50 
ECONOMIC GEOLOGY 
percentage of manganese ; the price in 1920 rose to 3s. 10d. 
per unit, and since then has varied between Is. and 2s. per 
unit. There is a deduction if the silica is more than 8 or 
10 per cent., and the phosphorus more than ‘I per cent. ; 
10 per cent. of alumina and potash is usually the maximuin 
accepted. 
The leading manganese-producing countries now are India, 
Brazil, Southern Russia and Georgia! and the Gold Coast. 
Extensive deposits are known elsewhere as in East Africa, 
and many low-grade ores were mined during the War in the 
southern part of the United States. [For literature see the 
monograph by Fermor, Mem. G.S. India, xxxvii, 1909; 
R. A. F. Penrose, Ann. Rep. G.S. Arkansas, for 1890, 
, 1891; and L. Demaret, Les Principaux gisements des 
Minerais du Manganise, Ann. Mines Belgium, x, 1905, 
pp. 809-001.] 
CHrOMIUM 
Curomium (Cr; at. wt., 52:5; sp. gr., 6:5 ; melting-point, 
above 4500° F.) is mainly used as an alloy with steel, which 
2 per cent. of chromium renders very hard and so tough 
that it may be bent cold. The addition of 13 to 14 per cent. 
of chromium makes rustless steel. Chromium compounds are 
used in dyeing, tanning, and bleaching. The chief mineral 
is chromite (FeO, Cry0,), which is a primary constituent 
of ultra-basic igneous rocks. The ore occurs as veins, 
lenticular nodules, and nodular masses in pre-Palzozoic 
igneous rocks that have been altered into serpentine or talc- 
schists, and occasionally as veins in unaltered basic rocks. 
The most important supply at present comes from Selukwe 
in Southern Rhodesia, where it occurs (A. E. V. Zealley, 
I'v. G. Soc. S. Africa, xvii, 1916, pp. 60-74) in lenses in talc- 
schists, and as granular patches in serpentine. Formerly 
the largest production was in New Caledonia, from veins 
or pockets in serpentine, and residual surface accumulations. 
The Canadian chromite mines may be illustrated by that at 
Lake Caribou near Thetford, Quebec. The ore isin serpentine, 
in a 30-foot band which has been intensely altered and charged 
! A recent account of the commercial aspect of the Georgian de- 
posits is given by D. Zeretelli, Manganese Ore with Special Reference to 
Georgian Ore, London, 1928, 136 pp.