220 THE POPULAR SCIENCE MONTHLY. 



water muddy by rendering the particles too numerous and gross. Such 

 a case occurred toward the close of my visit to Niagara. There had 

 been rain and storm in the upper-lake regions, and the quantity of sus- 

 pended matter brought down quite extinguished the fascinating green 

 of the Horseshoe. 



Nothing can be more superb than the green of the Atlantic waves 

 when the circumstances are favorable to the exhibition of the color. 

 As long as a wave remains unbroken, no color appears, but, when the 

 foam just doubles over the crest like an Alpine snow-cornice, under 

 the cornice we often see a display of the most exquisite green. It is 

 metallic in its brilliancy. But the foam is necessary to its production. 

 The foam is first illuminated, and it scatters the light in all directions ; 

 the light which passes through the higher portion of the wave alone 

 reaches the eye, and gives to that portion its matchless color. The 

 folding of the wave, producing, as it does, a series of longitudinal pro- 

 tuberances and furrows, which act like cylindrical lenses, introduces 

 variations in the intensity of the light, and materially enhances its 

 beauty. 



We have now to consider the genesis and proximate destiny of 

 the Falls of Niagara. "We may open our way to this subject by a few 

 preliminary remarks upon erosion. Time and intensity are the main 

 factors of geologic change, and they are in a certain sense convertible. 

 A feeble force, acting through long periods, and an intense force, act- 

 ing through short ones, may produce, approximately, the same results. 

 Here, for example, are some stones kindly lent to me by Dr. Hooker. 

 The first examples of the kind were picked up by Mr. Hackworth on 

 the shores of Lyell's Bay, near Wellington, in New Zealand, and de- 

 scribed by Mr. Travers in the Transactions of the New Zealand Insti- 

 tute. Unacquainted with their origin, you would certainly ascribe 

 their forms to human workmanship. They resemble flint knives and 

 spear-heads, being apparently chiselled off" into facets with as much 

 attention to symmetry as if a tool guided by human intelligence had 

 passed over them. But no human instrument has been brought to 

 bear upon these stones. They have been wrought into their present 

 shape by the wind-blown sand of Lyell's Bay. Two winds are domi- 

 nant here, and they in succession urged the sand against opposite 

 sides of the stone ; every little particle of sand clipped away its infini- 

 tesimal bit of stone, and in the end sculptured these singular forms. 1 



1 " These stones, which have a strong resemblance to works of human art, occur in 

 great abundance, and of various sizes, from half an inch to several inches in length. A 

 large number were exhibited, showing the various forms, which are those of wedges, 

 knives, arrow-heads, etc., and all with sharp cutting edges. 



" Mr. Travers explained that, notwithstanding their artificial appearance, these stones 

 were formed by the cutting action of the wind-driven sand, as it passed to and fro over 

 an exposed bowlder-bank. He gave a minute account of the manner in which the varie- 



