CONCLUSIONS 337 



(5) That it is unsafe to make a definite statement as to the origin of a 

 natural bridge, even when the rock and topography are known, without a 

 careful study in the field. 



(6) As concerns the stage of erosion, it may be said in general that 

 they may occur in all stages from youth to advanced maturity or early old 

 age. 



If natural bridges were to be classified according to the stage of erosion, 

 it would be found that (a) those formed by gravity and by deposition are 

 likely to be found in early stages of valley erosion; (&) those initiated by 

 solution, as in C 1, are more likely to occur in the youthful portion of a 

 valley, or, as in C 2, in an advanced stage of the erosion of a region; (c) 

 those formed by the perforation of the neck of an incised meander are 

 likely to occur in a submature stage of valley development, either in a 

 new cycle following an advanced stage of a previous cycle or in a more 

 resistant stratum upon which the stream meandered temporarily; (d) 

 those formed by pot-hole action may occur in any stage between youth 

 and advanced maturity, and that (e) sea erosion bridges are more likely 

 to occur only in rather early energetic stages of sea erosion. 



Catalog of North American Natural Bridges 



The following is a catalog of North American natural bridges, their 

 location, and the character of rock of which they are composed : 



Alabama. — A bridge composed of conglomerate (millstone grit) is re- 

 ported from Marion County. 



Arizona. — (1) The travertine bridge near Pine. 



(2) Bridge formed of a petrified log near Adamana. 



(3) A bridge near the New Mexico line reported by J. 



Gardiner, Jr.^^ 

 Canada. — The limestone bridge across the Kicking Horse Kiver near 

 Field, British Columbia. 



California. — (1) The Santa Cruz wave-cut bridge composed of shale. 

 (2) A bridge across Volcano Creek, a tributary of the 

 Kern in the Sierras. 

 Florida. — A number of bridges occur in the limestone of this State, of 

 which four are described in this paper. 



Indiana. — A sandstone ( ?) bridge near Attica. 

 Iowa. — Limestone bridges in Jackson County. 



Kansas. — Two gypsum bridges are described in volume v, 1899, page 

 73, plates 26, 27, of the Kansas Geological Survey. The best of these 



31 Science, vol. 6, 1885, p. 67. 



