CLAYS OF NEW YORK 497 



To define clay from the physical standpoint, we may say 

 that it is a fine-grained mixture of mineral fragments of variable 

 composition, possessing when wet plasticity which permits it to be 

 molded into any desired form and retaining that form when dry. 

 That furthermore, when heated above a certain temperature it loses 

 its chemically combined water and becomes converted into a rock- 

 like mass, which if reground and mixed with water no longer shows 

 plasticity. 



All clays found in nature contain, so far as known, a variable 

 amount of kaolinite, the hydrated silicate of alumina, which is 

 commonly spoken of as the clay base or clay substance. (It should 

 be stated that it is sometimes the custom, specially abroad in the 

 case of many impure clays to call the finest particles of the clay 

 irrespective of their composition the " clay-substance.") 



A mass of kaolinite would be termed Tcaolin. These two 

 terms are often used interchangeably, though the former is simply 

 the mineralogic name while the latter is a rock term. Pure 

 kaolin has not thus far been found in nature, though some very 

 nearly pure is known. Properly speaking the term haolin should 

 be restricted to white burning residual clays, a usage which is 

 widespread but has not become universal. " The name haolin is a 

 corruption of the Chinese Kauling, which means high ridge, and is 

 the name of a hill near Jauchau Fu, where the mineral is obtained." 

 (Dana, System of min., p. 687) 



Kaolinite is a secondary mineral formed by the decomposition 

 of feldspar. This is commonly caused by percolating waters aided 

 by disintegrating causes, the result being that the alkalies and alka- 

 line earths of the feldspar are carried off in solution, while the 

 alumina and silica, left behind, unite with water to form hydrated 

 silicate of alumina. The feldspars are essentially anhydrous sili- 

 cates of alumina, containing in addition varying amounts of lime, 

 potash or soda, and depending partly on their chemical composition 

 and partly on their physical characters. Nine varieties are usually 

 recognized, which fall into two groups known as the orthoclase 

 and plagioclase groups. 



