338 Proceedings of the Royal Physical Society. 
of the ocean. There is one circumstance in reference to the 
formation of these terraces on the borders of lakes which 
does not seem to be generally taken into consideration ; that 
is, the effect produced by floating ice. Water washing 
against a friable bank will no doubt wear it down, and in 
course of time will form a terrace ; but the operation will 
be much more speedily effected if it bear on its surface 
masses of ice which are carried by the waves against the 
banks, and act like the battering-rams of the ancient war- 
riors. The accounts that we read of the devastations some- 
times caused by floating ice brought down by the rivers in 
America amply confirm these remarks. It seems to us of 
great importance, in forming any hypothesis in regard to 
level terraces, raised beaches, sea-margins, or whatever other 
name may be given to them, to remember that they are of 
two kinds. One is formed below the surface of the water, 
and the other above it ; one is produced by the retreating, 
and the other by the advancing billow. The deposits left by 
rivers are altogether different in their origin and character 
from those which we find on the banks of lakes and on the 
shores of the sea. So long as the river is confined in a nar- 
row channel, the gravel and mud which it brings down are 
swept along together by the current. When the channel 
becomes wider, the sand and gravel are left behind, and 
form a bed, which has always a distinct downward slope 
according to the course of the stream. When the channel 
is still farther enlarged, the current becomes slower, the mud 
is deposited, and forms deltas and valleys, like those of the 
Mississippi and Ganges, with a surface nearly horizontal. 
Character and Appearance of these Terraces. 
The terraces found on the sides of lakes are regular and 
horizontal, making allowance, of course, for those places 
where the waves are particularly high ; but this is not the 
case in regard to the deposits left by the ocean. The upper 
terraces, formed by the advancing billow above ordinary high- 
water mark, sometimes exhibit a very remarkable horizon- 
tality of outline. When the material thrown up consists of 
shingle or gravel of such weight as to resist the action of 
