Part 20 (1/2)

Darwin has demonstrated, has nevertheless produced banks, or rather islets, of considerable extent, which at one time const.i.tuted veritable reefs rising out of the ocean. These reefs were princ.i.p.ally constructed in the Jura.s.sic period, and their extreme abundance is one of the characteristics of this geological age. The same phenomenon continues in our day, but by the agency of a new race of zoophytes, which carry on their operations, preparing a new continent, probably, in the _atolls_ of the Pacific Ocean. (See Fig. 108, p. 240.)

[Ill.u.s.tration: Fig. 117.

1, Otopteris dubia; 2, Otopteris obtusa; 3, Otopteris ac.u.minata; 4, Otopteris cuneata.]

The flora of the epoch was very rich. The Ferns continue to exist, but their size and bearing were sensibly inferior to what they had been in the preceding period. Among them Otopteris, distinguished for its simply pinnated leaves, whose leaflets are auriculate at the base: of the five species, 1, _O. dubia_; and 2, _O. obtusa_; and 3, _O.

ac.u.minata_; and 4, _O. cuneata_ (Fig. 117), are from the Oolite. In addition to these we may name _Coniopteris Murrayana_, _Pecopteris Desnoyersii, Pachypteris lanceolata_, and _Phlebopteris Phillipsii_; and among the Lycopods, _Lycopodus falcatus_.

The vegetation of this epoch has a peculiar facies, from the presence of the family of the Pandanaceae, or screw-pines, so remarkable for their aerial roots, and for the magnificent tuft of leaves which terminates their branches. Neither the leaves nor the roots of these plants have, however, been found in the fossil state, but we possess specimens of their large and spherical fruit, which leave no room for doubt as to the nature of the entire plant.

The Cycads were still represented by the _Zamias_, and by many species of Pterophyllum. The Conifers, that grand family of recent times, to which the pines, firs, and other trees of our northern forests belong, began to occupy an important part in the world's vegetation from this epoch. The earliest Conifers belonged to the genera _Thuites_, _Taxites_, and _Brachyphyllum_. The _Thuites_ were true _Thuyas_, evergreen trees of the present epoch, with compressed branches, small imbricated and serrated leaves, somewhat resembling those of the Cypress, but distinguished by many points of special organisation. The _Taxites_ have been referred, with some doubts, to the Yews. Finally, the _Brachyphyllum_ were trees which, according to the characteristics of their vegetation, seem to have approached nearly to two existing genera, the _Arthotaxis_ of Tasmania, and the _Weddringtonias_ of South Africa. The leaves of the Brachyphyllum are short and fleshy, with a large and rhomboidal base.

LOWER OOLITE ROCKS.

The formation which represents the Lower Oolite, and which in England attains an average thickness of from 500 to 600 feet, forms a very complex system of stratification, which includes the two formations, _Bajocien_ and _Bathonian_, adopted by M. D'Orbigny and his followers.

The lowest beds of the _Inferior Oolite_ occur in Normandy, in the Lower Alps (Ba.s.ses-Alpes), in the neighbourhoods of Lyons and Neuchatel. They are remarkable near Bayeux for the variety and beauty of their fossils: the rocks are composed princ.i.p.ally of limestones--yellowish-brown, or red, charged with hydrated oxide of iron, often oolitic, and reposing on calcareous sands. These deposits are surmounted by alternate layers of clay and marl, blue or yellow--the well-known _Fuller's Earth_, which is so called from its use in the manufacture of woollen fabrics to extract the grease from the wool. The second series of the Lower Oolite, which attains a thickness of from 150 to 200 feet on the coast of Normandy, and is well developed in the neighbourhood of Caen and in the Jura, has been divided, in Britain, into four formations, in an ascending scale:--

1. The _Great_ or _Bath Oolite_, which consists princ.i.p.ally of a very characteristic, fine-grained, white, soft, and well-developed oolitic limestone, at Bath, and also at Caen in Normandy. At the base of the Great Oolite the Stonesfield beds occur, in which were found the bones of the marsupial Mammals, to which we have already alluded; and along with them bones of Reptiles, princ.i.p.ally Pterodactyles, together with some finely-preserved fossil plants, fruits, and insects.

2. _Bradford Clay_, which is a bluish marl, containing many fine Encrinites (commonly called stone-lilies), but which had only a local existence, appearing to be almost entirely confined to this formation.

”In this case, however,” says Lyell, ”it appears that the solid upper surface of the 'Great Oolite' had supported, for a time, a thick submarine forest of these beautiful Zoophytes, until the clear and still water was invaded with a current charged with mud, which threw down the stone-lilies, and broke most of their stems short off near the point of attachment. The stumps still remain in their original position.”[65] See Fig. 1, PLATE XIX., p. 261. (Bradford, or Pear, Encrinite.)

[65] ”Elements of Geology,” p. 399.

3. _Forest Marble_, which consists of an argillaceous sh.e.l.ly limestone, abounding in marine fossils, and sandy and quartzose marls, is quarried in the forest of Wichwood, in Wilts.h.i.+re, and in the counties of Dorset, Wilts, and Somerset.

4. The _Cornbrash_ (wheat-lands) consists of beds of rubbly cream-coloured limestone, which forms a soil particularly favourable to the cultivation of cereals; hence its name.[66]

[66] See Bristow in Descriptive Catalogue of Rocks, in _Mus. Pract.

Geol._, p. 134.

[Ill.u.s.tration: Fig. 118.--Meandrina Daedalaea.

_a_, entire figure, reduced; _b_, portion, natural size.

(Recent Coral.)]

The Lower Oolite ranges across the greater part of England, but ”attains its maximum development near Cheltenham, where it can be subdivided, at least, into three parts. Pa.s.sing north, the two lower divisions, each more or less characterised by its own fossils, disappear, and the Ragstone north-east of Cheltenham lies directly upon the Lias; apparently as conformably as if it formed its true and immediate successor, while at Dundry the equivalents of the upper freestones and ragstones (the lower beds being absent) lie directly on the exceedingly thin sands, which there overlie the Lower Lias. In Dorsets.h.i.+re, on the coast, the series is again perfect, though thin. Near Chipping Norton, in Oxfords.h.i.+re, the Inferior Oolite disappears altogether, and the Great Oolite, having first overlapped the Fullers' Earth, pa.s.ses across the Inferior Oolite, and in its turn seems to lie on the Upper Lias with a regularity as perfect as if no formation in the neighbourhood came between them. In Yorks.h.i.+re the changed type of the Inferior Oolite, the prevalence of sands, land-plants, and beds of coal, occur in such a manner as to leave no doubt of the presence of terrestrial surfaces on which the plants grew, and all these phenomena lead to the conclusion that various and considerable oscillations of level took place in the British area during the deposition of the strata, both of the Inferior Oolite and of the formations which immediately succeed it.”[67]

[67] President's Address, by Professor A. C. Ramsay. _Quart. Jour.

Geol. Soc._, 1864, vol. xx., p. 4.

The Inferior Oolite here alluded to is a thin bed of calcareous freestone, resting on, and sometimes replaced by yellow sand, which const.i.tutes the pa.s.sage-beds from the Lia.s.sic series. The Fullers' Earth clay lies between the limestones of the Inferior and Great Oolite, at the base of which last lies the Stonesfield slate--a slightly oolitic, sh.e.l.ly limestone, or flaggy and fissile sandstone, some six feet thick, rich in organic remains, and ranging through Oxfords.h.i.+re towards the north-east, into Northamptons.h.i.+re and Yorks.h.i.+re. At Colley Weston, in Northamptons.h.i.+re, fossils of _Pecopteris polypodioides_ are found. In the Great Oolite formation, near Bath, are many corals, among which the _Eunomia radiata_ is very conspicuous. The fossil is not unlike the existing brain-coral of the tropical seas (Fig. 118). The work of this coral seems to have been suddenly stopped by ”an invasion,” says Lyell, ”of argillaceous matter, which probably put a sudden stop to the growth of Bradford Encrinites, and led to their preservation in marine strata.”[68] The Cornbrash is, in general, a cream-coloured limestone, about forty feet thick, in the south-west of England, and occupying a considerable area in Dorsets.h.i.+re and North Wilts, as at Cricklade, Malmesbury, and Chippenham, in the latter county. _Terebratula obovata_ is its characteristic sh.e.l.l, and _Nucleolites clunicularis_, _Lima gibbosa_, and _Avicula echinata_ occur constantly in great numbers.

Wherever it occurs the Cornbrash affords a rich and fertile soil, well adapted for the growth of wheat, while the Forest Marble, as a soil, is generally poor. The Cornbrash pa.s.ses downwards into the Forest Marble, and sometimes, as at Bradford, near Bath, is replaced by clay. This clay, called the Bradford clay, is almost wholly confined to the county of Wilts. _Terebratula decussata_ is one of the most characteristic fossils, but the most common is the Apiocrinites or pear-shaped encrinite, whose remains in this clay are so perfectly preserved that the most minute articulations are often found in their natural positions. PLATE XIX., p. 261 (Fig. 1), represents an adult attached by a solid base to the rocky bottom on which it grew, whilst the smaller individuals show the Encrinite in its young state--one with arms expanded, the other with them closed. Ripple-marked slabs of fissile Forest Marble are used as a roofing-slate, and may be traced over a broad band of country in Wilts.h.i.+re and Gloucesters.h.i.+re, separated from each other by thin seams of clay, in which the undulating ridges of the sand are preserved, and even the footmarks of small Crustaceans are still visible.

[68] ”Elements of Geology,” p. 400.

[Ill.u.s.tration: XVII.--Ideal Landscape of the Lower Oolite Period.]