layers of sand, clay, or marl, may be wanting, one or more of them having
thinned out and given place to others, or sometimes one of the masses first
examined is observed to increase in thickness to the exclusion of other beds.

The term "FORMATION," which I have used in the above explanation, expresses in
geology any assemblage of rocks which have some character in common, whether of
origin, age, or composition. Thus we speak of stratified and unstratified,
fresh-water and marine, aqueous and volcanic, ancient and modern, metalliferous
and non-metalliferous formations.

In the estuaries of large rivers, such as the Ganges and the Mississippi, we may
observe, at low water, phenomena analogous to those of the drained lakes above
mentioned, but on a grander scale, and extending over areas several hundred
miles in length and breadth. When the periodical inundations subside, the river
hollows out a channel to the depth of many yards through horizontal beds of clay
and sand, the ends of which are seen exposed in perpendicular cliffs. These beds
vary in their mineral composition, or colour, or in the fineness or coarseness
of their particles, and some of them are occasionally characterised by
containing drift-wood. At the junction of the river and the sea, especially in
lagoons nearly separated by sand-bars from the ocean, deposits are often formed
in which brackish and salt-water shells are included.

In Egypt, where the Nile is always adding to its delta by filling up part of the
Mediterranean with mud, the newly deposited sediment is STRATIFIED, the thin
layer thrown down in one season differing slightly in colour from that of a
previous year, and being separable from it, as has been observed in excavations
at Cairo and other places. (See "Principles of Geology" by the Author Index
"Nile" "Rivers" etc.)

When beds of sand, clay, and marl, containing shells and vegetable matter, are