These beds are not of any considerable thickness, but are very
persistent, and of great interest, inasmuch as they yield the remains of
the oldest known mammal (_Microlestes_), a small insect-feeder. They are
composed of limestones, shales and marls (_i.e._ limey clays), and are
best studied in Somersetshire and Dorsetshire. The "landscape marble"
belongs to this formation, which also contains a bone bed, or thin layer
made up of the bones and teeth, etc., of fish. Shells are not numerous,
though the casts of one species (_Avicula contorta_) is plentiful.
6. Trias, or New Red Sandstone, a thick series of sandstones and marls,
the great mass of which forms the subsoil of the western midland
counties, Birmingham being nearly in the centre, thence they extend in
three directions, one branch passing towards the north-west, through
Cheshire, to the sea at Liverpool, reappearing on the coast line of
Lancashire, Westmoreland, and Cumberland, where it also forms the Valley
of the Eden. Another branch extends through Derby and York to South
Shields, whilst the third may be traced southwards in isolated patches
down into Devonshire.
There are scarcely any fossils in it, but in Worcestershire and
Warwickshire the bivalve shell of a small crustacean (_Estheria minuta_)
occurs in the upper beds; whilst now and again the teeth and bones of
some strange amphibians (_Labyrinthodon_), or the impressions of their
feet (_Cheirotherium_) where they crawled on the then soft mud of the
foreshore, are found. The Trias is divided into Upper Trias or Keuper,
and Lower Trias or Bunter. The middle beds (Muschelkalk), which are
found in Germany, where they contain plenty of fossils, are wanting in
this country. In the lower beds of the Keuper, layers of rock salt,
sometimes of great thickness, occur, whilst casts (called pseudomorphs)
of detached salt-crystals are found abundantly in the sandy marls.
Northwich, Nantwich, Droitwich, and several other towns in Cheshire and
Worcestershire, are famed for their salt works, the salt being either
mined or pumped up as brine from these beds.
[Illustration: _Ceratites nodosus_ (from the Muschelkalk).]
PALAEOZOIC or PRIMARY.--Beds of this age generally possess a more
crystalline and slaty structure than any of those already mentioned, are
usually more highly inclined and disturbed, and form for the most part
more elevated ground. They are the principal store-houses of our mineral
wealth, containing as they do coal, iron, and other metals. The
Palaeozoic rocks are found in England to the north and west of the
secondary series, beneath which they disappear when traced to the
south-east. Wales, and the greater part of Scotland and Ireland, consist
of beds of this age.
1. Permian. Under this term are included beds of red sandstones and
marls, closely resembling those of Trias, and like them containing but
few fossils, as well as a very fossiliferous limestone, known as the
Magnesian Limestone, from the abundance of magnesia it contains. A
pretty polyzoan (_Fenestella retiformis_), a spiny brachiopod
(_Productus horridus_), various genera of fish, chiefly found in a marl
state underlying the limestone, some Labyrinthodonts and plant remains,
are the principal forms met with in this formation.
2. Carboniferous. This, from a commercial point of view, is the most
important of all the formations, comprising as it does the coal-bearing
strata. It is subdivided into--
_a._ _Coalmeasures_, a series of sandstones and shales with which are
interstratified the seams of coal, varying in thickness from six inches
to as much in one instance as thirty feet.
Coal is the carbonized remains of innumerable plants, chiefly ferns and
gigantic clubmosses, that grew in swamps bordering on the sea-coast of
the period. Each coal seam is underlain by a bed of clay called
"under-clay," containing the roots of the plants that grew on it. Some
of the best impressions of ferns, etc., are to be obtained in the shaley
beds forming the roof of the coal seam; many good specimens, however,
are to be got by searching the refuse heap at the pit's mouth. Besides
plants, the remains of fish are abundant in some of the beds of shale.
And in Nova Scotia the bones of air-breathing reptiles and land snails
have been discovered. Cockroaches and other insects were also denizens
of the carboniferous forests.