220 GEOLOGY. 
acters with the sandstone strata of the Cajon, for it was, no doubt, formed, in great part, from 
the abrasion of that sandstone. 
The moulding, or form of the slopes, although to a great extent the result of oceanic currents 
and waves, is mnch modified by the action of the streams which descend from the surrounding 
mountains. It becomes evident, from an examination of the surface, that at certain seasons 
these streams become so much swollen that they descend in floods, and spread out, fan-like, over 
the basin in a multitude of shallow channels, bringing down great blocks of granite and vast 
accumulations of pebbles and earth, and spreading them out over the surface in great 
confusion. 
These effects, and the general character of the results of the overflow of similar streams, were 
well shown by Johnson's River, which is a considerable torrent near the mountains; but a short 
distance below, ali timber and green vegetation disappear from its banks, and the bed of the 
stream is broad, shallow, and perfectly dry. Itis, however, paved with well-rounded boulders, 
-and intersected by numerous minor channels, between banks of gravel and sand, evidently accu- 
mulated by the force of a torrent. Great logs of drift-wood are also found lying on the surface 
and imbedded in the gravel. The coarser parts of this alluvial drift are found nearest the 
mountains, and as the stream descends the slope, and becomes more shallow and diffused, the 
drift is smaller and more uniform in size, until it becomes merely gravel and clay. The finer 
portions of the drift are thus carried far out from their source, and are deposited on the lower 
parts of the slopes. * . 
The tendency of all these mountain torrents appears to be towards a general diffusion over 
the surface, and not to the formation of a continuous channel. The character of the slope is 
thus preserved, although the result is a gradual filling up of all the depressions. In view of 
these facts, it becomes interesting to inquire how far the present condition of the surface is due 
to oceanic action, or to subsequent aqueous modifications. 
DRY LAKES OR PLAYAS. 
At the time of the overflow of the mountain streams, the excess of the water that is - 
absorbed in its passage over the extended slopes collects together in the numerous basin-like 
depressions formed by the intersection of opposite slopes. At such places temporary ponds or 
lakes of shallow depth are formed, and the lighter and clayey portions of the surface-drift that 
is brought down in suspension is deposited. We had evidence of these results in the numerous 
dry lake-beds that we passed over in the lowest parts of the valleys. One of these dry beds, 
lying between the Sierra and the range of the Lost Mountains north of the Bernardino Sierra, was 
seven miles in leñgth and four or five in width. "The soil was of fine clay, mixed with sand, 
the upper or surface layer being of nearly impalpable clay, which the water had left in the form 
of a brilliant pelliele. This was entirely dry, and not a trace of moisture could be detected. 
The surface was unbroken, and extended in a perfectly level plain, almost as smooth and hard 
as a polished table, and resembled the frozen surface of a lake. Such was the mirror-like polish 
of the thin clay pellicle, that the peaks of distant Lost Mountains, and small thorny shrubs on 
the margin, were as clearly and as distinetly reflected from the surface as they could be from " 
sheet of placid water, and it was, indeed, difficult to believe that water did not actually exist 
before me. The effects produced by simple reflection from a polished surface are sometimes Con” 
founded with those due to refraction, and they are indiscriminately called mirage. It is, how- 
