ORE-FORMATIONS 229 

rocks, while others are associated with derivative, and yet 
others with schistose, rocks. 
I. ORES OCCURRING IN IGNEOUS ROCKS 
Ores of this class are original or primary constituents, 
appearing sometimes as isolated grains or crystals, dissemin- 
ated through the body of a rock; at other times, as larger or 
smaller aggregates or masses which have obviously separated 
out from a molten magma. Not only ores but native metals 
occur under these conditions, more especially in plutonic basic 
igneous rocks. Acid plutonic rocks, on the other hand, are 
seldom rich in such constituents. 
(1) Native Metals.—Iron is irregularly disseminated 
through the basalt of Ovifak (Disco Island, West Greenland) 
in the form of scales, grains, nodules, and larger lumps and 
masses. Nickel-iron, in small grains, is met with in peridotite 
and serpentine in South Island (New Zealand). Platinum 
also occurs in similar small grains in peridotites, olivine- 
gabbros, and certain syenitic rocks in the Ural Mountains, 
and in peridotite in British Columbia. Gold, silver, copper, 
etc., have likewise been detected, generally as minute inclu- 
sions, in the constituent minerals of various igneous rocks— 
never in sufficient quantity, however, to invite mining 
operations. 
(2) Ores.—These are oxides and sulphides—the former 
being represented chiefly by magnetite, ilmenite, and 
chromite; and the latter by pyrite, pyrrhotite, and chalco- 
pyrite—each of which may contain variable percentages of 
nickel and cobalt. 
Oxides.—Magnetite, often titaniferous, is one of the commonest and 
most widely diffused constituents of igneous rocks. Now and again it 
forms massive aggregates in plutonic basic eruptives, as in certain 
gabbros, and gabbro-diorites in Sweden, Finland, Norway, and North 
America. In these rocks the mineral occurs disseminated in the usual 
way, along with other accessory constituents, but is so abundant as some- 
times to constitute a large percentage of the rock. Here and there, 
indeed, it becomes concentrated so as to form enormous aggregates. In 
some cases these aggregates are sharply marked off from the igneous 
rock in which they lie; in other cases, the disseminated ore gradually 
becomes more and more abundant at the expense, so to speak, of the 
other constituents of the gabbro, so that there seems to be, as it were, a 
passage from the latter into the ore-aggregate, Such aggregates are 
