THE OLDEST EROSIONS. 123 



2. The Ghor, on Deerfield river, a tributary of the Connecticut : or rather the 

 whole valley of Deerfield river west of Deerfield. 



3. The valley of Hudson river, for the most part in hypozoic and the oldest 

 Silurian rocks. 



4. The valley of Agawam river, from Mount Tekoa, in Westfield, to the summit 

 level of the Western railroad. 



5. The cut at the summit level of the Northern railroad, in New Hampshire. 



6. Dorset valley, on the west side of the Green Mountains, through which the 

 small streams called Otter creek and the Batten kill now run. The rocks are 

 hypozoic, or very old metamorphic. 



7. Gorge on Little river, in Russell and Blandford, No. 13, in hypozoic rocks. 



8. Gorge on the Potomac, below Great Falls, in Virginia, No. 15, in hypozoic 

 rocks. 



To these cases I might add probably nearly every valley through which rivers 

 of considerable size run in the hypozoic regions of our country, especially of New 

 England. But my object in this paper is not to describe all cases of erosions, but 

 only to give some good examples, in order to call the attention of geologists to the 

 subject. 



As, however, it does not follow because a gorge is found in hypozoic rocks, that 

 it is very ancient, I have thought that the following principles may enable us to 

 decide with much probability whether a valley is of the most ancient class. 



1. Such a valley must occur in the oldest rocks, viz: the hypozoic, early meta- 

 morphic, or Silurian. 



2. It will have great width in its upper parts, its slopes will be gentle, its sides 

 rounded, and with few precipitous gorges. Such effects could be produced only by 

 oceanic agency, as the continent was repeatedly submerged and raised from the 

 deep. The waves and currents, rushing back and forth through the gorges pro- 

 duced by streams, would give this breadth and rounded outline of the sides, and I 

 know of no other cause that could have produced the effect. 



3. The rivers in the oldest valleys have nearly ceased to' deepen their beds, 

 except perhaps where cataracts occur, and these are not usually of the most strik- 

 ing character. 



4. The drift agency in such valleys has smoothed and striated the rocks nearly 

 to the present level of the streams, and thus afforded proof that the beds have not 

 been much deepened since the drift period. 



2. The work of erosion in these oldest valleys must have been repeatedly inter- 

 rupted, varied, and renewed, by vertical movements. Some of the continents, 

 perhaps all of them, have been subject to such movements, as is obvious from 

 the character of the strata and their embedded organic remains. At one period, 

 for instance, dry land must have existed over wide areas, and then these must 

 have been submerged to receive a marine deposit now covering them. 



3. Some of the erosions that have been described in this paper are clearly the 

 beds of antediluvial rivers : that is, of rivers existing upon this continent before 

 its last submergence beneath the ocean ; which beds were deserted when the sur- 



