90 ON EROSIONS OF THE EARTH'S SURFACE. 



no such markings are the result of river action. The rock is smoothed sometimes 

 almost to a polish, but not distinctly scratched, unless something more than water, 

 or gravel and sand driven by water, has acted upon it. 



5. The drift agency sometimes operated in an up-hill direction, even to the 

 height of some hundreds of feet, whereas, rivers can operate only upon a level or 

 upon a descent. 



6. Pot-holes in the rocks are produced by rivers where they form cataracts, but 

 never by the drift agency. 



7. Where successive layers of rock are superimposed upon one another, some 

 are more easily worn away than others, and usually the central parts of a stratum 

 are the hardest. When currents of water act on such ledges, the edges of the 

 layers will be rounded and interspaces or grooves be produced: not regular, indeed, 

 but more or less deep and wide, according to the greater or less ease with which 

 the strata are disintegrated. But no such effects are produced by drift agency. 

 All parts of the surface, whether harder or softer, are swept down to the same 

 level, or nearly so. 



Marks by which to distinguish between Fluviatile and Oceanic Agencies. — This is a 

 much more difficult case than the last, and in some instances, I despair of determin- 

 ing by which of these agencies erosions were produced. In most cases, however, I 

 think the distinguishing marks are clear to a practised eye. 



1. Fluviatile action produces pot-holes, when rivers have cataracts, but they 

 never result from the action of oceanic waves, tides, or currents. 



2. When chasms or gorges are worn in the rocks by waves and tides, they are 

 usually almost straight, and generally follow the jointed structure of the rock, 

 producing purgatories. But when rivers wear out long chasms, they are usually 

 more or less crooked, as that of the Niagara river, for instance. 



3. Rivers have little power to form wide valleys. Sometimes, however, as a 

 stream cuts its bed deeper and deeper, either in consequence of the strata being 

 softer on one of its banks, or of a curvature in its course, it moves laterally so as to 

 leave a sloping bank on one side, perhaps to a great height. In that case, how- 

 ever, the opposite bank will be steep. If both banks slope nearly alike, so much 

 as to make the upper part of the valley quite broad, we must impute much of the 

 erosion to oceanic action; to the flux and reflux of the waters through the opening 

 for ages. The lower and narrower part of the same valley, in such case, may be 

 the result of river action. Or the river may have begun the work at a high level, 

 and it has been subsequently modified by the ocean. I apprehend that most of 

 the valleys in mountainous regions have been produced by this joint agency. 



4. If the crest of a mountain is crossed by parallel valleys of different heights, 

 evidently eroded, the presumption is, that the denudation was accomplished 

 mainly by oceanic action ; by the flux and reflux waves and tides, aided, perhaps, 

 by icebergs, during the upheaval of the land. For though a river, in such a case, 

 might sometimes change its bed, so as to wear each successive one deeper and 

 deeper, the supposition would imply that the successive beds had originally nearly 

 the same relative depth, and it is not easy to see why a lake should have so many 

 outlets at the same time. Lakes do, indeed, sometimes have two outlets, at oppo- 



