126 ON EROSIONS OF THE EARTH'S SURFACE. 



earliest and the antediluvial beds (and I might have added more for nearly equally 

 good reasons), are one or more of the following: — 



1. Some of these gorges are in rocks more modern than the oldest. 



2. Some of them have sides too precipitous for erosions of the earliest date. 



3. Yet some of them are situated too high above the present contiguous streams 

 to have been worn out so recently as the last sojourn of this continent above the 

 waters, previous to the present. 



5. The most numerous cases of erosion, which I have described, appear to be 

 postdiluvial, or produced during the alluvial period. 



1. All erosions in unconsolidated strata, lying above the tertiarj^ strata, must, 

 from the nature of the case, be of this description; since such deposits did not 

 exist certainly in their present position previously. Hence all those examples of 

 old river beds in alluvium, along the Connecticut and its tributaries, exhibited on 

 Plates III. and IV., and described in my paper on Surface Geology, because con- 

 nected with the subject of terraces, belong to this class of erosions. But they 

 are not limited to the unconsolidated strata. 



2. The gulf from Niagara Falls to Ontario through which the river now runs. 



3. The present bed of Genesee river, below Rochester. 



4. The same, between Portage and Mount Morris. 



5. The present bed of Black river, in Vermont, below Proctorsville, at least for 

 several miles. 



6. The present bed of Connecticut river, for some miles below Middletown. 



7. The present bed of Westfield river, in Russell, parallel to where an old bed 

 appears. 



8. A similar case, perhaps a mile long, on the same river, at Chester village. 



9. A similar case, four miles above Chester village, on the east branch of the 

 same river. 



10. Present bed of Delaware river, through the Gap. 



I am satisfied that a multitude of similar cases exist in our country as well as 

 on other continents, if care were only taken to trace them out. I judge so from 

 the ease with which I have found those above enumerated. 



6. The character of the rock, the position of the strata, their chemical charac- 

 ter, and the nature of the climate, as to heat and cold and moisture, are circum- 

 stances affecting the amount of erosion, to be taken into account in comparing the 

 work in different places. 



7. Hence we need a number of cases of erosion in different rocks, in countries 

 which we wish to compare together in this respect. 



8. Taking such an average as our guide, as far as we can do from the cases that 

 have been described, we infer that this work has not differed much in amount on 

 different continents. It has been great and long continued on them all. 



9. In rivers without cataracts or rapids, the work of erosion has nearly ceased, 

 and the marks of drift agency extend nearly or quite down to their present level. 



10. In some places, especially between cataracts, and in low alluvions, rivers are 

 filling up their beds. Ex. gr. The Mississippi near its mouth and the Po, whose 

 bed in some places is above the houses on its banks. 



