LAKES—INUNDATIONS. 363 
or 168 square miles. The lake of Xochimilco con- 
tains 494, that of Tezcuco 77, of San Christobal 274, 
and of Zumpango 9,°,, square miles. The valley 
itself is a basin enclosed by a wall of porphyritic 
mountains, and all the water furnished by the sur- 
rounding cordilleras is collected in it. No stream 
issues from it excepting the brook of Tequisquiac, 
which joins the Rio de Tula. The lakes rise by 
stages in proportion to their distance from its centre, 
or, in other words, from the site of the capital. Next 
to the lake of Tezcuco, Mexico is the least elevated 
point of the valley, the plaza mayor or great square 
being only 1 foot 1 inch higher than the mean level 
of its water, which is 112 feet lower than that of 
San Christobal. Zumpango, which is the most 
northern, is 29°211 inches higher than the surface 
of Tezeuco ; while that of Chaleo, at the southern 
extremity, is only 3:632 feet more elevated than the 
great square of Mexico. 
In consequence of this peculiarity the city has 
for a long series of ages been exposed to inundations. 
The lake of Zumpango, swelled by an unusual rise 
of the Rio de Guautitlan, flows over into that of San 
Christobal, which again bursts the dike that sepa- 
rates it from Tezcuco. The water of this last is con- 
sequently augmented, and flows with impetuosity 
into the streets of Mexico. Since the arrival of the 
Spaniards the town has experienced five great floods, 
the latest of which happened in 1629. In more 
recent periods there have been several alarming ap- 
pearances, but the city was preserved from any ac- 
tual loss by the desague or canal, which was formed 
for the purpose. 
The situation of the capital is more exposed to dan- 
¥ 
