For a century past, however, it has remained nearly in the present form; and as the ebullition of the water at the bottom of the cataract is so much greater at the center of this fall than in any other part, and as the water consequently acts with more force there in undermining the precipice than at any other part, it is not unlikely that it may remain nearly in the same form for ages to come. [Sidenote: NIAGARA FALLS.] At the bottom of the Horse-shoe Fall is found a kind of white concrete substance, by the people of the country, called spray. Some persons have supposed that it is formed from the earthy particles of the water, which descending, owing to their great specific gravity, quicker than the other particles, adhere to the rocks, and are there formed into a mass. This concrete substance has precisely the appearance of petrified froth; and it is remarkable, that it is found adhering to those rocks against which the greatest quantities of the froth, that floats upon the water, is washed by the eddies. We did not think of ascending the cliff till the evening was far advanced, and had it been possible to have found our way up in the dark, I verily believe we should have remained at the bottom of it until midnight. Just as we left the foot of the great fall the sun broke through the clouds, and one of the most beautiful and perfect rainbows that ever I beheld was exhibited in the spray that arose from the fall. It is only at evening and morning that the rainbow is seen in perfection; for the banks of the river, and the steep precipice, shade the sun from the spray at the bottom of the fall in the middle of the day.