192 FEAGMENTS OF SCIENCE. 



of suspended 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 favourable 

 to the exhibition of the colour. As long as a wave 

 remains unbroken no colour 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 di- 

 rections; the light which passes through the higher 

 portion of the wave alone reaches the eye, and gives to 

 that portion its matchless colour. 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 con- 

 vertible. A feeble force acting through long periods, 

 and an intense force acting through short ones, may 

 produce approximately the same results. To Dr. 

 Hooker I have been indebted for some specimens of 

 stones, the first examples of which were picked up by 

 Mr. Hackworth on the shores of Lyell's Bay, near Well- 

 ington, in New Zealand. They were described by Mr. 

 Travers in the ' Transactions of the New Zealand In- 

 stitute.' Unacquainted with their origin, you would 

 certainly ascribe their forms to human workmanship. 

 They resemble knives and spear-heads, being apparent- 



