86 , REPORT—1850. 
third, it has striated the surface, as rivers have not done; fourth, it has operated up- 
hill extensively, as rivers have not. 
Specific Effects of River Action First, pot-holes, sometimes twenty or thirty feet 
deep, and ten feet diameter; second, rounding the edges of the strata; third, the 
drift materials are less rounded than those by rivers. 
Distinction between Oceanic and Fluviatile Action—First, the latter produces pot- 
holes, but not the former ; second, the gorges made by the ocean are usually straight, 
those by rivers usually sinuous; third, rivers cannot produce wide valleys, as the 
ocean can ; fourth, the force of the ocean is directed towards the axis of mountain 
chains ; fifth, steep-worn cliffs, with no opposing cliffs, are due to the ocean; sixth, 
. several parallel valleys of erosion on the crest of a mountain evince oceanic agency. 
Mode and Extent of Erosion by Rivers.—First, when they began to flow, they would 
choose the lowest portions of the surface, and probably form a chain of lakes at first ; 
second, as the barriers wore away, the lakes would at the same time be filling with 
detritus ; third, deltas, constantly increasing, prove that the excavation exceeds the 
filling ap (the Mississippi carries to its mouth one cubic mile of mud in five years) ; 
fourth, rivers passing through loose materials have high banks, and are sinking deeper 
and deeper ; fifth, at cataracts, the water-worn appearance of the walls proves exca- 
vation ; sixth, rivers evade their beds,—first, by solution; second, by friction ; third, 
by producing disintegration; fourth, by freezing in crevices; fifth, by ice floods ; 
sixth, at cataracts chiefly do we see rivers lowering their beds. 
Modifying Circumstances.—First, the older the rock (ceteris paribus) the greater we 
should expect the erosion; second, the position of the strata in respect to the cur- 
rent ; third, in similar rocks the amount of erosion would depend considerably —first, 
upon the chemical composition ; second, upon the presence or absence of some pecu- 
liar mineral (ea. gr. sulphuret, or carbonate of iron) ; third, upon the relative posi- 
tion of the strata and the current. 
In applying these principles and means of discriminating the effects of river action, 
the author referred to numerous examples of erosion, chiefly in gorges, which he 
supposed were the result of river action. In gneiss and mica-slate he pointed out 
eighteen cases, one of which was on Deertield River, a tributary of the Connecticut 
in New England, where a gorge, called the Ghor, several hundred feet deep, cuts 
across mica-slate eight miles, and through a hill 400 or 500 feet high, eighty feet of 
which has been excavated since the drift period, although, probably, the river does 
not now lower its bed an inch in 100 years. In another case, he showed that Con- 
necticut River was once 682 feet above its present bed, and that the barrier through 
which it has worn its way shows marks of river action to that height. He pointed 
out eleven examples in Silurian and newer sandstone rocks, among which were the 
Niagara river, that has run back seven miles, Geneva River, and Oak Orchard Creek, 
that have worn an equal distance in the same rocks essentially. In limestone he 
quoted ten examples, including the natural bridges of Virginia and Armenia. In un- 
stratified rocks, he referred to twelve examples. He distinctly disclaimed referring 
all valleys to erosive action of any sort, much less to river action. He contended 
only, that the gorges to which he referred were thus produced; and that the rock 
was of enormous magnitude, although but seldom recognised in the writings of geolo- 
gists. He never had recognized them himself, till by learning how to distinguish 
river action from all other agencies, he had, as it were, acquired a new set of eyes. 
The paper closed with the following inferences :— 
1. Rivers in general, for the greater part of their course, have ceased to lower 
their beds, and in many places are filling them up. 
2, But wherever there are cataracts, the work of erosion is still going on. 
3. In many cases streams have worn backward through a succession of barriers, 
with intervening basins, and the sum of erosion through these barriers gives us its true 
amount, rather than the distance worn into the highest one. 
4, In some instances the gorges of rivers have been modified by vertical move- 
ments, and such cases should be omitted as examples of erosion. 
5. We ought to expect not a minute, but only a general resemblance between 
erosions in different parts of the globe. 
6. Yet the facts detailed show nearly the same amount of river erosion in different 
parts of the globe. 
es ΩΝ 
