THE NONMETALLIC MINERALS. 325 



material is cleaned, dried, and assorted, the drying taking place in the 

 open air, without artificial heat in summer, and recjuiring from five to 

 six days. The bulk of the material is sent direct to Vienna and Paris. 



15. Clays. 



• 



The term "clay," as commonly used comprises materials of widely 

 diverse origin and mineral and chemical composition, but which have 

 in common the property of plasticity when wet, and usually that of 

 becoming itidurated when dried either by natural or artificial means. 

 Of so variable a nature is the material thus classed that no brief 

 definition can be given that is at all satisfactory. One may perhaps 

 describe the cla3's, as a whole, as heterogeneous aggregates of hydrous 

 and anhydrous aluminous silicates, free quartz, and ever-varying quan- 

 tities of free iron oxides and calcium and magnesian carbonates, all in 

 a finely comminuted condition. 



Origin and mode of occurrence. — The clays are invariably of sec- 

 ondary origin — that is, they result from the decomposition of pre- 

 existing rocks and the accumulation of their less soluble residues, either 

 in place (as residual clays) or through the transporting power of ice 

 and water (drift clays). The fact that silicate of aluminum is so char- 

 acteristic a constituent of nearly all clays is due to the fact that this 

 substance is one of the most insoluble of natural compounds, and 

 hence when, under the action of atmospheric or subterranean agencies, 

 rocks decompose and their more soluble constituents — as lime, mag- 

 nesia, potash, soda, or even silica — are removed, the aluminous silicate 

 remains. 



The kaolins, which may perhaps be regarded as the simplest of clays, 

 are the product, as a rule, of decomposition in place of feldspathic 

 rocks, as gneisses, granites, and pegmatites. Those of Hockessin, 

 Delaware (Specimens Nos. 63427 to 63430), are mainly of gneissic origin, 

 though from some of the pits the material is in part at least derived 

 from the decomposition of feldspathic conglomerate. In other cases 

 the rock, as in the case of that from Blandford, Massachusetts (Speci- 

 mens Nos. 68219 and 68221, U.S.N.M.), is a quite pure pegmatite, com- 

 posed almost wholly of quartz and orthoclase. The samples show the 

 material in various stages of decomposition. In all these cases the 

 material as mined contains particles of free quartz and other substances 

 detrimental to its use as a clay, and which must be removed by washing. 

 It sometimes happens that the natural admixture of silica and unde- 

 composed silicates is of just the right proportions to be utilized after 

 merely griivding and bolting. The so-called "Cornwall stone" (Speci- 

 mens Nos. mi2>Q and 62118, U.S.N.M.) is but a granite, very free from 

 mica and ferruginous impurities, and in which the feldspar onh'has in 

 part decomposed to the condition of kaolin. In some instances the 

 natural conditions are such that running waters have assorted out the 



