EVIDENCES OF THE AGE OF ICE. 
113 
as in Greenland, pushed into the sea itself, for in Lancashire 
this deposit is associated with fragments of marine shells. 
Where this is the case, the finer particles have been carried 
away and re-assorted by sea-currents and mixed with marine 
organisms, as in the Clyde glacial beds ; the shells indicating 
arctic conditions. 
During one period of the glacial epoch we had a great de- 
pression of the land ; to such an extent, indeed, that these 
glacial deposits, associated with marine shells identical with 
species now existing, have been found in Cheshire up to a height 
of 1,200 feet. On Moel Tryfaen, near the Menai Straits, fifty- 
seven species of shells of marine mollusca have been obtained (all 
indicative of a colder climate than that of our present sea) at a 
height of 1,300 feet above the sea. 
It was at this period, no doubt, that icebergs and floes of ice 
laden with boulders and other foreign material were transported 
from the western and northern highlands and dropped their 
burdens, on melting, over and about where London now stands, 
especially in the neighbourhood of Finchley and Muswell Hill, 
where vast numbers of erratic blocks have been observed, and 
numbers of transported fossils have been collected by the late 
Mr. N. T. Wetherell, F.G.S., and other geologists. 
Various theories have been brought forward in explanation of 
the glacial epoch. Among these that of Mr. James Croll, F.R.S., 
of the Scottish Branch of the Geological Survey (also adopted 
in the main by Mr. James Geikie, F.R.S.), has been largely 
advocated. It is based on the calculation that at certain un- 
equal periods, owing to the eccentricity of our earth’s orbit 
around the sun, the earth is occasionally somewhat further dis- 
tant from the sun than at the present time (98,500,000 miles, 
instead of as at present, 90,500,000 ; or, to be exact, 8,641,876 
miles more distant). The last occasion Mr. Croll puts at 200,000 
years ago. 
The other cosmical cause advocated by Mr. Croll is the slight 
variation in the polar obliquity of our earth, which varies 
through long periods between 23J° and 24^°. 
When the earth, from these two causes combined, became 
subject to a slight variation in its two hemispheres, which would 
give to one 7-J days more of the sun’s presence in one tropic 
than the other now enjoys, then Mr. Croll concludes the ice on 
the more favoured pole would melt, and that on the less favoured 
would increase ; and this cause alternating, would give rise at 
long intervals to alternate glacial epochs in each hemisphere, 
accompanied by displacements of the earth’s centre of gravity, 
and a rising of the waters of the sea combined with an increase 
of ice at the pole.* 
* See Mr. Croll’s paper on the Glacial Epoch, “ Geol. Mag.,” 1874, p. 348. 
