1882] . 
Correspondence, 
1 77 
TIDAL THEORY OF GEOLOGICAL DENUDATION, 
To the Editor of the Journal of Science, 
Sir, — Some interest has lately been created by ingenious specu- 
lations regarding the Moon's action on the waters of the Earth 
in past ages, and especially in the creation of geological changes 
on the surface. 
The theory proposes to establish gigantic tides in the terres- 
trial oceans, upheaved by a closer approximation of the Moon, 
in its orbit, to the Earth, thus increasing its attractive power. 
It is believed there would be created a tremendous force of 
current aCIing as a powerful denuding agent on the superficial 
formations exposed to its aCtion. 
I am inclined to think that some illusion gilds this attractive 
idea , which looks well on paper to the physicist, but can hardly 
be so confidently accepted by the geologist. 
In the first place, the moon’s aCtion creates no currents in the 
open sea, the open tide being merely a temporary elevation of 
the sea-level at points where the moon is passing across its sur- 
face, and of course it can create no tides on the dry land, where 
there is no water. The water in the wave of oscillation up and 
down does not follow the moon at all, but, as soon as the moon 
has passed that spot, sinks down again. 
The tidal wave flows and ebbs in enclosed channels, and seas do 
not denude the bounding lands into hills and valleys, but level 
them flat into sand-banks and mud-flats. 
The rise and fall of 648 feet of the tides would be impracti- 
cable to measure, as they could only take place in the open ocean 
in sufficient depth of water, and would be impossible to realise 
in our narrow and shallow seas. 
In the case of tidal waves of oscillation there is but little 
analogy with storm waves, as the forces elevating the tidal are 
perpendicular to the surface of the sea, whereas in the latter 
they are horizontal. The impaCt of their momenta would corre- 
spond in direction, the tidal one taking a perpendicular line with 
lateral offset, and the storm one taking a horizontal direction 
with perpendicular subsidence. 
In the open ocean, if it could be gauged, a tidal rise and fall 
of 648 feet would take place gently and gradually, and slowly, 
and round the coasts the tidal flow and ebb would only be as 
high as the depth of water there would permit. A wave of 
water cannot be conceived to travel over land like a gale or 
dust storm, as it would collapse at once when brought into shal- 
lows of its own depth from the open sea. 
Most of the denudation we see on dry land has not been 
effected by the sea, but by fresh waters in some form, as ice and 
