4. Lias.
This for the most part consists of very regular alternations of argillaceous (clayey) limestone and clay, or shale. It is of great thickness, and hence for convenience has been divided into (a) Upper Lias, (b) Middle Lias or Marl-stone, and (c) Lower Lias. A large number of fossils are to be found in it. Lyme Regis and Whitby are perhaps the best known localities; the former, on account of the great number of specimens obtained of the huge fish-lizard (Ichthyosaurus, p. 24), and long-necked Plesiosaurus (p. 25), besides numberless fish; whilst the latter is renowned for its jet (or fossilized wood) and its "snake-stones" (Ammonites), concerning which curious old stories are told. Ammonites are plentiful in the Lias, which has been subdivided into zones, or layers, named after the ammonite occurring in greatest numbers in that particular zone. There is one thin limestone band in the Marlstone composed entirely of the shells of Ammonites planicostatus. A curious kind of oyster (Gryphæa incurva), locally known as the devil's toenail, a huge Lima (L. gigantea), a magnificent Encrinite (Extracrinus Briareus), and numerous other fossils, are also to be obtained by patient search.
Belemnitas elongatus (from the Lias).
5. Rhætic, Penarth Beds, or White Lias.
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.
