However, the most wonderful saline is the great _Salt Rock_, which he found further to the {188} southwestward, on the main Red Fork. "The whole cove on the right of the two forks of the river," says Capt. Boone, "appears to be one immense salt spring of water so much concentrated, that, as soon as it reaches the point of breaking forth, it begins depositing its salt. In this way a large crust, or rock is formed all over the bottom for perhaps 160 acres. Digging through the sand for a few inches anywhere in this space, we could find the solid salt, so hard that there was no means in our power of getting up a block of it. We broke our mattock in the attempt. In many places, through this rock-salt crust the water boiled up as clear as crystal ... but so salt that our hands, after being immersed in it and suffered to dry, became as white as snow. Thrusting the arm down into these holes, they appeared to be walled with salt as far down as one could reach. The cliffs which overhang this place are composed of red clay and gypsum, and capped with a stratum of the latter.... We found this salt a little bitter from the impurities it contained, probably Epsom salts principally." As it is overhung with sulphate of lime, and perhaps also based upon the same, might not this 'salt-rock' be heavily impregnated with this mineral, occasioning its excessive hardness? Capt. Boone also speaks of gypsum in various other places, both north and south of this, during his travel. Mr. Sibley (then of Fort Osage), who was quite familiar with the western prairies, visited {189} a saline, over thirty years ago, which would seem to be the 'Salt Plain' first mentioned by Capt. Boone. The former, it is true, found the salt much more abundant than as described by the latter; but this may be owing to Capt. Boone's not having [Pg246] penetrated as far as the point alluded to by Mr. Sibley,--whose description is in the following language:[153] "The Grand Saline is situated about 280 miles southwest of Fort Osage, between two forks of a small branch of the Arkansas, one of which washes its southern extremity, and the other, the principal one, runs nearly parallel, within a mile of its opposite side. It is a hard level plain of reddish colored sand, and of an irregular or mixed figure. Its greatest length is from northwest to southeast, and its circumference about thirty miles. From the appearance of the driftwood that is scattered over, it would seem the whole plain is at times inundated by the overflowing of the streams that pass near it. This plain is entirely covered in dry hot weather, from two to six inches deep, with a crust of beautiful clean white salt, of a quality rather superior to the imported blown salt. It bears a striking resemblance to a field of brilliant snow after a rain, with a light crust on its top."