CLAY WORKING INDUSTRIES. 71 



eroded again by the rains and frosts and ice of centuries, and again 

 deposited. No one knows or can guess how many times our clays have 

 been thus treated, and each transfer is of course accompained by the 

 further mixing and blending of clays with each other and with the 

 detritus of all other rocks. 



In the light of these few statements on the origin of our clays, it is 

 not a source of surprise that they vary but that they retain as much of a 

 family likeness as they do. 



(&) Composition. — As has just been indicated, we must expect to find 

 their chemical and mineralogical composition varying through a very 

 wide range. Indeed, there is some difficulty in determining how much 

 the pure clay base may be mixed with silica and other minerals and stiU 

 be called a clay. There is hardly any common rock which does not con- 

 tain it. Limestones when they disintegrate and dissolve away leave a 

 residue of clay behind them ; sandstones will generally become plastic if 

 ground fine with water, from the presence of the same mineral. 



This irregularity of composition led at first to great misunderstand- 

 ing of the nature of clays. It is only in the light of their origin that we 

 can understand the anomalies which their composition presents. Pure 

 clay or kaolinite has a percentage composition of 



Silica (Si O.,) 46.3. 



Alumina (Al 2 3 ) 39.8. 



Water (H 2 Oj 13.9. 



which may be represented by the chemical iormula Al 2 3 2Si 2 2H 2 0. 



Impure clays contain less alumina and water and more silica and 

 other elements beside. The researches of the late Professor Cook first 

 demonstrated that in any clay the silica could be separated into two kinds : 

 viz : — that combined with the alumina as a hydrous silicate, and that free 

 from combination and present as quartz or sand. It is possible that free 

 uncombined hydrated silica is present in some clays, but this form is cer- 

 tainly rare. By a chemical process it is possible to make a separation of 

 the silica into the two named classes, and on assembling the alumina, com- 

 bined water and the combined portion of the silica together, and finding 

 their ratio to each other it was found that a body was obtained having the 

 same percentage composition as kaolinite. There were naturally some 

 variations due to the errors of the chemical process employed and proba- 

 bly to the presence of other silicates than those of the clay proper, but 

 while the kaolinite bases thus found were sometimes too silicious and 

 sometimes too aluminous, nevertheless the essential identity of the base 

 of all clays was established. This chemical proof of the identity of the 

 presence of one kaolinite base in all clays has been a strong support of 

 the theory of their origin, and their origin in turn leads us to expect what 

 by research we find in the composition. 



Clay then is found, both by this theory of its origin and the discrim- 

 inating use of chemical analysis, to be a mixture of a kaolinite base with a 



