CLOSED LACUSTEINE BASINS. 



47 



during the Listoric i^eriod, when a city stood on an island in the central lagoon 

 at a lower level than several of the separate basins which had formed part of the 

 original lake, it became necessary to protect the habitations and temples from 

 the inundations by which the lower part of the depression might have been 



.Fig. 23. — Akea of the Mexican Lakes at Vaeiotjs Periods. 



Scale 1 : 550,000. 



'^ 'tm-- 





y9°io- 



^* Ayocinéo\'> ' ~1 



.■■^ H .Kk... N . I? ,/Afc^^' , .^ 



1520. 



1600. 1700. 1865. Highest Floods. 



Northern Lakes in 1880. 



12 Miles. 



liooded. The Aztecs had accordingly constructed strong defensive works, traces 

 of which may still be seen near the cities of Ixtapalapa and Guadalupe. 



But these embankments at last yielded to the pressure, and under the Spanish 

 rule the cajwtal was for a time exposed to all lacustrine floodings. Towards the 

 beginning of the seventeenth century the situation became so dangerous that it 



