The slow growth of the use of iron must be largely ascribed to the great difficulties in smelting it, especially when it occurs in impure forms. Iron does not occur in the pure state, as copper does to a small extent, but it is enormously abundant, being found, to a greater or less extent, in almost all rocks. Relatively pure ores are rare, most iron-bearing minerals containing a large number of impurities, some of which are very difficult to remove. Further, the process of smelting always requires much fuel, and, in the case of the more impure ores, remarkable skill and science. The result was that the early smiths could only employ a very high grade of ore; all others were useless to them. Even with a high grade of ore, they could only extract a relatively small amount of the iron present. A very curious little proof of this latter fact is furnished by the Roman iron workings in the Forest of Dean in Gloucestershire. The refuse thrown out of the ancient furnaces worked by the Romans here, was re-smelted by the British smiths long centuries afterwards, and this refuse fed their furnaces for a period of between two and three hundred years. The next point of interest in regard to iron is the source of the necessary fuel. At first wood or charcoal was always employed, and therefore iron could only be smelted in the vicinity of forests. Thus the Forest of Dean, already mentioned, supplied the wood used by the Romans in smelting, and the trees of the Weald or "wood" of Sussex and Kent were completely removed during the long centuries when the iron ores of that region were smelted. The Forest of Arden, near Birmingham, is another region where iron was long smelted by the aid of charcoal. The amount of fuel required, especially in the early days, was very great, and as the forests were cleared without any regard for scientific forestry, it naturally followed that in many districts the destruction of the necessary fuel led to a diminution of the industry. In England coal was not generally employed in the smelting of iron until after the middle of the eighteenth century, and long before that the British forests had been largely destroyed. The result was that the British iron industry had declined, and in the early part of that century considerably more iron was imported than was made in England. The countries which at this time were specially favoured in connection with the industry were those in which pure iron ores co-existed with extensive forests. This condition occurred especially in Germany, where the iron deposits formerly worked were those of the upland regions which have kept their forests till this day. Thus the wood and the ores of the Harz Mountains and of the Erzgebirge, or Iron Mountains, were of great importance before the industrial revolution, and up till the early part of the eighteenth century the German iron industry was more important than the British.