THE PHYSICAL GEOGRAPHY OF THE PAST. 
247 
other, will be more stable than the convex portions in which 
the strata incline outwards. In one case the force of gravity 
will retard denudation, in the other it will facilitate it. And 
thus it may, and often does come to pass that the summit of 
a hill is coincident with the trough of one of the folds, or the 
synclinal as it is called, whilst the valley runs along the 
anticlinal. Another reason for the difference is to be found 
in the frequent fracture of the tops of anticlinals allowing the 
freer access of water, and thus hastening the destruction of 
the arch. 
It now becomes easier to understand what I have said about 
the saucer-like shape of the western portion of our Coalfield 
conducing to its preservation. This is a structure common 
to nearly all our Coalfields, and is merely a geological instance 
as applied to strata of the “ survival of the fittest ” to with¬ 
stand denudation.* 
Having now briefly glanced at the physical features and 
geological structure of the Pennine Range and of its southern 
extension under the newer rocks of the Central Midlands, we 
must try to ascertain something about the conditions under 
which the sediments forming the various portions of this huge 
pile of Carboniferous Rocks were originally deposited. But 
before we attempt to do this let us turn for a moment to what 
is going on at the present day around our coasts, and see if 
we cannot deduce from our observations some guiding prin¬ 
ciples with regard to the phenomena of sedimentation, which 
may help us in our enquiry. 
We find that the material which is constantly beiifg 
brought down by streams and rivers, and which has of course 
been derived from the degradation and waste of the land, is 
deposited on the bottom of the sea when the velocity of the 
currents bringing it down has been sufficiently checked. The 
particles thus carried down to the ocean vary in size from 
large rounded pebbles to the finest possible mud; and, since 
* I do not know any better instance of the comparative stability of 
synclinally curved strata over strata curved in the opposite direction 
than is shown in a portion of our coast which most of you know well. 
The great Orme’s Head at Llandudno is a bold hill of massive 
Mountain Limestone connected with a smaller hill of the same rock, 
the Little Orme, by a very narrow neck of low-lying ground. When 
the two hills are seen from Llanfairfechau on the west, it can be 
clearly seen that the strata of the Great Orme are bent upwards in a 
synclinal or saucer-like form, and it is perfectly evident from the lines 
of the curved strata that they once bent over again in the form of an 
arch to the mainland, and joined those of the Little Orme. The 
result of denuding forces acting equally upon this once continuous 
mass, has been entirely to remove the comparatively weak arch or 
anticlinal portion, and to leave the saucer-like synclinal untouched. 
