216 Dana and Chambers on Ancient Sea-Margins. 



terraces, diflfering in height from any along the rivers (indi- 

 cative of the progress made at successive periods), and diiffer- 

 ing from those of a neighbouring lake. 



7. The terraces of a river vi^ithout lakes, in the case sup- 

 posed, would have the terrace-plains approximately parallel 

 with the bed of the stream. "Where there were lakes, the 

 terraces would be horizontal, and the stream left in the valley 

 might ultimately have (as in Glen Roy) a rapid descent. It 

 should be remembered, that the descent of large rivers, and, 

 consequently, the corresponding slope of their flats, is but one 

 or two feet to the mile ; and hence great accuracy in levelling 

 may be necessary to detect a variation from horizontality. 



By the way of further illustration, suppose North America 

 to rise sixty feet (or the sea-level to sink this amount). The 

 Mississippi has now a lower flat, in some places exceeding 

 twenty miles in width ; on the Ohio the flat is often over five 

 miles ; on the Connecticut, over a mile : and so with other 

 rivers. The great river of the west would soon work its bed 

 down the sixty feet. Its banks, — since their height has a 

 relation to the existing level of the river, — would be reduced 

 to their present elevation ; and, some miles back, a terrace of 

 sixty feet would mark the limits of the new-formed lower 

 flat. The same result, or something analogous, would take 

 place on the Ohio, and all the other rivers of the country. 

 Even the streamlets that constitute their head-waters, high 

 up among the hills, would each form its terrace where there 

 were proper alluvial shores ; for the slope of the whole, from 

 the summit rill to the mouths of the rivers at the sea, would 

 gradually have become increased by the elevation of the 

 country, and the process of excavation would therefore afl*ect 

 every part of the land. There would not necessarily be an 

 identity of height in the terraces formed, for the reasons stated 

 in sect. 2 and 5. 



If these are facts, — and who can doubt it, — the formation 

 of every elevated beach along a coast, must be attended by 

 the formation of numberless terraces along the rivers of the 

 elevated country. If so, — ^for the 60-feet rise Scotland has 

 experienced, there must be river terraces of contempora- 

 neous origin even over the higher lands of the country ; every 



