The Parallel Boads of Glen Hoy. 397 



course up to the head of Glen Spean, and down nearly to its 

 mouth, over a space of twenty miles, always preserving the 

 same level. Lastly, portions of terraces have been discovered 

 on the sides of the valley of the Spej-, many miles eastward 

 of those we have been describing, and at a computed eleva- 

 tion of 800 feet above the sea. 



The Memoirs of Sir Thomas Dick Lauder and Dr MacCul- 

 loch,* written nearly at the same time, prove that the terraces 

 were formed by water. In lochs which have pretty steep banks, 

 covered with materials somewhat loose, rains and storms hurry 

 down earth and stones, which have their motion checked when 

 they reach the water, and there form a projecting ledge or 

 shelf, which is nearly on a level with its surface close to the 

 shore, but slopes downwards as it advances, till it is some feet 

 or yards below the water at its outer margin. Sir Thomas 

 Dick Lauder observed shelves of this description under the 

 water in Loch Lochy, Loch Oich, Loch Ness, and also in Linnhe 

 Loch, which is an arm of the sea, and he inferred, that if by 

 any accident one of these lochs were drained, the shelf round 

 its margin would exactly represent one of the parallel roads of 

 Glen Roy. The inference was established, and the principle 

 beautifully illustrated, by a parallel case. At Subiaco in the 

 Apennines, forty miles from Rome, there is a small valley, 

 with a terrace of gravel and clay round its sides, similar to 

 those of Glen Roy. The stream flowing through it escapes by 

 a fissure in the rock at its lower end ; but the Roman writers 

 tell us that it was anciently a lake, and the fact is proved by 

 the remains of baths yet standing, and precisely at the level 

 of the terrace. Whether the fissm*e in the barrier, which is 

 twelve or fifteen feet wide, was opened by an earthquake, or 

 cut by the water, is not known. 



Sir Thomas supposes that Glen Roy, Glen Gloy, and Glen 

 Spean, formed distinct lochs, each being closed in by a barrier 

 at its lower end. The barrier at the foot of Glen Roy had 

 kept the water for a long period at the level of the dotted 

 line c C, till the alluvium collected and formed the shelf c. 



■" Transactions of Royal Society of Edinburgh, vol. ix., and of Geological 

 Society of London, vol. iv. first series. 



VOL. XXVII. NO. LIV. OCTOBER 1839. D d 



