CHARACTERISTTCS AND DISTRIBUTION OF LAKES 591 
of 82 square miles. The maximum depth is about 1220 feet, the 
mean depth 574 feet, and it is estimated to contain about 1,310,000 
milhon cubic feet of water. 
Lake of Varese lies 778 feet above sea-level, and covers an area 
of 6 square miles. The maximum depth is 85 feet, the mean depth 
36 feet, and it is estimated to contain about 5722 million cubic feet 
of water. 
Lake of Lugano lies at an elevation of about 889 feet above sea- 
level, and covers an area of 19 square miles. It has a maximum 
depth of 945 feet, a mean depth of 427 feet, and is estimated to 
contain about 231,699 million cubic feet of water. 
Lake of Como lies at an elevation of about 653 feet above sea- 
level, the variation in the water-level amounting to as much as 16 feet, 
and covers an area of 56 square miles. It has a maximum depth of 
1345 feet, a mean depth of 513 feet, and is estimated to contain 
about 794,700 million cubic feet of water. A bottom temperature 
of 42° -8 Fahr. has been recorded. 
Lake of Iseo lies 610 feet above sea-level, and covers an area of 
23 square miles. The maximum depth is 823 feet, the mean depth 
403 feet, and it is estimated to contain about 268,000 million cubic 
feet of water. 
Lake of Garda lies 213 feet above sea-level, and covers an area 
of 143 square miles. The maximum depth is 1124 feet, the mean 
depth 446 feet, and it is estimated to contain about 1,766,000 million 
cubic feet of water. 
The great rivers, the Rhine and the Rhone, have their origin in Rivers Rhine 
Switzerland, and the only important lakes drained by these rivers or Rhone, 
their tributaries occur in that country (see fig. 72). The explanation 
of this fact lies in the changes which Switzerland has undergone. 
The Swiss rivers are of very different ages, some being of comparatively 
recent origin, while others date back to very great antiquity, and parts 
of what is now considered a single river differ in age and history. The 
whole drainage of Switzerland north of the Alps originally found its 
way by the Danube to the Black Sea, and only after the subsidence 
which separated the Vosges and the Black Forest did the waters of 
the Rhine flow northward. After that the waters of the Rhone still 
joined the Rhine, and ran over the plains of Germany to the North 
Sea, till finally the Rhone broke its way by Fort de TEcluse, and 
falling into the Saone flowed to the Mediterranean. Another general 
change in the river-system is due to the fact that the watershed has 
retreated northward, because, the southern slope being much steeper 
than the northern slope, the Italian rivers have the greater power of 
erosion, and are gradually eating their way back. The way in which 
