224 FRAGMENTS OF SCIENCE 



tion next occurs, What was the character of the assumed 

 barrier which stopped the glens? There are at the pres- 

 ent moment vast masses of detritus in certain portions of 

 Glen Spean, and of such detritus Sir Thomas Dick-Lauder 

 imagined his barriers to have been formed. Bj some un- 

 known convulsion, this detritus had been heaped up. 

 But, once given, and once granted that it was subse- 

 quently removed in the manner indicated, the single road 

 of Grlen Griuoj and the highest and lowest roads of Glen 

 Eoj would be explained in a satisfactory manner. 



To account for the second or middle road of Glen Roy, 

 Sir Thomas Dick-Lauder invoked a new agency. He sup- 

 posed that at a certain point in the breaking down or 

 waste of his dam, a halt occurred, the barrier holding its 

 ground at a particular level sufficiently long to dam a 

 lake rising to the height of, and forming the second road. 

 This point of weakness was at once detected by Mr. Dar- 

 win, and adduced by him as proving that the levels of 

 the cols did not constitute an essential feature in the 

 phenomena of the parallel roads. Though not destroyed, 

 Sir Thomas Dick-Lauder' s theory was seriously shaken 

 by this argument, and it became a point of capital im- 

 portance, if the facts permitted, to remove such source 

 of weakness. This was done in 1847 by Mr. David Milne, 

 now Mr. Milne- Home. On walking up Glen Roy from 

 Roy Bridge, we pass the mouth of a lateral glen, called 

 Glen Glaster, running eastward from Glen Roy. There 

 is nothing in this lateral glen to attract attention, or to 

 suggest that it could have any conspicuous influence in 

 the production of the parallel roads. Hence, probably, 

 the failure of Sir Thomas Dick-Lauder to notice it. But 

 Mr. Milne- Home entered this glen, on the northern side of 



