7 eee DYNAMICAL GEOLOGY. [Book IIL. 
geological structure of the rocks powerfully influences the nature and 
rate of the encroachment of the sea. Where, owing to the inclina- 
tion of bedding, joints, or other divisional planes, sheets of rock slope 
down into the water, they serve as a kind of natural breakwater, up 

WHICH SERVES TO ARREST THE PROGRESS OF THE WAVES. 
and down which the surges rise and fall during calms, or rush in 
crested billows during gales, the abrasion being here reduced to the 
smallest proportions. In no part of the degradation of the land, can 
the dominant influence of rock-structure be more conspicuously 
observed and instructively studied, than along marine cliffs, Where 
the lines of precipice are abrupt, with, numerous projecting and 
retiring vertical walls, it will almost invariably be found, that these 
perpendicular faces have been cut open along lines of intersecting 
joint. The existence of such lines of division permits a steep or 
vertical front to be presented by the land to the sea, because, as 
slice after slice is removed, each freshly bared surface is still defined 
by a joint-plane. (See Book IV., Sect. ii.) 
But during the study of any rocky coast where these features are 
exhibited, the observer will soon perceive that the encroachment of. 
the sea upon the land is not due merely to the action of the waves, 
but that even on shores where the gales are fiercest and the breakers 
most vigorous, the demolition of the cliffs depends mainly upon the 
sapping influence of rain, springs, frosts, and general atmospheric 
disintegration. In Fig. 164, for example, which gives a view of a 
portion of the northern Caithness coast, exposed to the full fury of 
the gales and rapid tidal currents which rush from the Atlantic 
through the Pentland Firth, we see at once that though the base of 
the cliff is scooped out by the restless surge into long twilight caves, 
nevertheless the recession of the precipice is caused by the wedging 
off of slice after slice, along the lines of vertical joint, and that this 
process begins at the top, where the subaerial forces and not the 
waves are the sculptors. Undoubtedly the sea plays its part by 
removing the materials dislodged, and preventing them from accumu- 
lating against and protecting the face of the precipice. But were it 
not for the potent influence of subaerial decay, the progress of the 
sea would be comparatively feeble. The very blocks of stone which 
give the waves so much of their efficacy as abrading agents, are in 
great measure furnished to them by the action of the meteoric 

