496 NEW YOKK STATE MUSEUM- 



ORIGII^ AND ITATURE OF CLAY 



The term " clay " is difficult to define, being often used in a 

 rather loose manner. Recent investigations on the part of the 

 writer and others lead to the conclusion that the term " clay " 

 indicates a substance of peculiar physical characters, but hav* 

 ing absolutely no constancy of either chemical or mineralogic 

 composition. Two substances may appear to be entirely unlike from 

 a chemical standpoint, and yet their physical characters may be 

 such that both would be classed under the head of clay. 



Probably the best mineralogic definition of clay is that given h^ 

 Dr G. P. Merrill in his book, Rochs, rock-weathering and soils, in 

 which he defines it "as an indefinite mixture of more or less hy^ 

 drated aluminous silicates, free silica, iron oxid, carbonates of lime^ 

 and various silicate minerals which, in a more or less decomposed 

 and fragmental condition, have survived the destructive agencies to 

 which they have been subjected ". 



The only feature characteristic of all clays is that they are 

 plastic when wet and when burned harden to a rock-like mass. 

 This degree of plasticity has little probably to do' with the chemical 

 or mineralogic composition, for clays of either high or lo"^ 

 plasticity may vary widely in their make-up. It seems to depend^ 

 and this point will be discussed in more detail later, wholly on 

 texture and structure, that is on the shape and size of the^ particles. 

 As Dr Merrill points out, pure quartz, chalcedony, flint, feldspar oi 

 other silicates, will when reduced to an impalpable powder possess 

 a pastiness and even an odor similar to that of clay. Most of 

 these simple mineral mixtures of extreme fineness do not seem to 

 hold together like clay mixed by nature, probably because they 

 lack the plastic particles which true clay contains. Clay may 

 show all degrees of plasticity, and by the increase of one or anothei' 

 of its component minerals may pass into other rock types, as into 

 limestone on the one hand by the increase of carbonate of lime, ot 

 into sandstone by the addition of sand. 



