238 GEOLOGY. 
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results similar to those at the mouths of those rivers would, undoubtedly, have been produced; 
the sea would have been displaced, and a broad delta would have been formed, traversed by many 
channels. 
The explanation of the formation of the lake, and its disappearance, by evaporation, which 
has been presented, agrees with the traditions of the Indians. Their statement, that the waters 
retired little by little, is consistent with the gradual subsidence due to evaporation; and the 
sudden floods, of which they speak, undoubtedly took place. It is probable that the lake was 
long subject to great floods, produced either by overflows of the river at seasons of freshets, or 
by a change in its channel, or by a great freshet, combined with a very high tide, so that the 
river became, as it were, dammed up and raised to an unusual height. The present overflows, 
though but very slight, are probably similar; and yet it is possible that the interior of the 
Desert might be deluged at the present day, provided no elevation of the land has taken place, 
and the river should remain at a great height for a long time—long enough to cause the exca- 
vation of a deep channel for New River. . 
Many more facts and observations are required before a full explanation can be given of the 
progressive changes that have taken place during the formation of the delta and Ancient Lake. 
We need to know the actual level of different parts of the surface of the Desert, and an explora- 
tion of the terraces and channels between the Alamo and the head of the Gulf is desirable. 
The foregoing observations on the origin or formation of the Ancient Lake have been made 
without regard to the possibility of a change of level of the whole or a part of that region. It 
is very possible, and even probable, that there have been such changes. A slight elevation of 
the delta would have had the effect to deepen the channel of the river, and in this way would 
have hastened the isolation of the lake. There certainly has not been an elevation of the 
region sufficient to drain the lake, for the flow of New River shows us that the surface remains 
lower than the Colorado. A slight elevation of the whole region would probably have merely 
hastened the changes which have been described. If the precise elevation of the water-line, 
which remains so beautifully distinct on the sides of the mountains, could be ascertained, it 
would show at once whether there has been a change of level. The importance of determining 
its altitude for this object led me to make the attempt by the barometer; but there not having 
been simultaneous observations at the sea-level nearer than Benicia, over five hundred miles 
distant, the result cannot be regarded as conclusive. The height of the mercury was found to 
be 30.248 inches ; attached thermometer, 76° ; detached, 76°. The comparison of this obser- 
vation with the stationary barometer at Benicia, gives, as the altitude of the station, forty-six 
feet above mid-tide. The addition of one hundred feet, the estimated height of the water-line 
above the barometer, gives us as its elevation one hundred and forty-six feet. The result shows 
conclusively that the water-line is not far above the sea-level; but it is believed that the 
possible errors consequent upon the distance, and diverse local influences of the two barometers, 
may be greater than the altitude indicated. The water-line may, therefore, be at the sea-level ; 
its exact altitude cannot be ascertained without simultaneous observations at the shore of the 
Gulf, or by leveling. It is also very possible that the rough estimate of the elevation of the 
water-line above the barometer station is as much as forty or forty-five feet too low or t00 high; 
and this error, if too high, combined with one of one hundred feet in the barometrical indica- 
tions, would place the line about at the sea-level. If, however, the line is elevated as much as 
one hundred and fifty feet, there is still a wide area in the central part of the Desert which 
must be below the sea. The probability of a slight elevation is rendered greater by the fact 
