Further westward, the sandstone prevails; but some of the table plains are based upon strata of a sort of friable calcareous rock, which has been denominated 'rotten limestone:' yet along the borders of the mountains the base of the plains seems generally {185} to be of trap and greenstone. From the waters of Red River to the southwest corner of Missouri, throughout the range of the Ozark mountains, granite, limestone, flint and sandstone prevail. But much of the middle portion of the Prairies is without any apparent rocky foundation--we sometimes travel for days in succession without seeing even as much as a pebble. On passing towards Santa Fé in 1839, and returning in 1840, I observed an immense range of plaster of Paris, both north and south of the Canadian river, and between thirty and fifty miles east of the United States western boundary. The whole country seemed based upon this fossil, and cliffs and huge masses of it were seen in every direction. It ranges from the coarsest compact sulphate of lime or ordinary plaster, to the most transparent gypsum or selenite, of which last there is a great abundance. By authentic accounts from other travellers, this range of gypsum extends, in a direction nearly north, almost to the Arkansas river. [Pg243] Of metallic minerals, iron, lead, and perhaps copper, are found on the borders of the Prairies; and it is asserted that several specimens of silver ores have been met with on our frontier, as well as about the Witchita and the Rocky Mountains. Gold has also been found, no doubt, in different places; yet it is questionable whether it has anywhere been discovered in sufficient abundance to render it worth the seeking. Some trappers have reported {186} an extensive gold region about the sources of the Platte river; yet, although recent search has been made, it has not been discovered.[150] The most valuable perhaps, and the most abundant mineral production of the Prairies is _Salt_. In the Choctaw country, on the waters of Red River, there are two salt-works in operation; and in the Cherokee nation salt springs are numerous, three or four of which are now worked on a small scale; yet a sufficient quantity of salt might easily be produced to supply even the adjoining States. The _Grand Saline_, about forty miles above Fort Gibson, near the Neosho river, was considered a curiosity of its kind, before its natural beauties were effaced by 'improvements.'[151] In the border of a little valley, a number of small salt springs break out, around the orifice of each of which was formed, in the shape of a pot, a kind of calcareous saline concretion. None of the springs are very bold, but the water is strong, and sufficiently abundant for extensive works.