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coal strata, though the water must have been salt. The same 
is the case higher up, where genuine marine shells cease ; 
but in the upper parts, and in the lower new red sandstone, 
tidal action is very distinct. I shall now, however, confine 
myself to the consideration of tides and waves, and, after 
explaining my views of their motion, illustrate the applica- 
tion of those principles to the geology of particular districts. 
The movements of the particles of water in waves are not 
easily understood from mere description, and, therefore, I 
have constructed a moveable model to shew them. The chief 
points to be borne in mind are, that when the crest of the 
wave is passing over any spot, the water at the surface is 
advancing in the line of the wave's movement ; whilst, when 
the lowest part is passing over it, the motion of the surface is 
in the opposite direction. This is owing to the particles mov- 
ing in a circle, as will be seen from the model. The amount 
of this forward and backward movement depends upon the 
size of the wave and the depth from the surface, being equal 
to the height of the wave on the surface, and gradually vanish- 
ing downwards, so that at no great depth there is no motion 
of such a magnitude as to produce any material effect on the 
deposits at the bottom. However, when the wave advances 
from deep to shallow water, as for instance to the shores or 
a sandbank, there is produced, by the passage of each wave, 
a current moving in the line of the wave's motion, and alter- 
nately flowing forwards and backwards as the highest or 
lowest part of the wave is passing. When the tide rises and 
falls it is, in fact, a large low wave moving onwards, and 
when it advances into a shallower sea, a narrow estuary or 
strait, there is a current, oscillating in a similar manner in 
the line of the axis of the estuary or strait. Whatever be 
the direction of the wind and of the movement of the surface 
waves some distance from the shore, yet, when approaching 
the coast, and coming into shallower water, being retarded by 
