GEOLOGY OF ARRAK. 



of the river lorsa ; they are far more striking, indeed, than 

 anything of the kind in Arran. They consist throughout of 

 transported materials, some of the rocky masses being very 

 large; the sides are steep and the summits usually flat; and 

 the height of the highest is sixty or seventy feet above the 

 river, and at least thirty above the ancient sea level just 



Fig. ll. 



alluded to. Speaking of these (lorsa, and other such mounds) 

 in reference to river action, MacCulloeh remarks : " The 

 origin of such alluvia is very obscure a few may have been 

 deposited in particular situations by the same waters which 

 are now removing what they formerly laid down ; while in 

 other cases it is impossible to assign any mode of action by 

 which this double and opposite effect could have taken place 

 from one agent. . . . The quantity and quality of the 

 materials, their extremely rounded forms, the nature and 

 permanence of the hills above, and the want of a regular 

 gradation of size in the stones from the bottom upwards, 

 seem to show that other causes [than river action] of a tran- 

 sient, and probably of a diluvian nature, have in distant times 

 generated these deposits, which have been subsequently acted 

 on by the stream concentrated on the bottom of the glen by 

 the form of the ground" ( Western Isles, ii., p. 335; 1819). The 

 difficulties of these cases had thus presented themselves to 

 the mind of this distinguished geologist, and he offers the 

 best explanation that could then have been given. 



Similar mounds occur, but not in the terraced form, at the 

 openings of Glen Sannox and Glen Rosa, much elevated 



