Professor Beyrich has made known to us the existence of a long succession of marine strata in North Germany, which lead by an almost gradual transition from beds of Upper Miocene age to others of the age of the base of the Lower Miocene. Although some of the German lignites called Brown Coal belong to the upper parts of this series, the most important of them are of Lower Miocene date, as, for example, those of the Siebengebirge, near Bonn, which are associated with volcanic rocks.
Professor Beyrich confines the term “Miocene” to those strata which agree in age with the faluns of Touraine, and he has proposed the term “Oligocene” for those older formations called Lower Miocene in this work.
Lower Miocene of Italy.—In the hills of which the Superga forms a part there is a great series of Tertiary strata which pass downward into the Lower Miocene. Even in the Superga itself there are some fossil plants which, according to Heer, have never been found in Switzerland so high as the marine Molasse, such as Banksia longifolia, and Carpinus grandis. In several parts of the Ligurian Apennines, as at Dégo and Carcare, the Lower Miocene appears, containing some nummulites, and at Cadibona, north of Savona, fresh-water strata of the same age occur, with dense beds of lignite inclosing remains of the Anthracotherium magnum and A. minimum, besides other mammalia enumerated by Gastaldi. In these beds a great number of the Lower Miocene plants of Switzerland have been discovered.
Lower Miocene of England—Hempstead Beds.—We have already stated that the Upper Miocene formation is nowhere represented in the British Isles; but strata referable to the Lower Miocene period are found both in England, Scotland, and Ireland. In the Hampshire basin these occupy a very small superficial area, having been discovered by the late Edward Forbes at Hempstead near Yarmouth, in the northern part of the Isle of Wight, where they are 170 feet thick, and rich in characteristic marine shells. They overlie the uppermost of an extensive series of Eocene deposits of marine, brackish, and fresh-water formations, which rest on the Chalk and terminate upward in strata corresponding in age to the Paris gypsum, and containing the same extinct genera of quadrupeds, Palæotherium, Anoplotherium, and others which Cuvier first described. The following is the succession of these Lower Miocene strata, most of them exposed in a cliff east of Yarmouth:
1. The uppermost or Corbula beds, consisting of marine sands and clays, contain Voluta Rathieri, a characteristic Lower Miocene shell; Corbula pisum (Fig. 158), a species common to the Upper Eocene clay of Barton; Cyrena semistriata (Fig. 159), several Cerithia, and other shells peculiar to this series.
2. Next are fresh-water and estuary marls and carbonaceous clays in the brackish-water portion of which are found abundantly Cerithium plicatum, Lam. (Fig. 160), Cerithium elegans (Fig. 161), and Cerithium tricinctum; also Rissoa Chastelii (Fig. 162), a very common Kleyn Spawen shell, and which occurs in each of the four subdivisions of the Hempstead series down to its base, where it passes into the Bembridge beds. In the fresh-water portion of the same beds Paludina lenta (Fig. 163) occurs; a shell identified by some conchologists with a species now living, P. unicolor; also several species of Lymneus, Planorbis, and Unio.
3. The next series, or middle fresh-water and estuary marls, are distinguished by the presence of Melania fasciata, Paludina lenta, and clays with Cypris; the lowest bed contains Cyrena semistriata (Fig. 159), mingled with Cerithia and a panopæa.
4. The lower fresh-water and estuary marls contain Melania costata, Sowerby, Melanopsis, etc. The bottom bed is carbonaceous, and called the “Black band,” in which Rissoa Chastelii (Fig. 162), before alluded to, is common. This bed contains a mixture of Hempstead shells with those of the underlying Upper Eocene or Bembridge series. The mammalia, among which is Hyopotamus bovinus, differ, so far as they are known, from those of the Bembridge beds. Among the plants, Professor Heer has recognised four species common to the lignite of Bovey Tracey, a Lower Miocene formation presently to be described: namely, Sequoia Couttsiæ, Heer; Andromeda reticulata, Ettings.; Nelumbium (Nymphœa) doris, Heer; and Carpolithes Websteri, Brong.[[4]] The seed-vessels of Chara medicaginula, Brong, and C. helicteres are characteristic of the Hempstead beds generally.
The Hyopotamus belongs to the hog tribe, or the same family as the Anthracotherium, of which seven species, varying in size from the hippopotamus to the wild boar, have been found in Italy and other part of Europe associated with the lignites of the Lower Miocene period.