. 
103 
Another agent of destruction is the atmosphere. Composed 
of oxygen, nitrogen, and carbonic acid, its action is principally 
chemical, and is observable more or less on all exposed surfaces, 
Its gases, partly by themselves, and partly in conjunction with the 
moisture in which they are diffused, exert a wasting and weathering 
influence on rocks of every description—softening, loosening, and 
crumbling them down to be more readily borne away by currents 
of wind and water. Carbonic acid acts specially on all rocks 
containing lime. Oxygens rust or oxydises those impregnated with 
iron. Moisture insinuates itself everywhere, and thus in after 
years the hardest rock exhibits a wasted and weathered surface. 
Particle after particle is loosened, film after film falls away, a new 
surface is exposed to new waste, and in the course of ages the 
boldest mountain mass yields to this silent imperceptible agency. 
The operations of these various denuding agents are divided 
by geologists into two classes—chemical and mechanical; the 
former including the chemical action of the atmosphere and water, 
the latter, those agents which act either by abrasion or some other 
mechanical means—this class includes rain and rivers, waves of 
water and frost. 
Knowing now that these apparently powerless agents are 
capable of modifying the form of the earth’s surface, we have only 
to keep clearly before us the manner in which they act, to be 
convinced, once for all, that, other things being equal, their effect 
will be greatest on those parts of the earth’s crust which are 
softest, in the same way that a sculptor or a quarryman can work 
more rapidly in a soft stone than in one that is hard. And further, as 
the work ofa quarryman is greatly facilitated by the presence of joints, 
so is the action of these denuding agents. The more a rock is 
split up by joints, the greater is the destructive effect on it of both 
water and the atmosphere—chemically as well as mechanically— 
because the more readily do they gain access to its interior. 
Keeping these facts constantly before us, we shall, I think, 
be able now to appreciate fully the subject we have to consider, 
that is, the Influence of Geological Structure on Scenery. Let us 
commence with our sea-coasts. 
