CLAYS OF NEW YORK 523 



sulfate. This takes up water in eliemical combination and forms 

 gypsum. 



In many instances the presence of gypsum can be instantly de- 

 tected by the large transparent crystals scattered through the clay; 

 at other times it is found in the form of parallel fibres filling cracks 

 or cavities in the clay. So far as the writer is aware, only the 

 former type has been found in the ISTew York clays. Gypsum may 

 serve as a flux, but at the same time it may do considerable damage 

 in the burning by the liberation of sulfuric acid, which in its efforts 

 to escape may cause blisters on the surface of the ware. Lime may 

 be introduced into a clay by absorption, where a clay deposit rests on 

 a limestone or marl fo^rmation, the clay absorbing waters from below 

 that contain lime in solution, which the clay tends to separate. 



All clays do not contain lime, and indeed it sometimes happens 

 that the clays over very large areas are singularly free from it, 

 while in other regions the opposite may be true. The clays in many 

 parts of Alabama are remarkably low in lime. Those underlying 

 the region around Chicago', and again around Buffalo have an ap- 

 preciable amount of it. This material has been one of the chief 

 causes in restricting the utilization of the Hudson valley clays, 

 which for combining extent, location and accessibility are not sur- 

 passed by any other deposit. 



The range of lime in different clays is given below. 



Kind of clay 



Brick clay 



Pottery clay 



Fire clav 



Kaolin 



Magnesia 



Magnesia rarely occurs in clays in the same quantity as lime, and 

 in fact seldom exceeds 2^. The saime classes of compounds may fur- 

 nish it as furnish lime, viz, silicates, carbonates and sulfates. The 



Min. 



Max. 



Aver. 



.024 



23.20 



2.017 



.011 



9.90 



1.098 



.03 



15.27 



.655 



tr 



2.58 



.47 



