The shales of Missouri were formed from deposits of mud that settled out in sea water which in the past covered this state. Fossil remains of sea-living organisms which are preserved in the shale give evidence of the marine conditions once existent here. Like the muds that are accumulating along the Atlantic coast and in the Gulf of Mexico, where the Mississippi River is discharging its load of silt and clay, so did mud form layers on the bottom of geologically ancient interior seas. In some cases sand was later washed in and covered the mud; in other cases limestone-forming material (like off the coast of Florida today) was deposited on top of the mud. The weight of the overlying beds and the slow movement which raised the sea bottom up to land squeezed out the excess water, compressed and compacted the muds into thin layers, and brought about the shale rock which is exposed to us today. [Illustration: Soft, easily eroded bed of shale between two more resistant beds of limestone near Columbia.] Black muds, rich in humus and other organic material, formed black shales; red and yellow clays colored by red and yellow iron oxides (iron rusts) formed red and yellow shales; and sandy muds were compacted into gritty, sandy shales. All of them were derived from eroding land and soils just as today our eroding soils contribute to the formation of more shale now in the long, slow process of formation. The chemical composition of an average shale is not simple, as is shown by the subjoined composite analyses of sedimentary rocks taken from U. S. Geological Survey Professional Paper No. 127. 78 shales 253 sandstones 345 limestones SiO₂ 58.11 78.31 5.19 Al₃O₂ 15.40 4.76 .81 Fe₂O₃ 4.02 1.08 .54 FeO 2.45 .30 MnO 2.44 1.16 7.89 CaO 3.10 5.50 42.57 Na₃O 1.30 .45 .05 K₂O 3.24 1.32 .33 H₂O+ 3.66 1.32 .56 H₂O- 1.33 .31 .21 CO₂ 2.63 5.04 41.54 TiO₃ .65 .25 .06 P₃O₅ .17 .08 .04 SO₃ .65 .07 .05 Organic carbon .80 — — (100.) (100.) approx. (100.)