CLAY WORKING INDUSTRIES. 75 



no plates at all, only a homogenous matrix. These points indicate that 

 the plasticity of a clay depends on the extent to which this tendency to a 

 crystalline structure has been destroyed in the treatment that the clay 

 has received. This theory is in harmony with the facts as presented by 

 the clays of New Jersey, which are of more recent geological formation 

 and of different character from those found in Ohio. In Ohio, the clays and 

 shales all belong to the Carboniferous or older geological formations, and 

 it seems probable that some other agency than crystallization has made 

 many of our clays and shales non-plastic which once were so. In 

 default of any adequate explanation, it may be suggested, that long and 

 intimate association of the clay base with its silica and its other constit- 

 uents, the influence of enormous pressures for age after age of geological 

 time, possibly the influence of low metamorphic heat and the binding, 

 cementing action of the tarry organic compounds produced by the slow 

 change of the vegetable matters incorporated into the clays, have com- 

 bined to make many clays non-plastic. Most of these can be made plas- 

 tic again by grinding and kneading with water, but many fail to develop 

 much of their old plasticity. Beside these clays, which are now difficultly 

 plastic, there is a class of clays found in the Coal Measures of Ohio and 

 adjoining states which is entirely devoid of plasticity and no known 

 treatment is sufficient to induce any marked return to the plastic state. 

 Such clays are known as flint clays and are only known as clay at all by 

 their behavior under heat and by the fact that an analysis discloses that 

 they are generally nearly pure kaolins in composition. The action of 

 frost and water on these clays is to crumble them to a fine, sharp sand ; 

 grinding with water leaves them still gritty and with no more plasticity 

 than a sand rock would develop under similar treatment. 



The origin of these clays is altogether unexplained. Occurring as they 

 do, remarkably pure clays in the midst of very impure ones — they must 

 have been deposited in much the same way as the plastic clays around 

 them. They even contain pebbles sometimes, which shows the existence 

 of currents through the swampy tracts of which they formed the floor. 

 Organic acids, such as are, developed in swamps, are supposed by some to 

 have aided the vegetation in withdrawing everything of a soluble nature 

 from the clay, but this does not account for their freedom from silica. 

 The subject is full of interest and greatly needs more time and attention 

 than the Survey was able to devote to its study. 



Permanence in clay ware depends on the way in which the chemical 

 changes during burning are brought about and the temperature to which 

 the clay is ultimately subjected. If the heat applied has been only suffi- 

 cient to drive out the water of hydration, the resulting material 

 will be porous and friable, and while it will have shrunk very sensibly in 

 volume and is not soluble in water or organic acids, still the action of 

 frost is sufficient to crumble it to dust on account of its porosity and 

 attraction for moisture. If the heat has been carried up gradually above 



