Year: 1875

torn off by the advancing ice sheet from
the projecting rocks over which it moved,
and carried or pushed as bottom drift,
scratching and blowing the surface over
which they passed, and being scratched
and polished themselves in return, till
they were finally brought to rest by the
melting of the ice. They were not car
ried as far south as the ice sheet extend
ed, seldom beyond the parallel of 4O0 n.
The native copper of Lake Superior was
drifted four or five hundred miles south
and the pudding stones of Roxbury.
Massachusetts, were carried as far south
as the Island of Penikese.


The tough, elastic clay, called in this
country hard pan," and in Scotland
"till, is described as the oldest deposit
of ice agency. 1t was formed under the
great ice sheet as it ground along on the
earth's surface. This was often plowed
and forced forward in a confused mass,
was thickest in the valleys and on the
lee side of crags and other distractions,
and thins out towards the mountain tops,
where it appears only protected places.


During the existence of this ice sheet
over the earth's surface, geologists tell
US there was great depression of the
crust, which of course resulted in an
equally great encroachment of the sea
upon the land. Various reasons have
been assigned as the cause of this. Ad-
hemar, and later Croll, attribute it to a
displacement of the earths' centre of
gravity, due to an increased weight of ice
at the north pole, while at the same
time there is a diminution of ice at the
south pole. The latter authority esti-
mates that the melting of two miles of
ice from the antartic regions would
raise the ocean level one foot; and the
simultaneous abstraction of heat from
the arctic regions would add a mile to the
thickness of the ice at the north pole;
and that these results would so change
the earth's centre of gravity as to cause
a submergence of nearly 500 feet at the
northern polar religions, and a gradual
diminution of this amount towards the
equator But, unfortunately for this
theory, the facts show that the amount
of submersion does not diminish with
uniformity as we recede from the pole,
but with much greater rapidity than the
theory requires.


Another theory, advanced by Professor
Shaler, attributes this depression to the
weight of ice accumulated on the CONT
nents during the glacial period This
theory assumes that cap of ice, mile
or more in thickness, would, by its gravi-
-ty, depress the earth's crust at least 500
feet from the Great Lakes to the Arctic
Ocean. That this weight would tend to
produce depression, cannot be denied;
but it is hardly probable that it would
be sufficient to produce so great an effect
upon the earth's crust, which is probably
not less than 100 miles thick, and SO
shaped as to offer a resistance to pres
sure very similar to that of monstrous
arched bridge. 1t would seem more nat
ural and reasonable to attribute this de-
preston to the commonly accepted cause
of all the elevations and depressions on
the earths surface: the contraction of
its crust due to constant cooling.


After the disappearance. of the ice
sheet there were local glaciers which
conformed to the larger valleys, like
those now seen in the Alps. They car
ried down boulders torn from the rocks
along their sides, and deposited them,
with other debris, as terminal and lateral
moraines, which often damned up rivers
and formed many of the lakes which
now beautIfy our landscapes. Other
lakes were formed in the solid rock by
the gouging action of the boulders fro
zen into the ice. At perhaps about the
same time there was again an upheaval
or elevation of the land, or, more strict
ly, SUGGESTIONS of elevations. These
caused a rapid drainage, from the land,
of the water previously making up the
ice sheet, by which the drift was strait
fed and its pebbles assorted and round
ed; hills and ridges were formed by
deposition of material suspended in the
water, just as drifts of various shapes
and sizes are formed by the snow sus-
pended in the air; and the beautiful ter
races which ornament so richly our river
valleys are formed. These elevations are
also due to shrinkage of the earth's
crust, for depression in one place must
necessarily make an elevation in an
other.