Chapter 11

[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 generaDicksonia,Gleichenia, andAspidium.

“Cycads.—Two species, probably identical with the forms from Greenland described by Heer under the names ofPodozamites marginatusandP. tenuinervis.

“Conifers.—Fourteen species, belonging to the generaMoriconia,Brachyphyllum,Cunninghamites,Pinus,Sequoia, and others referred by Heer toJuniperus,Libocedrus,Frenelopsis,Thuya, andDammara. Of these, the most abundant and most interesting areMoriconia cyclotoxon—the most beautiful of conifers—andCunninghamites elegans, both of which occur in the Cretaceous clays of Aachen, Prussia, and Patoot, Greenland. TheBrachyphyllumwas a large and strong species, with imbricated cones, eight inches in length.

“The angiosperms form about seventy species, which include three ofMagnolia, four ofLiriodendron, three or four ofSalix, three ofCelastrophyllum(of which one is identical with a Greenland species), oneCelastrus(also found in Greenland), four or fiveAralias, twoSassafras, oneCinnamomum, oneHedera; with leaves that are apparently identical with those described by Heer as belonging toAndromeda,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 ofLiriodendron.

“Among the most interesting plants of the collection are fine species ofBauhiniaandHymenæa. 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.Bauhiniais a leguminous genus allied toCercis, and now inhabitstropical 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.

“Hymenæais 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 toBauhinia.

“But the most surprising discovery yet made is that of a number of quite large helianthoid flowers, which I have calledPalæanthus. 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 ofHelichrysum. 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 namedWilliamsonia, and referred to cycads by Carruthers. A similar fossil has been found in the Cretaceous rocks of Greenland, and named by HeerWilliamsonia cretacea, but he questions the reference of the genus to the Cycadeæ, and agrees with Nathorst in considering all the species ofWilliamsoniaas parasitic flowers, allied toBrugmansiaorRafflesia. The Marquis of Saporta regards them as monocotyledons, similar toPandanus. More specimens of the flowers now exhibited will perhaps prove—what we can now only regard as probable—that the Compositæ, like theLeguminosæ,Magnoliaceæ,Celastraceæ, and other highly organised plants, formed partof 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 mouthof 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.

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 namedBrasenia antiqua, occurs in the beds associated with one of the coals. It is a close ally of the modernB. peltata, an aquatic plant which occurs in British Columbia and in easternAmerica, 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 beenstudied 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 LowerMiocene. 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, andespecially 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 LowerLaramie 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 theFilicites Hebridicusof 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 associatedwith this species another and more delicate fern, the modernDavallia(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 aThujaor arbor vitæ (the so-called “cedar” of Canada). The Laramie species has been namedT. interruptaby Newberry, but it approaches very closely in its foliage toT. 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 modernS. giganteaandS. 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 generaGlyptostrobus,Taxodium, andTaxus; and the genusSalisburiaor 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 ofPlatanus, 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. AmericanaandC. 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.Sassafrasis represented by two species, and the beautiful group ofViburnum,, 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, theCatalpaandSapindus, and some curious leaves which seem to indicate the presence of the modern genusSymphorocarpus, 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 ofSequoia,Gingko,Taxus, andGlyptostrobusare also identical or closely allied, and so are many of the dicotyledonous leaves. For example,Platanoides Hebridicusis very near toP. nobilis, andCorylus Macquarriiis common to both formations, as well asPopulus ArcticaandP. 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 groupof 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 importantrésuméof 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 ofEurope. 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 ofthe 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.

Itmay 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 fromfloating 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 differentlatitudes. 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 conditionsallowing 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 ofSalisburia,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 surerefuge 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 likeSequoiaandLiriodendron, 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 someremains 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.,Œnanthe Tichenalii, Sm.,Potamogeton pterophyllus, Sch., andPinus 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, andPinus sylvestris, L.) are found also in Siberia, while six more (Sanguisorba officinalis, L.,Rubus fruticosus, L., 5Cornus sanguinea, L.,Euphorbia amygdaloides, L.,Quercus robur, L., andPotamogeton crispus, L.) extend into western Asia, and two (Fagus sylvatica, L., andAlnus 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: theseareThalictrum minus, L.,Thalictrum flavum, L.,Ranunculus repens, L.,Stellaria aquatica, Scop.,Corylus avellana, L.,Yannichellia palustris, L., andCladium 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., andOsmunda 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., andIsoetes lacustris, L.) extend round the north temperate zone, while three (Lycopus europæus, L.,Alisma plantago, L., andPhragmites 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 byRanunculus aquatilis, L., distributed over all the temperate regions of the globe, andScirpus 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, onlySalix 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 iceextended 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 itPrunus padus, L.,Quercus robur, L.,Corylus avellana, L.,Alnus glutinosa, L., andPinus sylvestris, L. In the white clay-beds at Bovey Tracey of the same age there occur the leaves ofArctostaphylos uva-ursi, L., three species of willow, viz.,Salix cinerea, L.,S. myrtilloides, L., andS. polaris, Wahl., and in addition to our AlpineBetula nana, L., the more familiarB. 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., andS. polaris, Wahl.; andBetula nana, L., has retreated to the mountains of Scotland. Three others (Dryas octopetala, L.,Arctostaphylos uva-ursi, L., andSalix 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:

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