BEMARKS ON GLEX ROY. 23 



beach of well-waterworn pebbles is probably due to tbe following 

 causes : — 



1. The comparatively feeble action of the waves, owing to the 



lake being narrow and surrounded by hills. 



2. The steepness of the slopes, which led to the stones being 



washed downhill before they had time to be worn, as 

 Lubbock points out. 



3. The transient existence of the lakes, whose duration was 



short compared with that of ordinary lakes, whose beaches 

 have been forming ever since the Glacial period came to 

 a close. 



4. The freezing of the lake-margin, which would arrest the 



action of the water. 



XII. DlSPEESIOlJr OF BOULDERS EROM THE GlEIST SpEAN SyENITE. 



This rock occupies the bottom of the Spean Valley for some dis- 

 tance to the east of the Eiver Treig, and Mr. Jolly has ascertained 

 that it extends farther to the south-east than is shown in my map, 

 and also rises to a higher level in that direction. It has given 

 off a profusion of fragments whose transport affords an excellent 

 means of tracking the flow of ice in that neighbourhood. These 

 blocks are often of large size, from 5 to 20 feet in length, and ex- 

 tend up the hills on the north side of the Spean to heights of fully 

 2000 feet above the sea and 1400 feet above the river. They are 

 scattered over Craig Dhu on to the very top ; I also traced them up 

 the bed of the Eough Burn all along from its mouth to an altitude 

 exceeding that of any of the Glen Roy lines. I found the rivulet 

 cutting its way through a great depth of unstratified earthy rubbish 

 full of large boulders, many of them belonging to this syenite ; 

 whereas the rock in the upper part of the stream is a gneiss full of 

 veins of red granite. The deep mass of moraine-like rubbish ceased 

 somewhat abruptly on following it up the hollow, and from this 

 point there is a fine view into Glen Treig, the mouth of which lies 

 right opposite. Ascending the hill on the west side of the Rough 

 Burn (or ' Allt-a-Chaoruinn ' of the Ordnance map) I found its 

 southern slope thickly covered with similar moraine-like debris to a 

 still greater height — to a level of about 1700 feet or more. On 

 reaching the bare rock at the brow of the hill I observed that it was 

 a reddish granite of rather small grain ; and on a projecting shoulder 

 of the ridge, at a height of nearly 2000 feet, I met with a large 

 angular block of the syenite, 9 feet in length, lying on the surface 

 of the granite in a place to which it could not have rolled down 

 from above, but must have been transported for some distance. 

 Proceeding westward to the next projecting point, which looks down 

 upon the Glen Glaster col, and is called ' Craig Uilleim ' on the 

 Ordnance map, I found it to be of gneiss full of granite veins. 

 Perched boulders of granite and syenite occur also on this hill. I 

 noticed none of the syenite boulders on the west side of the Treig, 



