178 J, LeConte—Old River-beds of Calrfornia. 
county on the north to Tuolumne county on the south inelu- 
sive, a distance of about 250 miles, and from the San Joaquin 
and Sacramento plains on the west to about 4000 feet elevation 
on the Sierra slope on the east, a breadth of about 35 miles. 
There is no problem in California geology more important, and 
ae none more difficult—none more enticing and yet none more 
affling—than the mode of filling of the old river-beds and the 
cause of the displacement of the streams. The opportunities 
of study are abundant, for in many places the old beds have 
been bared, and complete sections of their fillings made by the 
operations of hydraulic mining; but the phenomena are so ex- 
tremely complex and difficult of interpretation that we are not 
yet prepared for a final theory. I have on several occasions 
utilized my vacations by the study of these old channels and 
their fillings. In 1877 I examined those about Forest Hill; in 
telligent guidance and kind assistance of Mr. Hughes, the super- 
intendent of the Blue Tent mines, I have made a more ex- 
tended and thorough examination than ever before. Exten- 
sive gravel deposits exist on both sides of the South Yuba 
River for many miles. This is in fact the finest mining 
region in the State. I went up one side and down the other 
and examined these in succession. I wish now to very briefly 
resent the conclusions to which I have provisionally come 
y much reflection on the observations made on this and on 
previous occasions. I present them with some misgivings, 
well knowing that much more complete and detailed observa- 
tions are necessary before an entirely satisfactory theory can be 
reached. 
General Description. 
It is well known that in hydraulic mining the whole thick- 
ness of the old river-channel-fillings is worked down and cat- 
ried away by the prodigious force of the hydraulic jets. 42 
such a mine, therefore, we always have the old bed bared over 
the whole area cleared, and the fillings exposed from bottom t0 
top, on the face of an ever-receding vertical cliff 200 to 400 
feet: high. 
The Bed.—The old stream-bed thus exposed has a shallow, 
trough-like form, i. e. is lowest in the middle and rises gently 
on both sides. These higher sides of the trough are called the 
“rims.” The bed-rock, which is usually slate with nearly ver 
tical cleavage, retains usually its original soundness and hard 
ness, but in some places is more or less decomposed, and some 
times, while retaining its form, is completely changed into 
plastic clay. In all cases it is worn into irregular and fantasl@ 
hollows and channels, and often into deep pot-holes. As there 
is no apparent relation between the hardness or softness of the 
