The permian strata are sandstones, gypscous marls, and common and magnesian time stones.
With these beds terminate the palteozoic rocks. Before the commencement of the secondary epoch, great disturbances and depressions took place in the districts whose geological structure has been examined; and at the same time a great change took place in the character of the animal and vegetable life.
The typical rocks of the trias,sic period (q.v.), the earliest of the secondary epoch, exist in Germany. They are highly fossiliferous, contailiing the remains of marine animals of various kinds. In Britain, the rocks are chiefly red sandstones and red marls, the coloring matter of which seems to have been destructive to life; the only fossils they contain are a few land-plants, and some footprints and fragments of bones of reptiles.
The lias (q.v.), which follows, and forms the base of the oolite formation, consists of extensive clay deposits, with argillaceous limestones and sandstones—strata which indicate the existence of large tracts of land. The contained fossils have a mixed land, fresh-water, and sea character. With considerable numbers of plants and insects, there arc also marine brachiopods and cephalopods, and the remarkable swimming reptiles, that are so perfectly preserved as to supply materials for nearly perfect restorations.
The oolite series (q.v.) consists of alternating beds of limestone and clay, with very little sandstone. The abundance of dry land is testified to by the number and variety of the air-breathing fossils (amongst which mamnialia appear for the first time), and even by the occurrence of strata that have been ancient soils. The group is highly fossiliferous.
The cretaceous strata (see CRETACEOUS Gnour), which, as a whole, have had a deep sea origin, are introduced by fresh-water and estuary deposits, showing that great tracts of land were traversed by mighty rivers actively abrading and carrying off materials for delta deposits. The life of the period was abundant. The immense thicknesses of chalk, which give the name to the group, are composed to a very large extent of the perfect or comminuted shells of foraminifera and molluska. Besides these, land-plants, fresh-water and marine shells and fish, and large terrestrial and marine reptiles, occur.
Birds and mammalia have not yet been observed, but it is most probable that they did exist, as they have been found in older strata.
In passing to the tertiary epoch, there is not found so ,striking a change in the life of the globe as that which characterized the division between the paleozoic and sec ondary strata. From the thias, the fossils have been gradually assuming the appearance of existing organisms: many strange forms have existed and passed away without leav ing representatives in the later strata or in the living inhabitants of the earth. Still, the fades of the organic.remains gradually approaches that of the present fauna and flora, until the eocene period (q.v.), when some fossils appear, which. if not identical with recent species, so nearly approach them, as to make it impossible to distinguish them. The proportion of such species is from 3+ to 5 per cent. The seas in which the eocene beds were deposited were comparatively small, and consequently the deposits occur in scattered and isolated basins. The earlier strata are marine, but towards the middle of this period they become lacustrine or fluviatile.
The miocene period (q.v.) is said to contain above 25 per cent of living forms. It is doubtful whether there are in Britain any true representatives of this period. The strata are largely developed in France and Belgium. Besides abounding in marine molluska, the miocene strata contain the remains of many large mammalia. The deposits of the pliocene period (q.v.) contain from 50 to 70 per cent of existing forms. The strata are marly sands and gravels abounding with sea-spoils.
In the pleWocene strata (q.v.) the proportion of existing forms is still greater—indeed, all the principal generic forms now alive, except man, seem to have been in existence during this period. The strata consist of the sands, gravels, and boulder clay left by glaciers and icebergs, of marls and raised sea-beaches.
The newer strata belong to the human period, and have been, and are continuing to be, formed by agents now in operation. They contain the remains of species of plants and animals which still live on the globe.