a maximum thickness
of not more than 50 feet. It consists of soft sands, with occasional
intercalations of flaggy limestone. Though of small extent and
thickness, the Coralline Crag is of importance from the number
of fossils which it contains. The name "Coralline" is a misnomer;
since there are few true Corals, and the so-called "Corals" of
the formation are really _Polyzoa_, often of very singular forms.
The shells of the Coralline Crag are mostly such as inhabit the
seas of temperate regions; but there occur some forms usually
looked upon as indicating a warm climate.

The _Upper_ or _Red Crag_ of Suffolk--like the Coralline Crag--has
a limited geographical extent and a small thickness, rarely exceeding
40 feet. It consists of quartzose sands, usually deep red or
brown in colour, and charged with numerous fossils.

Altogether more than 200 species of shells are known from the
Red Crag, of which 60 per cent are referable to existing species.
The shells indicate, upon the whole, a temperate or even cold
climate, decidedly less warm than that indicated by the organic
remains of the Coralline Crag. It appears, therefore, that a
gradual refrigeration was going on during the Pliocene period,
commencing in the Coralline Crag, becoming intensified in the Red
Crag, being still more severe in the Norwich Crag, and finally
culminating in the Arctic cold of the Glacial period.

Besides the _Mollusca_, the Red Crag contains the ear-bones of
Whales, the teeth of Sharks and Rays, and remains of the Mastodon,
Rhinoceros, and Tapir.

The _Newer Pliocene_ deposits are represented in Britain by the
_Norwich Crag_, a local formation occurring near Norwich. It
consists of incoherent sands, loams, and gravels, resting in
detached patches, from 2 to 20 feet in thickness, upon an eroded
surface of Chalk. The Norwich Crag contains a mixture of marine,
land, and fresh-water shells, with remains of fishes and bones
of mammals; so that it must have been deposited as a local
sea-deposit near the mouth of an ancient river. It contains
altogether more than 100 marine shells, of which 89 per cent
belong to existing species. Of the Mammals, the two most important
are an Elephant (_Elephas meridionalis_), and the characteristic
Pliocene Mastodon (_M. Arvernensis_), which is hitherto the only
Mastodon found in Britain.

According to the most recent views of high authorities, certain
deposits--such as the so-called "Bridlington Crag" of Yorksh