<h2 class="nobreak" id="CHAPTER_XVII">CHAPTER XVII</h2>
<h3>FAMILIES OF ROCKS AND THEIR DESCENDANTS</h3></div>
<p>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,
<span class="pagenum" id="Page_198">-198-</span>
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.</p>
<p>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.</p>
<p><span class="pagenum" id="Page_199">-199-</span></p>
<p>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.</p>
<p>The first period was the Archæan, embracing the periods
of the earliest rocks wherein few or no traces of life occur.</p>
<p>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.</p>
<p>The third period was the Mesozoic (middle life), comprising
a series of ages when more advanced types of life
flourished.</p>
<p>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.</p>
<p>The fifth period is the Quaternary or Post-Tertiary and
Recent, and includes the time since man appeared on the
earth.</p>
<p>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.</p>
<p><span class="pagenum" id="Page_200">-200-</span></p>
<p>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,<SPAN name="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</SPAN> limestone, etc., these rocks underlying—and
being, therefore, older than—the Cambrian strata,
<span class="pagenum" id="Page_201">-201-</span>
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.</p>
<div class="footnote">
<p><SPAN name="Footnote_15" href="#FNanchor_15" class="label">[15]</SPAN> 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 <i>folium</i>, 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."</p>
</div>
<p>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.</p>
<p>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
<span class="pagenum" id="Page_202">-202-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_203">-203-</span>
of value about the world's climate in the earliest
eras.</p>
<p>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
<span class="pagenum" id="Page_204">-204-</span>
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,
<span class="pagenum" id="Page_205">-205-</span>
and certainly in this environment became more and more
fitted for the business of living.</p>
<p>In Mr. Henry R. Knipe's scholarly and well-informed
volume, <i>Nebula to Man</i> (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:—</p>
<div class="poetry-container">
<div class="poetry">
<div class="stanza">
<div class="verse indent0">Thus through the brine life manifold proceeds,</div>
<div class="verse indent0">Impelled to higher states by growing needs;</div>
<div class="verse indent0">And all these early life-types in the seas</div>
<div class="verse indent0">Will branch in time to many species;</div>
<div class="verse indent0">And some amid conditions too severe,</div>
<div class="verse indent0">Must, after stress and struggle, disappear.</div>
<div class="verse indent0">And when a species falls from Life's domain</div>
<div class="verse indent0">It never gains a place on Earth again.</div>
</div></div>
</div>
<p>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.</p>
<p>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
<span class="pagenum" id="Page_206">-206-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_207">-207-</span>
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
<span class="pagenum" id="Page_208">-208-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_209">-209-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_210">-210-</span>
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.</p>
<p>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
<span class="pagenum" id="Page_211">-211-</span>
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.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_212">-212-</span></p>
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