138 NEW YORK STATE MUSEUM 



glacial streams and finally deposited as beds of clay in the 

 numerous lakes which occupied the valleys during the early 

 Pleistocene period The erosion of the sandstones has added to 

 the grittiness of the clays. The extensive deposits of plastic clay 

 found on Long Island were elevated above sea level within 

 probably a comparatively short time after their deposition. 



Pure clay is of a white color and is very rare. The purest 

 clays known are the China clays, which have about the theoreti- 

 cal composition of kaolinite. 



Clays suitable for the manufacture of common brick are by no 

 means uncommon. The impurities in them often run quite high, 

 still in many cases they make a most excellent common brick. 

 The use of a clay for one thing or another is largely determined 

 by the impurities. The clay should be plastic, work easily and 

 burn to a good red color, giving a hard ringing product. 



Plasticity is the property which clay has of forming a pasty 

 mass when mixed with water, and changing to a hard mass when 

 subjected to a high heat. Burned clay if ground and mixed with 

 water is not plastic ; in fact clay loses its plasticity when the 

 water of combination is driven off. This property of clay is 

 largely due to the kaolinite base which occurs in the form of 

 minute hexagonal tablets. 



Dry kaolin is not plastic, and water seems therefore to like- 

 wise influence plasticity. Prof. G. H. Cook found that it was 

 also influenced by the degree of fineness of the kaolin. In clays 

 which are very slightly plastic the plates of kaolinite were found 

 to be collected in bunches, and a subsequent thorough grinding 

 in order to break up these aggregates increased the plasticity. 



A tough plastic clay is termed by the brickmakers " fat;" on 

 the other hand a clay of loose texture and possessing little plas- 

 ticity is said to be "lean" or "poor." 



Sand consisting of quartz, feldspar or mica destroys the plas- 

 ticity of clay and is one of the commonest impurities. The others 

 are lithium, titanium, iron, lime, magnesia and the alkalies potash 

 and soda. Sulphur is sometimes present, it having been found in 

 some of the Hudson River clays. 



Page* states that " the admixture of a proportion of siliceous 

 sand, which results in a combination containinsf as much as 90 



* Economic Geology, p. 186. 



