763 
its application to the case before us involves a great physical diffiz 
culty—-a depression of temperature, for which no. adequate cause 
has yet been assigned. The author does not admit the parallel 
which has been drawn between this case and that of places in equal 
latitudes in South America or that of the island of Georgia. 
Transport by Currents of Water.—It has already been remarked 
that the spreading out of diluvial matter into a horizontal stratum 
may be regarded as the necessary consequence of broad general 
currents, and that this has been the agency by which the mass of 
diluvium covering the surface of Lancashire has been carried there 
does not admit, in the author’s opinion, of the smallest doubt. He 
accounts for the existence of currents diverging from the centre of 
the district in question by a repetition of paroxysmal elevations. 
Suppose a certain area at the bottom of an ocean 'to be suddenly 
elevated; and, for the greater clearness, suppose the elevated area 
to be a circle of twenty miles in diameter, its elevation to be 50: 
feet, and the depth of the ocean 300 or 400 feet. If the elevation 
were sufficiently gradual no sensible wave would result from it, but 
if it were sudden the surface of the water above the uplifted area 
would be elevated very nearly as much as the area itself, and a 
diverging wave would be the consequence. Its front would be steep, 
like that of the tidal wave in some rivers called the bore, so that the 
highest part or summit of the wave would not be far from its front. 
The height at its summit would be approximately equal to the ele- 
vation of the uplifted area, or, in the case supposed, nearly 50 feet. 
The velocity with which the front would diverge would depend on 
the height of the wave and the depth of the ocean. In a certain time 
the water first raised above the general level of the ocean imme- 
diately over the elevated area would run off, leaving the surface of. 
the ocean there at its original level ; and when this should be com- 
pleted the posterior boundary of the wave would be immediately 
over the periphery of the elevated area. During the same time the 
front of the wave would move on through a certain space, still form, 
ing a circle of which the centre would be immediately over that o 
the elevated area. Thus the whole wave would at the instant now 
referred to be comprised between two concentric circles, the 
distance between which would be the breadth of the wave. A fter- 
wards, as the front or anterior boundary of the wave spread out- 
wards, so would the posterior boundary move forward in a similar 
manner, always preserving the annular form just mentioned. As 
the wave diverged its height would gradually diminish till it be- 
came finally insensible. 
The motion of the wave here spoken of is altogether distinct 
from the motion of translation of the aqueous particles. This 
latter motion, however, accompanies the former in the kind of wave 
here described, producing a current like that of a tidal river oppo- 
site to the usual course of the stream. Each particle begins to 
move onward the instant when the anterior boundary of the wave 
has reached it, but its motion being always slower than that of the 
