mass, and forced up on the beach, forming
a mound more than 16 feet high, which threw down the walls of several
buildings. "When I visited the spot next day, I saw ridges of ice, sand,
and pebbles, not only on the shore, but extending far out into the
bottom of the sea, showing how greatly its bed had been changed, and how
easily, where it is composed of rock, it may be furrowed and streaked by
stones firmly fixed in the moving ice."[303]

CHAPTER XVI.

PHENOMENA OF SPRINGS.

Origin of Springs--Artesian wells--Borings at Paris--Distinct causes
by which mineral and thermal waters may be raised to the
surface--Their connection with volcanic agency--Calcareous
springs--Travertin of the Elsa--Baths of San Vignone and of San
Filippo, near Radicofani--Spheroidal structure in travertin--Lake of
the Solfatara, near Rome--Travertin at Cascade of Tivoli--Gypseous,
siliceous, and ferruginous springs--Brine springs--Carbonated
springs--Disintegration of granite in Auvergne--Petroleum
springs--Pitch lake of Trinidad.

_Origin of springs._--The action of running water on the surface of the
land having been considered, we may next turn our attention to what may
be termed "the subterranean drainage," or the phenomena of springs.
Every one is familiar with the fact, that certain porous soils, such as
loose sand and gravel, absorb water with rapidity, and that the ground
composed of them soon dries up after heavy showers. If a well be sunk in
such soils, we often penetrate to considerable depths before we meet
with water; but this is usually found on our approaching the lower parts
of the formation, where it rests on some impervious bed; for here the
water, unable to make its way downwards in a direct line, accumulates as
in a reservoir, and is ready to ooze out into any opening which may be
made, in the same manner as we see the salt water flow into, and fill,
any hollow which we dig in the sands of the shore at low tide.

The facility with which water can percolate loose and gravelly soils is
clearly illustrated by the effect of the tides in the Thames between
Richmond and London. The river, in this part of its course, flows
through a bed of gravel overlying clay, and the porous superstratum is
alternately saturated by the water of the Thames as the tide rises, and
then drained again to the distance of several hundred feet from the
banks when the tide falls, so that the wells in this trac