[Some] shales are hard, tough, and strong enough to serve as temporary mine roofs. Hard shales are sometimes called “slate” but this name is technically incorrect. True slate is a metamorphic rock, composed chiefly of the mineral mica in very fine flakes, and will resist the action of water (weathering) for a long time. Therefore, it is a good roofing material for buildings, whereas shale is composed chiefly of clay minerals, and despite the strength and compactness of the more “slaty” varieties soon disintegrates in water. Missouri “slaty” shale would not serve as satisfactory roofing material.

The red “burned” shale found on burned-out coal mine dumps is called “shale” locally. It is, of course, shale which has been fired more or less to the condition of building brick by the hot burning waste coal. The same original shale could be crushed, molded into brick, “burned” in a kiln, and become a satisfactory building brick. The “burned shale” of the coal mine dumps is used in many places as a drive-way covering.

“Soapstone” is a name applied by some persons to some soft, slippery to greasy shales, but this name is incorrect in a technical sense. True soapstone is a metamorphic rock (shale is sedimentary) which is composed chiefly of the mineral talc. Soapstone occurs abundantly in certain parts of the Appalachian Mountains but is exceedingly sparse in Missouri.

The chief commercial uses of shale are in the manufacture of common brick, building brick, building tile, drain tile, sewer pipe, Portland cement, and other ceramic products. Many shale beds and occurrences are technically suitable for these uses but have no real commercial value because other necessary factors are lacking. In order to make brick, tile, or cement there must be sufficient fuel available at low cost, low-priced bulk transportation of the raw and finished products, available labor, capital for the erection of a plant, and above all a large near-by, dependable market for the manufactured product. The value of a shale deposit, therefore, depends as much upon outside conditions as upon the properties of the rock (shale) itself.

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.

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.