though already more than 350 species have been determined from
this newer series. The plants from the Amboy Clays, which form
the most important division of the Newer Potomac series and were
monographed in 1895 by J. S. Newberry, seem to belong to the commencement
of the Upper Cretaceous period. It is remarkable
that nearly 80% of the species are Dicotyledons, and that no
Monocotyledons have been found. The mere enumeration of the
genera will indicate how close the flowering plants are to living
forms. Newberry records Juglans, Myrica (7 species), Populus,
Salix (5 species), Quercus, Planera, Ficus (3 species), Persoonia
and another extinct Proteaceous genus named Proteoides, Magnolia
(7 species), Liriodendron (4 species), Menispermites, Laurus and
allied plants, Sassafras (3 species), Cinnamomum, Prunus, Hymenaea,
Dalbergia, Bauhinia, Caesalpinia, Fontainea, Colutea and other
Leguminosae, Ilex, Celastrus, Celastrophyllum (10 species), Acer,
Rhamnites, Paliurus, Cissites, Tiliaephyllum, Passiflora, Eucalyptus
(5 species), Hedera, Aralia (8 species), Cornophyllum, Andromeda
(4 species), Myrsine, Sapotacites, Diospyros, Acerates, Viburnum
and various genera of uncertain affinities. The points that suggest
themselves with regard to this flora are, that it includes a fair
representation of the existing orders of warm-temperate deciduous
trees; that the more primitive types—such as the Amentaceae—do
not appear to preponderate to a greater extent than they do in the
existing temperate flora; that the assemblage somewhat suggests
American affinities; and that when we take into account deficient
collecting, local conditions, and the non-preservation of succulent
plants, there is no reason for saying that certain other orders must
have been absent. The great rarity of Monocotyledons is a common
characteristic of fossil floras known only, as this one is, from leaves
principally belonging to deciduous trees. With regard to suggested
American affinities, it must be borne in mind that the Neocomian
Angiosperms are little known except in America and in Greenland,
and that we therefore cannot yet say whether families now mainly
American were not formerly of world-wide distribution. We
know that this was the case with some, such as Liriodendron; and
in Eucalyptus we see the converse, where a genus formerly American
is now confined to a far distant region. The Neocomian flora has
been collected from an area extending over about 30° of latitude;
but there is little evidence of any corresponding climatic change.
We cannot yet say, however, that the deposits are exactly contemporaneous,
and the great climatic variations that have taken
place in the northern hemisphere during the existence of our living
flora should make us hesitate to correlate too minutely from the
evidence of plants alone.
The highest division of the Dakota series (known as Dakota No. 1) which lies immediately beneath Upper Cretaceous strata with marine fossils, contains a flora so like that of the Tertiary deposits that only the clearest geological evidence has been considered sufficient to prove that Heer was wrong when he spoke of the plants as Miocene. These highest plant-bearing strata rest, according to Lester Ward, somewhat unconformable on the Dakota No. 2; they show also a marked difference in the included plants. The genera of Dicotyledons represented are Quercus, Sassafras, Platanus, Celastrophyllum, Cissites, Viburnites.
In the central parts of North America the lacustrine plant-bearing deposits are of enormous thickness, the Dakota series being followed by marine Cretaceous strata known as the Colorado and Montana groups, and these being succeeded conformably by a thousand feet or more of lacustrine shales, sandstones and coal-seams, belonging to the Laramie series. This also contains occasional marine Upper Cretaceous fossils, as well as reptiles of Cretaceous types. An extensive literature has grown up relating to these Laramie strata, for owing to the Tertiary aspect of the contained plants, geologists were slow to recognize that they could be truly contemporaneous and interbedded with others yielding Cretaceous animals. In addition to this, the earlier writers included in the Laramie series many deposits now known to be of later date and truly Tertiary, and the process of separation is even now only partially completed. It will be safest in these circumstances to accept as our guide to the true Laramie flora the carefully compiled “Catalogue” of F. H. Knowlton. According to this catalogue, the true Laramie flora includes about 250 species, more than half of which are deciduous forest trees, herbaceous Dicotyledons, Monocotyledons and Cryptogams, all being but poorly represented. Among the few Monocotyledons are leaves and fruits of palms, and traces of grasses and sedges. The Dicotyledons include several water-lilies, a somewhat doubtful Trapa, and many genera of forest trees still common in America. The genera best represented are Ficus (21 species), Quercus (16 species), Populus (11 species), Rhamnus (9 species), Platanus (8 species), Viburnum (7 species), Magnolia (6 species), Cornus (5 species), Cinnamomum (5 species), Juglans (4 species), Acer (4 species), Salix (4 species), Aralia (3 species), Rhus (3 species), Sequoia (3 species). Of trees now extinct in America, Eucalyptus and Ginkgo are perhaps the most noticeable. So large a proportion of the trees still belongs to the flora of North America that one is apt to overlook the fact that among the more specialized plants some of the largest American orders, such as the Compositae, are still missing from strata belonging to the Cretaceous period.
The imperfection and want of continuity of the records in Europe have made it necessary in dealing with the Cretaceous European Cretaceous. floras for us to give the first place to America. But it is now advisable to return to Europe, where Upper Cretaceous plants are not uncommon, and the position of the deposits in the Cretaceous series can often be fixed accurately by their close association with marine strata belonging to definite subdivisions. As these divisions of Cretaceous time will have to be referred to more than once, it will be useful to tabulate them, thus showing which plant-beds seem to be referable to each, and what are the British strata of like age. It has not yet been found possible so closely to correlate the strata of Europe with those of America, where distance has allowed geographical differences in both fauna and flora to come into play; therefore, beyond the references to Lower or Upper Cretaceous, no classification of the American Cretaceous strata has here been given. In Europe the most commonly accepted divisions of the Cretaceous period are as follows:—
| England. | France, &c. | ||||
| Wanting | Danian | ||||
| Upper Chalk | Senonian | ||||
| Middle Chalk | Turonian | ||||
|
Cenomanian | ||||
| Gault | Albian | ||||
| Aptian | |||||
| Lower Green-sand | Valenginian | ||||
| Urgonian | |||||
| Wealden | Neocomian |
In the continental classification the deposits from the Gault downwards are grouped as Lower Cretaceous; but in Great Britain there is a strong break below the Gault and none above; and the Gault is therefore classed as Upper Cretaceous. The limits of the divisions in other places do not correspond, the British and continental strata often being so unlike that it is almost impossible to compare them. The doubt as to the exact British equivalent of the Valenginian strata of Portugal, which yield the earliest Dicotyledon, has already been alluded to. The plant-bearing deposits next in age, which have yielded Angiosperms, appear to belong to the Cenomanian, though from Westphalia a few species belonging to the Cryptogams and Gymnosperms, found in deposits correlated with the Gault, have been described by Hosius and von der Marck.
|
| Fig. 3.—Credneria triacuminata. |
In Great Britain the whole of the Upper Cretaceous strata are of marine origin, and have yielded no land-plants beyond a few fir-cones, drift-wood and rare Dicotyledonous leaves in the Lower Chalk. Most of the deposits which have yielded Angiosperms of Cretaceous age in central Europe correspond in age with the English Upper Chalk (Senonian), but a small Cenomanian flora has been collected from the Unter Quader in Moravia. Heer described from this deposit at Moletein 13 genera, of which 7 are still living, containing 18 species, viz.: 1 fern, 4 Conifers, i palm, 2 figs, 1 Credneria, 2 laurels, 1 Aralia, 1 Chondrophyllum (of uncertain affinities), 2 magnolias, 2 species of Myrtaceae and a species of walnut. Saxony yields from strata of this period at Niederschoena 42 species, described by Ettingshausen. This small flora is most remarkable, for no fewer than 6 genera, containing 8 species, are referred to the Proteaceae. The Cenomanian flora of Bohemia is larger and equally peculiar. Among the Dicotyledons described by Velenovsky are the following: Credneria (5 species), Araliaceae (17 species), Proteaceae (8 species), Myrica (2 species), Ficus (5 species), Quercus (2 species), Magnoliaceae (5 species), Bombaceae (3 species), Laurineae (2 species), Ebenaceae (2 species), Verbenaceae, Combretaceae, Sapindaceae (2 species), Camelliaceae, Ampelideae, Minioseae, Caesalpinieae (5 species), Eucalyptus (2 species), Pisonia, Phillyrea, Rhus, Prunus, Bignonia,
