TRANSACTIONS OF THE SECTIONS. 61 
eras. forces, so as to cause them to excavate the valley along that line rather than 
any other. 
‘When, moreover, we examine faults below ground, we find no trace of any wide- 
gaping fissures; the walls of the fault, on the contrary, are jammed tightly against 
each other, and show frequent evidence of immense grinding force, proving the 
friction of the sides to have been enormous. In hard massive rocks there doubtless 
occur open spaces here and there between the walls, “pockets”’ or “bellies” between 
their projecting protuberances, or where they have been partly kept asunder by 
fragments detached from the sides. These are often full of crystalline minerals, 
and form “mineral veins” below, but seldom, if ever, form valleys or ravines at 
the surface. 
If these ideas as to the relative action of the internal and external forces at work 
upon the crust of the globe be well founded, it follows that none of the present 
features of the surface of the globe have been produced by the direct action of the 
internal forces, except volcanic orifices and cones, and that all others have been 
produced by the process of external erosion, except such as have been formed by 
external deposition, like hills of blown sand or alluvial flats and deltas. 
The surfaces of our present lands are as much carved and sculptured surfaces as 
the medallion carved from the slab, or the statue sculptured from the block. They 
have been gradually reached by the removal of the rock that once covered them, 
and are themselves but of transient duration, always slowly wasting from decay. 
Eyen, then, if the internal forces could produce such external features, it can always 
be shown that the surface which existed when they operated has long since dis- 
appeared, together with, in many cases, vast thicknesses of rock that intervened 
between it and the present one. 
It remains to say a few words on the nature of the erosive agencies which form 
these surfaces. 
The ocean is the grandest of these. The ceaseless breaking of its waves against 
the margin of the land constantly gnaws into and undermines it, and the tides and 
currents carry off the eroded materials and deposit them on some part or other of 
the ocean-bed. This action is that of a great horizontal planing-machine, always 
tending to the production of level surfaces, the cutting power being confined to 
the sea-level, while the matter carried off tends to fill up the hollows of the in- 
equalities that lie below it. The denuding action of the sea, therefore, produces 
“plains of denudation” on the parts it has passed over, and long lines of cliffs or 
steep banks along the margin where its influence ceased. It is essential for the 
energetic action of the sea that it should be the open sea, where a heavy swell can 
roll in upon the land, and where gales of wind can hurl furious waves against it. 
In sheltered bays and narrow inlets and fiords its erosive agency becomes compara- 
tively small, and in very protected places sinks to nothing. 
While, then, we look to marine denudation as the cause of wide plains, of long 
escarpments, of bold headlands and isolated hills, and of the general outline of 
mountain-chains, and as the remover of the great groups of rock that were con- 
tinuous over the area of the mountains before their elevation was commenced, I 
believe we err when we attribute to that cause the lesser features by which these 
greater ones are themselves modified. The river valleys that traverse the great plains, 
the gullies that run down the sides of the hills, the valleys, glens, ravines, and gorges 
that furrow the flanks of the mountain-chains, have, I believe, all been caused by 
atmospheric agency on the land, while standing above the level of the sea. 
The only case in which the sea tends to produce anything like a valley is that in 
which it forms open sounds or straits between islands, where the set of the tides 
and currents imparts to it a river-like action. Those depressions in the crest of a 
mountain-chain which are called “passes” or “ gaps” have doubtless been often 
eaused by this action, but it is obvious that this ceases as soon as the summit of 
the pass once rises above the sea-level and prevents the currents from sweeping 
through it. 
While the ordinary erosive action of the sea is a horizontal one, tending to the 
production of plains bordered by cliffs, that of the atmospheric agencies is a ver- 
tical one, always tending to the production of furrows, or more or less steep-sided 
channels, on all the land eet to their influence. 
