10 Review of Chambers's Ancient Sea Margins, 



might descend by a very gradual inclination to the lower flat, 

 instead of forming a proper terrace ; or part way it might be 

 gradually declining, and^ then fall off with the rapid slope that 

 usually bounds a terrace. These are all possible results of the 

 causes mentioned and are to be looked for in nature. 



6. The lakes of the country, where they could at once empty 

 themselves, would correspond with the rivers in the results pro- 

 duced, and each terrace might indicate in many cases a separate 

 elevation. When they remained closed by # a barrier, they would 

 either gradually become emptied, or by abrupt steps at intervals ; 

 and hence there might be several terraces differing in height from 



any along the rivers, (indicative of the progress made at succes- 



sive periods,) and differing from those of a neighboring lake. 





7. The terraces of a river without lakes, in the case supposed, 

 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 leveling may be necessary to detect a variation 

 from horizontality. * 



By way of farther 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 anal- 

 ogous 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 affect 

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

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

 $$ 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 form- 























