F
Fossils are chiefly found in rocks which have been formed of sediments laid down in water, such as sandstone, shale and most limestones. These rocks, broadly speaking, have been deposited in a horizontal position, though really slightly inclined from shore to deep-water. One layer has been formed above another, so that the oldest layer is at the bottom, and the newest at the top, of the series (Fig. 11). Let us, for instance, examine a cliff showing three layers: the lower, a sandstone, we will Call A; the intermediate, a shale or clay bed, B; and the uppermost, a limestone or marl, C (Fig. 12). In forming a conclusion about the relative ages of the beds, we shall find that A is always older than B, and B than C, provided no disturbance of the strata has taken place. For instance, the beds once horizontally deposited may have been curved and folded over, or even broken and thrust out of place, within limited areas; but occurrences like these are extremely rare. Moreover, an examination of the surrounding country, or of deep cuttings in the neighbourhood, will tell us if there is any probability of this inversion of strata having taken place.
Fig. 11—Horizontal Layers of Fossiliferous Clays and Sands.In Sea Cliff, Torquay Coast, Victoria, looking towards Bird Rock.(Original).
Fig. 11—Horizontal Layers of Fossiliferous Clays and Sands.In Sea Cliff, Torquay Coast, Victoria, looking towards Bird Rock.(Original).
(Original).
Fig. 12—Cliff-Section to Show Superposition of Strata.A = Sandstone. B = Shale. C = Limestone.
Fig. 12—Cliff-Section to Show Superposition of Strata.A = Sandstone. B = Shale. C = Limestone.
This law of superposition holds good throughout the mass of sedimentary rocks forming the crust of the earth.
(1). Thus, the position of the strata shows the relative ages of the beds.
Differences in Fossil Faunas.—
Turning once again to our ideal cliff section, if we examine the fossils obtained from bed A, we shall find them differing in the number of kinds or species common to the other beds above and below. Thus, there will be more species alike in beds A and B or in B and C. In other words the faunas of A and B are more nearly related than those of A and C. This is explained by the fact that there is a gradual change in specific forms as we pass through the time series of strata from below upwards; so that the nearer one collecting platform is to another, as a rule, the stronger is the community of species.
Guide Fossils.—
Certain kinds of fossils are typical of particular formations. They are known as guide fossils, and by their occurrence help us to gain some idea of the approximate age of rocks widely separated by ocean and continent. Thus we find fossils typical of the Middle Devonian rocks in Europe, which also occur in parts of Australia, and we therefore conclude that the Australian rocks containing those particular fossils belong to the same formation, and are nearly of the same age.
(2). The included fossils, therefore, give evidence of the age of the beds.
Value of Lithological Evidence.—
The test of age by rock-structure has a more restricted use, but is of value when taken in conjunction with the sequence of the strata and the character of their included fossils.
To explain both the valuable and the uncertain elements of this last method as a determinant of age, we may cite, for instance, the Upper Ordovician slates of Victoria and New South Wales as an example of uniform rock formation; whilst the yellow mudstones and the grey limestones of the Upper Silurian (Yeringian series) of the same states, are instances of diverse lithological structures in strata of similar age. A reference in the latter case to the assemblages of fossils found therein, speedily settles the question.
(3). Hence, the structure and composition of the rocks (lithology), gives only partial evidence in regard to age.
Strata Vertically Arranged.—
The Stratigraphical Series of fossiliferous sediments comprises bedded rocks from all parts of the world, which geologists arrange in a vertical column according to age.
A general computation of such a column for the fossiliferous rocks of Europe gives a thickness of about 14 miles. This is equivalent to a mass of strata lying edgewise from Melbourne to Ringwood. The Australian sediments form a much thicker pile of rocks, for they can hardly fall short of 37 miles, or nearly the distance from Melbourne to Healesville.
This vertical column of strata was formed during three great eras of time. The oldest is called the Primary or Palaeozoic (“ancient life”), in which the animals and plants are of primitive types. This is followed by the Secondary or Mesozoic (“middle life”), in which the animals and plants are intermediate in character between the Palaeozoic and the later, Cainozoic. The third era is the Tertiary or Cainozoic (“recent life”), in which the animals and plants are most nearly allied to living forms. These great periods are further subdivided into epochs, as the Silurian epoch; and these again into stages, as the Yeringian stage.
Vertical Column of Fossiliferous Strata, Australia.
1.—The classification of the Cainozoics as employed here is virtually the same as given by McCoy in connection with his work for the Victorian Geological Survey. The writer has obtained further evidence to support these conclusions from special studies in the groups of the cetacea, mollusca and the protozoa. The alternative classification of the cainozoics as given by one or two later authors, introducing the useful local terminology of Hall and Pritchard for the various stages or assises is as follows:—
2.—Or Permo-carboniferous. As the series is held by some authorities to partake of the faunas of both epochs, it is preferable to use the shorter word, which moreover gives the natural sequence. There is, however, strong evidence in favour of using the term Permian for this important series.
3.—Mr. W. S. Dun regards theLepidodendronbeds of W. Australia, New South Wales and Queensland as of Upper Devonian age. There is no doubt, from a broad view of the whole question as to the respective age of these beds in Australia, that the one series is continuous, and probably represents the Upper Devonian and the Lower Carboniferous of the northern hemisphere.
4.—These limestones contain a fauna of brachiopods and corals which, at present, seems to point to the series as intermediate between the older Silurian and the Upper Ordovician.
Vertical Column of Fossiliferous Strata, New Zealand.
Note 1.—Based for the most part, but with some slight modifications,on Prof. J. Park’s classification in “Geology of New Zealand,” 1910.
Fig. 13.Range-in-Time of Fossils in Australasian Sedimentary Rocks.E.M., del.]
E.M., del.]
Fig. 14—Skeleton of Diprotodon australis, Owen.Uncovered in Morass at Lake Callabonna, South Australia.(By permission of Dr. E. C. Stirling).
Fig. 14—Skeleton of Diprotodon australis, Owen.Uncovered in Morass at Lake Callabonna, South Australia.(By permission of Dr. E. C. Stirling).
(By permission of Dr. E. C. Stirling).
HOW FOSSILS ARE FOUND: AND THE ROCKS THEY FORM.
A
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 theDiprotodonremains 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 theGrypotheriumof South American caves.
Fig. 15—Bird Bones.Exposed on Sand-blow at Seal Bay, King Island.(Photo by C. L. Barrett).
Fig. 15—Bird Bones.Exposed on Sand-blow at Seal Bay, King Island.(Photo by C. L. Barrett).
(Photo by C. L. Barrett).
Fig. 16—Impression of a Bird’s Featherin Ironstone.About2/3nat. size. Of Cainozoic (? Janjukian) Age. Redruth, Victoria.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 17—Notochelone costata, Owen sp. (Anterior portion of carapace.)About1/4nat. size. A Marine Turtle from the Lower Cretaceous of Flinders River, Queensland.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
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). Fromthe Miocene beds of Florissant, Colorado, U.S.A., several hundred species of insects have been described.
Fig. 18.A Fossil Fish with Ganoid Scales (Pristisomus crassus, A.S. Woodw.).About1/2nat. size. Trias (Hawkesbury Series), of Gosford, New South Wales.(Nat. Mus. Coll.)
Fig. 18.A Fossil Fish with Ganoid Scales (Pristisomus crassus, A.S. Woodw.).About1/2nat. size. Trias (Hawkesbury Series), of Gosford, New South Wales.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig 19—A Fossil Insect (Tipula sp.) in Amber.Nat. size. Oligocene beds; Baltic Prussia.(F.C. Coll.)
(F.C. Coll.)
Fig. 20—A Fossil Lobster (Thalassina emerii, Bell).Slightly reduced. From the Pleistocene of Port Darwin, Northern Territory.(Nat. Mus. Coll.)
Fig. 20—A Fossil Lobster (Thalassina emerii, Bell).Slightly reduced. From the Pleistocene of Port Darwin, Northern Territory.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 21—An Ammonite (Desmoceras flindersi, McCoy sp.)Half nat. size. Showing complex sutures. L. Cretaceous: Marathon, Flinders River, Queensland.(Nat. Mus. Coll.)
Fig. 21—An Ammonite (Desmoceras flindersi, McCoy sp.)Half nat. size. Showing complex sutures. L. Cretaceous: Marathon, Flinders River, Queensland.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
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 straightOrthocerasshells 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).
Fig. 22.Belemnites (Belemnites diptycha, McCoy).1/3nat. size. Lower Cretaceous. Central South Australia.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 23—A Group of Lamp Shells (Magellania flavescens, Lam. sp.)Attached to a Polyzoan.About1/3nat. size. Dredged from Westernport, Victoria.(C.J. Gabriel Coll.)
(C.J. Gabriel Coll.)
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.
Fig. 24.— A Fossil Polyzoan (Macroporaclarkei, T. Woods, sp.)2/3nat. size. Flinders, Victoria.(F.C. Coll.)
(F.C. Coll.)
Fig. 25—A Fossil Polyzoan (Macroporaclarkei, T. Woods, sp.)About1/2nat. size. Cainozoic (Balcombian).Muddy Creek, Victoria.(F. C. Coll.)
(F. C. Coll.)
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 orgypsum, through which may be seen the loop or brachial support preserved in its entirety.
The Sea-mats or Polyzoa, represented byRetepora(the Lace-coral) (Fig. 24) andFlustra(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 frequentlybeing 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).
Fig. 26.—Fossil Worm Tubes (? Serpula.)Attached to a Pecten. Slightly Enlarged.Cainozoic (Balcombian).Muddy Creek, Hamilton, Victoria.(F.C. Coll.)
(F.C. Coll.)
Fig. 27—A Regular Sea-Urchin(Strongylocentrotus erythrogrammus, Val.)About2/3nat. size.Showing Spines attached.Living. Victoria.(F.C. Coll.)
(F.C. Coll.)
Fig. 28.—Sea-Urchin (Linthia antiaustralis, Tate).Test denuded of Spines.About2/3nat. size.Cainozoic (Janjukian):Curlewis, Victoria.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 29—Ophioderma egertoni, Broderip, sp.About1/2nat. size.A Brittle Star from the Liasof Seaton, Devon, England.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
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.
Fig. 30.A Fossil Crinoid (Taxocrinussimplex, Phillips sp.)About1/2nat. size.Wenlock Limestone (Silurian),Dudley, England.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 31—Graptolites on Slate (Tetragraptus fruticosus, J. Hall, sp.)Nat. Size. Lower Ordovician. Bendigo, Victoria.(Nat. Mus. Coll.)
Fig. 31—Graptolites on Slate (Tetragraptus fruticosus, J. Hall, sp.)Nat. Size. Lower Ordovician. Bendigo, Victoria.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 32.Polished Vertical Section of a Stromatoporoid. (Actinostroma).Nat. size. Middle Devonian. South Devon, England.(Nat. Mus. Coll.)
Fig. 32.Polished Vertical Section of a Stromatoporoid. (Actinostroma).Nat. size. Middle Devonian. South Devon, England.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
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. Thepolyps 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).
Fig. 33—Fossil Corals (Favosites).Photograph of a Polished Slab,2/3nat.size. In Devonian Limestone,Buchan, Victoria.
Fig. 34.
Fig. 34—Siliceous Skeleton of a LivingHexactinellid Sponge.Probably Chonelasma.× 4. Mauritius. (Viewed in TwoDirections.)(F.C. Coll.)
(F.C. Coll.)
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, andagain 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.
Fig. 35.Spicules of a Siliceous Sponge(Ecionema newberyi, McCoy sp.)Highly magnified. CainozoicShell-Marl.Altona Bay Coal-Shaft.
Fig. 36.Nummulites (N. gizehensis Ehr. var.champollioni, de la Harpe).About nat. size. Middle Eocene Limestone.Cyrene, Northern Africa.(Coll. by Dr. J. W. Gregory).
(Coll. by Dr. J. W. Gregory).
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 indicatedistinct life zones, being restricted to particular strata occurring in widely-separated areas.
Fig. 37—Siliceous Skeletons of Radiolaria.× 58. Late Cainozoic Age. Bissex Hill, Barbados, West Indies.(F.C. Coll.)
Fig. 37—Siliceous Skeletons of Radiolaria.× 58. Late Cainozoic Age. Bissex Hill, Barbados, West Indies.(F.C. Coll.)
(F.C. Coll.)
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 calledAraucarioxylonandDadoxylonin the Coal measures of New South Wales (seeantea,Fig. 3).
Fig. 38—Radiolaria in Siliceous Limestone.× 40. Middle Devonian: Tamworth, New South Wales.(From Prof. David’s Collection).
Fig. 38—Radiolaria in Siliceous Limestone.× 40. Middle Devonian: Tamworth, New South Wales.(From Prof. David’s Collection).
(From Prof. David’s Collection).
Fig. 39—Travertin Limestone with Leaves of Beech (Fagus).Nat. size. Pleistocene: near Hobart, Tasmania.(Nat. Mus. Coll.)
Fig. 39—Travertin Limestone with Leaves of Beech (Fagus).Nat. size. Pleistocene: near Hobart, Tasmania.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
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.
Fig. 40—Freshwater Limestone with Shells (Bulinus).About4/5nat. size. Mount Arapiles, Western Victoria.(Nat. Mus. Coll.)
Fig. 40—Freshwater Limestone with Shells (Bulinus).About4/5nat. size. Mount Arapiles, Western Victoria.(Nat. Mus. Coll.)
(Nat. Mus. Coll.)
Fig. 41—Fossiliferous Mudstone of Silurian (Yeringian) Age.With Brachiopods. About2/3nat. size. Near Lilydale, Victoria.(F.C. Coll.)
Fig. 41—Fossiliferous Mudstone of Silurian (Yeringian) Age.With Brachiopods. About2/3nat. size. Near Lilydale, Victoria.(F.C. Coll.)
(F.C. Coll.)
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, asLimnaea,Coxiella(brackish),CyclasandBulinus, 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 CO2tends 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).