Mode of Formation of' the Terr aces in Glen Roy, ^r- 33 



same, which a few days before I had observed on the sea-shore 

 at Dover. But the terraces, which are now the subject of in- 

 vestigation, are of a different class. All the circumstances agree 

 with the supposition, that the loose earth and stones are washed 

 down bv the rains, and in still greater profusion by the melting 

 snows ; that they change their course on reaching the point b 

 (Fig. 23.), and rush together over the inclined plane b c ; that 

 the water then pursues its course, diffusing itself for some dis- 

 tance over the surface of the lake ; and that all the larger par- 

 ticles, whether pebbles or sand, on arriving at the point c, fall 

 down the declivity towards d. I think it probable also, that 

 these terraces were chiefly formed, before the mountain-sides 

 were cultivated or much protected by vegetation. If any lake, 

 having such terraces, were drained, the appearances would ex- 

 actly agree with those in Lochaber and above Subiaco, which 

 Macculloch and Lauder have described with so much exactness 

 and ability*. 



The formation of these lengthened terraces, with a submerged 

 talus, is, however, much less important in modifying the earth's 

 surface than the production of those forms which are analogous 

 to the before mentioned acute cone, and which arise, wherever 

 the stream, instead of being diffused over a mountain side, di- 

 verges from a single point. As the heavier particles go on de- 

 positing themselves, they form a semicircular area, with a slope 

 from the centre, and with the different kinds of detritus arrang- 

 ed to a considerable degree in successive zones, according to their 

 comparative coarseness or fineness. The part of the area round 

 the centre continues to be raised, by successive depositions, ' 

 above the level of the standing water, while, on the other hand, 



• My view of the origin of these terraces seems to obviate the various dif- 

 ficulties which Dr ]\Iacciilloch states as attending his own hypothesis. 

 Judging from his maps, and those of Sir T. Lauder, and not having visited 

 the place, I think it clear, that there were, as Sir T. Lauder supposes, three 

 ancient lakes, Loch Gluoy, Loch Hoy, and Loch Spean ; that the barriers, 

 which produced the two former, were obtuse cones, formed by the streams 

 which now issue from ravines, o'- small lateral valleys, immediately below the 

 termination of the terraces; that the discharge of each lake was caused, not 

 by any " convulsion," but by the increase of the principal stream, wliich 

 carried away the base of the obtuse cone ; and that these valleys always dis- 

 charged their surplus water in the same direction as at present, and not from 

 their upper extremities, as Sir T. Lauder supposes. 



APRIL — JUNE 1831. C 



