NORTH-CAROLINA GEOLOGICAL SURVEY. 133 



city will be so far diminished that it cannot be carried from 

 place to place. 



Certain clays contain so little sand that in order to dry or 

 season well, it must be added ; but when clay is to be worked 

 by a machine, less sand is required than when it is worked by 

 hand. 



Clay that cuts smooth is probably a good clay. The sur- 

 face exposed by cutting should not exhibit ragged lines, or 

 show particles of coarse sand or hard spots. 



Good clay has a uniform color, and is not spotted with 

 ochrey matter. A clay may be red, blue, brownish or pur- 

 plish, and yet possess excellent properties. 



Clays for certain purposes should not effervesce with acids ; 

 this phenomenon denotes the presence of carbonate of lime, 

 which imparts fusibility to the compound. This tendency to 

 fuse in the kiln is increased when iron is present. All such 

 clays will require very great care in burning, and when burnt 

 into brick, are unfit for places where they will be exposed to 

 great heat. Fire clays consist of alumina and a fine or im- 

 palpable sand. For withstanding high heat, as much sand 

 must be mixed as the clay can bear and handled without 

 breaking. Sand increases the infusibility of the mass. 



100. A bed of fine clay overlies the shell marl. At cer- 

 tain places it is fine, plastic, cuts evenly, and may be moulded 

 readily into the form of any article in common use. On 

 Bogue sound, it is purplish and extremely fine, and is an ex- 

 cellent potter's clay. 



The composition of the infusible clays of the best kinds 

 have been determined by many anatyses. Thus, the celebra- 

 ted Stourbridge clay consists, according to the late Prof. 

 Johnston, of 



Alumina, 38.8 



Silex, 46.1 



Water, 15.1 



100.0 



The Woodbridge fire clay of New Jersey, according to 

 Prof. Cook, is composed of 



