with Observations on the study of Terraces. 9 



of descent was reached. Towards the mouths of the streams 

 the bed might be deepened the whole sixty feet. Above, the 

 amount would vary for any giv r en period according to the ability 

 of the waters in different parts to wear out the material over 

 which they pass. A hard rocky bed might prevent excavation ; 

 and if the material were yielding above such a place, the wear 

 might there level down the surface and produce a range of slow 

 waters, ending in rapids over the harder unyielding rocks. The 

 amount of excavation might thus vary from sixty feet, the maxi- 

 mum depth, to onlv a few feet along; the tributaries. 



3. Besides depressing their beds, the rivers would act laterally, 

 and carry off the alluvium of their banks, (when any existed,) 

 and every flood would aid in this result, until finally, a broad flat 

 or " bottom-land" in many places bordered the streams. Such a 

 flat, situated within the reach of the river floods, is common on 

 parts of all rivers where their descent is not too rapid. The 

 breadth of the flat would depend on the amount and force of the 

 waters during floods, and it would necessarily be bounded by a 

 steep slope rising to an upper level. Where the valley was nar- 

 row, all the former alluvium might be carried off; where broad, 

 some portions would be left. 



4. The result would be the same whether the rise were gradual 

 or abrupt. If the latter, the river would have more rending 

 power towards its mouth, and the deepening would go on more 

 rapidly. If a slow gradual rise should commence at any time, 

 the river, through its increasing excavating power, would begin 

 to sink between its banks, and the wear of the alluvial flat either 

 side by floods would also commence. The terrace slope would 

 also show its first beginnings, as an outline to the river flats. 

 Finally, the stream would have a bank of its former height, 

 this height being a constant quantity for the stream ; — it would 

 have more or less broad flats, which flats or bottom-lands would 



commonly be bounded by a slope ; and, if the former alluvium 

 remained, there would be a shelf or terrace bove. 



5. From the conditions mentioned in the last paragraph, it is 

 evident that during a gradual elevation, (and as gradual a sinking 

 of the stream,) the outline of the lower flat misht be varied in 

 consequence of a change in the bed or banks of the stream 

 which should vary its direction or the direction of its princi- 

 pal current; and, that therefore in the course of one and the same 

 rising, a terrace of sixty feet might form in one place, and in an- 

 other, perhaps not far distant, one of twenty, and another of 

 forty ; — or one of ten, and another of thirty, and another of 

 twenty ; and so on. In still other places the upper alluvial plain 



Ssgoitd Series, Vol. VII, No. 10.— Jan., 1849. 2 



