The Student's Elements of Geology Part 26

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 Hamps.h.i.+re 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 sh.e.l.ls. 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, Palaeotherium, 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:

(FIGURE 158. Corbula pisum. Hempstead Beds, Isle of Wight.)

(FIGURE 159. Cyrena semistriata. Hempstead Beds.)

1. The uppermost or Corbula beds, consisting of marine sands and clays, contain Voluta Rathieri, a characteristic Lower Miocene sh.e.l.l; Corbula pisum (Figure 158), a species common to the Upper Eocene clay of Barton; Cyrena semistriata (Figure 159), several Cerithia, and other sh.e.l.ls peculiar to this series.

(FIGURE 160. Cerithium plicatum, Lam., Hempstead.)

(FIGURE 161. Cerithium elegans. Hempstead.)

(FIGURE 162. Rissoa Chastelii, Nyst, sp. Hempstead, Isle of Wight.)

(FIGURE 163. Paludina lenta. Hempstead Bed.)

2. Next below are fresh-water and estuary marls and carbonaceous clays in the brackish-water portion of which are found abundantly Cerithium plicatum, Lam.

(Figure 160), Cerithium elegans (Figure 161), and Cerithium tricinctum; also Rissoa Chastelii (Figure 162), a very common Kleyn Spawen sh.e.l.l, and which occurs in each of the four subdivisions of the Hempstead series down to its base, where it pa.s.ses into the Bembridge beds. In the fresh-water portion of the same beds Paludina lenta (Figure 163) occurs; a sh.e.l.l identified by some conchologists with a species now living, Paludina 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 (Figure 159), mingled with Cerithia and a panopaea.

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 (Figure 162), before alluded to, is common. This bed contains a mixture of Hempstead sh.e.l.ls 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 Couttsiae, Heer; Andromeda reticulata, Ettings.; Nelumbium (Nymphoea) doris, Heer; and Carpolithes Websteri, Brong. (Pengelly, preface to The Lignite Formation of Bovey Tracey page 17. London 1863.) The seed-vessels of Chara medicaginula, Brong, and Chara 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 a.s.sociated with the lignites of the Lower Miocene period.

LIGNITES AND CLAYS OF BOVEY TRACEY, DEVONs.h.i.+RE.

Surrounded by the granite and other rocks of the Dartmoor hills in Devons.h.i.+re, is a formation of clay, sand, and lignite, long known to geologists as the Bovey Coal formation, respecting the age of which, until the year 1861, opinions were very unsettled. This deposit is situated at Bovey Tracey, a village distant eleven miles from Exeter in a south-west, and about as far from Torquay in a north-west direction. The strata extend over a plain nine miles long, and they consist of the materials of decomposed and worn-down granite and vegetable matter, and have evidently filled up an ancient hollow or lake-like expansion of the valleys of the Bovey and Teign.

The lignite is of bad quality for economical purposes, as there is a great admixture in it of iron pyrites, and it emits a sulphurous odour, but it has been successfully applied to the baking of pottery, for which some of the fine clays are well adapted. Mr. Pengelly has confirmed Sir H. De la Beche's opinion that much of the upper portion of this old lacustrine formation has been removed by denudation. (Philosophical Transactions 1863. Paper by W. Pengelly F.R.S. and Dr. Oswald Heer.)

At the surface is a dense covering of clay and gravel with angular stones probably of the Post-pliocene period, for in the clay are three species of willow and the dwarf birch, Betula nana, indicating a climate colder than that of Devons.h.i.+re at the present day.

Below this are Lower Miocene strata about 300 feet in thickness, in the upper part of which are twenty-six beds of lignite, clay, and sand, and at their base a ferruginous quartzose sand, varying in thickness from two to twenty-seven feet. Below this sand are forty-five beds of alternating lignite and clay. No sh.e.l.ls or bones of mammalia, and no insect, with the exception of one fragment of a beetle (Buprestis); in a word, no organic remains, except plants, have as yet been found. These plants occur in fourteen of the beds-- namely, in two of the clays, and the rest in the lignites. One of the beds is a perfect mat of the debris of a coniferous tree, called by Heer Sequoia Couttsiae, intermixed with leaves of ferns. The same Sequoia (before mentioned as a Hempstead fossil) is spread through all parts of the formation, its cones, and seeds, and branches of every age being preserved. It is a species supplying a link between Sequoia Langsdorfii (see Figure 153) and Sequoia Sternbergi, the widely spread fossil representatives of the two living trees Sequoia sempervirens and Sequoia gigantea (or Wellingtonia), both now confined to California. Another bed is full of the large rhizomes of ferns, while two others are rich in dicotyledonous leaves. In all, Professor Heer enumerates forty-nine species of plants, twenty of which are common to the Miocene beds of the Continent, a majority of them being characteristic of the Lower Miocene. The new species, also of Bovey, are allied to plants of the older Miocene deposits of Switzerland, Germany, and other Continental countries. The grape-stones of two species of vine occur in the clays, and leaves of the fig and seeds of a water-lily. The oak and laurel have supplied many leaves. Of the triple-nerved laurels several are referred to Cinnamomum. There are leaves also of a palm of which the genus is not determined. Leaves also of proteaceous forms, like some of the Continental fossils before mentioned, occur, and ferns like the well-known Lastraea stiriaca (Figure 154), displaying at Bovey, as in Switzerland, its fructification.

The croziers of some of the young ferns are very perfect, and were at first mistaken by collectors for sh.e.l.ls of the genus Planorbis. On the whole, the vegetation of Bovey implies the existence of a sub-tropical climate in Devons.h.i.+re, in the Lower Miocene period.

SCOTLAND: ISLE OF MULL.

In the sea-cliffs forming the headland of Ardtun, on the west coast of Mull, in the Hebrides, several bands of tertiary strata containing leaves of dicotyledonous plants were discovered in 1851 by the Duke of Argyll. (Quarterly Geological Journal 1851 page 19.) From his description it appears that there are three leaf-beds, varying in thickness from 1 1/2 to 5 1/2 feet, which are interstratified with volcanic tuff and trap, the whole ma.s.s being about 130 feet in thickness. A sheet of basalt 40 feet thick covers the whole; and another columnar bed of the same rock, ten feet thick, is exposed at the bottom of the cliff. One of the leaf-beds consists of a compressed ma.s.s of leaves unaccompanied by any stems, as if they had been blown into a marsh where a species of Equisetum grew, of which the remains are plentifully imbedded in clay.

It is supposed by the Duke of Argyll that this formation was acc.u.mulated in a shallow lake or marsh in the neighbourhood of a volcano, which emitted showers of ashes and streams of lava. The tufaceous envelope of the fossils may have fallen into the lake from the air as volcanic dust, or have been washed down into it as mud from the adjoining land. Even without the aid of organic remains we might have decided that the deposit was newer than the chalk, for chalk- flints containing cretaceous fossils were detected by the duke in the princ.i.p.al ma.s.s of volcanic ashes or tuff. (Quarterly Geological Journal 1851 page 90.)

The late Edward Forbes observed that some of the plants of this formation resembled those of Croatia, described by Unger, and his opinion has been confirmed by Professor Heer, who found that the conifer most prevalent was the Sequoia Langsdorfii (Figure 153), also Corylus grossedentata, a Lower Miocene species of Switzerland and of Menat in Auvergne. There is likewise a plane-tree, the leaves of which seem to agree with those of Plata.n.u.s aceroides (Figure 141 Chapter 14), and a fern which is as yet peculiar to Mull, Filicites hebridica, Forbes.

These interesting discoveries in Mull led geologists to suspect that the basalt of Antrim, in Ireland, and of the celebrated Giant's Causeway, might be of the same age. The volcanic rocks that overlie the chalk, and some of the strata a.s.sociated with and interstratified between ma.s.ses of basalt, contain leaves of dicotyledonous plants, somewhat imperfect, but resembling the beech, oak, and plane, and also some coniferae of the genera pine and Sequoia. The general dearth of strata in the British Isles, intermediate in age between the formation of the Eocene and Pliocene periods, may arise, says Professor Forbes, from the extent of dry land which prevailed in that vast interval of time. If land predominated, the only monuments we are likely ever to find of Miocene date are those of lacustrine and volcanic origin, such as the Bovey Coal in Devons.h.i.+re, the Ardtun beds in Mull, or the lignites and a.s.sociated basalts in Antrim.

LOWER MIOCENE, UNITED STATES: NEBRASKA.

In the territory of Nebraska, on the Upper Missouri, near the Platte River, lat.i.tude 42 degrees N., a tertiary formation occurs, consisting of white limestone, marls, and siliceous clay, described by Dr. D. Dale Owen (David Dale Owen Geological Survey of Wisconsin etc. Philadelphia 1852.), in which many bones of extinct quadrupeds, and of chelonians of land or fresh-water forms, are met with. Among these, Dr. Leidy describes a gigantic quadruped, called by him t.i.tanotherium, nearly allied to the Palaeotherium, but larger than any of the species found in the Paris gypsum. With these are several species of the genus Oreodon, Leidy, uniting the characters of pachyderms and ruminants also; Eucrotaphus, another new genus of the same mixed character; two species of rhinoceros of the sub-genus Acerotherium, a Lower Miocene form of Europe before mentioned; two species of Archaeotherium, a pachyderm allied to Chaeropotamus and Hyracotherium; also Paebrotherium, an extinct ruminant allied to Dorcatherium, Kaup; also Agrioch.o.e.rus, of Leidy, a ruminant allied to Merycopotamus of Falconer and Cautley; and, lastly, a large carnivorous animal of the genus Machairodus, the most ancient example of which in Europe occurs in the Lower Miocene strata of Auvergne, but of which some species are found in Pliocene deposits. The turtles are referred to the genus Testudo, but have some affinity to Emys. On the whole, the Nebraska formation is probably newer than the Paris gypsum, and referable to the Lower Miocene period, as above defined.

CHAPTER XVI.

EOCENE FORMATIONS.

Eocene Areas of North of Europe.

Table of English and French Eocene Strata.

Upper Eocene of England.

Bembridge Beds.

Osborne or St. Helen's Beds.

Headon Series.

Fossils of the Barton Sands and Clays.

Middle Eocene of England.

Sh.e.l.ls, Nummulites, Fish and Reptiles of the Bracklesham Beds and Bagshot Sands.

Plants of Alum Bay and Bournemouth.

Lower Eocene of England.

London Clay Fossils.

Woolwich and Reading Beds formerly called "Plastic Clay."

Fluviatile Beds underlying Deep-sea Strata.

Thanet Sands.

Upper Eocene Strata of France.

Gypseous Series of Montmartre and Extinct Quadrupeds.

Fossil Footprints in Paris Gypsum.

Imperfection of the Record.

Calcaire Silicieux.

Gres de Beauchamp.

Calcaire Grossier.

Miliolite Limestone.

Soissonnais Sands.

Lower Eocene of France.

Nummulitic Formations of Europe, Africa, and Asia.

Eocene Strata in the United States.

Gigantic Cetacean.

EOCENE AREAS OF THE NORTH OF EUROPE.

(FIGURE 164. Map of the princ.i.p.al Eocene areas of North-western Europe, showing: Shaded dotted: Hypogene rocks and strata older than the Devonian.

Shaded horizontal lines: Eocene formations.

NB.-- the s.p.a.ce left blank is occupied by fossiliferous formations from the Devonian to the chalk inclusive.)

The strata next in order in the descending series are those which I term Eocene.

In the map in Figure 164, the position of several Eocene areas in the north of Europe is pointed out. When this map was constructed I cla.s.sed as the newer part of the Eocene those Tertiary strata which have been described in the last chapter as Lower Miocene, and to which M. Beyrich has given the name of Oligocene. None of these occur in the London Basin, and they occupy in that of Hamps.h.i.+re, as we have seen in Chapter 15, too insignificant a superficial area to be noticed in a map on this scale. They fill a larger s.p.a.ce in the Paris Basin between the Seine and the Loire, and const.i.tute also part of the northern limits of the area of the Netherlands which are shaded in the map.

TABLE 16.1. TABLE OF ENGLISH AND FRENCH EOCENE STRATA.