7o GEOLOGY OF OHIO. 



SECTION I. 



GENERAL CONSIDERATION OF CLAYS. 



(a.) Origin. Pure clay is a hydrated silicate of alumina, composed of 

 one portion of the sesquioxide of aluminum united with two portions of 

 silica and two portion of water. Clay in this sense may be represented 

 by the chemical formula. 



Al 2 ,0 3) 2SiO a + 2H 2 0. 



Clays in this popular meaning of the term are something very differ- 

 ent from the pure mineral. They are compound minerals or mixtures 

 of minerals, having no definite proportions of base to acid, no definite 

 composition either chemically or mineralogically, but all containing some 

 of the real mineral clay and all retaining something of its physical and 

 chemical characteristics. 



Pure clay, Kaolin, or Kaolinite is not a natural mineral, but is 

 derived from the decomposition of another. Feldspar, which is a silicate 

 of alumina and some alkaline base like potash soda or lime, is a hard 

 stony rock bearing no resemblance to clay in any respect. This feldspar, 

 when exposed to the action of water containing carbonic acid gas in 

 solutiou, is dissolved or broken up and the alkaline base is carried away in 

 solution; leaving the silicate of alumina alone. 



This origin of kaolins from the decomposition of feldspar rock is 

 thoroughly established and cases are frequently found where a bed of 

 feldspar is covered or surrounded by the kaolin formed from its decom- 

 position and extending down into the crevices of the rock wherever the 

 water has had a chance to penetrate. 



But while it is likely that many of our beds of kaolin owe their 

 origin directly to beds of feldspar, still it is not to this source that we 

 owe our enormous deposits of clay. 



Feldspars are very widely disseminated in other minerals and form a 

 notable part of granitic and gneissic rocks, which form a large part of 

 the earth's crust. Granite consists of quartz, mica and feldspar. Quartz 

 is not affected by any of the natural atmospheric or aqueous agencies 

 now at work, and mica but slightly so, but the feldspar, as has already 

 been shown, is sensibly attacked. 



When the feldspar is thus decomposed the bond which held the 

 quartz and mica firmly together is gone, and these minerals are partly 

 washed away and partly retained in mixture with the kaolinite which 

 has been formed. Granite is not of any permanent composition chemi- 

 cally, but is always composed of varying proportions of these three 

 minerals. Hence, the clay resulting from granite would vary with the 

 variations of the parent rock, as well as from the great variations which 

 attend the conditions of its formation. Also, nearly all of the clay 

 deposits of the world are formed from the decomposition of the earlier 

 rocks, and have been successively deposited in water, hardened into rock, 



