Superficial Colouriuy Matter of Rocks. 327 



the stream, whether high or low ; a colour, not like the two 

 preceding, owing to a mineral stain, but to the growth and 

 death of minute cryptogamic plants.* 



On the shores of the lakes of this pre-eminently lake dis- 

 trict, the differences in the superficial colouring of the rocks 

 are chiefly two or three, and depending mainly on the cryp- 

 togamic vegetable covering. Black is the prevailing colour 

 of the rocks, at the very margin of the lakes, whilst white, 

 in many instances, is as conspicuously prevalent in the higher 

 adjoining situations, out of the reach of water, when the 

 lakes are at their greatest height. On the shores and islets of 

 Derryclare Lake so distinguished for its beauty, and on those 

 of Lough Inagh, a neighbouring lake, good examples are to 

 be seen of rocks thus coloured ; the white by a lichen, 

 Lichen lacteus ; the black by one or more of the lower cryp- 

 togamia undergoing decomposition, and acquiring a peaty 

 character, to which, in all of this tribe under the influence of 

 moisture and a comparatively low temperature, there appears 

 to be so great a disposition, as is indicated in the vast ex- 

 tent of bog for which Ireland generally, and Galway espe- 

 cially, is so remarkable. 



In the examples mentioned, the instances of the several 

 kinds of superficial colouring are well defined, occurring to- 

 gether. In other localities, occasionally only one colour is 

 found predominant, as different shades of red where the rocks 

 and gravel are stained by the peroxide of iron ; or of brown 

 and black where they are stained both by the peroxide of 

 iron and by the peroxide of manganese. Of the former a good 

 example offers in the bed of the river descending through 

 Glen Inagh into the lake of the same name ; and of the latter, 



* In the bed of the same rivulet, nearer the lake, where its course is less 

 rapid, an example occurs of a conglomerate rock in the act of formation, which 

 may be worth mentioning, — the pebbles washed down, and there resting, 

 finding as it were a matrix in the clay into which they are cemented by car- 

 bonate of lime. The induration of this conglomerate is not considerable ; but 

 it is easy to imagine how it may be greatly increased, either by the deposition 

 of more carbonate of lime, the proportion present being very small, only just 

 sufficient to effervesce slightly with an acid, or by the action of heat, or other 

 metamorphic agency. 



