4 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
northern part of its course do not really affect our impression of the 
persistence of the sheet. They not improbably indicate merely that in 
its protrusion it had a wavy irregular limit, which ill the progress of 
denudation has occasionally been not yet reached. For mile after mile 
the sill has been mapped by the Geological Survey in lines of crag across 
the moorlands, and as a conspicuous band among the limestones and shales 
that form the steep front of the Pennine escarpment, where it has long been 
known in the fine sections exposed among the gullies by which that noble 
rock-face has been furrowed. 
Along its main outcrop, the sill dips gently eastwards below the portion 
of the Carboniferous Limestone series which overlies it. But so slight are 
the inclinations, so gentle the undulations of the rocks in this part of the 
country, that far to the east of that outcrop the sill has been laid bare by 
the streams which in the larger dales have cut their way through the over- 
lying cake of Carboniferous strata down to the Silurian platform on which 
Fig. 176. — Section from tlie great Limestone escarpment on the west to the Millstone Grit hills 
east of Teesdale. 
1. Silurian strata; 2. Carboniferous Limestone series ; 3. The Great Whin Sill, which gradually rises to higher 
stratigrapliical position as it goes westward ; 4. Millstone Grit. 
they rest (Fig. 176). Among these inland revelations of the eastward con- 
tinuation of the sill under Carboniferous Limestone strata, the most striking i 
and best known are those which have been made by the River Tees, and of 
which the famous waterfalls of the High Force and Cauldron Snout are the j 
most picturesque features. The distance of the remotest of these denuded 
outcrops or “ inliers ” from the main escarpment is not less than 2 0 
miles. 
It is not possible to form an accurate estimate of the total underground 
area of the Whin Sill. In the southern half of the district, south of the 
line of the Roman Wall, where, the inclination of the strata being generally 
low, the same stratigraphical horizons are exposed by denudation far to the 
east of the main outcrops of the rocks, we know that the sill must have a 
subterranean extent of more than 400 square miles. Yet this is probably 
only a small part of the total area over which the molten material was 
injected. In the northern part of the district, the Carboniferous Limestone 
series is not exposed over so broad a stretch of country, and denudation has 
not there revealed the eastward extension of the sill. But there is no reason 
to suppose the sheet to be less continuous and massive there. We must 
remember also that the present escarpment has been produced by denudation, 
and that the intrusive sheet must have once extended westwards beyond its 
present limits at the surface. If, therefore, we were to state broadly that 
the Great Whin Sill has been intruded into the Carboniferous Limestone 
