8 WISCONSIN ACADEMY SCIENCES, AKTS, AND LETTERS. 



ments — e. g. limestone — by whose subsequent solution and removal 

 the clay may be left alone and pure. 



Whatever the exact origin, we may then group all clays conven- 

 iently into the bedded clays, and the clays accurring as disintegrated 

 rocks in ])lace. In both of these ways kaolin occurs. A. brief con- 

 sideration of each mode of occurrence will be of interest in the 

 present connection. 



Kaolin as a disintegrated rock in 2)lace. — Most kaolin of com- 

 merce comes from this kind of a deposit. Gneissic and other 

 felspathic rocks, frequently placed with their bedding planes verti- 

 cal, admit of deep penetration by the surface carbonated waters. 

 Their felspathic ingredient being thus decomposed, the whole rock 

 is converted into a soft admixture of kaolinite, quartz fragments, 

 particles of partly decomposed, and entirely undecomposed felspar, 

 and more or less altered particles of mica. This alteration has 

 been noticed to as great a depth as seventy feet and over. The 

 gneissic rocks of the Blue Ridge in Virginia, North Carolina, and 

 Georgia, are found altered to this depth over considerable areas, re- 

 taining still their original lamination and highly inclined position.* 

 Similar changes and to much greater depth are reported from Bra- 

 zil.f In such cases the once quartz veins are often seen occupying 

 their original position as great sheets in the soft clay. 



Should now the felspar be a largely predominating constituent 

 of the rock, or should the mica be present in inconsiderable quan- 

 tities only, there will result on decomposition a mixture of a very 

 pure white kaolinite with more or less quartz sand and undecom- 

 posed felspar fragments, which can readily be removed by leviga- 

 tion, and a valuable article obtained. Should, on the other hand, 

 the mica be largely present, or should there be any quantity of 

 hornblende or p3'rite in the rock, the resulting clay will be largely 

 contaminated with non-separable alkalies from the mica, or oxyd 

 of iron from the mica, hornblende or pyrite, and will be a mere red 

 brick-clay of no value. Thus it happens that whilst many locali- 

 ties of disintegrating granite are known, but few of them yield 

 good kaolin. In the case of much pyrite or other ferruginous con- 

 stituent in the rock, the weathering and leeching by the carbon- 

 ated waters, may result in the formation of deposits of the hydrat- 

 ed sesquioxyd of iron, in the shape of " bog iron ore." Such Dr. 



* Am. Jour. Sci. III. vii p. 60. -j- Hartt as quoted by Hunt, Loc. cit. 



