000,000. Because of increased production in some lines, and greatly increased prices for nearly all products, the value of the farm crops of 1917 was estimated at $750,000,000, and the mineral output was valued at $23S,000,000. Illinois is surpassed only by Pennsyl vania in the value of mineral production.
Studies in Illinois geology.—The Illinois State Geological Survey is organized as a part of the state government. Trained geologists, in the employ of the state, make extensive investiga tions and report concerning the rocks of the state and their use. The Geological Survey, through many years of research, has learned he story of the geologic processes which have gone on throughout the ages in Illinois. These studies reveal where
in the state the mineral resources are of sufficient value to offer profitable development. Experimental work carried on by the state discovers better methods of obtaining and using the mineral resources. This information is made freely available to all. Geology thus serves the practical needs of man, and its wide and systematic study can best be carried on by the state and national governments.
Kinds of rocks.—Nearly all the rocks of Illinois are sedi mentary, that is, they were formed in the sea, which for many ages covered Illinois. Sediments washed into the sea from surrounding lands were deposited in the water, and, when solidi fied, formed sedimentary rocks. These sedimentary rocks of the state arc so thick that the deepest well-borings, in many instances more than a thousand feet in depth, have not pene trated to the bottom of the sedimentary rocks.
Igneous rocks have been found in Illinois only at a few places where lava has been thrust as dikes into the fissures of the sedimentary rocks. Dikes are common in Pope and Hardin counties.
Metamorphic rocks are practically unknown in Illinois, although, in a coal mine in Saline County, a case of meta morphism has been found where coal has been changed to coke by geologic processes.
All sedimentary rocks rest upon igneous or metamorphic rocks. Therefore, at great depths, the whole of Illinois is underlain by rocks of other kinds than sedimentary, probably igneous rocks.
Divisions of geologic time.—Nearly all known rocks of Illinois belong to the Paleozoic era. Eras are divided into periods of geologic time. The rocks formed during an era make a group of rocks, those formed during a period make a system. The Paleozoic era which is so important in Illinois geology is divided into seven periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian.