560 
THE FRESH-WATER LOCHS OF SCOTLAND 
June 1904 the temperature of the surface water was 60° "8 Fahr. 
(16" C), and of the bottom water, where the depth was only about 
427 feet, 40° '2 Fahr. (4° '9 C). In August 1873 Le Conte i found the 
temperature of the surface water to be 66°'9 Fahr. (19° '4 C), falhng 
to 41° Fahr. (5° C.) at 772 feet, and to 39°-2 Fahr. (4° C.) at the 
bottom in about 1506 feet. The winters in this region are usually 
severe, so that the air probably remains far below the freezing point 
for a considerable period each year. Notwithstanding this fact, 
however, ice never forms on the lake except in the shallow bays. The 
water is very transparent ; a Secchi disc just disappeared from view 
at a depth of 66 feet (20 metres). When most of the snow has dis- 
appeared the transparency is said to be much greater, white objects 
being easily seen at a depth of more than 98 feet, and Le Conte 
records that in August 1873 he found that a white plate was still 
visible at a depth of 108 feet. 
Mexico and Mexico contains two types of inland drainage areas. The most 
Amerfca interesting is that in the volcanic highlands of Anahuac, in the latitude 
of the ci ty of Mexico, where the lakes are enclosures in the irregular 
topography of the piled-up volcanic material ; but this is now no 
lon^rer a true inland drainao;e area, for by means of immense artificial 
water-ways the lakes lying within the depression have been made to 
drain, as we shall presently have occasion to point out, with some 
detail, into the Gulf of Mexico. The principal lakes of this area are 
Tezcuco, near the shores of which the city of Mexico now stands, 
and Chalco. Both are noteworthy for their magnificent scenery, 
surrounded as they are by volcanic peaks and extinct craters of great 
height; but they, like the other lakes in the same basin, have shrunk 
greatly in size. The depth of water in Lake Tezcuco at the present 
day under normal conditions hardly exceeds 2 feet over a large part 
of its area. 
The ancient town of Tenochtitlan, formerly standing on the site 
of the modern Mexico (7524 feet above sea-level), was actually 
founded in the lake, like another Venice, cut up by canals and con- 
nected with the shores by narrow viaducts and bridges. It was built 
there by Aztec immigrants in order that they might defend themselves 
against surrounding enemies ; but, on gaining power and riches, they 
set to work trying to drive back the waters from their city by means 
of great dams, and thus free themselves from the danger of destruction 
by floods. They were only partly successful in this. After the 
complete destruction of the city at the conquest in 1521, Cortez 
built up the city of Mexico on the ruins of Tenochtitlan ; and, partly 
by means of dams, partly by the turning aside of streams, Mexico 
1 Cited by Juday, op. cit, p. 791. 
