CHAP. XLII 
THE BASIC SILLS OF ARDNAMURCHAN 
321 
tain, they do not attain the same coarseness of texture which the 
more massive beds there display. They generally possess fine-grained 
chilled selvages along their upper and under surfaces. 
These abundant sills may he traced up into the mass of Ben 
Hiant from which they have issued, 
and of the individual sheets of 
which they are a continuation. 
One of the most striking and 
easily - followed examples of this 
connection is to he seen on the 
north side of the mountain. A 
thick sheet in the middle of Ben 
Hiant descends from among its con- 
tiguous sheets and, as a prominent 
rib, runs down the scree -slope 
into the valley below, where it 
forms a prominent feature. Cross- 
ing the streamlet in the middle 
of the valley, where a section has 
been cut through its upper surface, 
it gradually bends round towards 
the north-east, mounts the side of 
Beinn na h-Urchrach until it 
reaches the crest of the ridge and 
joins the other sills of which this 
eminence is built up. The route of this band of rock will be understood 
from the annexed ground-plan (Fig. 326). 
That this prolongation of one of the thick beds of Ben Hiant is in no 
respect a superficial lava-stream but a true sill, is proved not only by its 
escarpment and dip-slope, but by its actually passing under and indurating 
the schistose grits, as may be seen in the stream-section. Again Beinn na 
h-Urchrach, which is mapped by Professor Judd as a northern expansion 
of Ben Hiant, is likewise not a lava but a true sill. Not only does it dip 
northwards at an angle of about 20°, having the schists immediately below 
its crest on the one side and descending with a long dip-slope on the other, 
but dwindling down rapidly from a thickness of 100 or 200 feet in the 
centre to no more than a few feet in a south-westerly direction, it there 
passes under schistose grits like those on which it lies. The strata that 
adhere to its upper surface are as usual indurated. 
A section drawn across this attenuated development of the Beinn na 
h-Urchrach sill and that from Ben Hiant shows the structure represented 
in the accompanying diagram (Fig. 327), which simply gives the facts as 
exposed on the ground. The lower sill is that which issues from the main 
body of Ben Hiant, massive at first but diminishing in thickness as it 
recedes from its source. 
Again, among the sheets which descend from the northern face of the 
VOL. 11 
Fig. 326. — Ground-plan of Sills at Ben Hiant, 
Ardnamurclian. 
a a, crystalline schists ; b 6, necks of volcanic agglomerate ; 
c, c, numerous thin sills ; D, massive sill of Beinn na 
h-Urclirach ; E, north side of Ben Hiant ; F, sill proceed- 
ing from the series forming Ben Hiant and joining that 
of Beinn na h-Urchrach. The arrows mark the dip. 
Y 
