266 SALMONIA. 



decaying vegetable 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 Septem- 

 ber, I found the character of the lakes en- 

 tirely 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 percepti- 

 ble 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 impuri- 

 ties in great lakes. Sometimes, though 

 rarely, mineral impregnations give colour to 

 water : small streams are sometimes green or 



