326 Dr Davy's Observations on the 



Tho greater number of the experiments connected with this in- 

 quiry were conducted in the laboratory of Dr George Wilson, to 

 whom I am deeply indebted for the kind manner in which he has af- 

 forded me every assistance in his power, during the whole course of 

 the investigations. 



So)ne Additional Observations on the Superficial Colouring 

 Matter of Rocks. By John Davy, M.D., F.R.S.S. Lond. 

 & Ed. Communicated by the Author. 



In an excursion recently made into the wilds of Connemara, 

 my attention was recalled to the superficial colouring matter 

 of rocks, from certain marked contrasts of colour observable 

 in adjoining rocks of the same quality, but differently situ- 

 ated. These contrasts presented themselves most conspi- 

 cuously in the beds and banks of certain streams, and on the 

 shores of certain lakes, especially at and near their margin. 



Of the first mentioned, a good example occurs in the bed 

 and banks of the small mountain-torrent which falls into 

 Singalla Lake, immediately below Flynn's or Half- way- house 

 (the designation on the map of the county Gal way) on the 

 road between the town of Galway and Clifden. There, in 

 the bed of the stream, on the same rocks — a variety of mica 

 slate — at least four distinct colours are noticeable. Of these 

 one is almost white, in localities exposed to the full force of 

 the stream when highest and of most force, or when swollen 

 after heavy rains ; a colour belonging to the rock in its worn 

 and weathered state. Another is of a light red or reddish- 

 brown hue, which appears on rocks in the middle of the little 

 stream, such as are commonly under water, and where the 

 water runs rapidly, — a hue owing, in this instance, to a 

 slight deposition of peroxide of iron, constituting a superfi- 

 cial stain. A third is black and glistening, noticeable more 

 partially, in spots here and there, towards the margin of the 

 stream, and amongst the pebbles in its marginal rocky hol- 

 lows, — a colour resulting also from a superficial stain, but 

 produced chiefly by adhering peroxide of manganese. An- 

 other, the fourth colour, is also black, but with little or no 

 lustre, occurring on the marginal rocks of the stream, sub- 

 ject to alternations of wet and dry, according to the state of 



