THE TEST OF INCLUDED ORGANISMS.
The second great law of the Stratigraphical Geologist is that
fossiliferous strata are identifiable by their included organisms, in
other words, that we can tell the geological age of deposits by
examination of the fossils contained in them, though the determination
of age must be given in more general terms in some cases than in
others. Considerable misconception has arisen concerning the value of
fossils as indices of age, and it is necessary therefore to discuss
the significance of the law of identification of strata by their
included organisms at some length.
The comparison between fossils and medals has frequently been made and
fossils have well been styled the "Medals of Creation"; and the
significance of fossils as guides to the age of deposits may perhaps
be made clearer if we pursue this comparison some way. In the first
place there is clear indication of a gradual increase in the
complexity of organisation of the fossils as one passes from the
earlier to the later rocks, and accordingly the general facies of a
fauna is likely to furnish a clue to the age of the rocks in which it
is found, even though every species or even genus represented in the
fauna was previously unknown to science. So an antiquary versed in the
evolution of art or metallurgy, might detect the general age of a
medal with whose image and superscription he was not acquainted. He
would know that a medal struck in iron was formed subsequently to the
bronze age, or that one formed of palladium appertained to the present
century. But quite apart from any theoretical knowledge, an antiquary
would find as the result of accumulated experience that certain medals
are characteristic of certain periods; he would learn that the
denarius is characteristic of a different period from that indicated
by the coin of the Victorian era, even though he had no knowledge of
the technicalities of numismatics. The same is the case with the
geologist. He may not be influenced by any knowledge of the evolution
of faunas and floras, but actual work amongst the rocks will show him
that the trilobite is not found with the belemnite or the ichthyosaur
with the elephant, save under exceptional circumstances, which only
prove the rule, as for instance when worn bones of ichthyosaurs are
washed from their proper strata into gravels with elephant-bones.
It must be distinctly understood that the determination of fossils as
characteristic of different periods is solely made as the result of
experience. No _à priori_ reasoning may give one indication of the
actual range in time of a species or genus; no one can say why
_Discina_ has a long range in time, whilst that of the closely related
_Trematis_ is very limited. This being the case, the greater the mass
of evidence which is accumulated as to the range of a fossil, the
greater will be the value of that fossil as a clue to the age of the
deposit in which it is found. This is so important, that it requires
more than mere notice. If a fossil is found in abundance in a group of
strata _B_ in any one area, and is not found in an underlying group
_A_ or overlying group _C_ in that area after prolonged search, we
may confidently speak of the fossil as characteristic of the strata
_B_ in that area, though without further work, the value of the fossil
as a clue to age in other areas would be unproved. It may nevertheless
happen, that after more prolonged search in _A_ or _C_, in the
original area a few specimens of the fossil which has been spoken of
as characteristic of _B_ may be found in one or other of them, in
small quantity. The value of the fossil as one characteristic of _B_
will be slightly diminished, though only slightly, as it is not likely
to turn up in numbers in the strata _A_ or _C_ after the prolonged
search. Should the fossil be found also to be characteristic of the
strata _B_ in areas other than the original one, it becomes of more
than local value, and if, after much study it is found to characterise
the same strata over wide areas, the cumulative evidence now obtained
will render the fossil peculiarly important to the stratigraphical
geologist. The detection of characteristic fossils is not quite so
simple as might be supposed from the above remarks, for examination of
the position of one fossil will not prove the contemporaneity of beds
in different places, to prove this, all the evidence at our disposal
must be considered, for reasons which will be presently pointed out.
As the result of accumulated knowledge, we can now compile lists of
characteristic fossils of the major subdivisions of the strata, which
are of world-wide utility and as our knowledge increases, we are
enabled to subdivide the strata into minor divisions of more than
local value.
_What is a fossil?_ Before discussing the value of fossils as aids to
the stratigraphical geologist, it may be well to make a few
observations as to what constitutes a fossil. It is difficult to give
any concise definition, and as is often the case in geology, an
explanatory paragraph is of more value than a mere definition. The
term fossil was originally applied to anything dug up from the rocks
of the earth's crust, and was used with reference to inorganic objects
as well as organic remains, for instance minerals were spoken of as
fossils. It is now applied essentially though not exclusively to
relics of former organisms, though one still reads of fossil
rain-drops, fossil sun-cracks, and so on. Furthermore, the relics need
not necessarily be parts of the organism, the track of a worm or a
bird's nest if embedded in the strata would be termed a fossil. It is
generally agreed that no sharp line can be drawn between recent and
fossil organic remains which is based upon the degree of
mineralisation (or as it was sometimes termed petrifaction) of the
relics, for many true fossils have not undergone mineralisation,
subsequent to their entombment.
It has been suggested that the name fossil should be applied to
organic remains which have been entombed by some process other than
human agency, but this restriction is undesirable. The stone-implement
of the river gravels is as genuine a fossil as the ammonite extracted
from the chalk, and the human relics of very recent date may give
information of a character quite similar to that supplied by other
remains, for instance, the occurrence of moa-bones in New Zealand in
accumulations below those containing biscuit-tins and jam-pots has
been used as a geological argument pointing to the extinction of the
moa before the arrival of Europeans in New Zealand. The biscuit-tin
here serves all the purposes of a fossil, and there is no valid reason
why it should not be spoken of as such.
This statement brings one to consider another method which has been
adopted in order to separate fossil organisms from recent ones,
namely the time-test. This again is inapplicable, for no line can be
drawn between the shell which was buried in yesterday's tidal deposit
and that which has lain in the strata through geological ages, and
each may be equally useful to the geologist.
Whilst, then, we can give no definition of fossil which is likely to
meet with general acceptance, the term can be so used, as not to give
rise to any doubts as to its meaning, and it is generally applicable
to any organic relics which have been embedded in any deposit or
accumulation by any agent human or otherwise.
_Mode of occurrence of fossils._ It will not be out of place to say a
few words as to the way in which fossils are found in strata, as beds
are often inferred to be unfossiliferous, because of ignorance of
methods which should be pursued in searching for organic relics. It is
unnecessary to dilate upon the actual modes of preservation of
organisms, which is treated of fully in other works. In the first
place, it is rash to assert that any deposit is unfossiliferous
because no fossils have been found in it, even after prolonged search.
The Llanberis slates had been eagerly searched for fossils for many
years without result, but that the search was not exhaustive was
proved by the discovery of trilobites in them some years ago. Seekers
after fossils are rather prone to confine their attention to strata
which are already known to be fossiliferous than to pay much attention
to those which have hitherto yielded no organic remains.
Some kinds of deposits are more often fossiliferous than others.
Limestones which are frequently largely of organic origin, are often
rich in remains, and muddy deposits more frequently furnish fossils
than those of a purely sandy nature. The difference in the yield is
not necessarily due to the original inclusion of more remains in one
rock than in another, but is often caused by the obliteration of
former relics owing to changes which have taken place in the rocks
subsequently to their deposition. No sedimentary rock must be regarded
as unfossiliferous, however unfitted it appears for the preservation
of fossils. The writer has seen fossils, not only in coarse
conglomerates, rocks which frequently contain no traces of organisms,
but in deposits composed largely of specular iron ore, and even in
intrusive igneous rocks, though in the latter case, the inclusion of
fossils was due to circumstances which cannot have occurred with
frequency.
In sandy strata, the substance of the fossils has often been
completely removed, leaving hollow casts, which may be almost or quite
unrecognisable. In these circumstances, much information may be
obtained by taking impressions of the casts in modelling wax or some
other material. The importance of this process may be judged from the
results it yielded to Mr Clement Reid in the case of the fossils of
the Pliocene deposits occurring in pipe-like hollows in the Cretaceous
rocks of Kent and the discovery of the remarkable reptiles described
by Mr E. T. Newton from the Triassic sandstones of Elgin.
In argillaceous rocks which have been affected by the processes
producing cleavage, the fossils may be distorted beyond recognition or
owing to the difficulty of breaking the rocks along the original
planes of deposition, may remain invisible. Under such circumstances,
small nodules of sandy or calcareous nature may sometimes be found
included in the argillaceous deposits and may perhaps yield fossils.
Oftentimes, also, where the argillaceous rock is in close proximity
to a harder rock, such as massive grit, the argillaceous rock in
close contiguity to the hard rock may escape the impress of
cleavage-structure, and fossils may be readily extracted from rocks in
this position when not obtainable from other parts of the deposit. It
was under these circumstances that the trilobites alluded to above
were obtained from the Llanberis slates.
The fossils of calcareous rocks are often very obvious, but difficult
to extract, as they break across when the rock is fractured. They are
frequently obtainable in a perfect condition when the rock is
weathered. Occasionally they may be extracted from certain
argillaceous limestones if the limestone be heated to redness, and
suddenly plunged into cold water. Fossils are often found in a state
which enables them to be readily extracted when a limestone is
coarsely crystalline, though they cannot be extracted in a perfect
condition when the same limestone is in a different state.
Many microzoa, which are invisible in rocks, even when viewed through
a lens, may be found in microscopic sections of calcareous and
silicious rocks, and plant structures may be detected under similar
circumstances in the case of carbonaceous rocks.
Various special methods of extracting fossils from rocks have been
described by different writers, many of which are very complex, and
require much time. The mechanical action of the sand-blast and the
solvent action of various acids as hydrochloric and hydrofluosilicic
have been found of use upon different occasions[11]. The various
processes which have been utilised in order to extract and develop
fossils can, however, be best learned by information obtainable from
curators of palæontological collections, and by actual experience,
and there is yet much information to be acquired as to the manner of
extracting fossils from various kinds of rocks.
[Footnote 11: For information concerning use of acids see especially
Wiman, C. "Ueber die Graptoliten," _Bull. Geol. Inst._, Upsala, No. 4,
vol. II. Part II.]
_Relative value of fossils to the Stratigraphical Geologist._ It has
been hinted above that no general rule as to the relative value of
fossils as guides to the age of strata can be laid down, and that the
ascertainment of their relative value is largely the result of actual
experience. It may be noted, however, that organisms which possess
hard parts are naturally more important to the geologist than those
which do not, as few traces of the latter are preserved in the fossil
state, and even when preserved are usually too obscure to be of much
practical use. Of the organisms which do possess hard parts, different
groups have been utilised to a different degree, and one group will be
more or less important than another, according to the use to which it
is applied. Groups of organisms which have a long range in time are
naturally useful for the identification of large subdivisions of the
strata, whilst those which have had a shorter range are valuable when
separating minor subdivisions.
Again, as the bulk of the sedimentary deposits has been formed beneath
the waters of the ocean, relics of marine organisms are naturally more
useful than those of freshwater ones. Other things being equal, the
more easily the organism is recognisable, and the more abundant are
its remains, the greater its value to the stratigraphical geologist,
and as the remains of invertebrates are usually found in greater
quantities and in more readily recognisable condition than those of
the vertebrates, they have been used more extensively as indices of
age. Of the invertebrates, the mollusca are often very abundant, their
remains are adapted for preservation, and their characteristics have
been extensively studied, and accordingly they have been and are of
great use to the geologist. Of other groups, the graptolites, corals,
echinids, brachiopods, and trilobites have been very largely utilised.
The Lower Palæozoic strata have been divided into numerous groups,
each characterised by definite forms of graptolites, and a similar use
has been made of the ammonites in the case of the Mesozoic rocks. It
is not to be inferred that these groups of organisms are naturally
more useful than other groups, on account of the extent to which they
have been used; we can merely state that they have been proved to be
useful as the result of prolonged study; when other groups have
received equal attention, they may well be found to be equally useful
for the purposes which we have in view.
_Contemporaneity and Homotaxis._ From what has been already stated, it
will be recognised that the ages of the various fossiliferous rocks of
the geological column[12] in any one area can be identified with
greater or less degree of certainty by reference to their included
organisms, the various subdivisions being marked by the possession of
characteristic fossils, and it will be naturally and rightly inferred
that the greater the number of characteristic fossils of any one
deposit, the more certain is the identification of that deposit. In
practice, geologists are wont to ascertain the age of the strata after
consideration of all the fossils found therein, some of which may be
actually characteristic whilst many may come up from the strata
below, or pass into those above. Having ascertained the order of
succession and fossil contents of the strata in various regions, it is
the task of the geologist to compare the strata of these two regions,
and this task is fraught with considerable difficulty. Much
controversy has arisen as to the degree of accuracy with which strata
of remote regions can be correlated, and the subject is one which
requires full consideration.
[Footnote 12: Although the rocks do not always lie on one another in
regular succession, it is often convenient to speak of them as though
they did, and as though a column of strata could be carved out in any
region consisting of horizontal bands of deposit one above another. We
speak of such an ideal arrangement as constituting a 'geological
column.']
Suppose that a series of strata which we will call _A_, _B_, and _C_
is found in any one area, each member of which contains characteristic
fossils which enable it to be recognised in that area, and we will
further suppose that in another area a series of strata _A´_, _B´_,
and _C´_ is discovered, of which _A´_ has the fauna of _A_ in the
former area, and similarly _B´_ the fauna of _B_, and _C´_ that of
_C_.
It cannot be assumed that the stratum _A_ is therefore contemporaneous
with _A´_, _B_ with _B´_, and _C_ with _C´_, but on the other hand, it
must not be assumed that they are not contemporaneous. This is a
statement which requires some comment. It has been urged that if the
deposits _A_ and _A´_ in different localities contain the same fauna,
this is a proof that the two are not contemporaneous, for some time
must have elapsed in order to allow of the migration of the organisms
from one area to another, it being justifiably assumed that they did
not originate simultaneously in the two areas. But everything depends
on the time taken for migration as compared with the period of
existence of the fauna. If the former was extremely short as compared
with the latter it may be practically ignored, for we might then speak
of the strata as contemporaneous, just as a historian would rightly
speak of events in the same way which occurred upon the same
afternoon, though one might have happened an hour before the other.
Let us then glance at the evidence which we have at our disposal,
which bears upon this matter.
The objection to identification of strata with similar faunas as
contemporaneous was urged by Whewell, Herbert Spencer, and Huxley, and
the latter suggested the term Homotaxis or similarity of arrangement
as applicable to groups of strata in different areas, in which a
similar succession of faunas was traceable, maintaining that though
not contemporaneous the strata might be spoken of as homotaxial.
Huxley went so far as to assert that "for anything that geology or
palæontology are able to show to the contrary, a Devonian fauna and
flora in the British Islands may have been contemporaneous with
Silurian life in North America, and with a Carboniferous fauna and
flora in Africa[13]," a statement which few if any living geologists
will endorse. If the statement be true, and the fauna which we speak
of as Devonian, when present be always found (as it is) above that
which we in Britain know as Silurian and below that which we term
Carboniferous, the faunas must have originated independently in the
three centres, and disappeared before the appearance of the next
fauna, or having originated at the same centre, each must have
migrated in the same direction, spread over the world, and become
extinct as it reached the point or line from which it started. Suppose
for instance a fauna _A_ originates at the meridian of Greenwich,
migrates eastward, and dies out again when it once more reaches
Greenwich, that _B_ and _C_ do the same, at a later period, then the
fauna _B_ will always be found above _A_ and _C_ above _B_, but if
_B_ did not become extinct when it reached the Greenwich meridian, it
would continue its eastward course, and _C_ having in the meantime
started on its first round, the fossils of the fauna _B_ would be
found both above and below those of _C_. It will be shown below that
cases of recurrence do occur, but nowhere do we find a Silurian fauna
above a Devonian one, or a Devonian one above one belonging to the
Carboniferous, nor is the fauna of a great group of rocks found in one
region above the fauna of another group, and in another region below
the same. And this is true not only of the faunas of one major
division, such as those of the Silurian and Carboniferous periods, but
also of the faunas of many minor subdivisions into which the large
ones are separated, for instance we do not find the Llandovery fauna
of the Silurian period which in Britain is found below the Wenlock
fauna embedded elsewhere in strata above the Wenlock. I have
simplified the statement by assuming that the faunas are identical in
the different localities, and exactly similar throughout the whole
thickness of the containing strata, which is naturally not the case,
but the additional complexity does not conceal the truth of what has
been stated. In the absence of actual inversion of well-marked faunas,
only one explanation is possible, namely, that the time for migration
of forms is so short as compared with the entire period during which
the forms existed, that it may be practically ignored, and the strata
containing similar faunas may be therefore spoken of truthfully as
contemporaneous and not merely homotaxial[14].
[Footnote 13: Huxley, T. H. "Geological Contemporaneity and Persistent
Types of Life," being the Anniversary Address to the Geological
Society for 1862; reprinted in _Lay Sermons, Addresses and Reviews_.]
[Footnote 14: For fuller discussion of this matter see a paper by the
Author 'On Homotaxis,' _Proc. Camb. Phil. Soc._, vol. VI. Part II. p.
74.]
_Apparent anomalies in the distribution of fossils._ There are several
occurrences which have tended to augment the distrust frequently felt
concerning the value of fossils as indices of the age of the beds in
which they occur, which may be here considered.
Though the greater number of fossil remains belonged to organisms
which lived during the time of accumulation of the deposits in which
they are now embedded, this is by no means universally the case, and
the occurrence of _remanié_ fossils, which have been derived from
deposits more ancient than the ones in which they are now found is far
from being a rare event. The existence of remains of this nature in
the superficial drifts and river-gravels of our own country has long
been recognised, and no one would suppose that the _Gryphæa_ and other
shells furnished by these gravels had lived contemporaneously with the
species of _Corbicula_, _Unio_ and other molluscs which are part of
the true fauna of the gravels. In this case the water-worn nature of
the remains is a good index to their origin, but in other cases, it is
by no means an infallible guide, for we sometimes find on the one hand
that remains of organisms proper to the deposits in which they occur
are water-worn, whilst on the other the relics of _remanié_ fossils
are not. The now well-known gault fossils of the Cambridge Greensand
at the base of the chalk were not always recognised as having been
derived from older beds, and there are certain fossils found in
nodules in the Cretaceous rocks of Lincolnshire, which still form a
subject for difference of opinion, for while some writers maintain
that they belong to the deposits in which they are now found, others
suppose that the nodules have been washed out of earlier beds.
Occasionally we find forms which occurring in a set of beds _A_ in an
area, are absent from the overlying beds _B_, and appear again in the
succeeding deposits _C_. Such cases of _recurrence_ are by no means
rare, though many supposed instances of recurrence have been recorded
as the result of stratigraphical or palæontological errors. The best
examples have been noted by Barrande among the Lower Palæozoic
deposits of Bohemia. The stage _D_ of Bohemia consists of five
'bandes' or subdivisions, the lowest (_d_ 1), central (_d_ 3) and
uppermost (_d_ 5) divisions are mainly argillaceous, whilst the second
(_d_ 2) and fourth (_d_ 4) are essentially arenaceous. Some of the
forms found in _d_ 1, _d_ 3 and _d_ 5 have not been found in _d_ 2 and
_d_ 4. The best-known example is the trilobite _Æglina rediviva_. It
is clear that this and other forms did not become extinct during the
deposition of the strata of _d_ 2 and _d_ 4, though they may have
disappeared temporarily from the Bohemian area, or else lingered on in
such diminished numbers that their remains have not been discovered.
The range of the organism is in fact right through the deposits of the
stage _D_, and the discontinuity of distribution is not a real
anomaly; it may be compared to some extent with cases of discontinuous
distribution in space. It is needless to remark that the whole fauna
does not disappear for a time and then reappear, but only a few out of
the many forms which compose it. The comparative rarity of examples of
recurrence after long intervals is an indication that the
palæontological record as it is termed is not so imperfect as some
suppose, for if our knowledge of fossils were very imperfect, we
should expect cases of apparent recurrence to be common, as the result
of the non-detection of fossils in the intermediate beds. One of the
most marked cases of apparent recurrence known some years ago was the
reappearance of a genus of trilobite _Ampyx_ in Ludlow rocks, found in
the Bala rocks, but not in the Llandovery or Wenlock strata. It has
since been discovered in Llandovery beds, and its eventual discovery
in beds of Wenlock age may be regarded as certain. A supposed case of
recurrence which would have been remarkable, that of the disappearance
of _Phillipsia_ in Ordovician rocks, its entire absence in those of
Silurian age, and its reappearance in the Devonian, has broken down,
for the supposed Ordovician form has been shown to belong to an
entirely different group of trilobites from that containing the genus
_Phillipsia_, and it has been therefore renamed _Phillipsinella_.
Many apparent anomalies of distribution have been explained as due to
migration, but it is doubtful whether any one of these supposed
anomalies is actual and not due to errors in determining the position
of the beds or the nature of their included fossils. Some of the
supposed anomalies have already been shown to be due to error, and the
others will almost certainly be cleared up. In speaking of anomalies
of distribution, the geologist can only be guided by experience as to
what constitutes an anomaly. For instance the existence of a complete
fauna in any one place in the beds of a system above that to which it
is elsewhere confined would be regarded as anomalous and as probably
due to error, whilst the reappearance of several forms in beds of a
system higher than that in which they had hitherto been found, could
hardly be considered as an anomaly. A geologist would suspect the
statement that after the disappearance of an Ordovician fauna in an
area and its replacement by a Silurian fauna, the Ordovician fauna
reappeared for a time, but would not regard the statement that a
Cenomanian fauna partly reappeared in the Chalk Rock with surprise.
The existence of a Silurian fauna in Ordovician times was maintained
by Barrande in the case of the Bohemian basin. Lenticular patches of
Silurian rocks having the lithological characters of the Silurian
strata are found in the Ordovician beds of that region, and they
contain fossils specifically identical with those of the Silurian
rocks. Barrande explained this appearance as due to the existence of a
fauna in other regions resembling the Silurian fauna of Bohemia,
during the Ordovician period, when the normal Ordovician fauna of
Bohemia inhabited that area. He supposed that in parts of the basin,
when favourable conditions arose, _colonies_ of the foreign fauna
settled for a time, but did not get a permanent footing in the basin
until the commencement of Silurian times. The theory of colonies has
now been rejected for the Bohemian area, and the phenomena shown to be
due to repetition of strata by folding and faulting, but it is a
theory which is again and again advocated in order to explain
apparently anomalous phenomena in other areas, and these apparent
anomalies which are so explained, must be regarded with grave
suspicion.
The various complexities alluded to in the foregoing pages increase
the difficulty experienced by the geologist in correlating strata in
different areas by their included organisms, but no one of them
disproves the possibility of making these correlations, which can be
carried on to a greater or less extent according to the nature of the
faunas.
A good deal of misconception has arisen concerning the geographical
distribution of former faunas, owing to the tendency to compare them
exclusively with the littoral faunas of the present day. These
littoral faunas have a comparatively limited geographical
distribution, the forms of one marine province often differing
considerably from those of an adjoining one, and still more widely
from one which is remote, so that anyone confronted with the relics
of faunas from the existing Australian and European seas, would find
no indications furnished by identity of species that the faunas were
contemporaneous. Recent researches have shown, however, that the
creatures whose remains are deposited at some distance from the
coast-line have a much stronger resemblance to one another than the
littoral organisms have, if the fauna of two distant areas be
compared. It is still a moot point which will be discussed in a later
chapter, how far the deep-sea deposits of modern times are represented
amongst the strata of the geological column by deposits of similar
origin. But it is certain that many of the ancient strata are not
littoral deposits, and it will be found that it is by comparison of
the faunas of the deeper-water deposits that the geologist correlates
the strata of remote regions: where shallow water deposits are formed,
the faunas differ markedly in different regions, and these
shallow-water forms can only be correlated owing to their occurrence
between deeper-water strata. Thus if strata _A_, _B_ and _C_ be found
in one area, and the fauna of _A_ and _C_ are deep-water forms, those
of _B_ being shallow-water forms, and in another area beds _A´_
contain the same fauna as _A_, and _C´_ the same fauna as _C_ whilst
the fauna of _B´_ is different from that of _B_, we can nevertheless
correlate the strata _B_ and _B´_ (if they be conformable with the
underlying and overlying beds), because of the identity of age of the
associated beds in the two areas. It will possibly be found that the
strata _A_ and _C_ can be further subdivided into _A_{1}_, _A_{2}_,
... &c. _C_{1}_, _C_{2}_, ... by the existence of minor faunas, which
are comparable in the two cases, but such subdivisions may not be
established in the case of the beds _B_ and _B´_.
To take actual examples:--The Llandovery beds of Dumfriesshire can be
subdivided into several minor divisions each of which can be
recognised in the Lake District of England, and to a large extent in
Scandinavia and elsewhere, for the deposits in these areas are of
deep-water character, and the sub-faunas of the subdivisions are
similar in the different areas, but the Llandovery rocks of the Welsh
borderland are shallow-water deposits, with a different fauna from
that of the deep-water deposits of this age, and can only be stated to
be contemporaneous with the Llandovery rocks elsewhere, because the
deeper-water faunas of the underlying Bala rocks and overlying Wenlock
rocks of the Welsh borders are respectively similar to those of the
Bala and Wenlock rocks of the other regions. The shallow-water
Llandoveries of the Welsh borders have only been separated into two
divisions, upper and lower, and have not been split up into a number
of subdivisions, each characterised by a sub-fauna, and each
comparable with one of the subdivisions of Dumfriesshire, Lakeland and
the other regions where the deep-water facies is found.
It will be seen that though the principle of William Smith that strata
can be recognised by their included organisms has been extended since
his time, and shown to apply to far smaller subdivisions of the strata
than was suspected, the method of application is the same, and is more
or less successful according to the amount of evidence which is
accumulated in support of it.