ght are
left untouched. And while we can thus sink our plummet from the summit
to the base of Mount Washington and measure the thickness of the mass of
ice, we have a no less accurate indication of its extension in the
undulating line marking the southern termination of the drift. I have
shown that the moraines mark the oscillations of the glaciers in Europe.
Where such accumulations of loose materials took place at its terminus,
there we know the glacier must have held its ground long enough to allow
time for the collection of these _debris_. In the same way we may trace
the southern border of our ancient ice-sheet on this continent by the
limit of the boulders; beyond that line it evidently did not advance as
a solid mass, since it ceased to transport the heavier materials. But as
soon as the outskirts of the ice began to yield and to flow off as
water, the lighter portions of the drift were swept onward; and hence we
find a sheet of finer drift-deposit, sand and gravel more or less
distinctly stratified, carried to greater or less distances, and fading
into the Southern States, where it mingles with the most recent
river-deposits.

One naturally asks, What was the use of this great engine set at work
ages ago to grind, furrow, and knead over, as it were, the surface of
the earth? We have our answer in the fertile soil which spreads over the
temperate regions of the globe. The glacier was God's great plough; and
when the ice vanished from the face of the land, it left it prepared for
the hand of the husbandman. The hard surface of the rocks was ground to
powder, the elements of the soil were mingled in fair proportions,
granite was carried into the lime regions, lime was mingled with the
more arid and unproductive granite districts, and a soil was prepared
fit for the agricultural uses of man. I have been asked whether this
inference was not inconsistent with the fact that a rich vegetation
preceded the ice-period,--a vegetation sufficiently abundant to sustain
the tropical animals then living throughout the temperate regions. But
the vegetation which has succeeded the ice-period is of a different
character, and one that could not have flourished on a soil that would
nourish a more tropical growth. The soil we have now over the temperate
zone is a grain-growing soil,--one especially adapted to those plants
most necessary to the higher domestic and social organizations of the
human race. Therefore I think we may believ