328 REPORT OF NATIONAL MUSEUM, 1899. 



of kaolin but much free quartz, scales of mica, bits of still fresh feld- 

 spar, and more rarely tourmalines and other of the less destructible 

 silicates. 



Iron in the hydrated sesquioxide state is found in nearly all clays, 

 even the whitest varieties. More than 1 per cent was found in a sili- 

 ceous clay from Ohio, although the clay itself was almost of snowy 

 whiteness. 



Iron also exists in the form of a silicate and protoxide carbonate, and 

 sometimes as a sulphide in the form of disseminated pyrite. Lime 

 and magnesia are also common constituents, either as free carbonates 

 or as lime-magnesia silicates, and may exercise an important bearing 

 upon the suitability of a clay for any particular purpose, as will be 

 noted later. The clay from which the well-known Milwaukee cream- 

 colored bricks are made contains sometimes as high as 23 per cent 

 carbonate of lime and 17 per cent carbonate of magnesia, together with 

 nearly 5 per cent of iron. 



The alkalies, potash and soda, are common constituents in small pro- 

 portions, and also lithia, the first named being most common as well as 

 most detrimental. It is a fair assumption that these substances are 

 constituent of still undecomposed fragments of feldspar and the micas. 

 To the presence of rutile needles and particles of ilmenite are due the 

 frequent traces of titanic acid revealed by chemical analysis. The 

 presence of any quartz and undecomposed feldspathic material in a 

 clay can as a rule be detected by the gritty feeling manifested when 

 tho material is rubbed between the thumb and fingers. Mica is, how- 

 ever, not readily detected by this means. 



The above remarks will explain why a purely chemical analysis of a 

 clay may be of little value for the purpose of ascertaining its suitability 

 for any particular purpose. It is essential that we know not merely 

 the presence or absence of certain elements but also how these elements 

 are combined. Further than this few clays are used in their natural 

 condition, being first purified by washing and usually mixed with other 

 constituents to give them body or fire-resisting properties. 



Kinds and classification. From a geological standpoint the clays 

 may be divided into two general classes, as above noted, (1) residual 

 and (2) transported, the first class including a majority of the kaolin, 

 halloysite, etc., and the second the ordinary brick and potter's clays, 

 the loess, adobe, Leda, and the bedded, alluvial deposits of the Cre- 

 taceous, Carboniferous, and other geological periods. Special names, 

 based upon such properties as render them peculiarly adapted to eco- 

 nomic purposes, are common. We thus have (1) the kaolin and 

 China clay, (2) potter's clay, (3) pipe clay, (4) fire clay, (5) brick, tile, 

 and terra cotta clays, etc., (6) slip clays, (7) adobe, and (8) fuller's 

 earth. These will be discussed in the order given, though they must 

 necessarily be discussed but briefly, since the subject of clays alone 



