236 GEOLOGY. 
alluvial deposits of the Colorado River were removed, it is probable that the Gulf would extend 
as far north as the base of San Gorgoiio and San Bernardino. Under the supposition that in 
this way the Gulf formed a long, narrow bay, reaching far above the mouth of the Colorado, the 
retrocession of the water and the formation of a great lake 1s readily explained. 
The raised banks of the Colorado show the vast amount of sediment that has accumulated 
along its course, and that has been discharged into the Gulf. The clay of the northwestern 
part of the Desert is similar in its character to that forming these banks, and it must be regarded 
as derived from the same source. Itis probable that only the heavier and coarser materials 
were deposited near the stream, and by their accumulation displaced the waters of the Gulf, 
and formed a broad delta. "The encroachment of this delta, and its final extension to the oppo- 
site shore, was sufficient to shut off the waters of the upper end of the Gulf, leaving them in the 
condition of a lake, connected with the river and the Gulf by a narrow channel or slough. The 
peculiar configuration of the valley was highly favorable to such a result. The waters of the 
Gulf formed a long and narrow bay, extending one hundred and seventy miles inland, north- 
west of its present limits ; the Colorado entered at its narrowest part, one hundred and forty 
miles below its upper extremity, and below its most contracted part; the current came in at 
right angles to its general direction, and thus the suspended alluvion was spread out in the 
waters at the point where the division of the Gulf could be most rapidly effected, and under 
circumstances favorable for its subsidence. Under these conditions, the channel connecting the 
upper and lower portions of the Gulf, must have gradually become more and more shallow, and 
the continued growth of the delta must have filled it up, so that the tide could no longer ebb 
and flow to the upper end, thus forming a lake, its only barrier on the south being the silt and 
mud of the Colorado. This barrier was probably an extended flat, and not a narrow bar, for 
the silt was undoubtedly much spread about by the tides, and the current caused by the influx 
of the river. A very considerable portion of the silt was doubtless carried to the extreme 
northern part of the Gulf, forming the foundation for the superstratum of clay of lacustrine 
origin which we now find there. The accumulation, however, was undoubtedly most rapid and 
deep opposite the mouth of the river, and it must have formed an effectual barrier between one 
part of the Gulf and the other. It must have been covered by only a few feet of water, and was 
thus left entirely bare at low tides. Such conditions were most favorable for the rapid growth 
and transformation of the flats to dry land or salt marsh. Every great freshet in the stream 
must have made great additions to it, until at length it was submerged only when the tides 
were very high, and the river was much swollen. In that climate, a surface of mud exposed to 
the sun and air, and so well watered, must have been covered with a luxuriant growth of tule, 
grass, and other vegetation ; and it doubtless existed for a long time as a low swamp, traversed 
in every direction by sloughs and channels. ۱ 
It is probable that even after the delta had so far grown as to be above the water, there were 
numerous, narrow, canal-like channels between the river and the lake, or between the lake and 
the Gulf; so that the water in the lake was constantly retained at the same level. That the 
lake received its supply of water, in great part, from the river, is shown by the fact that it was 
fresh-water, or but slightly saline ; the presence of salt or brackish water being proved by the 
fossil shell Gnathodon Lecontei. The great deposition of clay containing the shells probably 
took place in this way ; the current of the river being at times, if not constantly, turned in that 
direction. In this case the excess of water, if not removed by evaporation, must have flowed 
out into the Gulf by some channel further south. It is not impossible that the Colorado once 
