NORTH AMERICAN NATURAL BRIDGES 419 



spring. In the course of years this underground water might eat out a 

 channel which, in time, might accommodate a small portion of the vol- 

 ume 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 "NTiagara 

 Eiver and probably never will be, but they were completed in the forma- 

 tion of the Virginia natural bridge, and a bridge of this sort is actually 

 in the process of formation in Two Medicine Eiver, 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 Avails. It is small as natural 

 bridges go, the top being but 41 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 im- 

 portant particulars. 



Across the Kicking Horse Eiver in the Canadian Bockies, 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 al- 

 most 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 cross- 

 ing the bridge is almost a horizontal S (see Fig. 3). This bridge was 

 formed largely by " pot-hole " action. Almost everyone in New Eng- 

 land 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 Eiver 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 per- 

 mitted the water of the river to flow through the opening thus made. 

 In other words, the natural bridge across the Kicking Horse Eiver is 

 the sides of which were worn through so that the holes opened into one 

 another. 



