Sea-Weeds, Shells and Fossils Part 10

_c._ _Kimmeridge Clay._ This, by the pressure of the rocks subsequently deposited on it, has in greater part been hardened, and possesses a tendency to split in thin layers, and hence is termed by geologists a shale. It is seen at various points between Kimmeridge on the Dorsetshire coast and the Vale of Pickering in Yorkshire, and forms broad valleys. The princ.i.p.al fossils in it are Ammonites, a triangular-shaped oyster (_Ostrea deltoidea_), and one resembling a comma (_Exogyra virgula_).

Middle Oolites.

_a._ The _Coral Rag_, or _Coralline Oolite_, comprises a most variable set of beds, but princ.i.p.ally a series of limestone, with fossil corals still in the position in which they grew, and resembling in form the reef-building corals of the Pacific. They rest on

_b._ _Oxford Clay_, a dark blue or slate-coloured clay without any corals, but containing a great many _Ammonites_ and _Belemnites_. The _Kelloway Rock_, a sandy limestone at the base of the Oxford Clay, is well developed in Yorkshire, and furnishes amongst other fossils a large belemnite and an oyster (_Gryphaea dilatata_).

Lower Oolites.

_a._ _Cornbrash_, a very sh.e.l.ly deposit of pale-coloured earthy, and rubbly or sometimes compact limestone with plenty of fossils. The commonest are Brachiopods, Limas, oysters (_Ostrea Marshii_), Pholadomyas and Ammonites. It is best seen in Dorsetshire, Somersetshire, and near Scarborough in Yorkshire.

_b._ _Forest Marble_ and _Bradford Clay_. The former is an exceedingly sh.e.l.ly limestone, often splitting into thin slabs. On the surfaces of some of the beds may be seen the ripple marks the sea made countless years ago, and the tracks of worms and crabs that dwelt in the mud or crawled on its surface at a time when it was soft mud. The Bradford clay is a very local deposit, taking its name from Bradford in Wiltshire, where it is most developed, and its characteristic fossil is the pear-shaped Encrinite or "stone-lily" (_Apiocrinus Parkinsoni_).

_c._ The _Great_ or _Bath Oolite_, comprising a series of sh.e.l.ly limestones and fine Oolites, or freestones. The latter are largely quarried in the neighbourhood of Bath, and used for mantelpieces and the stone facings of windows. The great Oolite is rich in univalve mollusca, amongst which may be noted a limpet (_Patella rugosa_) and the handsome, tall-spired _Nerinaea Voltzii_, numerous bivalves belonging to the genera _Pholadomya Trigonia_, _Ostrea_ (_O. gregaria_), and _Pecten_, besides Brachiopods (_Terebratula digona_, which looks very like a sack of flour, and _T. perovalis_, etc.).

At the base of the Great Oolite are the "Stonesfield slates,"

so-called--a series of thin sh.e.l.ly Oolites, etc., that split readily into very thin slabs. They are princ.i.p.ally of interest to geologists on account of the discovery in them of the remains of small insect-feeding and possibly pouched mammals. With these are a.s.sociated the bones of that big reptile the _Megalosaurus_; the flying lizards called Pterodactyles; fish teeth and spines; lamp sh.e.l.ls; oysters, a _Trigonia_ (_T. impressa_); and the impressions of insects, including a b.u.t.terfly, and of plants.

_d._ _Fullers' Earth_, a clayey deposit occurring in the southwestern parts of England, but not in the north. It abounds with a small oyster (_O. ac.u.minata_) and Brachiopods (e.g. _Terebratula ornithocephala_), etc.

_e._ _Inferior Oolite_ (including the Midford Sands). As these beds are followed across the country from the south-west of England to Yorkshire, they are found to change greatly in character. Limestone and marine beds in the south are replaced by sandy and estuarine beds in the north.

Amongst other fossils from beds of this age may be found several Echinoderms, a crinkly lamp sh.e.l.l (_Terebratula frimbriata_), and a spiny one (_Rhynchonella spinosa_), bivalves belonging to the Genera _Ostrea_, _Trigonia_, _Pholadomya_, etc., and some very handsome Ammonites (e.g. _A. Humphresia.n.u.s_).

[Ill.u.s.tration: _Ichthyosaurus_, or Fish-lizard (from the Lias).]

[Ill.u.s.tration: _Plesiosaurus_ (from the Lias).]

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 sh.e.l.ls of _Ammonites planicostatus_.

A curious kind of oyster (_Gryphaea 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.

[Ill.u.s.tration: _Belemnitas elongatus_(from the Lias).]

5. Rhaetic, 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. Sh.e.l.ls 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 ma.s.s of which forms the subsoil of the western midland counties, Birmingham being nearly in the centre, thence they extend in three directions, one branch pa.s.sing towards the north-west, through Cheshire, to the sea at Liverpool, reappearing on the coast line of Lancashire, Westmoreland, and c.u.mberland, 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 sh.e.l.l 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 foresh.o.r.e, 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.

[Ill.u.s.tration: _Cerat.i.tes 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 princ.i.p.al 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 princ.i.p.al 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. c.o.c.kroaches and other insects were also denizens of the carboniferous forests.

The following are the princ.i.p.al coalfields:--

1. Northumberland and Durham coalfield.

2. South Lancashire coalfield.

3. Derbyshire coalfield.

4. Leicestershire and Staffordshire coalfields.

5. South Wales coalfield.

6. 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 coa.r.s.e sandstones, shales, and conglomerates with a few small seams of coal. Fossils are not very common in it.

_c._ Yoredale Rocks, a series of flagstones, gritstones, limestones and shales, with seams of coal, occurring in the northern counties. It is underlain by--

_d._ _Carboniferous_ or _Mountain Limestone_, which in places is upwards of 1,000 feet thick, and full of fossils. The stems of encrinites, or "stone-lilies," corals, brachiopods (_e.g._ _Productus_, _Orthis_, etc.), and Mollusca, including some Cephalopods, like _Goniat.i.tes_ and the straight Nautilus (_Orthoceras_), with fish teeth, etc., go to compose this tough, bluish-grey limestone which is largely quarried for marble mantlepieces, etc.

_e._ The _Tuedian group_ in the north, and _Lower Limestone Shale_ in the south, follow next, and consist of shales, sandstones, limestones, and conglomerates, varying greatly in different districts, and containing few fossils.

3. Devonian or Old Red Sandstone. To this age are a.s.signed a perplexing series of strata, the princ.i.p.al members of which consist of (_a_) a thick limestone, well seen in the cliffs and marble quarries of south Devon, and full of fossil-corals (_e.g._ _Favosites polymorpha_ [or _cervicornis_]) Brachiopods, and Mollusca, etc.

_b._ A series of sandstones, slates, and limestones in North Devon containing Trilobites (_Phacops_, _Bronteus_, etc.), Brachiopods, and other fossils.

_c._ The _Old Red Sandstone_ of Wales, the North of England, and Scotland, consisting of red and grey sandstone and marly beds, with remains of fish.

These fish, unlike most now living, were more or less covered with hard external plates, and possessed merely a cartilaginous skeleton. In one set of individuals, indeed (_Pterichthys_), the armour plates formed quite a little box. These creatures propelled themselves by means of two arm-like flippers, rather than fins. They were but a few inches long, and appear pigmies in contrast to the strange half-lobster-like crustacean, _Pterygotus_, that lived with them, and attained sometimes as much as five feet in length.

4. Silurian. Named by Sir Roderick Murchison after a tribe of Ancient Britons that dwelt in that part of Wales, where these rocks were first observed. Some of Murchison's Lower Silurian beds were included by Professor Sedgwick in his Cambrian, of which we shall have to speak next; and as these two geologists never could agree on a divisional line between their respective formations, and since succeeding observers have followed sometimes one and sometimes the other method of cla.s.sification, considerable confusion has resulted. Here, however, for several reasons, we propose to follow Sedgwick's arrangement; and hence, under the term Silurian, retain only Murchison's Upper beds. They consist of a series of sandstones, gritstones, conglomerates, shales, limestones, etc.

Amongst the more important fossils, which are very abundant in the limestones, are various corals (_e.g._ the Chain-coral _Halysites_), Star-fish, Crinoids, Trilobites (_Phacops_, etc.), Polyzoa, Brachiopods and Mollusca, especially Cephalopoda (_Orthoceras_, _Nautilus_, etc.).

These rocks occur princ.i.p.ally in the border land between England and Wales, and the adjacent counties; but are also represented in Westmoreland, Scotland, and Ireland. Their princ.i.p.al subdivisions are given in the Table on p. 16.

[Ill.u.s.tration: Trilobite (_Asaphus candatus_), (from the Silurian).]