Sea-Weeds, Shells and Fossils

The Mesozoic or Secondary rocks embrace a series of limestone, clays, sands, and sandstones that on the whole are well consolidated. The main mass of them lies to the west of a line drawn across the map of England from the mouth of the Tyne, in Northumberland, southwards to Nottingham, and thence to the mouth of the Teign in Devonshire. In the south-eastern counties they underlie the tertiary rocks of the London and Hampshire basins, as they are called, at no great depth from the surface. Outlying patches of secondary rocks occur in Scotland, where they are found near Brora on the east coast, and in the islands of Skye and Mull on the west. In Ireland they are scantily represented round about the neighbourhood of Antrim. The secondary rocks are divided into—

1. Cretaceous.

a. The Chalk is too well known to need description, though technically it may be described as a soft white limestone chiefly built up of the microscopic shells of Foraminifera, and characterized in its upper part by nodules and bands of flint. These flints frequently inclose casts of fossils (sponges, sea-urchins, etc.), and sometimes shells themselves. Fossils, too, are fairly abundant, scattered throughout the mass. Amongst the commoner may be noticed the sea-urchins, such as the "sugar loaf" (Ananchytes) and the heart-shaped Micraster, the Brachiopods or Lamp-shells (Terebratula, Rhynchonella), a "Thorny Oyster" (Spondylus spinosus), besides Ammonites, Belemnites (part of the internal shell of a kind of cuttle-fish), and the teeth of several species of sharks. Altogether the chalk is about 1,000 feet thick.

b. Upper Greensand is a series of greenish-grey sands and sandstones. The green colour, on close inspection, is seen to be due to the presence of innumerable small green grains of a mineral called glauconite. These are frequently casts of the chambers of the very same foraminifera that the chalk is so largely composed of.

Nodules and layers of "chert" (an impure kind of flint) occur in it, whilst in places it forms a hard rock called "firestone." The commonest fossils are Brachiopods, very similar to those in the chalk, a scallop-shell with four strongly marked ribs on it (Pecten quodricostatus), an oyster with a curved beak (Exogyra columba), and a pear-shaped sponge (Siphonia pyriformis). The Upper Greensand is better seen at places in the southern part of the Isle of Wight, in cliffs on the Dorsetshire coast, in Wiltshire, at Sidmouth, and in some parts of Surrey.

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, Limnæa, 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 (Gryphæa 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 Nerinæa 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 (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.

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.

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:—

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 Palæozoic down to Miocene times.