CHAPTER II.
THE DIFFERENT AGES OF STRATIFIED FORMATIONS.
THEIR SUCCESSIVE DEPOSITIONS.
THE next point to be considered is—Are stratified rocks of different ages? They are, and the
diagram, fig. 1, p. 13, will help to make this clear. There the bed No. 1 must be the oldest,
because it was deposited in the sea (or other water) before bed No. 2 was laid above it as layers
of mud, and so on to 3 and 4—taking the strata in order of succession. But that is not enough to
know. We are anxious to understand what is the actual history of the different stages which such
minor beds represent Now, if we had never found any fossil remains imbedded in the rocks, we
should lose half the interest of this investigation, and our discovery, that rocks are of different
ages, would have only a minor value. Turn again to the diagram. We find at the base, beds of
limestone, No. 1, perhaps composed of corals and shells. The organic remains in the upper part
of these beds lie above those in the lower part, and therefore the latter were dead and buried,
before the once living shells which lie in the upper part inhabited the area. Above the limestone
lie beds of shale, No. 2, succeeded by No. 3, a conglomerate, and then comes the bed of sandstone,
No. 4; therefore the shells (if any) in the bed of shale, No. 2, are of younger date
[24 Strata of Different Ages.]
than those in the bed of limestone, No. 1; the organic forms, plants or animals as the case may
be, in the conglomerate, No. 3, were buried among the pebbles at a later date than the shells in
the shale, and the remains of life in the sandstone, No. 4, were latest of all; and in each bed, each
particular form found there, lived and died before the sediment began to be deposited that forms
the bed above. All these beds, therefore, contain relics of ancient life of different dates, each bed
being younger or older than the others, according as we read the record from above or from
below. It is evident that the same kind of reasoning is equally applicable to the inclined strata of
fig. 2, or to the contorted beds of fig. 3.
But if we leave a petty quarry or sea cliff, and examine strata on a larger scale, what do we
find? On many a coast, where the cliffs consist of stratified rocks, a lesson may easily he learnt
on the method of understanding the order, or comparative dates of deposition of geological
formations. The Liassic, Oolitic, and Cretaceous cliffs of Yorkshire, from the Tees to
Flamborough Head, form excellent examples; or the coast of Devonshire and Dorsetshire, from
Torquay to Portland Bill. I take part of the latter as an example, from Lyme Regis to the eastern
end of the Chesil Bank.
If we eliminate those accidents called faults, we there find a succession of formations arranged
somewhat in the manner shown in diagram No. 5.
The horizontal line at the base represents the shore line. On the west (1) represents red marly
strata, known as the New red or Keuper marls. These pass under thin beds of white fossiliferous
limestone (2), known as the Rhætic beds. These in their turn pass
[Succession of Formations. 25]
or dip under beds of blue limestone and clay, called Lower Lias (3), which are seen to dip under
the Marlstone or Middle Lias (4), overlaid by the Upper Lias (5), on which rests the Inferior Oolite
sand and limestone (6), followed by the Fuller's Earth clay (7). Next comes a series 44 of
strata (8), which for present purposes I have massed together, and which are known when they
are all present as Great Oolite, Forest Marble, and Cornbrash.
FIG. 5
These dip under the Oxford
Clay (9), which dips under a limestone called the Coral Rag (10), and still going eastward this
dips beneath the Kimeridge Clay (11), which, in its turn, passes under the Cretaceous Series of
this district, consisting of Gault (12), Upper Greensand (13) and Chalk (14) which in a
bold escarpment overlooks the plain of Kimeridge Clay.1
Here, then, we see a marked succession
of strata of different kinds, or having different lithological characters,
formed, that is to say, of marls, clays, sands, and limestones, succeeding
and alternating with each other. They are all sediments originally deposited
in the sea, (if we except the New Red Marl, which was deposited in a Salt
lake), for the forms of old life found in them prove this. Some are only
forty or fifty feet thick, some are more than five or six hundred feet in
thickness.
If we leave the coast cliffs and turn to the middle of
1 The Portland beds being only occasionally present, are in this diagram purposely omitted, and
this does not affect the general
[26 Succession of Formations.]
England—from the borders of South Staffordshire and Warwickshire to the neighbourhood of
London—we discover that the whole series is made of strata, arranged in successive stages more
or less in the manner which I have already described, and they consist of similar materials.
Thus, through Warwickshire and South Staffordshire, we have rocks formed of New Red
Sandstone. The red sandstone dips to the east, and is overlaid by New Red Marl; the red marl dips
also to the east, under beds of blue clay, limestone, and brown marl, forming the various
divisions of the Rhætic beds and Lias; these pass under a great succession of formations of
limestones, clays, and sands, &c., known as the Oolites; these, in their turn, are overlaid by
beds of sand, clay, and chalk, named the Cretaceous series; which again, in their turn, pass
under the Tertiary clays and sands of the London Basin. All these pass fairly under each other in
the order thus enumerated. Experience has proved this, for though there are occasional
interruptions in the completeness of the series, some of the formations being absent in places,
yet the order of succession is never inverted, except where, by what may be called geological
accidents, in some parts of the world, such as the Alps, great disturbances have locally produced
forcible inversions of some of the strata. The Oolites, for example, in England, never lie under
the Lias, nor the Cretaceous rocks under the Oolites.
Observation of the surface in cliffs, railway cuttings, and quarries, therefore proves this
general succession of formations, and so does experience in sinking deep wells and mine shafts.
If, for example, in parts of the midland counties we sink through the Lower Lias, we pass
question. Some minor formations known further inland are added to make the series more
complete.
[Strata Fossiliferous. 27]
into the New Red Marl; if we pierce the red marl, we reach the water-bearing strata of the New
Red Sandstone. If in certain districts we penetrate the Cretaceous strata, we are sure to reach
the Upper Oolites, and under London many deep wells have been sunk through the Eocene beds, in
the certainty of reaching the chalk and finding water.
It is, therefore, not that the mere surface of the land is formed of various rocks, but the several
formations that form the land dip or pass under each other in regular succession, being, in fact,
vast beds placed much in the same way as a set of sheets of variously coloured pasteboard, placed
flat on each other, and then slightly tilted up at one end, may slope in one direction, one edge of
each sheet being exposed at the surface.
Vertical sinkings, therefore, in horizontal or slightly inclined strata, often prove practically
what we know theoretically, viz. the underground continuity in certain areas of strata one
beneath the other. Accurate but more difficult observation and reasoning has done the same for
more disturbed strata, so that our island and other countries have been proved to be formed of a
series of beds of rock, some many hundreds and some many thousands of feet in thickness,
arranged in succession, the lowest stratified formation being of older and the uppermost of
younger age.
Most of these strata are fossiliferous, that is to say
they contain shells, bones, and other relics of the creatures that lived and died in the waters or
water-laid sediments of each special period; or as sometimes happens, the remains of land
plants and terrestrial animals that have been washed into the sea or into lakes. What is the more
special evidence on this subject afforded by the rocks? As we proceed, we shall suppose,
[28 Succession]
from west to east across the Secondary and Tertiary strata, and examine the fossils found in
successive formations, we discover that they are not the same in all, and that most of them
contain marine organic remains, which are in each formation of species and sometimes of
genera more or less distinct from those in the formations immediately above or below.1
Thus turning again to fig. 5, p. 25, the Red Marly series No. 1, is rarely fossiliferous, and such
fossils as these beds may contain are chiefly land plants, footprints of Amphibia, and small
bivalve crustaceans. The Rhætic beds 2, contain sea-shells of a few genera and species, the latter
somewhat distinct from those found in the Lower Lias No. 3, the fossils of which are again
partly, but not altogether, of different species from those buried in the Marlstone No. 4, which
again partly differ from the forms in the Upper Lias clay No. 5, and so on, stage by stage,
through the remaining strata of the Oolitic rocks, up to the Kimeridge Clay No. 11. Throughout
the whole series from the Rhætic beds (2), upwards to the Kimeridge Clay (11), there is an
intimate relation, for in all the Liassic and Oolitic formations the general facies, that is to say,
the grouping of genera (Ammonites, Belemnites, Terebratuliie, Pholadomyas, Oysters, &c.) is
the same, and some species generally pass from each formation into the next above it; and not
only so, but sometimes through several formations. There is, however, generally enough of
difference in the species found in the different formations to enable anyone with sufficient
knowledge to tell by fossils alone, if he found enough of them.. what formation he may chance to
be examining. When,
1
There are also a few freshwater deposits, but the discussion of these is not essential to the
present argument.
[of Life. 29]
still ascending in the series, we come to the Cretaceous formations represented by 12, 13, and
14. a wonderful change takes place. None of the Oolitic species pass into these formations, and
some of the genera, especially of chambered shells (Cephalopoda) are new. There are no marine
passage beds in England sufficiently developed clearly to unite the two series. They were, in
fact, separated in their deposition by a long period of time during which our territory generally
formed land, and which is therefore unrepresented in the British area by marked marine
stratified deposits of dates between Oolitic and Cretaceous times.
I have selected the above instances, as affording a good type of the kind of phenomena that occur
again and again throughout the whole series of our geological formations. After a minute
examination, therefore, of the stratigraphical structure of our island, the result is, that
geologists are able to recognise and place all the rocks in serial order, so as to show which were
formed first and which were formed latest; and the following is the result of this tabulation,
omitting minor details.
It is a necessary part of the plan of this work to give some account of the range, structure, and
fossils of the formations enumerated in the following table, and I shall therefore in succeeding
chapters give a brief account of each formation or set of formations, beginning with the oldest,
so as in some degree to show their general relations to each-other, and, as far as I can, to give a
description of the physical geography of each prominent geological epoch.
[30 Table of British Formations.]
TABLE OF THE BRITISH FORMATIONS.
Recent . . . Alluvia, peat, and estuarine beds now forming, &c.
River and estuarine alluvia, and some peats, with
human remains and works of art; whales,
Postertia. .
seals, &c., bones of Mammoth, and other land mammalia; flint implements, raised beaches, and
bone caves, &c., in part. Latest traces of British glaciers.
Great glacier moraines, and boulder clays with marine and freshwater interstratiflcations.
Forest bed of Norfolk, Chillesford beds, and
Newer Pliocene. Norwich Crag, with land mammalia, &c. . . fled Crag.
Older Pliocene Coralilne Crag.
I MIDDLE . . Miocene . . . Bovey Tracey and Mull beds, with igneous rocks. 0 I Hem pstead beds
44 O4 >3 0 Osborne beds : J marine iuttratcation.
I (Upper Eocene Bembridge beds I Freshwater river beds, with
Headon beds Brackiesham and i
Middle Eocene 1 Bagshot beds Marine. tLowEn.
London Clay. Marine.
1Lower Eocene : { Woolwich and Reading beds and Thaiiet said.
Freshwater, estuarine, and marine.
(Chalk
(OnTAcEous. Gault Marine.
I Upper Greensand
I Lower Greensand I Atherfield Clay
WEAI Btings sands with marine intersfratiflcations.
(Weald clay and Freshwater river beds. estuarine and lagoon beds,
SERIES (Purbeck beds .)
0 I and sand . .Upp&
Portland Oolite .
N I I Kimeridge Clay
0I
I I Coral g. . Middle.
I Ioilordclay .
OoLrric I Cornbrash . .' Marine in middle and south of SEans 1 Forest Marble . England. Between
the Inferior
Oolite and Great Oolite, partly I Bath or Great I Oolite . . . Lower 1 Stonesfield Slate I
Northamptonshire, Uncoinshire.
freshwater and terrestrial, in
§ and Inferior Oolite ) and Yorkshire. and Sand Upper Lias Clay
J Marlstone (Middle Lias) I LLAS I Lower Lies Clay and Limestone I I (Rhætic beds. Passage beds
.J
I (Upper. New Red Marl (Keuper). Salt Lalce. tTRLkssIc Lower. New Red Sandstone (Bunter).
Lake deposits, probably . salt, but perhaps partly fresh or brackish.
i Magnesian limestone Salt lakes.
IPERMIAN . P, inn r Roth].iegende . . .
I (Coal-measures and Millstone grit. Partly terrestrial, freshwater, CARBowIF1- I and
marine. ROUS I Carboniferous limestone and shales. Chiefly marine, and in north
I Iof England, and Scotland, partly terrestrial and freshwater.
.-. o-1
45
N OLD Rim (Upper } Freshwater lakes. Devonian marine.
I SANDSTONE, & DEVONLA.N Lower (Upper Silurlan . } Marine. SAN . . Lower Silurian
(and Cambrian. Probably marine and freshwater beds Interatratifiod.
LAURENTIAN Marine.
01
3