518 GRADE OF THE CHANNELS. 
the general direction of their axes; but in which of the two opposite directions thus indicated, I 
do not know that there is any means of determining from the forms of the furrows alone. For 
this, recourse must be had to the general grade of the rock-surface. 
With reference to the value of barometric determinations of the grades of the ancient channels, 
as also with reference to the extent to which these grades themselves, even if accurately determined 
with the spirit-level, can be relied upon as pointing out the exact direction of flow upon the bed- 
rock, I will make a few remarks. 
The grades of the earliest gravel channels, though they undoubtedly varied in certain localities 
between the widest limits, and there were probably sometimes falls, that is, moderate cascades or 
cataracts in their courses, yet did not generally exceed three feet in a hundred, and from two to 
two and a half or even less was probably a commoner grade. 
Now, in the beds of the ancient streams there are many irregularities, and it is not at all uncom- 
mon, in the bottoms of the greater channels, to find irregular depressions and projecting knobs and 
ridges of bed-rock, with differences of twenty or twenty-five feet in height, and sometimes more, 
between the highest and the lowest points, though these are close together and in the very bottom, 
or even in the central portion of the deep, broad channel. Moreover, these irregularities are often 
extremely numerous. It is evident, therefore, that even the most accurate determinations of rela- 
tive heights could generally be relied upon as definitely determining the direction of the flow only 
when the extreme points were taken at a considerable distance apart. And with the errors to 
which the barometer at best is liable, and the rarity with which any single ancient channel can 
now be certainly traced for any considerable distance in the bed-rock, it is evident that its results 
can in ordinary cases be only of general value, and that the instances in which it alone can 
determine with certainty the original direction of flow in particular channels are compara- 
tively few and far between. But believing, as I do, that there has been no considerable and 
generally no appreciable disturbance of the bed-rock since the gravel period, I also believe, of 
course, that the present grades of the ancient channels, wherever the latter can be distinctly 
traced, — and a sufficient distance is taken into account to determine with certainty the direction 
of their fall, —are generally safe and certain evidence as to the direction of the ancient flow. 
It should have been mentioned in another connection that thin streaks of rich gravel, with 
coarse gold, do sometimes occur in the banks at considerable heights above the bed-rock. But 
such cases are somewhat rare, and I believe that, where-they do occur, they were spread over the 
surface of the older gravel by new streams coming from new directions, in which the bed-rock 
generally was not far off. 
Another thing which, though by no means universal, is nevertheless an extremely common fact 
throughout the ancient gravel hills, is that the gravel is richest upon the rim-rock. By the “xim- 
rock,” however, in this connection, is not meant the actual rim of-any particular ancient channel 
or channels, but simply the rock which underlies the gravel along its edges in the spurs and hill- 
sides fronting the present gulches and caiions. So common is this fact that it is often stated by 
the miners as a general if not almost the universal rule. 
There can be little doubt that this impression among the miners is partzally due, at least, to 
the fact that on the rim-rock the gravel is necessarily shallow, however steeply the banks may 
rise behind it, and it therefore can be more cheaply worked, and at far less cost per square yard 
of bed-rock surface, than farther back in the hill. On this account alone the profits of working 
the rim-rock would, of course, even on the supposition that there was no perceptible difference in 
the actual richness of the gravel, be much greater than those of working farther back beneath the hill. 
Yet, considering all the testimony, it can hardly be doubted that the rim-rock gravel has, oftener 
perhaps than otherwise, proved actually somewhat richer in gold than that farther back in the 
hills ; and if this be indeed the fact, it is a difficult thing to account for. We can easily under- 
stand, indeed, that, even on these steep hillsides, the rains, etc., in washing away the upper 
portions of the sloping banks, would have some tendency to concentrate and leave behind, upon 
or near the rim-rock, a portion at least of the gold which they contained. But this does not 
