542 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
of the granite belt, no doubt the belt would there be entirely buried; or 
if, on the other hand, the same thickness of rock could be stripped off 
some parts of the band of schist, the solid granite underneath would be 
laid bare. The extent of granite surface exposed must thus be largely 
determined by the amount of denudation, and by the angle at which 

4 
* 
a 
“f 
. 
-the upper surface of the granite is inclined beneath the schists. Where 
the inclination is high, prolonged denudation will evidently do com- 
paratively little in widening the belt. But where the slope is gentle, 
and especially where the surface undulates, the removal for some 
distance of a comparatively slight thickness of rock may uncover a ~ 
large breadth of underlying granite.1 Portions of the metamorphosed 
rocks left by denudation upon the surface of the granite boss, are - 
relics of the deep cover under which the granite no doubt originally 
lay, and being tougher than the latter rock they have resisted waste 
so as now to cap hills and protect the granite below, as at Lugnaquilla 
(L, in Fig. 269). ‘ 
Recent observations by Professor Hull and Mr. Traill, of the 
Geological Survey of Ireland, have shown that in the Mourne Moun- 
tains a mass of granite has in some parts risen up through highly 
inclined Silurian rocks, which consequently seem to be standing almost 
upright upon an underlying boss of granite. The strata are sharply 
truncated by the crystalline mass, and are indurated but not otherwise 
altered. The intrusive nature of the granite is well shown by the 
way in which numerous dykes of dark melaphyre are cut off when they 
reach that rock.2, The accompanying diagram (Fig. 270) is taken from 

Fia. 270.—Sr0rion oF SLIEVENAMADDY, Mourne Mounrtatns. 
a, a, Lower Silurian strata dipping at high angles; b, b, Dykes of basalt (melaphyre), 
cutting these strata but truncated by the granite c, which along the outer margin 
and in extruded veins passes into a quartz-porphyry, d, d. 
one of the sections in which this remarkable structure is portrayed by — 
these observers. 
In the Lower Silurian tract of the south of Scotland several large 
intrusive bosses of granite occur (Fig. 271). The strata do not dip away 
from them on all sides, but with trifling exceptions maintain their normal 
N.E. and §8.W. strike up to the granite on one side, and resume it 
again on the other. The granite indeed has not merely pushed aside 
the strata so as to make its way past, but actually occupies the place of 
so much Silurian greywacke and shale, which have disappeared as if 
they had been blown out or had been melted up into the granite. There 
is usually a metamorphosed belt of about a mile in width in which, as 
they approach the granite, the stratified rocks assume a schistose or 
gneissoid character. Numerous small, dark, often angular patches or 
fragments of mica-schist may be observed in the marginal parts of the 
* See Jukes’s Manual of Geology, 3rd ed. p. 243. 
* Horizontal Section No, 22, Geol. Surv. Ireland. 
