NORTH-CAROLINA GEOLOGICAL SURVEY. 135 



best tile clays, remarks that the adhesiveness of clay depends 

 mainly upon the proportion of alumina. Clays of an average 

 goodness will contain about 85 per cent of silex and alumina 

 when taken together. Much depends evidently upon the 

 coarseness of the sand, for when the sand is coarse the ten- 

 acity of the clay is very much diminished. Clays again in 

 which the infusible ingredients is greatest, other properties 

 being equal and favorable, are best adapted to the manufac- 

 ture of good tile, besides in this case they admit of being 

 moulded lighter and thinner. If lime and oxide of iron ex- 

 ist in large proportions, the clay is rendered more fusible, but 

 in that case, it possesses an advantage of being burnt with 

 less fuel. So with brick. The clay of the tertiary beds, it 

 will be perceived, contains but a small proportion of lime and 

 iron, or other elements which are calculated to confer fusibili- 

 ty. Hence it will probably be found that this clay will rank 

 with the most infusible of the clays, except the porcelain 

 clays, and being extremely fine and tenacious is well adapted 

 to the manufacture of many fine earthern wares which are so 

 necessary in house keeping. 



CHAPTER XII. 



The grasses and their functions Different objects attained by their culti- 

 vation Chemical constitution of the grasses Elementary organs, and 

 parts of the blossom. 



101. The grasses serve many important purposes. They 

 clothe the earth in green, a color easy and agreeable to the 

 eye. They protect the loose earth and prevent its washing 

 away and transportation into, the streams, or being cut into 

 gullies. They furnish food to the beasts and birds, and the 

 most important, the cereals, sustain the. millions of the human 



