On the other hand some iron mines now in operation are as much as two thousand feet in depth. In these mines, as in coal mines, pillars are left to support the rock above. A roof of the iron ore is often left also. The low-grade ore is left in the ground and no effort is made to preserve it for future use. These constitute the principal waste in iron mining. The pure iron of the ore is separated by washing out the clays and soft elements, but the harder substances must be smelted by means of heat. In the beginning this was done by charcoal, which is still used in Sweden. The latest method is to employ electricity manufactured by water-power, but most of the iron smelting in this country has been done by coal. Every ton of iron smelted requires its portion of coal for firing. If low-grade fuels in gas-producer engines, or water-power can be used it will be a great aid in conserving coal. If a limited supply of rather low-grade iron exists near a coal region, it can often be mined profitably, when, if it be far from an abundant fuel supply, it must be shipped to distant blast furnaces. The cost of shipping causes ore containing a small percentage of iron to be classed as "not available." Sometimes a large company with many mines has several varieties of ore of different strength and hardness. If these can be mixed to produce a medium grade by adding a small amount of high-grade ore to a large amount of lower grade, the value of the product will be doubled. Sometimes, too, the by-products can be made extremely profitable by manufacturing large amounts when the expense of undertaking the work is too great to be attempted with a small amount. So if iron mines are owned by a small company much ore may be classed as "not available" that could be used by a large company. All these things must be considered in estimating the iron resources. The first smelting of iron ore in this country was done at Lynn, Massachusetts, in 1645, using the low-grade bog-ores and smelting with charcoal from the surrounding forest. Now if we look over an iron map of the United States we shall find that there are four hundred and eighty blast furnaces, but that only nine of them are west of the Mississippi River and most of these are in Missouri. The greatest of all the iron regions now lies in upper Michigan and Minnesota. This furnishes eighty tons out of every one hundred mined in the United States, but the smelting is done along the southern shores of Lake Michigan. The reason for this is that the iron region itself is far distant from a cheap fuel supply. Pittsburg, Pennsylvania, has been the great iron city of the United States on account of its nearness to great supplies of both coal and iron. Birmingham, Alabama, is the heart of the great smelting region of the South. The iron is divided into districts as follows: