Chapter XVI: Part VI: Paleontological Geology

This division of the science deals with fossils, or the traces of plants and animals preserved in the rocks of the earth's crust, and endeavours to gather from them information as to the history of the globe and its inhabitants. The term "fossil" (Lat. _fossilis_, from _fodere_, to dig up), meaning literally anything "dug up," was formerly applied indiscriminately to any mineral substance taken out of the earth's crust, whether organized or not. Since the time of Lamarck, however, the meaning of the word has been restricted, so as to include only the remains or traces of plants and animals preserved in any natural formation whether hard rock or superficial deposit. It includes not merely the petrified structures of organisms, but whatever was directly connected with or produced by these organisms. Thus the resin which was exuded from trees of long-perished forests is as much a fossil as any portion of the stem, leaves, flowers or fruit, and in some respects is even more valuable to the geologist than more determinable remains of its parent trees, because it has often preserved in admirable perfection the insects which flitted about in the woodlands. The burrows and trails of a worm preserved in sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelid life among old geological formations. The droppings of fishes and reptiles, called coprolites, are excellent fossils, and tell their tale as to the presence and food of vertebrate life in ancient waters. The little agglutinated cases of the caddis-worm remain as fossils in formations from which, perchance, most other traces of life may have passed away. Nay, the very handiwork of man, when preserved in any natural manner, is entitled to rank among fossils; as where his flint-implements have been dropped into the pre-historic gravels of river-valleys or where his canoes have been buried in the silt of lake-bottoms.

A study of the land-surfaces and sea-floors of the present time shows
that there are so many chances against the conservation of the remains
of either terrestrial or marine animals and plants that if, as is
probable, the same conditions existed in former geological periods, we
should regard the occurrence of organic remains among the stratified
formations of the earth's crust as generally the result of various
fortunate accidents.

Let us consider, in the first place, the chances for the preservation
of remains of the present fauna and flora of a country. The surface of
the land may be densely clothed with forest and abundantly peopled
with animal life. But the trees die and moulder into soil. The
animals, too, disappear, generation after generation, and leave few or
no perceptible traces of their existence. If we were not aware from
authentic records that central and northern Europe were covered with
vast forests at the beginning of our era, how could we know this fact?
What has become of the herds of wild oxen, the bears, wolves and other
denizens of primeval Europe? How could we prove from the examination
of the surface soil of any country that those creatures had once
abounded there? The conditions for the preservation of any relics of
the plant and animal life of a terrestrial surface must obviously be
always exceptional. They are supplied only where the organic remains
can be protected from the air and superficial decay. Hence they may be
observed in (1) the deposits on the floors of lakes; (2) in
peat-mosses; (3) in deltas at river-mouths; and (4) under the
stalagmite of caverns in limestone districts. But in these and other
favourable places a mere infinitesimal fraction of the fauna or flora
of a land-surface is likely to be entombed or preserved.

In the second place, although in the sea the conditions for the
preservation of organic remains are in many respects more favourable
than on land, they are apt to be frustrated by many adverse
circumstances. While the level of the land remains stationary, there
can be but little effective entombment of marine organisms in littoral
deposits; for only a limited accumulation of sediment will be formed
until subsidence of the sea-floor takes place. In the trifling beds of
sand or gravel thrown up on a stationary shore, only the harder and
more durable forms of life, such as gastropods and lamellibranchs,
which can withstand the triturating effects of the beach waves, are
likely to remain uneffaced.

Below tide-marks, along the margin of the land where sediment is
gradually deposited, the conditions are more favourable for the
preservation of marine organisms. In the sheets of sand and mud there
laid down the harder parts of many forms of life may be entombed and
protected from decay. But only a small proportion of the total marine
fauna may be expected to appear in such deposits. At the best, merely
littoral and shallow-water forms will occur, and, even under the most
favourable conditions, they will represent but a fraction of the whole
assemblage of life in these juxta-terrestrial parts of the ocean. As
we recede from the land the rate of deposition of sediment on the
sea-floor must become feebler, until, in the remote central abysses,
it reaches a hardly appreciable minimum. Except, therefore, where some
kind of ooze or other deposit is accumulating in these more pelagic
regions, the conditions must be on the whole unfavourable for the
preservation of any adequate representation of the deep-sea fauna.
Hard durable objects, such as teeth and bones, may slowly accumulate,
and be protected by a coating of peroxide of manganese, or of some of
the silicates now forming here and there over the deep-sea bottom; or
the rate of growth of the abysmal deposit may be so tardy that most of
the remains of at least the larger animals will disappear, owing to
decay, before they can be covered up and preserved. Any such deep-sea
formation, if raised into land, would supply but a meagre picture of
the whole life of the sea.

It would thus appear that the portion of the sea-floor best suited for
receiving and preserving the most varied assemblage of marine organic
remains is the area in front of the land, to which rivers and currents
bring continual supplies of sediment. The most favourable conditions
for the accumulation of a thick mass of marine fossiliferous strata
will arise when the area of deposit is undergoing a gradual
subsidence. If the rate of depression and that of deposit were equal,
or nearly so, the movement might proceed for a vast period without
producing any great apparent change in marine geography, and even
without seriously affecting the distribution of life over the
sea-floor within the area of subsidence. Hundreds or thousands of feet
of sedimentary strata might in this way be heaped up round the
continents, containing a fragmentary series of organic remains
belonging to those forms of comparatively shallow-water life which had
hard parts capable of preservation. There can be little doubt that
such has, in fact, been the history of the main mass of stratified
formations in the earth's crust. By far the largest proportion of
these piles of marine strata has unquestionably been laid down in
water of no great depth within the area of deposit of terrestrial
sediment. The enormous thickness to which they attain seems only
explicable by prolonged and repeated movements of subsidence,
interrupted, however, as we know, by other movements of a contrary
kind.

Since the conditions for the preservation of organic remains exist
more favourably under the sea than on land, marine organisms must be
far more abundantly conserved than those of the land. This is true
to-day, and has, as far as known, been true in all past geological
time. Hence for the purposes of the geologist the fossil remains of
marine forms of life far surpass all others in value. Among them there
will necessarily be a gradation of importance, regulated chiefly by
their relative abundance. Now, of all the marine tribes which live
within the juxta-terrestrial belt of sedimentation, unquestionably the
Mollusca stand in the place of pre-eminence as regards their aptitude
for becoming fossils. They almost all possess a hard, durable shell,
capable of resisting considerable abrasion and readily passing into a
mineralized condition. They are extremely abundant both as to
individuals and genera. They occur on the shore within tide mark, and
range thence down into the abysses. Moreover, they appear to have
possessed these qualifications from early geological times. In the
marine Mollusca, therefore, we have a common ground of comparison
between the stratified formations of different periods. They have been
styled the alphabet of palaeontological inquiry.

There are two main purposes to which fossils may be put in geological research: (1) to throw light upon former conditions of physical geography, such as the presence of land, rivers, lakes and seas, in places where they do not now exist, changes of climate, and the former distribution of plants and animals; and (2) to furnish a guide in geological chronology whereby rocks may be classified according to relative date, and the facts of geological history may be arranged and interpreted as a connected record of the earth's progress.

1. As examples of the first of these two directions of inquiry
reference may be made to (a) former land-surfaces revealed by the
occurrence of layers of soil with tree-stumps and roots still in the
position of growth (see PURBECKIAN); (b) ancient lakes proved by beds
of marl or limestone full of lacustrine shells; (c) old sea-bottoms
marked by the occurrence of marine organisms; (d) variations in the
quality of the water, such as freshness or saltness, indicated by
changes in the size and shape of the fossils; (e) proximity to former
land, suggested by the occurrence of abundant drift-wood in the
strata; (f) former conditions of climate, different from the present,
as evidenced by such organisms as tropical types of plants and animals
intercalated among the strata of temperate or northern countries.

2. In applying fossils to the determination of geological chronology
it is first necessary to ascertain the order of superposition of the
rocks. Obviously, in a continuous series of undisturbed sedimentary
deposits the lowest must necessarily be the oldest, and the plants or
animals which they contain must have lived and died before any of the
organisms that occur in the overlying strata. This order of
superposition having been settled in a series of formations, it is
found that the fossils at the bottom are not quite the same as those
at the top of the series. Tracing the beds upward, we discover that
species after species of the lowest platforms disappears, until
perhaps not one of them is found. With the cessation of these older
species others make their entrance. These, in turn, are found to die
out, and to be replaced by newer forms. After patient examination of
the rocks, it has been ascertained that every well-marked "formation,"
or group of strata, is characterized by its own species or genera, or
by a general assemblage, or _facies_, of organic forms. Such a
generalization can only, of course, be determined by actual practical
experience over an area of some size. When the typical fossils of a
formation are known, they serve to identify that formation in its
progress across a country. Thus, in tracts where the true order of
superposition cannot be determined, owing to the want of sections or
to the disturbed condition of the rocks, fossils serve as a means of
identification and furnish a guide to the succession of the rocks.
They even demonstrate that in some mountainous ground the beds have
been turned completely upside down, where it can be shown that the
fossils in what are now the uppermost strata ought properly to lie
underneath those in the beds below them.

It is by their characteristic fossils that the stratified rocks of the
earth's crust can be most satisfactorily subdivided into convenient
groups of strata and classed in chronological order. Each "formation"
is distinguished by its own peculiar assemblage of organic remains, by
means of which it can be followed and recognized, even amid the
crumplings and dislocations of a disturbed region. The same general
succession of organic types can be observed over a large part of the
world, though, of course, with important modifications in different
countries. This similarity of succession has been termed _homotaxis_,
a term which expresses the fact that the order in which the leading
types of organized existence have appeared upon the earth has been
similar even in widely separated regions. It is evident that, in this
way, a reliable method of comparison is furnished, whereby the
stratified formations of different parts of the earth's crust can be
brought into relation with each other. Had the geologist continued to
remain, as in the days of Werner, hampered by the limitations imposed
by a reliance on mere lithological characters, he would have made
little or no progress in deciphering the record of the successive
phases of the history of the globe chronicled in the crust. Just as,
at the present time, sheets of gravel in one place are contemporaneous
with sheets of mud at another, so in the past all kinds of
sedimentation have been in progress simultaneously, and those of one
period may not be distinguishable in themselves from those of another.
Little or no reliance can be placed upon lithological resemblances or
differences in comparing the sedimentary formations of different
countries.

In making use of fossil evidence for the purpose of subdividing the
stratified rocks of the earth's crust, it is found to be applicable to
the smaller details of stratigraphy as well as to the definition of
large groups of strata. Thus a particular stratum may be marked by the
occurrence in it of various fossils, one or more of which may be
distinctive, either from occurring in no other bed above and below or
from special abundance in that stratum. One or more of these species
is therefore used as a guide to the occurrence of the bed in
question, which is called by the name of the most abundant species. In
this way what is called a "geological horizon," or "zone," is marked
off, and its exact position in the series of formations is fixed.

Perhaps the most distinctive feature in the progress of
palaeontological geology during the last half century has been the
recognition and wide application of this method of zonal stratigraphy,
which, in itself, was only a further development of William Smith's
famous idea, "Strata identified by Organized Fossils." It was first
carried out in detail by various palaeontologists in reference to the
Jurassic formations, notably by F.A. von Quenstedt and C.A. Oppel in
Germany and A.D. d'Orbigny in France. The publication of Oppel's
classic work _Die Juraformation Englands, Frankreichs und des
sudwestlichen Deutschlands_ (1856-1858) marked an epoch in the
development of stratigraphical geology. Combining what had been done
by various observers with his own laborious researches in France,
England, Wurttemberg and Bavaria, he drew up a classification of the
Jurassic system, grouping its several formations into zones, each
characterized by some distinctly predominant fossil after which it was
named (see LIAS). The same method of classification was afterwards
extended to the Cretaceous series by A.D. d'Orbigny, E. Hebert and
others, until the whole Mesozoic rocks from the Trias to the top of
the Chalk has now been partitioned into zones, each named after some
characteristic species or genus of fossils. More recently the
principle has been extended to the Palaeozoic formations, though as
yet less fully than to the younger parts of the geological record. It
has been successfully applied by Professor C. Lapworth to the
investigation of the Silurian series (see SILURIAN; ORDOVICIAN
SYSTEM). He found that the species of graptolites have each a
comparatively narrow vertical range, and they may consequently be used
for stratigraphical purposes. Applying the method, in the first
instance, to the highly plicated Silurian rocks of the south of
Scotland, he found that by means of graptolites he was able to work
out the structure of the ground. Each great group of strata was seen
to possess its own graptolitic zones, and by their means could be
identified not only in the original complex Scottish area, but in
England and Wales and in Ireland. It was eventually ascertained that
the succession of zones in Great Britain could be recognized on the
Continent, in North America and even in Australia. The brachiopods and
trilobites have likewise been made use of for zonal purposes among the
oldest sedimentary formations. The most ancient of the Palaeozoic
systems has as its fitting base the _Olenellus_ zone.

Within undefined and no doubt variable geographical limits
palaeontological zones have been found to be remarkably persistent.
They follow each other in the same general order, but not always with
equal definiteness. The type fossil may appear in some districts on a
higher or a lower platform than it does in others. Only to a limited
degree is there any coincidence between lithological variations in the
strata and the sequence of the zones. In the Jurassic formations,
indeed, where frequent alternations of different sedimentary materials
are to be met with, it is in some cases possible to trace a definite
upward or downward limit for a zone by some abrupt change in the
sedimentation, such as from limestone to shale. But such a precise
demarcation is impossible where no distinct bands of different
sediments are to be seen. The zones can then only be vaguely
determined by finding their characteristic fossils, and noting where
these begin to appear in the strata and where they cease. It would
seem, therefore, that the sequence of palaeontological zones, or
life-horizons, has not depended merely upon changes in the nature of
the conditions under which the organisms lived. We should naturally
expect that these changes would have had a marked influence; that, for
instance, a difference should be perceptible between the character of
the fossils in a limestone and that of those in a shale or a
sandstone. The environment, when a limestone was in course of
deposition, would generally be one of clear water, favourable for a
more vigorous and more varied fauna than where a shale series was
accumulating, when the water would be discoloured, and only such
animals would continue to live in it, or on the bottom, as could
maintain themselves in the midst of mud. But no such lithological
reason, betokening geographical changes that would affect living
creatures, can be adduced as a universally applicable explanation of
the occurrence and limitation of palaeontological zones. One of these
zones may be only a few inches, or feet or yards in vertical extent,
and no obvious lithological or other cause can be seen why its
specially characteristic fossils should not be found just as
frequently in the similar strata above and below. There is often
little or no evidence of any serious change in the conditions of
sedimentation, still less of any widespread physical disturbance, such
as the catastrophes by which the older geologists explained the
extinction of successive types of life.

It has been suggested that, where the life-zones are well defined,
sedimentation has been extremely slow, and that though these zones
follow each other with no break in the sedimentation, they were really
separated by prolonged intervals of time during which organic
evolution could come effectively into play. But it is not easy to
explain how, for example in the Lower Lias, there could have been a
succession of prodigious intervals, when practically no sediment was
laid down, and yet that the strata should show no sign of
contemporaneous disturbance or denudation, but succeed each other as
if they had been accumulated by one continuous process of deposit. It
must be admitted that the problem of life-zones in stratigraphical
geology has not yet been solved.

As Darwin first cogently showed, the history of life has been very
imperfectly registered in the stratified parts of the earth's crust.
Apart from the fact that, even under the most favourable conditions,
only a small proportion of the total flora and fauna of any period
would be preserved in the fossil state, enormous gaps occur where no
record has survived at all. It is as if whole chapters and books were
missing from a historical work. Some of these lacunae are sufficiently
obvious. Thus, in some cases, powerful dislocations have thrown
considerable portions of the rocks out of sight. Sometimes extensive
metamorphism has so affected them that their original characters,
including their organic contents, have been destroyed. Oftenest of
all, denudation has come into play, and vast masses of fossiliferous
rock have been entirely worn away, as is demonstrated by the abundant
unconformabilities in the structure of the earth's crust.

While the mere fact that one series of rocks lies unconformably on
another proves the lapse of a considerable interval between their
respective dates, the relative length of this interval may sometimes
be proved by means of fossil evidence, and by this alone. Let us
suppose, for example, that a certain group of formations has been
disturbed, upraised, denuded and covered unconformably by a second
group. In lithological characters the two may closely resemble each
other, and there may be nothing to show that the gap represented by
their unconformability is of an important character. In many cases,
indeed, it would be quite impossible to pronounce any well-grounded
judgment as to the amount of interval, even measured by the vague
relative standards of geological chronology. But if each group
contains a well-preserved suite of organic remains, it may not only be
possible, but easy, to say exactly how much of the geological record
has been left out between the two sets of formations. By comparing the
fossils with those obtained from regions where the geological record
is more complete, it may be ascertained, perhaps, that the lower rocks
belong to a certain platform or stage in geological history which for
our present purpose we may call D, and that the upper rocks can in
like manner be paralleled with stage H. It would be then apparent that
at this locality the chronicles of three great geological periods E,
F, and G were wanting, which are elsewhere found to be intercalated
between D and H. The lapse of time represented by this
unconformability would thus be equivalent to that required for the
accumulation of the three missing formations in those regions where
sedimentation was more continuous.

Fossil evidence may be made to prove the existence of gaps which are
not otherwise apparent. As has been already remarked, changes in
organic forms must, on the whole, have been extremely slow in the
geological past. The whole species of a sea-floor could not pass
entirely away, and be replaced by other forms, without the lapse of
long periods of time. If then among the conformable stratified
formations of former ages we encounter sudden and abrupt changes in
the _facies_ of the fossils, we may be certain that these must mark
omissions in the record, which we may hope to fill in from a more
perfect series elsewhere. The complete biological contrasts between
the fossil contents of unconformable strata are sufficiently
explicable. It is not so easy to give a satisfactory account of those
which occur where the beds are strictly conformable, and where no
evidence can be observed of any considerable change of physical
conditions at the time of deposit. A group of strata having the same
general lithological characters throughout may be marked by a great
discrepance between the fossils above and below a certain line. A few
species may pass from the one into the other, or perhaps every species
may be different. In cases of this kind, when proved to be not merely
local but persistent over wide areas, we must admit, notwithstanding
the apparently undisturbed and continuous character of the original
deposition of the strata, that the abrupt transition from the one
_facies_ of fossils to the other represents a long interval of time
which has not been recorded by the deposit of strata. A.C. Ramsay, who
called attention to these gaps, termed them "breaks in the succession
of organic remains." He showed that they occur abundantly among the
Palaeozoic and Secondary rocks of England. It is obvious, of course,
that such breaks, even though traceable over wide regions, were not
general over the whole globe. There have never been any universal
interruptions in the continuity of the chain of being, so far as
geological evidence can show. But the physical changes which caused
the breaks may have been general over a zoological district or minor
region. They no doubt often caused the complete extinction of genera
and species which had a small geographical range.

From all these facts it is clear that the geological record, as it now
exists, is at the best but an imperfect chronicle of geological
history. In no country is it complete. The lacunae of one region must
be supplied from another. Yet in proportion to the geographical
distance between the localities where the gaps occur and those whence
the missing intervals are supplied, the element of uncertainty in our
reading of the record is increased. The most desirable method of
research is to exhaust the evidence for each area or province, and to
compare the general order of its succession as a whole with that which
can be established for other provinces.