METAMORPHISM 379 
up a considerable supply of metal. An analysis of lava 
will always show much iron, and very often measur- 
able quantities of copper, lead, silver, ete. 
As the lava rocks decay at the surface, and enter 
into the formation of the sedimentary strata, this ore 
supply also goes into these accumulations; and with 
the later action of water, in favorable places, may ‘be 
gathered into beds which can be mined with profit. 
Still, where there is one such accumulation worth min- 
ing, there are many hundreds too poor to work. 
Classification of Ore Deposits. — We may divide ore 
deposits into three classes: (1) Erupted, (2) Mechanical, 
(3) Chemical. The latter, by far the most important, 
admit of much subdivision. 
Erupted Ore Deposits. While every lava contains goodly 
quantities of ore, this is usually too scattered to attract attention. 
In only a very few cases is this a source of extensive deposits, 
and these instances are mainly of iron. Some nickel deposits 
are believed to be of the same origin. In Greenland there are 
bunches of pure metallic iron in a lava. 
Mechanical Ore Deposits. As rocks decay, the more durable 
minerals resist disintegration. If for instance, a rock containing 
gold begins to disintegrate, since the gold is not readily de- 
stroyed, it endures in its metallic condition, while the other 
constituents of the rock crumble to bits. 
Being heavy, the gold resists removal by running water more 
than do the lighter minerals. So in being carried down stream, 
or in being washed backwards and forwards by the waves on a 
beach, the gold accumulates in pockets or layers, from which it 
may be mined with profit. This is the source of the stream 
