?374, 375, 376] clays — different kinds — colors. 211 



374. The next succeeding element — Aluminium (which, in its metallic state,, 

 has but lately been introiluced into the arts, and is distinguished by its extra- 

 ordinaiv lightness) is the most abundant ingredient of the earth's crust, next to 

 ox\gen and silicon. Its combination with oxygen, Alumina, when still further 

 combined with Silica, forms the main mass of all Clays, and of a great number 

 of rocks, by the decomposition of which, clays are formed. 



375. Clays. — There are innumerable varieties of clays, which have received 

 numerous names, both popular and scientific. The differences arise partly frorm 

 the various proportions in which silica and alumina may combine, partly, and 

 most frequently, from the admixture of foreign (i. e. pot chemically combined) 

 substances. The purest of all clays is Kaolin or porcelain earth ; next to this, 

 the white pipe-clay, such as those founJ chiefly in Tishomingo, but also in other 

 counties of this State, in the deposits of the Orange Sand formation (if 60, ff.)„ 

 In some of these pipeclays, there is a strong mechanical admixture of finely- 

 divided silica ; in some instances this ingredient gradually becomes so prominent 

 that the material loses its title to the name of clay, and becomes a soft, fine- 

 grained, white, siliceous rock, the transition being so gradual, that it is difficult 

 to say where the pipeclay ceases and the rock begins (1162, 66). 



A similar uncertainty very frequently attaches to the more common mixtures 

 of clay and sand, known as brick-clays, loams, silts, etc., including the majority 

 of soil-; ; there is no definite line of distinction, so that it is often optional with 

 the individual, whether some material he describes shall be called a clayey sand 

 or a sandy clay. 



376. The different colors of clays are generally owing to either or both of the 

 two last named elements of the list, viz : Manganese and Iron, and to Carbon, or 

 finely divided particles of coaly substance. 



Iron (in its two combinations with oxygen, called the protoxide and peroxide) 

 and Carbon are by far the most commonly occurring coloring matters. Peroxide 

 of iron imparts yellow, orange and red tints (the two former tints pass into 

 red whenever the substance is heite 1 to redness); protox de of iron, blue and 

 gieen. The two last mentioned cJors pass over into those of the peroxide, 

 whenever the material they color is exposed to the action of the air ; hence it 

 is that in the southern portion of the Stite, where blue and green clays are so 

 very common (in the upper Tertiary, or Grand Gulf Group, 1T-30, ff.), we 

 frequently sae nothing but yellow or orange surfaces on thj exposures, the blue 

 or green portion being found at the depth of several inches at times. The same 

 frequently happens wnh reference to the blue marls, both of the cretaceous and 

 tertiary formations; they have escaped notice in miny cases, simply on account 

 of the surface having undergone the change just referred to, which rendered them 

 Similar in aspect to the soil or subsoil. 



Clays colored by Manganese alone, are of rare occurrence ; according to the 

 amounts of oxygen comb ned with the element, it imparts delicate pnk, browrn 

 or black tints. But it frequently modifies the tints produced by iron, and is 

 rarely wanting where the other occurs. 



CI iys colored by Carbon alone, which imparts to them a black, gray or some- 

 times bluish tinge, become white by burning. Such is the case with many clays 

 from the Lignitic formations, which, although very dark at first, on burning 

 yield ware almost equal in whiteness to that made of the Tishomingo pipeclays. 



Of course it often happens, that several or all of these coloring matters are 

 present in one and the same specimen, thus producing an endless variety of 

 tints — from the purest white through all shades of gray to almost jet black; 

 from the light and delicate rose tints of the Orange Sand clays frequently found 

 in South Mississippi, to the deep red of the ferruginous clay occurring in Tisho- 

 mingo county, which has been designated as " Terra sigillata," and comes prop- 

 erly under the head of coloring materials (1f7l). Clays of all shades of blue 

 are frequen 1/ found associated with our lignite beds in North Mississippi, and 

 are very Ctmmm in the upper (fresh water) Tertiary of South Mississippi; &C 

 also are those of the several shades of green (If 231, 237, 243, etc). 



