16 



Prof. J. Prestwich. On the Origin 



[May 1, 



"road," No. 4, which extends through Glen Roy and Glen Spean, thab 

 need discussion, but they are not considered more serious than these 

 which attend the other hypothesis. 



The theoretical inferences of structure agree fairly well with the 

 facts so far as they are known. Mr. Brown's " clay with boulders 

 indistinctly stratified with thin (lenticular) layers of sand," repre- 

 sents the sliding detrital mass, — the finely stratified sand and clay ; 

 the sediment which subsided from the muddy lake waters after their 

 fall ; and the 2 to 3 feet of stones with clay, — the suba.erial fall of debris 

 from the slopes above. In the substratum and overlying sediment, 

 Mr. Brown found four species of fresh-water diatoms, while he found 

 none in the upper bed. This fact serves to confirm the subaqueous 

 origin of the body of the ledge, while it. tends further to disprove the 

 marine hypothesis. 



It has been before observed that, in all the cols, there is an entire 

 absence of a defined water channel, such as would be worn by the 

 long-continued flow of a river, but in all of them there are traces of 

 strong water action, such as might result from the temporary passage 

 of a large and rapid body of water. 



With respect to the main barriers acting as dams to Glen Gluoy, 

 Glen Roy, and Glen Spean, they were due, as already pointed out, to 

 the circumstance of an accumulation of ice at these spots so excessive 

 and so high as to last long after the ice generally in the lower tracks 

 had given way. Not, however, that any ice barrier could have been 

 permanent for a great period of time, but this the author's hypothesis 

 does not require. In any case, an ice-barrier in a state of rest will 

 form a more effective barrier than when in motion. 



Passing over the barrier at the entrance to Glen Gluoy, it is shown 

 that the point where the Glen Roy barrier existed is that where a 

 glacier coming down Glen Roy would, meet in opposition the ice from 

 Ben Nevis and the Spean Valley ; and that this glen was occupied by 

 a glacier is proved by the occurrence of glacial striae on the rocks 

 forming the bed of the valley near Dalriach, and of Till, or bonlder 

 clay, lower down the valley, nearer Achavady. Eut the great mass 

 of the latter lies precisely on the spot where the Ordnance Survey 

 have placed the line of barrier ; it was there heaped up by the same 

 conflicting causes that produced the barrier of ice. That it was 

 originally larger and higher is proved by its occurring on the two sides 

 of the valley, the river having worn a channel through it, and by the 

 presence upon it of thick beds of water- worn and water-strewn gravel, 

 the materials of which have been derived from the underlying deposit, 

 and which was formed, in all probability, by the rush of the waters 

 on the bursting of the barrier, for, in many places, the gravel is 

 thrown back and over, as though by downward and outward pressure 

 of water in motion. This detrital mass extends for a length of 2 miles 



