1 879.] Relations to that of the Glacial Epoch . 21 
The Great Diminution in the Thickness of the Ice-strata from 
the Top downwards not due , as supposed , either to Compression 
or to Melting. — The thinness of the lower as compared with 
the more superficial strata of the ice-sheet is considered by Sir 
Wyville Thomson to be mainly, if not altogether, due to 
two causes, Compression and Melting of the Ice, particu- 
larly the latter. “ The regularity of this diminution,” he 
says, “ leaves it almost without a doubt that the layers ob- 
served are in the same category, and that therefore the 
diminution is due to subsequent pressure or other aftion 
upon a series of beds which were at the time of their depo- 
sition pretty nearly equally thick. About 60 or 80 feet 
from the top of an iceberg the strata of ice are a foot or so 
in thickness, although of a white colour, and thus indicating 
that they contain a quantity of air and that the particles of 
ice are not in close apposition, are still very hard, and the 
specific gravity of the ice is not very much lower than that 
of layers not more than 3 inches thick nearer the water-line 
of the berg. Now it seems to me that this reduction cannot 
be due to compression alone, and that a portion of the sub- 
stance of these lower layers must have been removed.” 
If the layers 3 inches thick near the water-line were once 
a foot in thickness, as no doubt they were, then this great 
diminution in thickness cannot have been due to compres- 
sion, for had it been so the density of those layers would be 
more than double that of water. But Sir Wyville has found 
that the specific gravity of the layers 3 inches thick is not 
much lower than of those a foot in thickness, which proves, 
as he has pointed out, that compression cannot account for 
their thinness ; but it does not, as we shall presently see, 
necessarily prove “ that a portion of the substance of these 
lower layers must have been removed.” 
Assuming that the lower layers were all originally of the 
same thickness as the upper, it can nevertheless be shown 
that the gradual diminution in thickness of the layers from 
the top downwards follows independently altogether of com- 
pression or of the removal of any portion of the substance 
of the layers, either by melting or by any other means. 
Ice radiating from a Centre of Dispersion becomes thin- 
ner , because the space over which it is spread becomes greater . — 
There is this peculiarity in Continental ice, that there is a 
centre of dispersion from which the ice radiates in all direc- 
tions. This is particularly true in reference to the Antardlic 
ice-cap. It does not necessarily follow that the centre of 
