Lignite coal deposits of varying thickness underlie no less than 32,00o sq.m. of surface in the western half of the State. The U.S. Geological Survey estimates that of total lignite deposits of 965,902 million tons in the United States 633,329 million are in North Dakota, and about 5oo,000 million are capable of being mined. Beds 6 to 8ft. thick are common. In places they lie so close to the surface that the overlying soil can be stripped and the coal mined in open cuts ; in other places the room and pillar method is used. In 1934 there were 200 mines in operation of which 15o were underground. The coal output amounted to 1,753,888 tons valued at $2,363,000. The increase in production is shown by comparison with 416,580 tons in 1910 and 878,969 tons in 1920. An economically practicable briquetting process has been perfected which doubles the heat value of the coal, makes it less liable to deterioration and more convenient to handle.
Clay is found in a large number of varieties ranging from common brick through tile to high grade art pottery clay. De posits are large especially in the western part of the State where they are found near lignite beds, a proximity which enhances the value of both. Stone and gravel products in 1934 were valued at $130,813. The great gravel deposits of the State, found mainly in the ridges of glacial moraines, are becoming increasingly impor tant with the construction of modern highways.
corner diagonally north-westward to make a transcontinental con nection with the Canadian Pacific at the Canadian border. Branches of these main lines intersect the wheat-raising areas in every direction. The railroad mileage increased from 4,201 in 1910 to 5,311 in 1920 but fell to 5,276 by There were in 1934 106,846m. of rural roads and 7,6o3m. in the State highway system. Of the latter about 2,000M. were earth graded and provided with drainage and 5,878m. were surfaced with gravel. Motor vehicles registered increased from 144,972 in 1925 to 183,019 in 1930 and decreased to 164,217 in History.—Frenchmen of an expedition led by Pierre de la Verendrye crossed from the Assiniboine river in Canada to the Missouri river in 1738 searching for a "river to the west" and were probably the first Europeans in North Dakota. Two of Veren drye's sons returned in 1742 on the same mission but after months of wandering in the region south-west of the Missouri they also were forced to give up their quest and return. After 1790 this journey overland from the Assiniboine to the Missouri became common, parties of traders making the visit nearly every year to trade with the Indians for furs. Also after 1790 penetration of the Red river valley in eastern North Dakota was commenced by North-west and Hudson bay traders. Charles Chaboillez in 1797 established the first North Dakota trading post on the Pembina river near its junction with Red river, but it was soon abandoned. Alexander Henry, Jr. built a post in 18o0 near the mouth of Park river which he occupied until 18o8, and shortly after John Cameron built a post on the site of the present East Grand Forks.
After North Dakota, as a part of Louisiana territory, was acquired by the United States in 1803, exploration by that country commenced. The first Government expedition was that by Lewis and Clark which ascended the Missouri in 1804 and spent the winter at the Five Villages of the Mandan and Hidatsa tribes, pressing onward into Montana in the spring of 1805. Their return in 1806 stimulated the interest of St. Louis fur traders in the region, one of whom, Manuel Lisa, ascended the river in 1807 and opened trade relations with the Missouri Indians. His traders or those of his rivals thereafter ascended the river annually.