THE PERMIAN AGE AND CLOSE OF THE PALÆOZOIC.
The immense swamps and low forest-clad plains which occupied the
continental areas of the Northern Hemisphere, and which we now know
extended also into the regions south of the equator, appear at the
close of the Carboniferous age to have again sunk beneath the waves,
or to have relapsed into the condition of sand and gravel banks; for a
great thickness of such deposits rests on the coal measures and
constitutes the upper coal formation, the upper "barren measures" of
the coal-miners. There is something grand in the idea of this
subsidence of a world of animal and vegetable life beneath the waters.
The process was very slow, so slow that at first vegetable growth and
deposition of silt kept pace with it; and this is the reason of the
immense series of deposits, in some places nearly 15,000 feet thick,
which inclose or rest upon the coal beds; but at length it became more
rapid, so that forests and their inhabitants perished, and the wild
surf drifted sand and pebbles over their former abodes. So the
Carboniferous world, like that of Noah, being overflowed with water,
perished. But it was not a wicked world drowned for its sins, but
merely an old and necessarily preliminary system, which had fully
served its purpose; and, like the stubble of last year, must be
turned under by the plough that it may make way for a new verdure. The
plough passed over it, and the winter of the Permian came, and then
the spring of a new age.
The Permian and the succeeding Triassic are somewhat chilly and
desolate periods of the earth's history. The one is the twilight of
the Palæozoic day, the other is the dawn of the Mesozoic. Yet to the
philosophical geologist no ages excel them in interest. They are times
of transition, when old dynasties and races pass away and are replaced
by new and vigorous successors, founding new empires and introducing
new modes of life and action.
Three great leading points merit our attention in entering on the
Permian age. The first is the earth-movements of the period. The
second is the resulting mineral characteristics of the deposits
formed. The third is the aspect of the animal and vegetable life of
this age in their relation more especially to those which preceded.
[Illustration: DIAGRAM OF FOLDINGS OF THE CRUST IN THE PERMIAN PERIOD.
(The vertical scale of heights and depressions exaggerated more then
six times.) The lower figure shows a portion of folded strata in the
Appalachians--after Rogers.]
With respect to the first point above named, the earth's crust was
subjected in the Permian period to some of the grandest movements
which have occurred in the whole course of geologic time, and we can
fix the limits of these, in Europe and America at least, with some
distinctness. If we examine the Permian rocks in England and Germany,
we shall find that everywhere they lie on the upturned edges of the
preceding Carboniferous beds. In other words, the latter have been
thrown into a series of folds, and the tops of these folds have been
more or less worn away before the Permian beds were placed on them.
But if we pass on to the eastward, in the great plain between the
Volga and the Ural mountains, where, in the "ancient kingdom of Perm,"
the greatest known area of these rocks is found, an area equal in
extent to twice that of France, and which Sir R. I. Murchison, who
first proposed the name, took as the typical district, we find, on the
contrary, that the Permian and Carboniferous are conformable to one
another. If now we cross the Atlantic and inquire how the case stands
in America, we shall find it precisely the same. Here the great
succession of earth-waves constituting the Appalachian Mountains rises
abruptly at the eastern edge of the continent, and becomes flatter and
flatter, until, in the broad plains west of the Mississippi, the
Permian beds appear, as in Russia, resting upon the Carboniferous so
quietly that it is not always easy to draw a line of separation
between them. As Dana has remarked, we find at the western side of
Europe and the eastern side of America, great disturbances
inaugurating the Permian period; and in the interior of both, in the
plains between the Volga and the Ural in one, and between the
Mississippi and Rocky Mountains in the other, an entire absence of
these disturbances. The main difference is, that in eastern America
the whole Carboniferous areas have apparently been so raised up that
no Permian was deposited on them, while in Europe considerable patches
of the disturbed areas became or remained submerged. Another American
geologist has largely illustrated the fact that the movements which
threw up the Appalachian folds were strongest to the eastward, and
that the ridges of rock are steepest on their west sides, the force
which caused them acting from the direction of the sea. It seems as if
the Atlantic area had wanted elbow-room, and had crushed up the edges
of the continents next to it. In other words, in the lapse of the
Palæozoic ages the nucleus of the earth had shrunk away from its
coating of rocky layers, which again collapsed into great wrinkles.
Such a process may seem difficult of comprehension. To understand it
we must bear in mind some of its conditions. First, the amount of this
wrinkling was extremely small relatively to the mass of the earth. In
the diagram on page 162 it is greatly exaggerated, yet is seen to be
quite insignificant, however gigantic in comparison with microscopic
weaklings like ourselves. Secondly, it was probably extremely slow.
Beds of solid rock cannot be suddenly bent into great folds without
breaking, and the abruptness of some of the folds may be seen from our
figure, copied from Rogers (page 162), of some of the foldings of the
Appalachian Mountains. Thirdly, the older rocks below the
Carboniferous and the Devonian must have been in a softened and
plastic state, and so capable of filling up the vacancies left by the
bending of the hard crust above. In evidence of this, we have in the
Lower Permian immense volcanic ejections--lavas and other molten
rocks spewed out to the surface from the softened and molten masses
below. Fourthly, the basin of the Atlantic must have been sufficiently
strong to resist the immense lateral pressure, so that the yielding
was all concentrated on the weaker parts of the crust near the old
fractures at the margins of the great continents. In these places
also, as we have seen in previous papers, the greatest thickness of
deposits had been formed; so that there was great downward pressure,
and probably, also, greater softening of the lower part of the crust.
Fifthly, as suggested in a previous chapter, the folding of the
earth's crust may have resulted from the continued shrinkage of its
interior in consequence of cooling, leading after long intervals to
collapse of the surface. Astronomers have, however, suggested another
cause. The earth bulges at the equator, and is flattened at the poles
in consequence of, or in connection with, the swiftness of its
rotation; but it has been shown that the rotation of the earth is
being very gradually lessened by the attraction of the moon.[T] Pierce
has recently brought forward the idea[U] that this diminution of
rotation, by causing the crust to subside in the equatorial regions
and expand in the polar, might produce the movements observed; and
which, according to Lesley, have amounted in the whole course of
geological time to about two per cent, of the diameter of our globe.
We thus have two causes, either of which seems sufficient to produce
the effect.
[T] Sir William Thomson, who quotes Adams and Delaunay.
[U] "Nature," February, 1871.
Viewed in this way, the great disturbances at the close of the
Palæozoic period constitute one of the most instructive examples in
the whole history of the earth of that process of collapse to which
the crust was subject after long intervals, and of which no equally
great instance occurs except at the close of the Laurentian and the
close of the Mesozoic. The mineral peculiarities of the Permian are
also accounted for by the above considerations. Let us now notice some
of these. In nearly all parts of the world the Permian presents thick
beds of red sandstone and conglomerate as marked ingredients. These,
as we have already seen, are indications of rapid deposition
accompanying changes of level. In the Permian, as elsewhere, these
beds are accompanied by volcanic rocks, indicating the subterranean
causes of the disturbances. Again, these rocks are chiefly abundant in
those regions, like Western Europe, where the physical changes were at
a maximum. Another remarkable feature of the Permian rocks is the
occurrence of great beds of magnesian limestone, or dolomite. In
England, the thick yellow magnesian limestone, the outcrop of which
crosses in nearly a straight line through Durham, Yorkshire, and
Nottingham, marks the edge of a great Permian sea extending far to the
eastward. In the marls and sandstones of the Permian period there is
also much gypsum. Now, chemistry shows us that magnesian limestones
and gypsums are likely to be deposited where sea water, which always
contains salts of magnesia, is evaporating in limited or circumscribed
areas into which carbonate of lime and carbonate of soda are being
carried by streams from the land or springs from below;[V] and it is
also to be observed that solutions of sulphuric acid, and probably
also of sulphate of magnesia, are characteristic products of igneous
activity. Hence we find in various geological periods magnesian
limestones occurring as a deposit in limited shallow sea basins, and
also in connection with volcanic breccias. Now these were obviously
the new Permian conditions of what had once been the wide flat areas
of the Carboniferous period. Still further, we find in Europe, as
characteristic of this period, beds impregnated with metallic salts,
especially of copper. Of this kind are very markedly the copper slates
of Thuringia. Such beds are not, any more then magnesian limestones,
limited to this age; but they are eminently characteristic of it. To
produce them it is required that water should bring forth from the
earth's crust large quantities of metallic salts, and that these
should come into contact with vegetable matters in limited submerged
areas, so that sulphates of the metals should be deoxidized into
sulphides. A somewhat different chemical process, as already
explained, was very active in the coal period, and was connected with
the production of its iron ores; but, in the Permian, profound and
extensive fractures opened up the way to the deep seats of copper and
other metals, to enrich the copper slate and its associated beds. It
is also to be observed that the alkaline springs and waters which
contain carbonate of soda, very frequently hold various metallic
salts; so that where, owing to the action of such waters, magnesian
limestone is being deposited, we may expect also to find various
metallic ores.
[V] Hunt, "Silliman's Journal," 1859 and 1863.
Let us sum up shortly this history. We have foldings of the earth's
crust, causing volcanic action and producing limited and shallow
sea-basins, and at the same time causing the evolution of alkaline and
metalliferous springs. The union of these mechanical and chemical
causes explains at once the conglomerates, the red sandstones, the
trap rocks, the magnesian limestones, the gypsum, and the
metalliferous beds of the Permian. The same considerations explain the
occurrence of similar deposits in various other ages of the earth's
history; though, perhaps, in none of these were they so general over
the Northern Hemisphere as in the Permian.
From the size of the stones in some of the Permian conglomerates, and
their scratched surfaces, it has been supposed that there were in this
period, on the margins of the continents, mountains sufficiently high
to have snow-clad summits, and to send down glaciers, bearing rocks
and stones to the sea, on which may have floated, as now in the North
Atlantic, huge icebergs.[W] This would be quite in accordance with
the great elevation of land which we know actually occurred; and the
existence of snow-clad mountains along with volcanoes would be a union
of fire and frost of which we still have examples in some parts of the
earth's surface, and this in proximity to forms of vegetable life very
similar to those which we know existed in the Permian.
[W] Ramsay has ably illustrated this in the Permian conglomerates of
England.
With the exception of a few small and worthless beds in Russia, the
Permian is not known to contain any coal. The great swamps of the coal
period had disappeared. In part they were raised up into rugged
mountains. In part they were sunken into shallow sea areas. Thus,
while there was much dry land, there was little opportunity for coal
production, or for the existence of those rank forests which had
accumulated so much vegetable matter in the Carboniferous age. In like
manner the fauna of the Permian waters is poor. According to
Murchison, the Permian limestones of Europe have afforded little more
then one-third as many species of fossils as the older Carboniferous.
The fossils themselves also have a stunted and depauperated aspect,
indicating conditions of existence unfavourable to them. This is
curiously seen in contrasting Davidson's beautiful illustrations of
the British Lamp-shells of the Permian and Carboniferous periods.
Another illustrative fact is the exceptionally small size of the
fossils even in limestones of the Carboniferous period when these are
associated with gypsum, red sandstones, and magnesian minerals; as,
for instance, those of some parts of Nova Scotia. In truth, the
peculiar chemical conditions conducive to the production of magnesian
limestones and gypsum are not favourable to animal life, though no
doubt compatible with its existence. Hence the rich fauna of the
Carboniferous seas died out in the Permian, and was not renewed; and
the Atlantic areas of the period are unknown to us. They were,
however, probably very deep and abrupt in slope, and not rich in life.
This would be especially the case if they were desolated by cold
ice-laden currents.
During the Permian period there was in each of our continental areas a
somewhat extensive inland sea. That of Western America was a northward
extension of the Gulf of Mexico. That of Eastern Europe was a
northward extension of the Euxine and Caspian. In both, the deposits
formed were very similar--magnesian limestones, sandstones,
conglomerates, marls, and gypsums. In both, these alternate in such a
way as to show that there were frequent oscillations of level,
producing alternately shallow and deep waters. In both, the animal
remains are of similar species, in many instances even identical. But
in the areas intervening between these sea basins and the Atlantic the
conditions were somewhat different. In Europe the land was interrupted
by considerable water areas, not lakes, but inland sea basins;
sometimes probably connected with the open sea, sometimes isolated. In
these were, deposited the magnesian limestone and its associated beds
in England, and the Zechstein and Rotheliegende with their associates
in Germany. In America the case was different. In all that immense
area which extends from the Atlantic to the plains east of the
Mississippi, we know no Permian rocks, unless a portion of those
reckoned as Upper Carboniferous, or Permo-carboniferous in Northern
Nova Scotia, and Prince Edward Island, should be included in this
group. If such existed, they may possibly be covered up in some places
by more modern deposits, or may have been swept away by denudation in
the intervening ages; but even in these cases we should expect to find
some visible remains of them. Their entire absence would seem to
indicate that a vast, and in many parts rugged and elevated, continent
represented North America in the Permian period. Yet if so, that great
continent is an absolute blank to us. We know nothing of the animals
or plants which may have lived on it, nor do we even know with
certainty that it had active volcanoes, or snow-clad mountains sending
down glaciers.
Our picture of the Permian World has not been inviting, yet in many
respects it was a world more like that in which we live then was any
previous one. It certainly presented more of variety and grand
physical features then any of the previous ages; and we might have
expected that on its wide and varied continents some new and higher
forms of life would have been introduced. But it seems rather to have
been intended to blot out the old Palæozoic life, as an arrangement
which had been fully tried and served its end, preparatory to a new
beginning in the succeeding age.
Still the Permian has some life features of its own, and we must now
turn to these. The first is the occurrence here, not only of the
representatives of the great Batrachians of the coal period, but of
true reptiles, acknowledged to be such by all naturalists. The animals
of the genus _Protorosaurus_, found in rocks of this age both in
England and Germany, were highly-organised lizards, having socketed
teeth like those of crocodiles, and well-developed limbs, with long
tails, perhaps adapted for swimming. They have, however, biconcave
vertebræ like the lizard-like animals of the coal already mentioned,
which, indeed, in their general form and appearance, they must have
very closely resembled. The Protorosaurs were not of great size; but
they must have been creatures of more stately gait then their
Carboniferous predecessors, and they serve to connect them with the
new and greater reptiles of the next period.
Another interesting feature of the Permian is its flora, which, in so
far as known, is closely related to that of the coal period, though
the species are regarded as different; some of the forms, however,
being so similar as to be possibly identical. In a picture of the
Permian flora we should perhaps place in the foreground the
tree-ferns, which seem to have been very abundant, and furnished with
dense clusters of aërial roots to enable them to withstand the storms
of this boisterous age. The tree-ferns, now so plentiful in the
southern hemisphere, should be regarded as one of the permanent
vegetable institutions of our world--those of the far-back Lower
Devonian, and of all intervening ages up to the present day, having
been very much alike. The great reed-like Calamites have had a
different fate. In their grander forms they make their last appearance
in the Permian, where they culminate in great ribbed stems, sometimes
nearly a foot in diameter, and probably of immense height. The brakes
of these huge mares'-tails which overspread the lower levels of the
Permian in Europe, would have been to us what the hayfields of
Brobdingnag were to Gulliver. The Lepidodendra also swarmed, though in
diminished force; but the great Sigillariæ of the coal are absent, or
only doubtfully present. Another feature of the Permian woods was the
presence of many pine-trees different in aspect from those of the coal
period. Some of these are remarkable for their slender and delicate
branches and foliage.[X] Others have more dense and scaly leaves, and
thick short cones.[Y] Both of these styles of pines are regarded as
distinct, on the one hand, from those of the coal formation, and on
the other from those of the succeeding Trias. I have shown, however,
many years ago, that in the upper coal formation of America there are
branches of pine-trees very similar to Walchia, and, on the other
hand, the Permian pines are not very remote in form and structure from
some of their modern relations. The pines of the first of the
above-mentioned types (Walchia) may indeed be regarded as allies of
the modern Araucarian pines of the southern hemisphere, and of the
old conifers of the Carboniferous. Those of the second type (Ulmannia)
may be referred to the same group with the magnificent Sequoias or
Redwoods of California.
[X] Walchia.
[Y] Ulmannia.
It is a curious indication of the doubts which sometimes rest on
fossil botany, that some of the branches of these Permian pines, when
imperfectly preserved, have been described as sea-weeds, while others
have been regarded as club-mosses. It is true, however, that the
resemblance of some of them to the latter class of plants is very
great; and were there no older pines, we might be pardoned for
imagining in the Permian a transition from club-mosses to pines.
Unfortunately, however, we have pines nearly as far back in geological
time as we have club-mosses; and, in so far as we know, no more like
the latter then are the pines of the Permian, so that this connection
fails us. In all probability the Permian forests are much less
perfectly known to us then those of the coal period, so that we can
scarcely make comparisons. It appears certain, however, that the
Permian plants are much more closely related to the coal plants then
to those of the next succeeding epoch, and that they are not so much a
transition from the one to the other as the finishing of the older
period to make way for the newer.
But we must reserve some space for a few remarks on the progress and
termination of the Palæozoic as a whole, and on the place which it
occupies in the world's history. These remarks we may group around the
central question, What is the meaning or value of an age or period in
the history of the earth, as these terms are understood by geologists?
In most geological books terms referring to time are employed very
loosely. Period, epoch, age, system, series, formation, and similar
terms, are used or abused in a manner which only the indefiniteness of
our conceptions can excuse.
A great American geologist[Z] has made an attempt to remedy this by
attaching definite values to such words as those above mentioned. In
his system the greater divisions of the history were "Times:" thus the
Eozoic was a time and the Palæozoic was a time. The larger divisions
of the times are "Ages:" thus the Lower and Upper Silurian, the
Devonian, and the Carboniferous are ages, which are equivalent in the
main to what English geologists call Systems of Formations. Ages,
again, may be divided into "Periods:" thus, in the Upper Silurian, the
Ludlow of England, or Lower Helderberg of America, would constitute a
period. These periods may again be divided into "Epochs," which are
equivalent to what English geologists call Formations, a term
referring not directly to the time elapsed, but to the work done in
it. Now this mode of regarding geological time introduces many
thoughts as to the nature of our chronology and matters relating to
it. A "time" in geology is an extremely long time, and the Palæozoic
was perhaps the longest of the whole. By the close of the Palæozoic
nine-tenths of all the rocks we know in the earth's crust were
formed. At least this is the case if we reckon mere thickness. For
aught that we know, the Eozoic time may have accumulated as much rock
as the Palæozoic; but leaving this out of the question, the rocks of
the Palæozoic are vastly thicker then those of the Mesozoic and
Cainozoic united. Thus the earth's history seems to have dragged
slowly in its earlier stages, or to have become accelerated in its
latter times. To place it in another point of view, life changes were
greater relatively to merely physical changes in the later then in the
earlier times.
[Z] Dana.
The same law seems to have obtained within the Palæozoic time itself.
Its older periods, as the Cambrian and Lower Silurian, present immense
thicknesses of rock with little changes in life. Its later periods,
the Carboniferous and Permian, have greater life-revolution relatively
to less thickness of deposits. This again was evidently related to the
growing complexity and variety of geographical conditions, which went
on increasing all the way up to the Permian, when they attained their
maximum for the Palæozoic time.
Again, each age was signalized, over the two great continental
plateaus, by a like series of elevations and depressions. We may
regard the Siluro-Cambrian, the Silurian, the Devonian, the
Carboniferous, and Permian, as each of them a distinct age. Each of
these began with physical disturbances and coarse shallow-water
deposits. In each this was succeeded by subsidence and by a sea area
tenanted by corals and shell-fishes. In each case this was followed by
a re-elevation, leading to a second but slow and partial subsidence,
to be followed by the great re-elevation preparatory to the next
period. Thus we have throughout the Palæozoic a series of cycles of
physical change which we may liken to gigantic pulsations of the thick
hide of mother earth. The final catastrophe of the Permian collapse
was quite different in kind from these pulsations as well as much
greater in degree. The Cambrian or Primordial does not apparently
present a perfect cycle of this kind, perhaps because in that early
period the continental plateaus were not yet definitely formed, and
thus its beds are rather portions of the general oceanic deposit. In
this respect it is analogous in geological relations to the chalk
formation of a later age, though very different in material. The
Cambrian may, however, yet vindicate its claim to be regarded as a
definite cycle: and the recent discoveries of Hicks in North Wales,
have proved the existence of a rich marine fauna far down in the lower
part of this system. It is also to be observed that the peculiar
character of the Cambrian, as an oceanic bottom rather then a
continental plateau, has formed an important element in the
difficulties in establishing it as a distinct group; just as a similar
difficulty in the case of the chalk has led to a recent controversy
about the continuance of the conditions of that period into modern
times.
But in each of the great successive heaves or pulsations of the
Palæozoic earth, there was a growing balance in favour of the land as
compared with the water. In each successive movement more and more
elevated land was thrown up, until the Permian flexures finally fixed
the forms of our continents. This may be made evident to the eye in a
series of curves, as in the following diagram, in which I have
endeavoured to show the recurrence of similar conditions in each of
the great periods of the Palæozoic, and thus their equivalency to each
other as cycles of the earth's history.
There is thus in these great continental changes a law of recurrence
and a law of progress; but as to the efficient causes of the phenomena
we have as yet little information. It seems that original fractures
and shrinkages of the crust were concerned in forming the continental
areas at first. Once formed, unequal burdening of the earth's still
plastic mass by deposits of sediment in the waters, and unequal
expansion by the heating and crystallization of immense thicknesses of
the sediment, may have done the rest; but the results are surprisingly
regular to be produced by such causes. We shall also find that similar
cycles can be observed in the geological ages which succeeded the
Palæozoic. Geologists have hitherto for the most part been content to
assign these movements to causes purely terrestrial; but it is
difficult to avoid the suspicion that the succession of geological
cycles must have depended on some recurring astronomical force tending
to cause the weaker parts of the earth's crust alternately to rise and
subside at regular intervals of time. Herschel, Adhémar, and more
recently Croll, have directed attention to astronomical cycles
supposed to have important influences on the temperature of the earth.
Whether these or other changes may have acted on the equilibrium of
its crust is a question well worthy of attention, as its solution
might give us an astronomical measure of geological time. This
question, however, the geologist must refer to the astronomer.
[Illustration: CURVES SHOWING THE SUCCESSIVE ELEVATIONS AND
DEPRESSIONS OF THE AMERICAN CONTINENT, IN SEVERAL CYCLES OF THE
PALÆOZOIC TIME.]
There are two notes of caution which must here be given to the reader.
First, it is not intended to apply the doctrine of continental
oscillations to the great oceanic areas. Whether they became shallower
or deeper, their conditions would be different from those which
occurred in the great shallow plateaus, and these conditions are
little known to us. Further, throughout the Palæozoic period, the
oscillations do not seem to have been sufficient to reverse the
positions of the oceans and continents. Secondly, it is not meant to
affirm that the great Permian plications were so widespread in their
effects as to produce a universal destruction of life. On the
contrary, after they had occurred, remnants of the Carboniferous fauna
still flourished even on the surfaces of the continents, and possibly
the inhabitants of the deep ocean were little affected by these great
movements. True it is that the life of the Palæozoic terminates with
the Permian, but not by a great and cataclysmic overthrow.
We know something at least of the general laws of continental
oscillations during the Palæozoic. Do we know anything of law in the
case of life? The question raises so many and diverse considerations
that it seems vain to treat it in the end of a chapter; still we must
try to outline it with at least a few touches.
First, then, the life of the Palæozoic was remarkable, as compared with
that of the present world, in presenting a great prevalence of animals
and plants of synthetic types, as they are called by Agassiz that is,
of creatures comprehending in one the properties of several groups
which were to exist as distinct in the future. Such types are also
sometimes called embryonic, because the young of animals and plants
often show these comprehensive features. Such types were the old
corals, presenting points of alliance with two distinct groups now
widely separated; the old Trilobites, half king-crabs and half
Isopods; the Amphibians of the coal, part fish, part newt, and part
crocodile; the Sigillariæ, part club-mosses and part pines; the
Orthoceratites, half nautili and half cuttle-fishes. I proposed, in
the illustration in a former article, to give a restoration of one of
the curious creatures last mentioned, the Orthoceratites; but on
attempting this, with the idea that, as usually supposed, they were
straight Nautili, it appeared that the narrow aperture, the small
outer chamber, the thin outer wall, often apparently only membranous,
and the large siphuncle, would scarcely admit of this; and I finished
by representing it as something like a modern squid; perhaps wrongly,
but it was evidently somewhere between them and the Nautili.
Secondly, these synthetic types often belonged to the upper part of a
lower group, or to the lower part of an upper group. Hence in one
point of view they may be regarded as of high grade, in another as of
low grade, and they are often large in size or in vegetative
development.[AA] From this law have arisen many controversies about
the grade and classification of the Palæozoic animals and plants.
[AA] It seems, indeed, as if the new synthetic forms intermediate
between great groups were often large in size, while the new special
types came in as small species. There are some remarkable cases of
this in the plant world; though here we have such examples as the
pines and tree-ferns continuing almost unchanged from an early
Palæozoic period until now.
Thirdly, extinctions of species occur in every great oscillation of
the continental areas, but some species reappear after such
oscillations, and the same genus often recurs under new specific
forms. Families and orders, such as those of the Trilobites and
Orthoceratites, appear to have a grand and gradual culmination and
decadence extending over several successive periods, or even over the
whole stretch of the Palæozoic time. Toward the close of the
Palæozoic, while all the species disappear, some whole families and
orders are altogether dropped, and, being chiefly synthetic groups,
are replaced by more specialised types, some of which, however, make
small beginnings alongside of the more general types which are passing
away. Our diagram (page 183) illustrates these points.
[Illustration: DIAGRAM SHOWING THE ADVANCE, CULMINATION, AND DECADENCE
OF SOME OF THE LEADING TYPES OF PALÆOZOIC LIFE.]
Fourthly, the progress in animal life in the Palæozoic related chiefly
to the lower or invertebrate tribes, and to the two lower classes of
the vertebrates. The oldest animal known to us is not only a creature
of the simplest structure, but also a representative of that great and
on the whole low type of animal life, in which the parts are arranged
around a central axis, and not on that plan of bilateral symmetry
which constitutes one great leading distinction of the higher animals.
With the Cambrian, bilateral animals abound and belong to two very
distinct lines of progress--the one, the Mollusks, showing the
nutritive organs more fully developed--the other, the Articulates,
having the organs of sense and of locomotion more fully organized.
These three great types shared the world among them throughout the
earlier Palæozoic time, and only in its later ages began to be
dominated by the higher types of fishes and reptiles. In so far as we
know, it remained for the Mesozoic to introduce the birds and mammals.
In plant life the changes were less marked, though here also there is
progress--land plants appear to begin, not with the lowest forms, but
with the highest types of the lower of the two great series into which
the vegetable kingdom is divided. From this they rapidly rise to a
full development of the lowest type of the flowering plants, the pines
and their allies, and there the progress ceases; for the known
representatives of the higher plants are extremely few and apparently
of little importance.
Fifthly, in general the history tells of a continued series of
alternate victories and defeats of the species that had their birth on
the land and in the shallow waters, and those which were born in the
ocean depths, The former spread themselves widely after every
upheaval, and then by every subsidence were driven back to their
mountain fastnesses. The latter perished from the continental plateaus
at every upheaval, but climbed again in new hordes and reoccupied the
ground after every subsidence. But just as in human history every
victory or defeat urges on the progress of events, and develops the
great plan of God's providence in the elevation of man; so here every
succeeding change brings in new and higher actors on the stage, and
the scheme of creation moves on in a grand and steady progress towards
the more varied and elevated life of the Modern World.
But, after all, how little do we know of these laws, which are only
beginning to dawn on the minds of naturalists; and which the
imperfections of our classification and nomenclature, and the defects
in our knowledge of fossil species, render very dim and uncertain. All
that appears settled is the existence of a definite plan, working over
long ages, and connected with the most remarkable correlation of
physical and organic change: going on with regular march throughout
the Palæozoic, and then brought to a close to make room for another
great succession. This following Mesozoic time must next engage our
attention.
We may close for the present with presenting to the eye in tabular
form the periods over which we have passed. The table on page 187, and
the diagram (page 179), mutually illustrate each other; and it will
be seen that each age constitutes cycle, similar in its leading
features to the other cycles, while each is distinguished by some
important fact in relation to the introduction of living beings. In
this table I have, with Mr. Hull,[AB] for simplicity, arranged the
formations of each age under three periods--an older, middle, and
newer. Of these, however, the last or newest is in each case so
important and varied as to merit division into two, in the manner
which I have suggested in previous publications for the Palæozoic
rocks of North America.[AC] Under each period I have endeavoured to
give some characteristic example from Europe and America, except
where, as in the case of the coal formation, the same names are used
on both continents. Such a table as this, it must be observed, is only
tentative, and may admit of important modifications. The Laurentian
more especially may admit of division into several ages; and a
separate age may be found to intervene between it and the Cambrian.
The reader will please observe that this table refers to the changes
on the continental plateaus; and that on both of these each age was
introduced with shallow water and usually coarse deposits, succeeded
by deeper water and finer beds, usually limestones, and these by a
mixed formation returning to the shallow water and coarse deposits of
the older period of the age. This last kind of deposition culminates
in the great swamps of the coal formation.
[AB] "Quarterly Journal of Science," July, 1869.
[AC] "Acadian Geology," p. 137.
CONDENSED TABULAR VIEW OF THE AGES AND PERIODS OF THE PALÆOZOIC AND
EOZOIC.
Key to Symbols
### Tabulate and Rugose Corals, abundant.
*** Age of Algæ.
=== Age of Acrogens and Gymnosperms.
+++ And God said, "Let the waters bring forth abundantly
the swarming living creatures."
--- And God created great reptiles.
TIMES.
AGES. PERIODS. ANIMALS AND PLANTS.
PALÆOZOIC
{ {Newer. Red Sandstones, #
{ Rauchwacke, etc. # Beginning = -
{Permian {Middle. Zechstein, or # of Age = -
{ Magnesian Limestone. # of Reptiles. = -
{ {Older. Conglomerates, etc., # = -
{ Rotheliegendes. # = -
{ # = -
{ {N. Coal Formation. # = -
{Carboniferous {M. Carboniferous Limestone. # Age of = -
{ {O. Lower Coal Measures and # Batrachians. =
{ Conglomerates. # =
{ # =
{ {N. Upper Old Red, Chemung. # =
{Devonian {M. Eifel and Corniferous # =
{ { Limestones. # Age of Fishes. =
{ or Erian {O. Lower Old Red, Oriskany # =
{ { Sandstone. # = +
{ # +
{ {N. Ludlow, Lower Helderberg. # +
{Upper Silurian {M. Wenlock and Niagara # +
{ { Limestones. # Age of +
{ {O. Mayhill, etc., Oneida # Mollusks. +
{ { Conglomerates. # +
{ # +
{ {N. Caradoc, Hudson R. # +
{Lower Silurian {M. Bala and Trenton # +
{ or { Limestones. # * +
{Siluro-Cambrian {O. Llandielo, etc., Chazy. # * +
{ # * +
{ {N. Lingula Flags, etc., * +
{ { Potsdam Sandstone. * +
{ { {Acadian, etc.? Age of * +
{Cambrian {M. (Uncertain){ Crustaceans. * +
{ { {Menevian? * +
{ {O. Longmynd, Huronian? +
+
EOZOIC +
+
{ {N. Anorthosite Gneiss, etc. +
{Laurentian {M. Eozoon Limestones, etc. Age of +
{ {O. Lower Gneiss. Protozoa. +