98 f!eol(jyical Vhanyts from Alteration of the 



colour of water is a peculiar and not accidental characteristic 

 of that substance. This natural colour of water will also 

 afford us an easy explanation of a light green tint which is even 

 more strongly manifested in the crystal-like siliceous springs 

 of Iceland than in the lakes of Switzerland ; for the yellow 

 colour derived from traces of the hydrated oxide of iron, in 

 the siliceous sinter walls surrounding the water, blends with 

 the original blue to produce the same gresenish tint, which, 

 in the Swiss lakes, is derived from the yellow bottom ; the 

 most different rocks experiencing a superficial decomposition 

 from the continued action of water, and becoming tinged with 

 yellow by the formation of hydrated oxide of iron. Hence it 

 will be easily conceived that the blue, which continues to in- 

 crease in intensity with the increased depth of the strata of 

 water, may obliterate the action of this yellow reflection, and 

 thus either weaken or wholly destroy this greenish tint. The 

 green grotto on the shores of Capri affords a most striking 

 proof of this fact. The green colour, which is produced by 

 the reflection at an inconsiderable depth of water, from the 

 yellowish limestone constituting the bottom and the walls of 

 the grotto, illuminated by the light from without, wholly dis- 

 appears in the enormous depths of the water of the blue 

 grotto ; there a pure blue colour takes the place of the green, 

 observed in the shallower cavern, although the water and 

 rocks are the same in both cases.* 



Geological Changes from Alteration of the Earth's Axis of 

 Botation.\ 



Respecting a possible change of climate resulting from a change 

 in tlie earth's axis of rotation, — an hypothesis which has fi'om time 

 to time engaged attention, as one which might serve to account for 

 the occurrence of organic remains, supposed to be those of animals 

 and plants requiring a higher temperature than tliat of the regions 

 where such remains are found, we have had two communications. 

 In one from Mr SauU, he calls attention to the undoubted evidences 



* Vide Works of the Cavendish Society, vol. i., 1848. 



t Sir H. J. ))e la Beches (President of the Geological Society) Anniversary 

 Address to the Society for 1849. 



