CRUST OF THE EARTH—GEOLOGICAL SYSTEMS—EOZOIC, PRIMARY, SECONDARY,
TERTIARY, PREHISTORIC FORMATIONS.
The crust of Great Britain has been carefully examined, and from
the results of investigations at various periods, the earth has
been divided into a series of strata which follow the same order of
succession. Sometimes certain strata may not be present, and they may
be replaced by others, but the same order of succession will be found.
The order is as follows, commencing at the _lowest_. The illustration
is taken in the opposite direction:—
{ Laurentian } Until recently believed to
“Eozoic.” { Cambrian } be without traces of living
{ } creatures; hence “Eozoic.”
Palæzoic, { Silurian }
or { Old Red Sandstone } Shell-fish, seaweed,
Primary. { Carboniferous } ferns, fish, low reptiles.
{ Permian }
Mesozoic, { Triassic (Upper }
or { Red Sandstone) }
Secondary. { Oolitic } Birds, marsupials, reptiles.
{ Cretaceous }
Kainzoic, { Eocene } Superior life. Mammals,
or { Miocene } with great vegetable life, on
Tertiary. { Pliocene } to plants and animals now
{ Post-tertiary } existing. Man.
Quaternary. Recent—Prehistoric
[Illustration:
} TERTIARY STRATA.
} UPPER AND MIDDLE SECONDARY STRATA.
} LOWER SECONDARY STRATA.
} PRIMARY STRATA.
Fig. 646.—Systems.]
THE PALÆZOIC SYSTEMS.
LAURENTIAN SYSTEM. It will be perceived from the above list that
the Laurentian Rocks are the oldest. The name is derived from the
St. Lawrence formations, and was given to the strata by Sir William
Logan. They are metamorphosed rocks older than the Cambrian. These
rocks are sedimentary, of very old deposition, and of a crystalline
nature, consisting of quartz, gneiss, etc. The granite was probably
formed by the fusion of its component constituents, quartz, mica, and
felspar, which become crystallized by the excessive heat. For a long
time no traces of organisms could be detected in this or the Cambrian
systems, but modern research has been rewarded with a little success.
The original deposits of micaceous gneiss, etc., have been altered,
and many true igneous rocks, such as syenite and granite, are found
in them. These very old rocks must have been originally deposited in
strata converted by heat and pressure into crystalline rocks. These
rocks have been divided into two series, under the names of lower and
upper Laurentian. They are metamorphic, and consist “mainly of gneiss
interstratified with mica-schist, with great beds of quartz, and
massive beds of crystalline limestone, of which one varies from 700
to 1,500 feet in thickness. Conglomerates also occur, and there are
vast deposits of magnetic and specular iron. Graphite, or blacklead,
is disseminated in strings, veins, and beds through hundreds of feet
of the lower Laurentian, and its amount is calculated by Dr. Dawson
to be equal in quantity to the coal seams of an equal area of the
carboniferous rocks” (Nicholson).
[Illustration: Fig. 647.—Upward Granite (Section).]
[Illustration: Fig. 648.—Conformable Strata.]
[Illustration: Fig. 649.—Unconformable Strata.]
Hitherto, no distinctly recognisable fossil has been discovered, with
the important exception of the _Eozoön Canadense_, which has been
pronounced to have been a gigantic foraminifer, growing layer upon
layer, and thus forming reefs of limestone; the subject, however, is
still a matter of dispute. The eozoön was discovered by Mr. J. M’Mullen
in 1858 in Canada.
Granite was at one time considered to be the true primitive rock.
Gneiss is a word of Saxon origin, and consists of the same materials as
granite in different proportions. Mica-schist is made up of two of the
same constituents as the granite and gneiss. They are without fossil
traces.
[Illustration: Fig. 650.—Nereites Cambrensis.]
The CAMBRIAN system of aqueous origin may be said to contain evidence
of the dawn of organic life. It is part of the clay-slate system, and
the term “Cambrian” is taken from the ancient name of Wales, where
slate is plentiful. Mica-slate is also very important. These Cambrian
rocks are of the next oldest formation to the Laurentian, and all
the various deposits may be examined in Wales, where also traces of
volcanic and ice action may readily be perceived. In the pass of
Llanberis one immense ice-borne block is very prominent; no agency
but ice could have put it there as it rests. It is estimated that the
Cambrian and “Lower” Silurian rocks are from 20,000 to 30,000 feet in
thickness, and must embrace a very lengthened period. The fossils of
these formations show that zoophytes and certain primitive crustacea
lived in the remote ages when these rocks were formed by sedimentary
deposition. We have scarce a trace of plant-life. The organic remains
include annular worms, the first arrangement of the articulated animals
according to Cuvier. The flora and fauna are, of course, very low in
the scale of creation, when land and sea were so differently arranged.
[Illustration: Fig. 651.—Section across Snowdon.
A, Fossiliferous grits (Bala series); B, Greenstone (intrusive); C,
Porphyry; D, Volcanic ashes, sometimes calcareous and fossiliferous
Bala limestone.]
[Illustration: Fig. 652.—Silurian fossil.]
The SILURIAN system was so named by Sir. R. Murchison, after the
territory formerly occupied by the Silures, but the system is, of
course, universal. We have here sandstones, limestones, and shales,
deposits lying upon the Welsh slate. The Upper and Lower Ludlow beds,
and the “May Hill” sandstone, then the Lower Silurian, with Caradoc
beds, and the Tremadoc slate, etc. In this system volcanic action is
observable, and all the organic remains are those of marine animals,
such as corals, shell-fish, marine worms, encrinites, molluscs, and
other zoophytes in great variety. We find also a number of graptolites,
trilobites, echinus (sea-urchin), terebratula, and many other forms.
[Illustration: Fig. 653.—Trilobite.]
[Illustration: Fig. 654.—Terebratula.]
The Trilobites were amongst the first creatures inhabiting our globe,
and it is a curious fact to contemplate, that their eyes (fig. 655)
should have been preserved perfect; they present one of those wonderful
objects which carry one’s thoughts backwards to the early ages of the
world, probably many millions of years, and yet it is found by the
peculiar structure of the eyes of these Trilobites that they were
placed at the bottom of the sea with perfect power to look upwards at
the light of the sun through the transparent waters. The same hand and
the same power had then Divine care and solicitude for the well-being
of His creatures, as great as He has for those of later ages, and these
animals are mentioned in Genesis—“Let the waters bring forth abundantly
the moving creature that hath life.”
[Illustration: Fig. 655.—Eye of Trilobite.]
Thus we see that ages of comparative quiet succeeded the first great
contraction of the earth’s crust, probably millions of years, during
which time the tides and currents of the ocean had to wash and wear
down all the thousands of projecting rocks or inequalities, and
dissolve (as before described) all the lime, depositing the sand and
clay in those immense strata which form the old-named “transition
series”; this appears to have taken place over nearly the whole world
at that time, and ages upon ages must have elapsed to form such
deposits as the sandstone, claystone, and limestone, in alternation,
forming the “Llandilo,” “Caradoc,” and “Wenlock” strata, more than a
mile in thickness; these are by some geologists reckoned among the
primary series (by some called the “transition rocks”), and in England
form the “Cambrian” and “Silurian” systems which are so rich in
minerals and metallic veins.
[Illustration: Fig. 656.—Limestone made up of encrinite.]
The OLD RED SANDSTONE, or DEVONIAN System, is the next of the series
of layers which built up our earth, but a great gap of years separates
it from the Silurian. We find the “Old Red” in the Mendips and in
Scotland. The rocks in the West of England are apparently of later
deposition and of marine origin, while the “Old Red” is apparently a
fresh-water deposit. It is very “arenaceous,” and owes its tint chiefly
to iron, although there are circumstances in which it appears neither
as a sandstone nor with a red colour; but red sandstone describes the
true formation very accurately.
[Illustration: Fig 657.—Graptolites.]
In this water-deposited system, whether in lakes or by the sea, we
find a considerable advance upon the Silurian. We have flora in more
variety—seaweeds and ferns. The remains are all aquatic. We have
nothing higher in the scale of creation than the fish, the first
vertebrates; and judging by varieties a very considerable time must
have elapsed during which the Old Red Sandstone was deposited. The
dipterus—or double-winged—is herewith shown as an example of the fossil
fish of the Old Red Sandstone period.
[Illustration: Fig. 658.—“Double-winged” Fish.]
It is curious that no remains of any land-inhabiting animals have
been discovered in this system—whether in the Old Red or the Devonian
formations (the Lower and Upper Red Sandstones). We can only
distinguish the remains of aquatic animals or plants. We may picture
the great cuttle-fish, the nautilus, and the dipterus, with various
orders of mollusca, and the gradual approach to the crustacea, but no
terrestrial animals have been discovered. We may take it for granted
then that the Old Red Sandstone and Devonian systems are different—the
former being found in Scotland and parts of England, and formed of
deposits in fresh or brackish water, while in the Devonian system the
marine deposits are corals, and all the indications of ocean life,
separated from the great inland lakes by a range of hills. Neither
of the terms (Old Red Sandstone or Devonian) limit geographically
or descriptively the formations of this system. All the rocks are
clearly distributed between the Silurian and the Carboniferous. The
invertebrates in this last system have not developed very much, but
corals are very abundant, and fish of some armoured species are
plentiful and curious, while the crustacea were enormous.
[Illustration: Fig. 659.—Palæozoic Fish. Trilobites, Brachiopods, Coral
and Graptolite.]
We now arrive at the most important of all the rock formations, the one
to which we owe our national prosperity—we mean the COAL System.
THE CARBONIFEROUS FORMATION.
While the foregoing depositions were being made the earth was still
undergoing changes. The sandstones were deposited, and the corals
making use of the lime carried into the waters began to build and form
masses of limestone under the sea, pushing back the water and changing
the forms and positions of land and water. All this went on apparently
very quietly—volcanic action was not very frequent—the water was warm.
But sometimes earthquakes would heave up the submarine formations into
mountains, and therefore we find the fossils of the tiny sea-animals
on the hills. Extensive swamps were formed by partially retreating
sea-water, and their vegetation became luxuriant. Tree ferns and all
the floral appearance of the tropics grew up and formed dense forests,
far thicker than any we know of at the present time. It will readily
be understood that the condition of the atmosphere must have been
particularly favourable to the growth of plants, and therefore not
suitable for air-breathing animals. Heat and carbonic acid must have
been greatly developed.
[Illustration: Fig. 660.—Tree Ferns.]
We can now perceive how the gradual filling up of the earth for man’s
reception was taking place. The rain was taking carbonic acid from the
air, for the dead plants gave it out in enormous volumes. The carbonate
of lime dissolved in the water-springs, etc., was carried to the sea
for polypi to build shells from. The trees were absorbing carbonic
acid, too, and while purifying the air, were retaining the carbon in
their stems and leaves and branches, which (when they decayed) remained
untouched, and accumulated in thick layers to sink down, and by
pressure be turned into coal. This great effect was carried out several
times; and it is a remarkable fact that we find coal, limestone, and
ironstone so near together, all useful to us and to each other in the
course of the working of the minerals—so we come to the Carboniferous
system.
[Illustration: Fig. 661.—Limestone made up of corals (_Favosites
polymorpha_).]
Coal we have already treated of in _Mineralogy_, and coal looks at
times very different from our preconceived ideas of a _sedimentary_
rock, which we know is regularly deposited in layers. But when we
split or break the coal we find its cleavage in a certain direction.
Coal is wood squeezed and petrified by ages between enormous layers of
sedimentary rocks, and the coal-seam rests upon the soil in which the
plants once grew—perhaps more than six hundred thousand years ago!
Of course coal, as we burn it, was not all made at once. We can trace
it from the swamp as Peat, on to “Lignite,” or woody coal, through the
Tertiary and New Red Sandstone to the coal measures themselves. Even
lower down we find the remains in more or less pure carbon forms—the
anthracite and the graphite of the primary formations.
Coal appears not to have been formed equally in all places during the
period in which it originated. The remains of plants found in these
strata lead us to infer, that, during that period there existed an
exceedingly vigorous and crowded vegetation, consisting principally of
tree ferns and equisetaceæ, of which the _Sphenopteris Hœninghausii_
(fig. 662), _Pecopteris aquilina_ (fig. 663), and _Neuropteris Loshii_
(fig. 664), are amongst the most beautiful that have been found, and
the flora and fauna of this period were of a more or less primitive
kind or low order, but very luxurious. The former display a decided
advance, and reptiles of aquatic forms appear with large and predaceous
fishes. Mountain limestone, which is usually found in the coal
formations, includes metallic deposits, and organic remains are very
abundant in it. The following are specimens of the fossils—
[Illustration: Fig. 662.]
[Illustration: Fig. 663.]
[Illustration: Fig. 664.]
[Illustration: Fig. 665.—
1. Bellerophon costatus.
2. Spirifer glaber.
3. Productus Martini.
4. Orthoceras lateralis.]
The Carboniferous system is a very important one, as may be seen. In
these beds we have coal, limestone, sandstone, and shale. The _Coal
Measures_ consist of grit and sandstone and shale, with coal seams. The
Carboniferous Limestone, or “Mountain” Limestone, has no coal in it.
The sandstone has been termed “Millstone Grit,” because millstones
are made from it. We then have limestone shales, and the sandstone
beds. Each coal seam indicates a subsidence of the land and a regular
series of underclay or soil in which the plants grew,—the plants
themselves,—iron, coal, and then the shale, and so on again, indicating
frequent changes and a long lapse of ages.
[Illustration: Fig. 666.—Fern (_Pecopteris ligata_) from upper shale,
Scarborough.]
These “Coal Measures” occupy an area of five hundred square miles in
Great Britain alone; and as we have already said, the period which
elapsed while these deposits were being laid down represents hundreds
of thousands of years. The deposit would increase at the rate of about
three feet in a thousand years. And this is only one period of the many
changes to which our world has been subjected since it first began its
revolution in space around the sun.
[Illustration: Fig. 667.—Section across the carboniferous rocks of
Derbyshire and Lancashire (After Ramsay).
1. Carboniferous Limestone.
2. Yoredale Shales.
3. Millstone Grit.
4. Coral Measures.
5. Permian Limestone.
6. New Red Sandstone.]
Coal is usually found in “basins” or depressions—a sort of trough owing
to the upheaval of surrounding strata which became in time denuded (or
washed away) with any coal that was there. So it is in places where it
is concealed by overlying beds that protect it, that we now find the
coal saved from disturbance. When we search and come upon red sandstone
and grey-wacke, we may be almost certain that we are near coal,
particularly if the surrounding rocks form a “basin.”
[Illustration: Fig. 668.—Labyrinthodon.]
We have now briefly sketched the Carboniferous system, for in our
recreations we can do little more. We shall find in many places in
England tree trunks in the sandy shore, and ample evidence that a
forest has been at one time submerged in that spot. So inland the land
sank down again and again in successive periods—water, mud, soil,
vegetable growth succeeded, to be again submerged and form a new coal
seam for the use of man, who was destined to appear after the lapse of
ages.
THE PERMIAN PERIOD.
[Illustration: Fig. 669.—Impressions of feet of Cheirotherium].
At length the land remained undisturbed. It sank no more, and the trees
waved luxuriously over the buried forests of past ages, and another
cycle set in called the PERMIAN, from the ancient kingdom of Permia,
where all the features of that period are exhibited on a very grand
scale, and which extended for several hundred miles between the Ural
mountains and the Volga. In the Permian period we have the progression
of animal life more distinctly developed than in the Carboniferous
system, which immediately preceded it. It is true there are indications
in the latter that curious animals, called the Cheirotherium and the
Labyrinthodon, were alive then, and the remains of numerous insects,
such as beetles and crickets, have been found; but the Permian
developed them, and reptiles, saurians, and lizards have been traced;
but as Sir R. Murchison states, throughout the whole extent the animals
are of a single type. We have the hand-like impressions of the feet
of the Cheirotherium, so called from the Greek “cheir,” a hand. The
soil appears to have been very soft, and peculiarly adapted to receive
impressions, and which having been in many places covered over with
a stratum of fine sand, and then abandoned by the sea, the whole
has hardened into stone, and being now separated, the one contains
their footprints, and the other perfect casts of them! Nor are these
footmarks all that these sandstones have to tell us of their day; for
the ripples of the waves, and even the little pits made by drops
of rain as they fell, are in this most marvellous manner preserved,
forming objects of wonder and admiration.
The organic remains during this period are not very abundant, and many
of the fauna of the previous systems appear to have died out, while
others appeared to meet with fuller development in succeeding ages. The
Permian is also known as the New Red Sandstone, or Magnesian Limestone
group. “Dias” has also been suggested with reference to the “Trias”
group, “the Upper New Red Sandstone,” which comes next. The Permian
rocks are very varied, and contain minerals, such as copper and sulphur.
As the strata below the new sandstone formation was called the
“Carboniferous” system, from its containing much carbon in the
form both of coal and carbonic acid, so this has been called the
“Saliferous” system, from the occurrence in many places of strata
of “rock-salt,” or crystallised chloride of sodium, and (where the
rain finds its way down and dissolves it) of brine springs; these (in
England) exist chiefly in Cheshire and Warwickshire, but in Poland and
Hungary they exist on a much larger scale, the rock-salt being nearly
a thousand feet thick. It has been said that these strata of salt were
formed by the evaporation of salt lakes, but it is much more probable
that salt is one of the natural materials of the earth, and that both
salt lakes and oceans have become salt from dissolving out these strata
wherever they have come into contact.
It is supposed that during the Permian period the greater portion of
the continent of Europe was raised above the ocean, and the deposits
were formed in salt lakes, for the appearance of the organic remains
tends to establish the fact that the creatures of that period were not
far from dry land even in their watery existence, and the reptiles
found confirm this view. We have now to examine the Mesozoic, or
Secondary System.
[Illustration: Dinotherium giganteum.]