294 Dr. CO. S. Du R. Preller—Lake Zurich and Wallen. 
which subsequently facilitated the intrusion and purely abrasive 
action of the glaciers ; and these were, therefore, the secondary, but 
not the primary agency; or, as Prof. Bonney aptly expresses it, the 
glaciers must be regarded as the file rather than the chisel of nature.' 
2. The superficial area of the Wallen-lake and that of Zurich is 9 and 
34 square miles; their mean depth is 500 and 280 ft. respectively, and 
their average contents may therefore be computed at 3500 million 
and 6000 million tons (or cubic metres) of water respectively. It is 
evident that the Wallen-lake acts in relation to the Lake of Zurich, 
as a vast storage and clearing reservoir of the waters descending 
from the Alps; and its purifying action is rendered all the more 
effectual by the equable discharge of the Alpine rivers due to their 
origin in the glaciers. In both lakes, but more especially in the 
larger’ Lake of Zurich, the natural purification of water is effected 
by the precipitation of impure matter, favoured by the slow motion 
of the water; by extensive oxidation promoted by the large surface 
of water in contact with the air; by the water being kept in motion 
by occasional storms, and in the Lake of Zurich, moreover, by 
navigation; by solar heat, which extracts oxygen from aquatic 
plants; by an aquatic fauna of great variety, which collects at the 
outflow of drains and streams and feeds on the impure matter with 
which they are charged; and lastly, by the innumerable quantities 
of voracious micro-organisms living at the bottom of the lake. The 
extremely slow motion of the water through the lake as a purifying 
agency is evidenced by the fact that, the mean discharge from the 
lake into the river Limmat at Zurich being eight million tons per 
day, and its contents being 6000 million tons, it takes 750 days, or 
more than two years, before the water flowing into the lake at the 
upper end reaches the point of outflow at the lower or Zurich end. 
It is owing to this, in conjunction with the other chemical and 
mechanical agencies enumerated, that the water of the Lake of 
Zurich contains, on an average, only 170 bacteria (collectively about 
the size of the head of a small pin) per cubic centimetre, and 
that of the other Swiss lakes has been found to contain even less, 
e.g. the Lake of Constance 58, that of Lucerne 50, and that of 
Geneva only 38; whereas the spring water of the Zurich hills, 
especially after rain, often contains as many as 2000 bacteria per 
cubic centimetre. Having regard to the comparative size and 
depth of these several Alpine lakes, it may be legitimately inferred 
that, generally speaking, the larger their superficial area and the 
greater their depth, the more effectual is the natural process of 
purification of the water contained in them. 
3. An instructive instance of the deviation of water-courses as 
the result of geological changes is afforded by the river Sihl, which 
rises in the Glarner Alps of the Canton of Schwyz, beyond the Con- 
vent of Hinsiedeln, about thirty miles from Zurich. This river at 
one time emptied itself direct into the lake of Zurich near the upper 
end, not far from the island of Ufenau and opposite Rapperswil, 
where the accumulated detritus brought down by it projects into the 
1 The Building of the Alps, Royal Institution, 1884. 
