Obrazy na stronie
PDF
ePub

blue marl. It contains only a few irregular and rubbly limestone beds. In the lower part, the limestone beds are more frequent, and assume the peculiar aspect of lias, namely a series of thin stony beds separated by narrow argillaceous seams, giving the quarries of this rock, a striped and ribband-like appearance at a distance. In the undermost beds of this limestone, the argillaceous partings often become so thin as almost to disappear, a circumstance observable in the lias tract of South Wales. Beds of blue marl with irregular calcareous masses, generally separate these strata from the red marl of the subjacent new red sandstone formation. The limestone beds towards their centre when freest from admixture, contain fully 90 per cent. of carbonate of lime; the rest is alumina and iron.*

When the limestone beds come in contact with the alternating strata of clay, more alumina gets introduced into the mineral. The lias limestone is particularly characterised by its dull earthy aspect, and large conchoidal fracture. Its colour varies

Westbury cliff on the west bank of the Severn, Gloucestershire, illustrates the lower beds of the lias formation.

[blocks in formation]

Green siliceous grit, highly micaceous, and containing plenty of bones, well known by the name of the bone bed,

Black shale,

Green grit,

Black shale,

Greenish marlstone decomposing into balls,

Red marl of the new red sandstone formation.

2026

18

SHALE ALTERNATING WITH THE LIAS.

199

in different beds from light slate-blue or smoke-gray to white, the former shades belong to the upper, the latter to the lower layers of the formation. The blue lias contains much iron, and affords a lime capable of setting under water. The white is susceptible of a fine polish, and may be employed in lithography. It must be distinguished, however, from the lithographic stone imported into this country from the continent. This comes from the quarries of Solenhofen, and is of much more recent formation, or stands higher in the order of superposition.

The slate-clay or shale with which the lias alternates is of a gray-brown or black colour, it is frequently bituminous, and readily divides into plates as thin as pasteboard. The lias is never of a variegated hue like common marble, nor brecciated, neither does it take brilliancy or depth of tint by polishing. It sometimes exhibits dendritical appearances (Cottam marble). The irregular beds consist of fibrous limestone and cement stones (septaria), so called from being used in making Hydraulic mortar. When they are large and flat, these septaria are called girdles. Along the Whitby coast these girdles have afforded a partial protection to the shale, occasioning a number of grotesque insulated masses, which often turn red on exposure to the air.

The lias is nearly destitute of metallic or earthy minerals, except iron-pyrites which is very abundant. By the acidification of its sulphur the argillaceous strata acquire that efflorescence of the aluminous sulphate, which is so extensively worked at Whitby.

To the same pyritous decomposition, must be ascribed the spontaneous inflammation often observed in the cliffs near Charmouth in Dorsetshire.

Organic Remains.-The organic remains contained in the lias are peculiarly interesting. They afford more animals of a higher order (viz. of the vertebral class) than are contained in the catalogue of any other formation, excepting perhaps the Stonesfield beds of calcareous slate, in the great oolitic series about to be described.

In this class we have first to notice the remains of two very remarkable extinct genera of oviparous quadrupeds, evidently belonging to the same group with the great natural order Lacerta, or lizard; but differing most essentially in structure from all the genera at present known to exist, and in such a way as must have fitted them to live exclusively in the sea. They appear, therefore, to bear the same relation to living lacertæ, that the cetacea (whale tribe) bear to other mammalia, and form a division of the order lacerta to which the name Enalio-Sauri (marine lizards or fish-crocodiles) may be conveniently applied. The investigation of their comparative anatomy, or rather osteology, is very interesting. It has laid open various new links in the chain of animated nature. See the end of this section.

LIST OF LIAS FOSSILS.

3. The Whitby alum shale contains also abundant bones of the ichthyosaurus and plesiosaurus.

4. Bones and palates of the turtle have been found in this formation. 5. Fish of several species also occur in its strata.

6. The radius of a species of balista is of common occurrence.

EXTENT OF THE ENGLISH LIAS FORMATION. 201

7. There are some crabs, of the crustaceous tribe.

8. Among testaceous animals we have species of ammonites, nautilites, belemnites, helecinæ, the trochus, modiola, unio, cardita, terebratula, spirifer, gryphea, ostrea, pecten, plagiostoma, lima, plicatula; but the most characteristic shells, are the ammonites Bucklandi, gryphea incurva, and the plagiostoma gigantea. See Plate II.

9. Echinus, a variety of cidaris papillata.

10. Encrinites. Many species of pentacrinite occur in the of the lias formation.

11. Corals-madrepora turbinata.

upper beds

The vegetable remains consist of fossil wood, sometimes charred and occasionally silicified. Gigantic reeds resembling arundo donar, are found in the sea cliffs opposite High Whitby. Trunks and branches of fossil trees, the bark and softer parts of which have been changed into jet, are frequently met with in the alum shale; and leaves and impressions like those of the palm, occur in the sandstone and iron-stone.

The lias formation stretches across from the coasts of the German Ocean in Yorkshire to those of the Channel in Dorsetshire. The best places for studying this stratum are the cliffs of Whitby in Yorkshire, those of Fretherne and Westbury on the estuary of the Severn in Gloucestershire, of Watchett in Somersetshire, of Aberthau in Glamorganshire, and of Lyme in Dorsetshire. The lias generally forms broad and level plains at the foot of the oolitic range of hills. It has been remarked that the argillaceous formations in most cases, constitute low tracts in the present configuration of the earth's surface; a circumstance which may be ascribed to their offering less resistance, after deposition under the antediluvian ocean, to the sweeping action of the agitated waters. They must have also suffered a similar operation from the rains, ever since they were brought to the day. Near the Mendips, the lias sometimes occurs on the brow of tolerably steep escarpments, but its maxi

This may

mum elevation does not amount probably to five hundred feet above the level of the sea. also be regarded as its thickness. The inclination of the strata is generally very small, not exceeding 40 feet in the mile, which conformably with all the strata that range across the island, from north-east to south-west, is in a south-easterly direction.

As a soil it is usually cold and tenacious, fitter for pasture than tillage. In Glamorganshire, however, it bears very fine wheat; and the marl of the rag, or gray lias, is reckoned the richest in the country.

The springs are generally thrown out by the marl above the lias, near its junction with the lower beds of the sand that underlie the inferior oolites and it is therefore uncertain, to which formation the flow of water should with most propriety be referred.

Completely within the district of the lias marls, water can be had only by sinking to the bottom beds.

GREAT ANTEDILUVIAN AMPHIBIA.

I shall annex to the present section, a somewhat detailed account of this interesting order of antediluvian animals, all long since extinct, which occur in the lias or the adjoining mineral beds. A few words may be premised on the great cetaceous fishes (the whale tribe). From an investigation of their fossil remains, and from a skilful comparison of them with the osteology of their living types, M. Cuvier draws the following conclusions.

Thus is more and more confirmed the proposition already deduced from the examination of fossil shells,

« PoprzedniaDalej »