PROBABLE CHANGES OF LEVEL—CALCAREOUS DEPOSITS—TRAVERTIN. 239 
that the coast at San Pedro, and along the Santa Barbara channel, has undergone a compara- 
tively recent elevation of about thirty feet, as shown by fossils in the banks. This elevation 
may have been independent of the region at the head of the Gulf, but it is more probable that 
it was not. A powerful cause for an elevation, sufficient to hasten the changes which have taken 
place, is indicated to us by the earthquake disturbances which are so common in that region. 
The earthquake of 1852 was sufficiently violent to break off a portion of Chimney Peak, and to 
open fissures in the clay of the Desert. Such agitations can scarcely fail to affect the level of 
parts of the delta or to modify the course of the river. 
The water of the Ancient Lake was remarkable for the great amount of carbonate of lime 
which it held in solution and deposited on its shores. The calcareous water does not, however, 
appear to have been confined to the limits of the lake, for thick deposits of calcareous sinter or 
travertin were found in the crevices in the granitic and metamorphie rocks of Pilot Knob, and 
in the stratum of conglomerate forming the upper plain of the Desert. Near Pilot Knob, this 
conglomerate was formed of pebbles imbedded in a calcareous paste, which filled all the inter- 
stices, so that a mass of the rock looked like pebbles imbedded in mortar. The stones could be 
detached, but left cavities or casts of their forms. The stones or boulders found on the slopes 
of the Desert, below the water-line, were also invested with a calcareous crust. "These facts all 
show the prevalence of calcareous water over that region in former times. The deposits of Pilot 
Knob, and in the bank of conglomerate forming the broad upper plain, may at first be regarded 
as an indication that the water of the Ancient Lake extended over them, or that it was not 
confined to the long, narrow valley of the Gulf, but spread out over a broad surface along the 
Colorado and Gila, far above their point of confluence. "Whether this was so, can only be 
determined by measurements of the altitudes of the plain and water-line. The water, if thus 
extended, could not, however, have been a lake, for there is no barrier between the upper plain 
and the Gulf. It is probable, therefore, that the calcareous deposits of the Ancient Lake, and 
those of Pilot Knob and the conglomerate, are entirely distinct and of different age. 
There is much variation in the thickness of the travertin at different parts of the shore of the 
lake. At the point where it was first observed, in the northern part of the Desert near Deep 
Well, its thickness was not over four or five inches ; it was also less compact and hard than that 
afterwards seen. At the next point where the elevation was taken, it was nearly two feet 
thick, covering the rocks completely, so that all the edges and angles were rounded off. "This 
deposit was extremely hard, and yet filled with long tubular cavities, opening upon the surface, 
80 as to give it an appearance similar to a coarse coral. Small shells, of the same species as 
those found in the clay, were enveloped in the mass of this crust at least six inches below its 
surface ; they may occur at a greater depth but were not observed. The outer part of the 
deposit, if not the whole, was thus formed by the lake while the water was fresh or only slightly 
brackish. 
This great quantity of carbonate of lime, once diffused in the water of the lake, was probably 
supplied by springs, either at the bottom of the lake or near its borders. This is rendered more 
than probable by the existence of a group of springs in the valley of the Desert, with the water 
highly charged with carbonic acid, and surrounded by conical mounds which are probably 
calcareous sinter. The quantity of water and carbonate of lime which they former ly delivered 
may have been much greater than the present supply. The springs of San Felipe Creek may; 
also, have furnished a great quantity of the calcareous water, for they are still depositing 
travertin, and a thick layer is found on all the rocks of the ravine. Insome places the thickness 
