ORE DEPOSITS 



375 



the streams, the heavy gold particles quickly sank to the bed of the 

 streams, while the lighter minerals were borne on by the current. 

 In this way much of the gold contained in a large quantity of rock 

 has sometimes been concentrated in a small area. It consequently 

 happens occasionally that rich placer deposits are found in regions 

 in which none of the rock contains gold in sufficient quantity to pay 

 for its extraction. 



When conditions are favorable, gold-bearing (auriferous) gravels 

 are worked by dredging, even when the gravel yields only twenty- 

 five or thirty cents to the cubic yard. Ancient gravels which have 

 been buried beneath sheets of lava are sometimes mined for their 

 gold (Fig. 359 £)• 



Sedimentary Iron Deposits. — Extending in a broken belt from 

 Nova Scotia and New York to Alabama, beds of iron ore (Clinton 



Fig. 360. — Iron deposits in the Lake Superior region, Mesabi Range, Minnesota. 



(U. S. Geol. Surv.) 

 • 

 iron ore) occur which have the same position and much the same 

 character as other sedimentary beds, and in some cases contain ma- 

 rine fossils. These beds of iron ore may have been precipitated 

 from salt or from fresh water, just as iron is being deposited to-day 

 in fresh-water ponds and lakes. The iron contained in small quan- 

 tities in the rocks (usually igneous) of the land is leached out by per- 

 colating waters in the form of ferrous compounds. These compounds 

 upon exposure to the air are oxidized and ferric oxide (Fe 2 Os) is 

 precipitated, usually in the form of limonite (2 Fe 2 3 • 3 H 2 0). In this 

 way iron accumulates in bogs and is called bog ore, and similar deposits 

 are laid down in lakes. Another suggestion which, however, is not 

 widely accepted, is that the Clinton iron ore has been derived from 

 lavas rich in iron minerals which were extruded beneath the sea. 



The great iron deposits of the Lake Superior region (Fig. 360) 

 are believed to have been accumulated in beds as impure iron car- 

 bonates and silicates, too low in iron to pay for their extraction. 

 When the deposits were- uplifted to form land, they were exposed 



