REMARKS ON GLEN BOY. 21 



dimensions should have been mnch greater, because in that case 

 the depth of the slope would have been equal to the vertical range 

 of the waves in a freshwater lake plus the variation in level caused 

 by the tide. Now there are some places in the most sheltered parts 

 of Glen Eoy (as on the west side above Achavady) where I remarked 

 that the depth of the slope on some of the shelves does not seem to 

 exceed 3, or at the most 4, feet, which is quite inconsistent with the 

 movement of water in a sea-loch or fiord subject to a rise and fall 

 of tide. 



The horizontal breadth of a cross -section of the ' roads ' will be 

 governed by two elements, the depth of slope and its angle of 

 inclination. Por a given depth it will be greatest where the angle 

 is least, and for a given angle it will be greatest where the depth is 

 greatest. The extreme breadth will therefore occur where the 

 maximum depth coincides with the minimum angle. 



The angle of inclination seems to represent the slope at which 

 the materials could sustain the action of the waves. For a bank of 

 loose materials that angle is smaller in agitated water than it would 

 be in perfectly still water or simply in the air. This fact explains 

 why the • roads ' dip at a less degree than the hillsides above and 

 below them. The surface on which the lines are traced varies, it is 

 true, greatly in different places, but in those where the ' roads ' are 

 best marked the hillsides are generally steep. The amount of the 

 angle would depend chiefly on the nature of the materials and partly 

 also on the force of the waves : the looser the stuff the smaller the 

 angle, and vice versa. Accordingly it varies with the structure and 

 composition of the hillsides, and also with the exposure. In some 

 places the surface seems to be nearly flat, or at any rate as low as 

 5°, but from 12° to 25° is a more common angle,^ and where it is 

 rock it may reach the perpendicular. The above observations apply 

 to a lake that is not frozen. Some modification would occur if the 

 margin was converted into ice during part of the year and the hills 

 covered with snow. But, as the water would resume its sway during 

 summer and autumn, the result upon the whole would probably be 

 still much the same. 



The action of the waves upon a beach forms an interesting spec- 

 tacle. The impulse given by the wind drives the advancing wave 

 up above the normal level of the water. As soon as the onward 

 impulse is exhausted the water falls back again and constitutes the 

 retreating wave, which, rushing down the slope, meets the base of the 

 one next advancing and checks its onset, causing a turmoil in the 

 water. When the vrave curls over and breaks upon the beach there 

 is a downward stroke, which stirs up the bottom still more, and 

 mingles the finer material with the water. The muddy sediment 

 thus suspended has a tendency to float off from the shore and sub- 

 side into the deeper and quieter parts of the bottom, leaving the 

 washed sand and stones on the beach ; but if the fine mud cannot 

 get away it remains suspended in the water until the agitation 



1 See the Ordnance Survey measurements, &c., given by Prof. Prestwich in his 

 memoir, Phil. Trans. Eoy. Soc. for 1879, vol. clxx. p. 698. 



