244 FRAGMENTS OF SCIENCE. 



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 direc- 

 tions ; 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 protuber- 

 ances 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 conver- 

 tible. 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 LyelPs Bay, near Wellington, in New 

 Zealand. They were described by Mr. Travers in the 

 6 Transactions of the New Zealand Institute.' Un- 

 acquainted with their origin, you would certainly ascribe 

 their forms to human workmanship. They resemble 

 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 



