HOW FOSSILS ARE FOUND: AND THE ROCKS THEY FORM.

As already noticed, it is the hard parts of buried animals and plants that are generally preserved. We will now consider the groups of organisms, one by one, and note the particular parts of each which we may reasonably expect to find in the fossil state.

MAMMALS.—The bones and teeth: as the Diprotodon remains of Lake Callabonna in South Australia (Fig. 14), of West Melbourne Swamp, Victoria, and the Darling Downs, Queensland. Rarely the skin, as in the carcases of the frozen Mammoth of the tundras of Northern Siberia; or the dried remains of the Grypotherium of South American caves.

BIRDS:—Bones: as the Moa bones of New Zealand and the Emu bones of the King Island sand-dunes (Fig. 15). Very rarely the impressions of the feathers of birds are found, as in the ironstone occurring in the Wannon district of Victoria (Fig. 16), and others in fine clays and marls on the continent of Europe and in England. Fossil eggs of sea-birds are occasionally found in coastal sand-dunes of Holocene age.

REPTILES.—Skeletons of fossil turtles (Notochelone) are found in Queensland (Fig. 17). Whole skeletons and the dermal armour (spines and bony plates) of the gigantic, specialised reptiles are found in Europe, North America, and in other parts of the world.

FISHES.—Whole skeletons are sometimes found in sand and clay rocks, as in the Trias of Gosford, New South Wales (Fig. 18), and in the Jurassic of South Gippsland. The ganoid or enamel-scaled fishes are common fossils in the Devonian and Jurassic, notably in Germany, Scotland and Canada: and they also occur in the sandy mudstone of the Lower Carboniferous of Mansfield, Victoria.

INSECTS.—Notwithstanding their fragility, insects are often well preserved as fossils, for the reason that their skin and wings consist of the horny substance called chitin. The Tertiary marls of Europe are very prolific in insect remains (Fig. 19). From the Miocene beds of Florissant, Colorado, U.S.A., several hundred species of insects have been described.

CRUSTACEA.—The outer crust, or exoskeleton, of these animals is often hard, being formed of a compound of carbonate and phosphate of lime on an organic, chitinous base. The earliest forms of this group were the trilobites, commencing in Cambrian times, and of which there is a good representative series in Australian rocks. Remains of crabs and lobsters are found in the various Cainozoic deposits in Australia (Fig. 20), and also in the Jurassic in other parts of the world.

MOLLUSCA.—The Cuttle-fish group (Cephalopoda, “head-footed”), is well represented by the Nautilus-like, but straight Orthoceras shells commencing in Ordovician times, and, in later periods, by the beautiful, coiled Ammonites (Fig. 21). The true cuttle-fishes possess an internal bone, the sepiostaire, which one may see at the present day drifted on to the sand at high-water mark on the sea-shore. The rod-like Belemnites are of this nature, and occur abundantly in the Australian Cretaceous rocks of South Australia and Queensland (Fig. 22).

Elephant-tusk shells (Scaphopoda) are frequent in our Tertiary beds: they are also sparingly found in the Cretaceous, and some doubtful remains occur in the Palaeozoic strata of Australia.

The shells of the ordinary mollusca, such as the snails, whelks, mussels, and scallops, are abundant in almost all geological strata from the earliest periods. Their calcareous shells form a covering which, after the decay of the animal within, are from their nature among the most easily preserved of fossil remains. There is hardly an estuary bed, lake-deposit, or sea-bottom, but contains a more or less abundant assemblage of these shell-fish remains, or testacea as they were formerly called (“testa,” a shell or potsherd). We see, therefore, the importance of this group of fossils for purposes of comparison of one fauna with another (antea, Fig. 1).

The chitons or mail-shells, by their jointed nature, consisting of a series of pent-roof-shaped valves united by ligamental tissue, are nearly always represented in the fossil state by separate valves. Fossil examples of this group occur in Australia both in Palaeozoic rocks and, more numerously, in the Cainozoic series.

MOLLUSCOIDEA.—The Brachiopods or Lamp-shells consist generally of two calcareous valves as in the true mollusca (Fig. 23), but are sometimes of horny texture. Like the previous class, they are also easily preserved as fossils. They possess bent, loop-like or spiral arms, called brachia, and by the movement of fine ciliated (hair-like) processes on their outer edges conduct small food particles to the mouth. The brachia are supported by shelly processes, to which are attached, in the Spirifers, delicate spirally coiled ribbons. These internal structures are often beautifully preserved, even though they are so delicate, from the fact that on the death of the animal the commissure or opening round the valves is so tightly closed as to prevent the coarse mud from penetrating while permitting the finer silt, and more rarely mineral matter in solution, to pass, and subsequently to be deposited within the cavity. At the Murray River cliffs in South Australia, a bed of Cainozoic limestone contains many of these brachiopod shells in a unique condition, for the hollow valves have been filled in with a clear crystal of selenite or gypsum, through which may be seen the loop or brachial support preserved in its entirety.

The Sea-mats or Polyzoa, represented by Retepora (the Lace-coral) (Fig. 24) and Flustra (the Sea-mat) of the present sea-shore, have a calcareous skeleton, or zoarium, which is easily preserved as a fossil. Polyzoa are very abundant in the Cainozoic beds of Australia, New Zealand, and elsewhere (Fig. 25). In the Mesozoic series, on the other hand, they are not so well represented; but in Europe and North America they play an important part in forming the Cretaceous and some Jurassic strata by the abundance of their remains.

WORMS (VERMES).—The hard, calcareous tubes of Sea-worms, the Polychaeta (“many bristles”) are often found in fossiliferous deposits, and sometimes form large masses, due to their gregarious habits of life; they also occur attached to shells such as oysters (Fig. 26). The burrows of the wandering worms are found in Silurian strata in Australia; and the sedentary forms likewise occur from the Devonian upwards.

ECHINODERMATA.—Sea-urchins (Echinoidea) possess a hard, calcareous, many-plated test or covering and, when living are covered with spines (Fig. 27). Both the tests and spines are found fossil, the former sometimes whole when the sediment has been quietly thrown down upon them; but more frequently, as in the Shepherd’s crown type (Cidaris), are found in disjointed plates, owing to the fact that current action, going on during entombment has caused the plates to separate. The spines are very rarely found attached to the test, more frequently being scattered through the marl or sandy clay in which the sea-urchins are buried. The best conditions for the preservation of this group is a marly limestone deposit, in which case the process of fossilisation would be tranquil (Fig. 28).

The true Starfishes (Asteroidea), are either covered with calcareous plates, or the skin is hardened by rough tubercles; and these more lasting portions are preserved in rocks of all ages. The shape of the animal is also often preserved in an exquisite manner in beds of fine mud or clay.

The Brittle-stars (Ophiuroidea) have their body covered with hard, calcareous plates. Their remains are found in rocks as old as the Ordovician in Bohemia but their history in Australia begins with the Silurian period (Fig. 29). From thence onward they are occasionally found in successive strata in various parts of the world.

The bag-like echinoderms (Cystidea) form a rare group, restricted to Palaeozoic strata. The plates of the sack, or theca, and those of the slender arms are calcareous, and are capable of being preserved in the fossil state. A few doubtful remains of this group occur in Australia.

The bud-shaped echinoderms (Blastoidea) also occur chiefly in Devonian and Carboniferous strata. This is also a rare group, and is represented by several forms found only in New South Wales and Queensland.

The well known and beautiful fossil forms, the Stone-lilies (Crinoidea) have a very extended geological history, beginning in the Cambrian; whilst a few species are living in the ocean at the present day. The many-jointed skeleton lends itself well to fossilisation, and remains of the crinoids are common in Australia mainly in Palaeozoic strata (Fig. 30). In Europe they are found abundantly also in Jurassic strata, especially in the Lias.

HYDROZOA.—The Graptolites (“stone-writing”) have a chitinous skin (periderm) to the body or hydrosome, which is capable of preservation to a remarkable degree; for their most delicate structures are preserved on the surfaces of the fine black mud deposits which subsequently became hardened into slates. In Australia graptolites occur from the base of the Ordovician to the top of the Silurian (Fig. 31).

Another section of the Hydrozoa is the Stromatoporoidea. These are essentially calcareous, and their structure reminds one of a dense coral. The polyps build their tiers of cells (coenosteum) in a regular manner, and seem to have played the same part in the building of ancient reefs in Silurian, Devonian and Carboniferous times as the Millepora at the present day (Fig. 32).

ANTHOZOA.—The true Corals have a stony skeleton, and this is capable of easy preservation as a fossil. There is hardly any fossiliferous stratum of importance which has not its representative corals. In Australia their remains are especially abundant in the Silurian, Devonian (Fig. 33), and Carboniferous formations, and again in the Oligocene and Miocene.

SPONGES.—The framework of the sponge may consist either of flinty, calcareous, or horny material (Fig. 34). The two former kinds are well represented in our Australian rocks, the first appearing in the Lower Ordovician associated with graptolites, and again in the Cretaceous and Tertiary rocks (Fig. 35); whilst the calcareous sponges are found in Silurian strata, near Yass, and again in the Cainozoic beds of Flinders, Curlewis and Mornington in Victoria.

PROTOZOA.—The important and widely-distributed group of the Foraminifera (“hole-bearers”) belonging to the lowest phylum, the Protozoa, generally possess a calcareous shell. The tests range in size from tiny specks of the fiftieth of an inch in diameter, to the giant Nummulite, equalling a five shilling piece in size (Fig. 36). Their varied and beautiful forms are very attractive, but their great interest lies in their multifarious distribution in all kinds of sediments: they are also of importance because certain of the more complex forms indicate distinct life zones, being restricted to particular strata occurring in widely-separated areas.

Members of the allied order of the Radiolaria have a flinty shell (Fig. 37); and these organisms are often found building up siliceous rocks such as cherts (Fig. 38).

PLANTS.—The harder portions of plants which are found in the fossil state are,—the wood, the coarser vascular (vessel-bearing) tissue of the leaves, and the harder parts of fruits and seeds.

Fossil wood is of frequent occurrence in Palaeozoic, Mesozoic and Cainozoic strata in Australia, as, for instance, the wood of the trees called Araucarioxylon and Dadoxylon in the Coal measures of New South Wales (see antea, Fig. 3).

Fossil leaves frequently occur in pipe-clay beds, as at Berwick, Victoria, and in travertine from near Hobart, Tasmania (Fig. 39). Fossil fruits are found in abundance in the ancient river gravels at several hundreds of feet below the surface, in the “deep leads” of Haddon, Victoria, and other localities in New South Wales, Queensland and Tasmania.

FOSSILIFEROUS ROCKS.

Section I.—ARGILLACEOUS ROCKS.

Under this head are placed the muds, clays, mudstones, shales and slates. MUDS are usually of a silty nature, that is, containing a variable proportion of sand (quartz) grains. Such are the estuarine muds of Pleistocene and Recent age, containing brackish water foraminifera and ostracoda, and those shells of the mollusca usually found associated with brackish conditions. Lacustrine mud can be distinguished by the included freshwater shells, as Limnaea, Coxiella (brackish), Cyclas and Bulinus, as well as the freshwater ostracoda or cyprids (Fig. 40).

CLAYS are tenacious mud deposits, having the general composition of a hydrous silicate of alumina with some iron. When a clay deposit tends to split into leaves or laminae, either through moderate pressure or by the included fossil remains occupying distinct planes in the rock, they are called SHALES.

Clays and Shales of marine origin are often crowded with the remains of mollusca. The shells are sometimes associated with leaves and other vegetable remains, if forming part of an alternating series of freshwater and marine conditions. An example of this type of sediments is seen in the Mornington beds of the Balcombian series in Victoria.

MUDSTONE is a term applied to a hardened clay deposit derived from the alteration of an impure limestone, and is more often found in the older series of rocks. Mudstones are frequently crowded with fossils, but owing to chemical changes within the rock, the calcareous organisms are as a rule represented by casts and moulds. At times these so faithfully represent the surface and cavities of the organism that they are almost equivalent to a well preserved fossil (Fig. 41).

SLATE.—When shale is subjected to great pressure, a plane of regular splitting called cleavage is induced, which is rarely parallel to the bedding plane or surface spread out on the original sea-floor: the cleavage more often taking place at an appreciable angle to the bedding plane. The graptolitic rocks of Victoria are either shales or slates, according to the absence or development of this cleavage structure in the rock.

Section II.—SILICEOUS ROCKS.

In this group are comprised all granular quartzose sediments, and organic rocks of flinty composition.

SANDSTONES.—Although the base of this type of rock is formed of quartz sand, it often contains fossils. Owing to its porous nature, percolation of water containing dissolved CO₂ tends to bring about the solution of the calcareous shells, with the result that only casts of the shells remain.

FLINTS and CHERTS.—These are found in the form of nodules and bands in other strata, principally in limestone. In Europe, flint is usually found in the Chalk formation, whilst chert is found in the Lower Greensands, the Jurassics, the Carboniferous Limestone and in Cambrian rocks. In Australia, flint occurs in the Miocene or Polyzoal-rock formation of Mount Gambier, Cape Liptrap and the Mallee borings. Flint is distinguished from chert by its being black in the mass, often with a white crust, and translucent in thin flakes; chert being more or less granular in texture and sub-opaque in the mass. Both kinds appear to be formed as a pseudomorph or replacement of a portion of the limestone stratum by silica, probably introduced in solution as a soluble alkaline silicate. Both flint and chert often contain fossil shells and other organic remains, such as radiolaria and sponge-spicules, which can be easily seen with a lens in thin flakes struck off by the hammer.

DIATOMITE is essentially composed of the tiny frustules or flinty cases of diatoms (unicellular algae), usually admixed with some spicules of the freshwater sponge, Spongilla. It generally forms a layer at the bottom of a lake bed (Fig. 42).

Section III.—CALCAREOUS ROCKS.

LIMESTONES FORMED BY ORGANISMS.—Organic limestones constitute by far the most important group of fossiliferous rocks. Rocks of this class are composed either wholly of carbonate of lime, or contain other mineral matter also, in varying proportion. Many kinds of limestones owe their origin directly to the agency of animals or plants, which extracted the calcareous matter from the water in which they lived in order to build their hard external cases, as for example the sea-urchins; or their internal skeletons, as the stony corals. The accumulated remains of these organisms are generally compacted by a crystalline cement to form a coherent rock.

The chief groups of animals and plants forming such limestone rocks are:—

(a) FORAMINIFERA.—Example. Foraminiferal limestone as the Nummulitic limestone of the Pyramids of Egypt, or the Lepidocyclina limestone of Batesford, near Geelong, Victoria (Fig. 43).

(b) CORALS.—Ex. “Madrepore limestone,” or Devonian marble, with Pachypora. Also the Lilydale limestone, with Favosites, of Silurian age, Victoria (Fig. 44).

(c) STONE-LILIES.—Ex. Crinoidal or Entrochial limestone, Silurian, Toongabbie, Victoria (Fig. 45). Also the Carboniferous or Mountain limestone, Derbyshire, England.

(d) WORM-TUBES.--Ex. Serpulite limestone of Hanover, Germany. Ditrupa limestone of Torquay and Wormbete Creek, Victoria.

(e) POLYZOA.—-Ex. Polyzoal limestone, as the so-called Coralline Crag of Suffolk, England; and the Polyzoal Rock of Mount Gambier, S. Australia.

(f) BRACHIOPODA.—Ex. Brachiopod limestone of Silurian age, Dudley, England. Orthis limestone of Cambrian age, Dolodrook River, N. E. Gippsland.

(g) MOLLUSCA.—Ex. Shell limestone, as the Turritella bed of Table Cape, Tasmania, and of Camperdown, Victoria (Fig. 46), or the Purbeck Marble of Swanage, Dorset, England.

(h) OSTRACODA.—Ex. Cypridiferous limestone, formed of the minute valves of the bivalved ostracoda, as that of Durlston, Dorset, England (Fig. 47).

(i) CADDIS FLY LARVAE.—Ex. Indusial limestone, formed of tubular cases constructed by the larvae of the Caddis fly (Phryganea). Occurs at Durckheim, Rhine District, Germany.

(j) RED SEAWEEDS.—Ex. Nullipore limestone, formed by the stony thallus (frond) of the calcareous sea-weed Lithothamnion, as in the Leithakalk, a common building stone of Vienna.

(k) GREEN SEAWEEDS.—Ex. Halimeda limestone, forming large masses of rock in the late Cainozoic reefs of the New Hebrides (Fig. 48).

(l) (?) BLUE-GREEN SEAWEEDS.—Ex. Girvanella limestone, forming the Peagrit of Jurassic age, of Gloucester, England.

Section IV.—CARBONACEOUS and MISCELLANEOUS ROCKS.

COALS and KEROSENE SHALES (Cannel Coal).—These carbonaceous rocks are formed in much the same way as the deposits in estuaries and lagoon swamps. They result from the sometimes vast aggregation of vegetable material (leaves, wood and fruits), brought down by flooded rivers from the surrounding country, which form a deposit in a swampy or brackish area near the coast, or in an estuary. Layer upon layer is thus formed, alternating with fine mud. The latter effectually seals up the organic layers and renders their change into a carbonaceous deposit more certain.

When shale occurs between the coal-layers it is spoken of as the under-clay, which in most cases is the ancient sub-soil related to the coal-layer immediately above. It is in the shales that the best examples of fossil ferns and other plant-remains are often found. The coal itself is composed of a partially decomposed mass of vegetation which has become hardened and bedded by pressure and gradual drying.

Spore coals are found in thick deposits in some English mines, as at Burnley in Yorkshire. They result from the accumulation of the spores of giant club-mosses which flourished in the coal-period. They are generally referred to under the head of Cannel Coals. The “white coal” or Tasmanite of the Mersey Basin in Tasmania is an example of an impure spore coal with a sandy matrix (Fig. 49).

The Kerosene Shale of New South Wales is related to the Torbanite of Scotland and Central France. It occurs in lenticular beds between the bituminous coal. It is a very important deposit, commercially speaking, for it yields kerosene oil, and is also used for the manufacture of gas. The rock is composed of myriads of little cell-bodies, referred to as Reinschia, and first supposed to be allied to the freshwater alga, Volvox; but this has lately been questioned, and an alternative view is that they may be the megaspores of club-mosses (Fig. 50).

The coals of Jurassic age in Australia are derived from the remains of coniferous trees and ferns; and some beautiful examples of these plants may often be found in the hardened clay or shale associated with the coal seams.

The Brown Coals of Cainozoic or Tertiary age in Australia are still but little advanced from the early stage, lignite. The leaves found in them are more or less like the present types of the flora. The wood is found to be of the Cypress type (Cupressinoxylon). In New Zealand, however, important deposits of coal of a more bituminous nature occur in the Oligocene of Westport and the Grey River Valley, in the Nelson District.

BONE BEDS.—The bones and excreta of fish and reptiles form considerable deposits in some of the sedimentary formations; especially those partly under the influence of land or swamp conditions. They constitute a kind of conglomerate in which are found bone-fragments and teeth (Fig. 51). These bone-beds are usually rich in phosphates, and are consequently valuable as a source of manure. The Miocene bone-bed with fish teeth at Florida, U.S.A., is a notable example. The nodule bed of the Victorian Cainozoics contains an assemblage of bones of cetaceans (whales, etc.).

BONE BRECCIAS.—These are usually formed of the remains of the larger mammals, and consist of a consolidated mass of fragments of bones and teeth embedded in a calcareous matrix. Bone-breccias are of frequent occurrence on the floors of caves which had formerly been the resort of carnivorous animals, and into which they dragged their prey. The surface water percolating through the overlying calcareous strata dissolved a certain amount of lime, and this was re-deposited on the animal remains lying scattered over the cave floor. A deposit so formed constitutes a stalagmite or floor encrustation. As examples of bone-breccias we may refer to the limestone at Limeburners Point, Geelong (Fig. 52); and the stalagmitic deposits of the Buchan Caves.

IRONSTONE.—Rocks formed almost entirely of limonite (hydrated peroxide of iron) are often due to the agency of unicellular plants known as diatoms, which separate the iron from water, and deposit it as hydrous peroxide of iron within their siliceous skeletons. In Norway and Sweden there are large and important deposits of bog iron-ore, which have presumably been formed in the beds of lakes.

Clay ironstone nodules (sphaerosiderite) have generally been formed as accretions around some decaying organic body. Many clay ironstone nodules, when broken open, reveal a fossil within, such as a coprolitic body, fern frond, fir-cone, shell or fish.

Oolitic ironstones are composed of minute granules which may have originally been calcareous grains, formed by a primitive plant or alga, but since replaced by iron oxide or carbonate.

The Tertiary ironstone of western Victoria is found to contain leaves, which were washed into lakes and swamps (Fig. 53); and the ferruginous groundmass may have been originally due to the presence of diatoms, though this yet remains to be proved.