Mr ITufjh Miller on River- Terracing. 273 



as those of the Earn and Tcith.* I am informed by my col- 

 league, Mr J. Grant Wilson, that of the former it is some- 

 times an effective explanation ; but to the latter it can apply 

 only in the absence of any other more suitable. 



Fig. 5 will explain Mr Jamieson's supposition of morainic 

 gravel washed alongside a melting glacier, and left to fall 

 into terrace shape when the support of the ice is withdrawn. 



Summary of Opinions on the Origin of Valley Terraces, 

 chiefly known as river terraces. 



That they are ancient sea margins. 



That they are portions of ancient river-flats, deserted from 

 level to level as the land arose stage by stage. 



That they are portions of ancient river-flats, deserted from 

 level to level in consequence of periodic increase of erosion 

 due to cycles of climate. 



That they are indented margins of ancient and monstrous 

 floods, rising from about the level of our present rivers. 



That some of them are the margins of ancient lakes, 

 tapped in stages. 



That some of them represent fluvio-marine banks, once 

 fringing estuaries near river-mouths and suddenly upheaved. 



That some of them are due to morainic debris, banked-up 

 against the sides of melting glaciers. 



That they were submarinely formed, and answer to 

 Kames or ^sar. 



That they are chiefly due to the . unaided operations of 

 rivers worming themselves from level to level in the 

 ordinary course of valley-excavation, liable to impulses 

 towards increased erosion due to various causes. 



* On the Last Stage of the Glacial Period in Great Britain (Quart. Jour. 

 GeoL Soc, 1874, p. 333). 



VOL. VII. S 



