1916-17.] The Bone-Cave in the Valley of Allt nan Uamh. 335 
On the north side of the Traligill between the river and the outcrop of 
the Glencoul thrust-plane the Cambrian strata form an arch, on whose 
southern limb there is a normal ascending sequence from the basal quartz- 
ites (3 in fig. 3) to the limestones of the Eilean Dubh group (8 in fig. 3) 
exposed on the northern bank of the river channel. Here they are trun- 
cated by a thrust which has caused the basal limestones (7 in fig. 3) to 
override the members of the younger group (8 in fig. 3). 
After flowing underground for about a quarter of a mile, the Traligill 
reappears but with diminished volume (PI. I). About two hundred yards 
cate structure), chiefly of Eilean Dubh group; P, porphyrite sills; T.P., Glencoul thrust-plane; 
S, soles or major thrusts ; t, minor thrusts. 
farther down the stream a sudden increase in the volume of water is per- 
ceptible, the probable cause of which will now be indicated. 
About half a mile up the valley from the point where the Traligill 
enters its subterranean channel, and about two hundred yards south from 
the river, caves occur which are locally named Uamh an Tartair (Cave of 
Roaring, literally Great Noise) and Uamh an Uisge (Cave of Water). They 
lie along the outcrop of the Traligill thrust-plane just described, which is 
prolonged south-eastwards in the direction of Cnoc nan Uamh. Descending 
one of these caves, the observer encounters an underground river which is 
seen to leap over a waterfall and pass down a steep slope into a wide 
cavern. It is probably fed by several streams, which, during heavy rains, 
flow down the western slope of Breabag and plunge into large swallow- 
holes on reaching the limestone plateau (fig. 4). It is highly probable also 
