CLAYS OF jNEW YORK 841 



run of paving brick clays, but this is no serious objection. Tbe lean 

 ciiaracter of many can be overcome by the addition of plastic clay, 

 as in tbe case of tbe Cairo shale, in which instance the mixture, as 

 already stated, had a tensile strength of 100 pounds a square inch. 

 The amount of fluxes present permits their vitrifying at com- 

 paratively low temperature. But if necessary their refractoriness, 

 could be easily increased by the addition of a certain amoimt of 

 fire clay. 



Feldspar and quartz 



Mineralogic characters. Eeldspar, or " spar " as it is com- 

 mercially called, is one of the commonest of rock-forming min- 

 erals, and yet, owing to its usual intimate association with other 

 mineral species, commercially valuable deposits of it are compara- 

 tively rare. The deposit must be large and of snfiicient purity. 

 Its most common associate is quartz, but the two possess properties 

 which render them easily distinguishable. 



Feldspar is usually of a cream or red color, bnt at times may be 

 white. It cleaves readily in two directions nearly at right angles 

 to each other, so that fragments often show two smooth cleavage 

 surfaces, the result of this property of splitting. Chemically, feld- 

 spar is a complex silicate of alumina and potash, soda or lime. 



Quartz differs from feldspar in lacking cleavage, and being 

 harder. Its hardness is 7 in the scale, and it easily scratches glass. 

 It has also a bright glassy luster, and breaks with a conchoidal or 

 shell-like fracture. 



There are two well marked groups of feldspar minerals, the 

 potash feldspars, of which orthoclase is the type, and the lime soda 

 feldspars, or plagioclases. 



It is the orthoclase feldspar that is usually mined, though there 

 is undoubtedly some plagioclase in some of our commercial feld- 

 spars, but a systematic chemical investigation of the American 

 materials has, however, never been carried out. 



While there is but slight variation in the hardness of these feld- 

 spars, there is a variation in the chemical composition and fusibility. 



