EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED, CONTINUED.
In the preceding chapter, attention was drawn to the indications as to
conditions of deposition furnished by the sediments of any one
locality, and only passing reference was made to variation in the
nature of the sediments and their organic contents, when the deposits
are traced laterally from place to place; some attention must now be
paid to this matter.
It is sometimes inferred that, whereas similarity of organisms is a
dangerous guide in correlating the strata of two areas, accurate
correlations may be made, if the deposits can be traced continuously
through the intervening interval; no doubt the task is simplified when
this can be done, but the continuity of deposit of one particular
composition is no more proof of contemporaneity than the occurrence of
the same fossils continuously through the interval, imbedded in strata
of different character, indeed probably not so much so. The existence
of widespread masses of conglomerate, which are not found as linear
strips, but which extend in all directions, is in itself an indication
of this; the Oldhaven pebble bed for instance, in the Tertiary rocks
of the London basin, is very widely distributed. We cannot suppose
that coastal conditions prevailed far away from the shore-line, and
accordingly when a conglomerate occurs in a widespread sheet, and not
in a linear strip, this is indicative that the deposit has not been
formed continuously but that strip has been added to strip along an
advancing or receding shore line, and if this happens with
conglomerates, it must occur also in the case of other deposits.
[Illustration: Fig. 13.]
In fig. 13[44] let _A_ represent a shore line of a continent which is
undergoing gradual elevation. A deposit of pebbles _a_ will be formed
against the coast, one of sand _b_ further away, then one of mud _c_
and lastly limestone _d_, may be formed in the open sea away from
land. Naturally there may be intermingling of two kinds of deposit at
the junctions, but for the sake of simplicity this may be disregarded.
During the accumulation of the deposits _a_, _b_, _c_, _d_, certain
sporadic forms may be distributed throughout all the deposits, and
some of them may become extinct before the deposition of these beds is
completed, if the process is carried out on a large scale; we may
speak of the characteristic fossils of this period as fauna I. As the
result of elevation or of mere silting up of the sea-margin, or of
both combined, the next mass of pebble-deposit will be laid down
further away from the original shore, for the shore line will now be
at _A´_ and not at _A_, and it will partly overlap the mass of sand
_b_; the sand _b_^1 will also be deposited somewhat further out and
partly overlap the mud _c_, and similarly the mud _c_^{1} will partly
overlie the limestone _d_. During the formation of _a_^{1}, _b_^{1},
_c_^{1}, _d_^{1}, other sporadic forms belonging to a fauna II may
replace those of the first fauna. In the same way _a_^{2}, _b_^{2},
_c_^{2}, _d_^{2} will be deposited, and in the meantime a new fauna
III may arise and replace II. So the process will go on until we
finally have a group of deposits lying one over the other, consisting
of a basal accumulation of limestone, succeeded by mud, sandstone and
pebble-beds in succession. Each of these will be continuous, though
the inner part of the pebble-deposit was formed long before the outer
part of the limestone, which is nevertheless beneath a mass of
pebble-deposit continuous with that formed first, and the various
deposits will be separated by fairly horizontal planes _x_, _y_, _z_,
which might be regarded as bedding planes, but which are not so,
strictly speaking. The true bedding planes will occur at a slight
angle to these planes of separation, for the structure resembles false
bedding on a gigantic scale, but of course, the lines separating two
masses of similar deposit will be practically horizontal and parallel
to the planes of demarcation of two distinct kinds of material. The
lines separating two faunas would, under the conditions postulated,
run approximately parallel to the planes of separation of adjoining
deposits of the same lithological character but would pass from
conglomerate, through sandstone, mud and limestone, as indicated by
the lines 1, 2, 3, ... and the deposits between adjoining lines would
be contemporaneous[45]. In nature, complications will arise, owing to
the gradual appearance and disappearance of forms, and the existence
of endemic species in contemporaneous deposits formed in different
stations and having different lithological characters.
[Footnote 44: The writer gratefully acknowledges his indebtedness to
Prof. Lapworth for some of his views concerning deposition of strata.]
[Footnote 45: The lines 1, 2, 3 ... are incorrectly drawn in the
figure. Line 1 should be drawn so as to separate _a_, _b_, _c_, _d_
from _a_^{1}, _b_^{1}, _c_^{1}, _d_^{1}, line 2 to separate _a_^{1},
_b_^{1}, _c_^{1}, _d_^{1} from _a_^{2}, _b_^{2}, _c_^{2}, _d_^{2}, and
so with the others.]
If elevation ceased and were succeeded by depression, the exact
opposite would occur, and the pebble beds would be overlain by
sandstones, these by muds, and lastly limestones would appear. It
follows that during a marine phase occurring between two
unconformities we should have a =V=-shaped accumulation of deposits
with the apex pointing to the part of the shore line which was last
submerged before the commencement of elevation, as shewn in fig. 14,
though the beds of the apex will in most cases be denuded during the
re-emergence.
[Illustration: Fig. 14.]
Indications of the non-coincidence of the planes separating faunas and
those which separate deposits of one lithological character from those
of another have already been detected, for instance the 'greensand'
condition of the Cretaceous period occurs in some places during the
existence of one fauna, and in others during that of another, though
the planes have not been traced continuously. Mr Lamplugh has
furnished another example amongst the Cretaceous rocks of Yorkshire
and Lincolnshire, but as has already been observed, a great deal
remains to be done in this direction, and geologists are much in want
of two sets of stratigraphical maps, in one of which the lines are
drawn with reference to the differences of lithological character,
whilst in the other they separate different faunas.
The student will notice the normal recurrence of deposits in definite
order; conglomerate succeeded by sandstone, mud and limestone, in a
sinking area, and limestone succeeded by mud, sandstone and
conglomerate in a rising area. Naturally many instances of departure
from this rule are seen, owing to local conditions, but on a large
scale, it is very frequently noted, and recognition of this will
enable the student to remember the variations in the lithological
characters of the deposits more easily, than if he simply acquired
them from a text-book without taking heed as to their significance.
Upon the variations in the lithological characters of deposits and of
their faunas, when the beds are traced laterally depends very largely
the successful ascertainment of the existence of former coast-lines,
the restoration of which constitutes an important part, of
Palæo-physiography, concerning which some observations may here be
made[46]. If a set of deposits having different lithological
characters can be proved to be contemporaneous, the coarser detrital
accumulations will point to the approach to a coast-line, and the
actual position of the coast during the period of accumulation of the
deposits may be very accurately fixed. The pebble-beds at the base of
the Cambrian rocks of Llanberis indicate the existence of a
coast-line in that position during the accumulation of those
pebble-beds. Similar pebble-beds occur at St David's, at the base of
the Cambrian, but it is impossible in the case of these rapidly
accumulated sediments to say that two deposited so far away from one
another were actually contemporaneous, and therefore although we
might draw a line through Llanberis and St David's to indicate the
old coast-line of the period, it does not follow that the actual
beach existed simultaneously at the positions indicated. The
palæo-physiographer, however, attempts to restore the physical
conditions of greater thicknesses of deposit; for instance, the
distribution of land and sea during Lower Carboniferous times over the
area now occupied by the British Isles is often taken to illustrate
the methods of restoration of ancient features, and all admit that the
lithological and palæontological characters of the rocks indicate a
shallowing of the Carboniferous sea when passing northwards towards
Scotland. For conveying an idea of the restorations to the student, it
is almost imperative to portray the distribution of land and sea upon
a map, and this can only be done by drawing definite lines. It must be
distinctly understood that these lines are necessarily only an
approximation to the actual position of the ancient shore-lines, which
must have shifted again and again during the long period occupied by
the accumulation of the Lower Carboniferous strata, so that a true
idea of the positions of the Lower Carboniferous shore-lines could
only be obtained by placing on a series of maps the successive
shore-lines of different parts of the Lower Carboniferous period, and
taking a composite photograph of these, which would appear as a wide
belt of shaded portion of the map with no definite boundaries. The
utmost that the maker of palæo-physiographical maps can expect to
indicate, when dealing with considerable thicknesses of strata, is an
approximation to the mean position of the shore-lines of the period
when these strata were deposited. This is extremely valuable in
enabling the student to understand the significance of the variations
in the characters of the strata and their organic contents, if he
distinctly recognises the generalised nature of the map. Examination
of any two palæo-physiographical maps of the same period by different
authors will shew wide divergences in the details, but a general
resemblance of the main features. The reader will do well to consult
Prof. Hull's restoration of the physical features of Old Red Sandstone
and Lower Carboniferous Times on Plate VI. of his _Contributions to
the Physical History of the British Isles_, and compare it with the
map drawn by Prof. Green (_Coal: its History and Uses_, by Profs.
Green, Miall, Thorpe, Rücker, and Marshall, Fig. 3, p. 38), which will
be found to bear out this statement.
[Footnote 46: On this subject, the student may consult Prof. E. Hull's
_Contributions to the Physical History of the British Isles_.]
Valuable as the published maps of palæo-physiography are as an aid to
the student in understanding the significance of the variations of
characters amongst the sediments, he will do well to supplement them
by maps which he fills in for himself. He is recommended to procure a
number of outline maps of England, or of the British Isles, and when
studying in detail the characters of the British sedimentary rocks
formed during the various periods, to place a blank map by his side
when beginning the study of each period or important portion of a
period. On this map he should jot down the geographical distribution
of the different kinds of sediments, using the conventional signs
indicated at p. 90: thus, in the case of the Lower Carboniferous
rocks he would place the conventional sign for limestone in
Derbyshire, a combination of those for limestone and shale in
Yorkshire, and would add to these the sandstone sign in
Northumberland. He should also note the general character of the
fossils, using abbreviations for such terms as fresh-water fossils,
shallow-sea fossils, deep-water fossils. After reading the account of
the group of rocks in a comprehensive text-book, and inserting his
notes on the map, he should proceed to insert the probable position of
the coast-lines. He should also take notes of any indications of
contemporaneous volcanic action, though these might well be inserted
on a separate map. If this course be pursued, the student will not
only have the significance of the variations amongst the strata
impressed upon his mind, but he will have a means of obtaining at a
glance the distribution of sediments and faunas of different kinds in
the British area during the principal geological periods. On another
set of maps he may indicate the axes of the orogenic movements which
have occurred at different times, and when his various maps are
completed, he will have the materials for the construction of a
general account of the various geological processes which have been
concerned with the building of the British area.
When an area like Britain has been studied, the student may proceed to
construction of maps of wider regions, and he will find that in doing
this, new sets of facts must be taken into consideration, as for
instance the occurrence of different faunas on opposite sides of
once-existing continental masses, and the problems connected with the
present distribution of the faunas and floras. For an instance of the
importance of the former distribution of life the reader may consult
the twelfth section of the first part of Professor Suess' _Das
Antlitz der Erde_, whilst a good account of the value of recent
geographical distribution of organisms in supplying a clue to former
distribution of land and sea will be found in Mr A. R. Wallace's
_Island Life_, Chapter xxii.
Should the method suggested above be adopted, the student is likely to
acquire a much more coherent idea of the significance of the facts of
stratigraphical geology than can be obtained by a mere perusal of the
accounts of the strata given in those portions of the various
text-books which are devoted to a consideration of the stratigraphical
branch of the science.