48 ' GEOLOGY OF OHIO. 



proportions of lime, iron and other elements, together with fragments of 

 various minerals and rocks in all stages of disintegration and decomposi- 

 tion. Clay, therefore, is not silicate of alumina pure and simple. For 

 this compound we have the specific name already given, viz., kaolin. 

 This word comes to us in a somewhat modified form from the Chinese 

 language. It is said to be derived from two words (Kau-ling), meaning 

 "high ridge," reference being made to some locality from which large 

 deposits were derived. 



Physical Properties of Kaolin. Kaolin is a definitely constituted min- 

 eral and can therefore be represented by a chemical formula, which is as 

 follows: Al 2 3 , 2SiO a , 3 H 2 O. The proportions of the different 

 elements are shown herewith, viz: 



Alumina 39.80) 



Silica, 46.30 \ Geikie's Text Book, p. 81. 



Water 13.90 J 



or, discarding fractions, we have, Alumina, 39, Silica, 47, Water, 14. 

 The color of the mineral is white. Its specific gravity ranges from 1.5 to 

 2.2. Its hardness varies between 1 and 2. It is soft and meagre to the 

 touch when dry, and plastic when wet. It takes, also a definite crystal- 

 line form, that of thin plates. This, however, it very infrequently assumes. 

 Derived, as it is in most instances, from the decomposition of felspathic 

 rocks and chiefly from the decomposition of granite and gneiss, kaolin is 

 very seldom found pure in nature. In other words, the usual product of 

 such decomposition is clay and not kaolin. But the name of the mineral 

 is commonly applied to masses of high-grade clays, from which the finest 

 work can be manufactured. The deposits are generally found bordering 

 masses of easily decomposed felspathic rocks. The particles of kaolin 

 are exceedingly fine and hence are easily held in suspension in water 

 and can be transported for long distances ; # but part of them, carried by 

 water to lower levels, accumulate there in such settling basins as are 

 naturally furnished. With the clay, fine particles of sand and rock are 

 also transported which are subsequently removed from the so-called 

 kaolin deposits by repeated washings. Most of the kaolin used in the 

 arts has been subjected to this last mode of treatment. A new method 

 has recently been invented in which separation of the clay is effected by 

 regulated air blasts. 



In the decay of felspar, it happens that many of the grains of the 

 undecomposed mineral are set free along with the products of chemical 

 change. A fine mechanical division of the mineral is also accomplished 

 by ordinary abrading agencies, as those of streams and waves. The fel- 

 spar particles constitute almost as fine-grained a mass as the kaolin itself, 

 and they are often mistaken for clay, but they do not become plastic in water, 

 and by this means can be distinguished from clays proper. It is thus 

 seen that finely divided felspar, known as felspathic mud, must be dis" 



