XII.] ORIGIN OF METALLIFEROUS DEPOSITS. 227 



gin to explain my meaning by the case of iron. This, you 

 are aware, is one of the most widely diffused elements in 

 nature ; all soils, all plants, contain it ; and it is a necessary 

 element in our blood. Clays and loams contain, however, at 

 best, two or three hundredths of the metal, but so mixed with, 

 other matters that we could never make it available for the 

 wants of this iron age of ours. How does it happen that we 

 also find it gathered together in great beds of ore, which fur- 

 nish an abundant supply of the metal 1 The chemist knows 

 that the iron, as diffused in the rocks, exists chiefly in combi- 

 nation with oxygen, with which it forms two principal com- 

 pounds : the first, or protoxide, which is readily soluble in 

 waters impregnated with carbonic acid or other feeble acids; 

 and the second, or peroxide, which is insoluble in the same 

 liquids. I do not here speak of the magnetic oxide, which 

 may be looked upon as a compound of the other two, neutral 

 and indifferent to most natural chemical agencies. The com- 

 binations of the first oxide are either colorless or bluish or 

 greenish in tint, while the peroxide is reddish-brown, and is 

 the substance known as iron-rust. Ordinary brick-clays are 

 bluish in color, and contain combined iron in the state of 

 protoxide, but when burned in a kiln they become reddish, 

 because this oxide absorbs from the air a further proportion 

 of oxygen, and is converted into peroxide. But there are clays 

 which are white when burned, and are much prized for this 

 reason. Many of these were once ferruginous clays, which 

 have lost their iron by a process everywhere going on around 

 us. If we dig a ditch in a moist soil which is covered with 

 turf or with decaying vegetation, we may observe that the 

 stagnant water which collects at the bottom soon becomes 

 coated with a shining, iridescent scum, which looks somewhat 

 like oil, but is really a compound of peroxide of iron. The 

 water as it oozes from the soil is colorless, but has an inky 

 taste, from dissolved protoxide of iron. When exposed to the 

 air, however, this absorbs oxygen, and the peroxide is formed, 

 which is no longer soluble, but separates as a film on the sur- 

 face of the water, and finally sinks to the bottom as a reddish 



