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.)
[Illustration: Fig. 11.
_A._ Monomyary shell with one scar.
_B._ Dimyary shell with two scars.]
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.]
[Illustration: Fig. 12.
_A._ Holostomatous shell.
_B._ Siphonostomatous shell.]
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.]