1885.] Sound near the City of Mexico. 741 
on account of the bad state of preservation in which they were 
ound, 
In the region to the south of the hill more modern calcareous 
rocks are seen, and thicker deposits of recent ground with 
remains of Aztec ceramics. 
Not being therefore able to utilize the palzontological data for 
determining the age of these calcareous layers, we must fall back 
on the inspection of the ground. 
Two facts seem at once to reveal that even supposing the forma- 
tion to belong to the present age, it must be of remote antiquity. 
These facts are: The elevation of the ground above the actual 
level of the Lake of Tezcoco, and the remarkable hardness of 
the rock in which the bones are found, different from the other 
calcareous rocks that contain remains of ceramics or roots® of 
plants clearly modern. The upheaval of the lacustrine layers 
which contain the human remains might have taken place through 
the diminution and retirement of the waters of the lake, or by 
the upheaval of volcanic rocks. 
In the first case it could have been occasioned either by a vio- 
lent filtration of the water, or a slow evaporation ; but nowhere 
in the valley of Mexico are any traces to be found of a crack or 
opening through which the waters could have escaped, and which 
ought to appear outside of the present level of the lake, as if it 
were below, all the water would have disappeared. If the lower- 
ing of level was due to evaporation, a theory which would be 
more admissible, because from the time of the conquest of Mex- — 
ico to the present the submerged surfaces have notably dimin- 
ished, the time necessary to have elapsed in order that the level 
of the lake might fall three meters to its present one, must have 
been very long. What is most probable is, that the upheaval is 
due to volcanic action; for although until now no basalt has been 
discovered immediately underneath the place occupied by the 
hardened layers, yet dikes of that rock are to be seen in different 
directions at the foot of the hill, and even the volcanic masses 
which constitute it are found upheaved and inclined, demonstrat- 
ing the succession of geological phenomena in that vicinity. 
Let us now trace the origin of the silicified calcareous rock in 
which the bones were found, and which is different from the 
majority of the lacustrine rocks which occupy the valley of 
Mexico, these latter being, for the most part, thick and extensive 
i 
