especially important to copper mining where new methods

of concentration such as the flotation process, intro-

duced in the United States in earnest after 1910,

permitted the working of very low-grade copper ores.

In this process, ores were finely ground and put into

an oily liquid bath. Air was then let into the bottom

of the container, and oily bubbles collected the metal,

which had an affinity for the oil, and brought it to

the surface in the form of a froth; non-metallic material

remained behind as sediment.

 The second major advance, open pit mining,

eliminated the need for underground lighting and

ventilation, for expensive timbering, and the other

high cost operations and paraphernalia required in

underground mining.

 The introduction of open pit, non-selective

mining techniques, coupled with the introduction of the

flotation process, permitted extensive but low-grade

copper deposits to be mined at reasonably low cost;

changing techniques in mining and refining were

complementary.

 In iron mining, too, open pit operations greatly

increased productivity. The unique feature of Mesabi

ore in particular was its softness and proximity to the

surface where it could be "mined" with steam shovels.

Thus, it was not only the great extent of the rich ore

deposits that made the Mesabi so valuable; their closeness

to the surface made them more valuable still.