375 
their relative proportion by the length of the arrows; for 
there are several circumstances which cause the effective 
force of the current from one to be greater than that from 
the other. From the facts I have described, and others con- 
nected with the motions of the tides and waves, we may- 
determine the lines of axis of estuaries or straits ; the direc- 
tion of the more open sea, from which the tide wave pro- 
ceeded ; the line of the coasts, or positions of sandbanks or 
shoals ; the prevailing quarters of the wind, as well as 
probably the actual depth of the water ; the velocity of the 
currents affecting it, and the rate at which deposits were 
formed at the bottom. The knowledge of these facts is, of 
course, extremely valuable in ascertaining the circumstances 
under which any particular formation was accumulated. 
I purpose now to illustrate the application of these 
principles to particular cases ; but it is impossible to do so 
in a complete manner in the compass of this paper without 
maps. I shall, therefore, merely give some of the leading 
conclusions. I would first call attention to the upper group 
of the lower green sand in the neighbourhood of Folkestone, 
shewing a continuous simple current in one nearly uniform 
direction, over a considerable space, as being a very good 
example of the kind I referred to, as most probably not due 
to tidal or wave action. I am now employed in investigating 
the currents during the accumulation of the Magmesian 
Limestone, East of Sheffield ; and, though I have in no way 
completed my observations, yet I think they will throw much 
light on the subject generally, as well as on the circum- 
stances that occurred during its deposition. So far as I 
have yet examined, there appears to have been a rise 
and fall of the tide running North-east and South-west, 
amongst a number of shoals on which the surface waves 
stranded, that are elongated in the line of the tidal currents. 
The intervening channels are sometimes narrow, but in other 
