Geological and Typographical Features — Wittmann. 175 
years. As the precipitation would be the same whether 
the water was 5 feet or 150 feet deep we cannot estimate 
the true thickness of the bed from the difference between 
the highest and the lowest level, but we ma}- safely assume 
that the deposit below ^Monterey represents the least possi- 
ble thickness. 
Reasoning as above, a period of not less than 22,472 
years would be necessary to deposit the existing bed of 
sillar. But how many thousand years have passed since 
that lake disappeared and the rivers took its place and com- 
menced their own work of destruction and reconstruction? 
Our surest guide in geological study is lacking in this case. 
No fossils are found in the sillar, no shells of Crustacea 
^vhich would help to fix the period of the deposit. At least 
the author has never succeeded in finding any. But the 
undisturbed condition of the horizontal beds of sillar indi- 
cate that since their deposition commenced there has been 
no further volcanic eruptions in the valley. 
Below the gravel beds of undetermined thickness, we 
would surely find the disrupted and uplifted lime rock, and 
below that again the limestone of Topo Chico. An artesian 
well sunk to a considerable depth in former years failed to 
give definite results, for the drill broke and the work was 
stopped when a depth of 2,000 feet was said to have been 
reached. 
A possible explanation of the origin of the Topo Chico 
mountain may be that a mass of plutonic rock in a state of 
refusion and under enormous pressure partly penetrated 
and partly uplifted the overlying stratum of diluvial rock. 
The rounded top of the mountain leads to the belief that 
the whole mass while in eruption was in a semi-fluid con- 
dition. The eruption was followed by an outburst of sul- 
phurous gases which must have continued for a long time 
to penetrate the cracks and fissures caused by the contrac- 
tion of the crust while in process of cooling. These fissures, 
from half an inch to over three feet in width, have been 
nearly all closed by the action of the fumes of sulphur 
which changed the carbonate of lime to sulphate of lime. 
Xot all of these cracks and fissures have been thus 
nearly all closed by the action of the fumes of sulphur 
