276 SALMONIA. [NINTH DAT. 



matter, it had a tint of faded grass green ; and these 

 changes had occurred in a space not much more than 

 a mile in length. These observations I made in 

 1815 : on returning to the same spot twelve years 

 after, in August and September, I found the 

 character of the lakes entirely changed. The pine 

 wood washed into the second lake had disappeared ; 

 a large quantity of stones and gravel, washed down 

 by torrents, or detached by an avalanche, supplied 

 their place : there was no perceptible difference of 

 tint in the two upper lakes; but the lower one, 

 where there was still some vegetable matter, seemed 

 to possess a greener hue. The same principle will 

 apply to the Scotch and Irish rivers, which, when they 

 rise or issue from pure rocky sources, are blue, or 

 bluish green; and when fed from peat bogs, or 

 alluvial countries, yellow, or amber-coloured, or brown 

 even after they have deposited a part of their 

 impurities in great lakes, Sometimes, though rarely, 

 mineral impregnations give colour to water : small 

 streams are sometimes green or yellow from ferruginous 

 depositions. Calcareous matters seldom affect their 

 colour, but often their transparency, when deposited, 

 as is the case with the Yelino at Terni, and the Anio 

 at Tivoli ; but I doubt if pure saline matters, which 

 are in themselves white, ever change the tint of 

 water. 



