356 BATHYMETRICAL SURVEY OF 



Arkaig) is 12 miles in length; five of them exceed 100 feet in depth, 

 and three exceed 300 feet in depth, while one of them (Loch Lochy) 

 exceeds 500 feet in depth. It has been found convenient to include 

 also two small lochs which drain directly' into Loch Linnhe, viz., 

 Lochan Limn da-Bhra on the east and Loch nan Gabhar on the west. 

 Loch nan Gabhar is in Argyllshire, while all the remaining lochs are 

 situated in Inverness-shire. The relative positions of the lochs and 

 rivers within the area under discussion are shown in the little index 

 map (Fig. 51), from which it will be seen that Loch Arkaig drains into 

 Loch Lochy by the short river Arkaig, while the other lochs within 

 the basin drain into the river Spean, which joins the river Lochy 

 shortly after its exit from Loch Lochy, the junction of the two rivers 

 being marked by the pretty falls of Mucomir. 



The Lochy basin, only a small portion of which has been mapped 

 by the Geological Survey, lies wholly within the region of the crystal- 

 line schists of the Central Highlands. It is intersected by the powerful 

 north-east and south-west fault that traverses the Great Glen from 

 Inverness to the shores of Loch Linnhe. In the area west of this 

 dislocation the rocks, so far as known, consist of quartz-biotite granu- 

 lites and muscovite-biotite schists, which are believed to represent 

 altered sediments. These are traversed by acid and basic intrusions 

 and numerous veins of granite and pegmatite. 



East of the Great Glen several of the metamorphic groups of the 

 Eastern Highlands are represented, including the Perthshire quartzite, 

 black schist, limestone, Ardrishaig phyllites, and the associated quart- 

 zite, the beds striking generally in a north-east and south-west direction. 

 The schists are pierced by various masses of igneous rock, of which 

 the most important is the large intrusion of granite forming the lower 

 part of Ben Nevis. It is capped by andesitic lavas, breccias, and tuffs, 

 presumably of Lower Old Red Sandstone age. 



Loch Lochy (see Plate LXXXIII.). Loch Lochy is the southern- 

 most of the chain of lochs occupying the Great Glen which were 

 utilized in forming the Caledonian Canal. Its southern end is about 

 8 miles north of Fort William. It is a straight loch, running nearly 

 north-east to. south-west. In form Loch Lochy is a narrow triangle, 

 with the apex at the north end, gradually widening southward to near 

 Bunarkaig, where the greatest breadth is found, after which it rapidly 

 narrows for the remaining 2 miles to the outflow at Gairlochy. A good 

 road runs along the eastern shore, a rough cart-road on the western 

 side, and several stations of the Invergarry and Fort- Augustus railway 

 now give easy access to the loch on the east side. The surroundings are 

 wild, gloomy, and solitary (see Fig. 52). No village is found on its 

 shores, an occasional house being passed on the east side, while the 

 west side is uninhabited, save for one or two distant cots. 



