ORE-FORMATIONS 231 
It has never been mined in these rocks, but this is probably one of the 
sources of the tin-ore of “ placers.” 
Sulphides.—Pyrite and pyrrhotite appear now and again as in- 
gredients of certain igneous rocks, and chalcopyrite has also been 
recorded as occasionally occurring under similar conditions. While, in 
some cases, such metallic sulphides may be of secondary origin, there 
seems no reason to doubt that they are frequently primary constituents 
of the rocks in which they appear. It is highly probable, therefore, that 
the massive aggregates of sulphide ores met with in certain plutonic 
rocks are examples of magmatic segregation, and as truly syngenetic as 
the magnetic and titaniferous iron-ores referred to above. In some 
of the Norwegian gabbros, pyrrhotite, pyrite, and chalcopyrite, each 
containing a variable percentage of nickel and cobalt, are disseminated 
in small grains through the rock, but now and again they have segregated 
to form large masses of irregular form, which are grouped chiefly along 
the line of junction between the gabbros and the adjacent rocks (see Fig. 
81). Similar examples of the magmatic segregation of nickeliferous 
sulphide-ores are met with in Sweden, Piedmont, and North America. 
It is believed by some authorities that the auriferous pyrite of Rossland, 
British Columbia, and the high-grade copper-ores occurring in the 
peridotites and serpentines of northern Italy have originated in the same 
way. 
. 
2. ORES OCCURRING IN BEDDED ROCKS 
Under this head are included precipitates from aqueous 
solution, certain alluvial deposits, and ores interstratified with 
crystalline schists. 
(1) Precipitates from Aqueous Solution.—The most im- 
portant ores of this origin are iron- and manganese-ores. The 
iron-ores in question are well represented by the formations 
which are taking place now in marshy land and lakes. These 
consist essentially of hydrated ferric oxide, but usually contain 
many impurities. Sometimes they form continuous beds; in 
other places they occur as nodular concretions of some size, 
or as aggregates of odlitic and pisolitic spherules. They are 
the products of the alteration of ferriferous minerals and rocks, 
and owe their origin mainly to the action of water containing 
organic acids, which act as powerful solvents of iron-salts, 
Rocks exposed to the action of such acidulated water are 
bleached white by the removal of their iron, which is carried 
away in solution as a bicarbonate. From this solution, the 
iron tends to be precipitated as ferric hydrate, unless much 
decomposing organic matter be present; when such is the 


