gravel are found along; their margins, being brought in by streams
and worn by waves from the shore, but there are no tidal or other
strong currents to sweep coarse waste out from shore to any
considerable distance. Where fine clays are now found on the land
in even, horizontal layers containing the remains of fresh-water
animals and plants, uncut by channels tilled with cross-bedded
gravels and sands and bordered by beach deposits of coarse waste,
we may safely infer the existence of ancient lakes.

MARL. Marl is a soft, whitish deposit of carbonate of lime,
mingled often with more or less of clay, accumulated in small
lakes whose feeding springs are charged with carbonate of lime and
into which little waste is washed from the land. Such lakelets are
not infrequent on the surface of the younger drift sheets of
Michigan and northern Indiana, where their beds of marl--sometimes
as much as forty feet thick--are utilized in the manufacture of
Portland cement. The deposit results from the decay of certain
aquatic plants which secrete lime carbonate from the water, from
the decomposition of the calcareous shells of tiny mollusks which
live in countless numbers on the lake floor, and in some cases
apparently from chemical precipitation.

PEAT. We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants
of the surface of the bog lies a spongy mass composed of dead
vegetable tissue, which passes downward gradually into PEAT,--a
dense, dark brown carbonaceous deposit in which, to the unaided
eye, little or no trace of vegetable structure remains. When
dried, peat forms a fuel of some value and is used either cut into