Superficial Colouring Matter of Rocks. 329 



by the peroxide of manganese to the rocks and pebbles in 

 the beds of rivers on which it is found, to be owing to the 

 precipitation of the oxide from its state of solution as a sub- 

 oxide, on its becoming saturated with oxygen, and passing 

 into that of the peroxide, after the analogous manner in 

 which the stain by iron is produced in similar localities, and 

 under similar circumstances. The further observations I 

 have had an opportunity of making seem to corroborate this. 

 The superficial discolouration of rocks from the causes 

 assigned is, I believe, of wide extent, and consequently not 

 unimportant, considered merely in relation to the aspects of 

 nature. The mineral stains — the ochry of iron, and the rich 

 black of manganese, may be expected to be seen wherever 

 water impregnated with carbonic acid gas, — as all rain water, 

 the feeder of springs, more or less is,— percolates through, 

 before appearing at the surface, strata containing these 

 metals in the state of suboxide. And the dark vegetable 

 stain, that resulting from the partial and peculiar decom- 

 position essential to the formation of peat ; may be looked 

 for wherever the circumstances of average moisture and 

 temperature of climate are favourable to the production of 

 peat — a wide extent, comprising most parts of England, 

 Ireland, and Scotland, and the greater portion of the north 

 of Europe. These are not merely theoretical inferences ; 

 they are in accordance with many observations made both 

 in the Lake District of England and in the Highlands of 

 Scotland, and in the former much extended, as to localities, 



L bniil 9JlJ TO 890fl\6J2fll . cBjIIiBjUT 9 ft J lO .8070X1 



since I made the first communication on the subject, pub- 

 lished in a former number of the Philosophical Journal. 

 As regards the dark discolouration from decomposing ve- 

 getable matter, I may add that I have found it not only on 

 rocks, on the shores of lakes and moors, where the circum- 

 stances have favoured, but also on the sea-shore and on 

 inland precipices, where there has been a growth and de- 

 composition of minute cryptogamic plants. A good example 

 of the kind may be mentioned as occurring in the neighbour- 

 hood of Oban, at the entrance of Loch Etive, in Argyleshire. 

 Specimens of rocks, so discoloured superficially as to be of a 

 dead black, brought from thence, which I collected myself, 



