spring. In the course of years this underground water might eat out a channel which, in time, might accommodate a small portion of the volume of the river. This might eventually be enlarged to such an extent that all the water of the river would pass under the old bed of the river at the fall, leaving the present brink dry land; in other words forming a natural bridge under which the river would flow. The conditions above described have never been fulfilled in the case of the Niagara River and probably never will be, but they were completed in the formation of the Virginia natural bridge, and a bridge of this sort is actually in the process of formation in Two Medicine River, Montana. The height of a bridge of this origin will depend both upon the height of the original fall and upon the amount the stream deepens its valley after the formation of the bridge. The Virginia natural bridge is more than 200 feet high, but the original fall was probably less than that, since the stream has cut down its bed to some extent subsequent to the formation of the bridge.
Within the city limits of the manufacturing city of North Adams, situated in a valley which is beautiful in spite of the efforts of man to render it unsightly, is a natural bridge which well repays a visit. It is one of the most picturesque of natural bridges (Fig. 2) composed, as it is, of white marble with nearly vertical walls. It is small as natural bridges go, the top being but 44 feet above the stream bed and the cavity beneath only about 10 feet wide and 25 feet long. This bridge was formed somewhat as the one just described but differs in some important particulars.
Across the Kicking Horse River in the Canadian Rockies, a short distance from Field, B. C, amid some of the grandest scenery on the continent, within sight of primitive forests and glaciers, is a curious natural bridge and one which, at first sight, does not fulfill our conception of such a structure. In this case the opening is almost too small for the volume of the river, so that during floods the water probably flows over the top. The path which one follows in crossing the bridge is almost a horizontal S (see Fig. 3). This bridge was formed largely by "pot-hole" action. Almost everyone in New England has seen those interesting round holes which have been formed in the beds of swift streams by the whirling of pebbles in a permanent eddy until, after many years, a hole is bored which may be several feet in diameter and many feet deep. In the Kicking Horse River there was formerly a rapid or fall on which pot-holes were developed. These holes deepened and broadened at their bottoms until at length (Fig. 4) the walls of two of them were worn through near their bases and permitted the water of the river to flow through the opening thus made. In other words, the natural bridge across the Kicking Horse River is the sides of which were worn through so that the holes opened into one another.