182 J. LeConte—Old River-beds of California. 
been filled with detritus. 1st. They may have been filled by 
glaciers slowly and steadily retreating up the valleys, dropping 
their debris on their way, the debris perhaps afterwards modi- 
ed by currents from the melting glacier. 2d. They may have 
been filled successively from mountain foot toward mountain 
crest with detritus brought down by the rivers into a bay or 
fiord which steadily moved up the valley by subsidence of the 
land until the sea stood 4,000 feet above its present level. 
3d, they may have been filled in all parts nearly simultaneously 
by true river action in the same way as river-channels elsewhere 
have been or under certain conditions are now 
I shall not discuss the first and second. I mention them 
because each, but especially the second, has been held by some 
ersons. I have kept them constantly in mind during all my 
observations but have been compelled to abandon them as 
untenable. I am quite sure that no one can examine these 
energy is usually divided between the work of transportation , 
and the work of erosion. If the load of transported matter be 
moderate, a large amount of energy is left over for erosion ; 
but if the load of transported matter be very great, the whole 
energy may be expended in transportation and none is left for 
ion—the limit is reached at which erosion ceases an 
a certain amount of detritus which produces the maximum 
It is evident therefore that all that is necessary to cause 
any stream to deposit is to increase its load beyond the 
* Gilbert, this Journ., vol. xii, pp. 16 and 85, 1876. 
