334 
Proceedings of the Royal Society of Edinburgh. [Sess. 
III. Drainage of Underground Water and Formation of Caves 
in the Plateau of Cambrian Limestone. 
The limestone plateau lying between the most westerly thrust-plane or 
“ sole ” and the great lines of displacement to the east (Glencoul, Ben More) 
is admirably adapted for the circulation of underground water, as the lime- 
stone is soluble and is traversed by innumerable minor thrusts. These 
piled-up calcareous masses, as already indicated, lie between impervious 
strata (Cambrian quartzites and fucoid-beds) to the west and the displaced 
Lewisian Gneiss, Torridon Sandstone, and Cambrian quartzites forming the 
eastern range of high ground. The streams descending from the high 
ground to the west that feed the Skiag Burn and the Loanan river, flow 
over impervious rocks and suffer no diminution before reaching Loch 
Assynt. On the other hand, the tributaries draining the western slopes 
of the Glas Bheinn and Breabag range, on reaching the limestone plateau, 
either suffer diminution or disappear to issue again at lower levels before 
sinking to the underlying floor of impervious strata (see map, fig. 1). 
Near the northern limit of the limestone plateau north of Achumore, 
no streams descend from the western slope of Glas Bheinn between Loch 
Gainmhich and Allt a Chalda Mor. The plateau, which is here compara- 
tively narrow, is dotted with swallow-holes. The water that falls on its 
surface disappears below ground and probably supplies Allt a Chalda Beag, 
which issues near the outcrop of the Glencoul thrust-plane, and, flowing 
across the limestone belt, enters Loch Assynt near Castle Bay. 
The Big Chalda stream (Allt a Chalda Mor) draining Glas Bheinn and 
Beinn Uidhe, on reaching the outcrop of the Glencoul thrust-plane at the 
eastern margin of the limestone belt, loses part of its waters along this 
plane. Between the Big Chalda and Allt Poll an Droighinn, a tributary 
of the Traligill river, swallow-holes and open chasms occur along the out- 
crop of the Glencoul thrust-plane where the water descending the quartzite 
slopes to the east disappears. 
The phenomena connected with the drainage of underground water in the 
limestone plateau are best displayed in the area drained by the Traligill 
river and its tributaries and Allt nan Uamh. About a mile and a half up 
the Traligill from Inchnadamff Hotel the stream suddenly plunges into a 
cavern along the outcrop of an important thrust-plane or “ sole.” This line 
of disruption forms a prominent feature in the landscape (PI. I), the surface 
of the plane of movement giving rise to a well-marked slope on the northern 
side of the channel. The accompanying section (fig. 3) shows the relations 
of the strata where this “sole” appears at the surface in the Traligill. 
