ORDOVICIAN.
Lower Magnesian.—At the close of the Potsdam the condition of the sea had become such that lime-secreting animals could grow in abund ance, but the waters were not completely or continuously clear. During this time 100 to 500 feet of a strongly argillaceous limestone was de posited over the entire area of the State. Its presence has not been actu ally demonstrated in the extreme southern portion, but there is every reason to believe it is there. It appears at the surface only in LaSalle, Ogle and Calhoun counties. Near Utica, in LaSalle county, it is used in the manufacture of hydraulic cement.
St. Peters.—After a geologically brief but really long period following the deposition of the Lower Magnesian, during which northern Illinois was dry land, the State was again covered by a shallow sea and about 150 feet of a charactertistic white sand was deposited. This material is much used in the manufacture of glass and forms, above the Potsdam, the principal source of artesian water in the northern portion of the state. It appears at the surface in the lower ten miles of the valley of Fox river, and along the Illinois from Ottawa almost to LaSalle; also in the valley of Rock river from north. of Oregon almost to Dixon and
at Cap-a-Gres near the southern end of Calhoun county.
Galena-Trenton.—Following the period of deposition of the St. Peters, the sea became again suited to an abundant growth of organisms, al though the waters still contained considerable quantities of clayey sedi ment. During this period 300 to 400 feet or more of dolomite and limestone were formed. This formation carries the lead and zinc de posits of the northwestern portion of the State and corresponds with the oil-bearing limestone of eastern Ohio and Indiana. While usually com posed of limestone, the formation contains pockets of shale and clay.
Cincinnatian or Maquoketa.—At the close of the Trenton period the seas became very muddy, but with clearer patches here and there in which lime secreting organisms could flourish. During this period 50 to 200 feet of shales and shalt' limestones were formed. While in Illinois the Cincinnatian consists, for the most part, of argillaceous limestones and limy shales, where it outcrops in the northern part of the State, it contains local deposits of very pure shale.