264 Proceedings of the Royal Physical Society. 



This paper is an essay towards the elaboration of an out- 

 line sketch produced by one of the firmest hands that have 

 ever worked in geological landscape. The hand was that of 

 Playfair, and it is fitting that his outline should here stand 

 first. 



*' When the usual form of a river is considered," says 

 Playfair, " the trunk divided into many branches which rise 

 at a great distance from one another, and these again sub- 

 divided into an infinity of smaller ramifications, it becomes 

 strongly impressed upon the mind that all these channels 

 have been cut out by the waters themselves ; that they have 

 been slowly dug out by the washing and erosion of the land ; 

 and that it is by the repeated touches of the same instrument 

 that this curious assemblage of lines has been engraved so 

 deeply on the surface of the globe. The changes which have 

 taken place in the courses of rivers are to be traced in many 

 instances by successive platforms of flat alluvial land, rising 



Fig. 1. 



Ideal Section of a Terraced Valley. 



1, 2, 3, 4, and I., II., III., Terraces of Gravel and Alluvium, excavated 

 in boulder clay and rock. Highest terrace without alluvium. 



one above another, and marking the different levels on which 

 the river has run at different periods of time. . . . Each 

 change which the river makes in its bed obliterates at least 

 a part of the monuments of former changes ; . . . only 

 a part of the progression can leave any distinct memorial 

 behind it." * 



* Illustrations of the Huttonian Theory, Edinb. 1802, p. 102. "Such 

 platforms, or haughs, as they are called in Scotland," adds Playfair, at 

 another page of his classic work, ** are always proofs of the waste and detritus 

 produced by the river, and of the different levels at which it has run ; but 



