434 Introduction to the Study of Science 



in the United States. The output in 1912 was little more than 

 10,000 long tons. The important iron ores to-day are those of 

 the oxid group. These are usually classified in three groups. 



The first of these groups is called the hematite. This is the 

 most valuable of the iron ores, the production of 1912 being more 

 than 51,000,000 long tons. It is composed of 70 per cent iron 

 and 30 per cent oxygen. 



The second group is the magnetite. It is made up of 72.4 

 per cent of metallic iron and 27.6 per cent of oxygen. Although 

 the richest in iron of the used ores, it is a very poor second in 

 output and industrial importance. The 1912 production was 

 less than 3,000,000 long tons. 



The third group of iron ores is called the brown hematite, 

 or simply the brown iron ores. They are distinguished from 

 the other groups chiefly by the fact that they contain water. 

 Because of their water content and the fact that they are found 

 in places that either are or had been swamp land, they are often 

 described as bog-iron ores. 



There are five distinct kinds of the brown ores, which form a 

 remarkable series on the basis of their regular increase in water 

 content. The first of the series contains about 5 per cent of 

 water, the second 10 per cent, and so on at regular intervals to 

 the fifth, which contains about 25 per cent of water. If now we 

 include the ordinary hematite, which contains no water, as the 

 first, we have a complete series from to 25 per cent water 

 content. This is an interesting example of the way in which 

 certain substances in nature combine in definite and multiple 

 proportions. The industrial importance of the brown iron ores 

 is shown by the 1912 production, which was about one half the 

 tonnage of the magnetite ores. 



209. Production by states. Iron ores have been found and 

 sometimes worked in every one of the states of our country; 

 but they are to-day produced in considerable quantities in only 

 about thirty states. These are included in four regions or dis- 

 tricts : first, the Lake Superior region, which includes Minne- 



