FAMILIES OF ROCKS AND THEIR DESCENDANTS
Thus far, in accordance with the principles of the great geologists
from Sir Charles Lyell onwards, we have tried to disclose the
history of the earth's crust by observing the processes which are
going on to-day under our eyes. That is not, however, the only way
in which history has to be written. The documents on which history
rests are often lamentably incomplete. The records have great
gaps in them, and very often the gaps have to be filled by that
exercise of the imagination which Bishop Creighton once described
as the rearrangement of facts. We shall later in this book show
how naturalists can reconstruct the skeleton and even the general
appearance of an animal which for ages has not been seen alive on
the earth, from a consideration of fragments of the bony structure.
Similarly the archæologists who inquire into the history of forgotten
peoples can picture to us their lives and habits and manners from a
consideration of the fragmentary weapons and pottery and architecture
which they left in their buried cities; and similarly the geologist,
knowing, or partly knowing, how the forces of nature are at work
to-day, can attempt to describe the conditions under which rocks were
laid down before man ever trod them.
In speaking or writing of the earliest stages of the world's history
we have to adopt what seems to be the most likely history, modestly
qualifying what we say by adding that these speculations are only
the fruits of an inquiry that man has pushed beyond the ascertained
facts. But we are on firmer footing when we come to deal with that
portion of the globe which we can examine. The crust of the earth
has been found to consist of successive layers of rock which, though
far from constant in their occurrence, and though often broken and
crumpled by subsequent disturbances, have been recognised over a
large portion of the globe. They are the earth's own chronicle of its
history. Had these rocks of the Geological Record remained in their
original positions we should have known little of them, because only
the most recent would have been visible. Owing, however, to the way
in which the earth's crust has been twisted and cracked and broken,
portions of the bottom layers have been pushed up to the surface,
and the lower rocks have been inclined so that we can examine their
upturned edges. Instead, therefore, of being restricted to examining
a few hundred feet of earth crust we can examine many thousand
feet. The total thickness of the rocks of Europe which contain
fossil remains has been estimated at 75,000 feet, or fourteen miles.
This vast depth of rock has been laid open to our observation by
disturbances, twists, contortions, upsettings of the crust.
We shall not press on the reader in a volume of this kind any
detailed classification of the strata, but he will like to know the
names of the five great periods into which geologic time is divided.
The first period was the Archæan, embracing the periods of the
earliest rocks wherein few or no traces of life occur.
The second period was the Palæozoic (ancient life) or Primary, which
includes the long succession of ages during which the earliest types
of life existed.
The third period was the Mesozoic (middle life), comprising a series
of ages when more advanced types of life flourished.
The fourth period was the Cainozoic (recent life) or Tertiary period,
when such types of life as we know and see now appeared. This period,
however, does not include man.
The fifth period is the Quaternary or Post-Tertiary and Recent, and
includes the time since man appeared on the earth.
These divisions were not of the same length. The Palæozoic ages were
probably far, far longer than those of any other division, while
the Quaternary period is shorter than any of those which preceded
it. Each of these main divisions is divided further into systems or
shorter periods (just as the dynasties of ancient Egypt could be
subdivided into reigns). Though the broad outlines of the sequence
of the living things which existed in those periods has been the
same all over the world, many local differences may be traced in the
nature and grouping of the sedimentary materials in which the remains
of the living things of these epochs have been preserved.
To find the oldest rocks, we must seek those which lie at the bottom
or underneath all the others. Judged by this test, the oldest
rocks in Great Britain are certain hard rocks (like gneiss, or the
material of which volcanic veins are composed) which crop out in the
north-west of Scotland, and which form the outer Hebrides. They are
also known in Anglesea, and in the extreme west of Wales, at St.
David's. Similar strata form the Malvern Hills of Worcestershire,
the Longmynd Hills, Caer Caradoc and the Wrekin Hills of Shropshire,
and the hilly district of Charnwood Forest in Leicestershire. For a
long time the Cambrian rocks of Wales, so called from North Wales's
ancient name of Cambria, were believed to be the oldest on the face
of the earth. Up to the year 1830 even these rocks had no name or
recognition, for geologists believed that it was impossible to
classify them. But in 1831 Professor Adam Sedgwick, of Cambridge,
began the diligent study of the rocks in North Wales, and after five
years' work he was able to announce in 1836 that he had determined
the general order of succession in that district of a certain ancient
group of slaty, gritty, and flaggy strata. However, eighteen years
later, in 1854, Sir William Logan, who was then engaged in mapping
the rocks of Canada, found along the River St. Lawrence an enormous
thickness (30,000 feet or more) of gneiss, quartzite, schist,[15]
limestone, etc., these rocks underlying--and being, therefore, older
than--the Cambrian strata, which are also well developed in that
country. To these "bottom" rocks Logan gave the name of Laurentian.
For some time afterwards the same name was also applied to the
somewhat similar rocks which were found to underlie the Cambrian
formation in Britain, but it was felt safer to give the English
rocks a more general name. They are therefore now usually called
Pre-Cambrian, which simply means older than the Cambrian strata, or
Archæan.
[Footnote 15: Hard rocks are sometimes composed of different
minerals, which are arranged in a way that reminds us of a bed
of fallen leaves, and are called "foliated," from the Latin word
_folium_, a leaf. Gneiss is a good example of a foliated rock. It is
composed of the three minerals, quartz, felspar, and mica, arranged
in this foliated manner. Mica schist, talc schist, and other rocks
have a similar structure, and are sometimes briefly called "schists."]
In Canada the total thickness of the Laurentian, Pre-Cambrian,
or Archæan rocks is now estimated at 50,000 feet. In Britain it
is nothing like so great as this (though still considerable);
but the thickness of these extremely old and altered rocks is a
very difficult matter to determine, for all signs of the original
stratification in them have often been destroyed, and the rocks have
been so bent and folded that it is possible the same beds may have
been measured more than once in the same section.
It will be understood from some of the foregoing sentences that the
task of dating or classifying these early rocks is one which is far
from simple, and which has given rise to many different opinions. We
may here give another example. "During the years between 1831 and
while Sedgwick was occupied in studying the rocks of North Wales,"
writes Mr. W. Jerome Harrison, "another geologist, Mr. (afterwards
Sir) Roderick Murchison, was engaged in the examination of the
strata which occupy the south-east of Wales and the adjoining border
counties of England. To these rocks Murchison gave, in 1835, the
name of Silurian, from the ancient British tribe of the Silures, who
inhabited that part of the country when the Romans invaded Britain."
Later in last century, in order to distinguish more clearly the
periods of the rocks which began or ended in these areas, the name
of another ancient British tribe was called into requisition--the
Ordovics; and thus for certain strata which were neither Silurian nor
Cambrian Professor Lapworth proposed the name Ordovician.
Let us, however, now leave these geological controversies,
enthralling as they are to those who have taken part in them, to
consider briefly what was the aspect of the earth during the ages
when these rocks were being laid down. The earliest rocks do not
generally contain fossils, though there is no doubt that life existed
during the later part of the time when they were laid down. The
few fossils that have been preserved are those of crustacea (the
species from which shrimps, for example, are derived), and there are
certain tracks of two kinds of burrowing worms. It is noticeable
that crustacea, the oldest definite fossils yet found, belong to a
family which is well up in the animal kingdom, and therefore we know
that lower forms of life must have been long in existence. Since we
can only draw conclusions of the climate of a period from its fossil
remains, and as these fossil remains are so scarce, we cannot say
really anything of value about the world's climate in the earliest
eras.
When we come to the Cambrian, however, we are on firmer ground. In
the Cambrian rocks there is, for the first time, a fair preservation
in fossil form of the life of the period. Even here the record is far
from complete, but it is an immeasurable advance on the records of
previous periods. The most striking thing about this comparatively
plentiful appearance of life is that while the animal kingdom is
fairly well represented the plant remains are hardly to be recognised
at all. Yet there must have been plants if only to feed the animals,
and we have very good reasons for believing that the surface of the
land was clothed with some form of vegetation. Not a few of the
Cambrian animals were fixed to the bottom of the sea, and therefore
there must have been enough matter of some organic kind floating
in the water to bring them their daily food. Possibly many of the
plants were of the minute kind which forms scum on rivers and ponds,
and so would not readily leave fossil impressions. Turning to the
record of animal life, it appears that nearly every division of
the animal kingdom, except such as had backbones, had some kind of
a representative in Cambrian times. Crustaceans, molluscs, worms,
corals, jelly fish, sponges, quite a large variety of sea-animals,
suddenly make their appearance, and although no traces of land
animals have yet been found, we have reason to believe that some land
animals may have existed. Our reason is that in the next era but one
(Silurian) scorpions and insects appear, and these are such highly
developed forms of land-life that they probably had some primitive
ancestors in the Cambrian. No real fish have been found in the
Cambrian rocks, but they appeared in the next era (Ordovician). It
is the trilobite which is the characteristic animal of the Cambrian
times. They were crustaceans; they had eyes; and they gave the
promise of development; but there is no reason for believing that
they were as high in the order of creation as the commonest lobster
of the sea-shore. Nothing remains to us of them except their bony
structure, but we believe that they could both swim and walk on the
sea-bottom; that some were swift of movement, and that they acquired
the habit of moulting their shell. They may have been sociable
animals, for the shells of trilobites are sometimes found together
in large numbers, occasionally closely packed, "spoon fashion," and
though these may be moulted shells, we are warranted in supposing
that the early trilobites lived in colonies, hunted for food, and
made war like their descendants millions of years after. What were
the actual conditions of life in this world of Cambrian days we
do not know positively. The first beginnings of life, the simple
one-celled plants, may have first dwelt in the deep ocean. The land
was barren, its lakes unfitted to support life. On the other hand, it
is equally likely that the first beginnings of life may have been the
simple plants growing in inland waters and gradually spreading down
to the sea. We do not know, but it is most probable that life began
in some great body of water, where plants and insignificant animals
grew together, perhaps fought together, and certainly in this
environment became more and more fitted for the business of living.
In Mr. Henry R. Knipe's scholarly and well-informed volume, _Nebula
to Man_ (J. M. Dent & Co.), to which we are indebted not only
for several of our illustrations but for many extremely valuable
suggestions, the struggle for existence in the early ocean is well
summed up:--
Thus through the brine life manifold proceeds,
Impelled to higher states by growing needs;
And all these early life-types in the seas
Will branch in time to many species;
And some amid conditions too severe,
Must, after stress and struggle, disappear.
And when a species falls from Life's domain
It never gains a place on Earth again.
We may speculate with some approach to certainty on the general
appearance of the earth in those days. There was far more water
on the surface of the globe; the land surfaces were small and
infrequent. The seas may have been shallower than those which we
know, but they were far greater in extent. There must have been far
more rain and a very much greater number of violent storms arising
from the constant condensation of the waters by the rays of the sun.
The sun was probably seen far less often in those days, and there
are some geologists who believe the earth to have been perpetually
covered with cloud, as the planet Venus is now.
Europe as a continent did not exist. A few islands showed their heads
above the waves where Germany and Switzerland, Eastern France and
Spain now stand. Scotland's rocky islets were probably visible, on
the extreme west, though these islands were destined to sink again
below the waves. Thence the ocean stretched without a break, as at
present, to Canada. A great part of Canada's bleak lands was above
the waters; but the United States, except for a few great islands,
were submerged. In the southern hemisphere South America, split into
numerous long reefs and islands, gave promise of the continent to
be; and there were great stretches of land over Brazil, extending
to the west where the great chains of mountains now rise. Asia was
largely covered with shallow waters, and the whole extent of the
northern plains of Africa was sea. So far as we are able to judge
the distinctions of climates were less marked then than now, and
the conditions seem to have been much more uniform over all the
northern hemisphere. This equality of climate lasted into the next or
Ordovician period.
The Ordovician period glides insensibly into the Cambrian. There was
no distinct break in the succession of life. The species seem to have
slowly extended and developed from one of these great periods into
another. But the life of the Ordovician era, which has been preserved
for us, is much more abundant. Land was beginning to emerge from
the sea in greater bulk; life was springing up on the land and was
emerging from the sea, perhaps to take up its habitation there. The
first insect life appears in the Ordovician. It is not an imposing
relic except when seen through the eye of imagination. It is just an
obscure wing of an insect which was found impressed on some shales
found in the upper Ordovician rocks of Sweden, and all we can say of
it is that it belonged to the same class of insects as lady-birds.
The existence of this insect shows that there must have been land
vegetation and an atmosphere which was suited to active air-breathing
things. The other appearance of great interest in the Ordovician
rocks is that of the first fish. They were found in Colorado, but
they are very much shattered and tell us very little about the
animals they represent. These fish were covered with plates, and were
evidently thus defended against attack, so that we may surmise the
existence of some other animal that preyed on fishes. Whether these
fishes were themselves ferocious we cannot, however, say. But that
which was the chief characteristic of the Ordovician era was the
climax of the trilobite. More than half of all the known trilobites
were present in Ordovician times. Only a few of these came over from
the Cambrian, while the others make their first appearance in this
period. In the next period (Silurian) their numbers fell to one half,
and in later periods declined still further, till they disappeared
altogether at the close of the Palæozoic era. Some of these curious
animals appear to have been able to move very quickly; others would
roll themselves up like hedgehogs to defend themselves against
attack; and some of the larger ones were from eighteen inches to
two feet in length. Next to these in interest were the cephalopod
types, marine animals, that may have resembled the swimming nautilus
of to-day in some of their developments. They attained to enormous
sizes, some of the shells being twelve to fifteen feet in length and
a foot in greatest diameter. From this maximum they ranged down to
forms smaller than a pipe-stem. Their habits are to be gathered only
from their structure and from the habits of their relations in the
present seas. Perhaps they floated, shell uppermost, or crawled upon
the bottom and preyed on a variety of the weaker forms of life. There
appear to have been fewer worms, perhaps because the muddy and chalky
sea bottoms of the Ordovician period were less congenial to them than
the Cambrian sands.
The changes in the structure of the earth's crust which brought
the Ordovician period to an end marked also the beginning of the
Silurian period. These changes affected sometimes small areas
and were very intense; sometimes they affected larger areas more
slightly. It must not be assumed, however, that these changes were
necessarily sudden or violent. In examining the rocks now, we see
merely the effects, and of these effects it is the more remarkable
alone which have survived the march of ages. There was more water on
the earth's surface then than now; and side by side with continuous
storms of tropical violence, it is extremely likely that volcanoes
and earthquake movements were more frequent and more considerable
in their effects. The tide of movement by material things may have
been faster; and certain it is that the land was now lifting itself
up above the shallow seas. Mountains were being built along the
coast-lines; behind the coast-lines the continents were shouldering
their way upwards in large land areas. North America began to show
in this period the first signs of becoming a continent; Europe's
countries, or some of them, assumed a distinct existence. With the
advent of mountains came streams and rivers; and the streams, fed
by the abundant rainfalls, rushed down to the seas in torrents that
performed the work of erosion with a rapidity perhaps unequalled by
even the greatest rivers of our present-day knowledge--though at
first the land areas were not large enough to give rise to streams
as long as great rivers like the Amazon or Mississippi. We cannot
say exactly what the areas and localities of the water and the land
were; but it is safe to assume that at the beginning of the Silurian
period beds of sediment brought down by the rivers and the rain
were accumulating about the borders of the land, and as far out as
the waves and currents were able to convey the earth materials. The
climate was still equable and was much the same over great areas of
the world's surface, for the forests of warm temperate latitude are,
in part, the same as those in Arctic regions. Certain parts of the
land appear to have been desert.
Life began to change a great deal in Silurian times. The extensive
withdrawal of the sea from great stretches of submerged surface
reduced the area of shallow water available for the forms of life
that had so richly peopled it during the Ordovician period. Then
there came an age during which the sea invaded some of the regions
of the earth's crust, and again withdrew, leaving behind it great
stretches of water which gradually grew more intensely salt. All
these things had naturally a great effect on the development of the
plants and animals of Silurian times. We cannot in a brief summary
of this kind do more than indicate some of the more conspicuous
features. Corals began to spread through the clearer seas: and reef
building on a great scale took place, generally some distance from
the shores of the land. Other life in great abundance and variety
gathered upon or about these reefs, and they became rich depositories
of the animals of their day. The Crinoids, which, though animals,
are sometimes called the lilies of the sea, developed strongly;
sea urchins appeared, and forms akin to barnacles. The ancestors
of the pearl oyster and the mussel date from Silurian times; and
so do the first Ammonites, those creatures known to the youngest
collectors of fossils, and deriving their names from the Canaanitish
god Ammon, which had a ram's head. Sea scorpions, sand fleas, king
crabs, sea squirts, and worms and fishes of various kinds haunted
the Silurian seas. The Silurian fish were most of them armed for
defence, some with plates of bone; some of them had their tails
stiffly joined to their backbones; some had skin like a prickly
pear; some were not unlike the modern shark. The plants have left
us many records--liverworts, ferns, and club-like mosses. The
growing vegetation gave a new impulse to insect life--plant-lice
and cockchafers and the scorpions we have named: and the vastness
of their numbers is shown by the fact that they have outlasted the
changes and vicissitudes of a myriad generations.
We may conclude this chapter by saying what we imagine of the general
appearance of our own islands to have been. At the close of the
Silurian period Britain was probably an archipelago, ranging over
ten degrees of latitude, like many of the island groups now found
in the great Pacific Ocean; the old gneissic hills of the western
coast of Scotland, culminating in the granite range of Ben Nevis,
and stretching to the Southern Grampians, forming the nucleus of
one island group; the South Highlands of Scotland, ranging from
the Lammermuir Hills, another; the Pennine chain and the Malvern
Hills, the third and most easterly group; the Shropshire and Welsh
mountains, a fourth; and Devon and Cornwall stretching far to the
south and west. Every spot of the island lying now at a lower
elevation than 800 feet above the sea was under water at the close of
the Silurian period, except in those instances where depression by
subsidence has since occurred.