The fighting forces in the northeast, where the plain slopes gradually into the Suwalki Province, must pass over a country dotted with lakes and lagoons, which farther on take on the character of marshes, stagnant ponds, peat bogs, with small streams flowing lazily from one to the other. Here and there are patches of stunted pine forests, with occasional stretches of fertile, cultivated soil. Throughout this section many rivers flow along broad, level valleys, separating into various branches which form many islands and, during the rainy seasons, flood the surrounding country. Farther west the armies pass through broad valleys or basins, once the beds of great lakes, whose rich, alluvial soil give forth abundant crops of cereals. Here, too, flows the Niemen, 500 miles in length, watering a basin 40,000 square miles in area and separating Poland from Lithuania. It advances northward in a great, winding pathway, between limestone hills covered with loam or amid forests, its banks rising to high eminences in places, past ruined castles built in the Middle Ages. In the yellowish soil along its banks grow rich crops of oats, buckwheat, corn, and some rye. Naturally such a section would be thickly populated, not only on account of the fertile soil, but because the Niemen, like the Vistula, is one of the country's means of communication and transportation. As many as 90,000 men earn their livelihoods in navigating the steamers and freight barges passing up and down this great waterway. At Yurburg the Niemen enters East Prussia on its way to the Baltic. CHAPTER XLII THE STRATEGIC VALUE OF RUSSIAN POLAND It is in the southern part of Russian Poland, among the foothills of the Carpathians, that the armies come into possession of its mineral resources, a fact which will have some influence on the German military movements in this region. Up in the Kielce hills copper has been mined for 400 years, though the value of these mines has decreased on account of the much greater quantity found in America. A hundred years ago the Kielce mines produced nearly 4,000 tons of copper a year. Brown iron ore is also found here in deposits 40 per cent pure, while there are also veins of zinc sometimes 50 feet thick, yielding ore of 25 per cent purity. Sulphur, one of the ingredients for the manufacture of explosives, is found at Czarkowa in the district of Pinczow. In the southwest, in Bedzin and Olkuz, there are coal deposits about 200 square miles in area. In the southern districts wheat is also grown in some abundance.