Query V.

feafons to turn a grift-mill, though its fountain is not more than two miles above*^ * Don Ulloa mentions a break, fimilar to this, in the province of An- garaez, in South America. It is from i6 to 22 feet wide, iii feet deep, and of 1.3 miles continuance, Englifh meafure. Its breadth at top is not fen(ibly greater than at bottom. But the following fail is remarkable, and will furnifh fome light for conje£luring the probable origin of our natural bridge. < EUa caxa, 6 cauce efta cortada en pena viva con tanta preclfion, que las defigualdades del un lado entrantes, correfponden a las del otro lado falientes, como ft aquella altura fe hubiefe abieito exprefameute, con fus bueltas y tortuofidades, para dark tranfito a los aguas por entre los dos mural -• lones que la forman ; fiendo tal fu igualdad, que li llegafen a juntarfe fe en- dentarian uno con otro fin dexar hueco.' Not. Amer. II. § 10. Don Ulloa "inclines to the opinion, that this channel has been effefted by the •wearing of the water which runs through it, rather than that the moun- tain fhould have been broken open by any convulfion of nature. But if it had been worn by the running of water, would not the rocks which form the fides, have been worn plane? or if, meeting in fome parts with veins of harder ftone, the water had left prominences on the one fide, would rot the fame caufe have fometimes, or perhaps generally, occafioned prominences on the other fide alfo ? Yet Don Ulloa tells us, that on the other iide there are always correfponding cavities, and that thefe tally with the prominences fo perfedly, that, were the two fides to come together, they would fit in all their indentures, without leaving any void. I think that this does not refemble the effedl of running water, but looks rather as if the two fides had parted afunder. The fides of the break, over which is the natural bridge of Virginia, confifting of a veiny rock which yields to time, the correfpondence between the falient and re-entering inequalities, if it exifted at all, has now difappcared. This break has the advantage of the one defcribed by Don Ulloa in its fineft circumftance; no portion in that infiance having held together, during the feparation of the ether parts, fo as to form a bridge over the abyfs.