1883. ] Geological Effects of a Varying Rotation of the Earth. 17 
water or ice, by some secular change in the earth. For example, 
the accumulation of ice at the poles during the glacial period. It 
should be noted here, that the circulation of ocean currents and 
aérial currents have no effect, so long as the sea-level remains 
constant. Every current, however strong, or whatever its direc- 
tion, is counteracted by others. (3) A third case under this head, 
is the transfer of sediment, either by river or ocean currents. All 
streams flowing toward the poles conspire to this effect. The 
higher the latitude the more efficient the stream, other things be- 
ing equal, but as the erosion is diminished by the cold, those in 
middle latitudes are probably the most efficient. 
On the other hand, retarding influences are as follows: 
First, and most unquestioned—the friction of the tides. The 
conditions in which this force would be most efficient have not, so 
far as the writer is‘aware, been satisfactorily stated. An alterna- 
tion of oceans broad enough to accumulate the wave to its utmost, 
and of narrow continents, with shores adapted to raise the water to 
its highest point, and wholly check its flow, would seem perhaps 
the most, favorable. 
Second. The transfer of matter of any kind from higher to 
lower latitudes. The remarks made under the head correspond- 
ing, above, will apply equally well in this case. 
Third. Any elevation of the earth’s crust, either local or gen- 
eral, in lower latitudes, resulting either from increased heat, as near 
volcanoes, or from any bending of the earth’s crust. If in the lat- 
ter case, both the anticlinal and synclinal folds of the crust are in 
the same latitude, no change in rotation would result. 
Fourth, and last, but by no means least, we would rather say 
greatest, a distortion of the earth’s body by the attraction of the 
sun and moon. The degree of viscosity assumed by Mr. Darwin 
would seem to be little enough to satisfy the most ultra rigida- 
tian or uniformitarian, and if a more yielding condition be predi- 
cated of the earth, certainly its effect will be indefinitely magni- 
fied. Whether instruments will ever be invented delicate enough 
to measure its amount is doubtful. 
1 That this force is really efficient at the present time is attested by the influence 
of a variation in distance of these bodies on the occurrence of earthquakes. Fron» 
the researches of Perrey, Volger and others, we learn that earthquakes are much 
more numerous when the earth is near perihelion than when near aphelion, and 
that they occur more frequently and with more violence when the moon is in perigee 
than a: other periods, 
VOL. XVII.—no. I. 2 
