SIMULATION OF STRUCTURES.
Although it is easy to give an account of the structures which are of
importance to the student of the stratified rocks, actual observation
of these structures is frequently attended with difficulties owing to
the close imitation of one structure by another, and the past history
of the science shows that erroneous conclusions have been reached
again and again on account of the incorrect interpretation of
structures.
Simulation of organisms has frequently been the cause of error.
Inorganic substances take on the form of organisms with various
degrees of closeness. The dendritic markings produced by
efflorescences of oxide of manganese are familiar to all, and as the
name implies, they simulate, to some extent, plant remains. More
complex chemical changes have resulted in the production of
rock-masses in which, not the outward form alone but, the internal
structure of organisms is reproduced with more or less approach to
fidelity, as the rocks which contain the supposed organisms described
as _Eozoon bohemicum_, _E. bavaricum_, and, we may add, _E.
canadense_. Mechanical changes in rocks subsequent to their formation
may also cause the simulation of organisms by inorganic substances.
Prof. Sollas has given reasons for considering the structure
described as _Oldhamia_ to be inorganic, and in the Carboniferous
Sandstones of Little Haven, Pembrokeshire, every stage in the
formation of tubular bodies resembling worm-tubes, as the result of
complex folding of the strata, may be observed, whilst in other cases
we find imitation of worm-tracks, as has been observed before.
It is when one inorganic structure is simulated by another that the
stratigraphical geologist is most likely to be led astray, and
accordingly it is worth noting some cases where this has occurred, as
a warning, for it must not be supposed that the cases here noted are
the only ones which are likely to occur.
It has been seen that the existence of bedding-planes is of prime
importance to the geologist, and their detection is a matter of
supreme moment. Under ordinary circumstances there is no great
difficulty in distinguishing bedding-planes from other planes, but the
importance of discovering them is often greatest when the difficulty
is most pronounced. In rocks which have undergone no great amount of
disturbance the planes of stratification are often marked by their
regular parallelism, the separation of layers having different
lithological characters by these planes, the arrangement of the longer
axes of pebbles parallel to them, and the occurrence of fossils and
also of rain-prints, ripple-marks and other structures produced during
deposition, upon the surfaces of the strata, but none of these
appearances is necessarily conclusive, especially in areas where the
rocks have been subjected to orogenic movements. In regularly-jointed
rocks, jointing may well be mistaken for bedding, and there is often
great difficulty in discriminating between bedding and cleavage,
especially when the exposures of rock are of small extent. Fossils may
be dragged out along planes at an angle to the true bedding, pebbles
will be compressed by cleavage so that their longer axes do not remain
parallel to the bedding-planes but now lie parallel to the
superinduced planes of cleavage, and a structure closely resembling
'ripple-mark' may be produced on planes other than those of original
bedding, as the result of puckering. The alternation of rocks having
different lithological characters may also be misleading. Intrusion of
dykes along cleavage-planes, followed by decomposition of the
dyke-rock causing it to resemble a sediment, and formation of mineral
veins along the same planes, may give rise to an apparent succession
of rocks of different lithological characters which could easily
mislead an observer and cause him to mistake the cleavage-planes for
planes of stratification. In rocks which have undergone great lateral
pressure, the beds of different lithological character may be folded
in such a way as to give very erroneous ideas of the true dip of the
rock on a large scale. In Fig. 3 the dip of the rocks in a small
exposure might appear to be in the direction indicated by the
unfeathered arrow, whilst the true dip of the strata as a whole,
leaving the minor foldings out of account, is in the direction of the
feathered arrow, at the inclination represented by the dotted line.
The minor folds in a case like that represented may extend upwards for
scores or even hundreds of feet, so that an error as to the direction
and amount of dip may be made, even if the observer faces a cliff of
considerable height.
[Illustration: Fig. 3.]
False-bedding on a large scale may be a cause of error. In the Penrith
Sandstone of Cumberland, the planes of deposition are often found
dipping in one direction in a large quarry, but inspection of a wider
area shows that this is not the true dip of the beds as a whole, but
merely a local dip due to deposition on a slope, and any one
attempting to calculate the total thickness of the beds by reference
to these divisional planes might be seriously led astray. A reference
to Fig. 4 will explain this. The lines _AA´_, _BB´_ are the true
bedding-planes cut across in the section, whilst the lines sloping to
the right from _xx_ are only lines of false-bedding on a large scale.
An exaggerated estimate of the thickness of the deposit would be made
by measuring the thickness of each of these stratula from _A_ to _A´_
and adding these thicknesses together, whereas the actual thickness of
the middle bed is the distance between _A_ and _B_ or _A´_ and _B´_.
[Illustration: Fig. 4.]
When rocks have been affected by thrust-planes, the simulation of
bedding may be carried out to a very full extent. Not only do the
major thrust-planes resemble bedding-planes but the minor thrusts
produce an appearance of divisional planes separating stratula or
laminæ, and a close approximation to false-bedding is the result. To
this structure Prof. Bonney has given the name
'pseudo-stromatism[17].' It may be developed in rocks of all kinds,
whether possessing original planes of stratification or not, and as a
result of its existence the geologist may be seriously misled, not
merely by mistaking the direction of the strata, but also the nature
of the rock, for we may find it produced in an unstratified glacial
till, and in a massive igneous rock, and in each case the resulting
rock will resemble a sedimentary deposit, and of course the observer
may be confirmed in his erroneous opinion by the formation of apparent
fossils, ripple-marks or other objects which he might expect to
discover in sediments. As illustrative examples, reference may be made
to a number of schistose rocks, in which the planes of discontinuity
(which are in truth planes of foliation) have been taken for
bedding-planes and the rocks claimed as sedimentary though they are in
reality igneous; for instance many of the rocks of the Laurentian of
Canada, of the Hebridean of the North West Highlands, and some of the
ancient rocks of Anglesey.
[Footnote 17: Bonney, T. G., _Quart. Journ. Geol. Soc._, vol. XLII.
_Proc._ p. 65.]
A foliated structure may, as is now well known, be simulated by a
structure developed in a rock prior to its consolidation. The
similarity of flow structure of some lavas to the foliated structure
of a schist was long ago pointed out by Darwin and Scrope, and recent
work has proved that parallel structure due to differential movement
prior to consolidation may be developed in plutonic rocks, as shown
by Lieut.-General McMahon in the Himalayan granites, and by Lawson
amongst the plutonic rocks of the Rainy Lake Region; and as the
foliated structure may be mistaken for original stratification the
same may occur, and has occurred, when dealing with this
flow-structure.
This is not the place to discuss the truth of the old theory of
progressive metamorphism, in which it was maintained that a gradual
passage could be traced between ordinary sediments and plutonic rocks,
but it may be pointed out that much of the evidence which was relied
upon to prove the theory was fallacious and due to the confusion of
the parallel structure set up in plutonic rocks prior to, or
subsequent to, consolidation, with original stratification. Recent
study of metamorphic rocks has proved that the parallel structures
developed in the rocks of an area which has undergone metamorphism may
be produced by three distinct processes; they may be original planes
of deposition, or formed in a solid rock subsequently to its
formation, or in an igneous rock before its consolidation, and
although it is sometimes possible to separate the structures produced
by these processes, this is not always the case[18]. When a plutonic
rock contains large phenocrysts and an eye-structure is developed in
it, it may simulate a conglomerate, the rounded phenocrysts being
taken for pebbles[19]. Still closer simulation of an epiclastic
conglomerate may be produced in other ways and will be referred to
immediately.
[Footnote 18: It must be noticed that the rock in which parallel
structure is produced before consolidation, if it undergoes no further
change, though often associated with metamorphic rocks, is not itself
metamorphic. The term _gneiss_ applied to these rocks is a misnomer,
unless the term be used even more vaguely than it is at present.]
[Footnote 19: See Lehmann, _Untersuchungen über die Entstehung der
Altkrystallinischen Schiefergesteine mit besonderer Bezugnahme auf das
Sächsische Granulitgebirge_, Plate XI. fig. 1.]
We have already seen that the existence of unconformities has been
utilised in the demarcation of large divisions of strata in various
regions, and whether they be utilised in this manner or not, their
detection is a matter of importance to the stratigraphical geologist,
as they afford information concerning the occurrence of great physical
changes during their production. These unconformities may also be
closely simulated by structures produced in very different manner.
The occurrence of an unconformity implies the denudation of one set of
beds before the deposition of another set upon them, and accordingly
the denuded edges of the lower set will somewhere abut against the
lower surface of the lowest deposit or deposits of the overlying
set[20]. The existence of an unconformity may often be detected in
section, but when the unconformity is upon a large scale this may not
be possible, but it will be discovered by mapping the strata and will
be apparent on a map owing to the deposits of the lower set of beds
abutting against the others. This is well seen where the Permian rocks
of Durham, Yorkshire, and Nottinghamshire rest upon different members
of the underlying Carboniferous series, and will be noticed on any
good geological map of England. But a similar effect may be caused by
a fault, so that mere inspection of a map or even of the strata in the
field and discovery of one set of beds ending off against another does
not prove unconformity. When the fault is a normal one, with low hade
(that is, having a fissure approaching the vertical position), the
outcrop of the fault-fissure will approximate to a straight line if
the fault has a straight course, even if the ground be very uneven,
whereas, if the plane of unconformity has not been tilted to a high
angle from its original horizontal position, it will crop out in a
sinuous manner across uneven ground, in a way similar to that of beds
which are nearly horizontal, so that though the general trend of the
outcrop of the plane of unconformity may be fairly straight, its
deviation from a straight line will be frequent and marked, as seen in
the case of the Permian unconformity above referred to. But if the
unconformable junction has been highly inclined its outcrop will
resemble that of a normal fault, or if the fault be a thrust-plane
with high hade, the outcrop of this will resemble that of an
unconformable junction which has not been greatly tilted from its
original horizontal position. In these cases we require more evidence
before we can decide whether we are dealing with an unconformable
junction or a faulted one.
[Footnote 20: An unconformity may be simulated or an actual
unconformity rendered apparently more important, as the result of
underground solution of the underlying strata subsequently to the
deposition of the upper set upon them, and any insoluble materials in
the underlying strata may be left as an apparent pebble-bed at the
base of the upper beds. This is seen at the junction of the Tertiary
beds with the chalk near London. Subterranean water has dissolved the
upper part of the chalk, increasing the unconformity which naturally
exists between chalk and Tertiary beds, and the insoluble flint of the
dissolved chalk is left as a layer of 'green-coated flint' at the base
of the Tertiary deposits.]
The lowest deposits of the newer set of strata lying above an
unconformity have probably been laid down in water near the
shore-line. As the unconformity, if large, implies elevation above the
sea-level, the deposits first formed after this elevation has ceased,
and depression commenced, will necessarily be littoral in character
and possibly of beach-formation, and accordingly we often find that an
unconformity is marked by the existence of an epiclastic conglomerate
immediately above the plane of unconformity and, although this need
not be continuous, it is usually found somewhere along the line of
junction. The conglomeratic base of the Lowest Carboniferous strata
when they repose upon the upturned edges of the Lower Palæozoic rocks
of the dales of West Yorkshire is well known, and may be cited as an
example. The association of conglomerates with unconformities is
indeed so frequent that its possible occurrence will always be
suspected and sought by the geologist. Unfortunately the result of
recent observation is to show that along thrust-planes of which the
outcrop simulates those of unconformable junctions, the difficulty of
discrimination may be increased by the existence of cataclastic rocks
which bear a close resemblance to epiclastic conglomerates, and which
may be and have been styled conglomerates. It is well known that
fragments of the adjoining rocks are knocked into a fault-fissure
during the occurrence of the movements which cause the fault, to
constitute a _fault-breccia_, and as the result of the abrasion of
these fragments by chemical or mechanical agency, the angular
fragments may become rounded and converted into rounded pebble-like
bodies, when the rock is changed into a _fault-conglomerate_. Fig. 5,
from a photograph kindly supplied by Prof. W. W. Watts, shows a stage
in the formation of a conglomerate of this nature from a
fault-breccia; the fragment on the right remains angular, whilst those
on the left have become much more rounded. The illustration is from a
case described by Mr Lamplugh occurring in the slaty rocks of the Isle
of Man, and Mr Lamplugh's paper[21] furnishes the reader with
references to other examples of the production of similar rocks. No
general rule can be laid down for distinguishing the true from the
apparent unconformity, for the attendant phenomena will differ in each
case; but if a fault-conglomerate should be suspected, the observer
should try to ascertain whether fragments of a newer rock are imbedded
in an older one, which sometimes occurs; he should note the existence
of extensive slickensiding along the plane of junction and along
planes of faulting, though the existence of these, implying as it does
the occurrence of differential movement along the plane, does not
prove that the movement was necessarily great, or that it did not take
place along a plane of original unconformity; above all, he should
look for structures such as mylonitic structure, pseudo-stromatism,
development of new minerals, crushing out and stretching of fossils
and fragments and, in short, for any structure which is familiar to
him as a result of orogenic movements.
[Footnote 21: Lamplugh, G. W., "On the Crush-Conglomerates of the Isle
of Man," _Quart. Journ. Geol. Soc._, vol. LI. p. 563.]
[Illustration: Fig. 5.]
The effects of thrusting not only give rise to appearances suggestive
of unconformity, but naturally also to a simulation of overlap. The
thrust-planes are often parallel to original bedding-planes for some
distance, but must cut across them sooner or later, producing
lenticular masses which might be supposed to be due to the thinning
out of beds as the result of cessation of deposition in a lateral
direction.
Attention has already been directed to the deceptive appearance of
great thickness of strata which is due to repetition of one stratum or
set of strata by a series of thrust-planes, so that there is no actual
inversion of any part of a bed. When masses of limestone are affected
in this way, the thrust-planes may become sealed up, as the result of
chemical change, and a compact irregular mass of limestone devoid of
any definite divisional planes may be the consequence, and beds of
grit sometimes exhibit the same feature to some extent.
Enough has been said to show that simulation of one structure by
another has frequently occurred in rocks in so marked a degree as to
render mistakes easy; and that these examples of 'mimicry' in the
inorganic world are particularly frequent in rocks which have been
subjected to great orogenic movements. The student will do well to
acquaint himself with the macroscopic and microscopic structures
which may be taken as characteristic of the rocks which have been thus
affected, some of which can usually be detected with ease, and when he
discovers them he may suspect that many phenomena which appear
explicable in one way were in reality produced in a different one, for
it is frequently very true of a region in which the rocks have been
violently squeezed, stretched and broken that 'things are not what
they seem.'