CHAP. XIII. PARALLEL ROADS OF GLEN ROY. 255 



I should exceed the limits of this work, were I to attempt 

 to give a full description of all the geographical circumstances 

 attending these singular terraces, or to discuss the ingenious 

 theories which have been severally proposed to account for 

 them by Dr. Macculloch, Sir T. Lauder Dick, and Messrs. Dar- 

 win, Agassiz, Milne, and Chambers. There is one point, how- 

 ever, on which all are agreed, namely, that these shelves are 

 ancient beaches, or littoral formations, accumulated round the 

 edges of one or more sheets of water which once stood for a 

 long time successively at the level of the several shelves. 



It is well known that wherever a lake or marine fiord 

 exists surrounded by steep mountains subject to disintegra- 

 tion by fi'ost or the action of torrents, some loose matter is 

 washed down annually, especially during the melting of snow, 

 and a check is given to the descent of this detritus at the 

 point where it reaches the waters of the lake. The waves 

 then spread out the materials along the shore, and throw some 

 of them upon the beach ; their dispersing power being aided 

 by the ice, which often adhei-es to pebbles during the winter 

 months, and gives buoj^ancy to them. 

 The annexed diagram illustrates Fig. 37. 



the manner in which Dr. Maccul- 

 loch and Mr. Darwin suppose "the 

 roads" to constitute mere excres- 

 cences of the superficial alluvial 

 coating which rests upon the hill- 

 side, and consists chiefly of clay 

 and sharp unrounded stones. 



Among other proofs that the a b. Supposed original surface 



of rock. 



parallel roads have really been c d. Roads or shelves in the 



... outer alluvial covering 



formed along the margm oi a sheet of the hill. 



of water, it may be mentioned that 



wherever an isolated hill rises in the middle of the glen above 



the level of any particular shelf, as in Mealderry, Plate II., a 



