Dr. Mac Cull och on the Parallel Roads of Glen Roy. 369 



Besides these lines, terraces of similar alluvial matters, with 

 surfaces of considerable dimensions, and more or less level within 

 themselves, are found in the lower parts of the valley, (vertically- 

 considered j. These accompany the entrances of the different rivers 

 into it, whether principal or lateral, and, being proportioned to the 

 magnitude and power of the streams, they are most considerable at 

 the entrances of the Roy and the water of Glen Turit. At the 

 upper end of Glen Roy the most remarkable of these terraces coin- 

 cide with the lowest line, with which they present a level continu- 

 ously prolonged, as may be seen in the accompanying section and 

 drawings.* In these places, they are still subject to the action 

 of the river, from which cause they are gradually wasting away 

 and contracting their superficial dimensions ; while the lateral 

 wanderings of the river, acting now on the alluvial plain, multi- 

 plies their number, and produces a great series of inferior ones at 

 different levels, which skirt its course for a considerable way down 

 the stream. 



It is necessary in examining the correspondence of these pheno- 

 mena with the theory of a lake, to separate these latter appearances 

 from the former. They are plainly of posterior date, and the re- 

 sult of an action now daily going on, derived from the powers 

 already noticed which rivers possess of deepening their way through 

 alluvial plains. It is only the first class of appearances that we 

 have here to consider, for the purpose of examining how far they 

 agree with those which are found connected with lakes at the 

 present day. 



If we examine a lake inclosed by hills of a considerable declivity, 

 which, being formed of solid rock, are at the same time covered 

 with alluvial matter to a greater or less depth, we find that it is 



* PI. 21, Section L. and PI. 15 



