can also point to Tertiary representatives of the _S. gigantea_. Their
leases are stiff and sharp-pointed, are thinly set round the branches,
and lie forward in the same way: the egg-shaped cones are in some cases
similar.
There are, however, in the early Tertiary six species, which fill up
the gap between _S. sempervirens_ and _S. gigantea_. They are the _S.
Couttsiæ_, _S. affinis_, Lesq., _S. imbricata_, Hr., _S. sibirica_, Hr.,
_S. Heerii_, Lesq., and _S. biformis_, Lesq. Of these, _S. Couttsiæ_,
Hr., is the most common and most important species. It has short leaves,
lying along the branch, like _S. gigantea_, and small, round cones, like
_S. Langsdorfii_ and _sempervirens_. Bovey Tracey in Devonshire has
afforded splendid specimens of cones, seeds, and twigs, which have been
described in the "Philosophical Transactions." More lately, Count Saporta
has described specimens of cones and twigs from Armissan. Specimens of
this species have also been found in the older Tertiary of Greenland, so
that it must have had a wide range. It is very like to the American _S.
affinis_, Lesq.
In the Tertiary there have been already found fourteen well-marked
species, which thus include representatives of the two living types, _S.
sempervirens_ and _S. gigantea_.
We can follow this genus still further back. If we go back to the
Cretaceous age, we find ten species, of which five occur in the Urgon
of the Lower Cretaceous, two in the Middle, and three in the Upper
Cretaceous. Among these, the Lower Cretaceous exhibits the two types
of the Sequoia sempervirens and _S. gigantea_. To the former the _S.
Smithiana_ answers, and to the latter, the _Reichenbachii_, Gein. The
_S. Smithiana_ stands indeed uncommonly near the _S. Langsdorfii_, both
in the appearance of the leaves on the twigs and in the shape of the
cones. These are, however, smaller, and the leaves do not become narrower
toward the base. The _S. pectina_, Hr., of the Upper Cretaceous, has its
leaves arranged in two rows, and presents a similar appearance. The _S.
Reichenbachii_ is a type more distinct from those now living and those
in the Tertiary. It has indeed stiff, pointed leaves, lying forward, but
they are arcuate, and the cones are smaller. This tree has been known
for a long time, and it serves in the Cretaceous as a guiding star,
which we can follow from the Urgonian of the Lower Cretaceous up to the
Cenomanian. It is known in France, Belgium, Bohemia, Saxony, Greenland,
and Spitzbergen (also in Canada and the United States). It has been
placed in another genus--Geinitzia--but we can recognise, by the help of
the cones, that it belongs to Sequoia.
Below this, there is found in Greenland a nearly related species, the _S.
ambigua_, Hr., of which the leaves are shorter and broader, and the cones
round and somewhat smaller.
The connecting link between _S. Smithiana_ and _Reichenbachii_ is formed
by _S. subulata_, Hr., and _S. rigida_, Hr., and three species (_S.
gracilis_, Hr., _S. fastigiata_ and _S. Gardneriana_, Carr.), with leaves
lying closely along the branch, and which come very near to the Tertiary
species _S. Couttsiæ_. We have therefore in the Cretaceous quite an
array of species, which fill up the gap between the _S. sempervirens_
and _gigantea_, and show us that the genus Sequoia had already attained
a great development in the Cretaceous. This was still greater in
the Tertiary, in which it also reached its maximum of geographical
distribution. Into the present world the two extremes of the genus have
alone continued; the numerous species forming its main body have fallen
out in the Tertiary.
If we look still further back, we find in the Jura a great number of
conifers, and, among them, we meet in the genus Pinus with a type which
is highly developed, and which still survives; but for Sequoia we have
till now looked in vain, so that for the present we can not place the
rise of the genus lower than the Urgonian of the Cretaceous, however
remarkable we may think it that in that period it should have developed
into so many species; and it is still more surprising that two species
already make their appearance which approach so near to the living
_Sequoia sempervirens_ and _S. gigantea_.
Altogether, we have become acquainted, up to the present time, with
twenty-six species of Sequoia. Fourteen of these species are found in the
Arctic zone, and have been described and figured in the "Fossil Flora of
the Arctic Regions." Sequoia has been recognised by Ettingshausen even in
Australia, but there in the Eocene.
This is, perhaps, the most remarkable record in the whole history of
vegetation. The Sequoias are the giants of the conifers, the grandest
representatives of the family, and the fact that, after spreading over
the whole northern hemisphere and attaining to more than twenty specific
forms, their decaying remnant should now be confined to one limited
region in western America and to two species constitutes a sad memento
of departed greatness.[DJ] The small remnant of _S. gigantea_ still,
however, towers above all competitors, as eminently the "big trees ";
but, had they and the allied species failed to escape the Tertiary
continental submergences and the disasters of the glacial period, this
grand genus would have been to us an extinct type. In like manner the
survival of the single gingko of eastern Asia alone enables us to
understand that great series of taxine trees with fern-like leaves of
which it is the sole representative.
[DJ] The writer has shown that much of the material of the great lignite
beds of the Canadian Northwest consists of wood of Sequoia of both the
modern types.
Besides these peculiar and now rare forms, we have in the Mesozoic many
others related closely to existing yews, cypresses, pines, and spruces,
so that the conifers were probably in greater abundance and variety than
they are at this day.
In this period, also, we find the earliest representatives of
the endogenous plants. It is true that some plants found in the
coal-formation have been doubtfully referred to these, but the earliest
certain examples would seem to be some bamboo-like and screw-pine-like
plants occurring in the Jurassic rocks. Some of these are, it is true,
doubtful forms, but of others there seems to be no question. The modern
_Pandanus_ or screw-pine of the tropical regions, which is not a pine,
however, but a humble relation of the palms, is a stiffly branching
tree, of a candelabra-like form, and with tufts of long leaves on its
branches, and nuts or great hard berries for fruit, borne sometimes in
large masses, and so protected as to admit of their drifting uninjured
on the sea. The stems are supported by masses of aërial roots like those
which strengthen the stems of tree-ferns. These structures and habits
of growth fit the Pandanus for its especial habitat on the shores of
tropical islands, to which its masses of nuts are drifted by the winds
and currents, and on whose shores it can establish itself by the aid of
its aërial roots.
Some plants referred to the cycads have proved veritable botanical
puzzles. One of these, the _Williamsonia gigas_ of the English oölite,
originally discovered by my friend Dr. Williamson, and named by him
_Zamia gigas_, a very tall and beautiful species, found in rocks of this
age in various parts of Europe, has been claimed by Saporta for the
Endogens, as a plant allied to _Pandanus_. Some other botanists have
supposed the flowers and fruits to be parasites on other plants, like the
modern _Rafflesia_ of Sumatra, but it is possible that after all it may
prove to have been an aberrant cycad.
The tree-palms are not found earlier than the Middle Cretaceous, where
we shall notice them in the next chapter. In like manner, though a
few Angiosperms occur in rocks believed to be Lower or Lower Middle
Cretaceous in Greenland and the northwest territory of Canada, and in
Virginia, these are merely precursors of those of the Upper Cretaceous,
and are not sufficient to redeem the earlier Cretaceous from being a
period of pines and cycads.
On the whole, this early Mesozoic flora, so far as known to us, has a
monotonous and mean appearance. It no doubt formed vast forests of tall
pines, perhaps resembling the giant Sequoias of California; but they must
for the most part have been dark and dismal woods, probably tenanted by
few forms of life, for the great reptiles of this age must have preferred
the open and sunny coasts, and many of them dwelt in the waters. Still
we must not be too sure of this. The berries and nuts of the numerous
yews and cycads were capable of affording much food. We know that in
this age there were many great herbivorous reptiles, like _Iguanodon_
and _Hadrosaurus_, some of them fitted by their structure to feed upon
the leaves and fruits of trees. There were also several kinds of small
herbivorous mammals, and much insect life, and it is likely that few of
the inhabitants of the Mesozoic woods have been preserved as fossils.
We may yet have much to learn of the inhabitants of these forests of
ferns, cycads, and pines. We must not forget in this connection that in
the present day there are large islands, like New Zealand, destitute of
mammalia, and having a flora comparable with that of the Mesozoic in the
northern hemisphere, though more varied. We have also the remarkable
example of Australia, with a much richer flora than that of the early
Mesozoic, yet inhabited only by non-placental mammals, like those of the
Mesozoic.
The principal legacy that the Mesozoic woods have handed down to our
time is in some beds of coal, locally important, but of far less extent
than those of the Carboniferous period. Still, in America, the Richmond
coal-field in Virginia is of this age, and so are the anthracite beds of
the Queen Charlotte Islands, on the west coast of Canada, and the coal
of Brora in Sutherlandshire. Valuable beds of coal, probably of this
age, also exist in China, India, and South Africa; and jet, which is so
extensively used for ornament, is principally derived from the carbonised
remains of the old Mesozoic pines.
In the next chapter we have to study a revolution in vegetable life most
striking and unique, in the advent of the forest-trees of strictly modern
types.
NOTE TO CHAPTER V.
I append to this chapter a table showing the plant-bearing series of the
Cretaceous and Laramie of North America, from a paper in "Trans. R. S.
C," 1885, which see for further details:
(In Descending Order.)
Periods. Floras and sub-floras. References.
--------------+---------------------------+-----------------------------
Transition Upper Laramie or Porcupine { Platanus beds of Souris
Eocene to Hill. Fort Union { River and Calgary. Report
Cretaceous. group, U. S. territory. { of Geol. Survey of Canada
{ for 1879, and Memoir of
{ 1885.
--------------+---------------------------+-----------------------------
Middle Laramie or Willow
Creek beds.
{ Lemna and Pistia beds of
Lower Laramie or St. { bad lands of 49th parallel,
Mary River. { Red Deer River, &c., with
Upper { lignites. Report 49th
Cretaceous { Parallel and Memoir of
{ 1885.
(Danian and
Senonian). Fox Hill series Marine.
Fort Pierre series Marine.
{ Sequoia and Brasenia beds
Belly River { of S. Saskatchewan, Belly
{ River, &c. with lignites.
{ Memoir of 1885.
Coal measures of Nanaimo, { Memoir of 1883. Many
B.C., probably here. { dicotyledons, palms, &c.
--------------+---------------------------+-----------------------------
Middle Dunvegan series of Peace }
Cretaceous River. Dakota group, } Memoir of 1883. Many
(Turonian and U. S. Amboy clays, } dicotyledons, cycads, &c.
Cenomanian). U. S. }
Mill Creek beds of Rocky { Dicotyledonous leaves,
Mountains. { similar to Dakota group of
{ the U. S. Memoir of 1885.
--------------+---------------------------+-----------------------------
Lower Suskwa River beds and }
Cretaceous Queen Charlotte Island } Cycads, pines, a few
(Neocomian. coal series. Intermediate } dicotyledons. Report Geol.
&c). beds of Rocky } Survey. Memoir of 1885.
Mountains. Potomac }
series of Virginia. }
Kootanie series of Rocky { Cycads, pines, and ferns.
Mountains. { Memoir of 1885.
--------------+---------------------------+------------------------------
CHAPTER VI.
THE REIGN OF ANGIOSPERMS IN THE LATER CRETACEOUS
AND KAINOZOIC.
[Illustration: Fig. 68.--_Populus primæva_, Heer. Cretaceous, of
Greenland. One of the oldest known Angiosperms.]
It is a remarkable fact in geological chronology that the culmination
of the vegetable kingdom antedates that of the animal. The placental
mammals, the highest group of the animal kingdom, are not known till the
beginning of the Eocene Tertiary. The dicotyledonous Angiosperms, which
correspond to them in the vegetable kingdom, occur far earlier--in the
beginning of the Upper Cretaceous or close of the Lower Cretaceous. The
reign of cycads and pines holds throughout the Lower Cretaceous, but at
the close of that age there is a sudden incoming of the higher plants,
and a proportionate decrease, more especially of the cycads.
I have already referred to the angiospermous wood supposed to be
Devonian, but I fear to rest any conclusion on this isolated fact.
Beyond this, the earliest indications of plants of this class have
been found in the Lower Cretaceous. Many years ago Heer described and
figured the leaves of a poplar (_Populus primæva_) from the supposed
Lower Cretaceous of Komé, in Greenland (Fig. 68). Two species, a
_Sterculia_ and a _Laurus_ or _Salix_, occur among fossils described by
me in the upper part of the Kootanie series of the Rocky Mountains, and
Fontaine has recently found in the Potomac group of Virginia--believed
to be of Neocomian age--several angiospermous species (_Sassafras_,
_Menispermites_, _Sapindus_, _Aralia_, _Populus_, &c.) mixed with a rich
flora of cycads and pines. These are the early forerunners of the modern
angiospermous flora; but so far as known they do not occur below the
Cretaceous, and in its lower portions only very rarely. When, however, we
ascend into the Upper Cretaceous, whether of Europe or America, there is
a remarkable incoming of the higher plants, under generic forms similar
to those now existing. This is, in truth, the advent of the modern flora
of the temperate regions of the earth. A very interesting tabular view
of its early distribution is given by Ward, in the "American Journal of
Science" for 1884, of which the following is a synopsis, with slight
emendations. I may add that the new discoveries made since 1884 would
probably tend to increase the proportionate number of dicotyledons in the
newer groups.
Dicotyledonous Trees in the Cretaceous.
_Upper Senonian_ 179 species.
(Fox Hill group of America.)
_Lower Senonian_ 81 species.
Upper white chalk of Europe; Fort Pierre
group of America; coal-measures of Nanaimo?
_Turonian_ 20 species.
Lower white chalk; New Jersey marls;
Belly R. group.
_Cenomanian._ 357 species.
(Chalk-marl, greensand, and Gault, Niobrara
and Dakota groups of America); Dunvegan
group of Canada; Amboy clays of New Jersey.
_Neocomian_ 20 species.[DK]
(Lower greensand and Speeton clay, Wealden
and Hastings sands, Kootanie and Queen
Charlotte groups of Canada.)
[DK] Including an estimate of Fontaine's undescribed species.
Thus we have a great and sudden inswarming of the higher plants of
modern types at the close of the Lower Cretaceous. In relation to this,
Saporta, one of the most enthusiastic of evolutionists, is struck by
this phenomenon of the sudden appearance of so many forms, and some of
them the most highly differentiated of dicotyledonous plants. The early
stages of their evolution may, he thinks, have been obscure and as yet
unobserved, or they may have taken place in some separate region, or
mother country as yet undiscovered, or they may have been produced by
a rapid and unusual multiplication of flower-haunting insects! Or it
is even conceivable that the apparently sudden elevation of plants may
have been due to causes still unknown. This last seems, indeed, the only
certain inference in the case, since, as Saporta proceeds to say in
conclusion: "Whatever hypothesis one may prefer, the fact of the rapid
multiplication of dicotyledons, and of their simultaneous appearance in a
great number of places in the northern hemisphere at the beginning of the
Cenomanian epoch, cannot be disputed."[DL]
[DL] "Monde des Plantes," p. 197.
The leaves described by Heer, from the Middle Cretaceous of Greenland,
are those of a poplar (_P. primæva_). Those which I have described from
a corresponding horizon in the Rocky Mountains are a _Sterculites_ (_S.
vetustula_), probably allied to the mallows, and an elongated leaf,
_Laurophyllum_ (_L. crassinerve_) (Fig. 69), which may, however, have
belonged to a willow rather than a laurel. These are certainly older than
the Dakota group of the United States and the corresponding formations
in Canada. On the eastern side of the American continent, in Virginia,
the Potomac series is supposed to be of Lower Cretaceous age, and here
Fontaine, as already stated, has found an abundant flora of cycads,
conifers, and ferns, with a few angiospermous leaves, which have not yet
been described.
[Illustration: Fig. 69.--_Stercalia_ and _Laurophyllum_ or Salix, the
oldest Angiosperms Known in the Cretaceous of Canada.]
In the Canadian Rocky Mountains, a few hundreds of feet above the beds
holding the beforementioned species, are the shales of the Mill Creek
series, rich in many species of dicotyledonous leaves, and corresponding
in age with the Dakota group, whose fossils have been so well described,
first by Heer and Capellini, and afterward by Lesquereux. We may take
this Dakota group and the quader-sand stone of Germany as types of the
plant-bearing Cenomanian, and may notice the forms occurring in them.
In the first place, we recognise here the successors of our old friends,
the ferns and the pines, the latter represented by such genera as
_Taxites_, _Sequoia_, _Glyptostrobus_, _Gingko_, and even _Pinus_ itself.
We also have a few cycads, but not so dominant as in the previous
ages. The fan-palms are well represented, both in America and in the
corresponding series in Europe, especially by the genus _Sabal_, which
is the characteristic American type of fan-palm, and there is one genus
which Saporta regards as intermediate between the fan-palms and the
pinnately leaved species. There are also many fragments of stems and
leaves of carices and grasses, so that these plants, now so important
to the nourishment of man and his companion animals, were already
represented.
[Illustration: Fig. 70.--Vegetation of Later Cretaceous. Exogens and
palms. (After Saporta.)]
But the great feature of the time was its dicotyledonous forests, and I
have only to enumerate the genera supposed to be represented in order
to show the richness of the time in plants of this type. It may be
necessary to explain here that the generic names used are mostly based
on leaves, and consequently cannot be held as being absolutely certain,
since we know that at present one genus may have considerable variety
in its leaves, and, on the other hand, that plants of different genera
may be very much alike in their foliage. There is, however, undoubtedly
a likeness in plan or type of structure in leaves of closely allied
plants, and, therefore, if judiciously studied, they can be determined
with at least approximate certainty.[DM] More especially we can attain
to much certainty when the fruits as well as the leaves are found,
and when we can obtain specimens of the wood, showing its structure.
Such corroboration is not wanting, though unfortunately the leaves
of trees are generally found drifted away from the other organs once
connected with them. In my own experience, however, I have often found
determinations of the leaves of trees confirmed by the discovery of their
fruits or of the structure of their stems. Thus, in the rich cretaceous
plant-beds of the Dunvegan series we have beech-nuts associated in
the same beds with leaves referred to _Fagus_. In the Laramie beds I
determined many years ago nuts of the _Trapa_ or water-chestnut, and
subsequently Lesquereux found, in beds in the United States, leaves which
he referred to the same genus. Later, I found in collections made on the
Red Deer River of Canada my fruits and Lesquereux's leaves on the same
slab. The presence of trees of the genera _Carya_ and _Juglans_ in the
same formation was inferred from their leaves, and specimens have since
been obtained of silicified wood, with the microscopic structure of the
modern butternut. Still we are willing to admit that determinations from
leaves alone are liable to doubt.
[DM] Great allowance has to be made for the variability of leaves of the
same species. The modern hazel (_C. rostrata_) is a case in point. Its
leaves, from different parts of the same plant, are so dissimilar in form
and size that they might readily be regarded as of different species.
In the matter of names of fossil leaves, I sympathise very strongly
with Dr. Nathorst, of Stockholm, in his objection to the use of modern
generic names for mere leaves, and would be quite content to adopt some
non-committal termination, as that of "_phyllum_" or "_ites_" suggested
by him. I feel, however, that almost as much is taken for granted if a
plant is called Corylophyllum or _Corylites_, as if called _Corylus_. In
either case a judgment is expressed as to its affinities, which if wrong
under the one term is wrong under the other; and after so much has been
done by so many eminent botanists, it seems inexpedient to change the
whole nomenclature for so small and questionable an advantage. I wish
it, however, to be distinctly understood that plants catalogued on the
evidence of leaves alone are for the most part referred to certain genera
on grounds necessarily imperfect, and their names are therefore subject
to correction, as new facts may be obtained.
The more noteworthy modern genera included in the Dakota flora, as
catalogued by Lesquereux, are the following: _Liquidambar_, the
sweet-gum, is represented both in America and Europe, the leaves
resembling those of the modern species, but with entire edges, which
seems to be a common peculiarity of Cretaceous foliage.[DN] _Populus_
(poplar), as already stated, appears very early in Greenland, and
continues with increasing number of species throughout the Cretaceous and
Tertiary. _Salix_ (willow) appears only a little later and continues. Of
the family _Cupuliferæ_ we have _Fagus_ (beech), _Quercus_ (oak), and
_Castanea_ (chestnut), which appear together in the Dakota group and
its equivalents. Fruits of some of the species are known, and also wood
showing structure. _Betula_ (birch) is represented by a few species,
and specimens of its peculiar bark are also common. _Alnus_ (alder)
appears in one species at least. The genus Plat anus (Fig. 71), that of
the plane-trees, represented at present by one European and one American
species, has several species in the Cretaceous, though the plane-trees
seem to culminate in the early part of the succeeding Eocene, where
there are several species with immense leaves. The large leaves,
known as _Credneria_, found in the Cenomanian of Europe, and those
called _Protophyllum_ (Fig. 72) in America, appear to be nearer to the
plane-trees than to any others, though representing an extinct type. The
laurels are represented in this age, and the American genus Sassafras,
which has now only one species, has not one merely but several species in
the Cretaceous. _Diospyros_, the persimmon-tree, was also a Cretaceous
genus.
[DN] With reference to this, something may be learned from the leaves
of modern trees. In these, young shoots have leaves often less toothed
and serrated than those of the adult tree. A remarkable instance is the
_Populus grandidentatus_ of America, the young shoots of which have
entire leaves, quite unlike except in venation those of the parent tree,
and having an aspect very similar to that of the Cretaceous poplars.
[Illustration: Fig. 71.--_Platanus nobilis_, Newberry, variety
_basilobata_. Laramie. Much reduced.]
[Illustration: Fig. 72.--_Protophyllum boreale_, Dawson, reduced. Upper
Cretaceous, Canada.]
The single species of the beautiful _Liriodendron_, or tulip-tree, is
a remnant of a genus which had several Cretaceous species (Figs. 74,
75). The magnolias, still well represented in the American flora, were
equally plentiful in the Cretaceous (Fig. 73). The walnut family were
well represented by species of _Juglans_ (butternut) and _Carya_, or
hickory. In all, no less than forty-eight genera are present belonging
to at least twenty-five families, running through the whole range of the
dicotyledonous exogens. This is a remarkable result, indicating a sudden
profusion of forms of these plants of a very striking character. It is
further to be observed that some of the genera have many species in the
Cretaceous and dwindle toward the modern. In others the reverse is the
case--they have expanded in modern times. In a number there seems to have
been little change.
[Illustration: Fig. 73.--_Magnolia magnifica_, Dawson, reduced. Upper
Cretaceous, Canada.]
Dr. Newberry has given, in the "Bulletin of the Torrey Botanical Club" an
interesting _résumé_ of the history of the beautiful _Liriodendron_, or
tulip-tree, which may be taken as an example of a genus which has gone
down in importance in the course of its geological history.
"The genus _Liriodendron_, as all botanists know, is represented in the
present flora by a single species, 'the tulip-tree' which is confined
to eastern America, but grows over all the area lying between the Lakes
and the Gulf, the Mississippi and the Atlantic. It is a magnificent
tree, on the whole, the finest in our forests. Its cylindrical trunk,
sometimes ten feet in diameter, carries it beyond all its associates in
size, while the beauty of its glossy, lyre-shaped leaves and tulip-like
flowers is only surpassed by the flowers and foliage of its first cousin,
_Magnolia grandiflora_. That a plant so splendid should stand quite alone
in the vegetation of the present day excited the wonder of the earlier
botanists, but the sassafras, the sweet-gum, and the great Sequoias of
the far West afford similar examples of isolation, and the latter are
still more striking illustrations of solitary grandeur." (Figs. 74 and
75.)
[Illustration: Fig. 74.--_Liriodendron Meekii_, Heer. (After Lesquereux.)]
[Illustration: Fig. 75.--_Liriodendron primævum_, Newberry. (After
Newberry.)]
"Three species of _Liriodendron_ are indicated by leaves found in the
Amboy clays--Middle Cretaceous--of New Jersey, and others have been
obtained from the Dakota group in the West, and from the Upper Cretaceous
strata of Greenland. Though differing considerably among themselves in
size and form, all these have the deep sinus of the upper extremity
so characteristic of the genus, and the nervation is also essentially
the same. Hence, we must conclude that the genus _Liriodendron_, now
represented by a single species, was in the Cretaceous age much more
largely developed, having many species, and those scattered throughout
many lands. In the Tertiary age the genus continued to exist, but
the species seem to have been reduced to one, which is hardly to be
distinguished from that now living. In many parts of Europe leaves of
the tulip-tree have been found, and it extended as far south as Italy.
Its presence there was first made known by Unger, in his 'Synopsis,'
page 232, and in his 'Genera et Species,' page 443, where he describes
it under the name of _Liriodendron procaccinii_. The genus has also been
noticed in Europe by Massalongo, Heer, and Ettingshausen, and three
species have been distinguished. All these are, however, so much like the
living species that they should probably be united with it. We here have
a striking illustration of the wide distribution of a species which has
retained its characters both of fruit and leaf quite unchanged through
long migrations and an enormous lapse of time.
"In Europe the tulip-tree, like many of its American associates, seems
to have been destroyed by the cold of the Ice period, the Mediterranean
cutting off its retreat, but in America it migrated southward over the
southern extension of the continent and returned northward again with the
amelioration of the climate."
Leaves of _Liriodendron_ have been recognised in the Cretaceous of
Greenland, though it is now a tree of the warm temperate region, and
Lesquereux describes several species from the Dakota group. But the genus
has not yet been recognised in the Laramie or in the Upper Cretaceous of
British Columbia. In the paper above quoted, Newberry describes three new
species from the Amboy clays, one of which he considers identical with a
Greenland form referred by Heer to _L. Meekii_ of the Dakota group. Thus,
if all Lesquereux's species are to be accepted, the genus begins in the
Middle Cretaceous with at least nine American species.
In New Jersey the Amboy clays are referred to the same age with the
Dakota beds of the West. In these Dr. Newberry has found a rich flora,
including many angiosperms. The following is condensed from a preliminary
notice in the "Bulletin of the Torrey Botanical Club":[DO]
[DO] March, 1886.
"The flora of the Amboy clays is closely related to that of the Dakota
group--most of the genera and some of the species being identical--so
that we may conclude they were nearly contemporaneous, though the absence
in New Jersey of the Fort Benton and Niobrara groups of the upper
Missouri and the apparent synchronism of the New Jersey marls and the
Pierre group indicate that the Dakota is a little the older.
"At least one-third of the species of the Amboy clays seem to be
identical with leaves found in the Upper Cretaceous clays of Greenland
and Aachen (Aix la Chapelle), which not only indicates a chronological
parallelism, but shows a remarkable and unexpected similarity in the
vegetation of these widely separated countries in the middle and last
half of the Cretaceous age. The botanical character of the flora of the
Amboy clays will be seen from the following brief synopsis:
"_Algæ._--A small and delicate form, allied to Chondrites.
"_Ferns._--Twelve species, generally similar and in part identical with
those described by Heer from the Cretaceous beds of Greenland, and
referred to the genera _Dicksonia_, _Gleichenia_, and _Aspidium_.
"_Cycads._--Two species, probably identical with the forms from Greenland
described by Heer under the names of _Podozamites marginatus_ and _P.
tenuinervis_.
"_Conifers._--Fourteen species, belonging to the genera _Moriconia_,
_Brachyphyllum_, _Cunninghamites_, _Pinus_, _Sequoia_, and others
referred by Heer to _Juniperus_, _Libocedrus_, _Frenelopsis_, _Thuya_,
and _Dammara_. Of these, the most abundant and most interesting
are _Moriconia cyclotoxon_--the most beautiful of conifers--and
_Cunninghamites elegans_, both of which occur in the Cretaceous clays of
Aachen, Prussia, and Patoot, Greenland. The _Brachyphyllum_ was a large
and strong species, with imbricated cones, eight inches in length.
"The angiosperms form about seventy species, which include three of
_Magnolia_, four of _Liriodendron_, three or four of _Salix_, three of
_Celastrophyllum_ (of which one is identical with a Greenland species),
one _Celastrus_ (also found in Greenland), four or five _Aralias_,
two _Sassafras_, one _Cinnamomum_, one _Hedera_; with leaves that
are apparently identical with those described by Heer as belonging
to _Andromeda_, _Cissites_, _Cornus_, _Dewalquea_, _Diospyros_,
_Eucalyptus_, _Ficus_, _Ilex_, _Juglans_, _Laurus_, _Menispermites_,
_Myrica_, _Myrsine_, _Prunus_, _Rhamnus_, and others not yet determined.
"Some of the Aralias had palmately-lobed leaves, nearly a foot in
diameter, and two of the tulip-trees (_Liriodendron_) had leaves quite
as large as those of the living species. One of these had deeply lobed
leaves, like those of the white oak. Of the other, the leaves resembled
those of the recent tulip-tree, but were larger. Both had the peculiar
emargination and the nervation of _Liriodendron_.
"Among the most interesting plants of the collection are fine species of
_Bauhinia_ and _Hymenæa_. Of these, the first is represented by a large
number of leaves, some of which are six or seven inches in diameter.
They are deeply bilobed, and have the peculiar and characteristic form
and nervation of the leaves of this genus. _Bauhinia_ is a leguminous
genus allied to _Cercis_, and now inhabits tropical and warm temperate
climates in both hemispheres. Only one species occurs in the United
States, _Bauhinia lunarioides_, Gray, found by Dr. Bigelow on the Rio
Grande.
"_Hymenæa_ is another of the leguminosæ, and inhabits tropical America. A
species of this genus has been found in the Upper Cretaceous of France,
but quite different from the one before us, in which the leaves are much
larger, and the leaflets are united in a common petiole, which is winged;
this is a modification not found in the living species, and one which
brings it nearer to _Bauhinia_.
"But the most surprising discovery yet made is that of a number of quite
large helianthoid flowers, which I have called _Palæanthus_. These
are three to four inches in diameter, and exhibit a scaly involucre,
enclosing what much resembles a fleshy receptacle with achenia. From
the border of this radiate a number of ray florets, one to two inches
in length, which are persistent and must have been scarious, like
those of _Helichrysum_. Though these flowers so much resemble those
of the compositæ, we are not yet warranted in asserting that such is
certainly their character. In the Jurassic rocks of Europe and India
some flowers not very unlike these have been found, which have been
named _Williamsonia_, and referred to cycads by Carruthers. A similar
fossil has been found in the Cretaceous rocks of Greenland, and named
by Heer _Williamsonia cretacea_, but he questions the reference of the
genus to the Cycadeæ, and agrees with Nathorst in considering all the
species of _Williamsonia_ as parasitic flowers, allied to _Brugmansia_
or _Rafflesia_. The Marquis of Saporta regards them as monocotyledons,
similar to _Pandanus_. More specimens of the flowers now exhibited
will perhaps prove--what we can now only regard as probable--that the
Compositæ, like the _Leguminosæ_, _Magnoliaceæ_, _Celastraceæ_, and
other highly organised plants, formed part of the Cretaceous flora. No
composite flowers have before been found in the fossil state, and, as
these are among the most complex and specialised forms of florescence, it
has been supposed that they belonged only to the recent epoch, where they
were the result of a long series of formative changes."
The above presents some interesting new types not heretofore found in the
Middle Cretaceous. More especially the occurrence of large flowers of the
composite type presents a startling illustration of the early appearance
of a very elevated and complex form. Great interest also attaches to
these Amboy beds, as serving, with those of Aix and Greenland, to show
that the margins of the Atlantic were occupied with a flora similar to
that occurring at the same time in the interior plateau of North America
and on the Pacific slope.
The beds at Aix-la-Chapelle are, however, probably somewhat newer than
the Dakota or Amboy beds, and correspond more nearly in age with those
of the Cretaceous coal-field of Vancouver Island, where there is a
very rich Upper Cretaceous flora, which I have noticed in detail in
the "Transactions of the Royal Society of Canada."[DP] In these Upper
Cretaceous beds there are fan-palms as far north at least as the latitude
of 49°, indicating a very mild climate at this period. This inference
is corroborated by the Upper Cretaceous flora of Atané and Patoot in
Greenland, as described by Heer.
[DP] Vol. ii., 1884.
The dicotyledonous plants above referred to are trees and shrubs. Of the
herbaceous exogens of the period we know less. Obviously their leaves
are less likely to find their way into aqueous deposits than the leaves
of trees. They are, besides, more perishable, and in densely wooded
countries there are comparatively few herbaceous plants. I have examined
the beds of mud deposited at the mouth of a woodland streamlet, and have
found them stored with the fallen leaves of trees, but it was in vain to
search for the leaves of herbaceous plants.
[Illustration: Fig. 76.--_Brasenia antiqua_. Upper Cretaceous, South
Saskatchewan River. Natural size, _a_, _b_, Diagrams of venation,
slightly enlarged.]
The climate of North America and Europe, represented by the Cenomanian
vegetation, is not tropical but warm temperate; but the flora was more
uniform than at present, indicating a very equable climate and the
possibility of temperate genera existing within the Arctic circle, and it
would seem to have become warmer toward the close of the period.
The flora of the Cenomanian is separated in most countries from that of
the Senonian, or uppermost Cretaceous, by a marine formation holding few
plants. This depends on great movements of elevation and depression, to
which we must refer in the sequel. In a few regions, however, as in the
vicinity of the Peace River in Canada, there are plant-bearing beds which
serve to bridge over the interval between the Early Cenomanian and the
later Cretaceous.[DQ]
[DQ] See paper by the author in the "Transactions of the Royal Society of
Canada," 1882.
To this interval also would seem to belong the Belly River series of
western Canada, which contains important beds of Coal, but is Closely
associated with the marine Fort Pierre series. A very curious herbaceous
plant of this group, which I have named _Brasenia antiqua_, occurs in the
beds associated with one of the coals. It is a close ally of the modern
_B. peltata_, an aquatic plant which occurs in British Columbia and in
eastern America, and is also said to be found in Japan, Australia, and
India, a width of distribution appropriate to so old a type (Fig. 76).
In so far as vegetable life is concerned, the transition from the Upper
Cretaceous to the Tertiary or Kainozoic is easy, though in many parts
of the world, and more especially in western Europe, there is a great
gap in the deposits between the upper Chalk and the lowest Eocene.
With reference to fossil plants, Schimper recognises in the Kainozoic,
beginning with the oldest, five formations--Palæocene, Eocene, Oligocene,
Miocene, and Pliocene. Throughout these a flora, similar to that of the
Cretaceous on the one hand and the modern on the other, though with
important local peculiarities, extends. There is evidence, however,
of a gradual refrigeration, so that in the Pliocene the climates of
the northern hemisphere were not markedly different from their present
character.
In the first instance an important error was committed by palæobotanists,
in referring to the Miocene many deposits really belonging to the Eocene.
This arose from the early study of the rich plant-bearing Miocene beds of
Switzerland, and from the similarity of the flora all the way from the
Middle Cretaceous to the later Tertiary. The differences are now being
worked out, and we owe to Mr. Starkie Gardner the credit of pointing
these out in England, and to the Geological Survey of Canada that of
collecting the material for exhibiting them in the more northern part of
America.
In the great interior plain of America there rests on the Cretaceous
a series of clays and sandstones with beds of lignite, some of them
eighteen feet in thickness. This was formerly known as the lignitic or
lignite Tertiary, but more recently as the Laramie series. These beds
were deposited in fresh or brackish water, in an internal sea or group of
lakes and swamps, when the continent was lower than at present. They have
been studied both in the United States[DR] and Canada; and, though their
flora was originally referred by mistake to the Miocene, it is now known
to be Eocene or Palæocene, or even in part a transition group between
the latter and the Cretaceous. The following remarks, taken chiefly from
recent papers by the author,[DS] will serve to illustrate this:
[DR] See more especially the elaborate and valuable reports by Lesquereux
and Newberry, and a recent memoir by Ward on "Types of the Laramie
Flora," "Bulletins of the United States Geological Survey," 1887.
[DS] "Transactions of the Royal Society of Canada," 1886-'87.
On the geological map of Canada the Laramie series, formerly known as
the lignitic or lignite Tertiary, occurs, with the exception of a few
outliers, in two large areas west of the 100th meridian, and separated
from each other by a tract of older Cretaceous rocks, over which the
Laramie beds may have extended, before the later denudation of the region.
The most eastern of these areas, that of the Souris River and Wood
Mountain, extends for some distance along the United States boundary,
between the 102d and 109th meridians, and reaches northward to about
thirty miles south of the "elbow" of the South Saskatchewan River, which
is on the parallel of 51° north. In this area the lowest beds of the
Laramie are seen to rest on those of the Fox Hill group of the Upper
Cretaceous, and at one point on the west they are overlaid by beds of
Miocene Tertiary age, observed by Mr. McConnell, of the Geological
Survey, in the Cypress Hills, and referred by Cope, on the evidence of
mammalian remains, to the White River division of the United States
geologists, which is regarded by them as Lower Miocene.[DT] The age of
the Laramie beds is thus stratigraphically determined to be between the
Fox Hill Cretaceous and the Lower Miocene. They are also undoubtedly
continuous with the Fort Union group of the United States geologists on
the other side of the international boundary, and they contain similar
fossil plants. They are divisible into two groups--a lower, mostly
argillaceous, and to which the name of "Bad Lands beds" may be given,
from the "bad lands" of Wood Mountain, where they are well exposed, and
an upper, partly arenaceous member, which may be named the Souris River
or Porcupine Creek division. In the lower division are found reptilian
remains of Upper Cretaceous type, with some fish remains more nearly
akin to those of the Eocene.[DU] Neither division has as yet afforded
mammalian remains.
[DT] "Report of the Geological Survey of Canada," 1885.
[DU] Cope, in Dr. G. M. Dawson's "Report on the 49th Parallel."
The western area is of still larger dimensions, and extends along the
eastern base of the Rocky Mountains from the United States boundary
to about the 55th parallel of latitude, and stretches eastward to the
111th meridian. In this area, and more especially in its southern part,
the officers of the Geological Survey of Canada have recognised three
divisions, as follows: (1) The Lower Laramie or St. Mary River series,
corresponding in its character and fossils to the Lower or Bad Lands
division of the other area. (2) A middle division, the Willow Creek beds,
consisting of clays, mostly reddish, and not recognised in the other
area. (3) The Upper Laramie or Porcupine Hills division, corresponding
in fossils, and to some extent in mineral character, to the Souris River
beds of the eastern area.
The fossil plants collected by Dr. G. M. Dawson in the eastern area were
noticed by the author in an appendix to Dr. Dawson's report on the 49th
parallel, in 1875, and a collection subsequently made by Dr. Selwyn
was described in the "Report of the Geological Survey of Canada" for
1879-'80. Those of the western area, and especially collections made by
myself near Calgary in 1883, and by officers of the Geological Survey in
1884, have been described in the "Transactions of the Royal Society of
Canada" vols. iii. and iv.
In studying these fossil plants, I have found that there is a close
correspondence between those of the Lower and Upper Laramie in the two
areas above referred to respectively, and that the flora of the Lower
Laramie is somewhat distinct from that of the Upper, the former being
especially rich in certain aquatic plants, and the latter much more
copious on the whole, and much more rich in remains of forest-trees. This
is, however, possibly an effect rather of local conditions than of any
considerable change in the flora, since some Upper Laramie forms recur
as low as the Belly River series of the Cretaceous, which is believed on
stratigraphical grounds to be considerably older than the Lower Laramie.
With reference to the correlation of these beds with those of the United
States, some difficulty has arisen from the tendency of palæobotanists
to refer the plants of the Upper Laramie to the Miocene age, although
in the reports of Mr. Clarence King, the late director of the United
States Geological Survey, these beds are classed, on the evidence of
stratigraphy and animal fossils, as Upper Cretaceous. More recently,
however, and partly perhaps in consequence of the views maintained by the
writer since 1875, some change of opinion has occurred, and Dr. Newberry
and Mr. Lesquereux seem now inclined to admit that what in Canada we
recognise as Upper Laramie is really Eocene, and the Lower Laramie
either Cretaceous or a transition group between this and the Eocene. In
a recent paper [DV] Dr. Newberry gives a comparative table, in which he
correlates the Lower Laramie with the Upper Cretaceous of Vancouver
Island and the Faxoe and Maestricht beds of Europe, while he regards the
Upper Laramie as equivalent to European Eocene. Except in so far as the
equivalence of the Lower Laramie and Vancouver Island beds is concerned,
this corresponds very nearly with the conclusions of the writer in a
paper published last year[DW]--namely, that we must either regard the
Laramie as a transition Cretaceo-Eocene group, or must institute our
line of separation in the Willow Creek or Middle Laramie division, which
has, however, as yet afforded no fossil plants. I doubt, however, the
equivalence of the Vancouver beds and the Lower Laramie, except perhaps
in so far as the upper member of the former is concerned. I have also to
observe that in the latest report of Mr. Lesquereux he still seems to
retain in the Miocene certain formations in the West, which from their
fossil plants I should be inclined to regard as Eocene.[DX]
[DV] Newberry, "Transactions of the New Fork Academy," February, 1886.
[DW] "Transactions of the Royal Society of Canada," vol. ii.
[DX] While these sheets were going through the press I received a very
valuable report of Mr. Lester F. Ward upon the Laramie of the United
States. I have merely had time to glance at this report, but can see that
the views of the author agree closely with those above expressed.
Two ferns occurring in these beds are remarkable as evidence of the
persistence of species, and of the peculiarities of their ancient and
modern distribution. _Onoclea sensibilis_, the very common sensitive
fern of eastern America, is extremely abundant in the Laramie beds over
a great area in the West. Mr. Starkie Gardner and Dr. Newberry have also
shown that it is identical with the _Filicites Hebridicus_ of Forbes,
from the early Eocene beds of the Island of Mull, in Scotland. Thus we
have a species once common to Europe and America, but now restricted
to the latter, and which has continued to exist over all the vast ages
between the Cretaceous and the present day. In the Laramie beds I have
found associated with this species another and more delicate fern,
the modern _Davallia_ (_Stenloma_) _tenuifolia_, but this, unlike its
companion, no longer occurs in America, but is found in the mountains of
Asia. This is a curious illustration of the fact that frail and delicate
plants may be more ancient than the mountains or plains on which they
live.
There are also some very interesting and curious facts in connection with
the conifers of the Laramie. One of the most common of these is a _Thuja_
or arbor vitæ (the so-called "cedar" of Canada). The Laramie species has
been named _T. interrupta_ by Newberry, but it approaches very closely
in its foliage to _T. occidentalis_, of eastern Canada, while its fruit
resembles that of the western species, _T. gigantea_.
Still more remarkable are the Sequoias to which we have already referred,
but which in the Laramie age seem to have been spread over nearly
all North America. The fossil species are of two types, representing
respectively the modern _S. gigantea_ and _S. sempervirens_, and their
wood, as well as that of Thuja, is found in great abundance in the
lignites, and also in the form of silicified trunks, and corresponds
with that of the recent species. The Laramie contains also conifers
of the genera _Glyptostrobus_, _Taxodium_, and _Taxus_; and the
genus _Salisburia_ or gingko--so characteristic of the Jurassic and
Cretaceous--is still represented in America as well as in Europe in the
early Eocene.
We have no palms in the Canadian or Scottish Palæocene, though I
believe they are found further south. The dicotyledonous trees are
richly represented. Perhaps the most conspicuous were three species of
_Platanus_, the leaves of which sometimes fill the sandstones, and one
of which, _P. nobilis_, Newberry, sometimes attains the gigantic size of
a foot or more in diameter of its blade. The hazels are represented by a
large-leaved species, _C. Macquarrii_, and by leaves not distinguishable
from those of the modern American species, _C. Americana_ and _C.
rostrata_. There are also chestnuts and oaks. But the poplars and willows
are specially abundant, being represented by no less than six species,
and it would seem that all the modern types of poplar, as indicated by
the forms and venation of the leaves, existed already in the Laramie, and
most of them even in the Upper Cretaceous. _Sassafras_ is represented by
two species, and the beautiful group of _Viburnum_,, to which the modern
tree-cranberry belongs, has several fine species, of some of which both
leaves and berries have been found. The hickories and butternuts are
also present, the horse-chestnut, the _Catalpa_ and _Sapindus_, and some
curious leaves which seem to indicate the presence of the modern genus
_Symphorocarpus_, the snow-berry tribe.
The above may suffice to give an idea of the flora of the older Eocene
in North America, and I may refer for details to the works of Newberry,
Lesquereux, and Ward, already cited. I must now add that the so-called
Miocene of Atanekerdluk, Greenland, is really of the same age, as also
the "Miocene" of Mull, in Scotland, of Antrim, in Ireland, and of Bovey
Tracey, in the south of England, and the Gelinden, or "Heersian" beds,
of Belgium, described by Saporta. In comparing the American specimens
with the descriptions given by Gardner of the leaf-beds at Ardtown, in
Mull, we find, as already stated, _Onoclea sensibilis_, common to both.
The species of _Sequoia_, _Gingko_, _Taxus_, and _Glyptostrobus_ are
also identical or closely allied, and so are many of the dicotyledonous
leaves. For example, _Platanoides Hebridicus_ is very near to _P.
nobilis_, and _Corylus Macquarrii_ is common to both formations, as well
as _Populus Arctica_ and _P. Richardsoni_. I may add that ever since
1875-'76, when I first studied the Laramie plants, I have maintained
their identity with those of the Fort Union group of the United States,
and of the so-called Miocene of McKenzie River and Greenland, and that
the whole are Paleocene; and this conclusion has now been confirmed by
the researches of Gardner in England, and by the discovery of true Lower
Miocene beds in the Canadian northwest, overlying the Laramie or lignite
series.
In a bulletin of the United States Geological Survey (1886), Dr. White
has established in the West the continuous stratigraphical succession of
the Laramie and the Wahsatch Eocene, thus placing the Laramie conformably
below the Lower Eocene of that region. Cope has also described as the
Puerta group a series of beds holding vertebrate fossils, and forming a
transition from the Laramie to the Wahsatch. White also testifies that
a number of fresh-water mollusks are common to the Wahsatch and the
Laramie. This finally settles the position of the Laramie so far as the
United States geologists are concerned, and shows that the flora is to be
regarded as Eocene if not Upper Cretaceous, in harmony with what has been
all along maintained in Canada. An important _résumé_ of the flora has
just been issued by Ward in the bulletins of the United States Geological
Survey (1887).
Before leaving this part of the subject, I would deprecate the remark,
which I see occasionally made, that fossil plants are of little value in
determining geological horizons in the Cretaceous and Tertiary. I admit
that in these periods some allowance must be made for local differences
of station, and also that there is a generic sameness in the flora of the
northern hemisphere, from the Cenomanian to the modern, yet these local
differences and general similarity are not of a nature to invalidate
inferences as to age. No doubt, so long as palæobotanists seemed obliged,
in deference to authority, and to the results of investigations limited
to a few European localities, to group together, without distinction, all
the floras of the later Cretaceous and earlier Tertiary, irrespective
of stratigraphical considerations, the subject lost its geological
importance. But, when a good series has been obtained in any one region
of some extent, the case becomes different. Though there is still much
imperfection in our knowledge of the Cretaceous and Tertiary floras
of Canada, I think the work already done is sufficient to enable any
competent observer to distinguish by their fossil plants the Lower,
Middle, and Upper Cretaceous, and the latter from the Tertiary; and, with
the aid of the work already done by Lesquereux and Newberry in the United
States, to refer approximately to its true geological position any group
of plants from beds of unknown age in the West.
An important consequence arising from the above statements is that
the period of warm climate which enabled a temperate flora to exist
in Greenland was that of the later Cretaceous and early Eocene rather
than, as usually stated, the Miocene. It is also a question admitting of
discussion whether the Eocene flora of latitudes so different as those of
Greenland, Mackenzie River, northwest Canada, and the United States, were
strictly contemporaneous, or successive within a long geological period
in which climatal changes were gradually proceeding. The latter statement
must apply at least to the beginning and close of the period; but the
plants themselves have something to say in favour of contemporaneity. The
flora of the Laramie is not a tropical but a temperate flora, showing no
doubt that a much more equable climate prevailed in the more northern
parts of America than at present. But this equability of climate implies
the possibility of a great geographical range on the part of plants.
Thus it is quite possible and indeed highly probable that in the Laramie
age a somewhat uniform flora extended from the Arctic seas through the
great central plateau of America far to the south, and in like manner
along the western coast of Europe. It is also to be observed that, as
Gardner points out, there are some differences indicating a diversity
of climate between Greenland and England, and even between Scotland
and Ireland and the south of England, and we have similar differences,
though not strongly-marked, between the Laramie of northern Canada and
that of the United States. When all our beds of this age from the Arctic
sea to the 49th parallel have been ransacked for plants, and when the
palæobotanists of the United States shall have succeeded in unravelling
the confusion which now exists between their Laramie and the Middle
Tertiary, the geologist of the future will be able to restore with much
certainty the distribution of the vast forests which in the early Eocene
covered the now bare plains of interior America. Further, since the
break which in western Europe separates the flora of the Cretaceous from
that of the Eocene does not exist in America, it will then be possible
to trace the succession from the Mesozoic flora of the Trias and of the
Queen Charlotte Islands and Kootanie series of the Lower Cretaceous up
to the close of the Eocene; and to determine, for America at least, the
manner and conditions under which the angiospermous flora of the later
Cretaceous succeeded to the pines and cycads which characterised the
beginning of the Cretaceous period. In so far as Europe is concerned,
this may be more difficult, since the want of continuity of land from
north to south seems there to have been fatal to the continuance of some
plants during changes of climate, and there were also apparently in the
Kainozoic period invasions at certain times of species from the south and
east, which did not occur to the same extent in America.
In recent reports on the Tertiary floras of Australia and New
Zealand,[DY] Ettingshausen holds that the flora of the Tertiary, as a
whole, was of a generalised character; forms now confined to the southern
and northern hemispheres respectively being then common to both. It would
thus seem that the present geographical diversities must have largely
arisen from the great changes in climate and distribution of land and
water in the later Tertiary.
[DY] "Geological Magazine," August, 1887.
The length of our discussion of the early angiospermous flora does not
permit us to trace it in detail through the Miocene and Pliocene, but
we may notice the connection through these in the next chapter, and may
refer to the magnificent publications of Heer and Lesquereux on the
Tertiary floras of Europe and America respectively.
CHAPTER VII.
PLANTS FROM THE TERTIARY TO THE MODERN PERIOD.
It may be well to begin this chapter with a sketch of the general
physical and geological conditions of the period which was characterised
by the advent and culmination of the dicotyledonous trees.
In the Jurassic and earliest Cretaceous periods the prevalence, over the
whole of the northern hemisphere and for a long time, of a monotonous
assemblage of gymnospermous and acrogenous plants, implies a uniform and
mild climate, and facility for intercommunication in the north. Toward
the end of the Jurassic and beginning of the Cretaceous, the land of the
northern hemisphere was assuming greater dimensions, and the climate
probably becoming a little less uniform. Before the close of the Lower
Cretaceous period the dicotyledonous flora seems to have been introduced,
under geographical conditions which permitted a warm temperate climate to
extend as far north as Greenland.
In the Cenomanian or Middle Cretaceous age we find the northern
hemisphere tenanted with dicotyledonous trees closely allied to those of
modern times, though still indicating a climate much warmer than that
which at present prevails. In this age, extensive but gradual submergence
of land is indicated by the prevalence of chalk and marine limestones
over the surface of both continents; but a circumpolar belt seems to
have been maintained, protecting the Atlantic and Pacific basins from
floating ice, and permitting a temperate flora of great richness to
prevail far to the north, and especially along the southern margins and
extensions of the circumpolar land. These seem to have been the physical
conditions which terminated the existence of the old Mesozoic flora and
introduced that of the Middle Cretaceous.
As time advanced the quantity of land gradually increased, and the
extension of new plains along the older ridges of land was coincident
with the deposition of the great Laramie series, and with the origination
of its peculiar flora, which indicates a mild climate and considerable
variety of station in mountain, plain, and swamp, as well as in great
sheets of shallow and weedy fresh water.
In the Eocene and Miocene periods, the continents gradually assumed their
present form, and the vegetation became still more modern in aspect.
In that period of the Eocene, however, in which the great nummulitic
limestones were deposited, a submergence of land occurred on the eastern
continent which must have assimilated its physical conditions to those of
the Middle Cretaceous. This great change, affecting materially the flora
of Europe, was not equally great in America, which also by the north and
south extension of its mountain-chains permitted movements of migration
not possible in the Old World. From the Eocene downward, the remains of
land-animals and plants are found chiefly in lake-basins occupying the
existing depressions of the land, though more extensive than those now
remaining. It must also be borne in mind that the great foldings and
fractures of the crust of the earth which occurred at the close of the
Eocene, and to which the final elevation of such ranges as the Alps and
the Rocky Mountains belongs, permanently modified and moulded the forms
of the continents.
These statements raise, however, questions as to the precise equivalence
in time of similar floras found in different latitudes. However
equable the climate, there must have been some appreciable difference
in proceeding from north to south. If, therefore, as seems in every
way probable, the new species of plants originated on the Arctic land
and spread themselves southward, this latter process would occur most
naturally in times of gradual refrigeration or of the access of a more
extreme climate--that is, in times of the elevation of land in the
temperate latitudes, or, conversely, of local depression of land in the
Arctic, leading to invasions of northern ice. Hence, the times of the
prevalence of particular types of plants in the far north would precede
those of their extension to the south, and a flora found fossil in
Greenland might be supposed to be somewhat older than a similar flora
when found farther south. It would seem, however, that the time required
for the extension of a new flora to its extreme geographical limit is so
small, in comparison with the duration of an entire geological period,
that, practically, this difference is of little moment, or at least does
not amount to antedating the Arctic flora of a particular type by a whole
period, but only by a fraction of such period.
It does not appear that, during the whole of the Cretaceous and Eocene
periods, there is any evidence of such refrigeration as seriously to
interfere with the flora, but perhaps the times of most considerable
warmth are those of the Dunvegan group in the Middle Cretaceous, and
those of the later Laramie and oldest Eocene.
It would appear that no cause for the mild temperature of the Cretaceous
needs to be invoked, other than those mutations of land and water which
the geological deposits themselves indicate. A condition, for example, of
the Atlantic basin in which the high land of Greenland should be reduced
in elevation, and at the same time the northern inlets of the Atlantic
closed against the invasion of Arctic ice, would at once restore climatic
conditions allowing of the growth of a temperate flora in Greenland.
As Dr. Brown has shown,[DZ] and as I have elsewhere argued, the absence
of light in the Arctic winter is no disadvantage, since, during the
winter, the growth of deciduous trees is in any case suspended; while the
constant continuance of light in the summer is, on the contrary, a very
great stimulus and advantage.
[DZ] "Florula Discoana."
It is a remarkable phenomenon in the history of genera of plants in
the later Mesozoic and Tertiary, that the older genera appear at once
in a great number of specific types, which become reduced as well as
limited in range down to the modern. This is, no doubt, connected with
the greater differentiation of local conditions in the modern; but it
indicates also a law of rapid multiplication of species in the early life
of genera. The distribution of the species of _Salisburia_, _Sequoia_,
_Platanus_, _Sassafras_, _Liriodendron_, _Magnolia_, and many other
genera, affords remarkable proofs of this.
Gray, Saporta, Heer, Newberry, Lesquereux, and Starkie Gardner have all
ably discussed these points; but the continual increase of our knowledge
of the several floras, and the removal of error as to the dates of their
appearance, must greatly conduce to clearer and more definite ideas. In
particular, the prevailing opinion that the Miocene was the period of the
greatest extension of warmth and of a temperate flora into the Arctic,
must be abandoned in favour of the later Cretaceous and Eocene; and, if
I mistake not, this will be found to accord better with the evidence of
general geology and of animal fossils.
In these various revolutions of the later Cretaceous and Kainozoic
periods, America, as Dr. Gray has well pointed out, has had the advantage
of a continuous stretch of high land from north to south, affording a
more sure refuge to plants in times of submergence, and means of escape
to the south in times of refrigeration. Hence, the greater continuity
of American vegetation and the survival of genera like _Sequoia_ and
_Liriodendron_, which have perished in the Old World. Still, there are
some exceptions to this, for the gingko-tree is a case of survival in
Asia of a type once plentiful in America, but now extinct there. Eastern
Asia has had, however, some considerable share of the same advantage
possessed by America, with the addition, referred to by Gray, of a better
and more insular climate.
But our survey of these physical conditions can not be considered
complete till we shall have considered the great Glacial age of the
Pleistocene. It is certain that throughout the later Miocene and Pliocene
the area of land in the northern hemisphere was increasing, and the large
and varied continents were tenanted by the noblest vegetation and the
grandest forms of mammalian life that the earth has ever witnessed. As
the Pliocene drew to a close, a gradual diminution of warmth came on,
and more especially a less equable climate, and this was accompanied
with a subsidence of the land in the temperate regions and with changes
of the warm ocean-currents. Thus gradually the summers became cooler
and the winters longer and more severe, the hill-tops became covered
with permanent snows, glaciers ploughed their way downward into the
plains, and masses and fields of floating ice cooled the seas. In these
circumstances the richer and more delicate forms of vegetation must have
been chilled to death or obliged to remove farther south, and in many
extensive regions, hemmed in by the advance of the sea on the one hand
and land-ice on the other, they must have altogether perished.
Yet even in this time vegetation was not altogether extinct. Along the
Gulf of Mexico in America, and in the Mediterranean basin in Europe,
there were still some remains of a moderate climate and certain boreal
and arctic forms moving southward continued to exist here and there in
somewhat high latitudes, just as similar plants now thrive in Grinnell
Land within sight of the snows of the Greenland mountains. A remarkable
summary of some of these facts as they relate to England was given by an
eminent English botanist, Mr. Carruthers, in his address as President of
the Biological Section of the British Association at Birmingham in 1886.
At Cromer, on the coast of Norfolk, the celebrated forest-bed of newer
Pliocene age, and containing the remains of a copious mammalian fauna,
holds also remains of plants in a state admitting of determination. These
have been collected by Mr. Reid, of the Geological Survey, and were
reported on by Carruthers, who states that they represent a somewhat
colder temperature than that of the present day. I quote the following
details from the address.
With reference to the plants of the forest-bed or newer Pliocene he
remarks as follows:
"Only one species (_Trapa natans_, Willd.) has disappeared from our
islands. Its fruits, which Mr. Reid found abundantly in one locality,
agree with those of the plants found until recently in the lakes of
Sweden. Four species (_Prunus speciosa_, L., _[OE]nanthe Tichenalii_,
Sm., _Potamogeton pterophyllus_, Sch., and _Pinus abies_, L.) are found
at present only in Europe, and a fifth (_Potamogeton trichoides_, Cham.)
extends also to North America; two species (_Peucedanum palustre_,
Moench, and _Pinus sylvestris_, L.) are found also in Siberia, while six
more (_Sanguisorba officinalis_, L., _Rubus fruticosus_, L., 5 _Cornus
sanguinea_, L., _Euphorbia amygdaloides_, L., _Quercus robur_, L., and
_Potamogeton crispus_, L.) extend into western Asia, and two (_Fagus
sylvatica_, L., and _Alnus glutinosa_, L.) are included in the Japanese
flora. Seven species, while found with the others, enter also into the
Mediterranean flora, extending to North Africa: these are _Thalictrum
minus_, L., _Thalictrum flavum_, L., _Ranunculus repens_, L., _Stellaria
aquatica_, Scop., _Corylus avellana_, L., _Yannichellia palustris_, L.,
and _Cladium mariscus_, Br. With a similar distribution in the Old World,
eight species (_Bidens tripartita_, L., _Myosotis cæspitosa_, Schultz,
_Suæda maritima_, Dum., _Ceratophyllum demersum_,, L., _Sparganium
ramosum_, Huds., _Potamogeton pectinatus_, L., _Carex paludosa_, Good.,
and _Osmunda regalis_, L.) are found also in North America. Of the
remainder, ten species (_Nuphar luteum_, Sm., _Menyanthes trifoliata_,
L., _Stachys palustris_, L., _Rumex maritimus_, L., _Rumex acetosella_,
L., _Betula alba_, L., _Scirpus pauciflorus_, Lightf., _Taxus baccata_,
L., and _Isoetes lacustris_, L.) extend round the north temperate
zone, while three (_Lycopus europæus_, L., _Alisma plantago_, L., and
_Phragmites communis_, Trim), having the same distribution in the north,
are found also in Australia, and one (_Hippuris vulgaris_, L.) in the
south of South America. The list is completed by _Ranunculus aquatilis_,
L., distributed over all the temperate regions of the globe, and _Scirpus
lacustris_, L., which is found in many tropical regions as well."
He remarks that these plants, while including species now very widely
scattered, present no appreciable change of characters.
Above this bed are glacial clays, which hold other species indicating an
extremely cold climate. They are few in number, only _Salix polaris_, a
thoroughly arctic species, and its ally, _S. cinerea_, L., and a moss,
_Hypnum turgescens_, Schimp., no longer found in Britain, but an Alpine
and arctic species. This bed belongs to the beginning of the Glacial
period, the deposits of which have as yet afforded no plants in England.
But plants occur in post-glacial and upper-glacial beds in different
parts of England, to which Carruthers thus refers:
"The period of great cold, during which arctic ice extended far into
temperate regions, was not favorable to vegetable life. But in some
localities we have stratified clays with plant-remains later than the
Glacial epoch, yet indicating that the great cold had not then entirely
disappeared. In the lacustrine beds at Holderness is found a small
birch (_Betula nana_, L.), now limited in Great Britain to some of the
mountains of Scotland, but found in the arctic regions of the Old and
New World and on Alpine districts in Europe, and with it _Prunus padus_,
L., _Quercus robur_, L., _Corylus avellana_, L., _Alnus glutinosa_, L.,
and _Pinus sylvestris_, L. In the white clay-beds at Bovey Tracey of
the same age there occur the leaves of _Arctostaphylos uva-ursi_, L.,
three species of willow, viz., _Salix cinerea_, L., _S. myrtilloides_,
L., and _S. polaris_, Wahl., and in addition to our Alpine _Betula
nana_, L., the more familiar _B. alba_, L. Two of these plants have
been lost to our flora from the change of climate that has taken place,
viz., _Salix myrtilloides_, L., and _S. polaris_, Wahl.; and _Betula
nana_, L., has retreated to the mountains of Scotland. Three others
(_Dryas octopetala_, L., _Arctostaphylos uva-ursi_, L., and _Salix
herbacea_, L.) have withdrawn to the mountains of northern England,
Wales, and Scotland, while the remainder are still found scattered over
the country. Notwithstanding the diverse physical conditions to which
these plants have been subjected, the remains preserved in these beds
present no characters by which they can be distinguished from the living
representatives of the species."
One of the instances referred to is very striking. At Bovey Tracey the
arctic beds rest directly on those holding the rich, warm temperate flora
of the Eocene; so that here we have the evidence of fossil plants to show
the change from the climate of the Eocene to that of arctic lands, and
the modern vegetation to indicate the return of a warm temperature.
In Canada, in the Pleistocene beds known as the Leda clays, intervening
between the lower boulder clay and the Saxicava sand, which also holds
boulders, there are beds holding fossil plants, in some places intermixed
with sea-shells and bones of marine fishes, showing that they were
drifted into the sea at a time of submergence. These remains are boreal
rather than arctic in character, and with the remains of drift-wood often
found in the boulder deposits serve to indicate that there were at all
times oases of hardy life in the glacial deserts, just as we find these
in polar lands at the present day. I condense from a paper on these
plants[EA] the following facts, with a few additional notes:
[EA] "Canadian Naturalist," 1866.
The importance of all information bearing on the temperature of the
Post-pliocene period invests with much interest the study of the
land-plants preserved in deposits of this age. Unfortunately, these
are few in number, and often not well preserved. In Canada, though
fragments of the woody parts of plants occasionally occur in the marine
clays and sands, there is only one locality which has afforded any
considerable quantity of remains of their more perishable parts. This
is the well-known deposit of Leda clay at Green's Creek, on the Ottawa,
celebrated for the perfection in which the skeletons of the capelin
and other fishes are preserved in the calcareous nodules imbedded in
the clay. In similar nodules, contained apparently in a layer somewhat
lower than that holding the ichthyolites, remains of land-plants are
somewhat abundant, and, from their association with shells of _Leda
glacialis_, seem to have been washed down from the land into deep water.
The circumstances would seem to have been not dissimilar from those
at present existing in the northeast arm of Gaspé Basin, where I have
dredged from mud now being deposited in deep water, living specimens of
_Leda limatula_, mixed with remains of land-plants.
The following are the species of plants recognised in these nodules:
1. _Drosera rotundifolia_, Linn. In a calcareous nodule from Green's
Creek, the leaf only preserved. This plant is common in bogs in Canada,
Nova Scotia, and Newfoundland, and thence, according to Hooker, to the
Arctic circle. It is also European.
[Illustration: Fig. 77.--_Gaylussaccia resinosa_. Pleistocene, Canada.]
2. _Acer spicatum_, Lamx. (_Acer montanum_, Aiton.) Leaf in a nodule from
Green's Creek. Found in Nova Scotia and Canada, also at Lake Winnipeg,
according to Richardson.
3. _Potentilla Canadensis_, Linn. In nodules from Green's Creek; leaves
only preserved. I have had some difficulty in determining these, but
believe they must be referred to the species above named, or to _P.
simplex_, Michx., supposed by Hooker and Gray to be a variety. It occurs
in Canada and New England, but I have no information as to its range
northward.
4. _Gaylussaccia resinosa_, Torrey and Gray. Leaf in nodule at Green's
Creek. Abundant in New England and in Canada, also on Lake Huron and the
Saskatchewan, according to Richardson (Fig. 77).
5. _Populus balsamifera_, Linn. Leaves and branches in nodules at Green's
Creek. This is by much the most common species, and its leaves are of
small size, as if from trees growing in cold and exposed situations. The
species is North American and Asiatic, and abounds in New England and
Canada. It extends to the Arctic circle, and is abundant on the shores
of the Great Slave Lake and on the McKenzie River, and according to
Richardson constitutes much of the drift timber of the Arctic coast (Fig.
78).
[Illustration: Fig. 78.--_Populus balsamifera_. Pleistocene, Canada.]
6. _Thuja occidentalism_ Linn. Trunks and branches in the Leda clay
at Montreal. This tree occurs in New England and Canada, and extends
northward into the Hudson Bay territories. It is a northern though
not arctic species in its geographical range. According to Lyell it
occurs associated with the bones of Mastodon in New Jersey. From the
great durability of its wood, it is one of the trees most likely to be
preserved in aqueous deposits.
7. _Potamogeton perfoliatus_, Linn. Leaves and seeds in nodules at
Green's Creek. Inhabits streams of the Northern States and Canada, and
according to Richardson extends to Great Slave Lake.
8. _Potamogeton pusillus._ Quantities of fragments which I refer to this
species occur in nodules at Green's Creek. They may possibly belong to a
variety of _P. hybridus_ which, together with _P. natans_, now grows in
the river Ottawa, where it flows over the beds containing these fossils.
9. _Cariceæ and Gramineæ._ Fragments in nodules from Green's Creek appear
to belong to plants of these groups, but I cannot venture to determine
their species.
10. _Equisetum scirpoides_, Michx. Fragments in nodules, Green's Creek.
This is a widely distributed species, occurring in the Northern States
and Canada.
11. _Fontinalis._ In nodules at Green's Creek there occur, somewhat
plentifully, branches of a moss apparently of the genus _Fontinalis_.
[Illustration: Fig. 79.--Frond of _Fucus_. Pleistocene, Canada.]
12. _Algæ._ With the plants above mentioned, both at Green's Creek and at
Montreal, there occur remains of sea-weeds (Fig. 79). They seem to belong
to the genera _Fucus_ and _Ulva_, but I cannot determine the species.
A thick stem in one of the nodules would seem to indicate a large
_Laminaria_. With the above there are found at Green's Creek a number of
fragments of leaves, stems, and fruits, which I have not been able to
refer to their species, principally on account of their defective state
of preservation.
None of the plants above mentioned is properly arctic in its
distribution, and the assemblage may be characterised as a selection from
the present Canadian flora of some of the more hardy species having the
most northern range. Green's Creek is in the central part of Canada, near
to the parallel of 46°, and an accidental selection from its present
flora, though it might contain the same species found in the nodules,
would certainly include with these, or instead of some of them, more
southern forms. More especially the balsam poplar, though that tree
occurs plentifully on the Ottawa, would not be so predominant. But such
an assemblage of drift-plants might be furnished by any American stream
flowing in the latitude of 50° to 55° north. If a stream flowing to the
north, it might deposit these plants in still more northern latitudes, as
the McKenzie River does now. If flowing to the south, it might deposit
them to the south of 50°. In the case of the Ottawa, the plants could
not have been derived from a more southern locality, nor probably from
one very far to the north. We may therefore safely assume that the
refrigeration indicated by these plants would place the region bordering
the Ottawa in nearly the same position with that of the south coast of
Labrador fronting on the Gulf of St. Lawrence at present. The absence of
all the more arctic species occurring in Labrador should perhaps induce
us to infer a somewhat milder climate than this.
The moderate amount of refrigeration thus required would in my opinion
accord very well with the probable conditions of climate deducible from
the circumstances in which the fossil plants in question occur. At the
time when they were deposited the sea flowed up the Ottawa valley to a
height of 200 to 400 feet above its present level, and the valley of the
St. Lawrence was a wide arm of the sea, open to the arctic current. Under
these conditions the immense quantities of drift-ice from the northward,
and the removal of the great heating surface now presented by the low
lands of Canada and New England, must have given for the Ottawa coast
of that period a summer temperature very similar to that at present
experienced on the Labrador coast, and with this conclusion the marine
remains of the Leda clay, as well as the few land molluscs whose shells
have been found in the beds containing the plants, and which are species
still occurring in Canada, perfectly coincide.
The climate of that portion of Canada above water at the time when these
plants were imbedded may safely be assumed to have been colder in summer
than at present, to an extent equal to about 5° of latitude, and this
refrigeration may be assumed to correspond with the requirements of
the actual geographical changes implied. In other words, if Canada was
submerged until the Ottawa valley was converted into an estuary inhabited
by species of _Leda_, and frequented by capelin, the diminution of the
summer heat consequent on such depression would be precisely suitable to
the plants occurring in these deposits, without assuming any other cause
of change of climate.
I have arranged elsewhere the Post-pliocene deposits of the central
part of Canada, as consisting of, in ascending order: (1) The boulder
clay; (2) a deep-water deposit, the Leda clay; and (3) a shallow-water
deposit, the Saxicava sand. But, although I have placed the boulder clay
in the lowest position, it must be observed that I do not regard this as
a continuous layer of equal age in all places. On the contrary, though
locally, as at Montreal, under the Leda clay, it is in other places and
at other levels contemporaneous with or newer than that deposit, which
itself also locally contains boulders.
At Green's Creek the plant-bearing nodules occur in the lower part of the
Leda clay, which contains a few boulders, and is apparently in places
overlaid by large boulders, while no distinct boulder clay underlies
it. The circumstances which accumulated the thick bed of boulder clay
near Montreal were probably absent in the Ottawa valley. In any case we
must regard the deposits of Green's Creek as coeval with the Leda clay
of Montreal, and with the period of the greatest abundance of _Leda
glacialis_, the most exclusively arctic shell of these deposits. In other
words, I regard the plants above mentioned as probably belonging to the
period of greatest refrigeration of which we have any evidence, of course
not including that mythical period of universal incasement in ice, of
which, as I have elsewhere endeavoured to show, in so far as Canada is
concerned, there is no evidence whatever.[EB]
[EB] Notes on Post-Pliocene of Canada, "Canadian Naturalist," 1872.
The facts above stated in reference to Post-pliocene plants concur, with
all the other evidence I have been able to obtain, in the conclusion that
the refrigeration of Canada in the Post-pliocene period consisted of a
diminution of the summer heat, and was of no greater amount than that
fairly attributable to the great depression of the land and the different
distribution of the ice-bearing arctic current.
In connection with the plants above noticed, it is interesting to
observe that at Green's Creek, at Pakenham Mills, at Montreal, and at
Clarenceville on Lake Champlain, species of Canadian _Pulmonata_ have
been found in deposits of the same age with those containing the plants.
The species which have been noticed belong to the genera _Lymnea_ and
_Planorbis_.
The Glacial age was, fortunately, not of very long duration, though its
length has been much exaggerated by certain schools of geologists,[EC]
It passed away, and a returning cosmic spring gladdened the earth, and
was ushered in by a time of great rainfall and consequent denudation
and deposit, which has been styled the "Pluvial Period" The remains of
the Pliocene forests then returned--with somewhat diminished numbers of
species--from the south and again occupied the land, though they have
not been able, in their decimated condition, to restore the exuberance
of the flora of the earlier Tertiary. In point of fact, as we shall
see in the next chapter, it is the floras originating within the polar
circle and coming down from the north that are rich and copious. Those
that, after periods of cold or submergence, return from the south, are
comparatively poor. Hence the modern flora is far inferior to that of the
Middle Kainozoic. In America, however, and in eastern Asia, for reasons
already stated, the return was more abundant than in Europe.
[EC] This I have long maintained on grounds connected with Pleistocene
fossils, amount of denudation and deposit, &c., and I am glad to see that
Prestwich, the best English authority on such subjects, has recently
announced similar conclusions, based on independent reasons.
Simultaneously with the return of the old temperate flora, the arctic
plants that had overspread the land retreated to mountain-tops, now bared
of ice and snow, and back to the polar lands whence they came; and so it
happens that, on the White Mountains, the Alps, and the Himalayas, we
have insular patches of the same groups of plants that exist around the
pole.
These changes need not have required a very long time, for the
multiplication and migration of plants are very rapid, especially when
aided by the agency of migratory animals. Many parts of the land must,
indeed, have been stocked with plants from various sources, and by
agencies--as that of the sea--which might at first sight seem adverse to
their distribution. The British Islands, for example, have no indigenous
plants. Their flora consists mainly of Germanic plants, which must have
migrated to Britain in that very late period of the Post-glacial when
the space now occupied by the North Sea was mostly dry land. Other
portions of it are Scandinavian plants, perhaps survivors of the Glacial
age, or carried by migratory birds; and still another element consists
of Spanish plants, brought north by spring migrants, and establishing
themselves in warm and sheltered spots, just as the arctic plants do on
the bleak hill-tops. The Bermudas, altogether recent islands, have one
hundred and fifty species of native plants, all of which are West Indian
and American, and must have been introduced by the sea-currents or by
migratory birds.
And so the earth became fitted for the residence of modern man. Yet it is
not so good or Edenic a world as it once was, or as it may yet become,
were another revolution to restore a mild climate to the arctic regions,
and to send down a new swarm of migratory species to renew the face of
the earth and restore it to its pristine fertility of vegetable life.
Thus closes this long history of the succession of plants, reaching
from the far back Laurentian to the present day. It has, no doubt, many
breaks, and much remains to be discovered. Yet it may lead us to some
positive conclusions regarding the laws of the introduction of plants.
One of these, and perhaps the most remarkable of all, is that certain
principles were settled very far back, and have remained ever since.
We have seen that in the earliest geological periods all that pertains
to the structure, powers, and laws of the vegetable cell was already
fixed and settled. When we consider how much this implies of mechanical
structure and chemical and vital property, the profound significance
of this statement becomes apparent. The relations in these respects
between the living cell and the soil, the atmosphere and the sunshine,
were apparently as perfect in the early Palæozoic as in any subsequent
time. The same may be said of the structures of the leaf and of the
stem. In such old forms as Nematophyton these were, it is true, peculiar
and rudimentary, but in the Devonian and Carboniferous the structure
of leaves and stems embodied all the parts and principles that we find
at present. In regard to fructification there has been more progress,
for, so far as we know, the highest and most complex forms of flowery,
fruits, and seeds belong to the more recent periods, and simpler forms
were at least dominant in the older times. Yet even in this respect the
great leading laws and structures of bisexual reproduction were perfected
in the early Palæozoic, and the improvements introduced in the gymnosperm
and the angiosperm of later periods have consisted mainly in additions of
accessory parts, and in modifications and refinements suited to the wants
of the higher and more complex types.
CHAPTER VIII.
GENERAL LAWS OF ORIGIN AND MIGRATIONS OF PLANTS.--RELATIONS OF RECENT
AND FOSSIL FLORAS.
The origination of the successive floras which have occupied the northern
hemisphere in geological time, not, as one might at first sight suppose,
in the sunny climes of the south, but under the arctic skies, is a fact
long known or suspected. It is proved by the occurrence of fossil plants
in Greenland, in Spitzbergen, and in Grinnell Land, under circumstances
which show that these were their primal homes. The fact bristles with
physical difficulties, yet is fertile of the most interesting theoretical
deductions, to reach which we may well be content to wade through some
intricate questions. Though not at all a new fact, its full significance
seems only recently to have dawned on the minds of geologists, and within
the last few years it has produced a number of memoirs and addresses to
learned societies, besides many less formal notices.[ED]
[ED] Saporta, "Ancienne Végétation Polaire"; Hooker, "Presidential
Address to Royal Society," 1878; Thistleton Dyer, "Lecture on Plant
Distribution"; Mr. Starkie Gardner, "Letters in 'Nature,'" 1878, &c. The
basis of most of these brochures is to be found in Heer's "Flora Fossilis
Arctica."
The earliest suggestion on the subject known to the writer is that of
Prof. Asa Gray, in 1867, with reference to the probable northern source
of the related floras of North America and eastern Asia. With the aid
of the new facts disclosed by Heer and Lesquereux, Gray returned to
the subject in 1872, and more fully developed this conclusion with
reference to the Tertiary floras,[EE] and he has recently still further
discussed these questions in an able lecture on "Forest Geography and
Archæology."[EF] In this he puts the case so well and tersely that we may
quote the following sentences as a text for what follows:
[EE] Address to American Association.
[EF] "American Journal of Science," xvi., 1818.
"I can only say, at large, that the same species (of Tertiary fossil
plants) have been found all round the world; that the richest and
most extensive finds are in Greenland; that they comprise most of
the sorts which I have spoken of, as American trees which once lived
in Europe--magnolias, sassafras, hickories, gum-trees, our identical
southern cypress (for all we can see of difference), and especially
_Sequoias_, not only the two which obviously answer to the two
big-trees now peculiar to California, but several others; that they
equally comprise trees now peculiar to Japan and China, three kinds of
gingko-trees, for instance, one of them not evidently distinguishable
from the Japan species which alone survives; that we have evidence, not
merely of pines and maples, poplars, birches, lindens, and whatever
else characterise the temperate zone forests of our era, but also of
particular species of these, so like those of our own time and country
that we may fairly reckon them as the ancestors of several of ours. Long
genealogies always deal more or less in conjecture; but we appear to
be within the limits of scientific inference when we announce that our
existing temperate trees came from the north, and within the bounds of
nigh probability when we claim not a few of them as the originals of
present species. Remains of the same plants have been found fossil in our
temperate region as well as in Europe."
Between 1860 and 1870 the writer was engaged in working out all that
could be learned of the Devonian plants of eastern America, the oldest
known flora of any richness, and which consists almost exclusively of
gigantic, and to us grotesque, representatives of the club-mosses,
ferns, and mares'-tails, with some trees allied to the cycads and pines.
In this pursuit nearly all the more important localities were visited,
and access was had to the large collections of Prof. Hall and Prof.
Newberry, in New York and Ohio, and to those made in the remarkable
plant-bearing beds of New Brunswick by Messrs. Matthew and Hartt. In
the progress of these researches, which developed an unexpectedly rich
assemblage of species, the northern origin of this old flora seemed to be
established by its earlier culmination in the northeast, in connection
with the growth of the American land to the southward, which took place
after the great Upper Silurian subsidence, by elevations beginning in
the north while those portions of the continent to the southwest still
remained under the sea. The same result was indicated by the persistence
in the Carboniferous of the south and west of old Erian forms, like
_Megalopteris_.
When, in 1870, the labours of those ten years were brought before the
Royal Society of London, in the Bakerian lecture of that year, and in a
memoir illustrating no less than one hundred and twenty-five species of
plants older than the great Carboniferous system, these deductions were
stated in connection with the conclusions of Hall, Logan, and Dana, as
to the distribution of sediment along the northeast side of the American
continent, and the anticipation was hazarded that the oldest Palæozoic
floras would be discovered to the north of Newfoundland. Mention was also
made of the apparent earlier and more copious birth of the Devonian flora
in America than in Europe, a fact which is itself connected with the
greater northward extension of this continent.
The memoir containing these results was not published by the Royal
Society, but its publication was secured in a less complete form in the
reports of the "Geological Survey of Canada." The part of the memoir
relating to Canadian fossil plants, with a portion of the theoretical
deductions, was published in a report issued in 1871.[EG] In this report
the following language was used:
[EG] "Fossil Plants of the Devonian and Upper Silurian Formations of
Canada," pp. 92, twenty plates, Montreal, 1871.
"In eastern America, from the Carboniferous period onward, the centre
of plant distribution has been the Appalachian chain. From this the
plants and sediments extended westward in times of elevation, and to this
they receded in times of depression. But this centre was nonexistent
before the Devonian period, and the centre for this must have been to
the northeast, whence the great mass of older Appalachian sediment
was derived. In the Carboniferous period there was also an eastward
distribution from the Appalachians, and links of connection in the
Atlantic bed between the floras of Europe and America. In the Devonian
such connection can have been only far to the northeast. It is therefore
in Newfoundland, Labrador, and Greenland that we are to look for the
oldest American flora, and in like manner on the border of the old
Scandinavian nucleus for that of Europe.
"Again, it must have been the wide extension of the sea of the
corniferous limestone that gave the last blow to the remaining flora of
the Lower Devonian; and the re-elevation in the middle of that epoch
brought in the Appalachian ridges as a new centre, and established
a connection with Europe which introduced the Upper Devonian and
Carboniferous floras. Lastly, from the comparative richness of the later
Erian[EH] flora in eastern America, especially in the St. John beds, it
might be a fair inference that the northeastern end of the Appalachian
ridge was the original birthplace or centre of creation of what we may
call the later Palæozoic flora, or of a large part of that flora."
[EH] See pages 107 and 108.
When my paper was written I had not seen the account published by
the able Swiss palæobotanist Heer, of the remarkable Devonian flora
of Bear Island, near Spitzbergen.[EI] From want of acquaintance with
the older floras of America and western Europe, Heer fell into the
unfortunate error of regarding the whole of Bear Island plants as
Lower Carboniferous, a mistake which his great authority has tended to
perpetuate, and which has even led to the still graver error of some
European geologists, who do not hesitate to regard as Carboniferous the
fossil plants of the American deposits from the Hamilton to the Chemung
groups inclusive, though these belong to formations underlying the oldest
Carboniferous, and characterised by animal remains of unquestioned
Devonian age. In 1872 I addressed a note to the Geological Society of
London on the subject of the so-called "Ursa stage" of Heer, showing
that, though it contained some forms not known at so early a date in
temperate Europe, it was clearly, in part at least, Devonian when tested
by North American standards; but that in this high latitude, in which,
for reasons stated in the report above referred to, I believed the
Devonian plants to have originated, there might be an intermixture of the
two floras. But such a mixed group should in that latitude be referred
to a lower horizon than if found in temperate regions. Dr. Nathorst,
as already stated, has recently obtained new facts which go to show
that plants of two distinct horizons may have been intermixed in the
collections submitted to Heer.
[EI] "Transactions of the Swedish Academy" 1871; "Journal of the London
Geological Society," vol. xxviii.
Between 1870 and 1873 my attention was turned to the two sub-floras
intermediate between those of the Devonian and the coal-formation, the
floras of the Lower Carboniferous (Subcarboniferous of some American
geologists) and the Millstone Grit, and in a report upon these[EJ]
similar deductions were expressed. It was stated that in Newfoundland
the coal-beds seem to belong to the Millstone Grit series, and as we
proceed southward they belong to progressively newer portions of the
Carboniferous system. The same fact is observed in the coal-beds of
Scotland, as compared with those of England, and it indicates that the
coal-formation flora, like that of the Devonian, spread itself from the
north, and this accords with the somewhat extensive occurrence of Lower
Carboniferous rocks and fossils in the Parry Islands and elsewhere in the
arctic regions.
[EJ] "Fossil Plants of Lower Carboniferous and Millstone Grit Formations
of Canada," pp. 47, ten plates, Montreal, 1873.
Passing over the comparatively poor flora of the earlier Mesozoic,
consisting largely of cycads, pines, and ferns, and as yet little
known in the arctic, and which may have originated in the south,
though represented, according to Heer, by the supposed Jurassic flora
of Siberia, we find, especially at Komé and Atané in Greenland, an
interesting occurrence of those earliest precursors of the truly modern
forms of plants which appear in the Cretaceous, the period of the English
chalk and of the New Jersey greensands. There are two plant-groups of
this age in Greenland; one, that of Komé, consists almost entirely of
ferns, cycads, and pines, and is of decidedly Mesozoic aspect. This is
called Lower Cretaceous. The other, that of Atané, holds remains of many
modern temperate genera, as _Populus_, _Myrica_, _Ficus_, _Sassafras_,
and _Magnolia_. This is regarded as Upper Cretaceous. Resting upon
these Upper Cretaceous beds, without the intervention of any other
formation,[EK] are beds rich in plants of much more modern appearance,
and referred by Heer to the Miocene period, a reference, as we have
seen, not warranted by comparison with the Tertiary plants of Europe or
of America. Still farther north this so-called Miocene assemblage of
plants appears in Spitzbergen and Grinnell Land; but there, owing to the
predominance of trees allied to the spruces, it has a decidedly more
boreal character than in Greenland, as might be anticipated from its
nearer approach to the pole.[EL]
[EK] Nordenskiöld, "Expedition to Greenland," "Geological Magazine," 1872.
[EL] Yet even here the bald cypress (_Taxodium distichum_), or a tree
nearly allied to it, is found, though this species is now limited to
the Southern States. Fielden and De Ranee, "Journal of the Geological
Society," 1878.
If now we turn to the Cretaceous and Tertiary floras of western America,
as described by Lesquereux, Newberry, and others, we find in the lowest
Cretaceous rocks there known--those of the Dakota group--which may be
in the lower part of the Middle Cretaceous, a series of plants[EM]
essentially similar to those of the so-called Upper Cretaceous of
Greenland. They occur in beds indicating land and fresh-water conditions
as prevalent at the time over great areas of the interior of America.
But overlying this plant-bearing formation we have an oceanic limestone
(the Niobrara), corresponding in many respects to the European chalk,
and extending far north into the British territory,[EN] indicating that
the land of the Lower Cretaceous was replaced by a vast Mediterranean
Sea, filled with warm water from the equatorial currents, and not
invaded by cold waters from the north. This is succeeded by thick Upper
Cretaceous deposits of clay and sandstone, with marine remains, though
very sparsely distributed; and these show that further subsidence or
denudation in the north had opened a way for the arctic currents, killing
out the warm-water animals of the Niobrara group, and rilling up the
Mediterranean of that period. Of the flora of these Upper Cretaceous
periods, which must have been very long, we know something in the
interior regions, from the discovery of a somewhat rich flora in the
Dunvegan beds of the Peace River district, on the northern shore of the
great Cretaceous Mediterranean;[EO] and on the coast of British Columbia
we have the remarkable Cretaceous coal-field of Vancouver Island, which
holds the remains of plants of modern genera, and, indeed, of almost
as modern aspect as those of the so-called Miocene of Greenland. They
indicate, however, a warmer climate as then prevalent on the Pacific
coast, and in this respect correspond with a peculiar transition flora,
intermediate between the Cretaceous and Eocene or earliest Tertiary of
the interior regions, and which is described by Lesquereux as the Lower
Lignitic.
[EM] Lesquereux, "Report on Cretaceous Flora."
[EN] G. M. Dawson, "Report on Forty-ninth Parallel."
[EO] "Reports of Dr. G. M. Dawson, Geological Survey of Canada." Also,
"Transactions of the Royal Society of Canada," vol. i.
Immediately above these Upper Cretaceous beds we have the great
Lignite Tertiary of the West--the Laramie group of recent American
reports--abounding in fossil plants, at one time regarded as Miocene,
but now known to be Lower Eocene, though farther south extending upward
toward the Miocene age.[EP] These beds, with their characteristic plants,
have been traced into the British territory north of the forty-ninth
parallel, and it has been shown that their fossils are identical with
those of the McKenzie River valley, described by Heer as Miocene, and
probably also with those of Alaska, referred to the same age.[EQ] Now
this truly Eocene flora of the temperate and northern parts of America
has so many species in common with that called Miocene in Greenland that
its identity can scarcely be doubted. These facts have led to scepticism
as to the Miocene age of the upper plant-bearing beds of Greenland, and
more especially Mr. J. Starkie Gardner has ably argued, from comparison
with the Eocene flora of England and other considerations, that they are
really of that earlier date.[ER]
[EP] Lesquereux's "Tertiary Flora"; "White on the Laramie Group";
Stevenson, "Geological Relations of Lignitic Groups," American
Philosophical Society, June, 1875; Dawson, "Transactions of the Royal
Society of Canada," vol. iv.; Ward, "Bulletin of United States Geological
Survey."
[EQ] G. M. Dawson, "Report on the Geology of the Forty-ninth Parallel,"
where full details on these points may be found. "Transactions of the
Royal Society of Canada," vol. iv.
[ER] "Nature," December 12, 1878.
In looking at this question, we may fairly assume that no climate,
however equable, could permit the vegetation of the neighbourhood of
Disco in Greenland to be exactly identical with that of Colorado and
Missouri, at a time when little difference of level existed in the two
regions. Either the southern flora migrated north in consequence of a
greater amelioration of climate, or the northern flora moved southward as
the climate became colder. The same argument, as Gardner has ably shown,
applies to the similarity of the Tertiary plants of temperate Europe to
those of Greenland. If Greenland required a temperature of about 50°,
as Heer calculates, to maintain its Eocene flora, the temperature of
England and that of the Southwestern States must have been higher, though
probably more equable, than at present.
We cannot certainly affirm anything respecting the migrations of these
floras, but there are some probabilities which deserve attention. The
ferns and cycads of the so-called Lower Cretaceous of Greenland are
nothing but a continuation of the previous Jurassic flora. Now this was
established at an equally early date in the Queen Charlotte Islands,[ES]
and still earlier in Virginia,[ET] The presumption is, therefore,
that it came from the south. It has, indeed, the facies of a southern
hemisphere and insular flora, and probably spread itself northward as
far as Greenland, at a time when our northern continents were groups of
islands, and when the ocean currents were carrying warm water far toward
the arctic regions. The flora which succeeds this in the sections at
Atané has no special affinities with the southern hemisphere, and is of
a more temperate and continental character.[EU] It is not necessarily
Upper Cretaceous, since it is similar to that of the Dakota group farther
south, and this is at least Middle Cretaceous. This flora must have
originated either somewhere in temperate America or within the Arctic
circle, and it must have replaced the older one by virtue of increasing
coolness and continental character of climate. It must, therefore, have
been connected with that elevation of the land which took place at the
beginning of the Cretaceous. During this elevation it spread over all
western America at one time or another, and, as the land again subsided
under the sea of the Niobrara chalk, it assumed an aspect more suited
to a warm climate, but still held its place on such islands as remained
above water along the Pacific coast and in the north, and it continued
to exist on these islands till the colder seas of the Upper Cretaceous
had again given place to the warm plains and land-locked brackish seas
or fresh-water lakes of the Laramie period (Eocene). Thus the true Upper
Cretaceous marks a cool period intervening between the so-called Upper
Cretaceous (really Middle Cretaceous) and the so-called Miocene (really
Lower Eocene) floras of Greenland.
[ES] "Reports of the Geological Survey of Canada."
[ET] Fontaine has well described the Mesozoic flora of Virginia,
"American Journal of Science," January, 1879, and "Report on Early
Mesozoic Floras."
[EU] In the "Proceedings of the Royal Society of Tasmania," 1887, Mr. R.
M. Johnston, F. L. S., states that in the Miocene beds of Tasmania trees
of European genera abound. The Mesozoic flora of that island is of the
usual conifero-cycadean type. Ettingshausen makes a similar statement in
the "Geological Magazine" respecting the Tertiary flora of Australia and
New Zealand, stating that, like the Tertiary floras of Europe, they have
a mixed character, being partly of types now belonging to the northern
hemisphere.
This latter established itself in Greenland, and probably all around the
Arctic circle, in the warm period of the earliest Eocene, and, as the
climate of the northern hemisphere became gradually reduced from that
time till the end of the Pliocene, it marched on over both continents to
the southward, chased behind by the modern arctic flora, and eventually
by the frost and snow of the Glacial age. This history may admit of
correction in details; but, so far as present knowledge extends, it is in
the main not far from the truth.
Perhaps the first great question which it raises is that as to the causes
of the alternations of warm and cold climates in the north, apparently
demanded by the vicissitudes of the vegetable kingdom. Here we may set
aside the idea that in former times plants were suited to endure greater
cold than at present. It is true that some of the fossil Greenland
plants are of unknown genera, and many are species new to us; but we are
on the whole safe in affirming that they must have required conditions
similar to those necessary to their modern representatives, except within
such limits as we now find to hold in similar cases among existing
plants. Still we know that at the present time many species found in
the equable climate of England will not live in Canada, though species
to all appearance similar in structure are native here. There is also
some reason to suppose that species when new may have greater hardiness
and adaptability than when in old age and verging toward extinction. In
any case these facts can account for but a small part of the phenomena,
which require to be explained by physical changes affecting the earth as
a whole, or at least the northern hemisphere. Many theoretical views have
been suggested on this subject, and perhaps the most practical way of
disposing of these will be first to set aside a number which are either
precluded by the known facts, incapable of producing the effects, or
altogether uncertain as to their possible occurrence.
1. In this class we may place the theory that the poles of the earth have
changed their position. Independently of astronomical objections, there
is good geological evidence that the poles of the earth must have been
nearly in their present places from the dawn of life until now. From the
Laurentian upward, those organic limestones which mark the areas where
warm and shallow equatorial water was spreading over submerged continents
are so disposed as to prove the permanence of the poles. In like manner
all the great foldings of the crust of the earth have followed lines
which are parts of great circles tangent to the existing polar circles.
So, also, from the Cambrian age the great drift of sediment from the
north has followed the line of the existing Arctic currents from the
northeast to the southwest, throwing itself, for example, along the line
of the Appalachian uplifts in eastern America, and against the ridge of
the Cordilleras in the west.
2. Some of the above considerations, along with astronomical evidence,
prevent us from assuming any considerable change in the obliquity of the
axis of the earth during geological time.
3. That the earth and the sun have diminished in heat during geological
time seems probable; but physical and geological facts alike render it
certain that this influence could have produced no appreciable effect,
even in the times of the earliest floras, and certainly not in the case
of Tertiary vegetation.
4. It has been supposed that the earth may have at different times
traversed more or less heated zones of space, giving alternations of warm
and cold temperature. No such differences in space are, however, known,
nor does there seem any good ground for imagining their existence.
5. The heat of the sun is known to be variable, and the eleven years'
period of sun-spots has recently attracted much attention as producing
appreciable effects on the seasons. There may possibly be longer cycles
of solar energy, or the sun may be liable, like some variable stars, to
paroxysms of increased energy. Such changes are possible, and may fairly
be taken into the account, provided that we fail to find known causes
sufficient to account for the phenomena.
Of well-known causes there seem to be but three. These are: First, that
urged by Lyell--viz., the varying distribution of land and water along
with that of marine currents; secondly, the varying eccentricity of
the earth's orbit, along with the precession of the equinoxes, and the
effects of this on oceanic circulation, as illustrated by Croll; thirdly,
the different conditions of the earth's atmosphere with reference to
radiation, as argued by Tyndall and Hunt. As these causes are all founded
on known facts, and not exclusive of each other, we may consider them
together. I shall take the Lyellian theory first, regarding it as the
most important, and the best supported by geological facts.
We know that the present distribution of land and water greatly
influences climate, more especially by affecting that of the ocean
currents and of the winds, and by the different action of land as
compared with water in the reception and radiation of heat. The
present distribution of land gives a large predominance to the arctic
and sub-arctic regions, as compared with the equatorial and with the
antarctic; and we might readily imagine other distributions that would
give very different results. But this is not an imaginary case. We
know that, while the forms and positions of the great continents have
been fixed from a very early date, they have experienced many great
submergences and re-elevations, and that these have occurred in somewhat
regular sequence, as evidenced by the cyclical alternations of organic
limestones and earthy sediments in successive geological formations.