Sea-Weeds, Shells and Fossils Part 9

Small bones may be treated in like manner, but for large ones, weak glue is to be preferred to gelatine, which is only suitable for the finer and more delicate objects. Where it is desired to harden only a few things, it is better to mix the gelatine in a gallipot, which can be heated when required by standing it in a saucepan of water on the fire. In any case the gelatine need never be wasted, as it will keep almost any length of time, and can therefore be put by for future use. In default of the egg-boiler or wire-net spoon, an equally useful plan is to make a strainer from a piece of perforated zinc by turning up the edges all around, and attaching copper wire to it by which to lower the fossils into the gelatine, and raise them again.

When the fossils are quite dry they can be sorted, and those which have come to pieces may be mended with diamond cement (_i.e._ isingla.s.s dissolved in acetic acid), and then properly labelled and placed in trays, or mounted as previously described.

To the plant remains and Lignite there is little that can be done beyond tr.i.m.m.i.n.g them to suit the trays. Should there be much iron pyrites in the Lignite, it is sure, sooner or later, to decompose, when all that can be done is to throw it away. In the case, however, of valuable fruits and seeds, such as those from the London Clay of Sheppey, it is worth while to preserve them, if possible, in almost the only way known, viz. by keeping them in glycerine in wide-mouthed stoppered bottles, or by saturating them with paraffin.

Having prepared the specimens for the cabinet, the next thing is to arrange them in proper order. There are several ways of doing this, but for those who have not had much experience the following plan will be found the best:--Group the specimens according to the formations to which they belong, and arrange these groups in proper sequence (_vide_ Table, p. 16); then take each group, and arrange the specimens it comprises in columns. Beginning at the top of the left-hand corner, place first the specimens of the rock itself, and under it any examples of minerals, concretions, etc., found in that rock; next the fossil plants, if any; and finally, such animal remains as you have arranged according to their zoological sequence, beginning with the lower forms (_vide_ Table, p. 32). Unless cramped for room, each formation should begin a new box, its name being written on a slip of paper and placed at the head of the columns of trays. A label setting forth its contents should be fixed outside each of the boxes, which can then be put away on your cupboard shelves.

TABLE OF THE PRINc.i.p.aL FOSSILIFEROUS STRATA ARRANGED IN CHRONOLOGICAL ORDER.

_Land Plants._-----------+ _Invertebrata._--------+ _Fishes._------------+ _Amphibia._--------+ _Reptiles._------+ _Birds._-------+ _Mammalia._--+ _Man._-----+ {Alluvial Deposits, _Quaternary, { River Valley or { Gravels and Pleistocene._ { Cave Deposits. {Drift and Glacial { Deposits. V _Cainozoic, {Pliocene. or {Miocene. Tertiary._ {Eocene. { {Chalk. M { _Cretaceous._ {Upper Greensand. E { {Gault. S { O { _Neocomian._ {Lower Greensand. V Z { {Wealden. : O { : I { { {Purbeck. : C, { {_Upper._{Portland. : { { {Kimmeridge Clay. : or { { : { { _Mid._ {Coral Rag. : S { { _Oo- { {Oxford Clay. : E { {lites._{ : C { { { {Cornbrash and : O { { { { Forest Marble. : N { _Jura.s.sic._{ {_Lower._{Great Oolite. : D { { { {Fullers' Earth. : A { { { {Inferior Oolite. : R { { : Y { { Lias. : : { {Trias, or New : P { _Poikilitic._ { Red Sandstone. V ? V A { {Permian. L { ae { {Coal Measures. V O { {Millstone Grit Z { _Carboniferous._ { and Yoredale O { { Rocks. I { {Carboniferous C, { { Limestone, etc. { or { Devonian and Old { Red Sandstone. P { R { {Ludlow Beds. I { {Wenlock Beds. V M { _Silurian._ {Woolhope Beds. A { {Tarannon Shale. R { {Llandovery or May Y. { { Hill Group. V { { {Bala and { { Caradoc Beds. { {Llandeilo Flags. { {Arenig Group. { _Cambrian._ {Tremadoc Slates. { {Lingula Flags. { {Menevian Beds. { {Longmynd and { { Harlech Group. V { : { Pre-Cambrian and : { Laurentian. ?

NOTES ON THE DIFFERENT FORMATIONS MENTIONED IN THE TABLE.

RECENT.--The alluvial deposits of most river valleys and some estuaries still in course of formation, containing fossil sh.e.l.ls and mammals, all of living species.

QUATERNARY, POST-PLIOCENE, or PLEISTOCENE.--1. Including the raised beaches around the coast, the older gravels of river valleys and the cave deposits, in all of which the sh.e.l.ls are identical with those living in the rivers and seas of to-day, whilst the animals are many of them extinct, only a few being now found living on the spot.

2. The glacial drifts that cover all England north of the Thames, and which consist of sands, gravels, and clays, full of big angular stones frequently flattened on one side, scratched and sometimes polished from having been fixed in moving ice and forced over other rocks. A very interesting collection of these "boulders," as they are called, can be easily made, for they belong to almost every formation in England, and have some of them been brought from great distances, whilst the number and variety obtainable from a single pit is astonishing.

CAINOZOIC, or TERTIARY.--Beds of this age, in England at all events, are for the most part made up of comparatively soft rocks, gravels, sands, and clays, and are found in the eastern and south-eastern counties. They are divided into--

1. Pliocene, mainly consisting of a series of iron-stained sands, with abundant sh.e.l.l remains, and locally known as "crags." The sh.e.l.ls are very partial in their distribution, the beds in places being almost entirely made up of them, whilst in others scarcely one is to be found.

The great majority are of the same species as many still living. The Pliocene is subdivided into three groups:--

_a._ The _Norwich Crag Series_, sometimes called the "Mammaliferous Crag," as at its base the bones of mastodon, elephant, hippopotamus, rhinoceros, and some deer have been found. The sh.e.l.ls in it are such as still abound on the beaches of the eastern coast to-day--whelks, scallop sh.e.l.ls, c.o.c.kles, periwinkles, etc.

_b._ The _Red_ or _Suffolk Crag_, its two names indicating its characteristic colour (a dark red-brown) and chief locality. From the base are obtained the celebrated phosphatic nodules miscalled "Coprolites," whence is manufactured an artificial manure, and with them are found the rolled and phosphatized bones and teeth of whales, sharks, etc. Amongst the sh.e.l.ls the Reversed Whelks (_Fusus contrarius_), _Fecten opercularis_, _Pectunculus glycimeris_, several kinds of _Mactra_ and _Cardium_, etc., are the commonest. Walton-on-the-Naze, Felixstowe, and Woodbridge are the best known localities.

_c._ The _White_ or _Coralline Crag_ is generally of a pale buff colour, and is in places almost entirely composed of the remains of Polyzoa.

These (formerly called Corallines, whence the name Coralline Crag) are beautiful objects for a low-power microscope, or pocket lens, and are easily mounted in deep cells on slides. The bits of sh.e.l.l and sand that stick to them should be carefully removed with the point of a needle. A very large number of sh.e.l.ls occur in this crag: of bivalves, the _Pecten_ is very abundant, and its valves are frequently thickly grown over with Polyzoa; _Cyprina Islandica_, _Cardita Senilis_ are also plentiful; and of univalves, the genus _Natica_ is common. The Coralline Crag is best seen in the neighbourhood of Aldborough, Orford, Woodbridge, and other places in Suffolk.

2. Miocene, possibly represented in the British Isles by a small patch of clays and lignites at Bovey Tracey.

3. Eocene, divided into--

_a._ _Upper Eocene_, consisting of a series of very fossiliferous sands, clays, and limestones, exposed in the cliffs at the eastern and western ends of the Isle of Wight and on the neighbouring coast of Hampshire.

They are partly of freshwater origin, when they contain the remains of freshwater sh.e.l.ls such as _Limnoea Paludina_, _Planorbis_, etc.; partly of marine origin, when sh.e.l.ls belonging to such genera as _Ostrea_, _Venus_, etc., take their place; partly of estuarine, when the brackish water mollusca are found with bones and scutes of crocodiles and tortoises.

_b._ _Middle Eocene_, or the _Bagshot Beds_, composed of sands and clays. The beautiful coloured sands of Alum Bay, the sands of the Surrey and Hampstead Heaths, are familiar examples of the beds of this age.

Very few fossils indeed have been found in them. The clay-beds on the contrary as seen at Barton and Hordwell on the Hampshire coast and again in the Isle of Wight, abound with sh.e.l.ls belonging to genera such as _Conus_, _Voluta_ and _Venus_, that inhabit warm seas. With them are the Nummulites, looking externally very like b.u.t.tons, but on the inside divided into innumerable chambers in which the complex animal that formed the nummulite dwelt.

_c._ _Lower Eocene_, the well-known London clay, may almost be said to compose this division, for the underlying sands, gravels, and clays are in ma.s.s comparatively insignificant. The London clay contains plenty of fossils, only as they are disposed in layers (_zones_) at a considerable distance apart, they are not often hit upon. Layers of Septaria or cement-stones are of frequent occurrence. Sheppy is the great locality for London clay fossils, as the sea annually washes down large ma.s.ses of the cliffs and breaks them up on the beach. A great many fossil fruits and seeds, remains of crabs, sh.e.l.ls of Nautili, Volutes, and other mollusca, besides turtles, a species of snake, a bird with teeth, and a tapir-like animal, have at different times and in various places been found in this deposit, which sometimes attains a thickness of over 400 ft. The "Bognor Rock" is a local variety of the bas.e.m.e.nt bed of this formation.

[Ill.u.s.tration: _Aturia Zic-zac_ (from the London clay).]

The MESOZOIC or SECONDARY rocks embrace a series of limestone, clays, sands, and sandstones that on the whole are well consolidated. The main ma.s.s 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 sh.e.l.ls 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 sh.e.l.ls themselves. Fossils, too, are fairly abundant, scattered throughout the ma.s.s. Amongst the commoner may be noticed the sea-urchins, such as the "sugar loaf" (_Ananchytes_) and the heart-shaped _Micraster_, the Brachiopods or Lamp-sh.e.l.ls (_Terebratula_, _Rhynchonella_), a "Th.o.r.n.y Oyster" (_Spondylus spinosus_), besides Ammonites, Belemnites (part of the internal sh.e.l.l of a kind of cuttle-fish), and the teeth of several species of sharks. Altogether the chalk is about 1,000 feet thick.

[Ill.u.s.tration: _Ammonites various_ (from the chalk).]

_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.

[Ill.u.s.tration: _Rhynchonella depressa_ (a Brachiopod, from the Upper Greensand).]

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-sh.e.l.l 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.

[Ill.u.s.tration: _Ammonites auritus_ (from the Gault).]

_c._ _Gault_, a stiff blue clay abounding in fossils: Ammonites often retaining their pearly sh.e.l.l; 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 ma.s.s 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 sh.e.l.ly limestones that were deposited in the delta and off the mouth of a big river. The sh.e.l.ls 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.

[Ill.u.s.tration: _Inoceramus concentricus_ (from the Gault).]

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 sh.e.l.l 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 pa.s.sed. They contain numerous fresh-water sh.e.l.ls--_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_ sh.e.l.l; 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.