Ceratites nodosus (from the Muschelkalk).
Palæozoic 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 Palæozoic 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.
The following are the principal coalfields:—
- Northumberland and Durham coalfield.
- South Lancashire coalfield.
- Derbyshire coalfield.
- Leicestershire and Staffordshire coalfields.
- South Wales coalfield.
- Bristol and Somerset coalfields.
b. Millstone grit or Farewell-rock. The former term explains itself, the latter designation has been applied to it in the southern districts, because when it is reached, then good-bye to all workable coal-seams.
It consists of coarse sandstones, shales, and conglomerates with a few small seams of coal. Fossils are not very common in it.