and Zŭmpāngŏ, are formed. These lakes rise by stages as they approach the Northern extremity of the valley, the waters of the lake of Tezcuco being, in their ordinary state, four Mexican varas and eight inches lower than the waters of the lake of San Cristoval, which again are six varas lower than the waters of the lake of Zumpango, which forms the Northernmost link of this dangerous chain.
The level of the great square (Plaza Mayor) of Mexico, is exactly one vara, one foot, and one inch above that of the lake of Tezcuco, and is consequently nine varas and five inches lower than that of the lake of Zumpango; a disproportion, the effects of which have been the more severely felt because the lake Of Zŭmpāngŏ receives the tributary streams of the river of Gūāūtĭtlān, the volume of which is more considerable than that of all the other rivers, which enter the valley, combined.
In the inundations to which this peculiarity in the formation of the valley of Mexico has given rise, a similar succession of events has always been observed. The lake of Zŭmpāngŏ, swollen by the rapid increase of the river Guautitlan during the rainy season, forms a junction with that of San Cristobal, and the waters of the two combined burst the dikes which separate them from the lake of Tezcuco. The waters of this last again, raised suddenly more than a vara above their usual level, and prevented from extending themselves to the East and South-east by the rapid rise of the ground in that direc-
