ARRAN. WATER. 331 



seuce of peat on the stony surfaces whence these waters 

 are collected, prevents them, at least in summer and in 

 seasons of ordinary dryness, from being contaminated by 

 the brown colour so general in the Scottish rivers. They 

 thus present a state of purity rarely met with in similar 

 situations ; nor is there any reason to doubt that they 

 are as free of contamination as rain water has ever been 

 found; certainly free from all soluble matter capable of 

 communicating its own hue to them. The quantity of 

 colour is such as to be perfectly sensible at the depth 

 of five or six feet : at ten or twelve it possesses consi- 

 derable intensity. On comparing it with sea water in 

 similar circumstances of depth, it appeared to be fully 

 as green. The whiteness of the bottom on which the 

 stream flows, is essential to this observation; and the 

 rarity of that occurrence in fresh waters, is probably 

 the reason that this phenomenon has been generally 

 overlooked; sometimes, even denied. The courses of 

 all these streams are on a bottom of white or grey 

 granite; and as long as they flow over that rock, the 

 colour is visible ; vanishing when they arrive at the schist 

 or the red sandstone, the powerful colours of which are 

 transmitted, so as entirely to obscure their natural and 

 delicate tint. The same rule holds in sea water, 

 which, within small depths, and at rest, is only green 

 when on a bottom of white sand. In greater depths the 

 colour varies, according as the effects arising from the 

 nature of the bottom, are modified by different circum- 

 stances ; the chief of which are, the varying intensity of 

 light and the different degrees of agitation. Under fa- 

 vourable circumstances of this nature, and of the spec- 

 tator's position, the brown colour of rocks at the bottom, 

 is transmitted through many fathoms of sea; a fact well 

 known to the mariner who, on unknown and rocky shores, 

 is stationed aloft to direct the course of the vessel. In 

 less favourable cases the colour of the bottom becomes 

 invisible at small depths, and the same mass of water 



