Part 45 (2/2)

North of England. Thuringia.

1. Crystalline or concretionary,

1. Stinkstein.

and non-crystalline limestone.

2. Brecciated and pseudo-brecciated

2. Rauchwacke.

limestone.

3. Fossiliferous limestone.

3. Dolomit, or Upper Zechstein.

4. Compact limestone.

4. Zechstein, or Lower Zechstein.

5. Marl-slate.

5. Mergel-schiefer, or Kupferschiefer.

6. Inferior sandstones of various

6. Rothliegendes.

colours.

I shall proceed, therefore, to treat briefly of these subdivisions, beginning with the highest, and referring the reader, for a fuller description of the lithological character of the whole group, as it occurs in the north of England, to a valuable memoir by Professor Sedgwick, published in 1835.[302-A]

_Crystalline or concretionary limestone_ (No. 1.).--This formation is seen upon the coast of Durham and Yorks.h.i.+re, between the Wear and the Tees.

Among its characteristic fossils are _Schizodus Schlotheimi_ (fig. 333.) and _Mytilus septifer_ (fig. 335.).

[Ill.u.s.tration: Fig. 333. _Schizodus Schlotheimi_, Geinitz. Syn. _Axinus obscurus_, Sow. Crystalline limestone, Permian.]

[Ill.u.s.tration: Fig. 334. _Schizodus truncatus_, King; to show hinge. Permian.]

[Ill.u.s.tration: Fig. 335. _Mytilus septifer_, King. Syn. _Modiola ac.u.minata_, James Sow. Permian crystalline limestone.]

These sh.e.l.ls occur at Hartlepool and Sunderland, where the rock a.s.sumes an oolitic and botryoidal character. Some of the beds in this division are ripple-marked; and Mr. King imagines that the absence of corals and the character of the sh.e.l.ls indicate shallow water. In some parts of the coast of Durham, where the rock is not crystalline, it contains as much as forty-four per cent. of carbonate of magnesia, mixed with carbonate of lime. In other places,--for it is extremely variable in structure,--it consists chiefly of carbonate of lime, and has concreted into globular and hemispherical ma.s.ses, varying from the size of a marble to that of a cannon-ball, and radiating from the centre. Occasionally earthy and pulverulent beds pa.s.s into compact limestone or hard granular dolomite. The stratification is very irregular, in some places well-defined, in others obliterated by the concretionary action which has re-arranged the materials of the rocks subsequently to their original deposition. Examples of this are seen at Pontefract and Ripon in Yorks.h.i.+re.

_The brecciated limestone_ (No. 2.) contains no fragments of foreign rocks, but seems composed of the breaking-up of the Permian limestone itself, about the time of its consolidation. Some of the angular ma.s.ses in Tynemouth Cliff are 2 feet in diameter. This breccia is considered by Professor Sedgwick as one of the forms of the preceding limestone, No.

1., rather than as regularly underlying it. The fragments are angular and never water-worn, and appear to have been re-cemented on the spot where they were formed. It is, therefore, suggested that they may have been due to those internal movements of the ma.s.s which produced the concretionary structure; but the subject is very obscure, and after studying the phenomenon in the Marston Rocks, on the coast of Durham, I found it impossible to form any positive opinion on the subject. The well-known brecciated limestones of the Pyrenees appeared to me to present the nearest a.n.a.logy, but on a much smaller scale.

_The fossiliferous limestone_ (No. 3.) is regarded by Mr. King as a deep-water formation, from the numerous delicate corals which it includes.

One of these, _Fenestella retiformis_ (fig. 336.), is a very variable species, and has received many different names. It sometimes attains a large size, measuring 8 inches in width. The same zoophyte is also found abundantly in the Permian of Germany.

[Ill.u.s.tration: Fig. 336. Fenestella.

_a._ _Fenestella retiformis_, Schlot.

Syn. _Gorgonia infundibuliformis_, Goldf.; _Retepora fl.u.s.tracea_, Phillips.

_b._ Part of the same highly magnified.

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