ARRAT, DISTRIBUTION.
Keeping in mind the brief descriptions of the foregoing paragraphs, we may now divide the State into the following geological areas, Ordovi cian, Silurian, Devonian, Lower Carboniferous, Coal Measures, and Cre taceous or Tertiary.
Ordovician.—This area is roughly bounded by the parallel 41 30' and the meridian 88 36', covering the northeastern part of the State, together with a narrow strip running southward through western Will, Kankakee, Iroquois, and Ford counties. The Galena-Trenton now forms the sur face over the greater part of this area, but when it first became dry land it was also covered by the Cincinnatian and Niagaran. These have been removed from most of the area by erosion, but remnants of them still remains as mounds and elevated table lands. The lowermost beds of the Galena-Trenton include highly argillaceous limestones, and when ever the formation has been long exposed to the action of the weather the lime has been dissolved and carried away in solution, leaving the dis seminated clay as a mass of loose material. Where this is not exposed to the action of running water, it has accumulated in beds or pockets which sometimes cover a considerable area. Considerable deposits hav ing this origin are known to exist in the area, but their availability for the manufacture of pavers has never been tested so far as the writer knows.
In all the mounds and table lands referred to above the Cincinnatian or Maquoketa comes to the surface, either forming the apex of the eleva tion or lying immediately beneath a capping of Niagaran. This Cincin natian usually appears in this part of the State as a thin bedded limestone which carries a large percentage of clay, but locally it becomes a pure shale, often high in lime, but occasionally almost lime free, in which case it becomes a promising source of material (See H 18 and H 21.) The Cincinnatian also outcrops as shale near Wilmington, Will county. A considerable percentage of the Ordovician area is covered with glacial deposits, some of which may be found to be suited to the manufacture, of pavers. Small areas of rocks of this age may also be found near Batchtown, Calhoun county; Valmeyer, Monroe county; and Thebes, Alexander county. In some of these the rocks are shaly and look promising but have not been tested.
Silurian.—This formation covers the greater part of McHenry, Lake, Cook, Dupage, Kane, Will, Kankakee, and Iroquois counties, and small portions of Boone and DeKalb, all lying in the northeastern corner of the State. The surface rock is the massive Niagarean limestone which fur nishes no deposits of clay or shales in this State. It is everywhere over laid by thick deposits of glacial drift which permit the underlying rock to appear at the surface in small areas only.
The glacial clays of this area present the wide range in composition and properties so characteristic of deposits of that age. No studies of these clays have been made so far as the writer knows, but he feels certain that some of them would make pavers of excellent quality if properly treated.
Small areas of this age are also found at the mouth of the Illinois and east of Thebes in Alexander county, but they afford no clays or shales.
Devonian-.—The outcrops of Devonian rocks in Illinois are of small area and are confined to three localities, viz.: near Rock Island, on either side of the Illinois river near its mouth, and near Jonesboro in Union and Alexander counties. In all these areas shales are to be found which may prove valuable as paving brick material, but they have not been tested.
Lower Carboniferous or Mississippian.—The rocks of this area form a broad, but very irregular belt, along the Mississippi from New Boston, Mercer county, to the southern line of Union county and thence east across the state, forming the Ozark Ridge.
Most of the rocks of this age outcropping in the area described are limestones or sandstones, but nearly all the layers pass locally into shales, some of which, notably those of the Kinderhook and St. Louis, have been proved to have the qualities desired in paving brick material, but in only one locality, and that from the Kinderhook, so far as the writer knows, have they actually been used for this purpose.
Coal Measure or Pennsylvanian.—This area covers all that part of the, State lying between the parallel 41 30' and the Ozark Ridge except those portions included in the Silurian and Lower Carboniferous. Over all this area shales and fire clays are abundant along the banks of the streams. Many of them are all that could be desired as material for the manufacture of pavers, and they often present the added advantage of being underlaid by a bed of coal which may be used for fuel, thus lessen ing the cost of production. All the pairing brick plants now operating in the State use these shales and all but one of those in the western part of Indiana.
This area is everywhere overlaid by a deposit of glacial drift which is often from 50 to 300 feet in thickness, and it is only when this has been removed by erosion that the Coal Measure shales conic to the surface.
These glacial clays are now only used in the manufacture of drain tile and common building brick, but it is hoped that means will soon be devised which will make their use in the manufacture of vitrified ware of good quality commercially possible.
Cretaceous and Tertiary.—This area lies entirely south of the Ozark Ridge and includes the greater part of Alexander, Pulaski and Massac counties, and a small part of Pope. the.clay deposits of this area are of excellent quality and the tests to which some of them have been subjected indicate that they would make excellent pavers.
It is thus seen that there are no larger areas within the boundaries of the State that do not contain deposits of clay which are at least promising as sources of material for the manufacture of pavers. It is hoped that in the near future the survey will find itself in a position to demonstrate the availability of these deposits for this use.