lxXXVlii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9°4. 



the phenomena observable are to be assigned to the work of the sea 

 or of a lake. Nevertheless, on a review of the whole evidence, at 

 least as it is presented in this country, I feel very confident that 

 there is no risk of confusion in this matter. The marine terraces 

 maintain their distinctive features up to the very foot of the slopes 

 where the lake-terraces begin, while these in turn are marked 

 by other special peculiarities. 



Let any observer who has followed the great 50-foot raised beach 

 along the western coast of Scotland and up the Liunhe Loch to 

 the mouth of the Great Glen, look away to the right hand where 

 the wide Strath of Spean leads into the interior. While yet 

 standing on the platform of the raised beach, if the air be clear his 

 eye may detect the beginning of a line, drawn as with a ruler, at 

 the same height along the slopes on either side of the valley. This 

 is the lowest of the three great Parallel Roads of Glen Roy. and 

 runs at a height of 850 feet above the level of the sea. If he will 

 now ascend into Glen Roy, where the three terraces are best seen, 

 he will soon be struck by the distinctive differences between these 

 old lake-margins and the raised beaches with which he has already 

 made himself familiar. In the first place, he will remark their 

 faintness, as compared with the marine platforms of the coast. 

 Though readily traceable from a distance in their horizontal con- 

 tinuity, they are in many places hardly discernible when one is 

 actually standing upon them. A little examination soon reveals that 

 each of them has been produced mainly by the arrest of sediment 

 washed from the slopes above into the water of the vanished lake. 

 Instructive illustrations of this process may often be observed along 

 the sides of reservoirs which have been constructed in steep-sided 

 valleys : there each prolonged halt of the water at a particular level 

 is marked by a shelf of detritus which, blown by wind and washed 

 down the declivities by rain, is stopped when it enter's the water, 

 where it accumulates as a miniature beach. 



Here and there, especially on more exposed projections of the 

 hillsides, there has been a little cutting-back by the shore-waves 

 or drifting ice-floes of the old lake in Glen Roy. Occasionally also, 

 where a streamlet has entered the water, its arrested detritus has 

 accumulated as a broad, flat delta or terrace. But it is manifest 

 that, in such limited expanses of water, wind-waves could have had 

 comparatively little erosive power. Nor can we imagine that, even 

 if the water froze, its floe-ice could have had any potent influence 

 in sawing into the rocks of the declivities and producing seter or 



