ECONOMIC GEOLOGY 
cement, ciment fondu. This cement is very resistant to 
chemical attack, sets in as many hours as Portland Cement 
takes days, consists of calcium silicate mixed with calcium 
aluminate, and is made by the electric fusion of bauxite and 
limestone. Bauxite is usually mined by those who use it, 
so that there is no common market price; the value depends 
on the percentage of free alumina, with deductions for high 
percentages of silica, iron, and titanium oxide, The highest 
grade bauxite ranges from 60 to 65 per cent. of alumina, 
with I per cent. of iron oxide, and 2 to 5 per cent. of silica ; 
most of it is used for the manufacture of aluminium for which 
the usual minimum is 52 per cent. of alumina, not over 3 
or 4 per cent. of silica, 2 or 4 per cent. of titanium oxide, or 
over 6-5 per cent. of iron oxide. The second largest quantity 
is used for alum, for which the essentials are little titanium 
and iron, and no chromite, The qualities preferred are as 
follows —— 
For Manufac- 
ture of 
Aluminium 
Alum . | 
Refractories 
Cement . . 
Al,O,. 
+ 52 per cent. 
+ 52 percent. 
} 40 per cent 
+ 45 per cent 
SiO... 
— 4% per cent. 
-—~— 10 per cent. 
—~ 15 per cent. 
+ 10-15 per cent. 
Feo,Qq, 
— 61 per 
cent. 
low 
low 
10 per cent. 
TiO... 
— 2 per cent. 
or 4 per 
cent, in 
highest 
grades. 
low, no 
Cr,03.