of Edinburgh, Session 1871 - 72 . 
567 
bottle, during six months. But there is a more potent agent at 
work in the Yarrow. The dark, green-coated stones of the loch, 
with all their characters unreduced, pave the entire channel of the 
stream as low at least as the confluence of the Douglas Burn, and, 
with a less abundant covering, so low at least as Yarrow kirk, 
seven miles from the outlet of the lake. But there is nothing of 
the kind in the chief tributaries. At the junction, for example, 
of the Douglas Burn, there is an abrupt line of demarcation be¬ 
tween the dark green, slippery stones of the Yarrow, and the stones 
of the tributary, which are as naked as if they had been scrubbed 
clean with a brush. I do not well see how to escape the conclu¬ 
sion, that the confervac and diatoms of the stones live at the cost 
of the peaty matter from the loch,—that peat-extract is their food 
and is consumed by them. This is a ready explanation of their 
excessive growth on the stones of the loch. The want of such 
food equally explains the comparative absence of them from the 
stony banks of Loch Lomond, and the stony channels of all the 
streams of the adjacent mica-slate district.* Indeed, in the 
opposite circumstance'—in some mountain tarns of the district, 
resting, as they may, on peat, and surrounded by it—the slippery, 
dark green, stony bottom is no uncommon occurrence. 
If these views be correct, it is easy to appreciate both the un¬ 
favourable significance in a lake of a dark-green bottom of stones, 
densely covered with confervm and diatoms, and likewise their 
value in a running stream; and it may be well also not to let the 
imagination run away luxuriating in every “silver strand” that 
meets the eye. 
The Temperature of the Deep Fresh-water Lakes of this country 
has no connection with the preceding inquiries, further than that 
my observations on the subject arose incidentally while I was 
carrying on the inquiries in question. The results I have obtained 
may interest the cultivator of physical geography, if I am right 
* It lias been said that stones covered with green confervse and other 
diatoms do occur in Loch-Lomond. They do in bays and other shallows; 
but the covering is very thin ; and the line of such stones is narrow. Where 
deep water is near there are none at the edge, and where they do occur the 
dry stones close to the edge appear quite clean. 
