CHAPTER IX.

EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED.

The establishment of the order of succession of the strata, and the correlation of strata of different areas merely pave the way for the geologist. To write the history of the earth during various geological ages, he has to ascertain the physical and climatic conditions which prevailed during the successive geological periods, and to study the various problems connected with the life of each period. In the present chapter an attempt will be made to illustrate the methods which have been pursued in order to write to the fullest degree which is compatible with our present knowledge, the earth-history of various ages of the past. In making this attempt, the physical and climatic conditions may be first considered, and their consideration followed by that of the changes in the faunas, though it will frequently be necessary to refer to one set of conditions as illustrative of the other.

It will be assumed here that the great principle of geology, that the modern changes of the earth and its inhabitants are illustrative of past changes, is rigidly true. Reference will be made to this principle in a later chapter, but it is sufficient to state here that the study of the sediments which have been deposited from the commencement of Lower Palæozoic times to the times in which we now live bear the marks of having been formed under physical conditions, which, in the main, are similar in kind to those which prevail upon some part of the surface of the lithosphere at the present day.

One of the most important inferences of the stratigrapher relates to the existence of marine or terrestrial conditions over an area at any particular time, and we may, in the first place, consider the evidence which supplies us with a clue to this subject.

It has been previously stated that the ocean is essentially the theatre of deposition, the land that of destruction, and accordingly, the presence of deposit as a general rule indicates the evidence of marine conditions during the formation of those deposits, though this is not universally the case. Again, as denudation is practically confined to the land areas, and the shallow-waters at their margins, unconformity on a large scale gives evidence of the existence of terrestrial conditions in the area in which it is developed, during its production. Accordingly a mass of deposit separated from deposits above and below by marked unconformities shows the alternation of terrestrial conditions (during which the unconformity was produced) and marine conditions (during which the deposits were laid down). The deposits formed after an unconformity has been developed will naturally be of shallow-water character, as will also be those of the period immediately preceding the incoming of conditions which will cause the occurrence of another unconformity, and between these two shallow-water periods will occur a period when deeper-water conditions probably prevailed. We can therefore not only divide the history of any particular area into a series of chapters, of which every two successive ones will describe a continental period and a marine one, but each marine period may be divided into three phases--a shallow-water phase at the commencement, an intermediate deeper-water phase, and a shallow-water phase at the end. These phases are frequently complicated by the occurrence of a host of minor changes, but on eliminating these, the effects of the three great phases are shown by study of the nature of the strata, and their recognition does much to simplify the detailed study of the stratigraphical geology of various parts of the earth's surface.

In discriminating between terrestrial conditions and marine ones, the existence of unconformities is of great importance in marking terrestrial conditions and is often the only available evidence, for no accumulations or deposits formed on the land may be preserved to testify to the terrestrial conditions[31]. When terrestrial deposits and accumulations do occur, they are extremely important, and it is necessary to allude to the points wherein they differ from marine deposits.

[Footnote 31: The term terrestrial is used above in opposition to marine, to include the conditions prevalent above sea-level. The term continental would be better if it did not exclude insular conditions. Accordingly deposits formed in rivers, and fresh-water and salt-water lakes are spoken of as terrestrial.]

Apart from organic contents, the mechanically formed deposits of rivers and lakes resemble in general characters the shallow-water deposits of the ocean, though they are usually less widely distributed. It is the accumulations which have actually been formed as æolian rocks, or those which have been laid down as chemical precipitates in salt-lakes which, by study of lithological characters, furnish the most convincing evidence of their terrestrial origin.

Many æolian accumulations may be looked upon as soils, if the term soil be used in a special sense to refer to the accumulations which are produced as the result of the excess of disintegration over transportation in an area, whilst others are due to transport which has not been sufficiently effective to carry the material to the sea. When the weathered material accumulates above the weathered rock, it depends chiefly upon climate whether the disintegrated rock becomes mingled with much decayed organic matter forming humus. If this organic matter exists in quantity, the probability is that the accumulation is a terrestrial one, though this is by no means necessarily the case, for under exceptional circumstances a good deal of humus may be deposited in the sea, as beneath the mangrove-swamps which line the coasts of some regions, and to go further back, in the case of the Cromer Forest series of Pliocene times, or some coals, such as the Wigan Cannel Coal of the Carboniferous strata.

In addition to the work of water, which affects both land and sea-deposits, the land is especially characterised by the operations of wind and frost upon it, for these produce results which may frequently serve to differentiate a land-accumulation from a deposit laid down beneath sea-level. The effect of wind in rounding the grains of sand which are blown by it is well-known, and samples of the 'millet-seed' sands of desert regions are preserved in most museums. The greater rounding which characterises wind-borne as compared with water-borne sand grains is due, in great measure, to the greater friction between the grains when carried by the air than when swept along by the water. Under favourable circumstances water-worn grains may become rounded, especially when agitated by gentle currents sweeping over a shoal[32]; but a large mass of sand, in which most of the grains have undergone much rounding so as to give rise to 'millet-seed' sand, will nevertheless be probably formed by wind-action except where a marine deposit is formed of material largely derived from an earlier æolian one. The effect of frost is to split rocks into fragments which are more or less angular before they are subjected to water-action. The broken fragments are prone to collect on slopes as screes, and as any scree-material falling into the sea is likely to become rounded except under conditions which rarely prevail, the existence of much scree-material in a rock suggests its terrestrial origin. Glaciers gave rise to terrestrial moraines, which may occasionally be identified as land-accumulations by mere inspection of their physical characters, but all geologists are aware of the difficulties with which they are confronted when they attempt to discriminate between terrestrial and marine glacial deposits.

[Footnote 32: Cf. Hunt, A. R., "The Evidence of the Skerries Shoal on the wearing of Fine Sands by Waves," _Trans. Devon. Assoc._, 1887, vol. XIX. p. 498.]

The existence of much material amongst the stratified rocks which has been precipitated from a state of solution is an indication of the terrestrial origin of the rocks, which were laid down on the floors of the inland seas, separated more or less completely from the open ocean; for the waters of the ocean are capable of retaining in solution all of the material which is brought down to them, and accordingly precipitates of carbonate of lime, rock-salt, gypsum and other compounds formed from solution, are only formed on a large scale in inland lakes, though they may be formed to some extent when the water of a lagoon is only slightly connected with that of the open ocean, and the evaporation is great, for instance in the lagoons of coral reefs. Certain physical features often mark the deposits of chemical origin, cubical or hopper-crystals of rock-salt may be dissolved, and the hollow afterwards filled with mud, so that the rock surfaces are sometimes marked with pseudomorphs of mud after rock-salt. Sun-cracks and rain-prints impressed on the rock are not actual indications of terrestrial origin of the rocks on which they are found, for the shallow-water muds of an estuary may be deposited in the sea and yet exposed to the action of the air at low tide, but they mark very shallow-water deposits which have been exposed to the atmosphere immediately after their formation if not during the time they were formed, and they frequently occur amongst the deposits of inland lakes.

It will be observed that the characters of the terrestrial accumulations serve to distinguish them to some extent from the marine ones, but they also enable one to detect to some degree the actual conditions under which the accumulation was produced, whether on the mountain-slope, or in the plain, the desert or the fen, the river-bank or the lake-floor.

The conditions of formation of the marine deposits may be distinguished within certain limits with ease, by examination of their physical characters, for the near-shore deposits will generally be coarser and contain more mechanically-transported material than the sediments which accumulate at a greater distance from the shore, though it is not safe to infer that deposits are formed away from the shore on account of the absence of mechanically-transported sediments. In districts where the mechanically-transported material is rapidly deposited, organic deposits of great purity may form close to the coast-line; for instance, when the rivers of a country end in fjords, the mechanical sediments are deposited in the fjords, and the sea around the coast is free from this sediment, and there the organisms can build up deposits of great purity; and a similar thing may happen when the rivers on one side of a country have short courses, and do not carry down much sediment, which occurs when the watershed is near the coast. On the one hand, clay may be formed in considerable purity near the coast, where the supply of mud is so great that the organisms existing there can do little in the way of contribution to the mass of the deposit, or it may be formed on the other hand in great depths of the ocean, where the supply of sediment is extremely small, but where all the organic tests become dissolved; as the characters of the deep sea clays are mainly negative, a geologist examining the rocks of the geological column would have much difficulty in distinguishing a deep-water clay from a shallow-water one by its lithological characters only. In cases of difficulty, information of importance is likely to be furnished by examination of the relative thickness of equivalent deposits in adjoining areas, for if we find a mass of clay a few feet thick in one region represented by hundreds of feet of clay and limestone in another, the former mass probably accumulated slowly and at some distance from the land; again, the uniformity of lithological characters of a deposit over a very wide area is a possible indication of its formation away from land, but this is not a safe guide, for reasons which will eventually appear, unless it can be shown that the deposit is everywhere of the same age.

A clue to climatic conditions is frequently furnished by the physical characters of accumulations, especially terrestrial ones. The accumulations containing a large percentage of hydrocarbons have probably been formed under fairly temperate and moist climatic conditions, whilst the existence of millet-seed sandstones associated with chemical deposits points to desert conditions and inland lakes, requiring a dry climate and probably a warm one. Glaciated surfaces and glacial deposits of course indicate a low temperature. Some geologists profess that occasionally they can even determine the direction of the prevailing winds during past periods, by examination of the character of ripple-marks, rain-pits and other features, though it is doubtful whether much reliance can be placed upon these obscure indications.

Useful as is the physical evidence supplied by deposits, as an index to the conditions under which they were formed, it is usually only supplementary to the evidence derived from a study of the fossils. Fossils when present in the rocks, usually supply considerable information concerning the prevalent conditions during the deposition of the rocks. By them we can not only separate marine from terrestrial deposits, but also freshwater deposits from æolian accumulations; each kind of deposit will generally contain the remains of organisms which existed under the conditions prevalent in the area of formation of the rock, though it is of course a frequent thing for a terrestrial creature or plant to be washed into a freshwater area or into the sea. In an æolian deposit, the invertebrate remains may be those of any air-breathing forms, as insects, galley-worms, spiders, scorpions and molluscs. The land-molluscs are all univalve. Of vertebrates, we may find the bones and teeth of amphibians, reptiles, birds and mammals. Occasionally freshwater or even marine forms may be found in an æolian deposit, but they will be exceptional. Marine shells are often blown amongst the sand-grains of the coastal dunes, and seagulls and other birds frequently carry marine organisms far inland.

The creatures frequenting fresh water differ from those of the land and of the sea. The most abundant vertebrate remains will be those of fishes, and of the invertebrates we find mollusca preponderate. The variety of molluscs is not so great as in the case of marine faunas. The bivalves always possess two muscular scars on each valve (except adult _Mulleria_); whilst many marine shells as the oyster have only one muscular scar on each valve. (See Fig. 11.)

These scars mark the attachment of the adductor muscles, for drawing the valves together, and the shells with only one impression on each valve are called _monomyary_, those with two impressions _dimyary_. The discovery of monomyary shells indicates with tolerable certainty the marine character of the deposit in which they are found, though their absence cannot be taken as proof of freshwater origin. The beaks or umbones of the bivalves are often corroded in freshwater deposits, as may be seen by examining shells of the common freshwater mussel. "All univalve shells of land and freshwater species, with the exception of _Melanopsis_ and _Achatina_, which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing freshwater from marine strata; since if any univalves occur of which the mouths are not entire, we may presume that the formation is marine[33]."

[Footnote 33: Lyell's _Students' Elements of Geology_, Second Edition (1874), Chap. III. A good account of the differences between freshwater and marine organisms, from which some of the facts here cited are extracted, will be there found.]

In Fig. 12 _A_ shows a freshwater shell (_Vivipara_) with entire mouth, whilst _B_ exhibits the shell of a marine gastropod (_Pleurotoma_) with a notched mouth. The entire-mouthed shells are called _holostomatous_ whilst those which are notched, the notch being often prolonged into a canal, are termed _siphonostomatous_.

Many groups of invertebrates are seldom or never found in fresh water. Of exclusively or nearly exclusively marine creatures we may name the foraminifera, radiolaria, sponges with a hard framework, most hydrozoa which secrete hard parts, corals, echinoderms, cirripedes, king-crabs, locust-shrimps, most polyzoa, brachiopods, pteropods, heteropods, and cephalopods. Of extinct groups, the graptolites and trilobites seem to have been entirely confined to the sea.

In the modern and comparatively modern deposits, the forms frequently belong to existing genera, and we get fairly conclusive evidence of the conditions of deposit by determination of the genera. The terrestrial (including freshwater) molluscs have mostly a long range in time. We find pulmoniferous gastropods of living genera in the Carboniferous period, one (_Dendropupa_) belongs to a subgenus of the modern land-shell _Pupa_, the other (_Zonites_) to a subgenus of the snail group _Helix_. Many freshwater molluscs as _Unio_, _Cyclas_, and _Physa_ are found amongst the secondary rocks, and give a clue to the origin of the deposits which contain them. Many extinct genera are closely allied to modern genera, and their mode of existence may be assumed with fair certainty. With all these guides, we may sometimes be left in doubt as to the conditions of deposit when organisms are few in number; thus, it is yet a matter for discussion whether the Old Red Sandstone and many of the deposits of the Coal Measures of Britain were of freshwater or marine origin.

In considering the possibility of fossils having been carried from land to water or _vice versa_, it will be remembered that generally speaking they are more readily transferred from a higher to a lower level, so we are more likely to find remains of land-animals and plants in fresh water or the sea, and relics of freshwater animals and plants in the sea, than of marine or freshwater animals and plants in land, or marine organisms in fresh water. River-gravels and lacustrine deposits are especially prone to contain a considerable intermixture of land-forms with those proper to the station.

Fossils supply much information concerning the depth and distance from land at which the deposits were laid down. When portions of the ocean-water have been separated to form inland lakes, the water becomes saltier than that of the open ocean, if the evaporation is greater than the supply of fresh water, and the life of the inland sea undergoes change under the unfavourable conditions set up. Many forms disappear altogether, and those which survive tend to become stunted, and the shells of many of the mollusca are abnormally thin; the fauna of an inland sea though it may have abundance of individuals is apt to be characterised by paucity of species.

Turning now to the faunas of the open oceans, it is found that in addition to latitude, the distribution of organisms is affected by depth, and by the nature of the sea-floor, and accordingly we find different organisms in different areas; and in examining the same area the organisms inhabiting different depths are not all the same, and at the same depth some kinds of animals have different _stations_ from those of others, one creature being confined to a sandy floor, another to a muddy one, and so on[34]. The oceans have been divided into 18 _provinces_, each of which is more or less characterised by the possession of peculiar forms which are termed _endemic_, in contrast to the _sporadic_ forms which are widely distributed. In any area which is margined by a coast line, the molluscs are distributed in zones which were formerly classed as follows:--the _littoral_ zone between tide marks, the _laminarian_ zone from low water to fifteen fathoms, the _coralline_ zone between fifteen and fifty fathoms, and the _deep-sea coral_ zone from fifty fathoms to one hundred fathoms or more; this last depth was once supposed to mark the limit of the downward extension of marine life, but as the result of modern deep-sea soundings we know that organisms extend to a much greater depth, and the deep-sea fauna, owing to uniformity of conditions over wide areas, contains fewer endemic forms in proportion to the sporadic ones than the shallow-water[35]. The deep-sea deposits entomb the remains of these deep-sea organisms and also of numerous _pelagic_ organisms which live upon the surface of the ocean, whose remains sink to the ocean-floor after death. Amongst the deposits of the deeper parts of the ocean, we find many which are almost exclusively composed of the tests of foraminifera, radiolaria and pteropods, the spicules of sponges, and the frustules of diatoms; and accordingly the existence of foraminiferal, pteropodan, radiolarian, and diatomaceous oozes, amongst the strata of the geological column, has been taken by some as indicating the prevalence of deep-sea conditions during the formation of those deposits: as the purity of a calcareous ooze depends upon the absence of mechanical sediment, or volcanic dust, and as the component organisms of these oozes are pelagic forms which live near the continents as well as in the open oceans, the presence of calcareous oozes implies the existence of a _clear_ sea during their deposition but not necessarily of a deep one, for if the sea-area be far away from land masses, or if the sediment be strained off in fjords, calcareous oozes may be formed in shallow water. The existence of pure radiolarian or diatomaceous deposits is better evidence of deep water, for if they were formed in shallow water we should expect an intermixture of calcareous tests, whereas these are dissolved whilst sinking into the extreme depths of the ocean. As the deep-sea creatures are under very different conditions from those of shallower waters, we might expect marked structural differences between the deep and shallow-water creatures: one such difference has been emphasized, namely the occurrence of animals which are blind or have enormously developed eyes in the great depths of the sea, where the only light is due to phosphorescent organisms. This is well seen in the case of many recent crustacea, and has been noted by Suess in the case of the trilobites of some beds which he accordingly infers to be of deep-water origin, and it is interesting to find that these creatures are found in deposits which give independent evidence of an open-water origin. The _Æglinæ_ of the Ordovician strata are frequently furnished with enormous eyes, and they are often accompanied by blind trilobites, and in Bohemia the blind and large-eyed forms are sometimes different species of the same genus, for instance _Illænus_[36].

[Footnote 34: For an account of the distribution of one group of organisms see Woodward, S. P., _A Manual of the Mollusca_, from which many of the following observations are taken.]

[Footnote 35: For an account of the deep-sea fauna, see Hickson, S. J., _The Fauna of the Deep Sea_, 1894.]

[Footnote 36: Suess, E., _Das Antlitz der Erde_, 2^{er}. Bd., p. 266.]

As one would naturally expect, the actual depth at which deposits were formed can generally be calculated with a greater degree of certainty amongst the newer rocks than amongst the older ones. In the case of the Pliocene Crags, the depth in fathoms may be confidently given. In the Cretaceous rocks attempts have been made to give numerical estimates of the depths at which different accumulations were formed, but some differences of opinion have arisen in the case of these rocks. In the Palæozoic rocks, only a rough idea of the general depth can usually be obtained, and no attempt to calculate the depth in fathoms is likely to be even approximately correct in the present state of our knowledge.

The comminution of fossils has sometimes been taken as an indication of shallower water origin of the deposits which contain them, but although the hard parts of organisms in a broken condition have frequently been shattered by the action of the waves, they may also be broken at great depths by predaceous creatures, and in many instances the fracture is the result of earth-movements occurring subsequently to the formation of the deposits.

Turning now to the difference in organisms which results from difference of station, it will be sufficient to give a quotation from Woodward's _Manual of the Mollusca_ as an illustration:--"In Europe the characteristic genera of _rocky_ shores are _Littorina_, _Patella_, and _Purpura_; of sandy beaches, _Cardium_, _Tellina_, _Solen_; gravelly shores, _Mytilus_; and on muddy shores, _Lutraria_ and _Pullastra_. On rocky coasts are also found many species of _Haliotis_, _Siphonaria_, _Fissurella_, and _Trochus_; they occur at various levels, some only at the high-water line, others in a middle zone, or at the verge of low-water. _Cypræa_ and _Conus_ shelter under coral-blocks, and _Cerithium_, _Terebra_, _Natica_ and _Pyramidella_ bury in sand at low-water, but may be found by tracing the marks of their long burrows (Macgillivray)[37]."

[Footnote 37: Woodward, S. P., _A Manual of the Mollusca_, p. 151.]

The geologist will naturally select sporadic forms rather than endemic ones in comparing the strata of different areas, but how far differences in faunas are the result of existence at different times, and how far they are due to difference of conditions affecting contemporaneous organisms can only be discovered as the result of accurate observation. The main points to be regarded when comparing the successive faunas of different regions have been noticed in this and the preceding chapters, and it has been shown that as the evidence is cumulative, it requires the collection of a large number of facts obtained by observation of the strata before accurate inferences can be drawn.

The indications of climatic conditions furnished by organisms require some consideration. In the comparatively recent deposits it is not difficult to get some notion of the prevalent climatic conditions when the fossils belong to forms closely related to modern genera. The existence of the arctic birch and arctic willow, and of shells belonging to species now living north of the British Isles, in deposits of comparatively recent date in Britain would afford convincing evidence of the occurrence of colder climatic conditions than those which are now prevalent in the area, even if the evidence were not confirmed as it is, by physical proof of glaciation in deposits of the same age. Nevertheless, even in these recent beds, we have a useful warning, by finding species of elephant and rhinoceros associated with northern forms like the lemming, glutton, and musk-ox. We know that the species of elephant and rhinoceros (the mammoth and woolly rhinoceros) were provided with thick coverings which would enable them to resist the severity of an arctic climate, but had not these coverings been found, we might have been puzzled by the association of forms whose nearest allies are sub-tropical with others of arctic character. As we go back in time and deal with earlier deposits, the ascertainment of the climatic conditions becomes more difficult, as the fossils mostly belong to extinct species, genera or even families.

In these circumstances, it is very dangerous to draw conclusions as to climatic conditions from examination of a few forms, but when we find that plants and animals, terrestrial and marine forms, vertebrates and invertebrates alike point to the same conclusion, as in the London Clay, where all the fossils belong to forms allied to those now living under sub-tropical conditions, the state of the climate may be inferred with considerable certainty[38]. The character of the fossils must be taken into account rather than their size. There was a tendency amongst geologists to believe that large organisms probably indicate warm conditions. Recent researches in arctic seas have dispelled this belief. Marine algæ of enormous size are found in the cold seas, and the size of creatures, abundance of individuals and variety of forms in the arctic faunas of some regions is very noteworthy. In the Kara Sea, for instance, a variety of creatures were dredged up during the voyage of the Vega, and Baron Nordenskjöld makes the following pertinent remarks about them: "For the science of our time, which so often places the origin of a northern form in the south, and _vice versa_, as the foundation of very wide theoretical conclusions, a knowledge of the types which can live by turns in nearly fresh water of a temperature of +10°, and in water cooled down to -2·7° and of nearly the same salinity as that of the Mediterranean, must have a certain interest. The most remarkable were, according to Dr Stuxberg, the following: a species of Mysis, _Diastylis Rathkei_ Kr., _Idothea entomon_ Lin., _Idothea Sabinei_ Kr., two species of Lysianassida, _Pontoporeia setosa_ Stbrg., _Halimedon brevicalcar_ Goës, an Annelid, a Molgula, _Yoldia intermedia_ M. Sars, _Yoldia_ (?) _arctica_ Gray, and a Solecurtus[39]. "The temperatures were taken by a centigrade thermometer. Again we read of the results of dredging off Cape Chelyuskin. "The yield of the trawling was extraordinarily abundant; large asterids, crinoids, sponges, holothuria, a gigantic sea-spider (Pycnogonid), masses of worms, crustacea, etc. _It was the most abundant yield that the trawl-net at any one time brought up during the whole of our voyage round the coast of Asia_, and this from the sea off the northern extremity of that continent[40]."

[Footnote 38: For a discussion as to the value of plants as indices of climate see Seward, A. C., Sedgwick Essay for 1892.]

[Footnote 39: Nordenskjöld, A. E., _The Voyage of the Vega_, Vol. I. Chap. IV.]

[Footnote 40: _Ibid._ Chap. VII.]

Amongst the marine invertebrates reef-building corals and mollusca perhaps furnish the best evidence of climatic conditions. The coral-reefs of the Jurassic rocks with large gastropods and lamellibranchs clustered around them have been appealed to in proof of the existence of sub-tropical conditions during their formation; further back in time we find evidence of climate furnished by the fossils of the Silurian rocks of the Isle of Gothland in the Baltic Sea. Of these, Lindström writes "_The fauna had a tropical character_. In consideration of the great numbers of Pleurotomariae, Trochi, Turbinidae and the large Pteropods the assumption of a tropical character of the fauna may seem justifiable[41]."

[Footnote 41: Lindström, G., _On the Silurian Gastropoda and Pteropoda of Gotland_, Stockholm, 1884, p. 33.]

Structure may give some indication of climate even though the organism is not allied to living species. The bark of trees in arctic regions is often thicker than in more temperate regions, and the leaves of arctic plants often have special characters to enable them to resist the long periods during which they are deprived of water, though the fact that desert-plants frequently shew similar modifications deprives this test of any particular value except as a means of corroborating conclusions reached from other evidence[42]. The shells of arctic mollusca may become stunted, but this is not by any means universal, and the same result may be brought about by other abnormal conditions, as for instance the increase of salt in a water area by evaporation.

[Footnote 42: For an account of the modifications of the leaves of arctic plants, see Warming, Eug., _Om Grønlands Vegetation_, Meddelelser om Grønland, 12th part, p. 105.]

On the whole, an examination of the evidence available for ascertaining the character of climate by reference to included organisms, shews that inferences may be drawn within certain limits, but that the task is a difficult one not unaccompanied by danger, and every kind of available evidence derived from a study of physical phenomena and the included organisms should be utilised before any conclusion is drawn.

The likelihood of accurate inference is increased by comparing the faunas of various areas; should they seem to indicate a progressive lowering of climate when passing from lower to higher latitudes, it is probable that the indication is correct. The student is referred to a paper by the late Professor Neumayr for an account of the existence of climatic zones during the Mesozoic Period[43].

[Footnote 43: Neumayr, M., "Ueber klimatische Zonen während der Jura- und Kreidezeit," _Denkschrift. der Math.-Naturwissensch. Classe der k. Akad. der Wissenschaften_, Bd. XLVII. Vienna, 1883.]