THE FORMATION OF DEPOSITS 31 
SWamps by the reduction of metallic salts by decomposing 
Tganic matter or bacteria; thus are formed fp oo and 
ores which are periodically dredged in Swedish a on that 
umerous beds of iron ore formed in swamps, suc as, Pa 
at the Mesa de los Pinos at Rio Tinto. The gossans or ficial 
hats” that cover most lodes are also due to the die 
formation of iron oxide. The iron pyrites in 2 ods 1s ox in 
by descending rain-water and is removed in soution a o 
“he quarts fy] of cavities or, as the miners descri ® wn 
MOuse-eater the insoluble veinstones are stained gona 
by the iron oxide formed from the pyrites. Some po! or 
Minera]g leached out of the gossan may be carrie Ss 
Along t}¢ lode (Fig. 10), and on reaching water-level may 
Fig, 10.—~Di1acran ILLUSTRATING SECONDARY 
. Exricuament, 
Diagram illustrating secondary enrichment ; 
8, former surface of the country, which, 
with the top of the lode, ¢f, has been removed 
%Y denudation, Below the present surface, 
od, the rocks have been decomposed to the 
depth of thewater-table, WT. The upper part 
of the Jode has been altered to a gossan with 
Soncentrateq tich ore. Below the water- 
able is a fyrgh op secondary enrichment, below 
¥hich the lode continues with primary low- 
Brade ore. 
ry 
& 
rd 
- 
ToPrecipitageq as iron sulphide with metallic gold, or oe 
“OPper Sulphide, such ag chalcocite, or pockets of silver- ca 
2; thus haye been formed those secondary enrichments, 
Thich have yielded many of the richest prizes in mining 
1story, . 
Bp, MINERAL Drposits—The complete evaporation 
of lakes and lagoong may give rise to thick widespread beds 
of salts, The largest of them are formed by the evaporation 
*f arms of the sea, in which the salts are precipitated in order 
of their insolubility— gypsum rock salt, and potassium and 
Magnesium chlorides, which remain in solution till the last 
of the bittern (P. 206) has evaporated. Beds of salt are also 
formeg by the evaporation of lakes; the kind of salt depends 
“Pon the Composition of the adjacent rocks; thus potash salts 
ine depositeq Where the rocks contain much potash felspar. 
Miner, sheets or seams may also be formed by plants and 
Ama] Which groy on the earth’s surface or in shallow waters