Part 8

_c._ _Gault_, a stiff blue clay abounding in fossils: Ammonites often retaining their pearly shell; Belemnites, a bivalve with very deep furrows on it (_Inoccramus sulcatus_), and its first cousin (_I. concentricus_, p. 21), in which the ridge-like markings correspond with the lines of growth, besides many others, may be obtained in abundance from it. Layers of phosphatic nodules occur at irregular intervals. The gault is best studied at East Wear Bay, near Folkstone; it may also be seen in Dorsetshire, Wiltshire, and Cambridgeshire; lately it has been found as far west as Exeter.

2. Neocomian.

_a._ The so-called _Lower Green Sand_, named in contradistinction to the _Upper Green Sand_, includes a series of iron stained sands, sandstones and clays of great thickness. The clayey beds are seen at Atherfield in the Isle of Wight, and at Nutfield in Surrey, while the sandy beds are met with at Speeton, at Folkestone, and near Reigate. Besides brachiopods and oysters, these beds have furnished a species of _Perna_ (_P. Mulleti_), an elongated mussel (_Gervillia anceps_), a pretty _Trigonia_ (_T. cordata_), some _Ammonites_ and Nautili, with the teeth and bones of big reptiles. The celebrated "Kentish Rag" and the sponge gravels of Farringdon are of this age.

_b._ _Wealden._ The main mass of these rocks occupies the area inclosed between the North and South Downs, and forms the Valley of the Weald, whence they take their name. They consist of a series of sands, sandstones, clays, and shelly limestones that were deposited in the delta and off the mouth of a big river. The shells in them belong to freshwater genera, _Cyrena_, _Unio_, _Paludina_, etc. Bones of a huge lizard that hopped along on his hind legs (_Iguanodon_), and those of crocodiles, etc., are from time to time brought to light. The Wealden rocks occur also on both eastern and western sides of the Isle of Wight, and in Dorsetshire.

3. Oolites (or Roe-stones) are so named because the characteristic limestones of this formation resemble very much the roe of a fish. The small round grains, of which the typical examples are built up, when cut or broken through will be seen to be formed of numerous layers of carbonate of lime, disposed like the coats of an onion, around some central nucleus, generally a grain of sand, a fragment of coral, or the shell of one of the Foraminifera. They are divided into Upper, Middle, and Lower Oolites, and these again are subdivided as follows--

Upper Oolite.

_a._ _Purbeck Beds_, a series of fresh-water, with a few estuarine, or marine beds, which in point of fact connect the deposits we are next coming to with the Wealden just passed. They contain numerous fresh-water shells--_Paludina_, _Physa_, _Limnaea_, etc., with the microscopic valves of the little fresh-water crustacean _Cypris_, whose descendants are abundant in the rivers and lakes of to-day. An oyster occurs in the "Cinder Bed" and Plant remains in the "Dirt Beds." But the Purbecks are best known for the numerous remains of small mammals (_Plagiaulax_) allied to the kangaroo rat, at present living in Australia.

_b._ The _Portland Stone and Sand_, which come next in order, are largely quarried in the island whence they take their name. The quarrymen point out fossils in the stone, which they call "Horses'-heads" and "Portland screws." The former is the cast of a _Trigonia_ shell; the latter, that of a tall spired univalve (_Cerithium_).

In Wiltshire, a coral (_Isastrea oblonga_) is found in the sandy beds, the original calcareous matter of which has been replaced by silex.

_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 principal 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 principally 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 shelly 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 shelly 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 shelly 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 shelly Oolites, etc., that split readily into very thin slabs. They are principally 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 associated the bones of that big reptile the _Megalosaurus_; the flying lizards called Pterodactyles; fish teeth and spines; lamp shells; oysters, a _Trigonia_ (_T. impressa_); and the impressions of insects, including a butterfly, 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. acuminata_) 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 shell (_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. Humphresianus_).

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 (_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.

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. 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.

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.

The following are the principal 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 coarse 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 _Goniatites_ 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 assigned a perplexing series of strata, the principal 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 classification, 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 principally in the border land between England and Wales, and the adjacent counties; but are also represented in Westmoreland, Scotland, and Ireland. Their principal subdivisions are given in the Table on p. 16.

5. Cambrian. Under this term, derived from the old name for Wales, are included many sandstones, grits, slates and flags, with here and there a limestone band. They form the greater part of the western counties of Wales, where they rise to a considerable height above the sea level. The highest hills of Westmoreland and more than half of Scotland are composed of beds of this age.

The fossils, save in the limestone bands, are not easy to find, but in places they are fairly abundant. Brachiopods are far more numerous than the Mollusca properly so-called. Of these, the genus _Orthis_ was most abundant at about the close of this period. Certain beds of this age have received the name of Lingula Flags, owing this prevalence in them of the curious Brachiopod _Lingula_ so like the species now living in some of the warm seas of the tropics. The Trilobites included several forms, and one species (_Paradoxides Davidis_) attained the length of nearly two feet. A few star-fish, some Hydrozoans (_Graptolites_), and the tubes and casts of Annelides and tracks of Trilobites, complete the list of more remarkable fossils. The subdivisions of the Cambrian rocks will be found in the table on p. 16.

6. Pre-Cambrian.--Near St. David's Head and some other places in Wales, in Anglesea, Shropshire, etc., some yet older rocks have been found. They are probably for the most part of volcanic origin, but they have been so much changed since they were first deposited, and as hitherto no fossils have been found in them, little is known concerning them.

Parts of the western coast of Northern Scotland and the Hebrides are composed of a crystalline rock called Gneiss, and supposed to be the oldest member of the British strata. No fossils have been found in it.

VOLCANIC ROCKS. Although there are fortunately no volcanoes to disturb the peace of our country at the present day, there is abundant evidence of their existence in the past. Not only are some of the beds, especially those of Paleozoic age, composed of the dust and ashes thrown out of volcanoes, with here and there a lava flow now hardened into solid rock, but the stumps of the volcanoes themselves are left to tell the tale. The cones indeed are gone, carried off piecemeal by the rain and frosts, and other destructive agencies, in the course of countless ages: not so the once fluid rock within; _that_ cooled down into Granite, and though originally below the surface, it now, owing to the removal of the overlying softer strata, forms raised ground overlooking the surrounding country. The granite masses of Cornwall, of Dartmoor, in the south-west of Mt. Sorrel; the variety called Syenite at Malvern and Charnwood Forest; the Basalts of the Cheviot Hills and of Antrim; the volcanic rocks of Arthur's Seat, Edinburgh, and of the islands of Skye and Mull, etc., are examples of this class of rock. They are of different ages, and belong to different periods of the earth's history, from early Palaeozoic down to Miocene times.

TABLE OF THE PRINCIPAL DIVISIONS OF THE ANIMAL KINGDOM, TO SHOW THE ORDER IN WHICH THE FOSSILS SHOULD BE ARRANGED.

INVERTEBRATA.

_Foraminifera_, minute chambered shells like the Nummulite.

_Spongida_, Sponges.

_Hydrozoa_, Graptolites, etc.

_Actinozoa_, Corals.

_Echinodermata_, Sea-urchins, Stone-lilies, Starfish, etc.

_Annelida_, Worm tracks.

_Crustacea_, Trilobites, Crabs, etc.

_Arachnida_, Scorpions and Spiders.

_Myriapoda_, Centipedes.

_Insecta_, Beetles, Butterflies, etc.

_Polyzoa_ (_Bryozoa_) or Moss Animals.

_Brachiopods_, Lampshells.

{ _Lamellibranchiata_, Bivalves. _Mollusca_ { _Gasteropoda_, Univalves. { _Cephalopoda_, Cuttlefish, Ammonites.

VERTEBRATA.

_Pisces_, Fish.

_Amphibia_, Labyrinthodonts, Frogs, and Newts.

_Reptilia_, Reptiles.

_Aves_, Birds.

_Mammalia_, Mammals.

WORKS OF REFERENCE.

FOR NAMING COMMON FOSSILS.

=Tabular View of Characteristic British Fossils Stratigraphically Arranged.= By J. W. LOWRY. _Soc. Prom. Christ. Knowledge._ 1853.

=Figures of the Characteristic British Tertiary Fossils (Chiefly Mollusca) Stratigraphically Arranged.= By J. W. LOWRY and others. _London_ (_Stanford_). 1866.

PALAEONTOLOGY.

=The Ancient Life History of the Earth.= By H. A. NICHOLSON. 8vo. _Edinburgh and London._ 1877.

=A Manual of Palaeontology.= By H. A. NICHOLSON. 2nd edition. 2 vols. 8vo. _Edinburgh and London._ 1879.

PETROLOGY.

=The Study of Rocks.= By F. RUTLEY. (Text Books of Science.) 8vo. _London._ 1879.

FIELD GEOLOGY.

=A Text-Book of Field Geology.= By W. H. PENNING. With a Section on Palaeontology, by A. J. JUKES-BROWN. 2nd edition. 8vo. _London._ 1879.

GEOLOGY IN GENERAL.

=The Student's Elements of Geology.= By SIR CHARLES LYELL, Bart. 4th edition. 8vo. _London._ 1884.

=The Principles of Geology.= By SIR CHARLES LYELL, Bart. 12th edition. 2 vols. 8vo. _London._ 1875.

=Phillip's Manual of Geology.= 2nd edition. By SEELEY AND ETHERIDGE. 2 vols., 8vo. _London._ 1885.

=Tabular View of Geological Systems, with their Lithological Composition and Palaeontological Remains.= By D. E. CLEMENT. _London (Sonnenschein)._ 1882.

BRITISH GEOLOGY.

=The Physical Geology and Geography of Great Britain.= By SIR ANDREW C. RAMSEY. 5th edition. 8vo. _London._ 1878.

=The Geology of England and Wales.= By HORACE B. WOODWARD. 8vo. _London._ 1876.

=Geology of the Counties of England and Wales.= By W. J. HARRISON. 8vo. _London._ 1882.

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