The sandstone, which is the rock of most common occurrence about the rapids, very closely resembles that of Pittsburgh. It is commonly of a compact texture, having an argillaceous cement, with a laminated structure. At Shippingsport, and at Clarksville,[036] in Indiana, it is succeeded by bituminous clay-slate. While we were waiting at the rapids, several of the party made an excursion to visit the boiling spring, at the foot of the Silver Creek hills, in Indiana, at a little distance from New Albany.[037] This spring is small, discharging no water above the surface of the ground. It is an artificial excavation in the clayey bank of a small stream, called Fountain Creek. It is filled to the level of the water in the creek, the spring itself evidently discharging very little, if any water. That which fills the basin is turbid, being kept in constant agitation by the bubbles of inflammable air which rise through it. The {26} smell of sulphuretted hydrogen is perceptible at considerable distance about the spring; and a piece of silver, held near the surface of the water, was quickly tarnished. The Silver Creek hills are of argillaceous sandstone, and secondary clay-slate; and this spring seems to be placed near the meeting of the two strata. In the bed of the Ohio, opposite Shippingsport, is a tabular mass of rocks, visible above water for great part of the year, and called Corn Island.[038] On the highest parts of this, are remaining some small portions of the limestone stratum, which appears in many places to have been worn through, and removed by the river. Five or six acres of the surface of this island are of the smooth compact argillaceous sandrock before mentioned, lying horizontally, and divided into squares and parallelograms by the natural fissures. These fissures contain some soil which supports, in the summer, a dense growth of herbaceous plants. Among these, we noticed the hypericum sphæcrocarpum of Michaux, (apparently not the plant mentioned by Nuttall, under that name, which has been noticed near Philadelphia, by Collins and others, but without doubt that originally described by Michaux). Two species of andropogon, the panicum virgatum, solanum nigrum, polygala verticillata, leplanthus gramineus, chenopodium botrys, &c. The lower part of the island is covered with loose sand; bearing some small cotton-wood and willow trees.