THE NONMETALLIC MINERALS. 327 



Kaolinitc is in itself not properly a elay,noris it plastic. Further, 

 in many cases it is present only in nonessential quantities. More open 

 to criticism yet, because more concise, is the statement sometimes made 

 that day is a hydrated silicate of alumina having the formula Al^Og, 

 2SiO..+2H.O. It is doubtful if a clay was ever found which could be 

 reduced to such a formula excepting by a liberal exercise of the imagi- 

 nation. There is scarcely one of the silicate minerals that will not 

 when sufficiently finely conmiinuted yield a substance possessing those 

 peculiar physical properties of unctuous feel, plasticity, and color, 

 which are the only constant characteristics of the multitudinous and 

 heterogeneous compounds known as clays. Even pure vitreous quartz 

 when rubbed to the condition of an impalpable powder has when wet 

 the plasticity and odor of dny.^ Dau])ree so long ago as 1878*^ pointed 

 out the fact that by the mechanical trituration of feldspars in a revolv- 

 ing cylinder with water an impalpa])le nuid was obtained, which 

 remained many days in suspension, and on drying formed masses so 

 hard as to be broken only with a hammer, resembling the argillites of 

 the coal measures. 



The ever varying chemical nature of the materials classed as clays is 

 brought out to some extent by a comparison of the analyses in the 

 table (p. 349), but is even more evident in microscopic and mechanical 

 examinations. Indeed, as stated by Chamberlain:^ 



While it is convenient and customary to speak of the crude material of l>rick as 

 clay, that which is really made use of is a mixture of clay and sand, or, in the cream- 

 colored brick, of aluminous clay, calcareous clay or marl, and sand. The mixture is 

 really a loam and but for the appropriation of that term as the designation of a soil, 

 it would doubtless he more generally applied to such mixtures. 



Professor Crosb}^, as noted elsewhere, has shown that the blue-gray 

 brick clays of Cambridge contain only from one-fourth to one-third of 

 their bulk of "true clay," the remainder l)eing finely comminuted 

 material to which he gives the name rock Hour. 



An examination of certain English fire clays has shown * that they 

 can not properly be considered as mere hydrous silicates of alumina, 

 but are very complex mineral admixtures, among which scales of 

 hydrous micas, grains of feldspar, more rarely quartz and rutile needles 

 greatly preponderate over the kaolin. The Leda clays of Maine, as 

 the write)" has noted elsewhere, contain a comparatively small amount 



' Referring to the odor of clay when a shower of rain first begins to wet a dry, clayey 

 soil, Mr. C. Tomlinson has remarked that it is commonly attributed to alumina, and 

 yet pure alumina gives off no odor when breathed upon or wetted. The fact is, the 

 peculiar odor referred to belongs only to impure clays, and chiefly to those that con- 

 tain oxide of iron.. (Proceedings of the Geological Association, I, p. 242; quoted in 

 Woodward's Geology of England and AVales, p. 439.) 



^(ieologie Experimentale; 1879, p. 251. 



* Geology of Wisconsin, I, ]>. 673. 



* W. M. Hutchings, Geological Magazine, VII, 1890, p. 2()4, and VIII, 1891, p. Ifi4. 



