Principles of Geology Part 10

Now in the first place, it may be observed, that many naturalists are guilty of no small inconsistency in endeavoring to connect the phenomena of the earliest vegetation with a nascent condition of organic life, and at the same time to deduce from the numerical predominance of certain forms, the greater heat or uniformity of the ancient climate. The arguments in favor of the latter conclusion are without any force, unless we can a.s.sume that the rules followed by the Author of Nature in the creation and distribution of organic beings were the same formerly as now; and that, as certain families of animals and plants are now most abundant in, or exclusively confined to regions where there is a certain temperature, a certain degree of humidity, a certain intensity of light, and other conditions, so also a.n.a.logous phenomena were exhibited at every former era.

If this postulate be denied, and the prevalence of particular families be declared to depend on a certain order of precedence in the introduction of different cla.s.ses into the earth, and if it be maintained that the standard of organization was raised successively, we must then ascribe the numerical preponderance, in the earlier ages, of plants of simpler structure, _not to the heat_, or other climatal conditions, but to those different laws which regulate organic life in newly created worlds.

Before we can infer a warm and uniform temperature in high lat.i.tudes, from the presence of 250 species of ferns, some of them arborescent, accompanied by lycopadiacae of large size, and araucariae, we must be permitted to a.s.sume, that at all times, past, present, and future, a heated and moist atmosphere pervading the northern hemisphere has a tendency to produce in the vegetation a predominance of a.n.a.logous forms.

It should moreover be borne in mind, when we are considering the question of development from a botanical point of view, that naturalists are by no means agreed as to the existence of an ascending scale of organization in the vegetable world corresponding to that which is very generally recognized in animals. "From the sponge to man," in the language of De Blainville, there may be a progressive chain of being, although often broken and imperfect; but if we seek to cla.s.sify plants according to a linear arrangement, ascending gradually from the lichen to the lily or the rose, we encounter incomparably greater difficulties.

Yet the doctrine of a more highly developed organization in the plants created at successive periods presupposes the admission of such a graduated scale.

We have as yet obtained but scanty information respecting the state of the terrestrial flora at periods antecedent to the coal. In the carboniferous epoch, about 500 species of fossil plants are enumerated by Adolphe Brongniart, which we may safely regard as a mere fragment of an ancient flora; since, in Europe alone, there are now no less than 11,000 living species. I have already hinted that the plants which produced coal were not drifted from a distance, but that nearly all of them grew on the spots where they became fossil. They appear to have belonged, as before explained (p. 115), to a peculiar cla.s.s of _stations_,--to low level and swampy regions, in the deltas of large rivers, slightly elevated above the level of the sea. From the study, therefore, of such a vegetation, we can derive but little insight into the nature of the contemporaneous upland flora, still less of the plants of the mountainous or Alpine country; and if so, we are enabled to account for the apparent monotony of the vegetation, although its uniform character was doubtless in part owing to a greater uniformity of climate then prevailing throughout the globe. Some of the commonest trees of this period, such as the sigillariae, which united the structure of ferns and of cycadeae, departed very widely from all known living types. The coniferae and ferns, on the contrary, were very closely allied to living genera. It is remarkable that none of the exogens of Lindley (dicotyledonous angiosperms of Brongniart), which comprise four-fifths of the living flora of the globe, and include all the forest trees of Europe except the fir-tribe, have yet been discovered in the coal measures, and a very small number--fifteen species only--of monocotyledons. If several of these last are true plants, an opinion to which Messrs. Lindley, Unger, Corda, and other botanists of note incline, the question whether any of the most highly organized plants are to be met with in ancient strata is at once answered in the affirmative. But the determination of these palms being doubtful, we have as yet in the coal no positive proofs either of the existence of the most perfect, or of the most simple forms of flowering or flowerless vegetation. We have no fungi, lichens, hepatici or mosses: yet this latter cla.s.s may have been as fully represented then as now.

In the flora of the secondary eras, all botanists agree that palms existed, although in Europe plants of the family of zamia and cycas together with coniferae predominated, and must have given a peculiar aspect to the flora. As only 200 or 300 species of plants are known in all the rocks ranging from the Trias to the Oolite inclusive, our data are too scanty as yet to affirm whether the vegetation of this second epoch was or was not on the whole of a simpler organization than that of our own times.

In the Lower Cretaceous formation, near Aix-la-Chapelle, the leaves of a great many dicotyledonous trees have lately been discovered by Dr.

Debey, establis.h.i.+ng the important fact of the coexistence of a large number of angiosperms with cycadeae, and with that rich reptilian fauna comprising the ichthyosaur, plesiosaur, and pterodactyl, which some had supposed to indicate a state of the atmosphere unfavorable to a dicotyledonous vegetation.

The number of plants. .h.i.therto obtained from _tertiary_ strata of different ages is very limited, but is rapidly increasing. They are referable to a much greater variety of families and cla.s.ses than an equal number of fossil species taken from secondary or primary rocks, the angiosperms bearing the same proportion to the gymnosperms and acrogens as in the present flora of the globe. This greater variety may, doubtless, be partly ascribed to the greater diversity of stations in which the plants grew, as we have in this case an opportunity, rarely enjoyed in studying the secondary fossils, of investigating inland or lacustrine deposits acc.u.mulated at different heights above the sea, and containing the memorials of plants washed down from adjoining mountains.

In regard, then, to the strata from the cretaceous to the uppermost tertiary inclusive, we may affirm that we find in them all the princ.i.p.al cla.s.ses of living plants, and during this vast lapse of time four or five complete changes in the vegetation occurred, yet no step whatever was made in advance at any of these periods by the addition of more highly organized species.

If we next turn to the fossils of the animal kingdom, we may inquire whether, when they are arranged by the geologists in a chronological series, they imply that beings of more highly developed structure and greater intelligence entered upon the earth at successive epochs, those of the simplest organization being the first created, and those more highly organized being the last.

Our knowledge of the Silurian fauna is at present derived entirely from rocks of marine origin, no fresh-water strata of such high antiquity having yet been met with. The fossils, however, of these ancient rocks at once reduce the theory of progressive development to within very narrow limits, for already they comprise a very full representation of the radiata, mollusca, and articulata proper to the sea. Thus, in the great division of radiata, we find asteriod and helianthoid zoophytes, besides crinoid and cystidean echinoderms. In the mollusca, between 200 and 300 species of cephalopoda are enumerated. In the articulata we have the crustaceans represented by more than 200 species of trilobites, besides other genera of the same cla.s.s. The remains of fish are as yet confined to the upper part of the Silurian series; but some of these belong to placoid fish, which occupy a high grade in the scale of organization. Some naturalists have a.s.sumed that the earliest fauna was exclusively marine, because we have not yet found a single Silurian helix, insect, bird, terrestrial reptile or mammifer; but when we carry back our investigation to a period so remote from the present, we ought not to be surprised if the only accessible strata should be limited to deposits formed far from land, because the ocean probably occupied then, as now, the greater part of the earth's surface. After so many entire geographical revolutions, the chances are nearly three to one in favor of our finding that such small portions of the existing continents and islands as expose Silurian strata to view, should coincide in position with the ancient ocean rather than the land. We must not, therefore, too hastily infer, from the absence of fossil bones of mammalia in the older rocks, that the highest cla.s.s of vertebrated animals did not exist in remoter ages. There are regions at present, in the Indian and Pacific Oceans, coextensive in area with the continents of Europe and North America, where we might dredge the bottom and draw up thousands of sh.e.l.ls and corals, without obtaining one bone of a land quadruped.

Suppose our mariners were to report, that, on sounding in the Indian Ocean near some coral reefs, and at some distance from the land, they drew up on hooks attached to their line portions of a leopard, elephant, or tapir, should we not be skeptical as to the accuracy of their statements? and if we had no doubt of their veracity, might we not suspect them to be unskilful naturalists? or, if the fact were unquestioned, should we not be disposed to believe that some vessel had been wrecked on the spot?

The casualties must always be rare by which land quadrupeds are swept by rivers far out into the open sea, and still rarer the contingency of such a floating body not being devoured by sharks or other predaceous fish, such as were those of which we find the teeth preserved in some of the carboniferous strata. But if the carca.s.s should escape, and should happen to sink where sediment was in the act of acc.u.mulating, and if the numerous causes of subsequent disintegration should not efface all traces of the body, included for countless ages in solid rock, is it not contrary to all calculation of chances that we should hit upon the exact spot--that mere point in the bed of an ancient ocean, where the precious relic was entombed? Can we expect for a moment, when we have only succeeded, amidst several thousand fragments of corals and sh.e.l.ls, in finding a few bones of _aquatic_ or _amphibious_ animals, that we should meet with a single skeleton of an inhabitant of the land?

Clarence, in his dream, saw, "in the slimy bottom of the deep,"

----a thousand fearful wrecks; A thousand men, that fishes gnaw'd upon: Wedges of gold, great anchors, heaps of pearl.

Had he also beheld, amid "the dead bones that lay scattered by," the carca.s.ses of lions, deer, and the other wild tenants of the forest and the plain, the fiction would have been deemed unworthy of the genius of Shakspeare. So daring a disregard of probability and violation of a.n.a.logy would have been condemned as unpardonable, even where the poet was painting those incongruous images which present themselves to a disturbed imagination during the visions of the night.

Until lately it was supposed that the old red sandstone, or Devonian rocks, contained no vertebrate remains except those of fish, but in 1850 the footprints of a chelonian, and in 1851 the skeleton of a reptile, allied both to the batrachians and lizards, were found in a sandstone of that age near Elgin in Scotland.[212] Up to the year 1844 it was laid down as a received dogma in many works of high authority in geology, that reptiles were not created until after the close of the carboniferous epoch. In the course of that year, however, Hermann Von Meyer announced the discovery, in the coal measures of Rhenish Bavaria, of a reptile, called by him Apateon, related to the salamanders; and in 1847 three species of another genus, called archegosaurus by Goldfuss, were obtained from the coal of Saarbruck, between Treves and Strasburg.

The footprints of a large quadruped, probably batrachian, had also been observed by Dr. King in the carboniferous rocks of Pennsylvania in 1844.

The first example of the _bones_ of a reptile in the Coal of North America was detected so lately as September, 1852, by Mr. G. W. Dawson and myself in Nova Scotia. These remains, referred by Messrs. Wyman and Owen to a perennibranchiate batrachian, were met with in the interior of an erect fossil tree, apparently a sigillaria. They seem clearly to have been introduced together with sediment into the tree, during its submergence and after it had decayed and was standing as a hollow cylinder of bark, this bark being now converted into coal.

When Aga.s.siz, in his great work on fossil fish, described 152 species of ichthyolites from the Coal, he found them to consist of 94 placoids, belonging to the families of shark and ray, and 58 ganoids. One family of the latter he called "sauroid fish," including the megalicthys and holoptychius, often of great size, and all predaceous. Although true fish, and not intermediate between that cla.s.s and reptiles, they seem to have been more highly organized than any living fish, reminding us of the skeletons of saurians by the close suture of their cranial bones, their large conical teeth, striated longitudinally, and the articulation of the spinous processes with the vertebrae. Among living species they are most nearly allied to the lepidosteus, or bony pike of the North American rivers. Before the recent progress of discovery above alluded to had shown the fallacy of such ideas, it was imagined by some geologists that this ichthyic type was the more highly developed, because it took the lead at the head of nature before the cla.s.s of reptiles had been created. The confident a.s.sumption indulged in till the year 1844, that reptiles were first introduced into the earth in the Permian period, shows the danger of taking for granted that the date of the creation of any family of animals or plants in past time coincides with the age of the oldest stratified rock in which the geologist has detected its remains. Nevertheless, after repeated disappointments, we find some naturalists as much disposed as ever to rely on such negative evidence, and to feel now as sure that reptiles were not introduced into the earth till after the Silurian epoch, as they were in 1844, that they appeared for the first time at an era subsequent to the carboniferous.

Scanty as is the information hitherto obtained in regard to the articulata of the coal formation, we have at least ascertained that some insects winged their way through the ancient forests. In the ironstone of Coalbrook Dale, two species of coleoptera of the Linnaean genus curculio have been met with: and a neuropterous insect resembling a corydalis, together with another of the same order related to the phasmidae. As an example of the insectivorous arachnidae, I may mention the scorpion of the Bohemian coal, figured by Count Sternberg, in which even the eyes, skin, and minute hairs were preserved.[213] We need not despair, therefore, of obtaining eventually fossil representatives of all the princ.i.p.al orders of hexapods and arachnidae in carboniferous strata.

Next in chronological order above the Coal comes the allied Magnesian Limestone, or Permian group, and the secondary formations from the Trias to the Chalk inclusive. These rocks comprise the monuments of a long series of ages in which reptiles of every variety of size, form, and structure peopled the earth; so that the whole period, and especially that of the Lias and Oolite, has been sometimes called "the age of reptiles." As there are now mammalia entirely confined to the land; others which, like the bat and vampire, fly in the air; others, again, of amphibious habits, frequenting rivers, like the hippopotamus, otter, and beaver; others exclusively aquatic and marine, like the seal, whale, and narwal; so in the early ages under consideration, there were terrestrial, winged, and aquatic reptiles. There were iguanodons walking on the land, pterodactyls winging their way through the air, monitors and crocodiles in the rivers, and ichthyosaurs and plesiosaurs in the ocean. It appears also that some of these ancient saurians approximated more nearly in their organization to the type of living mammalia than do any of the reptiles now existing.[214]

In the vast range of strata above alluded to, comprising the Permian, the Upper New Red Sandstone and Muschelkalk, the Lias, Oolite, Wealden, Green-sand, and Chalk, scarcely any well-authenticated instances of the occurrence of fossil birds in Europe are on record, and only two or three of fossil mammalia.

In regard to the absence of birds, they are usually wanting, for reasons afterwards to be explained (see chap. 47), in deposits of all ages, even in the tertiary periods, where we know that birds as well as land quadrupeds abounded. Some at least of the fossil remains formerly referred to this cla.s.s in the Wealden (a great freshwater deposit below the chalk), have been recently shown by Mr. Owen to belong to pterodactyls.[215] But in North America still more ancient indications of the existence of the feathered tribe have been detected, the fossil foot-marks of a great variety of species, of various sizes, some larger than the ostrich, others smaller than the plover, having been observed.

These bipeds have left marks of their footsteps on strata of an age decidedly intermediate between the Lias and the Coal.[216]

[Ill.u.s.tration: Fig. 8.

_Natural Size_.

Thylacotherium Prevostii (_Valenciennes_). Amphitherium (_Owen_). Lower jaw, from the slate of Stonesfield, near Oxford.[218]]

[Ill.u.s.tration: Fig. 9.

Myrmecobius fasciatus (_Waterhouse_). Recent from Swan River. Lower jaw of the natural size.[219]]

The examples of mammalia, above alluded to, are confined to the Trias and the Oolite. In the former, the evidence is as yet limited to two small molar teeth, described by Professor Plieninger in 1847, under the generic name of Microlestes. They were found near Stuttgart, and possess the double fangs so characteristic of mammalia.[217] The other fossil remains of the same cla.s.s were derived from one of the inferior members of the oolitic series in Oxfords.h.i.+re, and afford more full and satisfactory evidence, consisting of the lower jaws of three species of small quadrupeds about the size of a mole. Cuvier, when he saw one of them (during a visit to Oxford in 1818), referred it to the marsupial order, stating, however, that it differed from all known carnivora in having ten molar teeth in a row. Professor Owen afterwards pointed out that the jaw belonged to an extinct genus, having considerable affinity to a newly discovered Australian mammifer, the _Myrmecobius_ of Waterhouse, which has nine molar teeth in the lower jaw. (Fig. 9.) A more perfect specimen enabled Mr. Owen in 1846 to prove that the inflection of the angular process of the lower jaw was not sufficiently marked to ent.i.tle the osteologist to infer that this quadruped was marsupial, as the process is not bent inwards in a greater degree than in the mole or hedgehog. Hence the genus amphitherium, of which there are two species from Stonesfield, must be referred to the ordinary or placental type of insectivorous mammals, although it approximates in some points of structure to the myrmecobius and allied marsupials of Australia. The other contemporary genus, called phascolotherium, agrees much more nearly in osteological character and precisely in the number of the teeth with the opossums; and is believed to have been truly marsupial. (Fig. 10.)

[Ill.u.s.tration: Fig. 10.

_Natural size._

Phascolotherium Bucklandi, _Owen_. (_Syn._ Didelphis Bucklandi, _Brod._) Lower jaw, from Stonesfield.[220]

1. The jaw magnified twice in length. 2. The second molar tooth magnified six times.]

The occurrence of these most ancient memorials of the mammiferous type, in so low a member of the oolitic series, while no other representatives of the same cla.s.s (if we except the microlestes) have yet been found in any other of the inferior or superior secondary strata, is a striking fact, and should serve as a warning to us against hasty generalizations, founded solely on negative evidence. So important an exception to a general rule may be perfectly consistent with the conclusion, that a small number only of mammalia inhabited European lat.i.tudes when our secondary rocks were formed; but it seems fatal to the theory of progressive development, or to the notion that the order of precedence in the creation of animals, considered chronologically, has precisely coincided with the order in which they would be ranked according to perfection or complexity of structure.

It was for many years suggested that the marsupial order to which the fossil animals of Stonesfield were supposed exclusively to belong const.i.tutes the lowest grade in the cla.s.s Mammalia, and that this order, of which the brain is of more simple form, evinces an inferior degree of intelligence. If, therefore, in the oolitic period the marsupial tribes were the only warm-blooded quadrupeds which had as yet appeared upon our planet, the fact, it was said, confirmed the theory which teaches that the creation of the more simple forms in each division of the animal kingdom preceded that of the more complex. But on how slender a support, even if the facts had continued to hold true, did such important conclusions hang! The Australian continent, so far as it has been hitherto explored, contains no indigenous quadrupeds save those of the marsupial order, with the exception of a few small rodents, while some neighboring islands to the north, and even southern Africa, in the same lat.i.tude as Australia, abound in mammalia of every tribe except the marsupial. We are entirely unable to explain on what physiological or other laws this singular diversity in the habitations of living mammalia depends; but nothing is more clear than that the causes which stamp so peculiar a character on two different provinces of wide extent are wholly independent of time, or of the age or maturity of the planet.

The strata of the Wealden, although of a later date than the oolite of Stonesfield, and although filled with the remains of large reptiles, both terrestrial and aquatic, have not yielded as yet a single marsupial bone. Were we to a.s.sume on such scanty data that no warm-blooded quadrupeds were then to be found throughout the northern hemisphere, there would still remain a curious subject of speculation, whether the entire suppression of one important cla.s.s of vertebrata, such as the mammiferous, and the great development of another, such as the reptilian, implies a departure from fixed and uniform rules governing the fluctuations of the animal world; such rules, for example, as appear from one century to another to determine the growth of certain tribes of plants and animals in arctic, and of other tribes in tropical regions.

In Australia, New Zealand, and many other parts of the southern hemisphere, where the indigenous land quadrupeds are comparatively few, and of small dimensions, the reptiles do not predominate in number or size. The deposits formed at the mouth of an Australian river, within the tropics, might contain the bones of only a few small marsupial animals, which, like those of Stonesfield, might hereafter be discovered with difficulty by geologists; but there would, at the same time, be no megalosauri and other fossil remains, showing that large saurians were plentiful on the land and in the waters at a time when mammalia were scarce. This example, therefore, would afford a very imperfect parallel to the state of the animal kingdom, supposed to have prevailed during the secondary periods, when a high temperature pervaded European lat.i.tudes.

It may nevertheless be advantageous to point to some existing anomalies in the geographical development of distinct cla.s.ses of vertebrata which may be comparable to former conditions of the animal creation brought to light by geology. Thus in the arctic regions, at present, reptiles are small, and sometimes wholly wanting, where birds, large land quadrupeds, and cetacea abound. We meet with bears, wolves, foxes, musk oxen, and deer, walruses, seals, whales, and narwals, in regions of ice and snow, where the smallest snakes, efts, and frogs are rarely, if ever, seen.

A still more anomalous state of things presents itself in the southern hemisphere. Even in the temperate zone, between the lat.i.tudes 52 and 56 S., as, for example, in Tierra del Fuego, as well as in the woody region immediately north of the Straits of Magellan, and in the Falkland Islands, no reptiles of any kind are met with, not even a snake, lizard, or frog; but in these same countries we find the guanaco (a kind of llama), a deer, the puma, a large species of fox, many small rodentia, besides the seal and otter, together with the porpoise, whale, and other cetacea.

On what grand laws in the animal physiology these remarkable phenomena depend, cannot in the present state of science be conjectured; nor could we predict whether any opposite condition of the atmosphere, in respect to heat, moisture, and other circ.u.mstances, would bring about a state of animal life which might be called the converse of that above described, namely, a state in which reptiles of every size and order might abound, and mammalia disappear.

The nearest approximation to such a fauna is found in the Galapagos Archipelago. These islands, situated under the equator, and nearly 600 miles west of the coast of Peru, have been called "the land of reptiles," so great is the number of snakes, large tortoises, and lizards, which they support. Among the lizards, the first living species proper to the ocean has been discovered. Yet, although some of these islands are from 3000 to 4000 feet high, and one of them 75 miles long, they contain, with the exception of one small mouse, no indigenous mammifer. Even here, however, it is true that in the neighboring sea there are seals, and several kinds of cetacea.[221]

It may be unreasonable to look for a nearer a.n.a.logy between the fauna now existing in any part of the globe, and that which we can show to have prevailed when our secondary strata were deposited, because we must always recollect that a climate like that now experienced at the equator, coexisting with the unequal days and nights of European lat.i.tudes, was a state of things to which there is now no counterpart on the globe. Consequently, the type of animal and vegetable existence required for such a climate might be expected to deviate almost as widely from that now established, as do the flora and fauna of our tropical differ from those of our arctic regions.

_In the Tertiary strata._--The tertiary formations were deposited when the physical geography of the northern hemisphere had been entirely altered. Large inland lakes had become numerous, as in central France and other countries. There were gulfs of the sea, into which considerable rivers emptied themselves, and where strata like those of the Paris basin were acc.u.mulated. There were also formations in progress, in shallow seas not far from sh.o.r.e, such as are indicated by portions of the _Faluns_ of the Loire, and the English _Crag_.

The proximity, therefore, of large tracts of dry land to the seas and lakes then existing, may, in a great measure, explain why the remains of land animals, so rare in the older strata, are not uncommon in these more modern deposits. Yet even these have sometimes proved entirely dest.i.tute of mammiferous relics for years after they had become celebrated for the abundance of their fossil testacea, fish, and reptiles. Thus the calcaire grossier, a marine limestone of the district round Paris, had afforded to collectors more than 1100 species of sh.e.l.ls, besides many zoophytes, echinodermata, and the teeth of fish, before the bones of one or two land quadrupeds were met with in the same rock. The strata called London and Plastic clay in England have been studied for more than half a century, and about 400 species of sh.e.l.ls, 50 or more of fish, besides several kinds of chelonian and saurian reptiles, were known before a single mammifer was detected. At length, in the year 1839, there were found in this formation the remains of a monkey, an opossum, a bat,[222] and a species of the extinct genus Hyracotherium, allied to the Peccary or hog tribe.

If we examine the strata above the London clay in England, we first meet with mammiferous remains in the Isle of Wight, in beds also belonging to the Eocene epoch, such as the remains of the Palaeotherium, Anoplotherium, and other extinct quadrupeds, agreeing very closely with those first found by Cuvier, near Paris, in strata of the same age, and of similar freshwater origin.

In France we meet with another fauna, both conchological and mammalian in the Miocene "faluns" of the Loire; above which in the ascending series in Great Britain we arrive at the coralline crag of Suffolk, a marine formation which has yielded three or four hundred species of sh.e.l.ls, very different from the Eocene testacea, and of which a large proportion, although a minority of the whole number, are recent, besides many corals, echini, foraminifera, and fish, but as yet no relic decidedly mammalian except the ear-bone of a whale.

In the sh.e.l.ly sand, provincially termed "Red Crag," in Suffolk, which immediately succeeds the coralline, const.i.tuting a newer member of the same tertiary group, about 250 species of sh.e.l.ls have been recognized, of which a still larger proportion are recent. They are a.s.sociated with numerous teeth of fish; but no signs of a warm-blooded quadruped had been detected until 1839, when the teeth of a leopard, a bear, a hog, and a species of ruminant, were found at Newbourn, in Suffolk, and since that time, several other genera of mammalia have been met with in the same formation, or in the Red Crag.[223]

Of a still newer date is the Norwich Crag, a fluvio-marine deposit of the Pleiocene epoch, containing a mixture of marine, fluviatile, and land sh.e.l.ls, of which 90 per cent. or more are recent. These beds, since the time of their first investigation, have yielded a supply of mammalian bones of the genera mastodon, elephant, rhinoceros, pig, horse, deer, ox, and others, the bodies of which may have been washed down into the sea by rivers draining land, of which the contiguity is indicated by the occasional presence of terrestrial and freshwater sh.e.l.ls.