appeared in recent years, the explanation of their origin has, for the most part, been omitted or has been unsatisfactorily given. In this paper an effort is made to show how the more important North American natural bridges were formed.^[1]

In the older geologies and geographies we were taught that all natural bridges were formed in one and the same way. According to this time-honored theory natural bridges resulted from the partial caving in of the roof of an underground tunnel or cavern, the portion of the Fig. 2. The North Adams, Mass., Natural bridge. roof left spanning the chasm being a natural bridge. That natural bridges must occasionally be formed in this way is evident. For example, in Edmonson County, Kentucky, where the Mammoth Cave is situated, it is estimated that there are 100,000 miles of underground passages. In the course of time these passages will be widened and the rocks above them will be worn down by surface erosion until, at length, the roofs will almost completely disappear, leaving portions standing here and there as natural bridges. What is happening in Kentucky now has been going on for countless ages in limestone regions in other parts of the world with the possible formation (Fig. 1) and later destruction of natural bridges. It is a rather curious fact, however, that although many small natural bridges have this history, as, for example, a number of bridges in Florida, Iowa, Missouri and other states, yet, as far as known at present, none of the world's great natural bridges has this origin.

The Virginia natural bridge may be taken as a type of natural bridge formed by solution aided by cracks (joints). This can best be explained by a theoretical case. Let us suppose that a short distance—100 or 200 feet—above the brink of Niagara Falls the water of the river should find a crack athwart its course in the limestone bed of the river and that the water seeping through this crack should flow along the top of a lower layer, and reappear underneath the fall as a